tube preamp using submini tube

[unclejed613]

i'm designing a preamp using a 6BF7 which is a submini dual triode. i'm using a 12v supply rail. i've come up with 2 possible solutions.... first, using the triodes as a diff amp, kind of like a tube input op amp, the second using 2 cascaded single ended stages. i'm trying to get as much gain as i can with a good clean sound. 20-20k flat isn't really necessary here since it's primarily a guitar preamp (and with the size of the tube, it could actually be built into a guitar in about the same space as a discrete SS preamp). the closest model i can find to this tube is a 12AU7 (6BF7 runs 100v max plate, 8ma, gm of 4800umhos, and mu of 35). the "op amp" seems to be a bit shy on gain with an overall gain of about 20. any suggestions? this is the first time in about 25 years i've designed anything using tubes.

[aletheian]

With 12v on the plate for a tube spec'd at 100v... you are not likely to ever get a clean sound, and at 12v, you don't have much headroom. 20x gain will clip against the rail with anything other than the weakest of single coil pickups. I'll hit peaks of almost a volt with the PUs on my main guitar.

You could try choke loading the thing... which would increase the swing and max out the gain at the same time.

[unclejed613]

i tried a preamp using a different tube (5703) and got a nice clean sound out of it, but it was a single stage SE with about 6db of gain, and i was trying for more like 10-12db. since it's a "tube sound" preamp 2-4% or so of 2nd harmonic is acceptable, and i may make a "tube fuzz" version of it as well, so if i run out of headroom, not a big problem there either. after i posted this thread i found a page that had more detailed info about tube diff amps.

[engels]

for a normal guitar pre you need at least two cascading stages and your double triode should do the job. I know 12AU7 may run on 12V plates but 6BF7 is something I've never tried, and as aletheian says you may get a saggy fuzz instead of clean headroom.

I'd suggest trying spacecharge tubes for a guitar preamp if you have 12V supply only. I've made a couple of those and they work awesome with 12V. Like, take two medium-mu 12FM6 or 12FK6 with 1M load and 1M grid-leak resistor. 12AJ6 is not recommended as it is a bit too fuzzy.

Building a tube preamp into a guitar is a brave idea, I think takamine recently issued an onboard tube preamp for their acoustics (12AU7 again). There's nothing wrong with this except for the heat that may damage a guitar.

I'm dreaming about making a guitar with built-in tube power amp, so if you have some solution for cooling I'd like to hear about it.

[aletheian]

Originally posted by unclejed613
i tried a preamp using a different tube (5703) and got a nice clean sound out of it, but it was a single stage SE with about 6db of gain, and i was trying for more like 10-12db. since it's a "tube sound" preamp 2-4% or so of 2nd harmonic is acceptable, and i may make a "tube fuzz" version of it as well, so if i run out of headroom, not a big problem there either. after i posted this thread i found a page that had more detailed info about tube diff amps.

Cool man. I just rigged up a little submini to see if I could get it down to 9v levels. It only has a few dB of gain and about 7.5% THD, but it works. A hot signal clips though.

[unclejed613]

the military solved the heat problem...... those tubes were sometimes mounted in aluminum blocks, fastened into a close-fit hole with RTV compound. to remove the tube, you force a sewing needle threaded with steel wire, and use the wire like a wire saw to cut the RTV compound. the aluminum heatsinks were very effective, besides, submini tubes don't create a lot of heat to begin with. actually i could (if i needed to) use old TO-5 heatsinks RTV'd to the tube, they're about the same diameter...... the 5703 prototype acually ran just a bit warm, not hot. you could grab the tube and hold it while it was operating. given that the max plate current is 10ma, and the plate voltage is 12v, that's only 120mw plus the filament wattage of 2.4w, for a total of 2.52w, not really much heat, even in an enclosed space. in the back compartment of a Strat that has been converted to a "hardtail", there's also some ventilation, since the back plate has holes in it.

i'm going to cascade the sections, and have an input gain and output level controls. the diff amp just doesn't do the job at 12v.

if i design a stomp box, i might try the diff amp with 24 or 30v (maybe add a flyback converter and get +/- 30v rails)., but that will be in a box with more room to work with, and more space for a second tube, etc...

[andyjevans]

You might think to try the 5676 - a very nice sounding directly heated triode. should solve the heat problem. but the other issue is that it may need to be mounted vertically.

[engels]

Originally posted by unclejed613
the military solved the heat problem...... those tubes were sometimes mounted in aluminum blocks, fastened into a close-fit hole with RTV compound. to remove the tube, you force a sewing needle threaded with steel wire, and use the wire like a wire saw to cut the RTV compound.

got a picture of that thing???
And - have you got it already mounted in that block?

[mandolin]

...in the back compartment of a Strat that has been converted to a "hardtail", there's also some ventilation, since the back plate has holes in it.

Very cool! I'd love to see photos of this. I actually did something similar involving a quartet of Nuvistors (two per pickup) in an old Gretsch Corvette solidbody, although my plate voltages were higher... would love to hear more about if you've actually tried putting this in a guitar yet.

[RE 604]

Hello ,
here are the pictures of the sockets , which I used in my little phono and line preamp :

http://www.ak-tubes.de/Vorverstaerke...ube_preamp.htm

Regards , Alexander .