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  1. 6

    Why was the bright and normal triodes (i.e. V1) designed with different currents?

    A guy named Paul once told me that there are two resistors at the cathode: The resistor that I paid for and the resistor that came for free with the tube. I took the free one to mean the internal cathode impedance and I used that to come up with my own equation for the "upper" cutoff frequency...
  2. 6

    Tube FX loop before or after tone stack

    The official SLO 100 schematic has a note on it that says to parallel the 2.2K resistor with a 1K resistor for a 0dBm send level and to bypass the cathode on the recovery stage. This seems to be something similar to what you are talking about. Maybe you could put that on a DPDT switch to...
  3. 6

    JCM 900 4100 DR DCV at pins 5 high

    Those are Russian 5881/6L6 WGC tubes. They were originally designed and built for the Soviet military and, as such, the specifications were classified secrets. They remain classified secrets to this day so that only those with a need to know in the Russian military have access to them...
  4. 6

    TAD 6L6GCM-STR Bias on JCM900

    The vintage GE info on Pg 3 does not include g2 current. You can tell by examining the Average Transfer Characteristics on Pg 5. The TAD curves are different than the 6L6GC curves, but the current is still clearly wrong on the TAD curves. They are supposed to be very similar to the original...
  5. 6

    TAD 6L6GCM-STR Bias on JCM900

    I think I figured out what is wrong with the TAD curves. Some 6L6 and 7581 curves have grid 1 current on the right side and plate current on the left side with a couple of curves for positive grid 1 voltages. The values in the plate current scale on the left are twice the grid 1 current values...
  6. 6

    JCM 900 4100 DR DCV at pins 5 high

    That is normal.
  7. 6

    JCM 900 4100 DR DCV at pins 5 high

    He is saying that the secondary readings are normal. What about the primary resistances?
  8. 6

    JCM 900 4100 DR DCV at pins 5 high

    The bias voltage is not the same with only the preamp tubes installed, but it is close enough for a preliminary bias setting. After the output tubes are installed and the amp has settled, the final bias voltage can be set.
  9. 6

    JCM 900 4100 DR DCV at pins 5 high

    You must have the tubes installed and the Standby switch closed in order to have the proper bias voltage. Set the bias voltage to "85vdc", install all of the tubes, power up and close the Standby switch (put it in the Play position). Now it should have the expected bias voltages.
  10. 6

    Help: JCM900-4501 issues

    The Points seem to be connected, but that is because the secondary winding looks like a straight piece of wire to a multi-meter.
  11. 6

    Sovtek mig 100 more low end please? too bright

    The 82K is going from the bass pot wiper to the treble pot wiper most likely. The tie point is insulated from the chassis.
  12. 6

    Sovtek mig 100 more low end please? too bright

    From the wiper to ground?
  13. 6

    Sovtek mig 100 more low end please? too bright

    I don't see it in their library. I know I posted that schematic somewhere, but I can't remember where. Was it in a post at music electronics forum?
  14. 6

    Sovtek mig 100 more low end please? too bright

    Probably not causing your problem, but the Presence is not completely off when the knob is on zero. It seems like every Sovtek amplifier was different and none of them were exactly like the schematic. (Nice schematic you posted, BTW. Where did you get that?) You might want to check the...
  15. 6

    Bias voltage by Cap

    Boy, I'm glad now that pdf64 repeated what I said, otherwise I would have been taking this tongue-lashing all by myself. I overlooked the 4.7K grid stoppers. It's up to 612.7K now.
  16. 6

    Bias voltage by Cap

    And some more:
  17. 6

    Bias voltage by Cap

    Here is some more information:
  18. 6

    JCM900 4500 EL34 Red plating/run away bias current

    You are talking about R30, right? R7 is also 56K, but reducing it to 47K will have very little effect on the bias voltage.
  19. 6

    Bias voltage by Cap

    That's where Marshall stole the circuit from. One of those dates back to 1967. I never much liked that design, either. I prefer the Vox one. You know, the one that shunts the positive side of the AC to ground via a diode.
  20. 6

    Bias voltage by Cap

    Unfortunate choice of terminology and symbol, but I know what you meant. The worst case resistance that I see in the bias circuit is 42K. The current from four tubes flows through this resistance, so the Effective resistance is 4 x 42K=168K. Two tubes are connected to each grid leak, so the...

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