Hiwatt DR-103 Clone PI Issues

[harry]

Hi there,

I built my Hiwatt DR103 clone and all was great until the other day when a click and a hum suggested something had gone wrong with the power section.

Schematic is here:
http://mhuss.com/Hiwatt/Schematics/DR_Pre4Input_v2.pdf

Sure enough, the scope shows that while the in-phase (82k, pin 1) section of the PI amplifies well when you turn up the volume, the out-of-phase (91k, pin 1) section hits a wall and clips quite early on.

All resistances measure good. Any ideas/suggestions much appreciated as always. Thanks, Harry.

[harry]

Also, removing NFB makes absolutely no difference, changing the PI tube made no difference and changing the 100n capacitor which leads to the NFB didn't help.

Problem persists with power tubes removed.

Bad output clips at 7V P-P
Good one goes off the screen of my scope, over 40V P-P

Changing 47nF coupling caps makes no difference

[jbrew73]

can you post the dc voltages on all the pins of that tube?

[harry]

At the moment:

1 - 255
2 - 0
3 - 4.8
4 - heater
5 - heater
6 - 253
7 - 0
8 - 4.8
9 - heater

Plates are a bit low, I took HT 3 down a bit because it was hitting this tube with over 350V before! Not good.

[jbrew73]

looks like v4 in not getting bias voltage from v3 pin 3. should be about 70 volts on pin 3 of v3 and on 2 and 7 of v4.

any chance that you mixed up the readings for pin 7 and 8? they look like they are reversed or maybe the tube is mis-wired.

[harry]

Interesting, I had completely forgotten to connect that part of the circuit.. but I had never noticed as I'd never had to occasion to turn the amp up that much! Quite sad really. Obviously at lower levels, the PI's response is reasonably linear without being biased.

I'm not sure if this is the problem though! We'll see. It still doesn't explain why it suddenly developed a hum.

Thank you for your help though, jbrew73!

[harry]

Much better.. edge of clipping at 56 watts but now I've fixed the PI problem, the plates are now at about 220/230V (cathodes at 40V).

Thanks again, jbrew73.

[harry]

sure enough, with plates restored to a fair voltage, onset of clipping is pretty much bang on 100 watts. Great.

[jbrew73]

you are welcome. glad you got it fixed.