parallel first stage in an aa964

[arjepsen]

Hey.
I'm playing with the idea of building a princeton from the blackface schematic, and are fiddling around with a layout of my own.
I'm trying to shorten the signal path, and I keep finding that I can shorten things, by putting in another 12ax7, and just use half of the first tube.
However, then I started wondering if there would be any benefit of using both halves of the first tube in parallel? If I recall Merlins book correctly, putting a 50K on the plates, and a 820R on the cathodes, it would pretty much work like a single triode - same gain, but also described as a "fuller" tone...?
Any comments?

Regards
Anders

[sbirkenstock]

Hi Anders,

just tried it out in my amp project (BF Vibroverb).
I left the two 100K plate resistors in place and connected them on the tube side with a short wire.
2 x 100K in parallel = 50k.
This way you can reverse it by unconecting them easily only to hear the difference.
I had my cathodes separated, with 2 times 1.5K and two 25/25 bypass caps.
The 1.5k bekome parallel and will be 750 ohm. Works well.
You just have to remove one 25/25 cap, otherwise you will have 50mf, which will get very bold.

Yes, it does sound "fuller", I´d say more "bold".
But it also takes away "sparkle" and "livelyness".
The current get´s doubled.

Took me 15 minutes, it´s worth a try to hear what it does, but I´ll not keep it.
I heard that Matchless is doing this in V1.
Also the parallel is used by Fender Reverb. But this drives the reverb pan, which is a different story.
Maybe worth trying to remove one side?
I did my project without reverb so I can´t easily try.

quest forth,

Stephan

[Meat&Beer]

6AV6 is a 1/2 12AX7 type worth looking at if you don't want to "waste" a triode.

Good prices on them, too. NOS, etc that aren't nearly in demand like the ever popular 12AX7.

Just a thought.

[Stevem]

Paralleling the triodes in a combo amp can make for a pain in the ass with microphonics depending on the the type of 12ax7 used!

[matt h]

(deleted)

[M Fowler]

I've built the Matchless Lightning and Spitfire which both have parallel first tube without any problems. I gigged that 18w Spitfire in small bars for years, cuts through the mix but required pedal board with boost and distortion pedal to get all the tones I needed.

I prefer to mix the two triodes of V1 with individual gain pots mix one as high and other as low. Naturally you need to select your cathode resistor and bypass cap for each triode.

Mark

[ampgeek]

I have homegrown built a handful of 59 Tweed Bassman (5F6A) circuits all with normal and bright channels jumpered at the tube in parallel using the volume knobs as an additional "tone" control option. Single input jack on all of them.

It is definitely fatter/warmer/fuller than either channel on it's own. Can't comment on the inability to cut-through the mix as I varied the contribution from each triode channel to make it work in the overall band mix.

Cheers,
Dave O.

[Stevem]

I get right from a customers custom built combo 100 watt rms amp with two 12 inch EV drivers that he plays flat out for the pasted 4 years , 3 to 4 nights a week.

[gingertube]

Try running the parallel input triodes at different operating points for a bit of harmonic emphasis.
Here is what I did on a "Junkbox Trainwreck". The local "Guitar God" loves this amp and uses it for all his gigs.

Cheers,
Ian

[mirage_indigo]

The mixed operating point is a really interesting idea. I've been poking around the idea of building a matchless inspired preamp. I hadn't ever thought of that.

Do you need to worry about the differing DC operating points? Or just get em "close enough"? Obviously the plate current will be split between the two halves unevenly, but does it just not matter?

And what does it sound like?