I've been working on a Scott 200 for the past few weeks now. It's a rather 'odd' amp IMHO in that it has some peculiarities to it that I've not run into in the past. Most notably is the fact that it had a single test point tied into the entire power supply side...which clearly was part of the bias adjust process, but I found it so inaccurate that I simply couldn't deal with it. The amp had a 12 ohm 7 watt power resistor tied into the HV Center-tap to ground.....and the test point came off at that junction. So, current from all 4 output tubes, plus all the signal tubes....ALL came through that one test point. The amp also had some kind of 'balance' pot set-up on it, but it was the cathode-to-ground side of things with 100 ohm wire-wound pots being used to balance cathode current. Again, just not how I like it. So, at the advice of a few other members, I'm attempting to 'mod' my amp (scary!!) and install the more 'typical' Scott Bias/Balance scheme.
Attached are a few pics of where I stand now in the process. What I've done so far is to remove the one bias pot, the two 'balance' pots, and the one 12 ohm resistor. I noted that my unit being an earlier A or B model 200, it had this 12 ohm resistor on the center tap side, where as the later 'C' model had a 33 ohm / 7 Watt resistor between the output of the rectifier and the first stage of the power supply. My amp was straight wired from the rectifier to the first PS stage. So, in removing the 12 ohm resistor, I also 'installed' a 39 ohm / 10 watt resistor off pin 8 of the rectifier socket to the PS stage 1. I also added 4 test points for the 4 output tubes, and installed 10 ohm 'matched' resistors between pins 7 of the 6GW8 output tubes and ground....with connections from Pin 7 to each of the 4 test points.
In removing the prior wire wound balance pots, it appears I've got good space to mount new balance pots feeding the input grid resistors. In addition, I'll also need to figure a way to mount 2 'bias' pots in the location where the former one pot was. May take a bit of cobbling, but I think I can get two pots in that location as well. Right now my problem is; What value pots do I use to begin with in establishing a whole new bias feed system for this amp? The old 'single' bias pot was a 10K. Each 'balance' pot was 100 ohms. So, previously I had essentially 50 ohms on each cathode (if the pot was 'centered'), plus what ever 'feed' was coming out of the bias pot. BTW, the function of the 'balance' pots is questionable as to if it was truly a 'bias' related function or if perhaps it was also simply a 'hum' or 'noise' related control. While the amp also has 'hum balance' pots off the filament wires, it was suggested that the 'balance' pots off the output tube cathodes were really more for additional 'hum' balance control than for use in setting bias.
In any event, I'm poised for being ready to start the 'build up' of the bias supply side of things. When I'm done I think this amp will be MUCH more controllable, and given the ability to find 'matched' 6GW8s (or lack there of), being able ti do bias/balance on each channel will be a true benefit. So, if anyone has any suggestions as to where I ought to go from here, please let me know. BTW, I'm looking at a Scott 299B Schematic, and using that as sort of a 'guideline' for doing this project. I noted some REALLY significant differences.....like 220K Grid resistor on the 299 vs 820K on my 200. But....hey....two completely different amps. I'm still VERY new to this 're-engineering' stuff....and don't feel real confident at this point. So, any and all help is appreciated. (note: last pic is a 'before' shot prior to my pulling the balance pots off the chassis). PPS: I've also run the center tap from the PT HV to one of the 'ground' lugs on the PS Filter multi-cap. So, that part of the circuit is now 'complete' as well. TSD
Many thanks, WC
Attached are a few pics of where I stand now in the process. What I've done so far is to remove the one bias pot, the two 'balance' pots, and the one 12 ohm resistor. I noted that my unit being an earlier A or B model 200, it had this 12 ohm resistor on the center tap side, where as the later 'C' model had a 33 ohm / 7 Watt resistor between the output of the rectifier and the first stage of the power supply. My amp was straight wired from the rectifier to the first PS stage. So, in removing the 12 ohm resistor, I also 'installed' a 39 ohm / 10 watt resistor off pin 8 of the rectifier socket to the PS stage 1. I also added 4 test points for the 4 output tubes, and installed 10 ohm 'matched' resistors between pins 7 of the 6GW8 output tubes and ground....with connections from Pin 7 to each of the 4 test points.
In removing the prior wire wound balance pots, it appears I've got good space to mount new balance pots feeding the input grid resistors. In addition, I'll also need to figure a way to mount 2 'bias' pots in the location where the former one pot was. May take a bit of cobbling, but I think I can get two pots in that location as well. Right now my problem is; What value pots do I use to begin with in establishing a whole new bias feed system for this amp? The old 'single' bias pot was a 10K. Each 'balance' pot was 100 ohms. So, previously I had essentially 50 ohms on each cathode (if the pot was 'centered'), plus what ever 'feed' was coming out of the bias pot. BTW, the function of the 'balance' pots is questionable as to if it was truly a 'bias' related function or if perhaps it was also simply a 'hum' or 'noise' related control. While the amp also has 'hum balance' pots off the filament wires, it was suggested that the 'balance' pots off the output tube cathodes were really more for additional 'hum' balance control than for use in setting bias.
In any event, I'm poised for being ready to start the 'build up' of the bias supply side of things. When I'm done I think this amp will be MUCH more controllable, and given the ability to find 'matched' 6GW8s (or lack there of), being able ti do bias/balance on each channel will be a true benefit. So, if anyone has any suggestions as to where I ought to go from here, please let me know. BTW, I'm looking at a Scott 299B Schematic, and using that as sort of a 'guideline' for doing this project. I noted some REALLY significant differences.....like 220K Grid resistor on the 299 vs 820K on my 200. But....hey....two completely different amps. I'm still VERY new to this 're-engineering' stuff....and don't feel real confident at this point. So, any and all help is appreciated. (note: last pic is a 'before' shot prior to my pulling the balance pots off the chassis). PPS: I've also run the center tap from the PT HV to one of the 'ground' lugs on the PS Filter multi-cap. So, that part of the circuit is now 'complete' as well. TSD
Many thanks, WC
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