Modded DR Too Much Filtering?

[thisismyname]

So you're talking about reducing the two series 100uf caps and not the 40uf at node B, right?

Would the amp have adequate filtering at that point?

[sluckey]

You keep wondering about too much filtering. Long time ago you said, "I have four 22uFs, three 16uFs...". Just use the three 16µFs to put the amp back to Fender's stock values. Might take 30 minutes. Then you can tell us if you had too much filtering.

[Stevem]

You might want to head in the other direction completely and drop back on the uf until the amp hopefully gets spongy and then work your way back up.

Just keep putting the filters you have in series.

[thisismyname]

You might want to head in the other direction completely and drop back on the uf until the amp hopefully gets spongy and then work your way back up.

Just keep putting the filters you have in series.

I'll do that. Thank you!

[JD0x0]

Something I noticed in your schematic is the 22nF couplers out of the PI along with 100k grid leaks. This may be part of why the amp is tighter and punchier than you desire. Those values will change the bias excursion significantly compared to the stock circuit and make the power amp much tighter and punchier than stock. Stock values are 220k grid leaks with oversized 100nF couplers out of the PI for a ~5hz roll off point. As is, it's about ~43hz roll off. The bias excursion and recovery will be much faster and the amp will be less prone to blocking distortion at loud volumes.

If the change back to stock makes it too boomy and farty, try changing the PI input cap from 22nF to the blackface value of 1nF

[Craig B]

My ears hear that you have far too much negative feedback and that makes your amp too sterile. 1k feedback resistor into 5k presence pot. Try setting your adjustable feedback resistor to it's other extreme (100k) and see how it sounds. 101k into that 5k pot would be about perfect.

[thisismyname]

I guess the main reason I don't use it that way is because of how much hum gets back into the sound. I had a 100K in the loop at one point (no pot) and REALLY liked the response, but the hum was a little ridiculous.

[FUCHSAUDIO]

While the excess filtering may be an issue unto itself, this appears to be clearly an instability.

You have either a layout issue someplace, missing a grid stopper or something too close to something else is talking to it. I would pull the reverb tubes as you might have an oscillation in the driver circuit, which can cause this. I've seen people chasing their tails thinking it was a blown speaker or something raunchy sounding, and it was the reverb driver or instability. Did you confirm your output stage FB is negative and not phase reversed ? You could try a 470 pf cap plate to cathode if it turns out to be the reverb driver. Check the power amp alone (inject a signal into the PI without the preamp in use) and see if the power amp does it alone. Does it do this with a load or a speaker or both ? Lots of possibilities....

[thisismyname]

The only locations w/o grid stoppers is the 2nd stage and before the PI. I'll try putting some there. I'll also add a 47pf PI snubber. I had one in there for awhile that made the response a little better at the cost of some treble. The amp is already pretty dark and bassy, requiring extreme settings to get it more usable.

The reverb circuit isn't connected to the circuit on either side, so unless I'm mistaken due to inexperience , I don't think instability would come from there, right?

Regarding positive NFB, what I do know is the pot does affect the NFB as it should per the pot wiring, so wouldn't it be negative? Doesn't it also howl if it's positive?

I'll finally start learning how to use my scope and test from there.

Regarding my thoughts on the layout, there are some things that stand out, however valid they may be. The Kendrick OT they installed runs it's primary wires under the cathodes of V4/the 3rd stage. They also go under all the PI wires. I kept the wires a moderate distance from those OT wires. Also, the standby switch's wires run right over those OT wires, perpendicular, but almost touching. The power tube grid wires are within an inch of the PT primary wires. I've adjusted them a bit. Perhaps I should just redo those wire runs.

[thisismyname]

I am starting to lose my patience with this amp. It's been over 5 years that I've had it and not been impressed by it.

I wired up weaker power filtering, and the amp is a little better, but it still sounds sick, as well as boomy and dark like it's always been. I tried a few other suggestions from people here. It's more usable now, but doesn't fare too well while distorted. Maybe I'm being too idealistic.

I have parts for a Liverpool coming next week. That amp is more in line with what I want in an amp. At least it knows what it wants to be and has a layout to support it. I also have good sounding lower-wattage amps I've restored or modified. Along with my Liverpool, I'll have good amps for every occasion.

Maybe sometime in the future I'll decide to get a new board and rebuild this amp, using the entire chassis to support good layout for a single channel, no FX circuit.

[thisismyname]

I haven't completely given up on it though.

I really appreciate all the help everyone here has given me, so thank you!

[Craig B]

That's a real dilemma. Your 100k feedback resister made for good tone but allowed too much hum. Your 1k feedback resister cancelled the hum but caused sterile tone. Neither one sounds acceptable.
It seems that you have to put the 100k back in there and find the cause of the hum. I vote for removing the 10 inches of OT wiring running within the chassis. Those wires carry high voltage and current--a bad combination. My preferred improvement would be to move the OT, power switch and standby switch much closer to the PT end of the chassis. Put the speaker output jack where the standby switch is. The layout goal is to move the hum generators far away from the sensitive small signal wires. Others may have other ideas...

[Stevem]

I may be blind, but why don’t I see the ground wire off the secondary side of your OT landed to the speaker jack?

Is that blue wire really the ground, or one of the other speaker taps?

PS, I just went to the Kendrick site and the picture of that 11 winding $225 buck OT DOES have a black wire that I am not seeing in your photo!

Also the tip of that speaker jack so close to that as yet unused tube socket could be a real problem!

[thisismyname]

I may be blind, but why don’t I see the ground wire off the secondary side of your OT landed to the speaker jack?

Is that blue wire really the ground, or one of the other speaker taps?

PS, I just went to the Kendrick site and the picture of that 11 winding $225 buck OT DOES have a black wire that I am not seeing in your photo!

Also the tip of that speaker jack so close to that as yet unused tube socket could be a real problem!

The blue wire is the ground. There is only a 8ohm tap.
I'll adjust the heater wires

[FUCHSAUDIO]

Move those transformer wires above the chassis, where they are is inviting trouble, bigtime....fwiw, without FB you should have a little more noise, perhaps a little more hum, but not much....maybe your grounding and/or filtering isn't right.....