The Difference Between Radio Craftsmen RC-2 and Model 2

[Selmerdave]

I was looking at the operating point specified for the Model 2 and I was a little shocked, but perhaps there's something I'm missing? The 6V6's are supposed to be operating with 325V across the tube, 315V at the screens and 42mA per tube, dissipating 13.5W. Maximum ratings I find are respectively; 315V, 285V, 12W. The one that stands out to me is the screens, running 30V beyond the limit.

Now, after rebuilding mine I put in a new matched set of TungSol (re-issue) 6V6's. Both sets crapped out in the same week, about 4 months after putting them in. After thinking about it I realized that the operating point given is a fair bit further than ratings and that might have something to do with the short lifespan of the tubes. I've since put in some older 6V6s but I didn't want to eat those up as well so tonight I put in a 250-ohm resistor before the first cap, bringing the voltages down to 300, 285 and 8.5W dissipation, 28mA per tube. I'll probably change out the cathode resistor for a smaller one at some point to bring up the current a bit.

Does that original operating point seem a little daring or was in common for manufacturers to go that far beyond ratings?

 

[tcdriver]

The 6V6's are supposed to be operating with 325V across the tube, 315V at the screens and 42mA per tube, dissipating 13.5W.

Are these the operating voltages specified in the schematic or as found in your amplifier? If they are the schematic voltages, what are your measured voltages?

One thing to keep in mind is that the standard AC line voltage, back when these amplifiers were made, was 10 to 15V lower than one typically finds today. This results in higher B+ voltages in all classic amplifiers from that period and this can result in higher power dissipation in the output tubes.

 

[Selmerdave]

Are these the operating voltages specified in the schematic or as found in your amplifier? If they are the schematic voltages, what are your measured voltages?

One thing to keep in mind is that the standard AC line voltage, back when these amplifiers were made, was 10 to 15V lower than one typically finds today. This results in higher B+ voltages in all classic amplifiers from that period and this can result in higher power dissipation in the output tubes.

These are specified in the Sam's folder, with the same parts values specified in the RC version of the schematic, which is why I asked about it. My voltages were very close, for example I believe 5V higher on the plates, and I should add that all parts except transformers were new last year. The only values I changed prior to adding the 250ohm resistor yesterday were making the first power supply cap 40uF rather than 60uF, both because of what I had on hand and what makes sense for a 5Y3 rectifier, and slight change in the cathode bypass cap on the 6V6s from 25uF to 30uF.

 

[Selmerdave]

Here is the RC schematic with the voltages given in the Sam's schematic marked in red.

 

[tcdriver]

Here is the RC schematic with the voltages given in the Sam's schematic marked in red.

Thank you, I will check my unit voltages and then post the results.

 

[Selmerdave]

Thank you/ I will check my unit voltages and then post the results.

Yours is a Model 2 or RC2? RC2 voltages are somewhat lower and screens are at around 287.5V, just a hair over the max and probably not terribly consequential.

I first started paying attention to it when the second amp's power tubes crapped out exactly one week after the first, about six months after first use (new Tung Sol's).

 

[tcdriver]

My amplifier is a RC-2. Yes, the schematic is different as well as the voltages. Here is a schematic that I found on the internet along with measurements of the B+, output tubes plate, screen grid and cathode voltages.

Measurements of my RC-2:

craftsmen RC-2 Voltages with VIN = 110V
6V6(1) Pin 3 = 325V Pin 4 = 327V Pin 8 = 17.45V
6V6(2) Pin 3 = 325V Pin 4 = 327V Pin 8 = 17.45V
5Y3 Pin 8 = 339V

craftsmen RC-2 Voltages with VIN = 115V
6V6(1) Pin 3 = 339V Pin 4 = 343V Pin 8 = 18.9V
6V6(2) Pin 3 = 348V Pin 4 = 342V Pin 8 = 18.9V
5Y3 Pin 8 = 354V

craftsmen RC-2 Voltages with VIN = 120V
6V6(1) Pin 3 = 354V Pin 4 = 355V Pin 8 = 20V
6V6(2) Pin 3 = 361V Pin 4 = 356V Pin 8 = 20V
5Y3 Pin 8 = 370V

note: The measured value of the output tubes cathode resistor, R15, is 231Ω

 

[Selmerdave]

Interesting changes with the line voltage, did you use a Variac? At 120 that's quite a ways past all the limits, dissipating 14.5W at idle!

 

[tcdriver]

Interesting changes with the line voltage, did you use a Variac? At 120 that's quite a ways past all the limits, dissipating 14.5W at idle!

Yes, I used a Variac to adjust the line voltage. The Craftsmen schematic specifies 115V at the input. Line voltage here typically runs 123V. I have installed a CL-80 inrush current limiter, but, that only drops the line voltage a little over 2V. I have a buck transformer that I can run with the Craftsmen to drop the line voltage to a safe level. It is either that or modify the power supply.