heater voltage missing - help needed!

[jimmy74]

Peavey classic 30 1995 model, put in the bias mod shown here:

http://blueguitar.org/new/articles/blue ... rigmod.pdf

I did follow the advice about the 1watt 1 ohm resistor replacing the jumper instead of cutting the trace. Bias voltage is there, b+ voltage is there and LV is there but no heater voltage( the tubes don't light up). What can the problem be? The amp was working before I did the mod but was eating up the tubes.

Schematic is here:
http://www.bustedgear.com/images/schema ... sic-30.pdf

thanks
J.

[jimmy74]

actually I just tried putting all 3 preamp tubes in and the heaters light up very faintly on all three. what's going on here?

[tsutt]

Your link doesnt show the bias mod.

[jimmy74]

http://blueguitar.org/new/articles/blue ... 0_mods.pdf

scroll down to page 35

[rdjones]

actually I just tried putting all 3 preamp tubes in and the heaters light up very faintly on all three. what's going on here?

Visible filament glow brightness can vary by quite a bit on various preamp tubes and is normal.
If they're glowing at all and the voltage is correct I'd say it's OK.
Check the voltage on the filament winding coming right off the power transformer's leads.
Compare that to the voltage at the farthest tube in the filament string to see if there's any drop.

rd

[jimmy74]

well as I said above, the only way to test the voltages is without the tubes in, and I'm getting correct reading for everything except the heaters that seems to be completely absent. There aren't any filament wires coming off the PT as one would expect, I think that the LV windings before the rect.bridge branch out to the heaters through two 3r 5W resistors.

thanks
J.

[Firestorm]

Looks like the preamp heaters are wired for 12V and put in series so probably running off 36VDC. Are they connected to the -36 volt source that feeds the bias supply? If you didn't do the bias mod just right, you may have reduced the filament supply enough to nearly shut down the tubes. Double check.

[jimmy74]

Yes well looking at the schematic the LV section that also feeds the bias supply does send off the heater voltage before the BR. I will do another checkup tomorrow but I'm fairly sure that I followed the bias mod correctly wiring the 3 wires to the right place and replacing the right jumper with the cathode resistor.
Another strange thing I found out is that there is no continuity between pins 4&5 and 9 of the preamp tubes and 4&5 of the power tubes... would the 12v series wiring explain this?

[Firestorm]

Pin 9 is not used. The output tubes seem to have a separate supply.

[M Fowler]

I just worked on an old Peavey classic 30 and have to say this amp is a killer tone machine for me anyway, in stock form.

I could not believe how well I liked this amp. I still have it waiting for payment and hoping he doesn't want it back.

Mark

[cbass]

Peavey has some super underated stuff and a lot of crap too.I snag up any of the old ones I can find for cheap.Very good Iron and I like scorpions.

[jimmy74]

well it seems as though you were right with all tubes in the all the tubes light up properly and B+ is around 325vDC. The bias mod? I truely think that adding the cathode resistor is pretty useless since all the measurements have to be taken with the circuit board mounted inside and that means the resistor or test point is impossible to reach. Can anyone tell me atleast whic resistor of the bias mod will give me the necessary bias voltage? Off the 22k resistor I'm getting around -10v and off the 39k resistor around -14v. Or would using the good old bias tester be the easiest thing to do?

thanks