Preamp stubes in "parallel?"

[jbefumo]

Hi all!

I'm a relative neophyte to amp building, by which I mean I have successfully completed 2 projects, both of which entailed following an existing schematic ("Ultimate Bassman" or some such from an old issue of Guitar Player Magazine), on existing Bassman chassis, leaving power supplies intact, etc.

This time around I'm going to attempt something a bit more ambitious, in that I've read up on tube theory & design procedures, am analyzing schematics of various amps I admire, and will attempt something a bit more original.

I was just looking over the schematic for a Matchless Chieftain, specifically in the preamp section (http://www.freeinfosociety.com/media/pdf/3223.pdf), and am a bit perplexed about the effect of having the two sides of the 12AX7s in both preamp sections wired as if they are a single tube. I'm sure this topology must be covered somewhere in the mass of documentation I have around, but this seemed like it might be a more time-efficient approach.

(Would have attached the schematic directly, but wasn't sure about copyright issues.)

Thanks.

Joe

[Structo]

They are just running the two triodes of each tube in parallel.

Must be a gainy little sucker.
Claims to be Class A.

Here is more schematic information:
http://www.schematicheaven.com/newamps/ ... ieftan.pdf

From the Matchless site:

The Chieftain preamp utilizes two 12AX7's for its interactive tone and two 12AX7's for the reverb circuits. This highly flexible preamp circuit is enhanced by a very active mid control. The preamp section is coupled to a pair of EL34's power tubes via a 12AX7 phase inverter for tremendous power reserve. Exceptional sound pressure levels can be attained with no appreciable loss of tone.

[John_P_WI]

The 12AX7 in parallel is an attempt to emulate the pentode EF86 tube.

A 12AX7 in parallel will have more gain than a single 12AX7 stage, less noise for the same gain, and a different colorization to the tone. New EF86's can be noisy / microphonic and may need o-rings or other to tame the microphonics, hence one of the reasons for the 12AX7.

John

[briane]

Well a parallalled 12ax7 has very little in common tonally with an ef86. I have to doubt johns take that its a poor mans substitute IMO. I know some have looked into various wirings of the 12ax to replace the ef86, but parallel was not one of their test beds (I forgot what was settled on, but when I inquired I was told dont try and substitute it for an ef86, it sounds different).

What it really does is double the current capacity of the tube. Essentially you can think of it as a beefier version of the 12ax, with less distortion, better bottom end, and in general a fuller sound. I have been known to use paralleled 12ax's in bass amps or clean only amps myself, where your looking for a very full sound with less distortion.

[jbefumo]

Thanks for the great info! I should have read further in my book, to the section on calculating gain, and I would have answered my own question. Lots to learn. When I last studied electronics, back in the late 1970s, transistors were treated in a perfunctory manner, as a dying technology, and most of what we learned started with Op Amps. Tubes were never covered at all. It's kind of gratifying to see that I can still dredge up some of the old math chops -- even if it is just simple algebra. Any need for differential equations may be a different story, though. I have a bunch more questions, but will try to resolve them myself before bothering you folks. Thanks again.

Joe

[jbefumo]

Okay, so, in figuring gain for parallel triodes, to you calculate the net transconductance as you would resistors in parallel?

[Andy Le Blanc]

I think it's behavior changes a bit because of what the current does.
Part of the reason that a 6sn7 have always been favored is the current handling
as opposed to gain. The distortion products change, and differences between
the two triode stages are smoothed out a little. Make your self a little table and
collect some data, it would be fun to see.