Major Score: 68 Pro Reverb for $400

[bob p]

Yes, on to better things.

Having somebody else demo the amp for you was a good idea. I like the idea of sitting out in the room and listening to your gear.

If I had the amp, the first thing that I would do would be to fix the power supply cord. You never know what kind of bad wiring you may run into in some clubs, and when you factor in the unpredictability of other people's gear (which may or may not be in good repair), there's the possibility of danger as you mentioned. I know that I'm funnier about this electrical safety business than most people, so I wouldn't waste any time getting the grounded cord / ground cap and switch replacement done. I also like to keep a OSHA Wiring Tester (the little plug-in module for the socket with the three indicator lights) in the gig bag. I always test the power on-stage before plugging in.

On a related note -- after changing the power supply wiring, you could retain the SPDT switch on the back of the chassis and use it for a NFB mod, as I alluded to earlier. Essentially, you can use the switch as a 2-way or 3-way selector to change the value of the NFB resistor. Its a good bang for the buck non-destructive mod.

[Regis]

Hi Bob,

I put a grounded cord on it today. Gave myself a good poke too, for the first time in years. I had finished the soldering and wiring, and was wrestling with the strain relief, trying to put it in. Even with the proper tool, which I have, sometimes it's tricky. Anyway I was grasping the chassis with one hand to sturdy it and trying to insert the strain relief with the other when my finger on the chassis touched something, probably the aux socket. Nice little 125 volt tingle. I had forgotten to unplug the amp after testing it. It would have been pretty dumb to kill myself installing a safety feature designed to prevent that.

About that NFB mod, what value resistors do you use? It has an 820 now.

Another thing about those cathode resistors on the power tubes, there is a 5mf cap connecting the two resistors. I still don't understand about all that.

[bob p]

... Anyway I was grasping the chassis with one hand to sturdy it and trying to insert the strain relief with the other when my finger on the chassis touched something, probably the aux socket. Nice little 125 volt tingle. I had forgotten to unplug the amp after testing it. It would have been pretty dumb to kill myself installing a safety feature designed to prevent that.

Yes, that would have been a tragedy! Its always easy to forget to do simple things, but the most important thing to keep in mind is this -- ALWAYS unplug the amp and drain the filter caps to ground through a resistive probe. ALWAYS! In order to make this sort of feature automatic, I build my amps with bleeder resistors whose grounds are switched into the circuit when the power mains are turned off. (I use a DPDT mains switch.)

Regarding the NFB mod, there are a couple of ways to do it. I would recommend keeping the stock value as one of your options. Then you need to decide whether or not you want more or less NFB in the amp as your selectable options. There's a limit to how much more NFB you can add before the circuit performance is degraded. Typically, options of higher resistance/less NFB are chosen, as the decreased NFB allows the amp to break up earlier. At the extreme, a really high value resistor makes the NFB loop look like there's no NFB at all.

The best way to choose the values is to crunch the numbers for NFB resistance. There are forumulae for this on Aiken's site that show you how to "scale" the amount of feedback. If you're not into math, a more direct method for experimentation would be to temporarily replace your 820-ohm resistor with a higher value pot. Start with the wiper set to about 1K and experiment with higher resistances. (Or maybe you should just add a pot in series with the existing resistor so that you never use too little Z in the loop). When you find a tone that you like, take a measurement and use that value.

One of the problems with NFB scaling is that you have to re-crunch the numbers for different OT tap impedances. An amp that has an 8-ohm tap to the NFB loop would require a different resistance than an amp that has a 4-ohm tap, so thare are no "universal" recommendations that I can make without first crunching the numbers myself.

I used to have a schem for this mod, but I can not find it. IIRC Dai Hirokawa was the person who drew it up, but I could be wrong about that. It employed an ingenious use of the existing SPDT switch to allow three selectable NFB values. Dai?

[hasserl]

Sweet amp! Congrats on a great deal.

Re the combo bias thing. I remember someone on the old Ampage board experimenting with adding this to his SF Bassman, an amp that did not have it to begin with. He added smallish cathode resistors that generated a bit of the bias voltage needed, then he tweaked the fixed bias voltage supply to add the required extra bias voltage. He loved the results, noting the added compression from the cathode biased part. I've always thought the idea had merit, I see nothing inherently "wrong" with it. Why it didn't work out for Fender may have more to do with other issues, not that there is anything wrong with the idea (kind of like UL, there is nothing inherently "wrong" with UL, Fender just wasn't very lucky with their implementation of it.

[Tom Phillips]

I think that the combo bias may very well have merit if properly tweaked. However, since an amp's performance is due to the sum of all it's parts...AND... the overall silverface performance was judged no good by most people...THEN...everything to do with SV circuits is judged no good. Call it the silverface curse if you will.
Tom