Reverb Circuit Hum

[mwaller]

I disconnected the reverb at the grid on V2 and added a 220k pull-down resistor. There is still hum and hiss unless I remove everything except the power tubes and rectifier. Can anyone recommend a tech in the Seattle area who might be willing to troubleshoot these issues? I'm running out of ideas...

[Stevem]

A long shot but worth the try here!
How is your PT mounted?
Can you pivot it 90 degrees without a major rework to see if it helps?

[mwaller]

A long shot but worth the try here!
How is your PT mounted?
Can you pivot it 90 degrees without a major rework to see if it helps?

Unfortunately, the PT fits into a rectangular hole in the chassis, so it can't easily be remounted. It does have a grounded hum shield, however.

[pompeiisneaks]

I disconnected the reverb at the grid on V2 and added a 220k pull-down resistor. There is still hum and hiss unless I remove everything except the power tubes and rectifier. Can anyone recommend a tech in the Seattle area who might be willing to troubleshoot these issues? I'm running out of ideas...

I'm in the seatle area, but been doing much less work lately. with the snowpocalypse it's not going to be fun getting around either, I'm not an official tech, either, just do builds on my own, and repairs for friends etc.

If you want to bring it over, we can put it on my scope etc and see if we can trace the hum.

It may take me a bit to be able to, just went through major surgery with my doggo, so she's needing help for the next week.

~Phil

[mwaller]

I disconnected the reverb at the grid on V2 and added a 220k pull-down resistor. There is still hum and hiss unless I remove everything except the power tubes and rectifier. Can anyone recommend a tech in the Seattle area who might be willing to troubleshoot these issues? I'm running out of ideas...

I'm in the seatle area, but been doing much less work lately. with the snowpocalypse it's not going to be fun getting around either, I'm not an official tech, either, just do builds on my own, and repairs for friends etc.

If you want to bring it over, we can put it on my scope etc and see if we can trace the hum.

It may take me a bit to be able to, just went through major surgery with my doggo, so she's needing help for the next week.

~Phil

Hi Phil -
I appreciate the offer! It would be great to have another set of eyes on this problem. A scope would be really helpful, since I don't have one.
The snow has really mucked things up around here, but I'm in no particular hurry. Perhaps we can get in touch in a week or so when the snow has subsided and your dog has recovered a bit?
Thanks!
Mika

[pompeiisneaks]

I disconnected the reverb at the grid on V2 and added a 220k pull-down resistor. There is still hum and hiss unless I remove everything except the power tubes and rectifier. Can anyone recommend a tech in the Seattle area who might be willing to troubleshoot these issues? I'm running out of ideas...

I'm in the seatle area, but been doing much less work lately. with the snowpocalypse it's not going to be fun getting around either, I'm not an official tech, either, just do builds on my own, and repairs for friends etc.

If you want to bring it over, we can put it on my scope etc and see if we can trace the hum.

It may take me a bit to be able to, just went through major surgery with my doggo, so she's needing help for the next week.

~Phil

Hi Phil -
I appreciate the offer! It would be great to have another set of eyes on this problem. A scope would be really helpful, since I don't have one.
The snow has really mucked things up around here, but I'm in no particular hurry. Perhaps we can get in touch in a week or so when the snow has subsided and your dog has recovered a bit?
Thanks!
Mika

Sounds like a plan. I also really need to sell an old hitatchi scope I got for 100$ since I've never used it since, I guess I should put that up here at some point, but I need to stop being so lazy

~Phil

[earl2]

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[earl2]

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[mwaller]

[/quote]

Sounds like a plan. I also really need to sell an old hitatchi scope I got for 100$ since I've never used it since, I guess I should put that up here at some point, but I need to stop being so lazy

~Phil
[/quote]

Hi Phil -
I sent you a PM, but it appears to be staying in my outbox. Perhaps this is user error... let me know if it came through.
Thanks!

[pompeiisneaks]

Sounds like a plan. I also really need to sell an old hitatchi scope I got for 100$ since I've never used it since, I guess I should put that up here at some point, but I need to stop being so lazy

~Phil
[/quote]

Hi Phil -
I sent you a PM, but it appears to be staying in my outbox. Perhaps this is user error... let me know if it came through.
Thanks!
[/quote]

I got it, it's a bad design choice by the phpbb people. Outbox means 'they haven't yet read it'

Except everywhere else it means 'it hasn't left to the user yet' Not sure why, but it shouldn't be there now.

[

everything okay earl2?

~Phil

[mwaller]

First - a big "thank you" to pompeiisneaks for taking a peek at my misbehaving amp! The issues have not been resolved, but we he did uncover some odd behavior that may hint at a solution. There appears to be an unusual DC offset occurring near the input to the reverb circuit:

Reverb Circuit.JPG

With approximately 80VDC at V2a Plate, we observed *negative* voltages (-10VDC) on the grid of the reverb driver, V3a. We tried replacing the reverb tube, dwell pot and the .001uf coupling cap, but had similar results.

Phil noted that leads going to the reverb driver are incredibly sensitive to touch - light taps with a chop stick are clearly audible in the output of the amp. Also, gentle movement of the reverb tube within the socket caused audible changes in the hum and squeals that are occurring.

We currently suspect that the reverb socket may be defective in some way. New parts are on order...

Has anyone seen this issue before? What could be causing it?!

[pompeiisneaks]

Yup this is the 10thtx's 1 tube reverb, super odd, I built an amp with it once and didn't have this kind of sensitivity. This was with the reverb pan removed. Also when inputting a sine wave, when the dwell pot was moved you could see the DC offset move the sine wave downwards more (the -DC offset increases)

What could be sending negative DC from the grid of the tube backwards into the pot etc?

~Phil

[pompeiisneaks]

another thing I did want to ask. The voltage above the 100k anode resistor was about 280 I think if I recall, but the voltage at the plate was only 80VDC is that also expected?

~Phil

[mwaller]

another thing I did want to ask. The voltage above the 100k anode resistor was about 280 I think if I recall, but the voltage at the plate was only 80VDC is that also expected?

~Phil

This is in reference to the gain stage immediately before the reverb circuit, V2a.

[pompeiisneaks]

I think I saw similar data for both the gain stage before as well as the anode of the first triode of the reverb circuit as well. But maybe I should have written it down hah.

~Phil