Robinette JCM 800 "Good" "Bad" & "How Ugly?"

[B-Rex]

The “good” news is that that my Robinette JCM 800 powers up nice, bulb limiter shows no shorts and I took readings of all B+ points , bias, and tube pins and things looked really good.

I was using 6L6s, biased at -55 and the 16 Ohm Speaker knowing that my Hammond 290cx PT is a 325-0-325 and more then the recommended Allen Amps (TP 25) 310-0- 310 which isn’t produced anymore and there seems to be a black hole in PTs of that size?

The “Bad” news is when I took off the limiter and took my first B+ reading I was getting 475 VDC and just shut it down. I had no idea it would be THAT HIGH. So now I’m wondering if I shelve the 290cx for another build or try to figure out how to bring the voltage down.

I managed to find a Heyboer APD-2000H from Amp Parts Direct that’s expensive but has the right taps, I’m waiting for information because in the spec sheet its listed as 325-0-325 and in the description its listed as 310-0-310? Anyhow any thoughts/good ideas on the best way to move forward would be appreciated. Thanks!

[pdf64]

... when I took off the limiter and took my first B+ reading I was getting 475 VDC and just shut it down. I had no idea it would be THAT HIGH.  ...

I don't see a problem per se.
With the output valves biased to cut off, bu definition they won't be drawing any HT current, and so the transformer HT winding voltage will rise towards its no load level, 357-0-357, which will rectify to 500V DC.

I suggest to get the output valves suitably biased to a reasonable idle current, and reassess the voltage levels.

My guess is that the Allen transformer HT winding voltage is noted at no load or low loading eg amp idling, whereas the Hammond is noted at full load.

What wall outlet mains voltage do you measure?
https://www.hammfg.com/files/parts/pdf/290CX.pdf?v=1697661948

[Latole]

To me, using 6L6s, biased at -55 do not mean if bias is the good one.

You must read cathode milli / amp and multiply by anode voltage to bais at 60 %power

[tubeswell]

475V is okay for 6L6GC if biased appropriately. Fender Showman Reverb is around 470

[B-Rex]

Thanks all for having a look much appreciated! With your comments it makes sense to at least see it through and see what I get, even if likely to be louder than I was aiming for. I ordered a Plate Bias Current Probe Tester try to do a little less poking around and make things easier moving forward.

I am getting an odd little whine/squeal coming from the amp though not real loud but I could isolate where it was coming from?

My wall outlet mains voltage is 121

Thank you very much! and hopefully have the tester tomorrow

[Latole]

Thanks all for having a look much appreciated! With your comments it makes sense to at least see it through and see what I get, even if likely to be louder than I was aiming for. I ordered a Plate Bias Current Probe Tester try to do a little less poking around and make things easier moving forward.

I am getting an odd little whine/squeal coming from the amp though not real loud but I could isolate where it was coming from?

My wall outlet mains voltage is 121

Thank you very much! and hopefully have the tester tomorrow

No bias probe;
You can also solder a 1 ohm resistor between the cathode and the ground on each 6L6 and read milli volts across these resistors.
You must disconnect tubes's cathode wire from the ground and put the resistor there.

On a 1 ohm, millivolts = millamps. And adjust the bias .
Resistors can remain in the circuit without disturbing the amp.

[B-Rex]

Thanks for the 1 ohm resistor tip. As I was looking into it I noticed the some suggest  1/2 watt to act a bit like fuse. Is this commonly agreed or is there a downside to consider?

[mresistor]

1 ohm 1% 1/2W current sense resistor on the K is a great idea to add to make bias checking easy and safe.  Here's a thread on it.


https://el34world.com/Forum/index.php?topic=28604.0

[pdf64]

Adding a HT fuse and using a diode in parallel with a 1R 1W cathode current sensing resistor seems best practice to me.
The diode and 1W rating helps to mitigate over dissipation if an output valve shorts.
https://el34world.com/charts/Schematics/files/Fender/Fender_65_super_reverb_guitar_amplifier_schematic.pdf

[Latole]

Adding a HT fuse and using a diode in parallel with a 1R 1W cathode current sensing resistor seems best practice to me.
The diode and 1W rating helps to mitigate over dissipation if an output valve shorts.
https://el34world.com/charts/Schematics/files/Fender/Fender_65_super_reverb_guitar_amplifier_schematic.pdf

[B-Rex]

Thanks again guys. So is the IN4006 correct for my situation, I have IN4007s left over from the other needs in the amp?

[tubeswell]

Thanks again guys. So is the IN4006 correct for my situation, I have IN4007s left over from the other needs in the amp?

1N4006 = 1A 800V rating
1N4007 = 1A 1000V rating

Both can be used if the diode will see less than 800V reverse voltage
But of the two, only a 1N4007 is good for 1000V

[B-Rex]

Thanks again! Great info.

I hope I'm not overdoing/overthinking this but I am using a old chassis that had a tremelo channel that has left me with 2 unused holes in , what is now the power section of the amplifier. I also have left over female banana clip inputs from the home stereo setup. Would there be any down side to use these to wire these resistors and diodes to bias the amp later without having to take apart the amp?

[Latole]

Thanks again! Great info.

 . Would there be any down side to use these to wire these resistors and diodes to bias the amp later without having to take apart the amp?

No.
I do it often on many amps I built

[Willabe]

Would there be any down side to use these to wire these resistors and diodes to bias the amp later without having to take apart the amp?

No, don't do that.

[B-Rex]

Thanks!