Regal Music amp

[lowell]

This amp is an old oddball. Guy brought it in after attempting a filter cap job. It say Regal Musical Instrument Co and the speaker is a 8" Rola.

I've rewired it correctly to the best of my knowlegde w/ no schematic but it still doesn't work. There is an odd preamp tube (big bottle) that has a 76 on it. The power tube is a 6L6 and the rectifier has an 80 or 8C on it, I cannot read it. The rectifier is a 4 pin tube w/ 2 of the pins a bit skinnier than the other 2. There is also another big tube next to the preamp tube that has a big shield over it and a ground wire from the tip of the tube to inside the chassis.

At this point I'm only getting 170v on the plate of the 6L6 and 140v on the screen grid, and 6v on the cathode. It's single-ended.

If anyone has any info or schematic for this please let me know. I can't figure out how the preamp is wired as I don't know what pins are what. I kind of can figure it out but my brain is starting to hurt and I don't know if the owner has messed w/ any of that wiring.

[g1]

Those 2 digit tubes are very old. So old I'm wondering what a 6L6 is doing in there. Do you know if it worked before the recap attempt? You said 6V on the cathode, what does that translate to in terms of current?
Any weird stuff like field coil/choke on spkr?

[Enzo]

76 is a triode, and 80 is a rectifier. You can find a data sheet for either one on Duncanamps.com in the TDSL.

Here is a ton of tube resources: Tube Data

The amp will be simple, and knowing the tube bases, you can trace out the circuit easily enough.

The shielded tube may be another triode, possible a pentode. The clip on the top is probably a grid connection. Look on the tube for a type number. Also, in that era, sometimes the sockets had the tube type stamped in the center.

[lowell]

thanks guys... I'll check down these paths and report back. G1 yes there is a choke, which I wired b/t plate and screens for the 6l6. The OT is mounted on the speaker. Guy says it worked pre cap job.

[lowell]

Ok so here's what I got. I finally have some noise when I touch the 6L6 w/ my meter probes. Seems the power amp is functional. When I removed the 76 tube that's when sound came back. The plate voltage on the 76 was around 10v until I removed it and then it came up to 100v. There is 200v on the 6L6 plate now. I then found a shorted cap in the 6f8 supply and removed it. The 6f8 still only had 7v on its plate until I removed it. Now it's 80v on the plate with no tube in the socket. So from what I can tell, it seems that both the 76 and 6f8 tubes are bad if they're drawing so much current as to load the supply that much. Seem right? I checked cathode resistors and they're fine. Grid leak connections seem good.

[Enzo]

Look at the grids of those "bad" tubes. Is there positive voltage there? A leaky coupling cap from a previous stage could put +V there, and that will turn the tube on hard.

[lowell]

Good point. Hopefully thats it.

[lowell]

So the coupling cap from first to second tube is indeed leaky, however the voltages are still low even with that cap disconnected.

[Enzo]

Bite the bullet, most likely every cap in it needs to be replaced. If you have to replace any of the couplers, then they are probably all waiting to die.

You have cathode resistors and plate resistors. If you measure the voltage drop across each you can determine the curent through it by Ohm's Law. That will tell you if the tube is dragging the voltage down or if the voltage is just going away before it gets there. Look for series B+ resistors that are way high in value.