Motorola VT-71 (TS4H)

[fox_rivers]

I haven't replaced the non electrolytic caps yet I just expected more then 1 tube to light at this point. I'll start replacing them and see if I have any luck.

[bandersen]

That's a bit odd since the tube filaments are wired in series. If one is glowing, others would have to be as well.
Unless perhaps there's a short somewhere causing the power to bypass some of the tubes and fry the 12AT7.

[kvflyer]

I believe there are two filament strings. If the ballast is not wired correctly, at least one string may not have power.

[fox_rivers]

Yeah I would think that if there was a problem that one string would only light. I don't understand this issue.

[polyphase]

Originally Posted by fox_rivers

Cool. Moving the ac line to 6 did the trick but now i only have one tube that illuminates which is the 6201 (12AT7). When i turn the lamp on the bulb glows dim and only the 6201 lights up but after a few seconds after the tube warms up the bulb suddenly brightens and the 6201 tube gets much brighter then its supposed to be. No other tubes light and this issue might explain why the 12AT7 that was originally in the set was dead.

Strange behavior indeed. With only a 40W bulb in series with the set, I doubt the tubes would glow visibly at all, although they may get hot enough to function, which would explain the sudden brightening of your bulb. However, the only thing I can think of that would cause a filament to get brighter when DC flows is a defective tube, or perhaps a filament-cathode short at the socket. Dirt in the socket could be an issue as well.

For now it might be useful to disable the DC supply so as to sort out any filament supply issues (If you want to see the tubes glow, you will need a bigger light bulb). The 6201 actally has two filaments, connected in //. Check that pins 4&5 are connected together.

BTW,if you are using the cap based ballast, it will take longer for the tubes to 'light up' in an obvious way, and they will begin to function before fully lit.

[polyphase]

re: Cap based ballast

Earlier in this thread I suggested that the value of the 30h 5w resistor in the ballast be 'tweaked' downward to set ideal string currents.

In fact this resistor is not in the filament circuit at all, it is part of the DC supply. I was writing from memory, and in my arrogance (or senility) I didn't check my notes or the schematic to verify this resistors function.

The correct value for this resistor is 10 to 20h, 5W. The 30h would overheat as originally stated.

My apologies for this lapse.

[fox_rivers]

Damn... No wonder why is set is misbehaving. Turns out there was 6SL7 where there should have been a 25L6. I assumed it was correct but I didn't know for sure since the print was barely legible. Thats pretty frightening.

[bandersen]

Ouch! That'll mess things up. I hope all the tubes in that string survived.

[fox_rivers]

I tested all the tubes and they are fine luckily. A 100 ohm resistor under the incorrect tube had popped in two at some point before I got the set and that wrong tube would explain that failure.

They must have figured since it fit in the socket that it was suitable replacement. Shame on me for not checking in the first place. *shrugs*