octal 10/12/2017 12:10 PM
Jonson Monarch M-4 (5D8 Fender Twin clone) B+ too high
So, I've got this Johnson Monarch M-4, which is a loose Canadian clone of the 5D8 Fender Twin.
Plugged straight in to the wall, the B+ is stupidly high (500V.) Original filter can cap was rated @500V, I replaced with a 525V cap. I would like to sell the amp, and while I can run it on a variac to get the B+ in line, I can't count on whoever I sell it to having the knowledge to do that, and I want to sell a reliable amp, not a time bomb. I'm using NOS 5Y3 recto tubes (it uses 2 in parallel.) A 250 ohm sag resistor at the PT centertap brings the B+ down to 432V, which is still really high. I'd really like to see something like 370-400V.
I bought a stud mount 39V/25W zener from Weber which for some reason only drops the B+ only about 10V when installed in the CT ground leg.

Ultimately the power transformer seems to be way overwound for this application. I don't want to make the supply too saggy by introducing tons of resistance. 5Y3s are pretty saggy to begin with...

Any ideas for the best way to fix this beast?

g1 10/12/2017 12:25 PM
Is the heater voltage high? A bucking transformer in the primary is an option.
pdf64 10/12/2017 12:52 PM
Is the 500V HT with the power tubes in place, warmed up and drawing HT current?
What are the PT Vac of the HT and yes, heater windings?
octal 10/12/2017 1:51 PM
I thought about a bucking transformer, but didn't want to drill holes. Maybe I should rethink that. Filament voltage is high, something like 6.8-6.9V. I don't have the HT AC voltage on hand, the amp is not at home with me at the moment. Will measure tomorrow and report back.

Yes, the B+ reading was taken with the amp on with all tubes in place. You don't even want to know what the B+ is when you turn the amp on cold...high 500s. There's a shared 250 ohm cathode resistor for the 2 6L6 which has about 32V across it (this is measured with the sag resistor in place and a B+ of 432) so the tubes are drawing a reasonable amount of current IMO- looks like that would be 128mA for the pair, so 64mA each, about 400V from plate to cathode for a total screen+plate dissipation of 25W/tube.
The Dude 10/12/2017 3:01 PM
Have you tried yanking one of the rectifier tubes? Obviously won't fix the higher heater voltage problem, but might give you more B+ sag.


Also, you might want to change the 25V rated cathode bypass cap in the output tube stage.
octal 10/12/2017 4:10 PM
I just tried that this afternoon! It pulled down the B+ a little, but it's still stupidly high, I also suspect this thing is going to sag REALLY badly now with a 250ohm sag resistor, only one rectifier tube- which happens to be a 5y3, which is one of the saggier tubes to begin with.

Great point on the cathode bypass cap- I think I changed it when I got this amp a couple of years ago- but did I bump it to a 50V? I will check tomorrow.
octal 10/13/2017 6:28 AM
I've got 427-0-427 AC coming from the power transformer under load. (Measured with a Fluke 77, I don't think it's a true RMS meter, but it should be close.)

Adjusting the primary down to 115V brings the filament voltage to 6.3 and the B+ to 470 w/o sag resistor and 410 with it.
pdf64 10/13/2017 6:36 AM
Unfortunately I think a bucking transformer, or a whole new PT, may be the best way forward.
Note that HT winding voltage is way above the 350Vac limit of a 5Y3 using a reservoir cap http://www.mif.pg.gda.pl/homepages/f...93/5/5Y3GT.pdf

A single 5U4 (450Vac limit) seems to be a better choice whatever http://www.mif.pg.gda.pl/homepages/f...93/5/5U4GB.pdf