12at7 preamp design

[raiken]

So, what do you with the rejects ? You got any you might want to unload ?

-g

They're still good for phase inverters, reverb drivers, etc. Just not quiet/non-microphonic enough to be used as the first gain stage in a mid-gain amp.

RA

[mooreamps]

They're still good for phase inverters, reverb drivers, etc. Just not quiet/non-microphonic enough to be used as the first gain stage in a mid-gain amp.

RA

Send me a PM.. I'll pick up a few from you..

-g

[walkman]

Hi Randall,

Thanks for your reply. I had a chance tonight to rewire my test amp . Its a single ended el84 with a single 12ax7 or 12at7 and 100k plate loads. With your suggested values of 47k and 1k5 for both sections and a 12at7, it took the amp from one that started to breakup at about 3-4 on the dial, to one that was just on the edge of breakup with the volume control at full. Its got a nice compression , regardless of which tube ax, at or au .

Ive got an old mullard el84 and mullard 12at7 in it and it does have a quite full clean sound .... ive only brief chance to play it due to the time now but its given me ideas / inspiration of how i can shape the sound further with some experimentation

Thanks very much for your help, I appreciate the advice and hope to get a chance to play one of your amps some time soon.

[kg]

Randall Aiken

blast from the past!

hey randall.

i agree on microphony concerns. sadly the fact that the ax7 is so ubiquitous means that it is also easy (relatively) to find good versions. not so true with other less common types, despite the fact that they have some real benefits wrt tone.

that's one of the reasons i switched over to russian military surplus types.

-ken

[Amp Kat]

Hey kg glad to see you guys posting again. That makes sense kg and with the 6N1P having a lower plate resistance of what about 4.4k is it and the mu around 33 you would have to either cascade or cascode to get the gm total back up to snuff. So do you think after doing that it would be less microphonic ? I would think the signal to noise ratio could be better also if done right.

[Steve Conner]

I've used the 12AT7 as a second and third stage in a dirty channel, with an EF86 driving it. I tried lots of different tubes as the "V2", and a Philips Miniwatt ECC81 was the winner.

I actually ended up with the 220k and 2.2k values I was using for the 12AX7s, but used combination bias. I tapped a little bias off the power amp bias supply, and fed it back to the 12AT7 grid through a pot, which I tweaked for the best overdriven tone.

My reasoning behind this was: With the too-big resistors, the tube will end up sitting at a lower current than it was designed for. This puts it down on the curved part of the characteristic, making lots of fat low-order distortion.

[mooreamps]

but used combination bias.

Finally, someone is starting to get it....

-g

[Guitarist]

Hey Steve, I sure would like to hear your amp. I bet it delivers. That pentode and the dialed-bias 12at7s seems like a brilliant approach.
I've got a Soldano'ish amp that is going that way partly.

[Steve Conner]

The irony is, I've ended up using it with a 12AX7 and the fixed bias turned down to zero. The 12AT7 made some nice tones at low volume, but the "feel" was wrong when playing loud. Sticky with no sustain.

[Gingertube]

Using the 12AT7
Look at page 3 of this for all the various operating point possibilities. I know it doesn't say 12AT7 but it is direct equivalent so data is good.
http://www.mif.pg.gda.pl/homepages/f...126/b/B309.pdf

I'm building an amp right now for a friend using 6SL7 which is the octal "12AT7". Its to be an exact copy of my own amp which he raves about. I'm basically using the London Power Standard Preamp LPSP circuit with some very minor component value changes (cathode bias resistor tweaks) to better suit the 6SL7 with a 6SL7 + PP 6V6 in a 5E3'ish power stage, less the tube rectifier. I asked Kevin if I could post the preamp circuit and he said NO.
Look it up in just about any of the TUT books.

Cheers,
Ian

[Gingertube]

One more hint about using 12AT7.
It has much higher grid current than a 12AX7 so you want to run it with high anode voltage in order to limit that grid current. That probably means 100K to 150K max. for the anode load resistor. Then to limit bias perturbations from grid current you want to stick to a lower Rg1 value. RDH recommends a Rg1 of not greater than 3 times the anode load resistor for high mu tubes like a 12AT7.
Cheers,
Ian

[Austin]

One more hint about using 12AT7.
It has much higher grid current than a 12AX7 so you want to run it with high anode voltage in order to limit that grid current. That probably means 100K to 150K max. for the anode load resistor. Then to limit bias perturbations from grid current you want to stick to a lower Rg1 value. RDH recommends a Rg1 of not greater than 3 times the anode load resistor for high mu tubes like a 12AT7.
Cheers,
Ian

I read this before. I wonder why the grid current in this is higher than other similar tubes? The grid spacing or something? Does this mean it would get even more power if you could run it in class a2 or does it mean it would damage the control grid?

[Merlinb]

I read this before. I wonder why the grid current in this is higher than other similar tubes? The grid spacing or something?

Yes, grid-cathode spacing is close (to get high gm), and the grid is also wound with close pitch (to get high mu). Recipe for more grid current!

Does this mean it would get even more power if you could run it in class a2 or does it mean it would damage the control grid?

These small noval triodes were not really designed for grid-current operation, so you could indeed burn out the grid. I suppose the 12AT7 might survive longer than its brothers since if uses more grid wire = more mass, but it's not something many people have ever attempted, I suspect.