The Amp Garage

I hadn't noticed that about the suppressor grids. They are referenced to ground but indirectly, thru the bias circuit 82k tail resistor. So at the top of the 82k, the suppressor grids are indeed seeing the -ve bias voltage. I've never seen that before either. If the suppressor grids are more negativ...

Current issue of Tonequest magazine has an interview with Dr. Z. where he discusses the DB4 amp. Without scanning & uploading a couple of pages from the article, which might infringe copyright, I read that Z points out that the 5879 is what Les Paul had Gibson put in the mid-'50s Les Paul GA-40, whi...

Here's the official Randy Bachman's Herzog schem. No idea what the layout looks like, but there may be pix on the net. Shouldn't be too hard to add on the existing chassis. Use one of the extra 12AX7s, and drill out another extra 12AX7 hole for an octal socket for the 6V6. You'll need a step drill w...

OK G400 PAR 5 Channel Preamp is what I have. This appears to connect thru point Y to the power section. Power section shown is only 2x6CA7 and uses 12AU7 PI instead of 6SN7, but notes also refer to 400 models with 4x6CA7. 12AU7 is just a 9-pin version of the 6SN7. I've also included the power sectio...

I think I found the 5-input preamp schem w/ 12AU7 reverb. The 4x6CA7 w/6SN7 PI power amp schem is a bit less well-defined. There are several versions and none have the exact same name as your nameplate but I'll sort thru them and post 'em in the morning.

Since it's a PA its inputs likely need a good bit more voltage in for full power out than a guitar can provide.

Would need a schematic at least. I have the Garnet amps book that came out a few years ago, I'll see if I can find it.

Those power tubes look like '70s GE 6CA7 "Fat Boys". If so, make sure you keep them, they are great sounding tubes! Assuming they are still OK, they are worth probably at least $50 each.

A 100W Plexi with those would be a worthwhile build!

Mine too. I only read the chapter on all-tube op amps so far but the number of *GEBs working on this stuff as part of the war effort is remarkable.

* Giant Earth Brain

And we're only halfway thru the year too....

Awesome! Thanx very much!

I bought a few of them when they were published but missed most of them. Hard copies seem to be unobtainium now.

Anyone ever try a 12AY7 in the PI? Or would that be too little gain?
Current draw would be closer to a 12AX7 and a triple mica 6072 should absolutely minimize noise.

Just saw this. He is having nerve system pain that is affecting his ability to play.

Mullard XF 2 works very nicely in the Express, as it also does in old Marshalls. Double getters are considered better than single getters. The Amperex brown base double getter EL34 is also very good indeed, but much less commonly seen, and even more expensive! I have always wondered what an Express ...

Too bad. Great amps, with "overdesigned" power supplies and excellent tone. He was a brilliant designer. Never found the cachet of Fender or Marshall, but IMO deserved it.

Joe, where did you get 50 ohms for Rt? The Philips GZ34 datasheet gives 75 ohms at 300VAC, the GE datasheet is 80 ohms. Half of 98 is 49 ohms, plus (300/120) sqd. * maybe 2 ohms (PT) = 12.5 ohms, so you are still shy a little of 75 ohms. I wouldn't worry too much about that difference though. A lot ...