Home brewed JTM 50 "Black Flag". High Pitched squeal on presence control.

[uk_blaster]

I have been lurking around on this board for the past few months. I was hoping I could solve my problem on my own, but I think input from those who know more than I would be a big help.

Over the last month I have built my own JTM 50 “Black Flag” amp from a kit. The 2-EL34 + 1-GZ34 Rectifier model. It’s my first build basically using what I’ve learned on the internet and what little attention I paid in high school science class.

My problem seems to be stemming from the presence control.

Basically, when I turn up the presence control past 5 or 6 I start getting a high pitched squeal, feedback type noise. (not like good guitar feedback, the really bad sounding kind.)

I went as far to replace the whole pot and .1uf cap on the back of the pot to no avail.

I did notice something on my board though.  And let me preface this with, I have no idea if  this is the cause of my issue.
The lead coming off the first lug of my presence pot to my board connects to what is supposed to be a 27k resistor.
I did a reading on this resistor (still attached to the board) with my multimeter and only get a reading of about 4.3k - 4.2k. 

Now while I know that carbon comps do tend to drift, I’m assuming this resistor is bad and needs to be replaced.

Can anyone with a solid electrical background confirm this for me? Would that be the cause of this high pitched squeal?

I just want to reiterate, The squeal only happens when I push the presence past 5 or 6ish. Other than that, I think the amp sounds good.

Any input from you guys would be appreciated.

[TIMBO]

Hi uk, You could try reversing your OT leads to the power tubes. 

[uk_blaster]

Hi,

Thanks for the reply.

I actually tried that last week after reading some of the posts here.
Soon as I flipped the stand by switch, I got an uncontrollable deep sounding buzzing that I could only describe as, "the sound someone would hear about half a second before being slammed into by the front end of a Mack truck."

Naturally I switched them right back.  

EDIT: I have to say, I really like the sound of the rest of the amp. Almost to the point that I just keep playing through it with the presence all the way down.

So, to post another question, would it be possible to negate the squealing by installing a lower value pot?

Maybe replace the 5Kl presence pot with a 1kl? Or is that incorrect thinking?

[Baguette]

Maybe your presence control works great but reveals some high frequency oscillation when turned up?
Does the same oscillation occurs when the treble control is turned up?
I would check lead dress, grid wires etc.

[uk_blaster]

Hi, Thanks for the reply.

No, the same does not happen with the treble control. In fact, I max out all the other controls and just use the volume as needed per the situation and control the overall level from my guitar's volume.

The only one I can't max out is the presence. 

[firemedic]

I had the exact same problem with my Laney 50W a couple years ago & wound up ripping the guts out & building a new amp. Then another, and another and another and another. Then 2 or 3 more.

Without negative feedback (presence turned up) you have a parasitic oscillation, which can be very hard to find. The presence control is removing enough NFB to make the oscillation manifest itself.

One solution could be to place a 2.2k or 3.3k resistor in series between the presence cap & ground. A smaller pot won't help. The series resistor there will keep the hi-freq. NFB in place but still leave you a usable presence control.
Kind of a band-aid but oscillation can be real hard to troubleshoot. But as always, retrace your lead dress to see if just moving a signal wire will eliminate the problem.

   

[uk_blaster]

Thank you for your reply.

It almost sounds like I should have a look around my input wiring and my 8ohm tap coming off my output transformer, and into the feedback resistor before it got into the phase inverter.

I''ll poke around these areas. Perhaps re-ground my inputs?

Thank you.

[Willabe]

But as always, retrace your lead dress to see if just moving a signal wire will eliminate the problem.

Look for grid wires (input) that are too long or laying on/near a plate wire (output) these 2 wires are 180deg. out of phase. Move the grid wire away with a wooden dowel or chop stick with the amp on, until the oscillation stops. You might have to shorten the grid wire as it acts as antenna, less antenna less it can pick up and inject in to the input to be amplified. If you have a long grid wire run, say input jack on the front of the amp going to a preamp tube at the back of the amp chassis, replace it with shielded wire. Only ground the shield at one end, input jack in this case.


                        Brad                

[uk_blaster]

Hi, Thanks for the tip. It's been a big help already.  

I just moved the input wires coming off the cliff jacks (which eventually hit the grid) away from the plate wires like you said (while the amp was off, though) and plugged back into the amp to find that I now only have this issue oscillation issue on the first input jack of treble channel. Also, the oscillations only start when I hit about 8 or 9 now on the presence control.

I think my next step is to at least replace as much of the input wire run coming off that first jack on the fist channel going back to the grid on the first tube (pin 7).
It's going to be a bit of a bitch as this is one of those wires that after it hits the two 68k grid resistors running in parallel on the board (if I even explained that correctly) it goes under the bored and thru two little holes on the board.
Kind of like a shoelace, they go through couple of holes in the board.

If I can get the old grid wire out of that board, how much harm do I pose (if any) by running that shielded wire above the board from those two 68k resistors to pin 7 of the 12ax7?

Thank you for your help.

[Willabe]

If I can get the old grid wire out of that board, how much harm do I pose (if any) by running that shielded wire above the board from those two 68k resistors to pin 7 of the 12ax7?

None.

Really those 2 grid stop 68K"s should be on the tube socket grid pins, soldered up as tight to the pin within reason. Leave as little lead as possible, they will work best that way. Put them on the tube socket and let the other end fly up in the air and connect the hot of the shielded wire to it.


                         Brad     

[uk_blaster]

Really? Wow.

I never though of doing it that way. I've been studying photos of other Marshall amps and some photos of Mike Germino's amps and even he still mounts those resistors to the turrets.

As soon as I get some shielded wire, I will hook it up like how you describe.

Thank you.

[uk_blaster]

Hay again,

Well, I did as you instructed. And while the issue did not go away entirely, I noticed this.

When plugged into the first input on the first channel, I still get that same oscillation. But, if I am plugged in with the presence turned up and the oscillations happening and I touch the metal cap pn the power transformer, the oscillation goes away entirely.

Now, I think I understand what ids going on. I'm some how grounding out the oscillation by touching the metal surface, but does that mean I have a band ground in the amp?

As per the wiring diagram that I followed (and many other like it that I have seen) I have both the filter caps, HT fuse and transformer itself grounded to a single lug attached to one of the transformer mounting screws.

And advice?

Thanks for reading this.

[Willabe]

And while the issue did not go away entirely.

How much did it improve/lessen? At what settings does it happen?

Can you post a few good pics of the insides of your build? It would help the guys to be able to offer ideas as to what needs to be changed/fixed.


                      Brad    

[uk_blaster]

Thanks.

It only happens as the very end of the presence control being maxed out.
Say from 9-10 of the dial. That last little bit of the pot.

I'll try getting some pics of the inside posted as soon as I can.

[uk_blaster]

Hi guys,

I just wanted to update you and let you know that the squealing issue has been eliminated.

I attached a 5.6k 1watt resistor to each pin 5 of the el34 output tubes then wired the other end back up to the board.
I re-biased the amp with a bias meter, loaded it into the case and the unit seems to be running fine. Sounds great. 

I'd like to thank everyone again who replied to my post and helped me out. I really appreciate it.

I'm already looking forward to my next build. I think an JTM45 with the KT66's is next.

Thanks again.

Uk

[Willabe]

the unit seems to be running fine. Sounds great. 

Great to hear!


         
                         Brad