Would this make my Hiwatt go kaboom?

[frank57]

I was thinking of moving the first filter cap and diodes off the pcb
and away from the preamp tubes. Get that Hv stuff away from the tubes as well.
It's definitely contributing hum.
I've probably got it wrong though, but I gave it a shot.
Would this work?
Schematic is here:
http://www.el34world.com/Forum/index.php?PHPSESSID=c186f5db1b40178

Right now I'm going to replace the 4004 diodes with 4007 anyway.
Maybe the 4004 might be part of the problem on this amp.

Anybody seen this new pcb hiwatt by the way:
http://www.hiwatt.co.uk/prod_details.aspx?pid=100038&cid=21

Might be an ok amp

[triode]

Quick question: is it giving you problems now? You say it is definitely contributing hum... I question
that slightly, even hiwatt would not like to design, build and sell amps that hum due to PCB layout
(at least not any real hum).

Still, what you drew _would_ work, although a lot of people would take the wire coming from the
+ of the first filter cap and run that through the standby switch, as you want to keep the ground
lines as short as possible. Additionally, if you lift the ground of the filter caps, it could cause the
voltage there to rise to a level higher than the cap can withstand (although most likely not).

Look at any of the hoffman boards to see how he does the standby switch, and ground the cap
to the closest good ground, IMHO.

[PRR]

> is it giving you problems now?

frank57 has been posting many-many messages on many forums asking for help with this low-price clone "HiWatt". He's had detailed responses representing hours of time from several gurus. I don't think he's understanding any of it.

[frank57]

This is not a simple solution.
It's a great learning tool which is why I've hung onto it.
It's a troubleshooting problem, hence difficult,  especially without a scope.
That hum is getting in there somewhere somehow.
There is also a tremolo like effect as well.

The suggestions have not solved the problem.
I have tried them all.
Some of the so called repair people who have looked at this amp have tried the absolute stupidest things to "fix" it like biasing at -29 and fiddling with mid cap values to reduce the hum and then insisting it's fixed.
Perhaps they don't "get it" either.One guy wrote pos and neg all over the board for the filter caps, although it's clearly marked anyway.Comical really.I doubt if any of them seriously tried to troubleshoot this thing with a scope.
The last guy actually admitted he didn't if you can believe it.Didn't get a refund either.

It's not a bad filter cap and it's not the heaters.

The layout is bad and it's a known issue with these amps.
It's certainly not helping.
Using IN4004 diodes isn't very bright either is it?

The red hv wires are contributing hum for sure.
Running wires like that right over the preamp tubes with the diodes right there is as dumb as you can get.
It's not the answer but it is part of the problem.Moving them as much as I can with a chopstick has an effect.

Don't assume Hiwatt or anyone else wouldn't sell you a piece of crap if they can get away with it.
I've had zero help from them on this amp.
Could you elaborate on the standby switch?
This amp has two wires on the standby, the green going to first filter cap negative or the board ground, and the black from the power transformer.

There is the odd thing about the power transformer ant the high voltages.
It has 2 red wires, one black and one green and the 2 yellow wires.
It may be that it's time to pack it in as well or go to rebuild ideas.
Great learning tool though.

[Danskman]

From my experience, you won't be able to correct this edsel... It's poorly designed, as often are these chinese amps. I bought some years ago a Behringer bass amp (SS) and tried to correct a strong buzzzz, but this POS was not designed to be modified or corrected. I sold it after some weeks.
Keep you "Hiwatt" as it is. Or replace the guts with a turret board, but this may imply putting mucho $$$...
Danskman

[frank57]

Yes it's a sad reality.
They are all POS.
Perhaps the new Hiwatt t20 amp is better designed having been designed in England.

Maybe ceriatone will come up with something.
Alteratively I can simply try to sell it as is.
Good solid wood Hiwatt style amp anyway.
The output transformer is huge like a 50 watt size.
Some good components on the board too.
Ok as a practice amp to save wear and tear on your main amp.

The jet city amps at least look alright inside and love him or hate him, they are designed by mike soldano.
What do you guys think?
Here's some shots of the inside of the 20 watt combo I found online.
No center tap on the transformer from what they told me.
Apologies to the original poster.

[gldtp99]

I think the amp in the pics has PCB-mounted tube sockets---- i may be old fashoned but i prefer chassis mounted tube sockets---it may be a good sounding amp, though.
Why not give up on the PCB in your Hiwatt, remove it and build a working circuit on a turretboard in its place.

[frank57]

I don't like those pcb tubes either but nothing we can do about it.
I've thinking about gutting it too and I think I might be able to pull that off.
But it's tough because the chassis is small.
It might be possible to rework a hiwatt preamp board, and get a 20 watt hiwatt style amp going based on this one.
and I did start that,but the power stuff throws me off, it's a little too complicated.
The jet city combo is awfully cheap right now because they have a 2 channel one in the works.
Main complaint is that set clean it's not that loud, it is one channel.
 here's a clip of it:

[Danskman]

No center-tapped PT is not really a problem; you have to use a bridge rectifier (SS) instead of a full wave one (SS or valve). The rest of the power supply circuit may be build as usual, IMHO.
BR,
Danskman

[frank57]

The only thing I wonder is if there's room in there for it.
But for the price they're going for, about 299.00 US,
they're pretty good value.
It is a soldano type circuit in there for sure.
It's not an astroverb like some people think though, it has  less gain which is a good thing.
More like an atomic 16 with reverb.

[frank57]

Okay here's the board idea with the standby switch in there.
V2 or v3 better?
The one with the switch in the B+ line, not the ground line, is better for fixed bias amp?

[RicharD]

You're totally chasing your tail.  1st off, your link to the schematic just throws me back to "tube amp building-tweaks-repairs.  Looking at the layout, I don't see how moving the rectifier off board is gonna do squat.  Are all those tubes really only a 1/2" apart?  What is the B+ voltage?  Since there are only 2 diodes tied full wave your HT has to have a center tap.  If they installed 1N4004 at the factory, I'm guessing this is a very low voltage amp.  It may not even be worth gutting.  A Chevette and Corvette are both Chevrolet but can't really be compared.  You can put phat tires and a cherry bomb on your Chevette but it's still gonna suck.

[frank57]

 Well, it's a great learning tool as I said before. I knew zero a year ago.
In 4007 diodes seem to have solved the tremolo issue.
Schematic is here:
http://www.el34world.com/Forum/index.php?topic=8727.msg78728#msg78728
Obviously if the genius who designed the amp had done it properly, v1 wouldn't even be there,
considering the efx loop could have been better implemented.That's the way to kill your signal eh?

Voltages all seem high if you ask me.
I've attached the full pcb board as well.
The plate voltage is 374-377.
Bias is about -15.3 or so.
Voltage on heaters is about 3.81
Other voltage is about 7.6.
Problem stops with v2 tube out.
With v2 in, hum goes up turning the volume control with tone controls engaged.
Buzz increases with gain.Mid pot makes hum worse especially.

With tone controls down and using only master and gain not as much of a hum buzz problem.
Hum goes down moving hv towards v3. Buzz goes up towards v4.
Moving output wires a bit further brings hum down a little too.
Yes the tubes are close but I don't think that's the problem
Transformers are not 90 degrees either but turning the power transformer doesn't do much, although it might affect the buzz.
Maybe transformer is amplifying the recto buzz because of the diode location?
In any event it's all centered on v2.
Any troubleshooting ideas?
Transformer has 2 red wires, 2 yellow ,one black (to standby) and one green to ground bolt where the power is grounded.
Is there a way to post a sound file?

[RicharD]

> http://www.el34world.com/Forum/index.php?topic=8727.msg78728#msg78728 

Ah... SSDT.  Everything has already been answered.
You are chasing your tail.  Fairy dust might work but that stuff is hard to come by.  You got 3 choices:

A.  Keep chasing your tail
B.  Sell it before you break it.
C.  Bulldoze it and build something new

B ain't gonna get you jack for scratch so C it is. 
Look at Doug's library of projects.  You've already got the right enough tubes & transformers for the Stout.  It's a simple circuit and kicks much ass.  Build it overdrive only.  Clean channel sucks unless you un-mod the PI.  The PI mod is where the "magic" is when coupled with the additional gain stage.  Now go to Doug's parts store and buy a turret board, a bunch of sockets, some caps -n- resistors and rock on.  You've already got all the expensive crap.  I bet you can get out for a little over a C note.

[frank57]

If I had tried to gut it at the beginning then I would have been chasing my tail,
 since I knew "squat" as you put it.You have to learn somewhere.


In actual fact it's a kick-ass little amp, with huge transformers,
and I mean huge, especially the output one which is like a 50 watt size.
You won't find these on a jet city amp or fender piece of junk blues junior let me tell you right now.
If you mod the circuit and take a out a gain stage even better but...


Yes I am considering putting something in there, but it's not easy because of the chassis size.
I would need a bit of help on it.
The amp itself is well made.
Maybe something Hiwatt derived could work.
I started a preamp idea with 3 tubes based on this amp but what to do about the power?
Maybe a brilliant input could be put in and I can use 4 tubes, but I would rather simplify it.
I can always post it.
It's rough but maybe you guys can help .
Copy the power stuff from this amp?


As you said you have to cost it out.
I'm also considering getting rid of it.
Certainly a builder can use it very easily.
It's also a useable practice amp.

As for the voltages on the power tubes:
pin 2: -15.32
pin3 :.001
Pin 7 :373
pin 9 :371
Not exactly low wattage.
The stout eh?
I'll have to check that out.
But I see a few problems with it on this chassis.

[RicharD]

I never said you didn't know squat.  I said moving the rectifier off board won't do squat.  OTOH, you said this amp was designed by idiots and now all of the sudden it's kick ass.  If it's kick ass, why mess with it?  In the other thread,  PRR pointed out a couple of the more grievous PCB errors.  There are more.  Your last post in the other thread has a nice colorful rendition of the PCB showing how to fix the grounding scheme.  It's still a Chevette.  Best of luck.

-Richard

[dynaman1]

> is it giving you problems now?

frank57 has been posting many-many messages on many forums asking for help with this low-price clone "HiWatt". He's had detailed responses representing hours of time from several gurus. I don't think he's understanding any of it.

Haha, that's politely understated. Incidentally, I've had to a bit of research on some Hiwatt stuff and have come across identical posts from this fella on more than a few forums. Some date to 2009. I think it's safe to say that he just enjoys the attention or is trolling for...I don't know what.

"Learning" is great, but that amp's not exactly user friendly. There's no shame in defeat if you learn something and just jump back up and get on a different horse. Seems like your trying to put new rings in a motor with a cracked block. And you've already (supposedly) wasted $$$ on a bunch of mechanics.

[frank57]

I'm not trolling for anything or need attention.
I've learned a tremendous amount so it's all been worth it.
The detailed responses and ideas have been checked out .
Including prr's idea to go straight to the first filter cap. Didn't do much.
The ground cuts I've been reluctant to do.
Now I gave that printout on those cuts to one of the repair techs I saw and he didn't do it. You figure it out.

This thing has dragged on mainly because I've wasted 7-8 months waiting for repair people to work on the amp only to discover that all they did was bias it -29 or - 24.

I have no idea if moving the diodes will work, but that is one of the big blunders.
As for the amp being designed by an idiot, I'm referring mainly to the poor pcb layout.
There is a flaw in the circuit design, but it depends on your point of view about that efx loop.
It could be that something wasn't done properly on a run of these amps.
ground track not cut properly?Screwy transformer?
 
I would like to fix the amp.What's wrong with that?
It sounds very, very good but it has hum and buzz issues.

And yes I am reaching the point to sell it and get on another horse.
I don't disagree about there being shame in defeat.You lose, you lose, fight another day.
I've learned about as much as I can from it.
That's why I was thinking of gutting it now that I have the knowledge.
But with the knowledge you see the problems this chassis will cause even going that route.
It's small and tight.

The theory behind the big output transformer on the small amp,in case you didn't know,
is that it will supposedly give you the dynamics of the 50 watt amp.Hogwash?
Maybe. But this amp sounds thick and full, not thin like Jet city or Egnater etc.
They're all the same stupid thing in a different case.


The dr210 is an interesting hiwatt variation and I think might fit in here.



I have finally found an excellent repair tech, but too late.

Still, at least I have someone I can bring the next amp to if it needs a real repair and not a con job.

[frank57]

Now can we stop getting sidetracked and take a shot at troubleshooting v2 at least?
If you had the amp in front of you,
 would you try something with a test clip on v2 to kill the hum?
If so what?
Try to figure out which part of v2 is the problem somehow?
V2a or v2b is the problem?
Is it coming in on the ground track?
If so how  to determine that?
Take out the scope and try to figure it out that way?
Replace the coupling caps?
Can't check the voltages from the top on them because of the way the amp is put together.
Connections might be part of the problem.
Replace some of the 473 caps that take on some of the ripple?
Contaminated pcb?
In which case no way to fix it.

[VMS]

To rule out one possible buzz source, remove the FX-loop wire and jump the send and return pins.

[frank57]

I'll give it a shot.
It's a bit tricky because they poked the connectors
all the way through the board and soldered them.
They don't just disconnect like on the output pcb where the send return jacks are.
Would simply disconnecting it there achieve the same end?

[VMS]

They don't just disconnect like on the output pcb where the send return jacks are.
Would simply disconnecting it there achieve the same end?

Yes (perhaps), try that and jump the red and white wire. Then move the wire away from the output jacks.

After you have tried that, and if the buzz is still there, try this: Move orange and black OT wires further away from the preamp tubes. And I would make the white and black output wires a bit longer and route those like this:

[frank57]

Here's a more recent shot.
The black orange wires are now flat along the chassis.
I think this was the original scheme from what I got on one forum.
I did try moving them before as you suggested but not much happened.

It may not be the best spot but at least this way they're out of the way, making the amp easier to work on.
Standby switch wire is also out of the way as well.
I have to get some wire, but definitely the speaker output wires are getting some hum where they are now.
We'll see what happens.
Maybe some of that buzz is from the unshielded power transformer?   

[67polara]

I don't know anything about that amp.  Does it have a solid state devices on it?  If not why not disconnect the heater wires and use a 6 volt lantern battery and see if it goes away.  The heater traces on the boards are close together and have jumpers over the top of them from the top side of the board many times they are signal wires.  Just a thought should be a quick check.

Tony

[frank57]

It's a good idea, but the hum is not from the heaters in this case.
That's pretty much been confirmed.

Could be a leaky coupling cap, so I'm going to replace one or two of those as well.
Also maybe cut that ground track near v2 and see if the ground is the problem.
The buzz issue seems to be related to the treble control and gain pot in particular.
It seems to be rectifier diode buzz which is being amplified somehow.
I'll try the efx loop take out and see what happens.

[VMS]

Maybe some of that buzz is from the unshielded power transformer?    

Probably not.

[VMS]

Also, I think it would make more sense to wire the OT wires like this:

[frank57]

You mean to the pads for the speaker outputs?
Might be worth a try too.

I don't know if the OT transformer wires are a big part of the problem,
but who knows with this amp.

That buzz seems to be generated by the treble pot in particular in combination with the gain.
That treble is connected to that efx loop so we'll see what happens there.
With the tone controls down and MV full blast you can hear the hum go down from the hv wires,
if you move them. I think when they're at 90 degrees to the output speaker wires.
It's not the main problem, I think, but there is some hum there.
Using the tone controls, you then get the hum out of control. Especially the mid.
Maybe the ground track has to be chopped near the bass control as was suggested.
If nothing happens then we know it's not the ground at least.

[VMS]

Have you tried this:

Turn gain and tone controls to zero and connect guitar to the fx-loop return. Does the amp buzz with full blast MV?

[frank57]

Just tried that right now.
With tone controls off, nothing plugged in,there is hum which increases as I turn the Master Volume up.
Plugging in the guitar into the efx loop return or send has no effect on it.
There might be very little buzz at full blast,but it doesn't seem to be the same kind of buzz.
Overall, buzz doesn't seem to be there.
Maybe there is heater related hum after all?

If I turn the tone controls on, I get the buzz with nothing plugged in.
Plugging into the return kills that buzz.
Only clue from one repair tech who at least took a quick look with the scope, was power supply problem,
and rectifier buzz.
Which pretty much could be anything I suppose.
Maybe that diode layout is getting buzz into the power supply going into tube 2 and then to the tone stack via the treble? Leaky cap there maybe?

If I tried the board idea with the diodes,what would the best way be to put that board together?
The centers of the screws are about 3" and 2 .75".Might be worth a shot.

[VMS]

It seems that the preamp is picking up the buzz from somewhere and then amplifies it.

If I tried the board idea with the diodes,what would the best way be to put that board together?
The centers of the screws are about 3" and 2 .75".Might be worth a shot.

Easiest way is to use terminal strips, like on the bottom of this page:

http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=BoardBuilding&ORDER_ID=!ORDERID!

One more wire that you could try moving is this red OTP wire:

[frank57]

Okay I moved the red wire and also the other HV a bit more in front of tube 4.
But I think I figured out something big here.
Going to the ground bolt from pin 2 with my multimeter on the v2 tube,
kills the hum and buzz. It doesn't get rid of it completely,
but wow does it knock it down a good 50-60 maybe 70percent.
Maybe that 223 cap there isn't working?
R6 R8 and c7 tend to change the frequency of that hum when I go to ground as well but they don't kill it.
I am getting the radio on pin 7 when I go to ground as well.
Maybe related to the repair?

So is the grid on the first half of v2 the big problem?
Wonder if the tube socket itself is crap?

[VMS]

So is the grid on the first half of v2 the big problem?

I think you are just shunting the signal to ground, which means no signal no buzz.

[frank57]

Anyway to know for sure?
The controls seem to all be working.
So far it's the only thing that's done anything.
There is some buzz and hum but nothing like before.
Plug in a guitar?

[dynaman1]

Have you checked for leaky coupling caps?

[frank57]

The problem is I can't check some of them, especially the 223 one on v2, from the top.
Whoever put the amp together squashed everything flush to the board.
So it's going to have to be the bulldozer route and just replace them.

I can measure all the voltages around there and post them.
But also if I rememeber the plate voltage is 201 on pin 1 and 134 on pin 6 for v2.
Some voltages:
c3 149  .001 ground
r2 289  149 towards c3
r9 314  289 towards r2
c12 200 going in .001 to bass and mid pot.
r10 is 313 going in 201 to r12.
R12 doesn't seem to do anything as I get 200 the other side towards c12.

[VMS]

Going to the ground bolt from pin 2 with my multimeter on the v2 tube,
kills the hum and buzz.

Just to make sure. Do you mean that while you are measuring voltage from pin2 to ground the buzz goes away?

[frank57]

Yes, exactly that.
I probably didn't explain it well.

Measuring the dc voltage, boom it's gone.
The minute you put the red lead on the grid with the black lead on the ground bolt, and press it, bam,
kills it big it time.

Touching the parts I mentioned before doing the same thing seems to change the frequency.
The buzz and hum is there, but at a much much lower level.
I had the master volume at high noon all controls about noon.

[VMS]

Ah, I misunderstood you first time.

Unfortunately I'm not sure what is happening there, but maybe someone smarter will chime in with an answer.

You could try if connecting 1M resistor between that ground bolt and pin2 has the same effect as the multimeter.

[frank57]

Crap. Looks like a rookie mistake.
I checked again, and I must have somehow knocked the meter to measure resistance.
So when you do that you kill the hum buzz.
Hopefully that helps?
If you measure the voltages, you still get a change in the frequency of the hum on that grid.
Also on r40 the diode resistor, the buzz goes down a little when I measure voltages.
On one side I get 124 on the other .08.
On the .08 side, the buzz goes down slightly.
Pretty sure that's rectifier buzz getting to the treble mainly somehow.

[Shrapnel]

So is the grid on the first half of v2 the big problem?

I think you are just shunting the signal to ground, which means no signal no buzz.

Crap. Looks like a rookie mistake.
I checked again, and I must have somehow knocked the meter to measure resistance.
So when you do that you kill the hum buzz.
Hopefully that helps?
If you measure the voltages, you still get a change in the frequency of the hum on that grid.
Also on r40 the diode resistor, the buzz goes down a little when I measure voltages.
On one side I get 124 on the other .08.
On the .08 side, the buzz goes down slightly.
Pretty sure that's rectifier buzz getting to the treble mainly somehow.

OK... pin 2 (input grid,) of the first half of V2, shorting to ground. (Feel free to just jumper it to ground for a minute, no harm will happen... IF the buzz disappears completely, and all the controls cannot bring it back, the buzz MUST be coming from either that grid, OR from somewhere before that grid.

Have you tried pulling V1 and/or V2 and seeing if the buzz is gone?  IF the buzz is gone with only V1 pulled the noise injection is located around that bottle only. (This is based on input starting with V1 and valve/tube ID incrementing from there as we get closer to the output.)

[frank57]

I'll have to recheck that about the tubes.
For sure the hum is gone with v2 out.
If I solder a wire to grid pin of v2 and and connect other end to ground bolt, I'm shorting it to ground?
Or better to put a 1m resistor to the pin and run the other side of the wire to the bolt? Makes no difference?
Buzz and hum definitely go waaay down when I did that resistance measurement.
They're still there but nothing like before.

[frank57]

Okay checked the tubes.
Pulling v1: Hum and buzz still there.
Pull V2, (v1 back in), hum and buzz are gone.
Maybe a little if you put the MV full blast and presence full blast.
Nothing to worry about .
Tone stack doesn't work so you're bypassing some of the problem there.

Phase splitter out nothing happening at all.


The original mod someone did on this amp had v1 out.
Might be a good idea circuit wise and so on but...
They had some kind of filters in there to kill the buzz.Didn't fix the issue obviously.

[Shrapnel]

Pulling v1: Hum and buzz still there.
Pull V2, (v1 back in), hum and buzz are gone.

The original mod someone did on this amp had v1 out.

Has that mod been completely undone? The problem could exist because of how V1 was removed and later restored.

OK V2 removed, no noise. All that soldering a 1M resistor between the grid and ground accomplishes is creating a 2nd grid resistor. Next step, ground pin 2 and pin 7 of V2 at separate times and see if hum and buzz is gone for only one of the two, or for both halves of V2.

If the buzz and hum are gone when you short V2 - pin 2 to ground  and also when you short V2 - pin 7 to ground, that means that it exists somewhere in how V1 and V2 interconnect. It doesn't sound like how the two halves of V2 are connected is cause of the problem. It sounds as if it is in there between the first input into V2 from the direction of V1 and the second stage of V1 feeding V2.

I'd check everything that connects those two tubes closely as to condition (the PCB too.)

As far as shorting it, a plain wire soldered to ground and the grid pin will work (MAKE SURE that it is the right pin number.  Shorting the plate instead will make the plate resistor work harder up to the point of possible failure, but shorting the cathodes to ground could mean the end of the tube. Shorting a heater pin to ground may or may not hurt the transformer depending on the connections.

Using a 1 Meg resistor will not equal a short. If you have a wire with clips on both ends this could work instead of soldering, but take care as to not let them touch any other connection during the test.

[VMS]

Do what Shrapnel said. The 1M resistor was intended to emulate that multimeter when you said that it removes the buzz.

If you have a wire that has a alligator clip on one end and pin on the other end (like in multimeters), then connect the clip to the ground bolt and check those pins that Shrapnel suggested and also check those ground pins on the tube connector wires.

[frank57]

What I did was I used the multimeter again on resistance.
I  didn't have a chance to check the forum again, but hopefully it's still valid test.
But the hum and buzz are still there when I go to ground on pin 2 and pin 7.
That is when I use the tone stack and gain the same problem is there.
But the volume of the problem (or the amp?)is down 50 percent.
So it's the same thing but 50 % percent lower.
Also when I did it on pin 7 and turned the gain a tremendous amount of pot noise came through the speaker.
Odd thing with the gain at say 9, and the master volume at say 3(so facing opposite directions)
things seem to cancel out and the buzz is gone.
The original cut traces were reconnected.
But with v1 out of the circuit and the cut traces,the problem was still there.
Also here are some voltage measurements and a shot of the pcb bottom side.
That jumper wire is gone.
The tube connector wires have been repaired it looks like too I'll scan them in and post a shot.

The original mod I think was an attempt by someone (HIwatt denies it was them) maybe Efkay the Canadian distributor,(who pretty much told me to you know what when I phoned them) to "fix" the issue by taking out a gain stage and putting low pass filters .
It didn't work . But it was intended I think  to fool anyone taking a quick look under the hood too.
Looking from the top you would never know the amp was modded.

[frank57]

Could the problem be right at the input jack?
Now theses jacks are odd.
The tabs are reversed.
How can I get a new jack to work ?
I have one that fits but if I put it in no signal will pass.

Maybe this is a clue to the problem:
Here are some mods that were tried.
The blue one was the original mod:
2 cut traces and a wire connecting input jack to gain pot.
red one was a v1 take out.
So the two parts were removed.
In this mod however the gain pot doesn't seem to be doing anything.
None of them worked.

I'm going to try a grid stopper and see if that works or does anything.
Good idea to cut the ground track near the bass control and run a wire to the ground bolt from the gain pot ground?

[Shrapnel]

IF you look at your schematic closely, that mod you are referring to with the Blue marking is just jumpering ground to that gain pot's ground. Remember that the one image is from the component side, not the trace side of the board.... mirror image. Your red mark is what bypasses V1 straight to the gain pot. I think I would put that ground wire back in without the rest of the mod, it might make a difference especially if you don't have that other black ground wire in place.

As far as your off-board recto & 1st filter  and Bias mods, I don't think your problems lies within that, or the original circuit.

IF you dead-short pin 7 of v2, and you still have your hum, your problem lies in how V2 is passing the signal around and back to itself. But if the hum is gone, it has to be somewhere between the input of V2's pin 7, which the gain knob CANNOT be removed from the circuit without putting in some other grid resistor, and your input jack.

IF your blue scratches across the traces was where traces were cut, then you would definitely disable the amp, and even cause other problems with V2's trace to pin 7 cut. (No grid resistor: The gain pot acts as a grid resistor as well as controlling the input level.)  Also as you picture it, the red wire would be on the wrong side of the cut trace to pass any signal.

Speaking of the input jack, you said it was still humming and buzzing with V1 removed. Your input jack isn't even part of the equation with V1 reinserted (properly) into the circuit and the tube itself pulled. As far as hacking up the given ground bus, the problem may lie there, then again it might not.  I would make sure that all grounds are connected to ground first.

IF the problem does lie in the ground bus, and it could (ground loop with some mod?), then hacking the ground bus must be done properly to fix the problem.

[frank57]

This amp is a tough problem, but at least we're narrowing it down.
I might not have explained the mods well.
The original mod was:
blue cuts on the traces with the  wire going across.
So v1 out of the circuit.
Also low pass filters of some kind on the in out of the preamp(not sure what they did there)
Now I was told that this was a ground loop, because of the wire going to pin 7 on tube 1.
The other red mod  was:
cuts reconnected , two parts out and jack grounded to back bolt.
Neither one worked.

The gain pot doesn't seem to do much with that one.

Some rookie questions:
How can I be sure the ground is the problem?
What's the best way to check all the ground connections?
Check the resistance and see if it's zero or close to it?
There was a suggestion and drawing to cut the ground track, but I don't know if that's the problem or the right way to do that.


When I short pin 7 to ground using the multimeter, as on pin 2,
the whole problem goes down 50% but is still there.
Could the problem as a long shot, be on the tube board or with the connectors?
How do the ground wires work on v2:
There are two independent wires in the grey sheath:
One goes to pin7 and the other one (which is cut on the side towards the tube pcb)goes to ground on the pcb.

I tried a grid stopper on v1 but that seemed to increase the buzz.
One thing I tried was turning the power transformer 90 degrees.
The buzz I think went down, but the hum went up.
There was an idea to take v1 out and try to rejig the circuit in between v2 a and v2b so that the gain pot is in between them, but I don't think that would solve it unless we know where that hum and buzz are coming in.
I think the bias mod might help overall but I have no idea either if it would solve anything.

One other thought:
is that 200 volts going into the tone stack from v2a too much?
On the real Hiwatt you'll find a 1000pf cap in between there. Is that to reduce the voltage?
Would it make more sense to go from the cathode?

[Shrapnel]

Do you have the pinout for a 12AX7 handy?

If Not:
pins 4 & 5 & 9 are heater (4 & 5 shorted and connected together, with the other connection on pin 9, is 6.3v @ 300mA. If 4 & 5 are not shorted, then you probably have 12.6v @ 150mA.)


Now let's hope this works.

Triode 1 Triode 2 Plate 6 1 Grid 7 2 Cathode 8 3

Looking at the image of the PCB being held up, I can't tell where the "mod" wires landed, or the added filter and what it consisted of. (Low pass btw would be to cut treble or higher frequencies and a Hi-pass (or High-pass) would cut bass and lower frequencies out.)

Pin 7 of v1 doesn't go to ground, EXCEPT through the input jack and the 1 Meg resistor (R1) If this pin, or pin 2 goes to ground, no signal can enter that half of the tube. (Pin 2 of V1 isn't a problem as that half of the tube isn't being put to work.) Pin 7 of V2 uses the gain pot to control input level, and act as the grid resistor. The ground pins, I believe are there for the shielded wires going to V1 and V2. (the wires that don't have a second wire connecting to the tube board, but is connected to the main PCB.)

For clarification, when you say the gain pot doesn't seem to do much, are you referring to cutting your noise, or attenuating the signal? Normal worst case scenario with V1 pulled is a hum/buzz controlled by the gain pot. (all the way down, should be practically no noise. All the way up is the best candidate for noise, but not guaranteed.)

Realistically, the problem *could* reside in the connectors or the board the tube is mounted on.

The capacitor you refer to, is probably part of the tone control circuit and other than perhaps act as a high pass filter (which would be normal for that spot) is to block DC voltages from being right on the pots.


As to your test, I don't believe your meter will give a perfect short, but the fact that it dumps (audibly) half of the noise compared to not "shorting" in 7 at all... The best way to check the ground is with the wire mentioned earlier in this discussion to short out pins 7 and 2. THEN using one hand (other away from the chassis) ONLY, carefully, and consciously "probe" the ground bus in different parts. IF it gets quieter, you've either found a spot that fixes a ground problem, or creates one that negates another. Zero-resistance testing works only to a point, it don't allow for slight changes in working voltages due to wire size and placement. I'd hold off on any more trace hacking until we find our source.