Scott Stereomaster distortion Type 340 Tuner/Amp

Low current draw in the triode. Basically anything off at pin 9 will seriously mess with how that stage balances and performs. Yours seems to be drawing so little current that I'm not surprised its not working well.

Check the voltage at pin 3. The pentode is direct coupled to the triode, and the voltages at the pentode plate are strongly influenced by the screen voltage at pin 3. Verify the 270K and 100K resistors are on value. If those are good, one option would be to adjust them to get the voltage at pin 9 where it needs to be. If pin 9 is close to right, the rest of it should fall in line nicely.

Trying other 6U8 or 6GH8 tubes may also yield some joy. The annoying part about those pentode/triode tubes is they are often pretty variable in terms of performance. Luckily yours happens to use fairly available and inexpensive ones so it won't be the end of the world to try others.
 
Low current draw in the triode. Basically anything off at pin 9 will seriously mess with how that stage balances and performs. Yours seems to be drawing so little current that I'm not surprised its not working well.

Check the voltage at pin 3. The pentode is direct coupled to the triode, and the voltages at the pentode plate are strongly influenced by the screen voltage at pin 3. Verify the 270K and 100K resistors are on value. If those are good, one option would be to adjust them to get the voltage at pin 9 where it needs to be. If pin 9 is close to right, the rest of it should fall in line nicely.

Trying other 6U8 or 6GH8 tubes may also yield some joy. The annoying part about those pentode/triode tubes is they are often pretty variable in terms of performance. Luckily yours happens to use fairly available and inexpensive ones so it won't be the end of the world to try others.
I did check that voltage on pin 3...it is at 51 volts. I will check those resistors. Could that resolve the very high voltage I found on pin 1? Thanks.
 
Voltage at pin 3 is the screen. Screen voltage strongly influences voltage at the plate of the pentode which is connected to the grid of the triode (pins 6 and 9). Higher voltage at pin 3 will make for lower voltage at the plate of the pentode. Grid voltage at pin 9 will determine voltages at pin 1 and 8. Basically the pentode biases the triode, and if the triode is way off the phase inverter behaves badly. It doesn't have to be dead nuts on the schematic values, but it should at least be in the neighborhood and yours are not.



Actually just for S&G, would you mind measuring all of the pins on the 6U8 and posting the readings? Wouldn't hurt to measure both channels too. I'm curious how it compares overall to the schematic.
 
It's not common- but it's not impossible that the 100pf cap between pins 6 and 9, and ground, is bad. If that's leaking, that would pull the voltage at pin 9 down to a bad level.

Also, check the tube socket- make sure that there's not a bad connection on pin 6. That could block current from flowing properly in the pentode, which would upset the voltage.

Once you make sure that's not an issue- I would then do what it takes, to get the voltage on pin 3 down somewhat. That will help get pins 6 and 9 up to their proper level (about 60v), which should then get pin 8 down to its proper level (about 185v).

Regards,
Gordon.
 
Greetings. I'm recapping a 340 (not a B) and found this this older thread helpful. I have one question that I think I know the answer to but am looking for reassurance.

I install 10 Ohm resistors between pin 5 of the 7591 and the grounds. On one 7591, they ran a white with black trace ground to pin 5. It attaches to a terminal strip where pin 7 (cathode & grid 3), 8 (cathode) and 9 (grid) of a 6U8 connect through various resistors.

Should the 6U8 ground stay with the black ground wires or remain on pin 5 and ground through the 10 Ohm resistor? I'm guessing it stays with the grounds or it could affect the reading measured across the resistor for that 7591, but not sure. Thx

20230925_210751.jpg
 
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When you add the 10 ohm resistors between pin 5 and ground, you raise the voltage at pin 5 above ground/zero potential. You want any other components that were previously grounded at pin 5 to be re-routed to the true ground bus at a point closest to the original connection, including the 6u8 connection you mentioned and the 330K grid resistors for the output tubes.
 
Thanks Dave! Another question. I've read some conflicting info on the Scott 340, or 340'A' because it's not a B.


"The bias supply in the Scott 340, like similar sets, is by way of a small full wave selenium bridge rectifier. I checked its output voltage. The manual and schematic do not show the voltage output from that bias rectifier, but it supplies the DC filament voltage for four 12AX7 series-connected preamp tubes. That means the supply must deliver about 48 volts output after the filter capacitors and resistors."

Output from the 75uf 75v x 4 daisy chained can also supplies the two DC balance pots for the 7591 tubes and the first 12AX7. (I used four 82uf 160 v and replaced the four out of spec 18-ohm 1 watt resistors). Schematic says -40V after that can and that is also mentioned in this thread.

I replaced the selenium rectifier with a diode bridge. I've read some replace the 18-ohm resistor from the rectifier to the first 75uf cap with a 33-ohm resistor to reduce the voltage. Voltage with the old selenium rectifier was -37VDC. I didn't have a 33 Ohm resistor, so I inserted a 10 ohm in series with the 18 ohm (28 total). I get -48.9VDC output from the daisy chained 75uf with the bridge.

So do I ignore the ohio ed guy and add a larger value resistor to reduce the output voltage to -40VDC? And no, I did not leave it powered on long.

Thx!
 

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