Scott 222C questions

[Ziradog]

I bought this from a fellow AK'er, who said that some work had been done to it & that it was working as-is. Both were correct, although it doesn't sound real good. The selenium rectifier has already been replaced, a 33 ohm 5 watt resistor replaced an 18 ohm, and the coupling caps were replaced with generic yellow film ones. Since I needed to order some parts for another project, I decided to get a parts list together for this too, even though I am still working on some other projects.

So some questions:
1. Some posts mention adding four 20 ohm, 2 watt resistors. Can someone spell out what this is about, pros & cons?
2. Current limiter? I have spare CL80's but see different ones used.
3. The diagram on the bottom of my case says it takes two 7199's, but has 6U8A's. I know that many folks switched from 7199's to 6U8A's or other similar tubes, but how do I tell if the correct changes were made to use the 6U8A? I don't see that any resistors were obviously changed around those two sockets.
4. None of the can caps have been replaced. Do I stick with the current values, If not, suggestions? One has two 20 mfd/450v sections & two 25 mfd/25v sections; one has four 75 mfd/75 voltsections, and one has four 20 mfd/450 volt sections.
5. I understand there are multiple schematics around, any easy way to know which version I need? Picture of the underside of the unit is below.
6. Any other specific suggestions/caveats?

Thanks for any help

 

[Brycecb750]

All that stuff is great, but power supply is important, I always replace the cans in my Scott's, you have all the coupling caps replaced, why not give it nice power. I always convert to 6U8's as well, mainly because I have a bunch and to run across them and like the way they sound.
The 20 ohm, are you talking about replacing the 18 ohm ones in the preamp heaters/bias? If so, I some times use 22 or whatever to set the bias, that will depend on the voltage increase after replacing the selenium rectifier and recapping (I replace four 75/75 can with four 100/100 caps under the chassis), always seems to have a pretty strong bias voltage after that. So that depends on where the output tubes are running.
I bump up filters down the way from the rectification, you can do that too much closer to the rectifier tho, can be hard on the tube in the first few stages.

 

[Ziradog]

I absolutely plan to replace the the can caps. Your suggestion to take the 75 mfd to 100 is exactly the kind of info I am looking for. You mention bumping up the other filter caps, what values do you go to (or, how high is too high)?

I understand the logic with going to 6U8A's or similar, but it does not look like anyone did any modifications to make the sockets fit that tube - resistors all look original. I know some 222C's used the 6U8A's from the factory, but the sticker on mine says 7199, hence my confusion.

Thanks for any suggestions.

 

[Ziradog]

Based on the best schematics I can find (none of which quite match my unit), it looks like the sockets are wired for 7199's, with a jumper added from Pin 6 to pin 9. See picture. So I either need to re-wire the socket & change some resistors & capacitors, or get some 7199 tubes. Any idea why the jumper?

It also looks like some of the replacement film capacitors are different values than the schematics I found. Baffling.

 

[gadget73]

6 to 9 would couple the pentode cathode to the triode grid on a 7199. Schematic says it should tie pin 2 to pin 9.

6 to 9 would be correct for a 6U8 though.

http://www.vintagevacuumaudio.com/schematics-manuals/hh-scott/hh-scott-222-c-schematics.pdf

 

[Ziradog]

Gadget, here are the relevant sections of the 2 schematics. The 7199 is the same one you posted, the 6AU8 came from the Scott manuals site. Neither seem to match what I have. What is actually attached to each pin is:
Pin 1 - 15K resistor, .22 mfd coupling cap (should be .1?)
Pin 2 - wire to balance pot
Pin 3 - 330K resistor, 150K resistor, .33 mfd (wrong value?) cap to pin 7
Pin 4 - Wire to pin 4 on output tubes (heater)
Pin 5 - Wire to pin 5 output tubes (heater)
Pin 6 - 220K resistor, wire to pin 9
Pin 7 - .33 mfd from pin 3, 1200 resistor, wire to a 180 cap & 10K resistor
Pin 8 - 15K resistor, .22 mfd coupling cap
Pin 9 - 100 cap, wire to pin 6

 

[BinaryMike]

Your wiring is correct for a 6U8/6GH8. It's possible that information about the change was present at one time, possibly in the form of one or more stickers that fell off. The most critical aspect of this circuit is pentode screen voltage, which has a strong effect on triode cathode and anode voltages, which must be near 25~30% and 70~75% of applied B+, respectively. Scott used a divider to stabilize screen voltage even though RCA recommended a single high-value dropping resistor. I always give close attention to this issue whenever I encounter direct-coupled pentode-triode amplifier input sections. Another issue to watch for, though it's more likely to be troublesome in homebrew amps, is that crosstalk from pin 1 to pin 2 will be in-phase and could easily cause stability problems.

 

[Ziradog]

It would be nice if these schematics had more voltages on them.

 

[gadget73]

Trouble with that is the voltages will vary considerably depending on the particlar driver tube, and the meter you measure it with. Honestly if the plate voltage is close and its making sound I let it be.

And the schematic chunks jive with the jumper. 7199 would be 2 to 9, 6U8/6GH8 would be 6 to 9 for functionally the same connection. A lot of the pentode/triode tubes used the same pinout, the 7199 being the odd man out even though functionally its not that different from some of the others.