Metropoulos Forum

built a 1987. about 435vdc on power tube plates at 120v.

i believe normal V1 plates should be around 150 (pin1) and 190 (pin 6) right?

i'm running 235 at pin 1. (Thats w/ a tube in the socket.) pin 6 is about 195....so thats prolly fine.

everything else appears to check out ok. wiring...componant values, etc. i dont THINK anything else is out of order.

both plate load resistors are right on the money...or just about. 101k what gives?

could i just use a 115k or 120k, or whatever, to drop the voltage on the plate? not really sure what else to do.

Amp is pretty clean....not as much gain as there should be. thanks-

Try a different tube in v1 and see if the measurements change at all. As for the gain being low, what 1987 circuit did you copy? Do you have a 500pf or 5000pf bright cap installed? What tap is your nfb connected to and what value nfb resistor? And finally, did you use a bypass cap across the 820r on V2? The amp will likely loosen up a bit after using it for several hours but they do have to be turned up pretty loud to get them to overdrive. I have to use a ppimv on mine to get the overdrive I like.

yeah...tried diff tubes. still about 235v. i was think "backwards" though with increasing the 100k plate load resistor- lol
duh......current is flowing the other way! so increasing it wouldnt drop the plate voltage.

i dunno with this thing............

When a triode is reading unusually high plate voltage, it can be a sign that the tube isn't conducting properly. This would normally be caused by a bad cathode resistor or a faulty solder joint at the cathode's ground or somewhere along the path. Yours is not extremely high though and the amp does seem to be playing fine. Different 12ax7's will also show different voltages but you have already tried that it seems. You can play with the dropping resistor values in the preamp circuit but it won't be a drastic change. Lowering preamp voltage will give a slightly browner sound and softer feel. You could also try raising the plate resistor value slightly (150k-220k) which will add some grit.