PI Trimpot in Express?

[David Root]

Anyone ever try this in the Express?

Tweaking the Express, I bet this is something KF might have done by selecting PI tubes to get the level of triode imbalance that sounded right in the particular instance. Why not put in a trimpot on the B+ to vary the PI triodes' voltage balance?

I realize some may see this as the camel's nose of Dumbleism under the sacred tent, but as a practical matter is it not worth looking at?

Truth in advertizing: I've built several ODS and an SSS and am preparing to do my first Express, so this in the Express is hypothetical for me at the moment.

[matt h]

(deleted)

[David Root]

I'd balance the AC signals off the caps then start unbalancing.

Of course I guess that should be done off the EL34 plates too, but the PS ripple would screw that up.

I have quite a few 12AX7s DC balanced within 0.5V per my tester.

"golden ratio Ken-approved......" Would you care to elaborate? I was thinking of just using my *ahem* golden ears.

[matt h]

(deleted)

[David Root]

I'm looking at it from the viewpoint of personal taste, rather than authenticity. Sorry I guess I didn't make that clear. Provided I can get the requisite bloom and dynamics of course. We shall see.

When I build one and it doesn't sound like Francesca (p=0.999) I'd like to be able to add PI imbalance to gridstoppers, extra .002 parallel cap, tube rolling, bias, plate voltages etc as tone modifiers.

From what I can see real Trainwrecks are like real Dumbles in that no two are exactly alike. It's what one would expect, and in both cases it seems to have been generally by design rather than accident, to meet either the client's or the builder's taste.

I can use the "same" tubes as in Gregg Levy's notes on what Ken liked in NOS. 60s to 70s Amperex short plate 12AX7s and Xf2 EL34s I can do. I expect to have to roll some to get closer to my taste. In this case it's the early Express less aggressive sound that my client is looking for, and that is what I think I might prefer myself. Client has a 4x12 cab so that is a good start.

[ampdoc1]

I assume you are referring to the Trim pot between the PI output pins?

I tried this years ago after I saw it here, and have added it to several Express type amps. I generally like the sound better with a little imbalance to find the "sweet" spot that accentuates the bloom of higher harmonics.

Besides, it's easy to implement, and if you like it ABSOLUTELY matched, you can always dial that in.

Attached is a schematic of my favorite amp "design" which uses primarily Express and Dumble "modules" and a Fender tube power supply. It's pretty simple but it gets the best of the mixed breeds, with a few additions, including the Trimpot to boot.



a'doc

[David Root]

Yes, Dumble style trimpot on top of the PI.
Not replacing the normal plate resistors like your design. Not sure I'd want that much voltage drop across a trimpot. OTOH your PI B+ is ~100V lower than a Dumble.

I had been thinking about mixing an Express and a Dumble, and here you've done it eight years ago! Very interesting, especially V1 & V2 as 12AT7s. Do you find the active loop is necessary or is it just for completeness?

[ampdoc1]

I put in the tubes as shown initially, but when I got the amp back a couple months ago (I borrowed it from the owner as I am making a copy of the original for myself) it had 12AXs throughout. I haven't changed them out, as the amp works fine.

Re the loop,..I'd recommend keeping it. That design was one of the first (supposedly) Dumble loops posted on this site. It is simple, quiet and useful.

If you follow the signal, you'll see every section of the amp has a following volume control. This really allows the player to adjust the amp for the tones vs. volume desired. Also, the 2 footswitchs give options of clean/ clean + boost/ clean + distortion, or clean + boost + distortion, all very touch sensitivity.

I'm not bragging as this was built alongside the guitarist/owner, to "trim" the sound and systems as we went along. But, I think we got a very good combination of the Express and Dumble tones. I have 20 some amps,..Fender, Peavey Pig Nose, Duncan, Trace Elliot, Gibson, Rivera, etc...and the DumbWrek can do ANYTHING I can get from any of these.

I going to see if I can get a friend to make some music shorts I can paste.

a'doc

[hans-jörg]

@ a´doc:

do you have maybe some pictures of this amp from inside
Just to get a overview what size of chassis or layout you used
Or, what I don't try to hope: a layout too? PDF?

If I would build an other amp - this would be my choice
Until now I built TW, Dumbles, Fender (small),Marshall (big and small). I thought, that's enough for me, but
sorry, but you waked up my hidden ghosts!

Hans-Jörg

[ampdoc1]

Hans,

Attached are a couple of pics of the original amp and my new build in progress.

The original amp was built in a Fender Bandmaster Reverb TFL chassis, which is 23.7" x 7.5" x 2". You'll get a laugh at the original,...again, it was pretty much built on the fly, a little at a time. HOWEVER, as crappy as it looks, it has virtually no hum.

The new chassis is a 20" x 8" x 2.5" Aluminum Pan with top, from Watts Tube Audio.
The layout is "tricky" at this point as I am trying to get things laid out as close as possible to the original. Once it is finished, I will be able to provide some raw figures for layout. Notice I am using spade connectors for most of the components, and small brass rails for the 6V heaters and Ground connections.

[hans-jörg]

Oh,
I appreciate!
Point to point, great.
Can´t wait the sound samples and all you like to share.
It will be definitive my next project. Probably a combo (Mark inspired )
Something where I can rest and work, a long term project, but with love

Regards
Hans-Jörg

[angelodp]

David, thats looking real good, keep em coming. Love to see the final.

[andresound]

I assume you are referring to the Trim pot between the PI output pins?

I tried this years ago after I saw it here, and have added it to several Express type amps. I generally like the sound better with a little imbalance to find the "sweet" spot that accentuates the bloom of higher harmonics.

Besides, it's easy to implement, and if you like it ABSOLUTELY matched, you can always dial that in.

Attached is a schematic of my favorite amp "design" which uses primarily Express and Dumble "modules" and a Fender tube power supply. It's pretty simple but it gets the best of the mixed breeds, with a few additions, including the Trimpot to boot.

Hi ampdoc,
Im fairely new here. I built a trainwreck express a few months ago (posted the build here). Sounds awsome, but would like to learn and try some tweaking. Very interested in the trim pot install. Please can you send me the schematic of how to implement this "trim pot" (and value to use) between the PI output pins. A hand drawn example would be fine. This would be MOST appreciated. I live in South Africa, and its VERY hard to get informed help with regards these amps.
Regards andresound



a'doc

[andresound]

stupid me The drawing WAS posted!

[andresound]

Yes, Dumble style trimpot on top of the PI.
Not replacing the normal plate resistors like your design. Not sure I'd want that much voltage drop across a trimpot. OTOH your PI B+ is ~100V lower than a Dumble.

I had been thinking about mixing an Express and a Dumble, and here you've done it eight years ago! Very interesting, especially V1 & V2 as 12AT7s. Do you find the active loop is necessary or is it just for completeness?

When will you attempt this? Do you think I should? Just a thought.
BTW, did you get my gut shots?
regards ANDRE