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Post subject: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:04 pm
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Under the direction of another member here, I've been working on a problem with my Super Reverb Reissue. The first time the problem (bad distortion at low frequencies) occurred, we tested all sorts of things, but wound up coming back to the power tubes. Replaced the 6L6 GT's with some JJ 6L6's, and it sounded good. Then I screwed something up. I think.

I've ordered another set of power tubes. I don't want to ruin these before I can figure out what's going on. So, besides too much voltage, are there other things that can ruin power tubes?

Thanks for any replies.


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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:28 pm
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I've read all of your posts.

Why do you think the powers tubes are bad? And if they are bad, you need to figure out what you did wrong before plugging in another set of tubes and destroying them.

I will repeat this post of mine one last time:

bluesky636 wrote:
stratele52 wrote:
Read power supply voltage and report.


This is one thing I have not seen addressed anywhere in this thread.

Whenever I am troubleshooting an amp, the very first thing I do is check all of the DC voltages and heater voltages. This includes the power supply (first with all tubes out of the amp, then adding tubes one at a time to see if there are any big drops in voltage). This includes all voltages at the power supply, the bias supply, plate, screen grid, and cathode voltages.

In my opinion, swapping tubes, checking microphonics, or anything else is a waste of time if the quiescent voltages are not known.


Take the measurements and post them for us to see. Use your old tubes .

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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:38 pm
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monkeyleg,

OK... to clue folks into what has been going on with your '65 Super Reverb Reissued amp.

1.) The original GT-6L6GC would not bias past a few mA of idle current (4mA and 17mA at 480VDC).

2.) While checking voltages at the pins on sockets with a DVM probe, the lines were shorted. Causing the 100-ohm centering resistors on the heater line to pop. And some other signal lines to burn.

3.) New resistors replaced the 100-ohm ones. Lines fixed & resoldered onto sockets.

4.) New JJ-6L6GC were inserted. Seemed ok, at first. Bias ok. Then, not working. Bias still ok. Poor sound.

OK... first a look at the burnt 100-ohm resistors off the heater line. NOTE: how these resistors are on the circuit board & not attached to the power lamp? New ones soldered in (red ones):


Image


Now, look at soldering fix on the sockets:


Image


Image



My guess is that the excess soldering onto the tangs of the sockets is causing the sockets to have intermittent poor contact. This is esp true with those newer white ceramic Chinese sockets. Resolder is in order. really new sockets are prolly best long-term cure.

The JJ-6L6GC are prolly ok. But, unless bought from a distributor that tests them under amp-like conditions, there maybe a question as to them being an issue. IHMO, better to go with the TS or EH 6L6GC, for long term reliability. This is just my opinion.


Last edited by BMW2002Ti on Sun Jan 26, 2014 11:01 pm, edited 2 times in total.

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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:40 pm
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One other note on this RI amp.

The main PSU caps under the hood ALL orient in the same direction (including the two 100mfd of first stage). As oppose to most Fender amps, that have two caps of the first stage in series.


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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:50 pm
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Other note.

monkeyleg is aware of the consequence of poor socket contact and loss of bias voltage.


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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:52 pm
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BMW2002Ti wrote:
Image


Ach dü lieber!

:shock: :shock: :shock: :shock: :shock:

I think a remedial course in "Soldering 101" is in order.

:lol:

Arjay

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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:53 pm
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BMW2002Ti wrote:
One other note on this RI amp.

The main PSU caps under the hood ALL orient in the same direction (including the two 100mfd of first stage). As oppose to most Fender amps, that have two caps of the first stage in series.


Image


So what? The schematic shows them in series.

http://support.fender.com/schematics/gu ... ematic.pdf

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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:55 pm
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BMW2002Ti wrote:
The JJ-6L6GC are prolly ok. But, unless bought from a distributor that tests them under amp-like conditions, there maybe a question as to them being an issue. IHMO, better to go with the TS or EH 6L6GC, for long term reliability. This is just my opinion.


Buy JJs from Eurotubes.

No guarantee that the others are any better.

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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:56 pm
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BMW2002Ti wrote:
Other note.

monkeyleg is aware of the consequence of poor socket contact and loss of bias voltage.


Should be pretty obvious if poor contact is being made when the tubes are removed/inserted. is the solder preventing the sockets from being retensioned?

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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:57 pm
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Bill,

When was the last time you worked on the PSU of a Fender amp? :) The first two caps are usually in series. BUT, those caps are oriented in OPPOSITE directions.

I know you knew this. But, when I first saw the photos of the SRRI PSU arrangment, I was amazed. As this most always leads to catastrophic failure. This is the first Fender amp I've seen with those caps in the same orietation.

The only other caveat are those amps that have the first two caps in parallel. Like DR. VLR, and Tremolux. Those caps orient in the same direction.



SF Super Reverb doghouse:

Image


Tremolux doghouse:

Image


Last edited by BMW2002Ti on Sun Jan 26, 2014 6:05 pm, edited 1 time in total.

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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 5:58 pm
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Retroverbial wrote:
Ach dü lieber!

:shock: :shock: :shock: :shock: :shock:

I think a remedial course in "Soldering 101" is in order.

:lol:

Arjay


As well as how to use a DVM without shorting things out.

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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 6:00 pm
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BMW2002Ti wrote:
Bill,

When was the last time you worked on the PSU of a Fender amp? The first two caps are usually in series. BUT, those caps are oriented in OPPOSITE directions.

I know you knew this. But, when I first saw the photos of the SRRI PSU arrangment, I was amazed. As this most always leads to catastrophic failure. This is the first Fender amp I've seen with those caps in the same orietation.


BMW,

I suggest you look at the schematic and board layout for the SRRI as I have posted several times.

http://support.fender.com/schematics/gu ... ematic.pdf

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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 6:04 pm
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So far, everything I have seen and read confirms my belief that the OP is in over his head with regard to working on this amp. Sorry.

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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 6:10 pm
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" BMW,

I suggest you look at the schematic and board layout for the SRRI as I have posted several times. "



Bill, ok understood. But, did you know this about the PSU cap layout? Are other reissued amp oriented this manner? After making amps with one orientation for 40+ years --- Fender decided to flip the first two caps? Almost sounds like a conspiracy!

:lol:


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Post subject: Re: What burns out power tubes?
Posted: Sun Jan 26, 2014 6:50 pm
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BMW2002Ti wrote:
" BMW,

I suggest you look at the schematic and board layout for the SRRI as I have posted several times. "



Bill, ok understood. But, did you know this about the PSU cap layout? Are other reissued amp oriented this manner? After making amps with one orientation for 40+ years --- Fender decided to flip the first two caps? Almost sounds like a conspiracy!

:lol:


Irrelevant how it was done for 40 years with a hand wired amp. We are now talking about a reissue where the concern is how to most effectively lay out a PCB trace.

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