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There should be a wattage reduction, not gain with the EL34/6CA7 tube. Plate dissipation is not as high as the good old 8417. Problem is getting a decent 8417 tube, and I have a JB-200 that needs 2 new tubes, also originally 8417. Going to be completely rebuilding the JB 200 as a project...

That's funny. I am not sure about the 220pf grid load cap and the 3.3K ohm resistor. I don't think mine has that. Must be for the JB12, not the 150. The relay thing is different in mine also, the bias is switched on the relay contacts in the middle (and lower left side of relay). The...

Finally got it working... I guess the tube is a little on the dead side, with the proper bias (-16V) after I fixed the voltage divider circuit, amp was more of a power absorber than an amplifier. Finally gave up and changed the bias to -36VDC on TX and RX, and amp works fine. Apply a...

Update: Finally got the bias straightened out on recieve. The wires were not in the right places on the relay for switching from -36VDC to -16VDC from receive to transmit. Tube no longer heats up during receive, bias stays at 35.9-36VDC. On transmit it's a little high, about 16.5-18VDC...

Someone else had said I could just run -47VDC to it all the time and not worry about switching it from TX to RX, just always -47VDC? Is that right? Thanks for the help guys, I may try some zener paralleling, just till I get the bigger part in or do something else to make the bias voltage.

Okay, looking at the schematic now, please since you said I have to visualize it backwards because of the FCC thing, should the tube have: Receive Bias: -32VDC Transmit Bias: -16VDC Or should it be the other way around like the schematic shows? Thanks again for the help. Greatly...

So I forgot to get my 5W zener diode order in on time to get them today, will have them next week. Question, will 1W Zeners work? I don't suppose I could parallel a couple? Isn't the thing that the first 1 starts to conduct and the others just sit there or will they actually work in parallel...

They actually look pretty good, though at some point I will probably replace them and a bunch of other stuff in there too. Someone tried to wire a radio shack 120VAC lightbulb into this amp as an indicator light to show the amp is "on" and they hooked it to the HV side, it smoked bad, and made...

I am assuming the ones in this amp (zener diodes) are 5 watt, they are pretty big, at least 3 times the size of the zener diodes I have here, which are not 1.0A as I stated earlier, I meant 1W(watt). So I don't know if that's big enough for the bias current, I did not measure it yet. Will...

Bias voltage in receive starts out at about -23.5VDC and after a few mins when I start to see the tube getting red hot, it has dropped to about -15VDC. You can watch it progressively drop. What should I do? How many amps are those zener diodes that are in there now? I have tons of new...

This is in reference to "chassis ground" I assume?

It looks like there is 2 diodes, and 2 resistors in that bias circuit. GE 5ZD15 GE 5ZD22 220K resistor 100K resistor The resistors tested good, the diodes passed the diode test function on the fluke, but I am wondering if one of those (probably the 15V) is leaking... I am assuming...

One thing I noticed was that someone has been fooling around inside this thing before, looks like they used some crappy lead free solder or some pipe solder or something really lumpy and chunky when you heat it up... I resoldered everything in the bias circuit, going to try it again. So I...

If the relay contacts were crappy, and it was losing connection on the recieve bias, would that cause it to do this? It's the center set of pins on the relay that switch the biasing, I can try cleaning them, but honestly the relay looks like it's been through WW2 and then some.... Maybe I...

Okay, here's an update on this. When it starts to glow really bright (during receive not transmit) I key up, and it stops getting brighter. I unkey and it goes right back to getting progressively hotter and hotter glowing red, until I key up again. So what is going on with that? Should...

Well, I have gone over it pretty well, maybe a few things were touching, or the tube wasn't making good contact in the socket or something, because now it seems fine. But yeah, please do post the info for me so I can check that.

Trying to figure out the schematic, it looks like the Bias is fed to the 8417 or at least changes voltage when you key up (T and R markings on schematic where it says "Bias" -32(T) -16 (R). Could the bias not be turning off, a problem with the relay? The relay inside looks raggedy, like it...

Schematic for the exact amp I have attached.

Update. Sat and watched it for about 5 mins without taking it off standby, keying up, or anything, and the tube started glowing on it's own, turned it off, right back on, starts immeadiately glowing real hot again, turn it off for a few mins turn it back on, and it takes about 3-5 mins again...

Okay, so I got my supposed "JB12" in the mail today. Turns out it's a JB150. The older one with the 6AQ5A keying tube. Had an RCA 8417 tube in it. After repairing a few broken solder connections inside and replacing the keying diode (1N60P, was 1N34A, and was broken) I talked on it for a...