Twin reverb filter caps

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Twin reverb filter caps

Postby strangedays on Fri Mar 11, 2016 12:17 am

Hi everyone,ve just purchased a 76 twin with jbl d120(in great condictions ,not reconed or anything). i notice that the sound is really muddy,bassy and presents an awful compression on the bottom end.Everything was still orignal when i purchased but to improve the sound i've changed all the tubes,the bypass caps(with the exception of the two 80-75 ones) but i nothing has changed,is stilll too far bassy and muddy.I'd like to change the filter caps,because they are 40 years old,hoping that would give brightness and trasparency to the amp.
Do you think it would be useful installing new filter caps to to clean up the sound or it's only useful to avoid damages on the amp?
Maybe should i replace the coupling caps first?
i'll apprecciate any suggestion.
Best regards,
Gabriele.
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Re: Twin reverb filter caps

Postby Unit_1 on Fri Mar 11, 2016 1:02 am

Wow, muddy with JBL's that's hard to do since they are so bright

The filter caps should definitively be changed. I've used Sprague, TAD and F&T with identical results. TAD's are the least expensive, Sprague cost the most.

Does it have a three prong power cord? I don't know when fender switched from 2 to 3, but if it has 2 then you should upgrade to 3. It makes the amp sound better....

when coupling caps are bad you get a lot of hum and buzz and clicking. If you don't have any of that then they're probably good. Testing them for leakage would confirm that. There should only be DC voltage on one side of the cap - blocking the HUM from the power side of the circuit is part of their function....
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Re: Twin reverb filter caps

Postby strangedays on Fri Mar 11, 2016 7:50 am

Unit_1 wrote:Wow, muddy with JBL's that's hard to do since they are so bright

The filter caps should definitively be changed. I've used Sprague, TAD and F&T with identical results. TAD's are the least expensive, Sprague cost the most.

Does it have a three prong power cord? I don't know when fender switched from 2 to 3, but if it has 2 then you should upgrade to 3. It makes the amp sound better....

when coupling caps are bad you get a lot of hum and buzz and clicking. If you don't have any of that then they're probably good. Testing them for leakage would confirm that. There should only be DC voltage on one side of the cap - blocking the HUM from the power side of the circuit is part of their function....


You 've said it right Unit_1,it's hard to believe that with Jbl's it can sound this way.But i can guarantee it's the absolute truth.It has three pong power cord but i notice that the midlle green wire is not connected to the terminal.
Thanks for reply.
Gabriele.
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Re: Twin reverb filter caps

Postby stratele52 on Fri Mar 11, 2016 9:27 am

Compression is from power supply. Not bypass cap or coupling caps.
Check Phase inverter circuit too
Did you do a output tubes bias ?

Muddy , compression at wich volume ? Please report

Filter caps last more than 40 years. You can check them to see how they work before.
1- Read AC volt ( ripple) on filter cap it should be less than 0.1 volts.
2- Read DC volts and play amp at "loud" volume to see voltage drop.
If 1 and 2 are ok , filter caps are good.

Test with another speaker.

You are on the way to change everything on this amp except the cab :laff:
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Re: Twin reverb filter caps

Postby pdf64 on Fri Mar 11, 2016 10:18 am

Safety first, get the amp chassis properly grounded.

Electrolytic caps may sometimes last 40 years or more, but the problem may indicate ineffective reservoir capacitance.

Note that some meters may not be able to read Vac when substantial Vdc is present. A dc blocking cap in series with the probe can be used to get around this, eg 0.1uF 600V.

However, 0.1Vac ripple seems incredibly low, I can't remember ever seeing it that low on an idling tube amp at the reservoir cap (Fluke 189 & 79 III)?
Around 1 to 2Vac seems more normal.
Even in standby, with no HT current being drawn, I'd expect more than 0.1Vac.

Later SF Fenders tended to be seem more bassy, due to the bass control taper being ~30% rather than ~10%, and due to the 0.01uF coupling cap to the LTP. http://bmamps.com/Schematics/fender/twin_reverb_sf_100_schem.pdf
I suggest replacing the bass control with a 10% type (eg regular CTS audio) and the above cap with 1nF; these should bring the tonal range more in line with classic BF.

If replacing power supply ecaps, ensure that those in the bias supply are done.
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Re: Twin reverb filter caps

Postby strangedays on Fri Mar 11, 2016 1:43 pm

Thanks everyone for advices.I'think i'll move gradually.I'll repace filter caps first and let's what happens.
Thanks again.
Best regards,
Gabriele.
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