(08-07-2021, 12:12 AM)Hangman Wrote: Hi ElectroNerd,
I don't remember the resistance exactly, but it should definitely be above 0.062ohms. something in the tens of ohms 20-80 would be my rough estimate.
Check the flyback diodes (D7-D10) these are sacrificial devices that will short if the flyback voltage goes too high, this results in the fuse blowing. if the fuse doesn't blow, its unlikely that these are the culprit, but I thought I'd mention it just in case.
-Hangman
Hi Hangman, thanks for the reply.
I've checked the flyback diodes both with and without power. They check out fine.
When I first powered this gal on I let her warm up, flipped on the standby, then flipped on my 1k signal with volume full down and it popped the fuse. I replaced the fuse and started over. This time everything stayed on, but I had some dusty smelling smoke rise out of the chassis on the side nearest the output transformer. I powered everything off and noticed one of the power tubes had gassed, I found a new/old set of matched 7027s and put them in and it worked, but I started getting some red plating on V5 and V8. Found that R54 had fried, so I replaced that, but wanted to get the plate and bias voltages sorted first before installing brand new tubes.
I've also found that R49 is more than 10% out of tolerance, so that's been ordered and will be replaced, but that shouldn't affect my resistances across the transformer. I've only biased one other amp and had about 60ohms, so I was expecting at least to see this across this transformer, or somewhere in that ballpark.
Now, I was reading that you want about 70% of the maximum plate dissipation. So if I go off the datasheet for the JJ 7027s, max plate dissipation is about 18 watts and 70% of that would be about 12 Watts. I did some math to reverse calculate what resistance I would need to drop 5V across each side of the transformer and to the plate, I came up with 220 Ohms, with a plate current of about 22mA. So I'm thinking even 60 Ohms from the transformer would be quite small.
I'm also having trouble tracking down some 3.6 Ohm Cement resistors. However, Mouser has some 3.9 Ohm ones that will work as I've seen these plate resistors can go up to 10 Ohm and be fine. I'm ok with using the 3R9, but I'd really like to keep the setup the exact same if possible. Know of any good places to buy some NOS components, besides eBay?
