Designing a preamp using octals.

[NFXP]

Anyone know of a good guitar preamp section using octals. My project has 3 6SF5's and a 6SL7. I think Ampeg uses octals but i would like to stick to the tubes i have since they are hard to come by. Anyone interested in designing a simple one id be interested to try it.

[ThermionicScott]

If it helps, 6F5/6SF5 has very similar characteristics to a 12AX7 section. The traditional preamp setup (100K plate, 1.5K cathode) works pretty well with 6SL7's, but yields a little less gain. So if you wanted, you could pick from a lot of standard preamp designs and just pretend you have big 12AX7's on hand.

- Scott

[NFXP]

thanks. im in the process of restoring then hopefully converting a Bogen P.A.
Thread Here

I've had to peice together 3 schematics since large sections of the orginal circuit were eliminated or reworked. I hope to be done the power amp section and a Phono input done soon. Then ill leave the 2 mic inputs to be converted to guitar. Ill post a schematic of what ive done when im done.

Before


After


Underneath


Its Home



Im not stuck on using 6sf5's. but i would like one channel to use 2 octals and 2 controls.

[ThermionicScott]

This guy originally used a 6AX5 rectifier? That's pretty cool, but it must have been a lower-power design, as it's only rated for 125mA or so (same as a 5Y3). What was the tube next to it? I can't make out the designation.

If you have the 6SF5's and 6SL7's on hand, I absolutely think you should use them. With 5 triodes in total, that's enough for each channel to have two gain stages, feeding a shared cathodyne PI.

- Scott

P.S. Is that little transformer on the inside a choke?

[NFXP]

This guy originally used a 6AX5 rectifier? That's pretty cool, but it must have been a lower-power design, as it's only rated for 125mA or so (same as a 5Y3). What was the tube next to it? I can't make out the designation.

If you have the 6SF5's and 6SL7's on hand, I absolutely think you should use them. With 5 triodes in total, that's enough for each channel to have two gain stages, feeding a shared cathodyne PI.

- Scott

P.S. Is that little transformer on the inside a choke?

Yup 6ax5 rectifier with 2x 6l6 output, 6sl7gt phase inverter then the rest is 6sf5's. and yes thats a choke not yet hooked up the circuit since i need to grab some caps first.

Ive built alot of stuff from schematics and have a basic understanding of this stuff. but i wouldnt dare try designing stuff or experimenting. i just dont know enough to be comfortable with that. ill post a schematic of what ive done and hopefully theres something that will work with it.

P.S the deleted holes in the chassis were for wierd power supplies or something. the one says H505 the other is H507

[ThermionicScott]

Ah, but you should experiment! Making mistakes is how we learn even more.

One thing that might be fun is to copy the input section from the Gibson EH-150 amp: http://www.harpamps.com/gibson/pg_0022.jpg

Mentally replace the 6SQ7's on the schematic with 6SF5's, and the 6N7 with a 6SL7. They're obviously different tubes, but the characteristics are in the ballpark.

- Scott

[NFXP]

Heres what i got. its what i could peice together from 2 schematics and whats there, I havent added the mic preamps.

Sorry if it isnt clear.

[ThermionicScott]

Oh, so you're rebuilding the original circuit? It looks pretty good, too. The mic preamps will probably be most "interesting" for guitar.

- Scott

[NFXP]

well i built up to the mic preamps. i added the phono input inorder to try it. Unfortunately the thing spits out crazy noise full blast as soon as the stand by is switched on. So ive sucessfully made the amp produce sound but its the wrong sounds. I've given up on restoring and i think ill source a pre-assembled turret board to drop in it after christmas.

[ThermionicScott]

Don't do that! Try disconnecting the negative feedback first, to see if you had it backward.

Does the crazy noise persist if you power it up without the preamp tubes installed?

- Scott

[NFXP]

Don't do that! Try disconnecting the negative feedback first, to see if you had it backward.

- Scott

I have no idea what your talking about. Looks like ill have to read a book or 2.

[ThermionicScott]

Ah, my bad. It's the 100k resistor connected between the output transformer and one of the 6SL7 cathodes. Try breaking that connection at either side and see if the amp is more stable when you power it up.

[NFXP]

Ah, my bad. It's the 100k resistor connected between the output transformer and one of the 6SL7 cathodes. Try breaking that connection at either side and see if the amp is more stable when you power it up.

wow yeah it must have been backwards. now it just hums with the preamp tube out. I still think i should throw a turret board in with some better components but that will wait till after christmas. so now im back to the subject of the thread. How can i go about making a preamp out of octals.

[ThermionicScott]

Rock on! At this point, you can leave the negative feedback disconnected (few of the Fender cathode-biased push-pull amps used it) or switch the OT wires going to each 6L6 so that the negative feedback can work properly if you decide to reconnect it.

If the amp were in my hands, I'd rebuild the rest of the amp from the schematic and try it out. It may sound good with guitar -- many of the classic early guitar amps had generic circuits.

- Scott

[NFXP]

Rock on! At this point, you can leave the negative feedback disconnected (few of the Fender cathode-biased push-pull amps used it) or switch the OT wires going to each 6L6 so that the negative feedback can work properly if you decide to reconnect it.

If the amp were in my hands, I'd rebuild the rest of the amp from the schematic and try it out. It may sound good with guitar -- many of the classic early guitar amps had generic circuits.

- Scott

thanks greatly. i probably would have given up. ill read up on a couple more things and hopefully have her ready for a test run soon.