Thanks for the research Andy and Wayne. The suppression cap and diode worked like a charm. I had to work around Marshalls values to get a range suited to the 7189 and it was kind of interesting how difficult that turned out to be. Normally it's just a voltage divider though in a normal scheme the resistor on the AC side is usually not predictable. With this supply the cap can only support so much total current before it gives up. I had to put pots in place of both the 8K4 and the 47K + 4.7K trimmer. I finally ended up with a 47K between the two filter caps and a 20K trimmer with an 8K2 tail to ground. Gives me about 7 volts to around 18. That's too much range I know but other size trimmers just caused the range to be in the wrong place or the supply to collapse all together. Anything smaller than 47K in the first part of the divider put the range too low. Oh, I'm running about 13.8V and about 24ma per tube for best tone.
The amp is really coming around now with the usual unlimited sustain and constant volume with marvelous harmonic content added by twisting the guitar volume control. The express tone stack works better than the Liverpool also with very fine adjustment available to bring out individual notes in the chords. It really snarls, with none of that nasal drip that it had with the 130 ohm cathode resistor. I'm not saying you can't get to where this amp is with a cathode bias scheme but I think it needs a higher grid center for the preamp voltage to swing across.
I've got a little noise I'm tracking down now. I suspect the new owner will be expecting a package next week. I'm going to do another one for me that's for sure. I'll take pictures before I ship it off. It's up to him to post clips. I'll just shame myself if I try.
Dan
More on bias etc.
Moderators: pompeiisneaks, Colossal
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: More on bias etc.
Last edited by skyboltone on Fri May 30, 2008 5:41 am, edited 1 time in total.
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: More on bias etc.
double. wierd.
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: More on bias etc.
I don't think the owner would mind if you posted clips. He may not have any way of doing that himself. 
He'd love to see more pics too.
He'd love to see more pics too.