Home brew troubles

[Valvehead]

I finally got off my lazy ass today and worked on my amp. It WAS a jcm 800 50w type circuit. I re-wired the preamp and added a tube stage and built it similar to the Bogner extacy OD channel.


Here: http://www.schematicheaven.com/newam...er_ecstacy.pdf


Anyways, its just not working. Ive been playing with resistors along the B+ rail to try and get close to the Bogner plate voltages for the pre-tubes. When i get the voltages in the ballpark, at each plate, the amp has NO output.

If i go back to like it was before (as far as resistors) , I get some clean output, but it quickly goes into "motorboating" when the gain is turned up or i hit the boost switch.

Note the plate volts on the bogner scem.

Now, heres what im getting :

V1a 49v !

V2a 220v
V2b 240v

V3a 140 v
V4b 290v

Not sure where the PI voltages should be.

Im baffled as to why the amp just has no output when i adjust to get the bogner plate voltages ? Its just going into some sort of cutoff ?

[Chuck H]

That Vp on V1a looks pretty funky. I'd recheck and make sure V1a cathode is actually 820 and not 82 or 8.2 ohms. I'd also check to make sure the bypass cap is not shorted AND in correct polarity.

Not being familiar with the Bogner, I cant say if the B+ for V1a is independant of V1b (or anything else) but you might check componants in that circuit also.

It looks like V1a is drawing major current for some reason.

HTH

Chuck

[PRNDL]

It would be helpful to post all of the voltages - B+ taps and cathodes.

Also, this amp has a lot of gain stages, which means its very easy to overload a stage, or have all kinds of preamp problems.

This preamp is very similar to the SLO, which has a ton of power supply filtering (more than ten electrolytics!).

[Valvehead]

havent messed with the amp for awhile . Last time i adjusted some of the plate voltages and gain. Its getting much better now---still tweaking though !

as far as filtering, im using the 2203 marshall style .