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Hi,
I'm finally getting around to refurbishing a Scott 222B that's been sitting on my shelf for the past year or so. I replaced all of the electrolytic and film caps and brought it up on a variac today to check out the voltages and make sure everything looked as it should. Well, overall not too bad but the 6BL8 phase inverter does not look right to me; the voltages are really off. One thing I noticed right away is that the plate resistors for the triode and the pentode are 220k ohms instead of the 100k ohms shown on the schematic. The voltages on the plates are not even close to what is given on the schematic: 40 VDC on the triode (should be 78 VDC) and a measly 7 VDC on the pentode (should be 60 VDC). The cathode voltages are both close to spec. Could this amp possibly work under these conditions? I guess I could try playing some music through it and see what happens. Should I change those 220k ohm plate resistors to 100k ohm as specified on the schematic? Any advice on how to proceed would be much appreciated. Thanks!
Bill
Follow Ups:
The only schematic that uses 220k plate resistors is the 222c and that uses a 7199.Make sure this is 222B and if it is,put the right plate resistors in and check the AC balance pots.That ties the two plates of your 6BL8 together and if you move that pot,see if you can change the voltage on the plate of the triode and pentode section of the 6BL8 tubes.You can do this even with the 220k resistors in it,
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 08/28/16
This is definitely a 222B. The circuit matches the schematic except for the 220k ohm plate resistors. I'll try moving the AC balance pots and see how that effects the voltages. Also, just to be safe I will change the plate resistors to 100k ohm. Thanks.
How did you make out?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Are the voltages the same on both 6bl8's? What is the DC on pins 2 and 3?
If the plate voltages are different on the 6bl8's do the 220K's measure 220k?
Hi,
Both 6bl8's are measuring the same. About 40 VDC on pin 1 (the triode plate) and 7 VDC on pin 6 (the pentode plate). Pin 8 (triode cathode) is 1.9 VDC and pin 7 (pentode cathode) is 1.5 VDC. Pin 3 is at 93 VDC. Pin 2 I haven't measured yet. The "top" of each 220k ohm plate resistor is at 312 VDC. All four 220k ohm plate resistors measure at about 178k ohm (but they are not isolated which may be causing the lower reading). So both tubes are behaving the same and both have current running through both halves. So maybe this is what the Scott engineers had in mind??? I've heard that the Scott schematics are not written in stone, i.e. they changed from time to time. I've run the amp and it sounds quite nice, definitely the circuit is working. So I'm inclined to just leave it as is.
If it's not broke, don't fix it. Seems reasonable.
If you are back in there for something else, check pin 2, I'm just curious.
Earlier Scott units, like the 222B, employ a paraphase splitter. Later models use a pentode voltage amplifier DC coupled to a "concertina" phase splitter. Dyna and Sherwood used that setup too.
You can look at later Scott units and "clone" their power section small signal circuitry. Don't be deterred by 7591 "finals". Small signal circuitry that works for 6BQ5s works for 7591s and vice versa. Several manufacturers took advantage of that fact to rebundle existing designs, while producing greater O/P power.
Eli D.
You changed the electrolytic caps in the power supply? Check the voltages off those PSU caps versus the schematic. May have a wiring or grounding issue.
Good luck!
Hi,
Yes I did change all the caps in the power supply. Here are the readings I got for the different nodes (in parentheses) along with the values shown on the schematic:
node 1 (right after the 5AR4)- 375 VDC (378)
node 2- 365 VDC (372)
node 3- (to center tap of output transformers)- 350 VDC (361)
node 4- (pin 9 of 6bq5)- 310 VDC (343)
node 5- (to the plate resistors of the 6BL8)- 280 VDC (312)
node 6a- (to pin 3 of 6BL8)- 105 VDC (93)
node 6b- (to the plate resistors of tone circuit 12AX7's)- 270 VDC (271)
node 7- (to the plate resistors of the phono amp 12AX7's)- 205 VDC (219)
So the voltages don't seem to be too far off, maybe a bit high. I actually plugged the amp in and played some music through it, doesn't sound too bad. I'm really surprised this thing will work with these low voltages on the 6BL8 plates. I guess I'll leave everything as is for now since it seems to be working. Thanks.
Bill
I would definitely make some adjustments even though it may pass a signal with those voltages. While the voltages coming from the can caps are within spec (+/- 15%) the 6BL8 voltages are way off.
Have you tested the 6BL8s? If the tubes are worn out that could have some influence.
I would try using the resistor values listed on the schematic as a starting point and then make further adjustments if necessary. A resistor substitution box is really handy for this if you have one. I just used mine to tweak the voltages on the 7199s in the Sherwood S-5000 I just finished restoring. On the pentode section the plate and G2 voltages are interactive so you may have to tweak both resistors a few times to get the combination of voltages that are reasonable.
I suspect the voltages are way off due to the change from 100k ohm plate resistors to 220k ohm plate resistors. I guess the question is, is this really what the Scott engineers had in mind? They look original so I assume they were installed there on purpose at the factory. Changing them out would be a pain so until someone tells me this is definitely a poorly designed circuit I'm inclined to just leave it as is. Like I said, the amp does sound quite nice.
Well, I can guarantee they didn't have 7 volts on the plate of the pentode in mind! The voltages you have can't be resulting in reasonable operating points.
I would assume the voltages listed in the schematic are correct and going to the 100 ohm plate resistors will get you closer. As I said, you will likely have to play with the resistor values depending on the condition of the tubes.
Replacing all the power supply caps doesn't do much good if the operating points of the tubes are way off.
I think you're right; 7 volts on the pentode plates can't be ideal. Maybe the person who assembled the amp just plain goofed up. I'm really surprised it sounds as good as it does. I haven't really pushed it yet so maybe at higher volumes it'll fall apart. Just to be safe, I think I will change the plate resistors to 100k ohm, maybe it'll sound even better. Thanks for the help, much appreciated.
Please do post the results after you change to the 100 ohm plate resistors.
I have a 222C that I found at a flea market a while back and now that I've finished with the Sherwood S-5000 I'm getting the itch to finally go through it. My 222C is apparently the 3rd version that uses 6U8s (or 6GH8s) in that position so it's bound to be a bit different from your 222B.
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