Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Wilks on July 01, 2010, 03:55:27 pm
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Hi, I have a SF Deluxe Reverb which according to the Chassis number (A42556) is a '75. I have owned and gigged it regularly for the last 20 years. It has been 'Black Faced' at some time in the past, the circuit appears to match AA763 layout (see attached pic). I had it serviced about 10 years ago and figure its time for another. I intend to do this myself. I have read the great guide to servicing Fender Amps article on this site which is really useful and intend to follow the recommendations it makes. The question I have concerns the blue ITW caps I have (see pic). Do you think these are the originals? Should I replace them as part of the service?
Thanks,
Wilks
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Blue chiclets are OK - but usually aren't ITW-branded. I'd keep them if they aren't leaky.
Chack the anode resistors and the electrolytics, they usually need to be changed.
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Hello,
Yes they are normally they are fine to leave in place and will help to keep your original tone of the amp. It's any electrolytic type that you should be concerned with as they will tend to dry up over time and will effect tone and/or biasing as they do. To keep your tone and performace of the amp as close to original as you can (if that's your objective), use the same values and note the polarity of how they are installed. Have fun and good luck with it.
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Make sure it is indeed blackfaced. Some guys don't go all the way. That happened to me before I started doing this myself and one reason I did start to service my own amps. I just never got along with the amp until I opened it up and realized the guy didn't do it right. Once I did it right it was much better, I added an original C12N Jensen and the amp was magic.
Replace ALL electrolytics......leave those blue caps unless they are leaking. Check all resistors for drift, easy to do and replace with carbon comp or metal film. I would replace some of the resistors on the power supply, like the the screen resistors and the dropping resistors with higher rated ones, Doug carries all that you need. Plus I would twist the green heater wires a little tighter if you can. It's easier to just remove the one green wire on pin 9 on the preamp tubes but this is not necessary unless the amp is a little noisy.
One issue I have with those amps is the cheap controls with the plastic shaft. If you have them I would replace them, but that's me. I also replaced the 50K resistor on the vibrato with one that had a switch so I could take it out of the circuit, that added some gain. Also the 1.5K resistors on the grid should be replaced, that's a lot of heat for a long time. I would do it a little different and just connect it to pin 5 not run it across the center of the socket. Use some shrink wrap to cover up the resistor to wire solder joint, that will keep it away from the heat.
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What are the ailments? I'm a huge believer in, "If it's not broke, don't fix it."
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Back to original question:
The question I have concerns the blue ITW caps I have (see pic). Do you think these are the originals? Should I replace them as part of the service?
The blue ITW caps are 100% stock and typical in silverface Fenders. You also see brown caps that are a similar size/shape but different texture. And all those white caps are Mallory electrolytics.
You could consider replacing them, but except for the one on the bias board, I would strongly consider leaving them in place. If you like the current low end of your amp, don't change them. E-lytics dry up over time and lower in value, which reduces bass response when the cathode bypass caps look like a smaller value. But many, many people fight farty bass by trimming the 25uF bypass caps down to around 1-2uF. Your caps (if they've changed value) are probably around 2-8uF, and likely sound pretty good as-is.
I have a SF Deluxe Reverb ... It has been 'Black Faced' at some time in the past ...
The Deluxe didn't change much. Grid resistors for the phase inverter become 330k instead of the blackface's 1M, phase inverter input cap becomes 0.01uF instead of BF's 0.001uF, and there are small caps from the 6V6 grids to ground. So the whole operation is changing 3 parts and removing 2 others. Yeah, there's a rectifier tube change, but as long as you don't burn up your filament winding, you can swap those at will; and the stock 5U4 in a silverface draws more current, so the GZ34 is always safe to install.
I had it serviced about 10 years ago and figure its time for another.
Is it humming? Are pots crackling? Are the tube sockets gripping the tubes so lightly that they're about to fall out? Are the filter caps originasl to the amp?
If you answer no to each of these (and the last one is somewhat conditional on the first question), then I wouldn't do anything except maybe install a new bias cap. I'd probably only do that if I had hum that did not stop when the phase inverter was pulled.
I have 2 amps I built maybe 10 years ago. I don't forsee servicing them for at least another 10 years (one, maybe not for the next 20 years or more, cause the filter caps aren't electrolytic).
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I'd definitely replace the 100k plate resistors. They have a tendency to start making that dreaded "frying bacon" sound. It's a cheap preventative maintenance measure, as long as you're in there anyway.
Dave
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Interestingly, the white Mallory cathode bypass caps in my 1970 SFDR have drifted up to the33-35 microfarad range. The plate and cathode resistors seem to be within about 10% of design values
Would the bypass caps account for the HUGE bass response of this amp?
I am hoping to replicate the tone of this amp in a 6BM8 version