Tracing noise problem, TwinRev.

[Chuck H]

When its turned up, i get significantly less volume than expected ( i need to turn to 3 to get say an equivalent acoustic gtr vol: usually vol 3 would be painfully loud).

If you typically only use the vibrato channel and never used the normal channel then this doesn't seem odd. The "vibrato" channel has about 7db more gain than the "normal" channel. But if you typically use the "normal" channel at volume setting 1 and now it needs to be on 3 then there may be something to troubleshoot.

[pdf64]

… pulling v2 to v5 because it's the simplest test.

Done this. Now with the vol pot at zero.. no noise.

Turning this vol pot... noise is directly affected, almost telling me the pot is dodgy (?).
…

How about if you try a few different 12AX7 (=7025=ECC83) in V1?

In this channel at least, the noise may be originating in the input stage.

What settings produce what result is somewhat irrelevant, the pots used are ‘cheap as chips’, not precision components.
All that matters is ‘does the amp get loud when set to max and very quiet (ideally mute) when set to minimum?’.

Bear in mind that Fender knobs are usually set so that ‘1’ is the minimum pot setting, full counter clockwise, there’s no zero setting per se.

[Sea Chief]

How about if you try a few different 12AX7 (=7025=ECC83) in V1?

In this channel at least, the noise may be originating in the input stage.

What settings produce what result is somewhat irrelevant, the pots used are ‘cheap as chips’, not precision components.
All that matters is ‘does the amp get loud when set to max and very quiet (ideally mute) when set to minimum?’.

Bear in mind that Fender knobs are usually set so that ‘1’ is the minimum pot setting, full counter clockwise, there’s no zero setting per se.

I've tried a few different 12ax7's in v1, I did this & swapping in known good tubes, & switching them around etc, are all I could do to be sufficiently sure before creating the thread, that I had covered this aspect.. which I outlined in #1 as having done.

So I was thinking the logical next progression, was to now try & isolate which component -if it is a component- the problem is due to.

There is definitely a huge difference in volume from the Normal channel, seemingly from having pulled v2 to v5: logic says to me that this is directly due to pulling of these tubes. Is this volume drop to be expected after pulling these tubes?

--

Is there any way of being able to conclude that this problem originates in the input stage? I can only you see add logic to this neverending problem.

I mean what would a technician do once ruling out the tubes, & pulling v2 to v5 ? (which is all I have done so far).

Is there a logical next step? Thanks, SC

[Sea Chief]

pdf64 Ive just redone the different 12 ax7 in v1.. to be super sure this base is covered. Same results. So I continue to have v2 to v5 pulled, guitsr plugged into Normal, guitar vol up.

With the amp vol knob at zero..

( IE before the 1 there is a 5 mm of rotation on to the furthest minimum point, which I call zero because it just seems logical to if usually on an amp the volume is cut to zero at this furthest point.. & furthermore if it is 5mm physically onwards past 1; I can't see any logic in calling this minimum position 1, if it is both physically past it, & mathematics says 1 = a value, a definite unequivocal progression past zero).

.. I get a silent amp.

Nothing whatsoever apart from a tiny usual mains hum. As soon as I touch the vol knob though I get crackling. If I venture to actually fractionally rotate it, I get very loud crackling/ noise. Even just wiggling the shaft a mite, the noise is introduced.

Is that any more help at all? ((Would this not simply point towards a defective vol pot?? Or is that just naiive))

Thanks, SC

[Chuck H]

You should check for DC voltage on the volume control wiper.

And removing tubes V2 through V5 shouldn't effect the normal channel volume significantly. Not sure you answered to matters I raised in post #16. You only say the gain is reduced. I only want to know if you are judging that based on an apples to apples comparison. Like I mentioned, the "vibrato" channel on that amp has a significant amount more gain than the "normal" channel you are using now. If you don't ordinarily use the normal channel your perception may be based on the gain level of the vibrato channel.

[Sea Chief]

You should check for DC voltage on the volume control wiper.

And removing tubes V2 through V5 shouldn't effect the normal channel volume significantly. Not sure you answered to matters I raised in post #16. You only say the gain is reduced. I only want to know if you are judging that based on an apples to apples comparison. Like I mentioned, the "vibrato" channel on that amp has a significant amount more gain than the "normal" channel you are using now. If you don't ordinarily use the normal channel your perception may be based on the gain level of the vibrato channel.

Hi Chuck, ok I will research which wiper is what & measure form where to where & hopefully do this test.

Thing is I occasionally used to play through the normal channel, & never noticed a difference in volume to the Vibrato channel.. on this, my musicman, my deluxe reverb build. So even if there may be a figure of "7" difference, audibly I have never noticed it. Or rather I may have had the vol knob on say 2.5 on the Normal channel to say equivalent 2 on the Vibrato channel, simply without noticing or knowing of there being any 7 figure difference..

But the findings doing the test pulling v2 to v5, the volume on Normal is in a totally different ballpark. A mile different. For eg if I usually turn the vol knob on either channel to 3, my toupe would lift up my eyes go crossed due to the sheer volume. FKN LOUD. Now, I have to turn it up to 3 just to get the equivalent volume to a strummed acoustic guitar. That defo ain't right IF you say pulling v2,4,5 shouldn't affect it's volume. If I can bear to turn it up to 3 because of all the damn noise pallaver scaring me to heck that is.

This surely signifies something amiss, no?

Thanks, SC

[pdf64]

On the face of things, operating the amp with V2-5 pulled should, if anything, increase the gain of the normal channel slightly.
Given the symptoms, checking for VDC on the pot is a great idea.

[Chuck H]

Hi Chuck, ok I will research which wiper is what & measure form where to where & hopefully do this test.

Thing is I occasionally used to play through the normal channel, & never noticed a difference in volume to the Vibrato channel.. on this, my musicman, my deluxe reverb build. So even if there may be a figure of "7" difference, audibly I have never noticed it. Or rather I may have had the vol knob on say 2.5 on the Normal channel to say equivalent 2 on the Vibrato channel, simply without noticing or knowing of there being any 7 figure difference..

But the findings doing the test pulling v2 to v5, the volume on Normal is in a totally different ballpark. A mile different. For eg if I usually turn the vol knob on either channel to 3, my toupe would lift up my eyes go crossed due to the sheer volume. FKN LOUD. Now, I have to turn it up to 3 just to get the equivalent volume to a strummed acoustic guitar. That defo ain't right IF you say pulling v2,4,5 shouldn't affect it's volume. If I can bear to turn it up to 3 because of all the damn noise pallaver scaring me to heck that is.

This surely signifies something amiss, no?

Thanks, SC

This is good reporting. Thank you. Ok, well,.. Then something else is wrong with the amp. May or may not have to do with what's causing the noise. Maybe the volume issue on the normal channel presented also with the noise issue and just went unnoticed till now if you weren't using it since the problems started.?. But one thing at a time. I don't want to rush ahead and foul Pete's process here. I've just been trying to shed light on any potential indicators in the matter in case it helps at all.

[g1]

We are assuming it has been rebuilt with stock twin wiring, but that is not necessarily the case.
If you don't find any significant DC on the vol. pot, put V2 back in and see if Normal ch. is still weak.

[Sea Chief]

On the face of things, operating the amp with V2-5 pulled should, if anything, increase the gain of the normal channel slightly.
Given the symptoms, checking for VDC on the pot is a great idea.

Hi pdf,

Ive got the beast up on bench all ready for tests again. V2 to v5 pulled, so ready to test VDC as Chuck suggests.

Now then, am I only needing to test it without rotating the knob? The noise is so loud & unsettling even touching the damn vol knob, so if this test is needed at various points on the rotation.. Im stuffed/ it's too scary a prospect, unless that is I briefly unplug the speakers.

Thanks all for your help, absolutely invaluable again. SC

[Sea Chief]

Chuck H I get 0 vdc across the Normal volume pot wiper, to the next lug going to ground.

Just by visual inspection, the board & all components inside seem perfectly dry/ the board is exactly as it was when new. I scratched the surface & it's rigid & dry without a hint of any film/ mould or any obvious nasties having accumulated on it.

I know it's not a foolproof test, but I certainly don't get continuity across the board using a DMM too.

Thanks, SC

[pdf64]

Chuck H I get 0 vdc across the Normal volume pot wiper, to the next lug going to ground.
…

How about across the track, the 2 end terminals, not the wiper?
ie one probe touching one end, the other probe to the other end.

[Sea Chief]

How about across the track, the 2 end terminals, not the wiper?
ie one probe touching one end, the other probe to the other end.

Will do 1st thing AM pdf. So just lug to lug missing out the wiper..

Thinking more on the dreaded conductive board scenario (which would be a huge disaster/ Im hoping massively it's a component failure instead)..

When I prized the baffle off the velcro, just tarting bits up etc, I was surprised to find lightgreen dry ( seems to be) mould: a thin uniform film had got in this gap, IE the 3mm gap due to the velcro patches separating the two chipboard surfaces.

So with the two main boards sandwiched together, the lower one to prevent components on theunderside shorting to the chassis.. I wonder if green mould might have similarly got in here. Similar tiny gap. Mould tends to thrive on wall in gaps behind cupboards, behind furniture very common here etc.

Its only a theory to consider if, hopefully not, I don't find the cause. Heck of a job to unsolder loads to lift it up to see. Last Resort Idea !!

Thanks, SC

[Sea Chief]

How about across the track, the 2 end terminals, not the wiper?
ie one probe touching one end, the other probe to the other end.

Just done this now. I get 0.07 vdc across the outer terminals.

SC

[Chuck H]

Ok. Just for the record I usually DON'T agree with "American" vs. British english, but... Here's one that I think we ARE actually doing better. In the US we use "mold" to indicate a fungus and "mould" to indicate a form that provides the shape for a temporarily viscous medium or the act of shaping such a medium. That way we're not using the same word for both applications. But I digress...

If you've got mold (mould) growing on the amp I'd say any board made from paper fiber is a terrible idea. I'm almost certain at this point that what you have is a conductive board. Be it new-ish or otherwise. You won't recognize a conductive board with a continuity test. The test voltage from your meter will be too low to instigate the problem. But the voltages present in that amp? Sure, you bet. I've experienced conductive boards that I could simply poke the red probe into (black probe grounded and meter set for VDC) at various locations and read voltages easily capable of ruining operating conditions for the tubes.

EDIT: My advice is to get on with the blow dryer operation I suggested. Except now I'd say do it for an hour. Just sit there and wave it very slowly back and forth. Don't worry about how hot it's getting. Keep going. Just watch the tele or read a book and sit there doing that. Then put the tubes back in and see if things haven't, at least, improved or even just changed.