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guitar amp loses volume

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tempus

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Hey all;

I've got a Carvin X100B tube amp that keeps losing volume. I've been messing with it for some time now, so I'll give you the rundown of what I know:

1. after about 10 minutes of use, the volume drops considerably (the actual time varies, but at times you can practically set your watch by it)
2. the volume drop may be gradual (i.e., over a period of a few seconds) or sudden, and is replaced with an increase in hum (but not excessively loud hum)
3. the tone stays pretty much the same (not all flabby and weak) it's just quieter
4. this problem is intermittent and sometimes a whole week will go by without any problems
5. there are 2 channels on this amp (dirty and clean) and the problem affects both

Here's what I've tried:

1. switching preamp tubes around - no effect
2. switching channels - no effect
3. resoldering most of the board - no effect
4. cleaning tube sockets - no effect
5. the amp also has a 60% power switch, which, when toggled, makes a staticy noise and usually resolves the problem. Thinking the switch might be the problem, I rewired it to be bypassed - no effect
6. changing speaker impedance switch - no effect

There may have been others, but that's about all I can think of for now. Figuring that there was an issue of supply voltage to the power tubes, I connected my meter to one of the tubes and measured the proper voltage. I left the meter attached for a long time, and the problem went away (while the meter was in the circuit). This is the only thing that has reliably solved the issue, and I've repeated it a couple of times to make sure it wasn't a fluke. I'm not sure how that might solve things, but it seems to have.

Any ideas on what to do here?

Thanks
 
Sounds like a filter cap failing (since your experiencing a "hum"), so I'd guess the filter cap C71, on the grid biasing circuits for the 6L6 final amps AND connected to the 60% power switch circuit: see schematic http://carvinbbs.com/userpix/8926_Carvin_X100B_1981_Schematic_Poweramp_1.jpg.

<EDIT> Might also be one or Both of the resistors in the 60% circuit since they're switched in and out and are integral to that same biasing circuit. If you can find them, do they look "toasty"? If so, replace them - note wattage values.

Might be a good idea to replace ALL the electroytics (and there a bunch), but I'd start with C71.
 
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Sounds like a filter cap failing (since your experiencing a "hum"), so I'd guess the filter cap C71, on the grid biasing circuits for the 6L6 final amps AND connected to the 60% power switch circuit: see schematic http://carvinbbs.com/userpix/8926_Carvin_X100B_1981_Schematic_Poweramp_1.jpg.

<EDIT> Might also be one or Both of the resistors in the 60% circuit since they're switched in and out and are integral to that same biasing circuit. If you can find them, do they look "toasty"? If so, replace them - note wattage values.

Might be a good idea to replace ALL the electroytics (and there a bunch), but I'd start with C71.

Thanks for the tips Cowboy. The 22K resistor looks and measures fine, and since the problem persisted with it out of the circuit, I'll assume it isn't the problem. The 470K is in a much more difficult to access place, but I do remember having to replace it a few years back, so I'll definitely check it again. The filter cap at C71 also crossed my mind, but it's in a multicap can, and so not easy to replace as a single unit, and pretty expensive (at those voltage ratings) to replace the whole can just to check. Might a much lower value (but still same voltage rating) cap in parallel solve the problem, just as a test to prevent ordering an expensive part?
I'll look into these things.
 
Check R103 and R104 (or more easily check the voltages on them), anode loads of triodes are the most common failure in valve amps.

Thanks for this Nigel.
I suspected R103 and 104 as well, but the problem I'm running into with is that as soon as I hook a meter up to the circuit, the issue resolves itself. I can't check them in circuit with the amp on, and if I measure resistance out of circuit, I'm sure they'll measure within spec, since the problem arises after a period of time.
 
as soon as I hook a meter up to the circuit, the issue resolves itself.
That should give you a clue. Is there any particular hook-up point where this happens? What is the meter impedance?

C64/C65 could be a culprit.
 
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Thanks for this Nigel.
I suspected R103 and 104 as well, but the problem I'm running into with is that as soon as I hook a meter up to the circuit, the issue resolves itself. I can't check them in circuit with the amp on, and if I measure resistance out of circuit, I'm sure they'll measure within spec, since the problem arises after a period of time.

What voltages are on the anodes?, and on the top ends of the 100K's.
 
That should give you a clue. Is there any particular hook-up point where this happens? What is the meter impedance?

C64/C65 could be a culprit.

Thanks Alec. I had the meter connected to pin 4 of one of the power tubes. The meter is a Fluke 73 DMM, so I'm assuming quite high impedance?
 
I've just done a little more exploring and I've found something really odd. C69 and C71 have been replaced (in a previous repair) by a single 100uF cap. Is it even possible for the amp to function properly like this or am I misreading something? That repair was done years ago. Also, I can't locate the 47K 2W resistor, although there is an extra 470K in there. Is this just functioning as a bleed resistor for that big cap?

Thanks again
 
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I've just done a little more exploring and I've found something really odd. C69 and C71 have been replaced (in a previous repair) by a single 100uF cap. Is it even possible for the amp to function properly like this or am I misreading something? That repair was done years ago. Also, I can't locate the 47K 2W resistor, although there is an extra 470K in there. Is this just functioning as a bleed resistor for that big cap?

Thanks again
My emphasis.

By the way, tempus,I misspoke about C71 being in the control grid biasing circuit of the 6L6s: it is feeding the 12AX7's plate voltages (thank you nigel for pointing that out).

But if you are correct about C71 AND C69 being tied together to a single cap, then the voltage at points "B" and "C" would be identical, shorting around the R122, R123 portion of the voltage divider network they make with R124 (not just a bleeder), and essentially putting the 12AX7s' plates (anodes) and the 6L6 screen grids at the same potential. Add that to a "leaky" (slowly, partially shorting as it heats up) C69 OR C71, or BOTH and you'll get the problem you describe.

AND, if the 47K you mention is R124 and it's missing (or replaced with a 470K) then the 12AX7 plates and 6L6 screens are going to be even further from design specs.

Finally, introducing the Fluke into the screen circuit of the 6L6s is, apparently, altering the screen grid bias, however subtlely, and allowing for correct operation.

Take the time to replace C69 & 70, as per the schematic (single caps for each is acceptable and considerably cheaper than a multiple can type), replace the 470K with the correct value and my bet is that you will have fixed it.

Lastly, paralleling a good electrolytic cap with a leaky electrolytic cap does not undo the effect of the leak.
 
C69 and C71 have been replaced (in a previous repair) by a single 100uF cap.
That would effectively short out R122/R123, which seems unlikely.
Is this just functioning as a bleed resistor
As I read it, R122-R124 form a voltage divider which sets the HT voltage for the V3 anodes, and C71 smoothes the voltage.
[Edit: Cowboybob beat me to it :)]

Check for dry joints at the ends of R121; also the value of R121.
What volts do you get at test points A,B,C?
 
My emphasis.

By the way, tempus,I misspoke about C71 being in the control grid biasing circuit of the 6L6s: it is feeding the 12AX7's plate voltages (thank you nigel for pointing that out).

But if you are correct about C71 AND C69 being tied together to a single cap, then the voltage at points "B" and "C" would be identical, shorting around the R122, R123 portion of the voltage divider network they make with R124 (not just a bleeder), and essentially putting the 12AX7s' plates (anodes) and the 6L6 screen grids at the same potential. Add that to a "leaky" (slowly, partially shorting as it heats up) C69 OR C71, or BOTH and you'll get the problem you describe.

AND, if the 47K you mention is R124 and it's missing (or replaced with a 470K) then the 12AX7 plates and 6L6 screens are going to be even further from design specs.

Finally, introducing the Fluke into the screen circuit of the 6L6s is, apparently, altering the screen grid bias, however subtlely, and allowing for correct operation.

Take the time to replace C69 & 70, as per the schematic (single caps for each is acceptable and considerably cheaper than a multiple can type), replace the 470K with the correct value and my bet is that you will have fixed it.

Lastly, paralleling a good electrolytic cap with a leaky electrolytic cap does not undo the effect of the leak.

Thanks for all this Cowboy.
Yup, it sounds like I need to roll my sleeves up and fix this mess. I'm agog that the amp has actually functioned at all for a number of years with this level of miswiring. I'm also concerned about the 470K and how it got there; is it possible that the value on the schematic is a misprint?
 
Tube type amp circuits are very forgiving of anomalies that develop with age. But only up to a point.

If you can properly identify to what, exactly, the 470K is currently atached, I'd trust the schematic value over the previous hack job "fix" value.
 
Have you ever measured the voltages on the anodes of the triodes yet?.

Sorry for the late reply on this Nigel - I figured I'd better try to get everything else in order first. By "triode" do you mean the 12AX7? If so, the voltage at pin 1 of V3 is 268V, and at pin 6 it's 258V, so about a 10V discrepency. Is this significant? The voltage at point A is 470V, point B is 464V, and point C is 392V. I have rewired the amp as per the schematic (I kept the 470K since I didn't have a 47K handy), and checked the bias on the power tubes, which was in spec.

Something else came up while I was checking these voltages: touching the tip of the power amp in jack with the plastic part of my probe created the fault condition, and it was corrected by touching the metal tip of the probe to any of points A, B or C, or to the anodes of V3. I repeated this several times with consistency. When I switch to 60% mode, however, touching the anodes of V3 or point C does not correct the fault, but touching point A or B does.

So... maybe something goofy with that jack?
One other thing I was wondering - am I correct in assuming that the amp is functioning at 60% when the 22K resistor is switched out of the circuit?
Thanks Nigel
 
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Sorry for the late reply on this Nigel - I figured I'd better try to get everything else in order first. By "triode" do you mean the 12AX7? If so, the voltage at pin 1 of V3 is 268V, and at pin 6 it's 258V, so about a 10V discrepency. Is this significant? The voltage at point A is 470V, point B is 464V, and point C is 392V. I have rewired the amp as per the schematic (I kept the 470K since I didn't have a 47K handy), and checked the bias on the power tubes, which was in spec.

The anode loads look fine then, they would be a LOT lower if they were duff.

Something else came up while I was checking these voltages: touching the tip of the power amp in jack with the plastic part of my probe created the fault condition, and it was corrected by touching the metal tip of the probe to any of points A, B or C, or to the anodes of V3. I repeated this several times with consistency. When I switch to 60% mode, however, touching the anodes of V3 or point C does not correct the fault, but touching point A or B does.

So... maybe something goofy with that jack?
One other thing I was wondering - am I correct in assuming that the amp is functioning at 60% when the 22K resistor is switched out of the circuit?

Yes you are.
 
:confused: Given those measured voltages, by my reckoning each of the V3 anode resistors has ~ 1.2mA through it. So I don't see how the amp could work if R122 were 470k as per the schematic. Even ignoring the current through R124, R122 would have 2.4mA through it and the voltage at B would have to be 1128V, which simply isn't available. IMO it would make more sense if the values of R122 and R124 were swapped.
I'm wondering if the amp is perhaps oscillating at an above-audio frequency, the application of the probe being enough to start/stop the oscillation. Such oscillation would likely screw up the measurements.
 
Well, this might be one of the weirdest fixes ever... I shut the amp down and plugged and unplugged a phone plug into the Power Amp in jack a few times, and now can no longer reproduce the problem. Maybe a spider laid an egg in the jack or something and that's been the problem all along. The jack is switched, so maybe it was making an intermittent connection or something. Ran the amp for over half an hour with no issues. The tone seems markedly beefier - so hard to tell without doing an actual A/B, so might just be the power of suggestion, although I suppose it makes sense now that the amp is running closer to design spec. As I mentioned in the original post, this has been a very intermittent problem (although more often present than not), so I may not be done with it yet, but we'll see how it goes.
I'd definitely be interested in anything anyone may have to say on the issue, and will post back if the problem arises again.

Thanks to everyone for all your help.
 
Good catch, tempus. You should be proud.

As for me, certainly a reminder to check the connections first, especially on older equipment.

Why is it that it seems so much easier (and wiser) to go for the difficult solution and ignore what should always be the first tests?
 
Good catch, tempus. You should be proud.

As for me, certainly a reminder to check the connections first, especially on older equipment.

Why is it that it seems so much easier (and wiser) to go for the difficult solution and ignore what should always be the first tests?

Because every time something's wrong with my car, I open the hood to see if there's one wire that's obviously come loose...and there never is.

I think that the switched jack must have been making only a weak and intermittent connection, and so when open, there was little or no signal hitting the power amp section (because normally with something plugged into that jack, whatever was coming back from the fx loop would be going straight to the power amp), thus causing the very low level signal and hum. I still can't figure out why this would only occur after a period of time, but I guess as long as it's fixed I'm happy.

Thanks again for all your help.
 
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