# mellow 100 watt SF Twin Reverb



## prairietelecaster (Oct 25, 2006)

I have an early/mid 70's Twin Reverb. It sounds great, good condition cosmetically for its age, recent cap/bias job etc. but it does not peel the paint when set on 5. I'd call it full but not as loud like you'd expect. In other words, quite mellow for a twin. Tubes are all good and firing. Nothing like a 12AU7 in the v1 v2, tubes are all spec. Speakers are good. I suspect it was tweaked at some point inside to tone it down somewhat. I will probably leave it as is as I like the way it works. I picked it up several years ago from the local music store before it was bought out by L&M. The reason I think it might have been custom tweaked is there was an "Amos Was Here" note taped inside the box. (as in Garrett's autograph, and I know this could also mean nothing). I had Keith M. go over it a few years ago but we never really looked at the volume thing as I said, I do like it as is so I never brought this to attention and I didn't crank it in his shop. It had the mellow thing going on before the cap job and any re-tubing. As a point of curiosity and learning, what should I look for to see what is reducing the volume? Thanks in advance.


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## loudtubeamps (Feb 2, 2012)

What kind of speakers are "In the Twin"?
When it had a cap job, were the cathode caps 22u/25v in the pre-amp section replaced as well?
What size is the resistor on the mid pot to ground?
If you could post some good pics. of the circuit board and output section, you will get some pretty good suggestions from some members.
Could be any number of mods applied to reduce the volume.
Cheers, doug


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## prairietelecaster (Oct 25, 2006)

Thanks. I will get around to sliding out the guts and take some pics. Stay tuned.


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## nonreverb (Sep 19, 2006)

What does it say where the speaker plugs in: 100 watts or 135 watts?


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## prairietelecaster (Oct 25, 2006)

100 as per the subject. I am away from the amp at the moment so I haven't opened it yet.


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## nonreverb (Sep 19, 2006)

prairietelecaster said:


> 100 as per the subject. I am away from the amp at the moment so I haven't opened it yet.


Right! Just checking 
How old are the power tubes? They might test OK on a tester and bias OK but that doesn't mean they sound good. That's a good place to start.
The speakers would be the second suspect. If it has the dreaded Utah speakers in it, that's going to have an effect on the quality of the sound.


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## washburned (Oct 13, 2006)

Had that problem once with my Orange. Turned out one speaker lead was bad, amp was pushing into one speaker @16 ohms instead of two at 8 ohms.


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## prairietelecaster (Oct 25, 2006)

Finally I got around to looking inside. Hope the link works. Looking at the cap on the bias supply, I think I should change that while I've got it open.

https://picasaweb.google.com/116439116481564367083/TwinReverbGuts


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## nonreverb (Sep 19, 2006)

I see it does indeed have the Utah speakers...also has the brown coupling caps which can be a problem. You can change both bias caps although one's been changed already.


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## prairietelecaster (Oct 25, 2006)

I've a new 100v100µ cap lying around. The schematic and layout show the originals as 80/75. The 100/100 should be ok to replace the other one I am assuming as this seems to be a stock answer "out there". I am talking about the silver cap on the trim pot in this photo. Yes?
https://picasaweb.google.com/116439116481564367083/TwinReverbGuts#5783276351671060594


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## loudtubeamps (Feb 2, 2012)

As mentioned by nonreverb,the brown blocking caps seem to have a bad reputation.
Their main function is to block high D.C. voltage.

*It has to be said that you should only try these suggestions if you are comfortable doing so.*

You can check them by putting a multi meter set to a low D.C. scale _on the low voltage side of the cap_.
One side of the cap has the 100K resistors with it and will give readings of 200 volts D.C.+- while the other side should show almost no D.C. voltage.
So: Amp in operate mode.
Positive meter test lead to the cap lead feeding the tone controls etc.and neg.meter lead to chassis ground.
If the D.C. readings are 1 volt or less and you do not have any signs of noisey pots during rotation, then they should be O.K.
You may want to upgrade to different blocking caps as an option for improved tonal response but if the D.C. readings are low on yours, then they are doing their job.

The white poly preamp cathode bypass caps you have ([email protected] volts) are original and do last quite a long time.

To test to see if they are failing (which would cause a significant drop in overall volume and response), you can solder a new electrolytic polarized cap in and A/B it against the original. 
To easily do this test:
With the amp disconnected from the mains..............
Solder the _positive_ lead of a fresh cap (same value) to the cathode eyelet while leaving the original cap in as well.
Leave the neg. lead free. 
Next,unsolder the original *neg.*bypass cap lead at the eyelet (going to chassis ground)and leave it free.

Clip one end of a test lead (with small insulated alligator clips) to the free end of the original neg. cap lead.

Now; While in the operate mode ,play an open string at normal volume and tone settings and while doing so touch the free end alligator clip to the chassis. Touch and then lift while the string is resonating. You will hear a difference.
Repeat the process with the _fresh cap *neg*.lead_.
If the original cap is failing, the difference between old and new will be quite obvious with this test. 
I would check the cap(s) closest to the input first (#1-12AX7) as these should give the most noticable results.
If your amp is as mentioned, sounds a bit sluggish but does not exibit any other significant problems then you may want to change all of the original cathode bypass caps.

http://i1132.photobucket.com/albums/m570/loudtubeamps/001.jpg


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## loudtubeamps (Feb 2, 2012)

prairietelecaster said:


> I've a new 100v100µ cap lying around. The schematic and layout show the originals as 80/75. The 100/100 should be ok to replace the other one I am assuming as this seems to be a stock answer "out there". I am talking about the silver cap on the trim pot in this photo. Yes?
> https://picasaweb.google.com/116439116481564367083/TwinReverbGuts#5783276351671060594


 The cap in question is your bias filter cap and unless your amp has a very noticable hum, then it's fine.
BTW> As far as one can tell from your pics. the preamp section components look stock and original.
Cheers, doug


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## nonreverb (Sep 19, 2006)

I'm hedging my bets on those Utah speakers. Every SF Fender I've owned and many I've serviced, vastly improved tone-wise when they were replaced...


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## prairietelecaster (Oct 25, 2006)

Thanks for all the info everyone. I'll check the speakers as well.


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