Help with musicman 2475 hd130 ss driver and biasing

[telejames23]

One of the old tubes shorted (Sylvania 6CA7), got new el34 quad and am finding there is no voltage drop across the 3r9 pair off the emitters (cant set bias). Looked everywhere, +/- 16volt line OK, found a diode that was conducting both ways off TR1 (took it out it was fine out of circuit, replaced anyway). Put in 2 of the 4 original (working) Sylvania's with same issue. Amp not biased properly, has a weird fiz-out as the notes decay. Any experience out there know what might cause no voltage drop across 3r9 pair? Checked je1692's with diode checker, test out fine. What gives?? Know I need 25mv across 3r9 pair, getting nothing.

[soundguruman]

Check to see that the driver transistor cases are not shorted to the chassis. Check the plastic insulator that reaches thru the screw hole. Make sure the screw is not shorted to the chassis.
Check the driver transistors. It is pretty common for them to short, I always install new ones.
The fizz sound could either be the bias, the transistor shorted to the chassis, shorted transistor, or a shorted coupling cap in the preamp.

Your MM Trivia:
Have you ever noticed a 60 cycle buzz in the speaker, when the amp is in standby? (standby is off)
The speaker frames need to be grounded, to the same point as the chassis.
Thought you might like to know this.

[telejames23]

Thank you guruman, found that while not shorted across the pins themselves, the center pins on the je1692's are shorted to the metal tops which attach to the heatsinks. I assume this is bad,.....
Is there a suitable replacement for these (2n6488, 6292), as local suppliers don't have the 1692? NTE replacement? Thanks in advance.

[soundguruman]

Thank you guruman, found that while not shorted across the pins themselves, the center pins on the je1692's are shorted to the metal tops which attach to the heatsinks. I assume this is bad,.....
Is there a suitable replacement for these (2n6488, 6292), as local suppliers don't have the 1692? NTE replacement? Thanks in advance.

One of the 3 pins is supposed to be connected to the heatsink. That's correct.
Check that the transistor conducts from the base to emitter and from the base to collector, with an ohm meter.
There should be no other path but those two. If there is reverse leakage, or a short replace them.
If you don't know how to test transistors, get a tech to do it.

[telejames23]

Yep, did that. That's what I meant by diode drop in my initial question, checked OK. Also checked pre for leaky (DC) caps, all look good. WTF?

[Enzo]

If you get no current through the one side resistor, then either the tubes are not conducting or the drive transistor is not turning on. If both sides are not reading current, then something is amiss.

You keep mentioning the "pair" of resistors and a reading across the pair. Just where are you taking this reading? You need to take a reading from the emitter of each drive transistor to ground, not to the other emitter. In other words you need to take the reading across ONE resistor at a time, not the PAIR. Please clarify what you are doing there.

Since your amp works, amplifies, then I have to assume the tubes are conducting at least some. But it never hurts to check. You have B+ at BOTH pins 3 and 4 of each power tube? You should have about +16 at pin 5 of each power tube. I forget what the tube cathodes ought to read, seems to me somewhere in the 75-90v area. What do you get there?

TR1 is the bias trimmer, note from +16v there is a 1.5k resistor and two diodes in series to ground. This forward biases the diodes so they conduct, and about a half a volt across each. That means that TR1 ought to have about half a volt across it, and the wiper grabs some part of that to turn on the transistor bases. D15 is the diode witrh TR1 in parallel. ANy diode with only 100 ohms in parallel with it is going to read "both ways" to a tester.

[Steve Conner]

Did you check the screen resistors on the tubes? A shorted tube can take one out, and the symptoms match what you described.