Obscure Tubes for Guitar Amplification?

[brokennarsil]

Hello everyone. I have the tubes from an old PA amp and was wondering how they would perfrom for guitar amplification. They are 5U4G x1 (rectifier), 6L6G x2 (output?), 6SC7 x2 (preamp?), and 6Sl7 x1 (another preamp maybe?)

I know the standard (and all I can ever find info on) is the 12AX7. I imagine I can figure out a way to wire them for my use, but if it's going to sound crappy I might as well go ahead and buy a few 12AX7s right?

If anyone knows anything about these tubes, please share. Thanks!

[loudthud]

The 6SC7 is a dual triode with a mu of 70. Problem is that the cathodes are tied together internally. Makes a good phase inverter. The 6SL7 is also a dual triode with a mu of 70 but the cathodes aren't tied together. Some people like the sound. It's not your modern heavy metal sound, but a mellower bluesy low tech organic tone. You'll find these tubes in early Fender tweed amps and early Gibson amps. The problem with converting to 12AX7s is changing the tube sockets to 9 pin miniture.

[imaradiostar]

so...sounds like you could build something that uses a parallel triode input, tweed fender cathode follower and a LTPI into the 6L6's. It'd be cool to keep it all octal!

jamie

[Bruce / Mission Amps]

I've built a fair number of Fendery 6L6 and 6V6 tweedy type amps using 6Sl7GTs in the preamp and phase inverter... they sound wonderful!!
They don't lack anything with respect to tone but yes, they do have less gain then a 12AX7... and I think that is good thing in most of these simple amps.

[brokennarsil]

Thanks for the help! It's good to know a little more about these tubes that are hard to find info for!

Bruce, do you know anywhere I can find a schematic using this tube setup? The sound you mentioned sounds rather like something I'd enjoy!

[brokennarsil]

so i found a schem. of a Rickenbacker M11

http://www.freeinfosociety.com/media/pdf/3293.pdf

It seems to use the same tubes as I have. Can't the 5y3 be subbed for the
5u4g? Would this be a fairly simple amp to build? I'm just looking for something nice and simple to get my foot in the door.

btw, the transformer voltages aren't listed on the diagram, but couldn't I just use the ones that came from the amp with my tubes? Would the affect operation of the new amp?

[imaradiostar]

You'd end up with much lower plate voltage with the 5y3 but it probably won't support the current needed to feed a 6L6 output section. If the power transformer is a high impedance affair like so many old PT's the voltage may be too high (not enough load) if you're using 6v6's rather than 6L6's. The only way to know is to build something and try it. With the durability and sweet tone of modern 6V6's you might keep the 5u4 but double up the 6V6's- impedance should work out nicely with an output TF that's built for a pair of 6L6's or 5881's.

You might wanna download Duncan's PSU designer. I've found that measuring the non-loaded voltages and resistances and feeding them to the program tends to yield pretty accurate results- at least enough to get yourself into trouble and start soldering.

I say stick with the tube complement you've got. If you don't already have a marshall jtm45 or 4x10 bassman type sound I'd head that way- a large percentage of recorded guitar tones are based on that topology and you could take advantage of the shared cathode connection by paralleling the input triodes for a slight thicker tone and higher gain.

One of the major forum contributors once suggested that the key to getting these large octal tubes to sound as good as possible is avoiding 12ax7 circuit values. It seems there needs to be a little current flowing before 6sl/sc/sn/7's start to sound really cool. I wish I could remember who suggested it.

jamie

[tubeswell]

do you know anywhere I can find a schematic using this tube setup?

Tweed Pro 5b5 or 5C5, Tweed Super 5C4 could use these tubes.

[Chuck H]

Brass tacks is that "tubes is tubes". You'll get tube tone using tubes. The tube type matters to the effect of -30 amplification with the tubes you propose. There is more than that lost on most tone stacks. The lower gain thing is really relative. If you build the right kind of topography with 6sc7 and 6sl7 tubes you won't even notice any loss of gain compared to 12ax... tubes. And as long as the tubes your using arent microphonic or have other issues you will end up with a very satisfying project... Even if your going for a high gainer. Really.

The tubes and sockets you have to work with are historically proven in guitar amps. With even three cascade channels using 12ax7 tubes there is the nessecity of attenuation. So I don't think you will have any problem generating gain.

Just go for it and tweak the design until it sounds the way you want it to. You really do have the right stuff to start with. no need to change to noval sockets and all that...

Chuck

[brokennarsil]

well thanks for all the info, everyone. it sounds like i'm on my way here. So, if my research has been correct, my next step (having deciding what sound and contols I want) would be to figure out the voltage of my transformer(s). And use the formulas to figure up my biasing?