Cathode biased amp

[rake]

Guys, I'll try not to ramble. Back in Jr high school (1972-73) I was given my first loud amp! 1959 Gibson GA-80 Vari-tone. Amp was supposedly stolen 9 years later. (ex-wife sold it for dope)
anywho, I got to play thru one last week. (still love the tone)

Thinking about doing a single channel version for myself. Would like to add a  switchable 3rd gain stage.
Stage 1 is 1/2 of a 12AX7. 2 is a 5879. My thought was to just use the other half of the 12AX7. (thoughts?)
Also, NO TREMELO! (I hate tremelo lol.) Will explore reverb though. Maybe master volume? This question is for our big brain members, (one of which I am not) Keep it cathode biased or switch to grid biasing?
Any and all thoughts are welcome and appreciated!
Thanks!

Schematic:
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga_80_varitone.pdf

[shooter]

what's the endgame? if I have a '59Ply, i probably wouldn't spend much time making it a '57 Chevy
 

[sluckey]

Put your ideas on paper and upload to the forum.

[bmccowan]

Small brain here. And I am also interested in seeing a sketch of your ideas. In the meantime. A pentode in the V2 position is a great sound IMO. I think that if you try to feed it with two gain stages you will overdo it. Maybe parallel that first stage. The Matchless Clubman circuit uses either a 6SH7 or EF86 in that fashion - both similar to the 5879. I've built a couple amps with the Clubman preamp and love the idea you are working toward.

[rake]

Small brain here. And I am also interested in seeing a sketch of your ideas. In the meantime. A pentode in the V2 position is a great sound IMO. I think that if you try to feed it with two gain stages you will overdo it. Maybe parallel that first stage. The Matchless Clubman circuit uses either a 6SH7 or EF86 in that fashion - both similar to the 5879. I've built a couple amps with the Clubman preamp and love the idea you are working toward.

Thanks. the added stage would be a lead boost. Parallel 1st stage may still accomplish this. Gonna start playing with this on paper. Thanks all. Who knows, maybe I can turn a VW bug into a Porshe 911 yet!

[Williamblake]

A 911 basically is a beetle with tremolo. 

[rake]

I started kicking the idea around and because of parts available vs. parts I have I decided to take  bit of a left turn! The turn is more of a single channel Super Reverb with a 5879 pentode in the second stage and no tremolo. In all the years playing clubs I never once used the tremolo on any amp. 
Any and all advice is appreciated!
Thanks!

[kagliostro]

Ciao Rake

if I try to open your file with Jschem the result is a warning saying that there aren't  valid schematic data

Franco

[tubenit]

Maybe something like this?   It's one of my favorite amps.  I have a layout for this also.  I have 3 amps that I've built, IF I could only keep one ......... it would be this one.  Tweed BluezMeister Lite


You said cathode biased, one channel, 5879 as an OD gain stage/lead boost and reverb ......... right?   


With respect, Tubenit

[rake]

Ciao Rake

if I try to open your file with Jschem the result is a warning saying that there aren't  valid schematic data

Franco

That's because I used ExpressSCH to draw it.

[rake]

Maybe something like this?   It's one of my favorite amps.  I have a layout for this also.  I have 3 amps that I've built, IF I could only keep one ......... it would be this one.


You said cathode biased, one channel, 5879 as an OD gain stage/lead boost and reverb ......... right?   


With respect, Tubenit

Thank you sir. I will study your schematics.

[rake]

Jeff; Thanks for sharing your schematic. It gave me some direction.
I laid it out this way in hopes of coming close to the GA-80 tone.
All help greatly appreciated!
Thanks in advance!

[tubenit]

I'll offer a couple of thoughts for your consideration.  Not saying any of these would be "better" than what you've drawn up. (Just based on my personal preference in experimenting with values.)

I don't think you "need" to change a thing from glancing over your schematic.  I would anticipate it being a fine sounding amp as you have drawn it.  Thank you for sharing the schematic! 


I'd use a 12DW7 for the reverb tube.  Using the "12AU7" (triode 1,2 & 3) for the drive and the "12AX7" (triode) for the return.  I'd skip the 470p across the reverb send pot. I've used a one tube reverb on maybe a dozen different amp designs and never needed that before. And I don't know why this amp needed it to lessen the brightness of the reverb?  Because the reverb pot is 1MA and there is a 330k resistor there, .............. you could lower the 470k "mixing" resistor down as far as 150k if you like having more gain there. 

You may prefer a 12AX7 as V1?  I would suggest maybe a 5751 and have V1b have 120k/1.8k  plate/cathode with 5uf instead of 10uf.  Again, just a preference thing on my part and not necessarily better. 

Your V1b may benefit from an internal trim pot either before or after that triode so you can control the gain there some more?  OR perhaps a fixed resistor?  That's certainly something you'd need to experiment with.  You need to add a ground on the "left bottorm" of the DPDT. 

The 5879 plate needs to connect to a filter cap (probably D?).  I'd probably add a grid resistor (pin 1) going into the 5879 ?  Maybe a 10k to 33k?

I know Kevin O'Connor talks about using that type of master volume in the LTPI.  I've never used one like that? So, I'd be curious about it. I do know the PPIMV works quite well in my experience. Since you don't have NFB (although you drew part of the NFB loop), the PPIMV wouldn't be impacting NFB.  IF you want to add NFB then put a 4.7k to 6.8k into the positive side of the speaker terminal.


With respect, Tubenit

[rake]

I'm always good for leaving at least 1 ground out! 
I used that MV on a Deluxe Reverb and a Pro Reverb when I was playing clubs
and it worked great for me. I'm sure they're will be some changes to make along the way.
The old GA-80 sounded killer with my 59 LP Special. All I used was that amp, the 59, Vox wah,
DOD Bi-FET and a MXR dist (no light) old one.
I copied your suggestions and will save them for reference when I start this build.
Hope to start in the spring. Still trying to get my dad's estate settled. That alone is enough
to give someone ulcers!

Thanks Again!

I did order a few things from Doug Sunday night.
Coming tomorrow. Now that's customer service!

[rake]

I truly appreciate your help and direction!
I gave your last post some careful consideration and came up with some revisions.
Much Appreciated!

[tubenit]

The schematic looks good. Again, your original design I think would sound good and my suggestions may just be different and not better. Their simply based on my preferences.


Regarding your "cross-line" master volume after the phase invertor. Please hear this respectfully because I truly mean it respectfully.  That master volume is my least favorite of any that I've tried.  I thought it was a real tone killer in my experience. Every one of those I've tried has sounded buzzy, fizzy or harsh and lacked smoothness to my ears.   IIRC,  Sluckey is not keen on that one either?   


The PPIMV, the typical Marshall pre phase invertor master volume and the VVR have been ones I've been pleased with the most.


with respect, Tubenit

[bmccowan]

The problem I have with this Forum is that it is always sucking me into a project. So is the case with this thread. I love the tone of early Gibson amps and also pentodes in the V2 position - so how do I resist. Anyways, a few years ago I built an amp that was loosely based on the Dr Z Z-28, but with a 5879 in V1 instead of an EF-86, and a VVR. I do not play it much. It has an empty 9 pin socket in the 1st position; so perfect for adding a 12A_7. Looking at the schematics in this thread I am thinking a similar preamp, keeping the 6V6 power amp. I will shoot for lower gain than what I see in your circuit Rake, as I am not much for high gain amps. I will draw it up and share, hoping to get thoughts from both you and Tubenit.

[rake]

The problem I have with this Forum is that it is always sucking me into a project. So is the case with this thread. I love the tone of early Gibson amps and also pentodes in the V2 position - so how do I resist. Anyways, a few years ago I built an amp that was loosely based on the Dr Z Z-28, but with a 5879 in V1 instead of an EF-86, and a VVR. I do not play it much. It has an empty 9 pin socket in the 1st position; so perfect for adding a 12A_7. Looking at the schematics in this thread I am thinking a similar preamp, keeping the 6V6 power amp. I will shoot for lower gain than what I see in your circuit Rake, as I am not much for high gain amps. I will draw it up and share, hoping to get thoughts from both you and Tubenit.

What I like about this place is that most everyone will share and speak up when something is off!
Just remember, like the first wife's uncle always said, "it's hahd tellin not knowin"!

[WiderGates]

Hello,
instead of MV: https://el34world.com/Forum/index.php?topic=4401.msg37964#msg37964

[tubenit]

WiderGates,


I've done that cathode attentuation also.  It's been alot of years ago but I remember being impressed how well it kept the original tone at lower volumes.


With respect, Tubenit

[ac427v]

Is the Presence pot connected to the wrong side of the 470 ohm resistor?

[rake]

Is the Presence pot connected to the wrong side of the 470 ohm resistor?

Could be. I have a habit of getting things bass ackwards from time ot time! LOL!