Tweet
I am working on refreshing a Masco MU 5 amp from the 40's. It's a very simple single ended circuit. I replaced the filter caps and coupling caps and none of the resistors seem to be out of spec. Most of the voltages seem to be within 10-20% of where they should be except for the plate voltages at V1. I am getting a reading at each plate of a little over 100V but the schematic calls for 215V here. The output seems a bit lower than I would expect for this amp. I replaced the tube and plate resistors and double checked the preamp filter cap with another capacitor but still getting the same reading... When I remove the 6SL7 tube and measure pin 2 or 5 I am getting about 240V. Any idea why the plate voltage is still low?
-
-
I am very much a newbie, but i would check/replace C4. -
Coupling caps were replaced...Originally posted by Foxfire View Post
I am getting normal voltages at the cathodes of V1 and V2.Comment
-
I assume you mean the two cathodes of V1?
SO you get about 0.9v at BOTH?
The current through the tube should be the same, plate and cathode. Calculate that current. If you know the cathode resistors are indeed 1000 ohms, check the voltagte across it and calculate. Then the voltage across the plate resistor and its resistance, calculate the current there. SHould be more or less the same.
Meanwhile, I don't believe your schematic. It says the screen of the 6V6, pin 4, is at 215v. OK, but then dropping through the 27k of R9 in the B+ supply rail, then through a 100k plate load, and we are to believe the voltage is exactly the same? Only if zero current flows through V1.
For sake of argument, I will assume 0.9 milliamp of current through each triode. I did that because the schematic calls for 0.9v at the top of the 1000 ohm cathode resistors. The pair of triodes then draws 1.8ma through the 27k resistor. 1.8ma through 27k gets me about 48v dropped there. Then from there, each 100k branch to a plate. 0.9ma through 100k means a voltage drop of 90 volts. SO I could reasonably expect a plate voltage of about 80v. I'd say your 100v isn't far off.
SO what voltages are there at the two ends of that 27k? Whatever is at the left end is going to be higher than the plates can be. And the 100ks will drop a lot more.
VOltage readings without the tubes means very little.Education is what you're left with after you have forgotten what you have learned.Comment
-
i really do hate it when i forget to read everything before i open my mouth...Originally posted by hearforever View PostComment
-
Ditto what Enzo says, 100vdc at 6SL7 plates seems ball-park, I'd be worried if I had 215vdc.Comment
-
There's something screwy with that schematic. It shows 275V at pin 6 of the 6V6 where there is no pin connection.Comment
-
Such discrepancies are pretty common on schems...pins 4 & 6 could be linked on the actual amp (perhaps a filter cap & the B+ supply resistor is mounted on the power tube socket?). A schem is drawn, then the amp is built....hopefully they all end up concurring at the end of the day, but it can be a real headache keeping up with revisions.Comment
Comment