Major voltage drop in power amp section

[EETStudent]

As soon as I thought everything was good with my 5F2 build, I came across an interesting observation. In my previous thread, I revealed that my screen voltage was about 20V higher(around 320) than my plate voltage. Today, before I experimented with a grid resistor, I measured the voltages again and noticed that my screen voltage was around 180, and my plate around 164. I pulled my 6v6 and checked the voltage coming off of the rectifier. I measured 370V. Put the tube back in, and it went back down to 180. I swapped in a 6K6 for the 6V6, both of which were good yesterday, and I got the same results. What happened here?

[Gtr_tech]

Sounds like yer pullin' a ton of current. Does the output tube redplate? FOr that matter, is it a known good tube? Plate supply will drop a bit under load...thats normal. But it shouldn't be half what you started with. Thats a pretty simplistic circuit, not that much to go wrong. Double check your wiring, make sure theres no DC on the control grid. You do have the 220k to gnd at the control grid right?

[EETStudent]

I am 99% sure the tubes are good. I bought a small lot of 6V6 and 6K6 tubes some time ago, and they all had tested good. Plus, a few days ago I was getting the voltages you would expect. Yes, I do have that 220k control grid resistor going to ground. With all tubes pulled, all transformer voltages check out correctly. With rectifier tube only, all measurements check out and I get about 370V coming off of the cathode(6x4). Other tubes inserted, and everything drops drastically. Could it be the rectifier? It is the only tube that I do not know for sure whether its good or not

[Chuck H]

I don't think it's the rectifier tube. They usually either work, or they don't. FWIW, with 370Vp unloaded, then add the drop from the rectifier tube resistance as current is drawn through it, you should probably see around 325 to 345 Vp. So I'm inclined to agree with Gtr tech that your amp is either drawing way too much current or your PT can't handle the load. Have you measured the bias for each tube???

Chuck

[Alex R]

- you could also check your centre tap voltage to see if it's a fault in the OT.

[EETStudent]

Both power tubes I tried are biasing around 3W, pulling about 20mA. I just ordered a new set up tubes, so we will see what that does. There is no center tap voltage btw

[Alex R]

Oh yeah 5F2, oops

[loudthud]

Verify that the cathode resistor of the 6V6 is the correct value and any cap across it isn't shorted. Verify that the 6V6 is getting not more that about 7 VAC on the heater. CAREFULLY verify that the rectifier tube is getting close to it's rated heater voltage.

[mooreamps]

sounds like "no bias"


-g

[EETStudent]

Loudthud, all of these things checked out fine. Rk is reading 468. Heaters are about 6.7vac on all three tubes. Like Gtr_tech mentioned, this circuit is so simple its hard to go wrong. All wiring is correct, all measurements are correct(except when loaded). Heaters, center taps, grounds are all in place. Could it be an issue with the OT? If not, I have to believe its the tubes

[EETStudent]

Nope, bias is good. Well, its about 20mA under biased at the time, but both the cathode resistor and cap are perfectly fine

[MWJB]

what Gary means is that high current might have been pulling down the B+, did you measure idle plate current during the low voltage episode?

What is your idle current in normal operation.

If it does it again, start at the B+ secondaries, then rectifier & work down the power supply rail, looking for causes of high current draw/loss of voltage to one side of the recto (halving of the voltage looks suspicious)?

[EETStudent]

Heres something that might help out. I experimented with leaving everything hooked up and all tubes in their sockets, and I disconnected the B+ from the second filter cap that feeds the screen grid on the 6v6. Powered up, and I measured that floating wire to have 370VDC on it. Powered down, ran a jumper wire, and it shot back down to 180-ish. What is going on here?

[EETStudent]

Right now its biased around 20mA/3W. Very cold for a 6V6. Previously, I had biased right at 42mA, perfect for a 6V6.

MWJB, Im not so sure I understand your troubleshooting method. Could you explain a little more?

[MWJB]

"Right now its biased around 20mA/3W. Very cold for a 6V6." interesting, out of curiosity, why so cold?

If everything is working fine & properly connected the voltage would usually drop if there was a huge current draw or a short to ground from B+ rail.

If there was no such heavy draw on the power supply, then perhaps a tenuous connection at one side of the PT B+ secondary could see your full wave recto become a half wave & halve the voltage (near enough)?