Single-channel AB763 Build(s)

[King Fan]

Heh, my friend, I start to think you've looked at almost all the 1,000 causes of hum, so forgive me if a) these are dumb ideas or b) you've reviewed them or c) this just adds a bunch of false leads you have to track down.

Is there still some hum that varies with the first volume knob and sounds like cable/guitar hum even with no cable inserted? That must have lots of potential causes, but have you confirmed your shielded cables to V1 are actually grounded only at one end (no stray shield whiskers) and, if I'm right, preferably to the ground bus near its anchor?

Re a small cap (bright cap?) that hums when you touch it, that may be normal. But have you considered noisy silver mica caps? Apparently pretty common, and if not noisy from the factory, easily subject to heat damage.

Did you say 'reverb filter cap'? I don't know this schematic at all. Or is that the cap you think may have the outer foil reversed? Sounds like the reverb sector deserves extra scrutiny. IIRC you and I agree Mallories are sometimes hard to test (hard to tell the difference) but in a higher-gain circuit it could make a difference. And I don't know if those xicons have an open foil construction.

Re the sheet of aluminum foil, good idea. For sure I've been fooled testing an open chassis, or testing in a noisy electric environment. But the video suggests your hum persists when you button up the amp and move it to a 'quiet' room? Do you have shielding tape on the inside 'ceiling' of your cab?

Re the scope needing a signal generator, recall I know nothing at all here, but would the kind of free sig gen app you can get for your phone help at all?

Now back to our regularly scheduled help thread...

 

[Snfoilhat]

Here’s some inspiration, uncle doug making the oscilloscope diagnosing a reverb noise problem look easy.


His reverb unit has oscillation associated with the transformer on the send/driver side, which isn’t necessarily what @joulupukki ’s amp has, but he notes that RF can be generated and beamed to other devices, so if this amp’s problem appears to be on the return/recovery side, i suppose it could be picking up a similar noise from an oscillation.

My own experience makes me think about the ground scheme, specifically the hybrid old-new ground schemes that DIYers who have learned from valvewizard and others build, partially updating traditional Fender designs. There’s no easy, obvious place to put the reverb grounds and recovery grid leak other than the traditional spot. Vintage Fender used the chassis as a wire and many Of them are reasonably are quiet. Maybe the DRRI can provide a clue how to implement a modern ground without adding a meter of extra wire and potentially making the amp noisier than the vintage style.

Has a method like removing tubes or chopsticking told you if the hum is in the send or return or elsewhere yet?

 

[ElliotKnapp]

Another question you've maybe already explored: how close is your spring tank to the amp during all of this testing, and how is it oriented? The tank will definitely pick up a lot of PT noise, especially in the cramped quarters of a head config if the input/white end of the tank is at the same end of the chassis as the PT.

Also, do you have any other electronics plugged into the same outlet/nearby? The description of little intermittent bits of interference does sound reminiscent of something being too close to a computer, phone charger, etc. I had my iPhone charging cable draped over the top of an amp and it caused all kinds of noise, even when it wasn't plugged into the phone! Since you have a DRRI handy, you could potentially experiment with placing it close to the same conditions as your project to see if anything external to the amp might be a culprit.

In my experience with this stuff, large issues seem to always be tied to a significant cause--I've never solved a problem like this with RG-174 or coupling cap orientation, but it doesn't mean I haven't tried to, every single time! I HAVE messed with grounding schemes, though, and I've always had better results splitting the power grounds from the preamp grounds.

 

[Lowerleftcoast]

You have mentioned the PT buzzing. Some do. Imo, this should be investigated further.

You started to eliminate parts of the circuit (reverb tank, tube etc.) with no luck. Leave them out until you find the noise... then add them back in.

 

[NTC]

Wild stabs in the dark:

1. Noise picked up by the footpedal wire? I forget if one is hooked up here.
2. An input shorting jack that isn't shorting? I have had this happen. Perhaps try a jumper wire to ground the input to insure the hum is getting in later.

 

[joulupukki]

Heh, my friend, I start to think you've looked at almost all the 1,000 causes of hum, so forgive me if a) these are dumb ideas or b) you've reviewed them or c) this just adds a bunch of false leads you have to track down.

I’ve only looked at 447 causes of hum so far, so I haven’t yet quite filled the quota.

Hum I’m not as concerned about at this point, but it’s more the buzz. I will make another video later to demonstrate.

Is there still some hum that varies with the first volume knob and sounds like cable/guitar hum even with no cable inserted? That must have lots of potential causes, but have you confirmed your shielded cables to V1 are actually grounded only at one end (no stray shield whiskers) and, if I'm right, preferably to the ground bus near its anchor?

The shielded cable is grounded directly to the input jack ground, which is now where the preamp bus grounds. So far as I can tell, there’s no little whiskers touching anything.

I still can’t track down the 7x 3-lug terminal strips I purchased from Mojotone in one of my orders. For that reason, my 33K resistor is just heatshrunk with the shielded cable before it reaches V1. I’d prefer the shielded cable to be attached to a terminal strip and from there to V1 with the 33K resistor.

Re a small cap (bright cap?) that hums when you touch it, that may be normal. But have you considered noisy silver mica caps? Apparently pretty common, and if not noisy from the factory, easily subject to heat damage.

Hadn’t considered that, but I do think the problem is elsewhere … as explained below.

Did you say 'reverb filter cap'? I don't know this schematic at all. Or is that the cap you think may have the outer foil reversed? Sounds like the reverb sector deserves extra scrutiny. IIRC you and I agree Mallories are sometimes hard to test (hard to tell the difference) but in a higher-gain circuit it could make a difference. And I don't know if those xicons have an open foil construction.

I think the buzz is getting into the circuit before the reverb stage. However, there is definitely a decent amount of hum that is introduced by the reverb stage. I’ll attack that once I get the buzz figured out (Even though I have tried to reduce that hum).

Re the sheet of aluminum foil, good idea. For sure I've been fooled testing an open chassis, or testing in a noisy electric environment. But the video suggests your hum persists when you button up the amp and move it to a 'quiet' room? Do you have shielding tape on the inside 'ceiling' of your cab?

I don’t have any shielding tape in my cab. Maybe eventually, but since it behaves the same with or without the alum foil, I haven’t yet bothered with that.

Re the scope needing a signal generator, recall I know nothing at all here, but would the kind of free sig gen app you can get for your phone help at all?

Yep. That’s what I ultimately decided. I have successfully hooked up my phone with a signal generator app and dialed it back so that it’s only about 150mV coming in (with this oscilloscope I found).

Here’s some inspiration, uncle doug making the oscilloscope diagnosing a reverb noise problem look easy.

Really interesting stuff here. I’ll study it more in depth.

My own experience makes me think about the ground scheme, specifically the hybrid old-new ground schemes that DIYers who have learned from valvewizard and others build, partially updating traditional Fender designs. There’s no easy, obvious place to put the reverb grounds and recovery grid leak other than the traditional spot. Vintage Fender used the chassis as a wire and many Of them are reasonably are quiet. Maybe the DRRI can provide a clue how to implement a modern ground without adding a meter of extra wire and potentially making the amp noisier than the vintage style.

I’ve tried grounding the power amp separately from the preamp and it neither hurts nor hinders any of the issues. In fact, I almost want to say that hum wise it’s better when the power amp is grounded to the preamp bus bar. But, any gain is so insignificant.

Has a method like removing tubes or chopsticking told you if the hum is in the send or return or elsewhere yet?

I’ve done a number of things like this with no success yet … though see below for what I think is the most likely cause.

Wild stabs in the dark:

1. Noise picked up by the footpedal wire? I forget if one is hooked up here.

The buzz happens without foot switch or reverb tank plugged in at all.

2. An input shorting jack that isn't shorting? I have had this happen. Perhaps try a jumper wire to ground the input to insure the hum is getting in later.

Have checked that.

Wild stabs in the dark:

1. Noise picked up by the footpedal wire? I forget if one is hooked up here.

The buzz happens without foot switch or reverb tank plugged in at all.

2. An input shorting jack that isn't shorting? I have had this happen. Perhaps try a jumper wire to ground the input to insure the hum is getting in later.

Have checked that.

You have mentioned the PT buzzing. Some do. Imo, this should be investigated further.

At this point, this is my biggest suspicion and educated guess on where the buzzing is originating, the vertical power transformer.

#1, it’s definitely a lot noisier than the DRRI with a dummy load. You can hear it come on, and it sounds buzzy all by itself. I’ve even tightened the screws on it that hold the plates together (no improvement). It’s that same buzziness that seems to be transferred through the entire amp.

#2, if I take out ALL preamp tubes, including the phase inverter and get my ear right down next to the speaker, I can faintly hear that same buzzing, even though it’s extremely quiet. When I do the same experiment with the DRRI in the same exact location (chassis is removed) plugged into the same external speaker, there is ZERO buzzing from the DRRI.

The one thing I have not yet done with the PT is to try rotating it 90° to see if that improves anything. I would need to splice wires on to the end of the PT wires since they are all trimmed to fit neatly in the amp. IF, when I try this, it fixes the problem, I will then just abandon the cap can and go with radial Nichicon filter caps mounted to a terminal strip or two (because the new orientation of the PT would take up the space of the existing cap can).

You started to eliminate parts of the circuit (reverb tank, tube etc.) with no luck. Leave them out until you find the noise... then add them back in.

Very good point. Thank you for that tip.

Next big question, is there a suitable vertical power transformer of a different brand that would work for this amp? I’d be willing to buy and try it at this point because why build an expensive amp only to not have it perform at its finest!?

Again, the one I’ve got right now is a Musical Power Supplies 660VAC center tapped 140mA (https://www.musicalpowersupplies.com/app/download/7115014784/PT330M_REVD_DWG.pdf).

 

[joulupukki]

I’ve ruled one thing out – the orientation of the PT. I unhooked one side of the wires and rotated it 90° and same exact issue. I should have taken a picture, but I would have probably been scorned … wire nuts connecting wires back to the board from the PT. Ghetto. But at least I can put that theory to bed.

However, while in that configuration I plugged in a cable into the POWER AMP INPUT jack on the back. The buzz goes away when the preamp is out of the picture. After restoring the PT to its proper location, the buzz stays away with something plugged into the power amp input jack. I think what this means is that the buzz lives somewhere in the preamp? But where or where can it be? Something is essentially picking up stray buzz, acting like an antenna. Hmm.

 

[joulupukki]

Decent progress! The buzz is still there, but significantly reduced. Here are some things I did that generally improved the overall condition of the whole amp:

1. Connected a ground wire from the OT’s common (speaker out ground) over to the PT’s ground
2. Disconnected the tremolo speed pot’s ground from the reverb pot and connected it (through the 100K resistor) to the ground bus where the cathode of the tremolo connects to the ground bus
3. Instead of running the ground from the reverb pot to the closest place on the ground bus, I ran it all the way over to where the V2 (reverb driver) cathode connects to the ground bus.

Questionable:
4. I restored having the power section (and cap can) having its own single ground over by the PT. I think there is a little more hum than there was before this change so I may switch it back to ground at the end of the preamp ground bus again to see if it brings the level of the hum back down a bit.

Question:
All along, manipulating the volume pot at certain locations introduces buzz. At all the way off, it is a little buzzy. When it gets to quarter of the way there the buzz from that goes away but then as it reaches max it comes back. Would this indicate a bad / inferior volume pot? It’s an Alpha brand.

 

[chas.wahl]

1. Connected a ground wire from the OT’s common (speaker out ground) over to the PT’s ground.

If you've done this, have you isolated the output jack from the chassis too? It's not a good idea to have something grounded to two different locations, in general.

 

[joulupukki]

If you've done this, have you isolated the jack from the chassis too? It's not a good idea to have something grounded to two different locations, in general.

Agreed. No, I haven’t isolated those output jacks. I don’t have any more shoulder isolating washers, nor are they cliff jacks (they are Switchcraft), but I do know that it significantly reduced hum, so at least it’s a step in the right direction.

 

[joulupukki]

Questionable:
4. I restored having the power section (and cap can) having its own single ground over by the PT. I think there is a little more hum than there was before this change so I may switch it back to ground at the end of the preamp ground bus again to see if it brings the level of the hum back down a bit.

Ok, confirmed. In this particular amp, there is significantly less hum if I keep the PT ground separate from the preamp ground.

 

[joulupukki]

Huge success!

So that ground from the reverb pot that I grounded to the same spot on the preamp ground bus where the reverb driver’s cathode grounds … I re-wired it again still connecting to the same place, but this time with a wire that twists with the wire bundle going to and from the reverb pot. Buzz is nearly, if not all the way gone!

State of the amp is pretty good. There’s more white noise than I’d prefer, but I’m gonna chalk that up to whatever resistors I used. With everything turned all the way up it does sound like it’s “ON” and a bit hummy so there could be more that I could do. I suspect that if I were to switch from a cap can with a common ground to individual radials with B+4 grounded on the preamp bus it’d improve a bit more. And perhaps there is more I could do localizing and isolating grounds. But I’m 95% happy with it at this point. Easily giggable and very enjoyable as an indoor amp too thanks to the master volume pot.

Thanks to all so far who have chimed in with excellent suggestions … and who have suffered through my insane amounts of thinking “out loud” in the thread.

Here’s the latest gut shot and a closer look at the reverb wire bundle:

 

[BigDaddy23]

Well done mate

I'm all for isolating everything and using this type of ground scheme principle - It's certainly been the way that has worked best for me.

I'm sure you've learned heaps from this arduous journey - thanks for documenting it too as I'm sure it will help others out!

 

[joulupukki]

Thanks @BigDaddy23! I sure hope it helps save someone some time, or at least gives more ideas of what not to do.

I hadn’t plugged in my Suhr strat much (have mostly been playing my tele). This thing, in its current config breaks up way too early. I think I’m gonna go back to the dropping resistors found in Rob’s Blackvibe 6V6 (6.8K and 1K). Right now it’s what the DRRI uses (10K and 10K).

Before I do, just gonna document the voltages of everything, for reference.

Wall voltage: 122VAC

V6 OT Plate to B+ Resistance: 33.3 Ohms
V7 OT Plate to B+ Resistance: 40.8 Ohms

Using 10K & 10K dropping resistors:

B+: 413V
B+2: 411V
B+3: 339V
B+4: 302V

Bias supply: 54.7VAC
Rectified bias: -44.4V
Bias after bias pot: -35.5V

	Pin 1	Pin 2	Pin 3	Pin 4	Pin 5	Pin 6	Pin 7	Pin 8
V1 7025 Preamp	201.6		1.54			209.3		1.42
V2 Reverb Driver	407	8.3mV	8.1			407	8.3mV	8.1
V3 Reverb Recovery	216.9		1.485			209.3		1.485
V4 Tremolo	208		2.052			411	208	214
V5 12AT7 Phase Inverter	201.3	46.1	76.7			195.9	50	76.7
V6 6V6 Power Tube			412	410	-35.36	412		17.8mV
V7 6V6 Power Tube			412	410	-35.32	412		15.8mV
V8 GZ34 Rectifier	413

Using 6.8K and 1K dropping resistors:
Wall voltage: 122VAC

B+: 414V
B+2: 412V
B+3: 358V
B+4: 354V

Bias supply: 54.8VAC
Rectified bias: -44.1V
Bias after bias pot: -35.2V

	Pin 1	Pin 2	Pin 3	Pin 4	Pin 5	Pin 6	Pin 7	Pin 8
V1 7025 Preamp	233.2		1.819			241.6		1.691
V2 Reverb Driver	408	8.6mV	8.1			408	8.6mV	8.1
V3 Reverb Recovery	251.1		1.769			243.2		1.769
V4 Tremolo	207.1		2.052			413	207.1	214.7
V5 12AT7 Phase Inverter	212	48.4	81.2			206.7	52.5	81.2
V6 6V6 Power Tube			414	413	-35.25	413		18.1mV
V7 6V6 Power Tube			413	413	-35.25	413		16.1mV
V8 GZ34 Rectifier	414

After making the change, I’m not sure exactly if it’s good or bad. It does seem to have a lot more bass response somehow. But maybe I just haven’t played it enough with my strat to get a sense of that.

One thing I forgot about while I was in the schematic and layout files is that I had only made one input jack on the faceplate. I would have liked a second input jack for my strat. But oh well.

I’ll have to crank it up tomorrow if/when people are mostly out of the house and check it out properly.

 

[mountainhick]

Glad you're having progress!

I am wondering about a couple of your turret connections into the top holes, the mica cap, and looks like an electrolytic cathode cap to the right of it.

:

 

[joulupukki]

@mountainhick is it these you're wondering about? Here's another picture from more of a birds-eye view (though it's grainy because I zoomed in). If you're wondering whether they are soldered in, yes. There's plenty of solder down that turret hole where the 10pF mica cap is connected to. I didn't fill it up to the brim on this one. But maybe you're referring to something else?

 

[mountainhick]

@mountainhick is it these you're wondering about? Here's another picture from more of a birds-eye view (though it's grainy because I zoomed in). If you're wondering whether they are soldered in, yes. There's plenty of solder down that turret hole where the 10pF mica cap is connected to. I didn't fill it up to the brim on this one. But maybe you're referring to something else?
View attachment 1071257

Close enough... also a different electrolytic cap to the right of the closeup, But if they are solid, no worries!

 

[joulupukki]

Yeah, gotcha. Initially when I first soldered everything up, I pretty much kept shoving solder in there 'til they were domed at the top. But, as I troubleshooted and re-heated to disconnect/connect something else, that solder would drop. But, in all cases, still plenty of solder in there to support them so I didn't bother re-filling them. Good eye spotting those. It does make me wonder if it'd be more reliable if I filled them all up again. But there is so much solder in this. Where does it all go!? Those are like ginormous sink holes.

 

[joulupukki]

If I wanted to convert my single input jack to one that could support having a “LO” input (for when I plug my humbucker guitar in), would it make sense to add a push-pull switch with the Volume potentiometer? Basically it’d switch in or out a 68K resistor in parallel. I’d have to ditch the idea of having the grid stopper resistor down next to V1A, but this would work right? I’d still run shielded cable from the junction of the two 68K resistors to V1A. Good/bad idea?

 

[joulupukki]

Tonight is the first night I compared my new B-Verb to the DRRI…

If I’m being honest, I’m a little disappointed. The DRRI easily wins in the volume department. My B-Verb breaks up way to early too. Not enough clean headroom. I’m still not entirely sure why that would be. I had assumed that if I went back to the original dropping resistors in the power supply, the extra plate voltage on the preamp tubes would help clean it up a bit.

I’m also wondering now if the PT and OT I used aren’t quite as beefy as the one used in the DRRI? Or perhaps I’ve done something else not quite right. In addition, it definitely has a LOT more hum than the DRRI, which almost has none.

I also compared against my Mojotone PR. It easily has more punch than the PR, but wow, the PR sounds very good. It’s got some magic in it, for sure.

Kind of perplexed.