|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