Hum in MV Twin Reverb

[MarkusBass]

So, I removed the external speaker jack from the chassis but left it connected electrically, just hanging in the air. If I took my chopstick and moved the jack so that it's sleeve touched the chassis, the sound would degrade/crackle and/or drop away almost to nothing.

I'm not sure whether you understood very precise explanation by Potatofarmer. He explained the role of both jacks, why one of them is isolated from the chassis and how SPEAKER JACK and EXTERNAL SPEAKER JACK can be used for different combinations of speakers. And you ignore his explanation and short the sleeve of the external jack to chassis. By doing this you short the output transformer to ground. And you are surprised that there is no output signal in this case. I wonder whether the amp has survived your experiments .
Please read the post by Potatofarmer once again and try to understand it. This is very important. Please look at the schematic and check how both jacks are wired (one different from the other).
If you want to check the jacks, just desolder them from the amp and check the required connection with Ohm-meter. If they are not OK, buy new ones (of correct type).

Mark

[earache]

I'm not sure whether you understood very precise explanation by Potatofarmer. He explained the role of both jacks, why one of them is isolated from the chassis and how SPEAKER JACK and EXTERNAL SPEAKER JACK can be used for different combinations of speakers. And you ignore his explanation and short the sleeve of the external jack to chassis. By doing this you short the output transformer to ground. And you are surprised that there is no output signal in this case. I wonder whether the amp has survived your experiments .
Please read the post by Potatofarmer once again and try to understand it. This is very important. Please look at the schematic and check how both jacks are wired (one different from the other).
If you want to check the jacks, just desolder them from the amp and check the required connection with Ohm-meter. If they are not OK, buy new ones (of correct type).

Mark

MarkusBass - I do understand and appreciate the thorough explanation by Potatofarmer. I should have further explained that the reason I did touch the sleeve of the jack to the chassis was to verify (for myself) that this was the cause of my problem.

The amp survived my experiments.

The last part of my post was asking if anybody else had this similar problem and how they resolved it. I should have added that I know that the fiber washers are the main cause of the problem. As Potatofarmer said, Fender expects a lot from those little washers. I am seeking guidance from the collective experience of the group...what have they used as a better replacement? Or is there a readily available replacement on line?

I am not entirely certain how it is that I can show continuity with my meter between the sleeve of the jack and the chassis whether it is touching the chassis or not; I guess I wanted to see it myself.

My post seems to have annoyed you somewhat. Let me apologise if I wasn't clear enough in my last post, I think I have, with your help, clarified my questions somewhat better.

[MarkusBass]

I am not entirely certain how it is that I can show continuity with my meter between the sleeve of the jack and the chassis whether it is touching the chassis or not; I guess I wanted to see it myself.

The basic test is to desolder the jacks and check resistance of all components: the output winding of the transformer, continuity between the OT winding and the chassis, continuity between various pins of the jacks (for example pins marked usually T and TS of the SPEAKER JACK should be shorted). The same applies to the other jack - it can be easily tested when desoldered from the circuit.

My post seems to have annoyed you somewhat.

Not at all. I was to harsh and I apologize for that. The reason for my reaction was that explanations by Potatofarmer were very precise (you couldn't think of better explanation) and by looking additionally at the schematic you know everything. There is no need for experiments (especially shorting OT to the ground ). In fact, wiring of these two jacks is quite tricky and it requires few minutes to look at the schematic to fully understand it.

Mark

[g1]

The amp's sound improved a lot but it seems that the hum as controlled by the "Output Tubes Matching" pot needs to be tweaked every time I turn off/turn on the amp. There is still a fair amount of crackling which comes and goes.

Is it possible that the external speaker jack affects the bias of the power tubes?

Has anybody else had and solved this or a similar external speaker jack problem with this style of amp?

Is there a better replacement part or external speaker jack arrangement?

Do any of these issues happen when the jack is not installed in the chassis? If not, then the issue is strictly the chassis mounting, and maybe dirty switch contacts in the jack.
Does it have the proper "shoulder" type fiber washer, or just a flat type?

[earache]

It sounds like you have a bunch of bad tubes and/or a bunch of bad grounds and/or drifted components and/or leaky coupling caps. Any of these things could cause the problems you're having. Check for DC under operating conditions at pins 2 and 7 of the questionable circuits. If you don't find anything over half a volt (check meter default) try replacing those tubes.

Backed away from this amp for a couple days to collect my wits, and order some shoulder washers and a set of output tubes. I just looked at the DC voltages on the phase inverter tube pins 2 and 7 as you suggested...found 46 VDC at pin 2 and 58 VDC at pin 7 ! I replaced the .01 ceramic at pin 2 and the .1 cap at pin 7 but no change. Will dig deeper

[g1]

I just looked at the DC voltages on the phase inverter tube pins 2 and 7 as you suggested...found 46 VDC at pin 2 and 58 VDC at pin 7 !

Sometimes these pins will not measure correctly if referenced to ground. Put your black probe on the cathode (pins 3&8) and measure 2 & 7 again. Then measure from 3&8 to ground and do the math.

[earache]

Sometimes these pins will not measure correctly if referenced to ground. Put your black probe on the cathode (pins 3&8) and measure 2 & 7 again. Then measure from 3&8 to ground and do the math.

Measured on the phase inverter:
Pin 2 With Respect To Pins 3 and 8 -1.26 VDC
Pin 7 WRT Pins 3 and 8 -1.25 VDC

Pins 3 and 8 measured WRT ground 63.7 VDC

Can you elaborate on "the math" that you refer to please?

Thanks in advance

[Enzo]

You measured like 46v on the grid of the tube and 64v on the cathode. That would be 18v of negative bias on that grid. But it is a false reading. It is a high impedance circuit, and your meter impedance is combining with it to make a voltage divider. Usually your meter is fine, but this is one case you cannot measure to ground. So you measure voltage from the cathode to grid. You got 1.25v. So that means the grid is 1.25v less positive than the cathode - 1.25v negative bias. SO the math is simple:

63.7 minus 1.25 equals 62.45. So your grid really has 62.4v on it. And that is a pretty normal amount of bias for a 12AX7 triode.

[earache]

Enzo -

Thank you very much for the explanation and the education. I always appreciate how you and many of the regular posters here take the time to write out the "how's and why's".

[g1]

Can you elaborate on "the math" that you refer to please?

Sorry, I wasn't trying to be snarky, but was pressed for time. Enzo explained it much better than I would have anyway.

[earache]

Sorry, I wasn't trying to be snarky, but was pressed for time. Enzo explained it much better than I would have anyway.

Not a problem at all, I didn't take it that way. I don't understand all of the concepts yet, having come at learning electronics in a sort of "sideways" manner. That's why I have to ask about some subjects that are probably pretty basic knowledge to most visitors here.

Not only do I still have a lot to learn, I have to break my bad habits of shotgun parts replacement and always going for the perceived "easy fix".

I hope that I have always expressed gratitude to you, G1, and all others who take the time to read and respond to my posts. I sincerely appreciate the help and the education.

Finally - Enzo always explains it better than most, doesn't he?

[earache]

I ordered and received the shoulder washers and a new set of power tubes.

Installed the washers and there is no improvement. The amp is still far too quiet and pretty distorted at full volume.
The new set of power tubes shows no improvement.

I am going to circle back around and try some of the earlier tests again to see if I might have missed something.
I am also going to focus around the Phase Inverter circuit to to see if there are bad components or if the tube I swapped in is also bad.

[lowell]

Just a thought but make sure your extension speaker jack and internal speaker jacks havent been swapped.

I also see no post indicating that you've measured the bias CURRENT of your power tubes. The bias voltage on pin 5 was said to be -55 (ish). How about the current through the tubes? This can affect the sound and volume.

[earache]

Hello All You Helpful Folks

For closure, I need to post the resolution to my problem here.

I had put the amp on the "back burner" for a while when I did other stuff. When I came back around to it, I decided that a good first step would be to review all the things I had done before and make sure they were correct before implementing some of the ideas and actions that were suggested to me by you folks.

There were two things I did at once between periodically checking to see if the volume returned to normal (loud), and one or both of the following actions returned the amp to it's proper output:

I replaced the Phase Inverter;
I resoldered the connections at the caps in the doghouse.

Now the amp is loud as it should be. I still have to set the bias and get rid of some crackling noises but I feel confident that I can solve those problems.

Thanks to all of you who have contributed your time and expertise. I really learned a lot on this one!

Earache

[Chuck H]

It's possible that replacing the PI scraped the contacts clean of oxide enough to work again. Unfortunately you don't mention which of those operations corrected your power issue, so we can't know what actually helped and we have confused circumstances for diagnosis. Was it re soldering under the doghouse, friction on the PI tube contacts, the new PI tube or some combination of these. So we still can't ID a positive course of action. I still think you should clean and re tension all the sockets and pins. Considering the amp fixed because it's "working now" and sort of correctly is kind of like thinking that falling is fine because it doesn't hurt like landing. You haven't positively identified and corrected anything so you can only hope that you've fixed something accidentally.

You should do one procedure at a time and test in between for more concise troubleshooting. Further, it would be an idea to clean the pins on the old PI tube and plug it back in. If it works you'll know that the new tube actually had nothing to do with any problems. It only seemed that way. It's better to know something than believe something.