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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 3:13 pm
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Ahhhh... this is good info!

Ok, what should be in the doghouse (which I believe is the pan where the big 500v caps live?)

From the looks of things, there are:

2 x 330v 200uf side by side
2 x 500v 47uf side by side
1 x 500v 22uf
The 2 small caps that are covered by foam, but I will follow up once I get back inside.

The amp is sounding worse and worse to me....This past weekend, I played a Blackface twin reverb re-issue (no master volume) that was part of the gig's in-house backline. I basically had it the volume at 2.5 and it was screaming loud and bright....and very very clean

My twin could compare if I cranked the master and channel volume to maybe 6 or 7....and is pretty distorted at that point.

For this amp (UL 135 watt rms), and for the layman, could you guys tell me what should be in the power rail capacitor wise...I think this might be the ticket. I'm not confident enough to try and test live caps and resistors, but I can definitely handle taking out the wrong part, and putting in the right one...

The tech changed these all out, and I think he doesn't know one twin from another...?

Thanks
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 3:32 pm
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You need to study the schematic that was posted earlier very carefully and trace out all of the components in the power supply. Keep in mind that even just changing a power supply cap requires that you drain the cap to be sure it is safe to handle. It would be virtually impossible, in my opinion, to walk you through step-by-step in a forum how to go about undoing all that appears to have been done to this amp. If you are not comfortable doing this, you should take that schematic and amp to a different tech to work on it.

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

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 4:08 pm
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Hi Bill

I get that, but on the schematic, I see nothing that matches the caps I have, so I'm not sure what I'm looking at.

I know that this symbol "||" means capacitor.

I assume that when the schematic says things like

"220
285v" (as it does in a couple of spots near the bottom right of the schematic) That it means the capacitor needs to be a 220uf 285v cap?

(unless it says PF, it which case it needs to be converted, http://www.justradios.com/uFnFpF.html )

Getting closer to what I assume to be the power rail, (bottom right) there appears to be a diamond configuration: (4) capacitors .002 Ikv...I have...no idea what this means. :)

Any resource online that showed an amplifier schematic, but then explained what everything does and everything means would be amazing....but I can't find that.

If someone could point be to where the caps I'm looking for, are on the schematic (specifically the power rail capacitors) I could at least, figure out whats supposed to be in there.

Thanks again
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 4:27 pm
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ParksideDrive wrote:
Hi Bill

I get that, but on the schematic, I see nothing that matches the caps I have, so I'm not sure what I'm looking at. That's just it. WE don't see the caps that are in your amp on the schematic either. That is the problem.

I know that this symbol "||" means capacitor. Yes.

I assume that when the schematic says things like

"220
285v" (as it does in a couple of spots near the bottom right of the schematic) That it means the capacitor needs to be a 220uf 285v cap? Yes.

(unless it says PF, it which case it needs to be converted, http://www.justradios.com/uFnFpF.html ) Yes.

Getting closer to what I assume to be the power rail, (bottom right) there appears to be a diamond configuration: (4) capacitors .002 Ikv...I have...no idea what this means. :) That is the solid state rectifier that converts the high voltage AC into the high voltage DC.

Any resource online that showed an amplifier schematic, but then explained what everything does and everything means would be amazing....but I can't find that. You will find that in the books I suggested you buy and study.

If someone could point be to where the caps I'm looking for, are on the schematic (specifically the power rail capacitors) I could at least, figure out whats supposed to be in there. The "power rail" caps are simply the high voltage power supply filter caps. The caps that are in your amp do not appear on the schematic. That is the problem. You need to trace out the circuit and figure out what SHOULD be there and change what is in your amp. We can't do that for you.

Thanks again
Dave


I'll be blunt: Based on the questions you are asking and your general lack of knowledge about the insides of a tube amp, you really need to do some studying about tube amps before you stick your hands any further into this one. You could very easily kill yourself and no one wants that to happen.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 4:44 pm
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Bluesky, for some reason this amp's schematic is one of the more difficult ones to read. The engineer/artist that drew it did not separate the power supply section from the rest of the amp.

Anyhow, I put gray dots by those power filter caps.

1.) The two 220mfd/330VDC caps are in series and feed the output section.

2.) I see only 20mfd/500VDC caps, for rest of PS supply (three total). Each one feeding one of the gain/EFX/input stages. It's hard to see the characteristic "pi-filter" (the resistor and caps in each stage ) in this drawing. But, they are there. You may need to redraw the PS section schematic, to better grasp cap and resistor layout and how the amp taps off their sections.

3.) I see no 47mfd/500VDC caps. This may have been an option done by a tech, when he re-did the amp).

4.) Those smaller caps under the foam maybe part of the Bias Power Supply (70mfd/100VDC and 80mfd/75VDC). If so... be very careful of the polarity of these caps. Just like the first two 220mfd ones.

Good luck. I'd go cap-by-cap and triple check things, before applying any voltage. You have a Variac to ramp this puppy up?

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 4:46 pm
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Fully understood and appreciated.

Thanks for the help.

Last question, do any of those books contain a schematic explanation, that will teach me to be able to make sense of what Im looking at? I like to learn hands on, and have no trouble reading a map, once I understand the legend/key

In all honesty, I have no idea what any of it means and can only intuit knowledge based on common sense and google. For example I get what v3 is, I get what a capacitor, resistor, ground looks like on the page and in the amp.

I have honestly no idea what a diode is or does, or a rectifier, opamp, and probably 10 more. Don't really understand how the tubes work or in what order. I don't know how the signal chain works, with regards to what happens between a patch cord hitting an input jack and sound coming out the speaker (looks like it takes a wild ride :)

What is the best book to start with. Im eager to learn and happy to take my time....Not as stoked to buy 8 different books that likely need to be understood in graduated sequence.

My honest thanks for the info, If anyone knows where to start for the novice, that would be great...in the meantime I guess I will have to bear with this tone. Or take it to tech 3, who unfortunately may be too busy to pour over the schematic line by line himself.

thanks again
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 4:53 pm
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Bmw, that's awesome...and shows I was on the right track, because the power supply caps seem to be the highest rating voltage-wise.

OK. so. None of whats in there is right, he basically grabbed caps that were closely rated and belong in a Hot Rod Deluxe and stuck them in my amp...because it was likely what he had in stock.

You guys have been amazing. Even if I don't do the work myself for the reasons that Bill mentioned (and I agree) I can take it to a tech, show him the schematic, show him the photo and say "this is wrong....you fix.."

Thanks again
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 4:56 pm
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ParksideDrive wrote:
What is the best book to start with. Im eager to learn and happy to take my time....


This:

http://www.amazon.com/Guitar-Amplifier- ... pd_sim_b_6

Good luck.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 4:57 pm
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BMW2002Ti wrote:
Bluesky, for some reason this amp's schematic is one of the more difficult ones to read. The engineer/artist that drew it did not separate the power supply section from the rest of the amp.


Resistor values don't jive either.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 4:58 pm
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ParksideDrive wrote:
You guys have been amazing. Even if I don't do the work myself for the reasons that Bill mentioned (and I agree) I can take it to a tech, show him the schematic, show him the photo and say "this is wrong....you fix.."

Thanks again
Dave


That would be a smart move. You picked a heck of an amp for your first troubleshooting experience. :lol:

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Last edited by bluesky636 on Mon Oct 22, 2012 5:00 pm, edited 1 time in total.

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 22, 2012 4:59 pm
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" there appears to be a diamond configuration: (4) capacitors .002 Ikv "

These are RF "Snubber" caps, to tame some soild-state hash generated by the diodes.

You know, I agress with Bluessky. Unfortunately, you really picked a difficult amp to redo. And these UL amps run big voltages and can be unforgiving, to tech errors.

Maybe, check out some good websites and get a couple books, if you decide to tackle the amp. One thing you need to keep in mind. This amp runs 500+ VDC in its circuit. You really need to understand the consequences of mis-wiring to the amp and your health.

Be safe!


Valve Wizard has some good intro points:

http://www.valvewizard.co.uk/bridge.html

http://www.valvewizard.co.uk/smoothing.html

http://www.valvewizard.co.uk/psu.html


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 8:47 am
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Hey guys

Is there a recommendation for a brand of filter caps. Arjay mentioned to steer clear of the IC's.

How about Sprague?

I'm hoping to eliminate further issues by providing the tech with the parts I want installed...

Thanks
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 9:09 am
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I've been using F&T electrolytic caps, for main power supply, bias supply section, & some bypass sections --- the last year. They seem pretty stable. I have yet to have an early failure (which occured on two amps with new Sprague Atoms).

http://www.ftcap.de/index.php/axial-lead-archiv.html


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 9:36 am
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OK, I will pick those up online.

2 x 330v 200uf side by side- this matches the schematic fairly closely. The schematic calls for 285v 200uf, but those don't seem available anymore. Will the extra 15volts resistance affect tone? The web seems to think this is an ok compromise.

2 x 500v 47uf side by side- I think these may be the biggest offender. The schematic makes no mention of 47uf, and he has 2 of them in there, more than doubling the capacitance. I would like to replace with 2 x 500v 20uf, but it seems F and T doesn't make those (they make a 22uf) but sprague does..... http://thetubestore.com/ca-at-20uf-500v.html. Does the 2 micro farads make any difference

1 x 500v 22uf as above....

The final 2 capacitors I'm concerned with were the smaller ones hidden by the foam. If I'm reading the schematic correctly, they should be:

80uf 75v (the bias Cap?) hard to find an 80/75...is 100/100 F&T the way to go?
70uf 100v Same...hard to find. Substitute?

Thanks again
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Tue Oct 23, 2012 10:08 am
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Yes, the 100mfd/100VDC cap will work fine for the 80mfd/75VDC and 70mfd/100VDC caps.

WATCH THE POLARITY! On all caps. I usually changed them out one-by-one to avoid disaster.

Also, check to be sure all resistors in the power rail (between the caps) are correct value (resistance & wattage). I'd go metal oxide or wire wound here --- for stability and long life.

After all is done, do a thorough check with a DVM (in resistance DC mode) to be sure all the caps are hooked up correctly. And you have no shorts --- before firing up.

A Variac or the old "light-bulb" trick should be used on first fire-up. It could save you a world of pain.

http://www.radiomuseum.org/forum/curren ... ester.html


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