How does Rk affect tone/gain?

[Blindog]

Hello everyone,

I've been playing around with an old Zenith phonograph chassis. It was pretty close to a Deluxe 5D3 circuit from the PI to the output, so I thought I would rebuild it to those specs. It sounds pretty good right now, although I have to confess I have never played through a tweed Deluxe circuit before, so I don't know what I'm comparing to.

My question is regarding Rk of the 6J5. I put in a 820R to match the Fender circuit. Would a different value be better suited for a 6J5? I left the stock 33K plate resistor...100K was only getting about 48V on the plate.

If someone could check my voltages on the schematic that I modified to match my circuit and give me any suggestions, I would really appreciate it.

Thanks!

Mark

[Chuck H]

Thats an odd tube for a first gain stage in a guitar amp. I've never had much luck with the metal tubes. It looks like 1/2 of a 12au7. It would probably make a good driver tube for a current dependant circuit like a reverb or a class B oscillator or something. But as a first stage preamp tube it's lacking in gain IMHO. Kind of like driving a tractor in the Indy 500. Yeah, it'll go around the track, but it's not the right vehicle for the job. Of course, I would hate to try plowing a field with a race car. It only has about half the available gain of the stock 5D3 12ay7, and one fifth the gain of a 12ax7. Also, the filament can't be wired in a hum bucking arrangement. And the first gain stage is the most important place to do this. I might consider replacing it with a 12**7 tube of some kind. Only because it's a more audio oriented tube, has more gain and you would also be able to reduce hum in the first stage because the filament is hum bucking. It will also be much easier to find a non microphonic tube for your first gain stage. You would also get the "other" preamp channel of the 5D3. Thats alot of extra good stuff just for changing one tube socket.

I don't have any practical experience with the 6j5. But I can say that if you only got 48 volts on the plate with a 100k resistor, that tube is drawing alot more current than it needs to for a preamp stage. You might try raising the cathode R to something like 2.7k or even 3.3k and then try the 100k plate R again. Don't worry about the cathode cap. At 22uf your bypassing everything the amp can reproduce by a good margin anyway. Unless you go quite small, like under 10uf, changing it's value will make little if any audible difference. Oh, and you'd probably better use a 1W plate resistor for this tube if you choose to keep it.

Chuck

[Enzo]

6J5 does come in glass too - 6J5GT.


If you are going to use Fender resistor values, you need to use Fender type tubes.

What were the original values around that stage?

I'm going with CHuck here on a differnt tube, like another 12AX7 or something or a 6AV6 for a single triode.

If you want to stick to the octal socket, you might rewire it for a 6SL7 and use one half of that. 6SL7 has an amplification factor of 70. Not quite 12AX7's 100, but a lot more gain than the 20 of the 6J5

[tubeswell]

What Enzo and Chuck H said

FWIW The cathode resistor in a pre-amp tube sets the bias for the tube. You have to know the what the tube's grid curves look like. There's an article here on triode gain stages that runs through how to work it out.

http://www.freewebs.com/valvewizard1...Gain_Stage.pdf

This site data on the 6J5 plate characteristics including a chart to enable you to work it out . Those grid curves look steep and it looks like it needs a comparatively high amount of bias voltage compared to what you would want for a 12A_7 type of tube

http://www.r-type.org/exhib/aam0016.htm

and the 6J5GT

http://www.r-type.org/exhib/aam0004.htm

[Blindog]

Chuck and Enzo, thanks a million for the responses. Now I need to tell you the REST of the story...

I agree with you both on changing out the 6J5. That was one of the first things i did. I rewired the octal for a 6SL7. I had major oscillation when the volume cracked about 1.5. Even with no input. I tried a different 6SL7, chopsticked a million wires, added 1K5 grid stoppers on the 6V6 (forgot to add them on the schematic), shielded cable to the input jack, and totally rewired the ground scheme. Nothing would make it go away! Changed the socket back to the 6J5, quiet as a church mouse. But, doing all of the above things really made the amp much more quiet. I just chalked it up to a layout situation, thinking the Zenith dudes knew what tube would work with this layout. (whew that was a mouthful...)

The only thing that is different from when I had the 6SL7 in and now is that I added a short piece of shielded cable to the input side of the 12AX7. I had some pretty good 60hz hum without that. Now I'm tempted to put the 6SL7 back in to see if that shielded cable will help (i doubt it )

Enzo, the original Rk was 680R bypassed with a 40uF. The 33K plate is stock.

I took some pics of the guts so you all can see what's going on. Oh, BTW, the OT is from a Bogen CHB 35. I fried the original drilling some holes underneath the chassis and ripping into the paper

Thanks again!

Mark

[Enzo]

Note the B+ for the 6J5 stage is NOT decoupled from the 12AX7 stages B+. I suspect that is part of the instability you encounterd. The low gaion circuit they designed doesn;t care. Higher gain circuits do care.

The Zenith guys didn't use the 6J5 because a higher gain tube won't work, they designed a circuit to suit their needs. The signal source they had didn;t need more gain. If they had needed higher gain, they certainly could have designed it easily enough. The very Fenders you seek to emulate are examples of that.

[Blindog]

The Zenith guys didn't use the 6J5 because a higher gain tube won't work, they designed a circuit to suit their needs. The signal source they had didn;t need more gain. If they had needed higher gain, they certainly could have designed it easily enough. The very Fenders you seek to emulate are examples of that.

Yeah, that makes more sense. Thanks for the insight.

Note the B+ for the 6J5 stage is NOT decoupled from the 12AX7 stages B+.

Ok, forgive my ignorance. Does decoupling that stage from the B+ mean adding another filter cap like in the dropping string?

Thanks

Mark

[Blindog]

http://music-electronics-forum.com/s...55&postcount=4

Thanks for that answer Enzo! Yes, I know, search is my friend

Ignorant question number 2: So I would just insert the cap between the 12AX7 plate resistors and the 6J5 (or 6SL7) plate resistor then? I'm a little confused on where to install it.

Mark

[Chuck H]

Something like this...(modification in red circle) Actually. I would make all three of the B+ rail resistors to 5K.

Chuck

[Blindog]

Thanks Chuck. I could see it in my head but sometimes I need picture to understand. I'm anxious to try that 6SL7 in there again!

Mark

[Enzo]

Yeah, like that.

I don't KNOW that is the problem, but is would be good practice to design it that way, and lack of proper decoupling is often a source of instability.

Surely wouldn;t hurt.

[Blindog]

Well considering everything else I tried, it does make sense. I've learned a lot playing with this circuit. Thanks for all the input!

Mark

[Blindog]

Oscillation Update

I decoupled the B+ to the input tube, and I still had the oscillation. I went ahead and installed a nine pin socket and wired it for a 12AY7 and guess what? No oscillation! I'm inclined to think that miniature tubes are just more stable than the octals (but what do I know!) I also lowerd the dropping string resistors to 5K and the 68k grid resistor to 33k. It's sounding like ZZ top in the shop Thanks for the input guys!

Mark

[Chuck H]

I decoupled the B+ to the input tube, and I still had the oscillation. I went ahead and installed a nine pin socket and wired it for a 12AY7 and guess what? No oscillation!

That is suspect because with no B+ on a tube it can't oscillate. But no matter because somewhere along the way the problem was fixed. Glad it sounds good.

IMHO the 12ay7 is a great sounding tube. Something about them just sounds really reactive and "tuby". But I've never designed for them specifically, I've just plugged them into a 12ax7 socket. So the operating parameters may have been unusual for that tube. But they all sounded good.

Are you using the other 12ay7 triode or is it just hanging there?

Chuck

[Blindog]

Chuck,

I think my term was wrong about decoupling. I meant to say I added another 20Uf cap to decouple the 12Ax7 from the Input tube per you and Enzo's suggestion.

Right now, the the other triode is hanging there unused. Thought about adding another volume control and input to make it like a real Deluxe.

I figured I would put the 12Ay7 in since that was really the only missing piece from my Deluxe circuit anyway.

I had some old Fender branded RCA 6V6's in my box and that's what's in there now, although I think I like it a bit better with 6l6's.