Using 12AT7 or 12AU7 as lower power output section

[armstrom]

I'm considering building a hybrid amp consisting of a simple, low power tube amp driving a dummy load (probably a weber MASS speaker motor). The output of the tube amp would be tapped to drive a very clean, high powered solid state amp (~60W).

So, with that in mind... I'm considering using the standard Fender reverb driver circuit as my tube power amp (along with 15K Ohm to 8 Ohm reverb tranny). Most of the Fender schematics I've looked at show both halves of the 12AT7 run in parallel sharing the 15K plate load from the transformer and a 2K2 cathode resistor bypassed by a 25uF cap. Here are my questions (finally!):

-What (if any) changes would be required on the cathode side (resistor and/or bypass cap) to run the output stage at a +B of ~300V?

-Would a push/pull 12AU7 output stage (as used in the Firefly amp from AX84.com) be superior in any way?

-Finally, does anyone have a schematic for the Ted Weber MASS pedal? I glanced at it a month or so ago and remember it using a 12AT "power" stage but since then all traces of the product have been removed from weber's site.


and maybe one bonus question Am I wasting my time? I hear all the time that to get "that tube sound" you need the whole deal (Preamp, power amp and output transformer) in the mix. I'm considering the cost (and weight!) of a very low power tube amp combined with a high power SS amp against a 60W tube amp.

[6267]

Ok, so there are a number of issues here. Do you want it to sound like a big tube amp? or to sound different?

If you want it to sound like a Marshall or fender whatever, you're going to want to use push-pull signal pentodes or low-power pentodes on the output...maybe 50c5/50b5s. Likely any transformer with a reasonable core size and the right ratio will work ok at these low currents. The firefly style push-pull works, but it doesn't sound quite like the other push-pull topologies (in my opinion, it seems to sound more like SE in many cases). Probably end up with around 1-4w output this way.

If you want it to sound more like a champ, the paralleled 12at7 is a good choice, but you may want a better transformer than the reverb one (reverb doesn't need to cover the full range of the guitar in many cases, so the transformers have compromises). I'd probably use something like a 6em7/6dn7/etc dissimilar triode, using the low-mu/rp half as the power stage.
Around .5-2w output this way.

The main decisions here would seem to be harmonic content (single-ended or push-pull), transformer quality, and of course the preamp. You will find that you do not need much signal to overdrive the hell out of the whole thing, so you'll want to use low mu tubes probably with only one gain stage or so.

For a 12au7/6sn7/6cg7/etc. at around 300v, I use about 600 ohms. 300-350 if it's a shared resistor firefly style. Google tubecad and read the grounded-cathode article, it has great equations for calculating these kind of things.

[HTH]

I'm using a Hammond 125ASE OT in my parallel SE 12AU7 amp. I've got the 10k (8ohm) tap hooked up to a 16 ohm load to refect the higher (20k) primary Z thats required.

Around 230v on the plates is the sweet spot imo.

You'll want to use a lower cathode resistor than 2k2 - try 1k for a SE triode, or 470ohms if you're parallelling both triodes in a 12AU7.

Also, if you're gonna be running it into a clean power-amp, instead of the Weber Mass, why not just load the amp's OT with a 15ohm/10w wirewound resistor and hookup a simple line-out to run the power amp.

[armstrom]

Thanks for the great info guys!

6267,
As for the sound.. I'm after something with mild overdrive for blues playing. The champ is generally regarded as producing good blues tones so something like that (But much louder due to SS amp) would do nicely Just FYI this isn't for me as I don't play guitar, I'm building this for a friend.

The preamp section will probably just be two 12AX7 gain stages with standard fender TMB tone stack between them (basically half of an Alembic F2B or any other basic fender preamp since the beginning of time )

HTH,
Regarding the MASS, I plan to use the speaker motor in place of a pure resistive load as you describe. I just want to use the motor rather than a full weber MASS attenuator circuit. Obviously, a simple power resistor would be MUCH cheaper so I may try that out first to see how it sounds. If it doesn't give up much in terms of quality I won't bother with the MASS.

[tedweber]

-Finally, does anyone have a schematic for the Ted Weber MASS pedal?

https://taweber.powweb.com/store/mass_pdl_schem.jpg

https://taweber.powweb.com/store/mass_pdl_layout.jpg


Ted

[armstrom]

Ah, Thanks Ted! What happened to the MASS pedal anyway? Was it just not selling? I considered picking up a kit for my "preamp".

[6267]

since 12au7/12at7/6sn7 etc are higher transconductance and gain than most power tubes, and use a lower bias voltage (I think it's around -5 to -10 for the au7 and sn7) you will almost certainly want a voltage divider to cut the max signal down to something close to this. Otherwise you will get very bad sounds when the preamp tries to put out voltages scaled to be run into a PI & power amp. It will try to drive the power tube grid positive, fail, and do something bad sounding as a result. Alternately, experiment with your volume pot and install series resistors to prevent the bad sound, same thing really. I think you're going to have a cool sound out of this thing, just remember to roll a bit of bass off the SS amp for a more classic sound. Big transformers for bass were expensive, especially so for the cheaper amps that are such famous blues performers.

[bluesman60]

bigger amp

armstrom,ive been a harp player for 40 some yrs and have used evrything you can imagine but for harp or guitar,the blackface vibrloux from the mid 60s is an excellent piece.for harp i changed the ax7 preamp tubes to 12at7 and i get just what i want and enough power to play with a full band,without feedback..