The Amp Garage

No no. I was just letting you know that before I built anything, I did the math and found that a 50R resistor in series with the primary would drop the B+ to something manageable while still leaving the filament voltage at an acceptable level. There's nothing really inherently wrong with the amp. Mo...

Filament voltages are about 5.5 or something. It's within tolerance though. I measured the unloaded voltages on the PT before starting the build and did the math to determine resistor size. I know I could regulate B+ using mosfets, but like I said earlier, not to familiar with SS. Zeners would be pr...

Alright. Still working out some bugs. Got a nasty oscillation as I turn up the master. It's more prone to it as the gain and tone knobs go more clockwise. I set it to where it's oscillating a small amount and then chopsticked around to see if I could get rid of it. When I fiddle with the input jacks...

Alright... I hope this isn't too confusing. Rt is dropping 50 ish volts. If I measure from the cathode to grids, I get 1.2 volts. Here's where it's unfamiliar territory for me, though. Measuring from the "top" of Rt to the grid, I get 35 volts. In a normal gain stage (with normal z in) this would be...

Got some good news and a question. Not a whole ton of hum now. Got the proper shorts sorted out. Amp is actually passing signal and playable! :D Here's my question, though: The PI grids (both of them) appear to have about 35VDC on them when referenced to ground (although it will be dragged down beca...

Phoenix, AZ... 73 degrees F.

Alright. Sounds good. Man... I'm going to be really frustrated if this build ends up sounding average.

I know that the grids get drawn down by the DMM, which I would imagine makes it even worse! I mean... if 13VDC at the least is getting drawn down... :? I don't even know what to look for now. I've pulled up this board so many times and had to rewire it. I know that the PI has been a huge problem for...

Alright. After a very busy work week and the holidays finally being over, I finally got around to putting some very long 18ga solid core leads on another 9 pin socket so that I can measure the voltages on the pins without having to rip up the board again. Here they are. V1 (parallel triode gain stag...

Just found something else interesting. The .02uF power tube coupling caps. The one that's connected to the Ra with the messed up voltage has only 45VDC across it while the other has 175VDC across it. The 175VDC seems more correct to me because one end is connected to an anode and the other end at gr...

So after unwiring the board and taking a good holiday break from it before rewiring it back up, I'm still having the same problem. Reflowed any suspicious looking joints on the PI socket and I'm still dropping about 192VDC on the same anode resistor with the tube plugged in or unplugged. Chopstickin...

So here's a bit of an update. The 22k resistor was swapped out when I posted that last comment and there was a change in cathode and anode (obviously) voltage. No change in signal though. Found a cold joint on one of the EL84 cathodes that was the cause of the filament voltage going in and out for t...

Hrm... I think most of us here actually have... you know... a job. And aren't squatting in our mom's living room.

Absolutely. I'll probably get those voltages posted tomorrow. Also,I need to specify there is not 310VDC sitting across that pot There should be somewhere around 6.3VAC with the heaters elevated, but I'll check tomorrow. Thanks for the help and insight so far guys!

Alright. Couple things have changed. The 4V on the V1 cathode somehow changed to 2V. Cathode resistor was confirmed at 1k5, although the "220k" anode resistor was actually 22k. :oops: Haste makes waste. Noise is still present, and now the PI is acting funny. Rt is now only dropping 30V, Rk is droppi...