Hoffman DR AB763 : Low b+, no sound, loud hum, etc...

[Wil]

Hello everyone,
(Sorry I just realised I posted in the wrong section -> Building Your Amp, so I move the thread here)
It's been a while ! I'm glad to see MEF is bigger and bigger every year ! As my guitar amp problems are, haha !
I rebuild my Deluxe to Hoffman's AB763 specs and have some problem.
It's an unusual design because I'm absolutely terrified by hum and noises and decided to shield every AC wire with copper tubes, haha. A bit radical as you can see.
Symptoms :
-Low b+ : 290V. Despite the low b+, the bias readings are fine : 23mV. (I use 6v6).
-Hums. Loudly. No guitar sound can be heard.
-Hums persists when the preamp tubes are removed
-I randomly measured some test point to ground and read 390VDC from red rectifier winding to ground.
-I get continuity between PT bias wire (blue and red) and ground. I desoldered it and measured and there's still continuity (don't know if that's normal...).
Then I resoldered it and got some higher-pitched "physical" (no speaker plugged) hum.
-It just fried my multimeter when I tried to measure the plate voltage after I resoldered the red/blue wire from the PT.
-I get 50ohm from heaters to ground.
-I double check every connection and I'm pretty sure everything is ok and at the right place (3rd Hoffman Deluxe Build).







I double-checked the PT and OT wiring and it's all good.
Changing the power tubes or the rectifier doesn't change anything.
Could this be the PT ? It's a Mercury Magnetics Axiom that was installed by the previous owner.
But the amp stood on my bench for 8 months without being touched so it would surprise me.

If you have any clue what the problem could be, please shine in :-)
Thank you once again.

[52 Bill]

I can't tell from your photos, but are the ends of the 1 ohm bias/cathode resistors grounded? I don't see a wire from the black terminal post going to ground.

I also think that you are going to have to reorganize the power supply grounding points to get the least amount of hum.

[Wil]

I can't tell from your photos, but are the ends of the 1 ohm bias/cathode resistors grounded? I don't see a wire from the black terminal post going to ground.

I also think that you are going to have to reorganize the power supply grounding points to get the least amount of hum.

Thank you for your answer !
The 1 ohm from 1+8 pins from power tubes are are bolt to the chassis (in order to be able to measure bias from outside of the chassis)
The 100ohm virtual tap for heaters are grounded.
What do you recommend for the grounding scheme ?

[52 Bill]

The 1 ohm from 1+8 pins from power tubes are are bolt to the chassis (in order to be able to measure bias from outside of the chassis

Yes, I understand this, but what I asked was - Is the black terminal post really connected to ground?

[Wil]

Yes, I understand this, but what I asked was - Is the black terminal post really connected to ground?

Yes it is (I re-checked) ! There is connectivity.
Is it normal bias tap PT to have connectivity to ground ? It surprised me.
I know PT rarely fails but since the wiring seems to be alright after double-checking, it raises questions.
Is that also normal I was able to measure approx 400VDC from the red secondary AC wiring to ground ?
Thanks

[Justin Thomas]

Just out of curiosity, what is the ACTUAL NUMERICAL RESISTANCE of the disconnected bias tap to ground? What rectifier are you using?
Here's my theory. The bias tap is on the same PT winding as the HT. If you have HT Center Tap that is grounded (depends on rectifier) then you certainly could have continuity to ground IF you are using your meter's continuity function. That particularly function is not exactly precise; usually anything under a few hundred ohms will read as "continuity." The normal resistance reading ofcross a full HT winding is maybe several hundred ohms, then if you cut that in half because of the grounded CT, and it's even less due to the bias tap... So I wouldn't be surprised if you gave continuity to ground from your bias tap.

If the resistance reads exceptionally low, then you have a short somewhere. But we need the actual number. And your 400VDC from the red secondary AC wiring to ground - is that AFTER the rectifier,or before? If it's after the rectifier, yes, that's normal. If it's before, your amp defies the laws of electronics, or your wall voltage is 400VDC...
Now that I think of it, do you have a light bulb limiter?

Justin

[Wil]

Just out of curiosity, what is the ACTUAL NUMERICAL RESISTANCE of the disconnected bias tap to ground? What rectifier are you using?
Here's my theory. The bias tap is on the same PT winding as the HT. If you have HT Center Tap that is grounded (depends on rectifier) then you certainly could have continuity to ground IF you are using your meter's continuity function. That particularly function is not exactly precise; usually anything under a few hundred ohms will read as "continuity." The normal resistance reading ofcross a full HT winding is maybe several hundred ohms, then if you cut that in half because of the grounded CT, and it's even less due to the bias tap... So I wouldn't be surprised if you gave continuity to ground from your bias tap.

If the resistance reads exceptionally low, then you have a short somewhere. But we need the actual number. And your 400VDC from the red secondary AC wiring to ground - is that AFTER the rectifier,or before? If it's after the rectifier, yes, that's normal. If it's before, your amp defies the laws of electronics, or your wall voltage is 400VDC...
Now that I think of it, do you have a light bulb limiter?

Justin

Hi, thank you for the answer.
Actually, for the moment, I'm a bit stuck. My multimeter was killed when I tried to measure the OT blue lead VDC...
So, before bothering you too much (haha!), I'll take relevant and trustful measurements with an electronics engineer to be sure to deal with actual, verified numbers.
Since then, have a great weekend !

[Wil]

Hi everybody,
If it can help someone, I discovered the problem :
I had no filtering before the choke. A and B filter capacitors were connected at the same point, so I only had the sound of my wall voltage going through the OT. Silly mistake.
Now I get a beautiful crispy Fender sound !
But still, the amp need some tweaks.
We've (my uncle who is a retired electronic engineer and I) browsed the amp with the oscillo and discovered a lack of filtering from the 2 first 22uF caps resulting in 100Hz hum (Caps are TADs and test OK). Why are we limiting the first stage to 2x 22uF ? Why not trying one single 100uF ?
My reverb also hums. Could it benefit from an increase in first stage filtering value since it takes power after only two stages of filtering ?
I also have a couple of questions about grounding :
I believe it is recommended to make a star grounding scheme. Do you have any sort of schematic that could illustrate star grounding for an Hoffman AB763 Amp ?
Since it's obviously impossible to individually ground each ground connection, what are the component that can be link together ?
Plus, since I have ground busses on the pots, is it "good" to have the pots directly bolt and therefore grounded to the chassis ? Shouldn't they be isolated ?
Thank you for your time.

Edit: Actually, Star grounding doesn't seem to be ideal.
Here's what Doug recommends and what I'll follow.
http://el34world.com/charts/grounds.htm
Very clear layout by the way.

[überfuzz]

I seem to remember telling you about your funky ired filter in one of your threads on this amplifier. Anyhow, it nice to see that you finally make some progress.

I'm not 100% sure of what you refer to when you write A and B caps. The only node with quite a lot of ripple should be the B+ node. Other nodes in the filter should have low ripple. In addition to this you only feed the output transformer with B+ (sometimes tremolo to get it to start oscillating). The power stage with the output transformer is not sensible to ripple. So the hum is probably introduced somewhere else. The way you've been treating your filter caps up to this point can be the cause. I wild guess is that they've taken quite a beating.

This is what I would do.
A thing you could try is to use a NON conducting stick, with one hand on the back, and carefully move in and out wires to preamp tubes.

1. Turn all pots to zero and up to see if you can nail the origin of the hum.
2. Turn off and pull one preamp tube at the time and see if the hum is gone when you turn it on again.
3. Preform the chop stick maneuver.

If you don't nail it with these points you're probably better off dragging it to a tech. It's time to swallow your pride and start playing that amp. :-)

[Wil]

I seem to remember telling you about your funky ired filter in one of your threads on this amplifier. Anyhow, it nice to see that you finally make some progress.

I'm not 100% sure of what you refer to when you write A and B caps. The only node with quite a lot of ripple should be the B+ node. Other nodes in the filter should have low ripple. In addition to this you only feed the output transformer with B+ (sometimes tremolo to get it to start oscillating). The power stage with the output transformer is not sensible to ripple. So the hum is probably introduced somewhere else. The way you've been treating your filter caps up to this point can be the cause. I wild guess is that they've taken quite a beating.

This is what I would do.
A thing you could try is to use a NON conducting stick, with one hand on the back, and carefully move in and out wires to preamp tubes.

1. Turn all pots to zero and up to see if you can nail the origin of the hum.
2. Turn off and pull one preamp tube at the time and see if the hum is gone when you turn it on again.
3. Preform the chop stick maneuver.

If you don't nail it with these points you're probably better off dragging it to a tech. It's time to swallow your pride and start playing that amp. :-)

Hi uberfuzz,
I added a 22uF cap in parallel to the two first 22uF filter caps that smooth the b+ node and the hum is now lower and acceptable.
The reverb hums over 3 but in need to investigate to solve that problem.
One strange thing is when I "play" with a switch (bright or custom feedback resistor switch), or unplug the guitar jack, I get a strange background noise, like parastatic noises. If I play a note on send something to the input then it shuts immediately and get back to quiteness.
I suspect the rectifer or the power tubes to be responsible of that. When I tap them, they make some scratching noises.
I'm a little afraid 66uF may be too much stress for the rectifier (GZ34). Do you think it is ?

[überfuzz]

no i dont think that extra cap will mess things up. Did you test all 'old' filter caps and are you sure the choke is ok? You should have tested them given the way they've been treated.

If you suspect something fishy in the reverb, unhook the reverb tank. Try starting the amp without reverb tube. I.e. Try the easy stuff before you try amything funky.