Garnet GS100tRW -- Very low output power

[Lectroid]

Thanks for reading,

Except my own, I've never debugged any broken amps, but a buddy has asked me to look at his Garnet amp.  He said it had just "stopped working."  He replaced all tubes with nice-looking NOS; for now I'm assuming they're okay.  I fixed a dangling bass cap right away and then the amp starting amping--but it's putting out very low power.  I have to crank the Volume and Master Volume to be able to hear it at all.

The 6L6 voltages I have are: 
plate      395V
screen    380V
grid        -28V   

The GS100R schematic calls for -48V bias at the 6L6 grids, so I will assume the bias circuit is bad. Picture and diagram below.  The physical bias circuit is like a Marshall bias except Garnet left off any bias adjustment pot.  Ha-ha.   On the diagram, the actual voltages I measured are shown in parentheses. 

The supply resistor has drifted a lot from 120K, to 26.6K. So it's probably the culprit here, right?  Or should I also be looking at another possibility?  All advice and suggestions welcome.

Does it make the most sense right now to focus on the bias circuit to unravel the low power scenario?  If I can restore the right bias voltage, could that be the fix (or most of it) for this low-output problem?

On a side issue, I don't understand fixed bias as well as I'd like to.  But it seems to me that, with a lower supply resistance from what was specified, the bias voltage coming in would be that much higher, not dropped down to -28V.  Can anyone explain what I'm missing? 

Thanks!

[shooter]

I don't understand fixed bias as well as I'd like to

Your PA tubes life depends on you understanding this!


-48 is what the manufacturer wants, that what you need, the closer to ZERO that number gets, the more current the PA tubes will dissipate til they glow red and melt.


TAKE OUT the power tubes and leave out til this is fixed.

[HotBluePlates]

The supply resistor has drifted a lot from 120K, to 26.6K. So it's probably the culprit here, right? ... it seems to me that, with a lower supply resistance from what was specified, the bias voltage coming in would be that much higher, not dropped down to -28V.

Bias filter cap (64µF) could be leaking current, dragging the output voltage down.
Coupling caps feeding the output tube grids (0.047µF) could be leaking +DC Volts, reducing the total bias voltage.

Unsolder each of the coupling caps to remove them as a possibility.  If the tube grids suddenly have much more-negative voltage, you've found at least one issue.

Also, the 120kΩ resistor might not be able to be accurately measured unless you unsolder one side.  120kΩ is in-series with 33kΩ to ground, and all that is in-parallel with half the transformer secondary.  That's going to booger your reading.

There are times when you don't need to unsolder one leg of a component to measure.  But at this early stage, assume you'll need to unsolder one leg of any part before attempting to measure it.  You can figure out the exceptions to this rule later.

[shooter]

schematic calls for -48V bias at the 6L6 grids, so I will assume the bias circuit is bad.

What HPB said...also;


IF...this was measured with tubes in, they could be bad, dragging down the bias.


your bias supply snippit doesn't have a ground symbol, add that just to keep ducks in a row

[Lectroid]

I don't understand fixed bias as well as I'd like to

Your PA tubes life depends on you understanding this!

-48 is what the manufacturer wants, that what you need, the closer to ZERO that number gets, the more current the PA tubes will dissipate til they glow red and melt.

TAKE OUT the power tubes and leave out til this is fixed.

Thanks, shooter.  I do know it's pretty critical even if I don't know the details yet.  I will remove the power tubes for now.

Also, ground symbol added.

[Lectroid]

Bias filter cap (64µF) could be leaking current, dragging the output voltage down.

So--I'll check for any voltage drop across the cap.  It does measure 156uF in circuit, fwiw.  I'll unsolder and re-measure.
It has about 2M resistance measured in circuit.

Coupling caps feeding the output tube grids (0.047µF) could be leaking +DC Volts, reducing the total bias voltage...Unsolder each of the coupling caps to remove them as a possibility.  If the tube grids suddenly have much more-negative voltage, you've found at least one issue.

Question: wouldn't any cap show a DC voltage drop across it, if it was leaking?

Also, the 120kΩ resistor might not be able to be accurately measured unless you unsolder one side.  120kΩ is in-series with 33kΩ to ground, and all that is in-parallel with half the transformer secondary.  That's going to booger your reading.

Understood.  I will take your advice and shooter's about unsoldering.  I also have voltage readings to post when I get them formatted.

[Lectroid]

As promised

Garnet GS100TRW

Tube	Plate	Screen	Grid-Cathode
6L6GC	386V	376V	-28.5
6L6GC	385V	377V	-28.5
12AX7a	173	-	-1.44V
12AX7b	168	-	-1.31V
12AU7a	102V	-	-4.28V
12AU7b	296V	-	-11.7V
12AU7a	51V	-	-1.81V
12AU7b	150V	-	-8.1V
12AX7a	219V	-	0V   (?)
12AX7b	249V	-	0V   (?)

The zero volts reading on the last tube's grids is weird, I agree, but it's what I got last night.  I will re-test them today. Thanks

[Willabe]

No, they want to see the voltage at pins 1,2,3 and 4,5,6 for all the preamp tubes. 

[HotBluePlates]

Coupling caps feeding the output tube grids (0.047µF) could be leaking +DC Volts, reducing the total bias voltage...Unsolder each of the coupling caps to remove them as a possibility.  If the tube grids suddenly have much more-negative voltage, you've found at least one issue.

Question: wouldn't any cap show a DC voltage drop across it, if it was leaking?

You're not measuring volts across the coupling caps.
Instead, unsolder one side of each of the coupling caps, then measure DC Volts from Pin 5 of the output tubes to ground.

If negative volts increase (say, from -28v to -40v), the voltage leaking through the coupling caps was counteracting the bias.

Bias filter cap (64µF) could be leaking current, dragging the output voltage down.

So--I'll check for any voltage drop across the cap.  It does measure 156uF in circuit, fwiw.  I'll unsolder and re-measure.
It has about 2M resistance measured in circuit.

The "2M resistance" probably doesn't mean anything useful.  It should also trend towards ∞Ω, but...

A better move is to unsolder this cap & tack-in any cap rated in µFs and with a high-enough voltage rating.  We're not seeking "perfect" just trying to eliminate a bias filter cap as being leaky.

[Lectroid]

No, they want to see the voltage at pins 1,2,3 and 4,5,6 for all the preamp tubes.

willabe, I hear you, but I've already removed the power tubes (per shooter), and dug into the the bias board low voltage issue.  Those are all the preamp voltage numbers I will have until the bias is corrected.  I hope the bias problem is the primary issue and any other amp issues will be minor..

@shooter, @HotBluePlates,
I unsoldered enough of the bias connections to be able to measure all of them with one lead free.  Results:

Nominal Value		Actual Value
63uF		145uF
120K		119.3K
33K		33.7K
68K		70.2K

The resistances seem to be in tolerance. But he filter cap measures 145uF, so--not good. 

I'm assuming I'll replace the cap, but I have a question:  There's a Vishay cap that would just fit the slot, just not in 63uF.  Go up to a 68uF or 70uF?  Or can I drop down to a 60uF?  Is there a rule of thumb here?  60uF seems like it should be enough. 

any suggestions?   

I'm curious about one thing, How does a failing, drifting cap lower the bias voltage?  What's the mechanism that makes that happen?

Thanks for all the pointers.  This has been a great help.

[Lectroid]

Question: wouldn't any cap show a DC voltage drop across it, if it was leaking?

You're not measuring volts across the coupling caps.
Instead, unsolder one side of each of the coupling caps, then measure DC Volts from Pin 5 of the output tubes to ground.

If negative volts increase (say, from -28v to -40v), the voltage leaking through the coupling caps was counteracting the bias.

Ahh, now I get it.  I'll test that and report back.

...The "2M resistance" probably doesn't mean anything useful.  It should also trend towards ∞Ω, but...

A better move is to unsolder this cap & tack-in any cap rated in µFs and with a high-enough voltage rating.  We're not seeking "perfect" just trying to eliminate a bias filter cap as being leaky.

I hate it when I wonder why I didn't think of that.  Dang.    I'll try that first thing tomorrow with a 47uF, if that's large enough for a test.   If I get a bias voltage of somewhere around -48V, then that shows the original cap was toast.  Perfect. 

Is there a "best" size for that bias filter cap?  How high should you go, or how low can you go, and still get adequate bias filtering? 

Thanks for the idea.

[HotBluePlates]

... I'll try that first thing tomorrow with a 47uF ...

Is there a "best" size for that bias filter cap?  How high should you go, or how low can you go, and still get adequate bias filtering?

47µF and as-many-volts (or more) than the original cap would be an excellent start.

[Lectroid]

@HBP,

I found a 100uF/100V cap in the box, so installed that one.  I re-checked all the resistors (unsoldered) and got the same values.  I tested continuities, then powered up the amp and the bias voltage is now -28VDC, just a shade higher than it was yesterday. It does hit its max voltage faster now.

So--I measured some voltages and put them in the bias circuit drawing, see below.  354V is tapped off one side of the PT, and the 120K resistor drops 300V.  The new voltage is now 53.6 VAC, measured at the junction of 120K, 33K, and the diode.  Line 1 on the drawing is just a simple equation to show that relation.   

Line 2 on the drawing is my question right now.  If I take the current bias voltage of -28V and fold in the voltage drop of 21.8, I get -49.8V, which is pretty close to spec.  Is it just a coincidence?

I did not yet unsolder the PI coupling caps and test for leakage.  Their leads are wrapped tightly round around the terminal strip post, so I went for the easier fix.  I'll do that tomorrow.

Thanks again.

[Calboy]

I have a suspicion the low output is not related to bias, but the bias should be fixed first.
Fixed bias with no adjustment is not ideal.
It would be better to make the bias adjustable from -53 to -28vdc, but I'm not sure how to do that at the moment. It can be done though.

Temp solution could be to remove/lift the 33k resistor where it meets the diode and 120k and see what new negative voltage is.
I think this is safe?

[shooter]

now -28VDC, just a shade higher than it was yesterday.

That's with Power tubes OUT?
what are you using for meters ground reference when you measure?

[Willabe]

https://el34world.com/charts/Schematics/files/Garnet/Garnet_gs100r_jammer.pdf

[Lectroid]

Thanks, Willabe,

I've seen this one. I use it where I can, but this amp is not exactly that amp on the schematic; it's like a GS100R, with differences.  Input jacks are like the S90, a bias-wiggle tremolo and the phase inverter from the  LB100TR.  The PI is a typical cathodyne PI--not the LTP on the GS100R schematic.  This amp has a gain-boost switch on the MV, and most of the .047uF caps on the Jammer schematic are actually .056uF in this amp.

If I get time, I'll sketch out a schematic of what I see.  I'm curious to hear how it sounds, once I get this [deleted] bias fixed.

[shooter]

If there are 220pF caps on the PA tubes grid to ground like Willabe's schematic, snip them out.

[stratomaster]

@HBP,

I found a 100uF/100V cap in the box, so installed that one.  I re-checked all the resistors (unsoldered) and got the same values.  I tested continuities, then powered up the amp and the bias voltage is now -28VDC, just a shade higher than it was yesterday. It does hit its max voltage faster now.

So--I measured some voltages and put them in the bias circuit drawing, see below.  354V is tapped off one side of the PT, and the 120K resistor drops 300V.  The new voltage is now 53.6 VAC, measured at the junction of 120K, 33K, and the diode.  Line 1 on the drawing is just a simple equation to show that relation.   

Line 2 on the drawing is my question right now.  If I take the current bias voltage of -28V and fold in the voltage drop of 21.8, I get -49.8V, which is pretty close to spec.  Is it just a coincidence?

I did not yet unsolder the PI coupling caps and test for leakage.  Their leads are wrapped tightly round around the terminal strip post, so I went for the easier fix.  I'll do that tomorrow.

Thanks again.

You need to dissect your bias supply. 

Stage 1 is to disconnect the yellow wire from the cap.  If your PI couplers are healthy then the voltage at the cap (-28v) will not change.  If it goes to the target voltage of -48v (or thereabouts) then you know you have at least one leaky coupler.

Stage 2 lift the cathode side of the diode.  The 120k/33k network is just a resistive voltage divider. If you're getting 345vAC into the 120k then you'll get about 0.22*345vAC at the junction between the two resistors.  If that's what you measure then this part of the circuit is working fine.

Stage 3 reconnect the diode and lift the 82k resistor. It doesn't matter which end, but the junction to the diode anode would be safest.
 Measure the voltage DC at the cap.  The 82k forms a complex voltage divider as it is kind of in parallel with the 33k resistor with diode rectification happening between the two.  Lifting the 82k will give you the highest DC allowable by the fixed voltage divider set by the 120k/33k divider.  If this number is at or below 48v then there's a design problem with this bias supply.
Note: using just round numbers this should actually be pretty close to the target as 345*0.2*0.7 is pretty close to 48.  I don't see how one gets -48v with the 82k resistor intact. 

I think there is a problem with the design of this bias supply.

I'd personally replace the 33k on the AC side with a 47k-68k and the 82k with a 25k pot with a 33k to ground.  Take the bias off the wiper of the pot ala Fender. You can add Merlin's 100k safety resistor and added cap to the wiper for additional smoothing and grid decoupling.

[Lectroid]

@stratomaster,

Thanks for explaining that, it helps me to understand how the voltages are related in the bias supply.  Please look at the attachment below; these are the voltages I just measured all around the bias board.  You may see something I don't.  I do know the voltage at the the 120K-33K junction is roughly 54 VAC, not the 77 VAC you predict (354V * 0.22)

Lifting the PI coupling caps doesn't change the bias voltage. I'll try your other suggestions tomorrow.



I did change the cap from a leaky 63uF/100V to a new 100uF/100V, and I lifted all the resistors in order to test them. They all measured within spec.  I re-soldered them, and re-flowed every solder junction I could see.  Didn't help--the junction of the 82K and 33K resistors still stuck at -28V.

I think there is a problem with the design of this bias supply...I'd personally replace the 33k on the AC side with a 47k-68k and the 82k with a 25k pot with a 33k to ground...

I don't like it either.  If it was mine, I'd probably make it adjustable.  But if I can get it to put  out -48V--or something close--I'll take it and move on. 

[stratomaster]

Thanks for explaining that, it helps me to understand how the voltages are related in the bias supply.  Please look at the attachment below; these are the voltages I just measured all around the bias board.  You may see something I don't.  I do know the voltage at the the 120K-33K junction is roughly 54 VAC, not the 77 VAC you predict (354V * 0.22)
Read again. That only applies after you've lifted the diode.
Lifting the PI coupling caps doesn't change the bias voltage. I'll try your other suggestions tomorrow.



I did change the cap from a leaky 63uF/100V to a new 100uF/100V, and I lifted all the resistors in order to test them. They all measured within spec.  I re-soldered them, and re-flowed every solder junction I could see.  Didn't help--the junction of the 82K and 33K resistors still stuck at -28V.
That cap was likely still functioning.

I think there is a problem with the design of this bias supply...I'd personally replace the 33k on the AC side with a 47k-68k and the 82k with a 25k pot with a 33k to ground...

I don't like it either.  If it was mine, I'd probably make it adjustable.  But if I can get it to put  out -48V--or something close--I'll take it and move on. Lift the 82k. If I'm right, that'll get you close.

[shooter]

from reply #14;

That's with Power tubes OUT?what are you using for meters ground reference when you measure?

If there are 220pF caps on the PA tubes grid to ground like Willabe's schematic, snip them out.

I'll try your other suggestions tomorrow.

Thanks
 

[Lectroid]

@shooter,
Sorry for the slow response.  I have been following your advice, I'm just bad at reporting back.  This bias problem its frustrating.  Plus life events eating into a man's sacred shop time.

That's with Power tubes OUT? what are you using for meters ground reference when you measure?

I measure between the PT ground bolt and pin 5 on the output tube.

If there are 220pF caps on the PA tubes grid to ground like Willabe's schematic, snip them out.

No caps on any preamp grid.  The input jacks circuit is from the S90 amp, not the Jammer schematic.  It's like Fender standard 68K input wiring. There are several differences between the amp I see, and the GS100R schematic.  However, they use this same bias circuit.   See reply #16, para. 2.

I did lift the PI - power tubes' coupling caps and saw no change in bias voltage.

As of today, the bias voltage still too low for power tubes.  I've replaced the filter cap.  Voltage is still stuck right at -28V, while B+ (unloaded) = 480V. 


@stratomaster,
I'll do these tests today.

• Disconnect yellow wire at cap and check DC voltage at the cap.
• Lift the cathode end of the diode and measure AC voltage the 120K-33K junction.
• Lift the 82K at the anode end and measure DC and AC.

Thanks, all!

[shooter]

Plus life events eating into a man's sacred shop time.

been there done that.

I measure between the PT ground bolt and pin 5 on the output tube.

just to confirm, the PA tubes are out while troubleshooting the Bias?

[Lectroid]

just to confirm, the PA tubes are out while troubleshooting the Bias?

Yes, power tubes are out.

[shooter]

I measure between the PT ground bolt and pin 5 on the output tube.

Is the main PS a grounded CT?
IF..so does it ground at that same bolt?


when you get a chance measure as you described above, but measure VAC, not VDC

[Lectroid]

@ shooter,
Yes, at the same bolt as the center tap coming out of the PT, also the 6.3V center tap.

@stratomaster,

1. Lift yellow wire from cap; measure DC at cap:     -45.7 VDC*

2. Lift diode's cathode; measure AC at junction of 120K and 33K resistors:     77.9 VAC

3. Lift 82K resistor at cathode anode and measure DC at the cap.     ?

Before I tested #3, I re-attached the diode cathode, didn't make sense otherwise. 

So--at power up, the bias voltage jumps instantly to around -50 VDC. But it doesn't stabilize.  It keeps rising steadily: -60V, -70V, -80V in ca. 10-15 secs.  I don't let it go any higher.  Am I doing it wrong?  Is the cap charging up, or what's happening here?

*Everybody was right.  The tremolo LFO output coupling cap was open: 0.5 Ω   I'll be replacing that one.  And testing every other coupling cap.

[shooter]

0.5 Ω

that's a short, not open

[Lectroid]

Maybe a milestone? 

shooter,
Guilty.  Short is not open. Gets me every time.   

Earlier today I replaced the toasted 0.1uF tremolo coupling cap with a 0.047uF. 

And now the bias voltage is up to -46V, tube pin 5 to ground at both sockets. 

Several of you were right.  It was a leaky cap messing up the bias voltage. It didn't sink in quite fast enough.  But I get it now.

So, it's not quite -48V, but is -46V good enough to put in the power tubes without undue risk?  I'd like to see how the amp behaves now.   I'd appreciate any advice

[stratomaster]

@stratomaster,

1. Lift yellow wire from cap; measure DC at cap:     -45.7 VDC*

2. Lift diode's cathode; measure AC at junction of 120K and 33K resistors:     77.9 VAC

3. Lift 82K resistor at cathode anode and measure DC at the cap.     ?

Sorry about the typo. Just lift the 82k. Lifting at the diode end is safest.

So it looks like with no load you're getting near the -48v target.  That would suggest something is loading down the bias supply a whole lot.

Nevermind, just read your last post. Looks like you found it.

[shooter]

so you're a learn'n, that's a good thing.
most of us suspected something "loading down" the bias supply, that's the reason I wanted to make sure it wasn't tubes.


a "leaky" cap becomes "resistive", good caps Block DCv, leaky caps act like a Resistor create a V-drop and  current flows.


I would say a NO SIGNAL tube install and check would be next.  You want to be monitoring the bias voltage to make sure it's "stable over time".


so if it starts dropping(moving towards ZERO) more than 2VDC, POWER OFF.
IF..It's stable for 5 minutes, plug in, play for 5 minutes, unplu, verify stablility, good?
IF..So, jam for 1/2 hour, watching the PA tubes for ANY sign of red-plating, crackling in the speaker, smoke.....

[Lectroid]

@statomaster, shooter, HotBluePlates

Thank you all for walking me through this. I’ll test it like shooter suggested. Stay tuned.

[Lectroid]

3. Lift 82K resistor at cathode anode and measure DC at the cap.     ?

. . .

Sorry about the typo. Just lift the 82k. Lifting at the diode end is safest.

No worries, you were clear.  I was the one who mis-typed it--it should have read "diode anode".  I blame spell-check. 

[Lectroid]

Checking in.

I replaced the 6L6s and ran the amp.  I got a steady B+ of 430 and a bias voltage right at -45.0V.  Ran it for five minutes quietly.  The bias voltage stayed locked at -45.0 for the whole 5 minutes.  Then I ran a tone through it, and the amp works fine.  Reverb and tremolo worked, everything right down to the Brite switch, and the amp's sound is strong and clear.  Other than the coupling cap that I replaced a couple of days ago, I don't think it had anything else wrong with it. 

I played steadily but not hard for many minutes and the bias voltage never jiggled unless trem was in use.  With trem, the bias varied between -42V and -47V.  Had the amp on for about half an hour altogether and the bias stayed at -45V.  I followed RobRob's OT resistance method to find the plate currents.  I got 39mA (56% max) and 46mA (66% max), fwiw.  Does that sound reasonable?

Never played a Garnet before.  It's pretty close to a VibroVerb or a ProReverb in power.  Preamp distorts smoothly.  Volume knob has a pull-switch for Bright, and MV has a  pull-switch for Boost.  A nice strong Middle control I like better than Fender's. The reverb is pretty intense at full intensity, too clattery and echo-y for my taste. If you turn down there's a decent Fender-like reverb.  More surf than jazz to my ears, but fully controllable. Might benefit from a dwell control.  Picture of the reverb's one-spring below.

[stratomaster]

Reasonable dissipation for the tubes, but a rather poor match.  Do you notice an audible hum in the noise floor?  Poorly matched tubes can result in higher hum than a more closely matched set. 

[Lectroid]

.. a rather poor match.  Do you notice an audible hum …

Nothing so far. I plan to swap the tubes and re-check it. Might get lucky.

[Lectroid]

Funny story.  I had this amp running strong and give it back to the happy and grateful owner.  He just brought it back.  The original problem was a bad tremolo LFO output cap.  My first draft of the circuit is attached if that helps.

It plays, but the amp is weak and mushy-sounding like before.   I checked the bias voltage, and it was -36V, too low.  So I assume I've got DC in the signal path again.  I took out the power tubes.  I lifted the grid end of the two PI output caps.

Immediately the bias voltage measured at pin 5 jumped back up to its nominal -45V.  Cool.

I measured the PI output caps.  Both are within spec, one's resistances was only 250K, but the other too high to measure.

That means the caps are okay, right?  Or should I replace them?  How to be sure they're the caps that failed?

Otherwise, I'll re-test every cap.  I've already gone through it once, but didn't find anything yet.  Where I found any DC voltages they were all random  millivolt noise.  Measured from the output lead of the coupling cap to transformer bolt ground.

If it isn't the PI caps, any other thoughts about how to run down a DC leak?  Any other places I might look?  Or techniques, rules of thumb, rude suggestions? 

[SEL49]

I measured the PI output caps.  Both are within spec, one's resistances was only 250K, but the other too high to measure.

That means the caps are okay, right?  Or should I replace them?  How to be sure they're the caps that failed?

The cap that measures 250K is bad. To be sure it is leaking, disconnect the lead that goes to the power tube grids. Turn the power on and measure VDC at the dangling lead of the suspect cap. If it's leaking, you will have a positive voltage on that dangling lead. The higher the voltage, the more its leaking. Replace both.

[Lectroid]

@SEL49,

Thanks.  Unfortunately, I made some mistake or other bc now they're both reading 'too high to display' on my meter.  I'll replace them to be safe.

Did replace them with orange drops and--no change.  Bias voltage still -36V, way too low.

So now I'm mostly focused on leaky output caps, but I'm not finding much out of line. Is there another part of the amp I should check, that could be causing this leakage?  I've never run into this problem before.

Q: Does every output cap need to be lifted to check for voltage leaking?

[shooter]

I checked the bias voltage, and it was -36V, too low.

too low should read; "too HOT" or too high

jumped back up to its nominal -45V.

I know it's pick'n nits, when you find yourself on the negative side of things, zero is always higher than you are

I checked the bias voltage, and it was -36V, too low.  So I assume I've got DC in the signal path again.

were the PA tubes in when you tested and found -36?

[Lectroid]

I checked the bias voltage, and it was -36V, too low.  So I assume I've got DC in the signal path again.

were the PA tubes in when you tested and found -36?

I pulled the power tubes, not the preamp tubes.  Not sure which you meant.  All readings so far taken with preamp tubes only, no power tubes. 

[shooter]

PA = Power Amp, so ya power tubes


I wanted to "verify" the PA tubes weren't the cause for high bias volts


so you're back to normal bias?


Or;
Is it -45 one time -36 the next time you measure?

[Lectroid]

When I got the amp back, it was weak and distorted.  I checked the bias. It was up (?) to -36V.  I then pulled the PA tubes at that point and they've stayed out.

I measured the bias voltage in isolation and it's still putting out a steady -45V. I'm pretty sure that the PI caps are good and not leaking DC.  I’ve tried other 6L6s but no improvement. . 

It must be a leaking output cap, right?  I tested every output cap for downstream voltage, didn’t find a bad one. Is there anything else to look at?

[SEL49]

It must be a leaking output cap, right?

What are you calling an output cap? There are only three caps that can leak and change the bias voltage on pin 5 of the output tubes. The two PI caps and the tremolo cap between the trem tube plate and the intensity pot. You said you replaced the trem cap. Did you actually replace the two PI caps?

Does the bias voltage remain steady while the output tubes are pulled? If so, none of those three caps are leaky. In this case, replace the 6L6s.

[Lectroid]

SEL49, shooter,

Thanks for the answers. I'll focus on the three caps.   

I read the cap's voltage with the black probe on a PT bolt, and the red probe on the cap's lead which is not connected to the tube's anode pin. 

I know it's necessary to lift one side of a cap the measure its capacitance.  Is it necessary to do that to read a DC voltage like I'm doing?  Am I measuring wrong.

[SEL49]

Am I measuring wrong.

I told you exactly how to correctly do it in reply #37.

[Lectroid]

Am I measuring wrong.

I told you exactly how to correctly do it in reply #37.

Yes, you did, about the PI caps.  I was asking more as a general cap question, to fill in my blank spots

Okay.  I replaced the PI caps again, with brand new Orange Drops.  Using a light bulb limiter and without power tubes, the bias voltage decreased to -44V and stayed steady.  When I do this without power tubes, the light bulb glows briefly at switch-on, then goes dim again.

I put in the power tubes.  The B+ jumped to 460, then began to roll back as the amp warmed up and the bias voltage began to get lower and lower.  The bulb glows brightly at switch-on, but then dims way down and glows slightly, not bright at all.

When the B+ drops down to around 410 or so, the B+ falls (and bias gets more positive).  I tried this with the original tubes, and with two other pairs of 6L6GCs.  Behavior with power tubes in was the same for all sets of tubes.  At first the B+ and the bias voltages jump right into the ballpark, but then they drop.

 




   

[SEL49]

Quit using the light bulb limiter. Put it under the bench. Repeat all your voltage checks.

[shooter]

^^^^  IF that does the same, leave limiter out of circuit and



were both the PS and OT transformers new on this build?


what you are describing is something sucking more current than should be sucked.  I would expect everything to be stable at about that 410-420vdc point, not bias continuing to rise n B+ continuing to drop.  That indicates a partial short.

PULL all the preamp tubes, lift the coupling caps from PI to PA on one side and repeat the test you just did.

[Lectroid]

@shooter, SEL49

Dang.  This is not the first time I've forgotten about the LBL.  Sorry for the wild goose chase.  B+ is now 430V and bias voltage is steady at -44V, and varies from -40.3V to 47.1V at full tremolo.  I played amp for a while--everything worked fine.  I put it back into its cabinet and plugged in the cab's internal speaker.

And suddenly the amp went quiet again--no sound.  What the--?

I unplugged the cab's speaker, and plugged in my shop 12", and the amp sounded great again.  I always used my shop speaker on my bench, so I never tested the cab speaker.  It may have been dead when he brought it to me. 

Continuity in the speaker cable is good.  There is a strong signal appearing at the speaker terminals.  The resistance of the speaker is 0.6 Ω  But I also tapped the terminals with a 9V battery and the speaker popped.

I'd say the speaker is bad, but if it is, I can't explain the 9V pop. 

I just need to know: Is the speaker bad, or not?  Is there a positive electrical test I could do to be sure?  I don't want to trash a good speaker by mistake

Thanks again for sticking with me through this.