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Old 8th May 2008, 02:01 PM   #1
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Default '81 Twin problems

New guy.

I'll put it out there right now that I don't know much at all about amp electronics. I understand the basics and can do simple solder repairs but beyond that, I'm lost. Is there a difference between a filter cap and a coupling cap? Sounds like it, but beats me!

So onto my question. I recently recieved a 1981 (I believe) Twin Reverb. It's the 135w model and apparently is the same as the older Silverface twins, but mine has a blackface, push/pull Master volume knob, etc...

The power and preamp tubes are all the same make/style (6L6GC's and JJ 12ax7/12at7's if memory serves correctly) but they were thrown together so they're definitely not matched sets.

My first problem arose while I was playing clean with the volume midrange, around 5. After about 10 minutes of playing I noticed my clean tone began to degrade very quickly and within a minute or two I went from a very clean sound to a weird, muddy light distortion. Not distortion as in a overdrive sound (which I dislike anyway) but more like breakup...like, something is wrong. At first I thought it was a blown speaker but I ruled that out. I pulled 3 of the preamp tubes (not the tremolo or reverb ones, the volume on these are always set to 0, FYI) and replaced them. I also noticed that the bias balance pot was all the way low, so I eyeballed it and set it at about the middle. After that, it was perfectly fine, clean tone was normal.

Yet, with my clean tone being back, if I hit my distortion pedal and play heavily for about 5 minutes, I can hear my distortion acting up. It sounds sort of muddy again. It's hard to explain, but weirdly, it sounds as though the sound waves are overlapping slightly and cancelling each other out. It's not the crisp and clear distortion I'm used to hearing.

(I'm also surprised that for a 135 watt amp, it's not as loud as I was expecting. I definitely think the '65 RI are much louder and they're what, 80 watts? I'm sure there's a myriad of fixes for this, but just throwing it out there. I'm sure getting it totally retubed with matching sets couldn't hurt. I also noticed most people disable the master volume knob to benefit their tone, is this so?)

Could my distortion problem be this parasitic oscillation I've been reading about? I opened the amp up and looked at the large caps that are in there (whichever those are) and they look pretty new. They look very clean, only one of them has a little bit of brownish stuff kinda stained on the + end about the size of a pencil eraser. Beyond that, I have no idea. Thanks alot for reading and any help you can give me. I'm definitely looking to learn more about what all goes on inside these things so I can diagnose and fix it myself. Thanks.
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Old 8th May 2008, 02:52 PM   #2
Trout is offline Trout  United States
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If you are not experienced in high voltage tube circuits I highly recommend a trip to a service tech.

The amp is of an age that the source could be any number of things.
Tube age and bias being something to look closely at.

It is a good idea to get the voltages at the schematic check points and or at the tube pins of each tube. Often it can lead you right to the source of the problem quicker than guessing.

Generally speaking based on your description, I would start by looking closely at the tubes. Even if they were new they could still be an issue.

Quote:
My first problem arose while I was playing clean with the volume midrange, around 5. After about 10 minutes of playing I noticed my clean tone began to degrade very quickly and within a minute or two I went from a very clean sound to a weird, muddy light distortion.
If that does not correct the problem, it might be a component that is failing or badly drifted.
You are nearing the age for a cap job. Amps that sit unused for periods of time often need them replaced earlier.
Edit: any cap that shows signs that it leaked or vented should be replaced.


If you are handy with high voltage electronics, it is not a hard job to replace those. I would change all of the electrolytics at that point., this includes all bypass and bias circuit caps.

You could also have a leaky coupling cap causing similar problems.

Now I will repeat myself, Safety First!

If you are not experienced in high voltage tube circuits I highly recommend a trip to a service tech.

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Old 8th May 2008, 05:08 PM   #3
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Are the 6L6's new? If not, replace them (with a matched set) and have a tech bias them correctly. You can't just "eyeball" the bias control. 6L6's (Or any other power tube) will behave badly if their bias is set incorrectly. And if the plates start to glow cherry red...turn it off! If your not getting a bad hum, you probably just have bad power tubes, but it would be a good idea to take it to a good tech and have the caps and other innards checked out.
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Old 8th May 2008, 10:37 PM   #4
Trout is offline Trout  United States
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Quote:
Originally posted by TubeHead Johnny
Are the 6L6's new? If not, replace them (with a matched set) and have a tech bias them correctly. You can't just "eyeball" the bias control. 6L6's (Or any other power tube) will behave badly if their bias is set incorrectly. And if the plates start to glow cherry red...turn it off! If your not getting a bad hum, you probably just have bad power tubes, but it would be a good idea to take it to a good tech and have the caps and other innards checked out.


I agree 100%

The part that caught my attention was a possibly vented cap

Quote:
I opened the amp up and looked at the large caps that are in there (whichever those are) and they look pretty new. They look very clean, only one of them has a little bit of brownish stuff kinda stained on the + end about the size of a pencil eraser.
Find a good local tech, Not a Guitar Center Tech, but a good Mom N pop operation. Check in over at a few guitar gear forums, those guys know where loads of techs are all across the country.

The Gear Page is a good place to start.
http://www.thegearpage.net
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Old 10th May 2008, 04:52 AM   #5
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Trout, well mentioned.
Shoptalk; A group had a problem with this unknown homemade amp which was in a poor state and were desperate to get it going for a Gig. They all said when it worked it gave a miracle sound. However they showed me the underneath and hesitated from poking about with it.
Underneath, the threat worked. The label isn't approved type, but it means, in effect a heart difibrilator. 250 Joules is common energy in a tube amp and this can put one squarely in a coffin. The amateur who constructed this amp had the foresight to think of others.

Take Heed!!

richj
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Old 10th May 2008, 09:41 AM   #6
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I've fixed lots of these amps of this vintage, not only are the caps due for replacement, but I find many of the carbon comp resistors have drifted in value, often 50 to 100% away from the original value, this will affect the way the amp performs.

This might be skirting the issue a little, but I typically by habit replace all the larger power caps, check all resistors and if the value is more than 20% out, I will replace it, generally all the more serious problems automatically disappear. Check all voltages against a schematic, this will help identify something funny if the votages are significantly out.

Then you will also quite likely have a bit of sizzle, hissing and snarling going on with an amp of this vintage, which can often only be shut up by replacing all the caps throught the PTP board.

If the thing is still changing tone after 5 minutes of use, you then need to go thru some sort of logical fault finding/elimination process.

Michael
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Old 12th May 2008, 02:26 AM   #7
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Thanks for the all the suggestions, from what else I've read online changing the caps and checking resistors etc seems to be the way to go.

Seeing as the amp is hand wired I am very confident in safely replacing some components in the amp, my problem is that I'm not very familiar with all the different types of resistors and caps and whatnot. If I had someone sit down with me and explain in more detail how it operates and what needs replacing, that would be fantastic, I know I could go from there fixing it myself. I know how to drain and replace the caps so I may do that myself, but as for checking the tube and resistor voltages for problems I may find a tech around here to do that sort of poking around.

It seems the consensus is that older parts probably need replacing and I need a matched set of power/pre amp tubes that are biased correctly for the amp. Thanks again for the help.
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