Blocking distortion/cutoff on reverb driver???

[rfgordon]

I just finished this new amp, which was to be a kind of tweed-meets-blackface deal. Signal goes into both sides of a parallel 12AX7, then vol/ton, then into a straight up blackface reverb circuit: parallel 12AT7, xfmr, tank, 1/2 12AX7 recovery, then mixed with dry into a mixer half of 12AX7, then to a LTP.

The 12AT7 has 327 VDC on the plates thru the verb xfmr.

Trouble is the 12AT7 cuts off when hit hard and the volume is up. Is it too much gain? It's a by-the-book Fender blackface verb circuit. It works great for the verb--light verb to truly cavernous--but it farts out when hit hard. What's really weird, if it is too much signal, is the amp ain't very loud, and it's a pair of 6L6s (cathode bias w/ 400 VDC on plate/screen).

The reason I know it's the verb driver, is that I hooked a speaker up to the reverb send (pulled the PI out for this test). Same deal. With the volume cranked, I hit a power chord hard and there's literally a "click" and the sound stops for a second or two, then comes back. Most unpleasant.

I don't have that much reverb experience, so any ideas would be helpful.

[skyboltone]

Well, stick a 12AT7 in the first hole. If it goes away then it's too much gain. I know you don't have drawing software, can you do one of your pencil sketches and scan it in? Post an tinypic.com then use to post.

Dan

[Darkbluemurder]

Maybe try a grid load of 1 Meg or 2M2 before the grid of the 12AT7?

[rfgordon]

I've come to the conclusion that the problem is an out-of-phase parasitic oscillation, so I'm stripping the board and starting over--paying much more attention to lead dress. I'm using a variation of the California Dreamer layout (with some tweaks) from AX84, since I know that one works and is stable. It's basic Fender.

We shall see once I finish the rebuild. There is a possibility that it's the reverb transformer. This is the replacement I got for one that had a shorted winding, so this one may be faulty, too, just in a different way. I got it from Tubedepot.com, and I don't know if anyone else has had trouble with their parts?

I will report back at a later date.

[rfgordon]

Fixed it by rebuilding it. Not sure what was wrong before, but the thing is a very aggressive and loud amp now! The reverb is deep enough to dive a U Boat into!

Not much in the way of headroom, but that's what I wanted: an aggressive blues/rock machine ready for a barroom brawl. It's destined for brown gator tolex and a cane grill. 15" Weber Chicago speaker. Gonna put a beer bottle opener on the grill, too.

Some of my connections tonight were kinda quick n dirty, so it's a bit noisy. So I'll up the neatness factor and chase out the noise later.

One thing I will say about the trouble, though, is that the Alpha 1 MA pot that was on the volume seems kinda funky. I had one with a push-pull switch on it go wonky a few months ago in another build. Anybody else have trouble with the Alpha pots?

[Tonegeek]

Anybody else have trouble with the Alpha pots?

Yep, I used them in my dclone and had one go bad early on. They sell these at Radio Shock which should tell you all you need to know...

[rfgordon]

Tonegeek,
Thanks for your comment. Do you have a favorite brand of pot? Hmmm, maybe I should have spelled that one out.....

What about a good source for pots with push-pull switches?