6SJ7 gain set up?

[casey73]

I just finished building a simple SE amp based on the Fender 5C1 Champ, that uses a 6SJ7 pentode for the preamp. I set up the output for an EL34 instead of a 6V6, which involved just putting a 300 ohm cathode resistor in so the EL34 runs around 19 watts dissapation. I can get a nice clean tone, but not as much breakup as I was hoping for at max volume. I then used the values that Matchless uses around the EF86 in the DC30, for the 6SJ7. Still not as much early breakup as I was hoping for. Any trick to squeezing more gain out of a
6SJ7?

[stokes]

I would suggest switching from that 6SJ7.It doesnt have a lot of gain and the EL34 isnt being driven hard enough.The 6sj7 has a grid leak bias scheme,which is going to be harder to tweak.I would suggest using a 6SL7 or 6SN7 since you have the octal pre amp socket.These are dual triodes.You would rewire the preamp like a later Tweed Champ with the dual stage 12AY7 but adapt the 6S?7 for it.Both the SL and SN are interchangeable.They will have different gain characteristics similar to the diff.between a 12AY7 and 12AX7.Fender switched to the dual stage preamp because the 6SJ7 didnt have enough gain.Your clone probably sounds just like the original would-weak.Even with the 6V6,the 5C1 is considered the weakest sounding of the Champs.

[J Martin]

The 6SJ7 has a huge amount of gain! You just need to set it up right. Don't try to graft values from other amp circuits. Get yourself a reprint tube manual and refer to the "resistance coupled amplifier" charts. The charts have plate and screen voltages vs plate and screen resistance values cathode resistor and cathode bypass cap values, etc.. Setting up pentodes is not as easy as setting up triodes. The balance between plate and screen and cathode is critical. The basic ratio is that the screen voltage shouldn't be more than two thirds the plate voltage, but you will need to experiment and find what works in your circuit. Pentodes really sing when set up right and its a sound you can't get with a triode. Your patience will be rewarded. I use a lot of small signal pentodes in my guitar amp designs: 6SJ7, 6J7, 6267(EF86), 6AU6, etc. One thing to watch out for is that as you get to the high end of the gain scale, you may have impedance matching problems with the next stage.

jm

[stokes]

"One thing to watch out for is that as you get to the high end of the gain scale, you may have impedance matching problems with the next stage."

Which is exactly the point of using the dual triode,in order to get as much gain from the 6SJ7 as from the dual triode you end up overloading the grid of the power tube.The 6SJ7 can be tweaked for more gain than the stock Champ but I think as you approach the higher gain of the dual triode you will start to get clamping from the grid of the power tube,better to bring it up over 2 stages,in my opinion.

[Matt T.]

casey,
Read this thread. It's regarding EF86 pentodes but there might be some good info in there...especially the old post from kg that I posted. HTH.

[J Martin]

I was writing my reply to address the architecture of the 5C1. Of course you could follow Fender's evolution of the Champ circuit by using a dual triode. You can also follow a first pentode stage with another gain stage and adjust accordingly. This is a good opportunity to experiment and learn pentode characteristics since it only involves a few resistor changes.

jm

[casey73]

Will try 6SL7.......

Thanks to all who replied. I hadn't thought about the 6SL7 option, mostly because I'd never used them before. I rummaged through my box of pulls and found both a 6SL7 and 6SN7 that test okay on the tester. I hope to rewire it this afternoon. I'll report back with my results.

[stokes]

In an earlier post I described the SL7 being like a 12AY7,but it is actually like the 5751 which is a lower gain AX7,between the AY7 and AX7.I am sure this will get you where you want to be with the gain in that amp.

[Zoe_N_Iain]

In case anyone's interested, here's how I'm using a pentode input stage in my current project. Admittedly, not very Champ-ey.

[stokes]

Zoe,what I was pointing out,the 6SJ7 just doesnt cut it as the only gain stage as in the 5C1.Although I have never experimented with it,I am sure it would make a good first stage with at least another half of a dual triode feeding the power tube,as you have there.I have played a couple of 5C1's and they are weak,I have had two different people ask me to mod their 5C1's for more gain,but in order not to reduce the collector value I advised them to get a later model Champ and leave the 5C1 as it is.

[Zoe_N_Iain]

Oh, no. No argument here. I cobbled up a testbed 5C1(ish) amp with an EL37 a while ago and was a bit underwhelmed with the performance. I was intending to add in a parallel gain stage, as in my current design, but never got around to it.

I recently got some 6BR7s from a mate, though, and thought I'd give the idea another bash with the new valve. It's still a bit of a work in progress yet but it's looking promising.

I read somewhere about a pentode/triode valve version of the Champ that is supposed to rock but I can't remember where I saw it. If I recall correctly, it was a pentode stage, driving a 20-ish mu triode, driving a 6V6. Anyone got an idea what valve that's likely to be? I'm always up for a nice simple build that does it's thing well.

[stokes]

Zoe,I dont recall an actual Champ with that tube line up but it sounds a lot like the pre amp you posted the schem for.The AU7 has a gain factor of 20.I'm with you on the simple build thing,the Champ circuit can be a lot of fun experimenting with.I just finished a build for my sons girlfriend with a 5751 pre amp that has an extra gain stage using a half of a 12AV7 that can be switched in or out,going into a single ended out put with a NOS RCA 6V6 and a NOS Siemens EL84 paralleled with it,biased in Class A, this thing really kicks ass.Hate to give it to her.

[Zoe_N_Iain]

I think it might have been one of those 'semi-boutique' limited production offerings that seem to turn up from time to time. One of the so called 'Turbo-Champs' (at least that's the nickname they got over here in the UK. Seem to be fairly common as studio amps around my region).

There's a guy about 30 miles down the coast from here that gigs with 4 Turbo-Champs in a stack, actually. Can't remember exactly what the specifics are now but I think it's something like a pair with 6L6GC SE output for clean and a pair with hotrod preamps and 6V6s for dirty. I've seen the band a couple of times and they sound excellent, proving that you can rock the house with a simple amp.

Since you mention the 12AV7, I've just bought a couple of them really cheap. Any recommendations for what types of circuit they work best in? I heard somewhere thay sound excellent in harp amps, so I figured it was worth a bash for guitar considering the postage was higher than the cost of 2 NOS valves. Any ideas you have are more than welcome.

I'm a fan of the disimilar output valve thing, too. One of my current projects is a 40-ish watt head with 2 6V6s and 2 6EZ5s in pushpull. I have the poweramp, PI and clean channel done but haven't quite nailed down the choice of dirty channel yet, so it's under the bench until something I really like comes along. I've a Sound City 120R and a Carlsbro 60TC under there too, so I should probably stop experimenting with new stuff and get round to rebuilding the ones I have, but well, ya know...

[stokes]

Havent used the AV7 a lot.I got a couple of sleeves of them years ago and I used one in this situation,have it after the volume control in front of the driver.I used another one in an experimental 1 or 2 watt amp I got a schem for somewhere,I dont recall where.It called for a 12AU7 out put,I ended up with the AV7, paralleled the 2 halves for the out put.I tried subbing them in for a 12AX7 in a couple different amps but wasnt crazy about them there.I remember reading somewhere that they can be more useful if you tweak the plate and cathode resistors some,but never really got around to experimenting with them.They have a gain factor of around 35,I think.I would imagine they would be good anywhere you want to lower the gain some,such as a harp amp.

[casey73]

Rewired and fired it up today. Lots more gain....but, I've got some oscillation issues now when fully cranked. I dropped the 6SN7 in, and it didn't excite me. I set up the 6SL7 as I would a 12AX7, 1.5K/25uf on the cathode and 100K on the plate for each triode, with .02 coupling cap between them. I may have bring the gain down a bit in the first stage.