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Post subject: Silver to Black Twin conversion question
Posted: Wed Apr 17, 2013 2:05 pm
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So I took a Push Pull Master Volume twin reverb to blackface specs. Customer wanted the master volume in place so I kept that the same. Sounds great except...........

There is a bit of hiss and crackle (intermittent) when the amp is in play mode.
So I started trouble shooting.
1. chopstick-nothing
2. Stage isolation Pulling tubes.
3. Voltage and parts evaluation- everything in spec
4. Changed speaker cab, cords, guitar......-no change.
5. Pull reverb tube-no change.
6. Pull Trem tube-no change.
Here's the weird part.........
Guitar plugged in with volume down or unplugged. Problem gone!!!! Doesn't matter if its in normal or vibrato channel it does it just the same.
I have checked all resistance readings and they are below an ohm.
Im assuming its in the audio path because it only does it when the amp is in play mode and the volume is up on the guitar. Turn volume down, on guitar, and the amp is Dead QUIET.
I have changed all parts in both audio stages on both channels. Tubes are good and have swapped even just to be safe. Could it be pots or jacks?
Remember it's intermittent. Just comes and goes. All of it above like 1khz or so.
What should I do next. Audio probe???????? Any good ideas of where to put the scope on it.

I've tried "NEARLY" Everything.
Does it when chassis is on bench and back in the cab
HELP PLEASE!!!!!!!!!!!!!!


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Post subject: Re: Silver to Black Twin conversion question
Posted: Wed Apr 17, 2013 3:17 pm
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Possible DC leakage from a coupling cap in the phase-inverter stage or an aging carbon-comp resistor there that's absorbed some moisture over the years.

Arjay

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Post subject: Re: Silver to Black Twin conversion question
Posted: Thu Apr 18, 2013 2:09 am
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goldstache wrote:
So I took a Push Pull Master Volume twin reverb to blackface specs. Customer wanted the master volume in place so I kept that the same. Sounds great except...........

There is a bit of hiss and crackle (intermittent) when the amp is in play mode.

HELP PLEASE!!!!!!!!!!!!!!



Good chance is bad solders in power supply rail ( B+ voltage)

Be sure bias in not too hot.


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Post subject: Re: Silver to Black Twin conversion question
Posted: Thu Apr 18, 2013 12:59 pm
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I rebuilt the PI. Problem persists. I will try the B+ solder points. Thanks for the tips.
It's 95% rebuilt at this point! LOL


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Post subject: Re: Silver to Black Twin conversion question
Posted: Sat Apr 20, 2013 11:32 am
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It was the two 220K 1watts that are in the filter section. I used NOS carbon comp and once I went to Metal, problem ceased!
Thanks!!!!!!!


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Post subject: Re: Silver to Black Twin conversion question
Posted: Sat Apr 20, 2013 2:43 pm
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There is really no good reason to use carbon comps anywhere in an amp these days. The fact that they were used in vintage amps is only because.....wait for it.....that's all they had! They weren't used because of some fabled tonal magic. :idea:

The inductive qualities of carbon comps that could add to tone would only be realized if the voltage drop across them was very high, much higher than will be seen in virtually any amp. The only place that might possibly be of any sonic benefit would be the Plate resistors of preamp tubes, and even then I wouldn't use them as the possibility of sound degradation due to noise is much greater than any possibility of tone improvement. Some people want to use them to keep the look of the amp "authentic" to the originals, but who will ever see them? :lol:

Some places that sell carbon comps might tout the superiority, but I ain't buyin' it. :wink: And "NOS" carbon comps......please! :lol:

Almost forgot to mention the best reason not to use carbon comps, they can catch on fire and actually burn! :shock:

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Post subject: Re: Silver to Black Twin conversion question
Posted: Sat Apr 20, 2013 5:27 pm
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" It was the two 220K 1watts that are in the filter section. "

These are really important resistors. They balance the voltage load over those two 100mfd/350VDC caps in the first stage of the main power rail.

I'd have to agree with Shimmy. Esp in the power rail. Good non-flammable metal oxide or wire-wounds would be nice. Next time around. :D

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


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Post subject: Re: Silver to Black Twin conversion question
Posted: Sat Apr 20, 2013 11:25 pm
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BMW2002Ti wrote:
" It was the two 220K 1watts that are in the filter section. "

These are really important resistors. They balance the voltage load over those two 100mfd/350VDC caps in the first stage of the main power rail.


+1

The reservoir caps are critical for dependable amp performance, as is every resistor on the power rail. And metal-film types there can help extend the TBO for a filter-cap array.

Arjay

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