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Post subject: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 7:12 pm
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Hi Everyone.

I recently purchased a 1980 Silverface Twin Reverb.

The issue I am having is that the 6th string, and any low register sounds sound farty, like a fuzz tone. Not what I was looking for in a notoriously clean amp. High E string, B, G, D all sound fine. A string is suspect, E is bad.

The only thing I can do to clean it up, is turn the channel volume down to 0.5-1, bass set to 2, treble set to 8, anything above that and I have a very heavy amp, with a 6th string that sounds like an old Black Sabbath cassette tape (cool right? yeah...but not for me... I like my clean to be clean)

Here's the work that has been done so far (from most obvious onward)

#1) Allllll the tubes have been changed with brand new, properly matched tubes. (I even went so far as trying a second set of new pre amp, and power amp tubes.)
#2) The tech changed the caps. Im assuming the filter caps, but I haven't taken the amp apart yet. He mentioned the word 'electrolytic' in describing what he changed. I believe we have brand new filter caps.
#3) I have hooked up a single 12" 4 ohm speaker to test whether or not I had blown speakers. Sounded the same.
#4) New screen resistor installed
#5 Biased

It has been worked on by two tech's at this point, and though I trust the second tech far more than the first, Im a little embarrassed to keep going back and saying "its not fixed yet", do it again.

I Know how to do some basic amp repairs, how to safely change capacitors, use a meter, and know my way around a soldering iron.

Anyone have any guesses?, any insight from the pros would be greatly appreciated.

(pls don't ask me if I have checked the tubes...I have checked the tubes...the tubes have been checked, twice :D )

Thanks guys
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 8:11 pm
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Does the symptom manifest itself on both channels?

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 8:32 pm
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Hey Arjay

I was hoping you would reply. I have read many of your posts and your clearly know your gear.

The problem does exist on both channels, and all inputs.

Thanks again
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 8:42 pm
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A couple of things spring to mind......

A coupling or bybass cap at or downstream of V6 has gone wonky (drift and/or DC leakage) OR one or more of the 470-ohm screen resistors (there's one across each of the power-tube sockets) is flaking out. Check out both areas visually and with a meter (you'll need to isolate each of the components for an accurate static measurement, ie: un-solder one side of each).

Best of luck, HTH

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 9:11 pm
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" The tech changed the caps. Im assuming the filter caps, but I haven't taken the amp apart yet. He mentioned the word 'electrolytic' in describing what he changed. I believe we have brand new filter caps. "

Good points Arjay! One other thing...

If your tech also changed the bypass electrolytic caps in the amp (those are the ones usually bypassing the 12AX7/7025 and some 12AT7 cathode-to-ground resistor) --- he may have replaced the wrong size. Too big a mfd cap here can lower the low frequency roll-off to the point that the section is causing the amp to "fart."

The usual OEM bypass cap is 25mfd/25VDC. I've seen as much as 100mfd/100VDC caps in sections where techs have tried to tailor the tone to get more "juice" and bass outta the amp. Esp at low volumes.

Not sure what circuit you have in your amp. But, if you look at this schematic, you can see those caps below each 12AX7/7025 section.

Welcome to the Forum & good luck with that amp!

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


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 9:16 pm
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His amp is the ultra-linear 135-watt platform, Steve (same as mine).

Here's the correct schematic for the amp......

http://web.archive.org/web/200610200955 ... _schem.pdf

Good point about over-spec bypass caps. I never quite figured out why most modders think they know more than Leo did.

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 9:37 pm
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Thanks, for the correct schemo.

There seems to be "quick-fix" cures running around the Net. Really, you mess with one section too much and it can cause issues up-&-downstream of the "mod." You know how everything is somewhat interactive.

Why anyone would want to make a TR pass more bass is beyond me. But, I've seen Marshall Majors with these kinda funky-doodle mods.

BTW... was this farting an issue before the second tech touched the amp?

A photo of the circuit may help.


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 10:20 pm
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Guess i'd check the .01uf cap that feeds the phase inverter. Art

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 10:40 pm
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BMW2002Ti wrote:
If your tech also changed the bypass electrolytic caps in the amp (those are the ones usually bypassing the 12AX7/7025 and some 12AT7 cathode-to-ground resistor) --- he may have replaced the wrong size. Too big a mfd cap here can lower the low frequency roll-off to the point that the section is causing the amp to "fart."

The usual OEM bypass cap is 25mfd/25VDC. I've seen as much as 100mfd/100VDC caps in sections where techs have tried to tailor the tone to get more "juice" and bass outta the amp. Esp at low volumes.


Slightly off topic and not intending to hi-jack the thread, but I just got through with a discussion on TDPRI explaining why replacing the 22 uf cathode bypass cap on the 6V6 of the Chinese-built Champion 600 with a 1000 uf cap ( :shock: )was a bad idea.

Now back to our regulalry scheduled program. :wink:

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 11:03 pm
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bluesky636 wrote:
......explaining why replacing the 22 uf cathode bypass cap on the 6V6 of the Chinese-built Champion 600 with a 1000 uf cap ( :shock: )was a bad idea.


GOOD GAWD......WTFWTT???

:shock:

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 07, 2012 11:30 pm
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Must admit that I have messed around with most every cap and resistor in my Fender amps. And you can either be gross or subtle with the frequency response. Not being an electronic engineer, I go by ear. IMO, for the most part, you can't hurt anything(except your ears). Obviously, I'm not speaking about the HV!!! But the signal chain has always seemed fair game, to me. Art

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 08, 2012 4:59 am
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Art, prolly better to say "fair-game" to a point. Amps are built to withstand quite a bit of tolerance. What does most Fender schematics say? +/- 20%?

But, when you start pushing the tolerance envelope past 20% (or have several sections in the chain beyond specs, so that the total tolerance imbalance is beyond 20%), then you risk an unstable amp.

Blessing of Fender amps, is the heavy duty PS and trannies. They seem to be able to take a world of crazy demands and not blow up. Schumachers don't fry with the frequency of those early Marshall trannies.

But, instabilities happen. Usually, ending up with a "tube & fuse eating" amp.

My philosophy on all this is keep the amp as close to specs (esp nice vintage ones) as possible. Use pedals and various speaker combinations to alter final tone. Then, you will tend to have a more happy amp, that has a long steady life. My opinion only! :D


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 08, 2012 7:51 am
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Retroverbial wrote:
bluesky636 wrote:
......explaining why replacing the 22 uf cathode bypass cap on the 6V6 of the Chinese-built Champion 600 with a 1000 uf cap ( :shock: )was a bad idea.


GOOD GAWD......WTFWTT???

:shock:

Arjay


Apparently this is a mod included in the Champion 600 upgrade kit sold on E-Bay by someone named "ALNICOMAGNET". He claims that it "boosts and tightens up the bass" of the C600 when used with a 12" cab. I explained that the cathode of the 6V6 is already fully bypassed by the 22 uf cap and that the frequency response is flat down to 37 Hz. Putting a larger cap in that position will only drop the -3 dB point lower. The amp already goes down an octave below the low E of a guitar so a larger cap in that position serves no purpose. As a matter of fact, I ended up installing slightly smaller coupling caps than the stock values in order to RAISE the bass -3dB point of the amp because the bass was TOO bloated and farty with a 12" cab.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 08, 2012 11:25 am
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Sounds like a real pud-knocker to me. I wonder what his day job is?

:lol:

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 08, 2012 6:01 pm
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Yeah Beemer, I'm with on the stability and happy amps. Most of the things I try are just curious experiments, that get reversed back to spec. A few minor adjustments do find a home in my amps. But like you said, the speakers and tubes are the main game-changers. The OT and cab are probably next in line. Part of the circuit or component mod deal, is that a change is made one at a time and given some time to let the rest of the amp adapt. It's all synergy, ehy dude??? Keep in mind my amps are not nice vintage amps. They're player's amps. Art

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Last edited by aclempoppi on Mon Oct 08, 2012 6:24 pm, edited 1 time in total.

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