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Post subject: Re: What burns out power tubes?
Posted: Wed Feb 05, 2014 10:12 am
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So you are getting the same hum regardless of patch cord or guitar? But, no hum with no guitar plugged in?

Sounds like a ground loop in the input stage of the amp. Did you do any work on the input and V1 stage of this amp?


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Post subject: Re: What burns out power tubes?
Posted: Wed Feb 05, 2014 12:04 pm
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No. No work on the V1 or input stage. First task was replacing the filter capacitors and their resistors. Then replacing wires at the V8 socket after shorting out a pin. Then replaced R70 and R71 heater resistors. Then replaced power tubes and things worked fine. Then they didn't. Then replaced sockets, wires R70 and R71 resistors again. The hum has been there all along, but I can't remember if it was there before my amp had problems. That was about five or six weeks ago.


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Post subject: Re: What burns out power tubes?
Posted: Wed Feb 05, 2014 4:39 pm
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Monkeyleg wrote:
Okay, back to the problem. I replaced the sockets for V7 and V8, the 6L6GC tubes. I replaced any wires that were scorched. I replaced the R70 and R71 resistors for the V7 and V8 tubes. I did pretty good solder joints, I think.

I put in new power tubes, and biased them to 32 on the V7 and about 35 on the V8. I kept swapping out preamp tubes with different ones. No change in the bad sound, though.

I took readings at the test points. I've put the values shown on the schematic in parentheses after the read values.

TP7 2.02VDC (2.02VDC)
TP10 2.15VDC (2.12VDC)
TP33 98.3VDC (93.6VDC)
TP36 -42VDC (-32VDC or -52VDC, can't read it)
TP37 35mVDC (33mVDC)
TP38 33mVDC (33mVDC)
TP39 469VDC (433VDC)
TP33 98.3VDC (93.6VDC)
TP36 -42VDC (-32VDC or -52VDC, can't read it)
TP13 2.08 (2.06VDC)
TP22 2.2VDC (2.01VDC)
TP26 -7.25VDC (-5.50VDC)
TP27 2.02VDC (2.02VDC)
TP28 12.3VDC (13,2VDC)
TP2 462VDC (433VDC)
TP3 450VDC (420VDC)
TP4 411VDC (385VDC)
TP5 -63VDC (-50.2VDC)
TP1 467VDC (435VDC)
TP18 7.7VDC (7.6VDC)

There's a hum that increases with the volume knobs on both the guitar and the amp. There's no hum if the guitar isn't plugged in, or if the guitar volume knob is set to 0.

Any ideas, anyone?


I have several concerns with these voltage readings:

1. I agree with shimmilou, that the amp is biased too cold. The measured plate and cathode voltage works out to be an idle power of 15 watts. This is 50% of the max power of a 6L6GC (30 watt tube) and is too cold.

2. TP36 on the schematic is -52 VDC. That corresponds to the cathode and plate voltage shown on the schematic. However, you are having to INCREASE the bias supply voltage to -42 VDC to get approximately the cathode and plate voltage values shown on the schematic. This could indicate a problem in the bias power supply.

3. You need to measure the DC voltage at TP40. It should be within a couple of volts of the DC plate voltage measured at TP39. If it is not, there could be a problem with V8.

4. TP1 through TP4 are all 30 VDC higher than the schematic value. I know the schematic says that voltages may vary by +/-20%, but that could indicate a problem in the power supply. What is the voltage of your incoming AC line?

5. If TP5 should be -50.2 VDC (I can't read it), the measured value of -63 VDC is too negative and could be the reason that you have to crank up the bias level as high as you did to get the schematic cathode voltage level. Another indicator of a possible bias supply problem.

I suggest you take a close look at the power supply and in particular the bias power supply. Hum could also be a problem if the power tubes are not balanced very well (we don't know the plate voltage of V8 so we can't come to a conclusion there). You might want to check the values of R62/63 and the voltages at pin 4 on each power tube (should be less than the measured plate voltage) to be sure a screen resistor didn't blow.

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Post subject: Re: What burns out power tubes?
Posted: Wed Feb 05, 2014 6:59 pm
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Bill,

This SRRI has bigger issues than simply poor tone. Hum, blowing fuses, popping resistors. The circuit board & sockets have been pretty heavily worked on. So, I think a lot of issues maybe related to poor conductivity and/or poor grounding. So, I'm trying to sort out these issues, first -- with the owner.


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Post subject: Re: What burns out power tubes?
Posted: Wed Feb 05, 2014 9:57 pm
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BMW,

No offense, but nothing that the OP has done seems to have fixed anything. And new problems are now appearing. There are definitely some anomalous voltage readings that shouldn't be ignored. Its the OP's choice to check them out or not. Guess I'll just bow out of this thread now.

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Post subject: Re: What burns out power tubes?
Posted: Thu Feb 06, 2014 9:42 am
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Bill,

Yeah, he was a bit scattered in his approach. I felt someone on another board gave him advice that didn't really narrow the problem or isolate it to a particular section of the amp. This happenstance approach lead to more damage throughout the amp.

So, now he's dealing with burnt resistors, wiring, and blown fuses... in addition to the original hum and bad tonal repsonse.

Good news, I think he maybe on his way out of the thick forest. To a less dense area of trees, at least. He says he will remeasure the points you described. As well as try to get an audio clip of the original problem. Which prolly would have been the best thing to do, at the start of all this. :D


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Post subject: Re: What burns out power tubes?
Posted: Thu Feb 06, 2014 5:04 pm
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Bill, I hope you haven't left the thread. If you have, I understand.

To address your points above, the bias readings measured at R72 and R73 are 34.2mV and 33.1mV. The reading at TP39 is 469 and at TP40 is 469. The readings at pin 4 on V7 is 469 and on pin 4 on V8 is 469. At R72 it's 470 and at R73 it's 471. The voltage fluctuates a little, so they're all essentially the same.

The voltage in the incoming house line is 121.6 VAC.

Is there somewhere on the internet that explains how to test a Super Reverb transformer, including the bias supply? I can't seem to find anything that's Super Reverb specific.

The hum is still there, louder than it was a few weeks back. It changes along with changes in the tone knobs, and increases in volume with the guitar and amp volume knobs. I've checked every ground on the schematic, and they're all at 0 resistance.


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Post subject: Re: What burns out power tubes?
Posted: Thu Feb 06, 2014 8:46 pm
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BMW2002Ti wrote:
Bill,

This SRRI has bigger issues than simply poor tone. Hum, blowing fuses, popping resistors. The circuit board & sockets have been pretty heavily worked on. So, I think a lot of issues maybe related to poor conductivity and/or poor grounding. So, I'm trying to sort out these issues, first -- with the owner.


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Is that solder splatter all around those traces and solder pads? You need to get that cleaned off!

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Post subject: Re: What burns out power tubes?
Posted: Fri Feb 07, 2014 2:33 am
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Monkeyleg wrote:
Bill, I hope you haven't left the thread. If you have, I understand.

To address your points above, the bias readings measured at R72 and R73 are 34.2mV and 33.1mV. The reading at TP39 is 469 and at TP40 is 469. The readings at pin 4 on V7 is 469 and on pin 4 on V8 is 469. At R72 it's 470 and at R73 it's 471. The voltage fluctuates a little, so they're all essentially the same.

The voltage in the incoming house line is 121.6 VAC.

Is there somewhere on the internet that explains how to test a Super Reverb transformer, including the bias supply? I can't seem to find anything that's Super Reverb specific.

The hum is still there, louder than it was a few weeks back. It changes along with changes in the tone knobs, and increases in volume with the guitar and amp volume knobs. I've checked every ground on the schematic, and they're all at 0 resistance.


1.) I think Bill suggested biasing the 6L6GC closer to 20 watts per tube. Maybe, start at around 18 watts/tube and see how the amp sounds. At 470VDC on the anode plates, the mV read at R72 and R73 points should be adjusted with the bias pot, to 38mV.

2.) If you are getting good B+, X, Y, and Z voltages (compared to the schematic), the PT has to be working well. Measurements will usually give you an idea of how well the upstream source of those readings is working. That is one of the principles of taking these readings.

3.) Redo those heater line layout. Proper twisting of the lines. Then, re-check the hum.

Good luck!


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Post subject: Re: What burns out power tubes?
Posted: Fri Feb 07, 2014 5:40 pm
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Hi, Steve.

Things have definitely taken a turn for the surreal. I took the amp to a Fender tech this afternoon. He said he couldn't find anything wrong with it. He said my solder joints were ugly, but solid. He didn't have a 2 ohm set of speakers, just 8 ohm. Through those, the amp didn't sound as distorted on the very low tone chords. He measured the bias, and said it was hot, with a reading of -59 at R56 and R57. That's where he measures bias. I figured that the impedance of the speakers might be a factor, and I was right. I get -42 with the 2 ohm speakers here.


My reply:

Darn it... you do NOT bias an amp at the junction of the 220k-ohm grid-to-ground resistors. I'm beginning to lose faith that there are any good techs out there, anymore. And -59VDC will make a COLDER, not hotter, bias than Fender's recommended -52VDC, at this point.

You measure idle bias current that passes through the specific tube. Each 6L6GC. And not the negative voltage applied by the bias supply. Each output tube will pass a unique & different current --- with the same negative bias voltage. One may pass 38mA with -52VDC on the grid. Another may pass 43mA with the same negative 52DC voltage.

The load makes little difference to the output tube's idle bias. Going from 2-ohm load to an 8-ohm load will alter the idle current flow (bias) very little.

How are you supposed to know what each tube is passing by measuring only at one point? This is crazy.


SRRI schematic:

http://www.webphix.com/schematic%20heav ... reverb.pdf


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Post subject: Re: What burns out power tubes?
Posted: Fri Feb 07, 2014 8:02 pm
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BMW2002Ti wrote:
I'm beginning to lose faith that there are any good techs out there, anymore.


+1

And most of those seem to be habitués of this forum.

:wink:

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Post subject: Re: What burns out power tubes?
Posted: Fri Feb 07, 2014 8:56 pm
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This has turned into a "Comic Strip" and "Dilbert" comes to mind!

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Post subject: Re: What burns out power tubes?
Posted: Fri Feb 07, 2014 9:11 pm
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Well, I hate to ask what role I play. ;)

There's still a lot of distortion down on the lower strings. There's definitely more than before this all started. I play "Pride and Joy" very frequently, and the shuffle on the E, A and D strings 2nd through 4th frets has more distortion than before. It doesn't sound good.

I told BMW I'd record the sound and upload it so anyone interested can listen. I'll try to do that this weekend.

I'd try to come to the tech's defense, but he had a customer come in and say that an amp the customer had been playing for 9 months had changed, and was now sounding bad, and he told the customer to turn down the base. And that there was nothing wrong with the amp. That's like a patient telling a doctor he gets shortness of breath when he walks, and the doc tells him not to walk as much.


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Post subject: Re: What burns out power tubes?
Posted: Fri Feb 07, 2014 10:04 pm
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Monkeyleg wrote:
Well, I hate to ask what role I play. ;)

There's still a lot of distortion down on the lower strings. There's definitely more than before this all started. I play "Pride and Joy" very frequently, and the shuffle on the E, A and D strings 2nd through 4th frets has more distortion than before. It doesn't sound good.


Could be an idle bias issue. A PSU issue. Bad tube(s) issue. "Farting" issue (from the amp, that is -- :lol: ---). Speaker issue. You get the picture. Differential diagnosis is in order!

Onward, cried the millions! :D :mrgreen: :D


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Post subject: Re: What burns out power tubes?
Posted: Fri Feb 07, 2014 10:12 pm
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BMW, I have a lot of tubes, and I've swapped out every one for a known good tube. I don't think that's it. I've tested the speakers using a different amp, and I've also disconnected (briefly) two from one side and then the opposite two to see if there was a difference in sound.

Bill seemed to be leaning toward a power issue. He sounded logical about it.

I hate to go resistor by resistor and capacitor by capacitor, in part because I'm worried about really screwing up the board (have to learn to solder well first), and also because of the time. The amp sounds okay to me, but I know it sounded better.

Oh, yeah. I don't fart around people or guitar equipment. ;)


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