Dlx Rvrb, ugly distortion at low volume

[sonofmickel]

Edit: I rebuilt the amp with a Hoffman eyelet board and new Belton sockets/RCA jacks. 4.7uf preamp bypass caps = better amp now/keeper.
The link https://ampgarage.com/forum/viewtopic.php?f=8&t=37712

Weber 6A20 Deluxe Reverb build.
The voltages are in the ballpark.
The ugly distortion is on both channels and at low volume.
At volume 2, playing with an extremely light touch the amp sounds nice and clean, try playing even just a little teeny wee bit bit harder and the "back end" of the notes distort, use a pick and it sounds horrific.
It's almost like as if a distortion pedal was letting clean signal bypass and distortion is creeping in from a mix knob.
Turning up to 3 and it is just an ugly mess.
Both channels react the same.

I am thinking the problem is in the phase inverter?
Screens 437v and Plates 438v are within a volt?

Swapped 6v6 tubes, no change
Swapped 12AT7 PI , no change
Swapped OT, no change
Swapped PT, no change

[Stevem]

If you pull the signal off the reverb send jack, but feed it thru about a 470k resistor into another amps input is the signal loud but clean?

Also what’s your negative voltage on pin 5 of each output tube, is it at least -34?

[sonofmickel]

Pin 5 is -39 for each tube.
6v6s biased at roughly 21/22mv.

I'll probably need a diagram for that.
What you are suggesting, take the reverb out "hot" and tack on a 470k resistor, clip that to the center of a guitar cable, ground jacket of guitar cable, plug that into an effects return of another amp?

What makes an amp ugly-distort with gentle picking dynamics?
Both channels do this, so I am thinking power amp.

With reverb and vibrato turned on the effects is less.
I don't think the reverb driver is part of the problem, the 220k resistors should be isolating the 3rd preamp stage out of the equation...yes? Therefore the Normal channel does not use the 3rd reverb triode.

I tear the PI apart and test all resistors.

[sonofmickel]

I have a different, higher 200ma PT wired into the chassis now, 424v at plates.
No change. Still ugly distortion at 3 when hitting hard chord or single notes.
The bigger PT tells me that it was not the original 150ma PT being current starved.

I pulled all the preamp tubes except the V1 and the PI.

I want to isolate the preamps. Tell me if this is methodically sound...
I clipped in a 22k voltage divider to ground after the Normal channel 220k mixer resistor to dump preamp to ground(input to PI), so I can test just the preamp.
Volume starts to just break up at around 7, telling me that the preamp is "clean"/working as it should. No harsh distortion when hitting hard chords.

When I pull the 22k, the distortion is harsh again, square wave?

I believe the PI is the problem.

How to test for DC, leaky caps?

So...It appears the problem is hitting strong chords that is overloading the PI.
One more thing, when turning the amp off with a ringing chord, the decay is horrific sounding grindy hash.

Please help with this, I am loosing my mind, having nightmares

[pdf64]

Are you sure that the global feedback loop is negative, rather than positive?
eg if the feedback loop is opened, does the system gain increase or decrease?

[sonofmickel]

Global feedback: amp gets louder when GNFBK is lifted, more quiet and "cleaner" when attached.

Here's an audio example at 3 volume.
Another at 5 volume, could have recorded at 4, same result if I hit strings harder, these are not with a pick, used thumb/fingers

[B Ingram]

One more thing, when turning the amp off with a ringing chord, the decay is horrific sounding grindy hash.

This is normal, and not something anyone can fix.

The decay becomes more distorted after the power is shut off because the heaters are cooling, and the tube-cathodes are conducting less & less as they fizzle out.

Still ugly distortion at 3 when hitting hard chord or single notes.
...
I clipped in a 22k voltage divider to ground after the Normal channel 220k mixer resistor to dump preamp to ground(input to PI), so I can test just the preamp.
Volume starts to just break up at around 7, telling me that the preamp is "clean"/working as it should. No harsh distortion when hitting hard chords.

When I pull the 22k, the distortion is harsh again, square wave?

I believe the PI is the problem.
...
So...It appears the problem is hitting strong chords that is overloading the PI.

Are you sure that the global feedback loop is negative, rather than positive?

Global feedback: amp gets louder when GNFBK is lifted, more quiet and "cleaner" when attached.

Does your distortion issue go away in the "no negative feedback" condition?



Things have probably changed since the photos you posted at the top of this thread.

That said, you had at least one ground wire over by the PT floating in the air connected to nothing (except ground), and the negative feedback wire from the speaker laying over top of the phase inverter tube. You might eat to route that wire along the back apron of the chassis from the Speaker jack to the Standby switch, then make a right-turn and pass straight to the feedback resistor on the board. That would keep speaker-signal away from the phase inverter tube (and possible unintentional coupling).

Other than that, I'd suggest setting your meter to read Peak AC Volts at Pin 5 of the output tubes when you "get ugly distortion at 3 when hitting hard chords." You're looking to catch the peak voltage delivered from the phase inverter to the output tubes.

- Next, compare this to the DC Volts measured at Pin 5 of the output tubes.
- If you get something like "43v peak" but the DC Volts show something like "-38v DC" (which is 6V6 bias voltage, then there is "nothing to fix."

The reason is that any time you send a drive signal whose peak is larger than the tube's bias voltage, distortion is certain. The fact you "fixed the distortion" with the voltage divider you rigged makes it sound like "distortion was due to a large driving signal" which is normal behavior.


It could be that you feel the transition in/out of distortion is too abrupt in this amp... and that's a normal byproduct of negative feedback around a power section. Amps with no negative feedback have a gradual transition from "clean" to "dirt" but negative feedback makes that more abrupt. It's the Cost of Doing Business. But the negative feedback also addresses other issues, like the speaker being boomy at its bass-resonant frequency, and it part of this amp circuit's character.

[sonofmickel]

The loose green ground wire was for a third 16uf power capacitor. I just upped the caps in the doghouse 40uf + 20uf.

Nasty "below the note " distortion still there, early, with negative feedback removed.

Pin 5 Ac to ground was peaking 36-41 volts.
Pin 5 DC -38v.

I hooked the speakers up to a Quilter 101, Surf channel, 50 watts. Loud and clean almost all the way up. Pretty convincing. Speakers get loud, clean, no distortion or breakup until very loud.

I used to play this amp live with the Gypsy Bastards, it was glorious one day it sounded horrible and lost volume. PT was too hot to touch 2-3 seconds.

I think I need to put a scope on the amp...somewhere...

[sonofmickel]

Changed today:
Swapped .1uf PI coupling caps for .022uf. No change, a little less bass.
Swapped 1.5k grid stoppers for 4.7k. No change.
Raised heater connects from transformer to powertubes to see if they were coupling to signal. No change.
Re-routed negative feedback wire towards back of chassis, away from PI. No Change.
Rewired Vibrato channel input jacks, solved signal bleed at zero volume!
Touched up suspect solder joints. No change.

There has to be a stupid simple thing I did wrong in here somewhere.
Could both sets of JJ 6v6 be bad?

Vibrato circuit problem?: V5 pin 1 is higher than it should be. 424vdc should be 270vdc?
The cathode is reading zero V5 pin 3, It should be 2.1vdc or so.

[sluckey]

Vibrato circuit problem?: V5 pin 1 is higher than it should be. 424vdc should be 270vdc?
The cathode is reading zero V5 pin 3, It should be 2.1vdc or so.

Not a problem. When the tremolo is disabled (ie, no footswitch to turn it on), the plate voltage will be very high and the cathode voltage will be zero because the tube is cutoff due to the high negative voltage on the grid (pin 2). Enable the tremolo by grounding the FS jack and the voltages will change drastically.

[sonofmickel]

Close this thread!
I can't take it anymore, trying to fix this janky bs homebuild junk.
I'm gutting it and starting fresh.

Try reusing the wax eyelet board or get a hoffman eyelet board?
Probably the Hoffman, that should take my iron abuse better than wax.

What's the best, easiest to work with hook up wire?

Who's got the best regular Deluxe Reverb build on here? Dorrisant, Sluckey, Xtian?
No trickery, just a straight Deluxe Reverb build.

[sluckey]

What resistance do you measure from V6 pins 3 and 8 to chassis?

Separate issue... Look at the attached pic. Looks like missing solder on one of the eyelets for the roach. And I don't see a jumper either.

[Stevem]

Good eye!

[lonote]

Close this thread!
I can't take it anymore, trying to fix this janky bs homebuild junk.
I'm gutting it and starting fresh.

Try reusing the wax eyelet board or get a hoffman eyelet board?
Probably the Hoffman, that should take my iron abuse better than wax.

What's the best, easiest to work with hook up wire?

Who's got the best regular Deluxe Reverb build on here? Dorrisant, Sluckey, Xtian?
No trickery, just a straight Deluxe Reverb build.

I am not sure what board you have (old or re-issue) but I would move to a G-10/FR4 board to eliminate any conductivity issues down the road. You could do either eyelet or turret. I have never used the Hoffman eyelet boards, but the Hoffman AB763 turret-board is good (I have used them) but be aware that one is a bias-vary trem circuit, not the stock Deluxe Reverb trem, if that is important.

Sluckey has drawings out there somewhere to adapt the Hoffman board turret layout to use his Trem-O-Nator circuit.


I just buttoned up a '79 Pro Reverb that, after the first re-build, had to have a new board made as the original was unusable due to the massive amount of wax that had been applied. It was leaking voltage all over the place & I just couldn't get it clean.

I used eyelets & some 1/16" Garolite I had on hand. It wasn't fiberglass but more old-school bakelite & I don't think I would use it again, though everything is working as it should.


As far as wire, my favorite is 22ga from MojoTone, as it is pre-tinned, strips & forms great. Very easy to use. It is a PVC jacket but tolerates a little more heat than many. I tried quite a few & like this the best, though I wish it came in a few more colors.

https://www.mojotone.com/22-Ga-Pre-Tinn ... oated-Wire

[sonofmickel]

The roach is connected under board.
I think I'll get the Hoffman eyelet board. Spendy but should save time, I'll do all wiring above board, nothing hidden.

Done, Hoffman Eyelet AB763 reverb incoming.