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Topic: Bias Caps |
Jon Light
From:
Saugerties, NY
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Posted 8 May 2004 3:57 pm |
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| I'm doing a rehab job on a Dual Showman Reverb. It was suggested to me to replace the bias cap with a 100uF/100v while I'm in there. Well I'm looking at a layout that shows me two caps coming off the bias (balance) pot, both at 50uF/75v. And the amp itself has the two, one 50/60, the other 80/75 and I'm over my head. Do I replace both? Can I use 100/100 that I have? |
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Ken Fox
From:
Nashville GA USA
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Posted 8 May 2004 6:25 pm
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| If you have a 135 watt model, the schematic shows two 80 at 75 volts. The 100uf should be fine. Is youra a 100 watt model? |
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Ken Fox
From:
Nashville GA USA
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Posted 8 May 2004 6:29 pm
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| The AA270 (no master volume) shows two 50uf/70 VDC. The later model 100 watt with master volumes is the same as the 135 watt above. [This message was edited by Ken Fox on 08 May 2004 at 07:31 PM.] |
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Jon Light
From:
Saugerties, NY
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Posted 8 May 2004 6:47 pm
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Hi Ken. This is a 100w master volume.
Do I replace each cap (one 50uF, one 80uF) with a 100/100? In other words, two 100uF/100v caps? I am researching this and am seeing recommendations to replace the two caps with one 100 but I believe this is part of a return to a true bias adjust setup. Frankly, too much, too soon for me.
BTW--the caps in there now look ok. But now I'm wondering about the mismatch. |
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Ken Fox
From:
Nashville GA USA
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Posted 8 May 2004 7:27 pm
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Well, my schematic for the 100 watt master volume model shows two 80uf at 75 volts. The two at 100uf should be fine.
This amp can be made to have a bias balance and a bias adjust. I have done several amp this way.
It requires removing the 15k resitor that is tied off the bias control to ground and replacing it with a 10K cermet pot that is in series with a 10K resistor. You would then have a bias control with a range of 10K to 20K. This makes it nice as you can match tubes and set bias both.
It is intersting to note that the method for setting the old bias balance was for "minimum hum". When doing this with unmatched tubes the voltage will be the same (phase cancelled) but the current will not actually be the same! With matched tubes you will get less hum when the currents are near equal, using the bias balance control.
I use a Bias King to watch the current of two of the tubes (one on each side of the O.T. windings. It is very easy with the Bias King to quickly balance and set the bias current.
[This message was edited by Ken Fox on 08 May 2004 at 08:51 PM.] |
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Jon Light
From:
Saugerties, NY
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Posted 8 May 2004 7:58 pm
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Cool. Thanks. I'm going to pass on the bias thing until I have a better grasp of this stuff. I have just spent some time doing my first amp job and I'm feeling pretty cool--replaced some chocolate blobs with orange drops, new filter caps, tone caps--wanted to do the slope resistor change to 56K but I forgot to order it. I hate that. I'll be ready to fire it up in the morning. Duck and cover.
Hey Ken---I was interested in---but not yet ready to absorb---your work with the reverb in the Dual Showman. I recall that you achieved significant improvement. Can you summarize the changes you made? |
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Ken Fox
From:
Nashville GA USA
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Posted 9 May 2004 9:02 am
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I basically converted the circuit back to a very early Silverface. The later Silverface was really a thin sounding reverb. A few resitor changes and removal of some caps was about all that was involved,, asw I recall. It made a huge difference.
You can just parallel a 100K around the 100K slope, if you have one. That will get you in the ball park. I will send you a mod for reverb on both channels. That way both are in phase, one channel can have the slope resistor and the other be a standard Fender tone with reverb.
It's easy, just move 1 wire!!!
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Jon Light
From:
Saugerties, NY
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Posted 9 May 2004 9:23 am
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Hey Ken. Thanks. The 2 channel reverb mod is the first thing I did a couple of months ago on this amp and a on a SFDR. It was a great way to get my feet wet and get confidence in mucking around inside.
Before closing this up I was hoping to try a thing or two to the reverb to restore some fatness to it (you & I discussed this in another thread some time ago--you had the same observation about the reverb. Thin, no bottom--just not classsic Fender. I don't know if I've got the parts in hand though to do it now anyway. |
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Jon Light
From:
Saugerties, NY
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Posted 9 May 2004 2:23 pm
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| Well--the greatest sound I ever heard was the sound of silence (as in no explosions) when I fired this up. My first amp job. A bunch of new caps and a couple of experiments (not of my own design, most assuredly). This includes modification of the reverb circuit and the use of a tantalum cap for the Ch 1 bypass (Brad--THANKS!!). Also that parallel 100K resistor for the slope res. in Ch 1 and a 25K pot for mid control of Ch 2. I never trust first impressions so I'll reserve judgement until after tonight's gig but it sounds quite good. But first and foremost, it works! |
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Jon Light
From:
Saugerties, NY
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Posted 9 May 2004 11:42 pm
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| wow |
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Jay Fagerlie
From:
Lotus, California, USA
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Posted 10 May 2004 7:06 am
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C'mon Jon, a simple 'wow' isn't gonna cut it......
Tell us the news!
Jay |
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Jon Light
From:
Saugerties, NY
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Posted 10 May 2004 9:33 am
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OK. First impressions after a long & late gig. I have played Twins at rehearsal studios and rental backlines. This is why I dig Fenders. But at home I've got a BF Showman and this Dual Showman Reverb (as well as a SF Deluxe and a messed up Vibrosonic Reverb, neither of which pertain to all this). All I could say when I got the DSR was that it sounded good but not great, very much like the Showman--kind of muddy in the lows & low-mids and just sort of lifeless. Disappointing. Low strings had no pop or separation. Highs were kind of shrill.
The first thing I noticed last night was that anyplace I played on the guitar, the sound had clarity. No murk. I could strum low 6th chords and hear the notes! With too much bass dialed in I could roll thunder but still--I could hear the strings!
Seems like Ch 1 with the "Healy" tantalums is, as I've seen others describe, really 'singing'. It actually may be too hi-fi for my ears. But I had to do some playing with the band last night on occasion  so I couldn't spend all night playing with the dials and the channels. The reverb has more depth now too although I may need to dial it in better--I didn't have the proper resistor value so I just threw something in there.
All in all, it's like taking windex to a very dirty window. And then putting up new blinds and shades.
I just opened up the old Showman for the first time and sure enough, it has its original caps from 19sixtysomething leaking and all. Which is why the Dual sounded a lot like the Showman. And is why the Showman will soon sound like the Dual.
Friends--if you have an old Fender and it disappoints--sounds lifeless, muddy, too shrill because you are turning up the treble (or down the bass) to fight the mud--do yourself a huge favor and get it recapped---filter caps and tone caps. I am, finally, a believer that this is not just a bunch of techs talking about subtle stuff. |
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