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Jaz,

I tacked in an additional 1m to ground, where the 68k, and two inputs for each channel merge. This didn't help with what we're looking for, but man, did it kill some hum! I notice no sonic difference with or without the resistor when playing a chord. (Just a quick test, but I like it! I think it'll stay there.)

I tried a 100pf across the 1m (effectively grid to gnd), no difference. I tried it on the socket to its circuits ground, nope.

Channel 2 still sounds better than 1 with respect to the RF/noise floor. Channel 1 doesn't even have a cathode bypass cap on stage 1, and it's the worst of the two channels.

I made up a little brass shield to go around the input jack and its resistors, giving it its own little grounded hut to live in. But that didn't help either.

Your suggestions were awesome, thanks!

[Update]

I'm changing the OT's out right now and noticed someone had the + and - on the hanging speaker input jack reversed. I'm pretty sure with one speaker this doesn't make a difference but because this amp has negative feedback would the blk ground wire from the old OT going to the input of the feedback loop (the 270K R) and the green, + going to ground have anything to do with the OT sounding weak with low volume...?

al

Update

I put the new OT in as recommended by HBP with the Negative Feed Back on a switch, I mounted the speaker jack inside the amp on the chassis, that's the ground for the speaker (not hanging like the pic). I put the NFB switch off the speaker jack + and before the 270K R (both mounted next to each other in the chassis). This is exactly like the schematic except the old OT had one sec winding with two pairs of wires like the pic I posted. The new one only had a pair, + and -.

The resistance between both green and both black sec on the old OT were less than 1 ohm, so they both most likely were solder at the same winding point.

Feedback switch off, the amp screams, very loud and good head room, basically all fixed, looks like it was the old OT...?  But when I turn on the feedback, I get a "huge pulsating hum from the speaker", I can feel the vibration and have to turn it off in seconds.

There are only two components in the feedback loop, the 270K to the cathode of the PI or driver tube (12AX7) then from the cathode to ground through an 8.2K R. Both resistors check out good, I checked the solder joints and cleaned the board in that area.

Still the big buzz...?  Could switching the OT primaries, brown and blue possibly help? But the amp plays great with the feedback off.

al

I look forward to following your progress with this project. Let me know if I can contribute. You may want to start a new thread since there are already 50+ messages in this thread.

OK,
 If you agree that the AB763 is a good base, i'll build it on the breadboard first, as-is, and we'll go from there.....I'm hoping this circuit choice will simplify some things for myself and others who might want to build one and just use a Hoffman board and layout

I'll start with a fresh schematic and incorporate those changes to the circuit that I mentioned, and we'll see where it goes.....

I've got a new Hammond 1760L (4.2K primary) that can't wait to push 2 6L6s around 
With my bench supply, I can start with any ac input voltage up to 480,,,,let me know where you'd like me to be

I also have a separate 230 vac supply (straight off the wall---if that's ok) if you'd like to work the negative supply separately at first...your call
I'll start with the standard fixed bias supply that I build onto the board,,,and get it up and running

I was looking for a reason to build a Deluxe Reverb Plus, and here it is........a close friend just bought a reissue that I have full access to, so I'll let you know just as soon as mine sounds better than his   

The part gathering has already begun and the good news is, I have most of it here 
As long as life co-operates, this shouldn't take long........I'll worry about chassis and cabinet details later

... I'm leaning towards a single channel AB763 with reverb ...
I was also looking at the '63 Blonde Twin, for it's lower voltages,,,but I'd like to have reverb

...

Can we do that within the voltage range you want to be?

Ideally, you'd have an existing amp you want to plug this stage into, rather than coming up with a design from scratch. Why? Because you know how the amp sounds now, and so will know what the change did to the amp's performance is the main reason. The other big reason is that out of the hundreds of little things to decide to design a complete amp, we would only need to figure out a few to add the new cathode follower stage.

Yeah,  my cherry tomato bushes are doing better this year then ever before. Should have a pretty nice batch of them maybe by this weekend?  Not growing anything else this year and decided to just do the local farmer's market more.

 

Jeff

You are welcome, I see (sorry if I make you repeat yourself), so it appears that you may have too much gain and/or picking up interference at the first stage for both channels. Some quick things you can try, in no particular order:

- move the 1M grid leak resistor to before the 68k, i.e., it goes between the input pin and ground;
- remove the 22uF cathode bypass capacitors or substitute a smaller one say 1uF, that will knock the gain down for both stages;
- add a capacitor from the input grids to ground, say 100-270pF, this will roll off the HF.

The Studio Bass uses a 12AT7 as a post-PI CF to drive 3 x 6L6GC on each side

so far, sim show 12AT7 has best numeric performance. best PSRR, largest swing driving 47K load and lowest Zout of the common 9pin bottles including 12AU7/6CG7/12BH7. maybe why fender chose it?

--pete

I'm adding a clear shot of stage(s) 1. Both are essentially the same, with some minor variants.

This is channel 2, as is. (Ignore the dotted capacitor on the 1 meg output pot)

For channel 1, subtract the 100pf across the plate resistor. Subtract the 22uf across the cathode resistor. The dotted cap on the 1 meg output pot is a 270pf.

Remember that the input jack goes to one single shared 68K, and then goes to both channel 1 and channel 2 grids of each "stage 1".

Jaz,

Yes, the bias knob was in control of the boating. However, since as previously mentioned, when I re-situated the pots ground, it helped immensely. Before, it would boat, after, I'd hear a "wubb" as I passed about 2 on the pot, if it were labeled 1-10 concentrically. Just now, I don't get the "wubb".

It is acting strange still. At zero and bringing it up, I can hear it increasing the hiss of the amp sharply, then quickly diminishing it before I even get to 12 o'clock.

I did reverse tube plucking as you asked, and it yielded the results I had assumed. V1, and V3 (first two stages of both channels) are the culprits for the dirty noise floor and RF. V1a, V3a, and V3b all have the 22pf grid to plate, and V3 is the worst. Even with having 1st and 2nd stages with this mod as opposed to just the 1st stage of the other.

V3 is channel 1
V1 is channel 2

In conclusion, the amp sounds pretty darn quiet given the settings I have it set at for this test, and with those two tubes pulled.

What is a better way to eliminate RF other than the grid to plate cap I'm doing? It's not all high freq stuff, there's a lot of low garble in there too. I think it's post three I made here in the beginning, the sound clip I attached a link to. It still sounds like that, but not as bad.

It goes to show I've been working on this amp a lot more than I should be paying attention to actually playing the guitar.

When we first determined the value of and implemented this 22pf grid/plate cap, we watched it on the scope. Adjusting the signal generator, at about 15k it would start to roll off, at about 20k the signal had shrank by at least 50%, and continued decreasing on the same curve as K further increased.

Thank you so much for continuing this battle with me, Jaz.