65' Deluxe reverb reissue Drri hum problem V4?

Menuiserie

Member
Messages
25
Hello,

I had a Deluxe reverb reissue for quite a long time, not using it much because it was a bit noisy. A mix of hiss, 50Hz and harmonics.

Changed tubes, nothing helped.

Last month I decided to recap it thinking it might be caps leaking.
No it was not. So I tried twist the AC wires, and added a humdinger balance pot for heaters. As per some advices, 6.3V is now soldered directly to tubes, and heater balance resistors are connected to humdinger, with wiper connected to V8 cathode (ground).

I could remove some of the hum, but not enough.

As I had ROHS tremolo circuit, I bought a Fender optoisolator and reverted it to the regular circuit (removing both added PCB and 7 10k grid resistors that were on the preamp grids, near the sockets.

No more hiss from the tremolo intensity volume, but still general hum and hiss.

Then I removed some of the tubes to isolate the problem.
With only PI and output tubes + rectifier : no hum, no noise, very very clean.

I add V1. I get the normal channel. It is clean, no problem, a bit of hiss when I crank the volume but nothing unusual, beautiful sound, no hum.

I add V2. No noise. The weird thing is I can get the vibrato channel (without vibrato and reverb of course) working, but with low level, and no treble, but still no hum. Looking at the schematic it should be muted as it passes through V4 (absent), isn't it?

I add V3, V5, nothing changes.

I add V4. Hum, hiss. When I crank the reverb button, a lot more of them (tank not attached). As I understand it, V4a is the reverb return driver and V4b is the fixed gain 3rd preamp stage of the second channel. So If V4 is wrong, this should affect both these fonctions.
I tried changing tube. not the solution.

I tried poking with a chopstick, reverb return up : every V4 wire but the heaters are microphonics, also every direct connexion with them (rca hot rev return, R28, etc). I reflowed those connexions. Checked good with DMM, but nothing changed when powered. Still Hum and microphonics wires, even when I change for a known good tube.

Checked again those new filter caps, they are good on the mesure.

I'm running out of ideas here...
What do you think?

Thanks a lot for your answers.
 

fiveightandten

Silver Supporting Member
Messages
6,571
Do you have a scope?

Have you tried manually grounding the ground points associated with that V4 tube? Have you checked for leakage at the coupling caps in those stages?
 

HotBluePlates

Member
Messages
14,080
... I add V1. ...

I add V2. No noise. The weird thing is I can get the vibrato channel (without vibrato and reverb of course) working, but with low level, and no treble, but still no hum. Looking at the schematic it should be muted as it passes through V4 (absent), isn't it? ...

The 2nd stage of the Vibrato channel shares a cathode resistor with the 2nd stage of the Normal channel. You're very likely hearing cross-talk between those 2 stages.

The cathode bypass cap across the shared cathode resistor should kill this effect, and "ground the input through the cathode of the Normal channel's 2nd stage." But caps are imperfect, and that is probably why you heard some bleed-through.

There may be another spot in the circuit where cross-talk was able to happen, but between close-spaced circuit board traces.

... I add V4. Hum, hiss. When I crank the reverb button, a lot more of them (tank not attached). As I understand it, V4a is the reverb return driver ...

It will be hard to pin down the hum-source in V4. There could be several of them.

The amp signal is the weakest here of any spot in the amp. Any little bit of noise/hum overwhelms the desired signal quickly.​

You could have poorly-shielded reverb cables, or poor connection from those cables to the amp's ground.​
The 220kΩ at the return jack may be grounded in a spot that has a noisy ground-current, or otherwise contributes to hum.​
Power supply filtering is probably okay.​
You could have a bum tube with heater-to-cathode leakage (it injects heater hum directly into the audio). A functioning, large (22-220µF) cathode bypass cap normally swamps this problem.​


Most amps that don't have a single major fault have many little hum sources all adding a bit to create a large hum problem.
 

Menuiserie

Member
Messages
25
Do you have a scope?

Have you tried manually grounding the ground points associated with that V4 tube? Have you checked for leakage at the coupling caps in those stages?
Thanks a lot!

I can have access to a scope. I will have a look at it.

When you say manually grounding V4, you mean :
V4 2/grid from reverb return?
V4 7/grid from V2?

Can you explain how should I look for leakage in coupling caps?
 

Menuiserie

Member
Messages
25
The cathode bypass cap across the shared cathode resistor should kill this effect, and "ground the input through the cathode of the Normal channel's 2nd stage." But caps are imperfect, and that is probably why you heard some bleed-through.
Thanks a lot !
Could this be that this cap (C18 .1uf/400v) looks suspicious?

You could have poorly-shielded reverb cables, or poor connection from those cables to the amp's ground.
This is humming even when cables are disconnected from the amp chassis.
Reverb return plug ground from the cinch socket shows continuity to the ground.
There is also a small ceramic cap between this ground and chassis next to it. As I don't see it on other amps, neither on the schematic, i tried disconnecting it, but nothing changed.
IMG_20220615_164432.jpg


The 220kΩ at the return jack may be grounded in a spot that has a noisy ground-current, or otherwise contributes to hum.
I can try grounding it elsewhere. First how could I look if this spot is noisy, with a scope across R28 (220kOhm)?

You could have a bum tube with heater-to-cathode leakage (it injects heater hum directly into the audio). A functioning, large (22-220µF) cathode bypass cap normally swamps this problem.
I tried at least 3 different tubes in V4, always with the same result, so unless I'm very unlucky, we can rule the tube out, can't we?


The amp signal is the weakest here of any spot in the amp. Any little bit of noise/hum overwhelms the desired signal quickly.
I will try to ground the Grids of V4 and report back my findings.

Thanks a lot, it's really nice to get some help after so many hours of lonely trial and error :)
 

fiveightandten

Silver Supporting Member
Messages
6,571
Thanks a lot!

I can have access to a scope. I will have a look at it.

When you say manually grounding V4, you mean :
V4 2/grid from reverb return?
V4 7/grid from V2?

Can you explain how should I look for leakage in coupling caps?
Hum can often be a result of a bad ground connection. Find the nodes that are grounded in the offending stage and just ground each of them manually (one by one) to see if the hum is reduced or goes away. A simple alligator clip lead will do. I like to string 2 DMM leads together (not connected to the DMM), an alligator clip and a probe tip. Then just touch the probe to each ground node with the alligator clip on a solid ground point.

Coupling caps should block DC, so check for DC voltage at their output.

And if you have access to a scope that can be very helpful to track down exactly where the noise is being introduced. Go through the signal path with the scope and find the point where the noise originates. Then you know where to concentrate your efforts. This is just a more accurate version of the tube pulling that you did.
 

Menuiserie

Member
Messages
25
Hum can often be a result of a bad ground connection. Find the nodes that are grounded in the offending stage and just ground each of them manually (one by one) to see if the hum is reduced or goes away. A simple alligator clip lead will do. I like to string 2 DMM leads together (not connected to the DMM), an alligator clip and a probe tip. Then just touch the probe to each ground node with the alligator clip on a solid ground point.
Thanks!
I suspect c16 cathode bypass ground indeed, PCB looked suspicious at this point as far as I remember...
I will try it when I get back home.
What would be the best reference ground to tie the alligator clip in your opinion?
 

Menuiserie

Member
Messages
25
Hum can often be a result of a bad ground connection. Find the nodes that are grounded in the offending stage and just ground each of them manually (one by one) to see if the hum is reduced or goes away. A simple alligator clip lead will do. I like to string 2 DMM leads together (not connected to the DMM), an alligator clip and a probe tip. Then just touch the probe to each ground node with the alligator clip on a solid ground point.
I add a chance to try again, with a fresh mind!

I try different groundings for V4 circuit but nothing positive.

Then I grounded V4/7 grid. Still a bit of hiss, but no more hum.
So hum must not come from V4 but from before.

I removed v2, still a bit of hiss but no hum. So I guess either hum is happening in v2b, or between volume and v2/7 grid. I swapped tubes, but still the same.

I will try to connect v2/7 directly to volume with a shielded coax wire and see if hum improves.
What are your thoughts?
Thanks
 

Menuiserie

Member
Messages
25
Well, I just did it, cut the trace from the volume wiper and solder a shielded coax to v2p7. And...

Nope. Still the same hum...

I m starting to wonder if I would be better by tossing away this PCB and putting new eyelet board inside...
 

HotBluePlates

Member
Messages
14,080
... I can have access to a scope. I will have a look at it. ...
Well, I just did it, cut the trace from the volume wiper and solder a shielded coax to v2p7. And...

Nope. Still the same hum...

I m starting to wonder if I would be better by tossing away this PCB and putting new eyelet board inside...

Did you ever use the oscilloscope to see where the hum begins? It would be wise to do some logical troubleshooting before trying random cures.
 

Menuiserie

Member
Messages
25
Looks like my cheap scope is broken and doesn't wanna go below 0.5v/div so I cannot see the noise...

Few things I know:
When I ground R25 at V4 grid. I got no noise, but hum (normal ground loop I think)
When I ground R25 at C12 jonction (v2 plate side) i got noise (I think resistor noise from the 3.3m ohm) but no more hum.

If I pull out v2, noise + hum is the same. If I do this and ground R25 at C12 jonction, then hum doesn't cut.
 
Last edited:

Menuiserie

Member
Messages
25
If I ground v2b grid ( return from the volume pot) appart from shorting the guitar signal I ve got no changes in the hum neither the hiss.

I guess it narrows the problem to v2b circuit, don't you think?
 

Menuiserie

Member
Messages
25
So If I'm not mistaken we are talking about:
C11 & R24 to Cathode
R23 from big c35 cap (High voltage)
C12 to R25.

V1 shares c11&R24 at his cathode, and got no problem, so I rule them out.

High voltage from c35 should be clean, as it feeds also v1 and there is no problem. R23 mesures 100k as specified (98k on the DMM)

That leaves C12...
IMG_20220622_143144.jpg
 

Menuiserie

Member
Messages
25
C12 mesures 7.5nf instead of 22nf on the schematic, but this is taken from the circuit at both r23 and r25 (with and without v2 in place).
Could it be the hum problem?
IMG_20220622_144335.jpg
 

Menuiserie

Member
Messages
25
V3 is out also, as R25 connects to both his cathodes and I wanted to rule that out as well.
Could also be r26 1mohm though, but it shows good continuity to ground and the right value...
 

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