Cathodine PI as input stage?

[Satamax]

Hi everybody!

Well, coming back with weird ideas.

I'd like to make an amp wich has only post PI tone and volume controls, or nearly. What i'm envisioning is cathodine PI as an input stage. I know this has been done before. then two gain stages on each leg of the PI, then output tubes. This, having an arangement of crossline tone controls and master volume. Anybody has examples of such amps? Schemos, sketches or anything. I think i realy have the idea in my head, but would like to document, to see what others have done.

Thanks a lot.

Max.

[MWJB]

I have done this, using dual pots for a traditional vol & tone controls between the PI stage & additional gain stage. It was a very smooth sounding amp, I put this down to the reduction in SE stages, it did a nice, compressed, warm clean tone, but not much crunch.

Danelectro (Red Bank) made JDR accordian amps with vol & tone controls before the first tube grid, then a master vol ("sensitivity") between the input/gain stage & PI triodes of a 12AX7, followed by a 6SN7 gain stage driving 4x6V6.

I guess, to cut down on parts, that a single vol/tone control could be put in place of the Dano sensitivity control, may be easier to keep balance, but balance didn't crop up as an issue.

PM me and I'll send you my sketched schem.

[DIY_Guy]

That sounds like a fun little amp.
I would like to hear what would happen if you used a separate tone control for each leg, messing with the phase a bit. Wonder what it would do?

[Satamax]

Thanks a lot for the replies guys.

DIY guy, you're giving me food for thoughts!

I thought this was intresting too.

http://satamax.free.fr/P663.jpg

[octal]

I think an LTP or paraphase would make a better first stage. The cathodyne has no gain, so the outputs will be 'guitar level' signals. I think it's important for noise reasons to get the signal level up in an amp quickly, as opposed to having lots of guitar level signals running around the place. Not sure how good the PSRR of the cathodyne is either.

Nathan

[Satamax]

Hi Nathan.

Verry nice of you to advise this. But i want no paraphase or long tail pair! Schmidt or else. I want cathodyne. I have thought of this. Well, i most certainly end up having the first half of V1 as a gain stage before PI, thought, i don't think it would mater that much, if the cathodyne is the first stage, as there would be a gain stage right after. No tone or else in between thoses two.

Hey guys. Just an idea which crossed my mind. Since, let say on V2, the stage after the cathodyne, signals will be out of phase, wouldn't it be possible to make something like a cathode volume control. Something like split cathode resistors and bypass caps. Then a pot feeding the signal from one cathode to the other cuting the volume.

Thanks again guys. Keep the ideas coming.

[tubeswell]

Merlin has a neat bootstrapped cathodyne in his 1st book which you could look at for added gain

[MWJB]

"I think an LTP or paraphase would make a better first stage. The cathodyne has no gain, so the outputs will be 'guitar level' signals. I think it's important for noise reasons to get the signal level up in an amp quickly, as opposed to having lots of guitar level signals running around the place. Not sure how good the PSRR of the cathodyne is either.

Nathan"

Hi Nathan,

You are overlooking the gain stage that typically preceeds the cathodyne PI, I envisaged Satamax as intending to do the same as me/Dano, by having an input/gain stage at the first triode of V1, then the PI as the 2nd triode. Gain wasn't an issue for me, I actually used a split load plate resistor at V1 triode A to make it manageable, the Dano (whilst not exactly a high gain amp) gets by with just a 6SN7 as the gain stage following the PI.

[redelephant]

Isn't the PI stage called a 'cathodyne' only the single triode (or transistor) doing the actual phase inverting? Therefore technically it has no gain.
But in guitar amps it is so common to have a common cathode stage preceding the cathodyne and in that case the PI-stage as a whole does have gain, but it is not the cathodyne that gives it the gain...???
The way I understand Max's idea is having only the cathodyne without the gain-triode as the first stage in the preamp. But I agree with the others that having gain first is a good idea to increase the S/N ratio.
Kinda like this: (I'm using 6922s, so the various resistors will need to be changed with other tubes)

I've just completed this stage in my preamp, tho as the last stage before the MV, and I really haven't had a chance to give it a good testing yet. It certainly gets a bit more fizzy in the distortion since it mainly gives odd order artifacts. I guess it has a right to exsist but it is different than SE preamp distortion. Clean I hear no difference.

[octal]

"I think an LTP or paraphase would make a better first stage. The cathodyne has no gain, so the outputs will be 'guitar level' signals. I think it's important for noise reasons to get the signal level up in an amp quickly, as opposed to having lots of guitar level signals running around the place. Not sure how good the PSRR of the cathodyne is either.

Nathan"

Hi Nathan,

You are overlooking the gain stage that typically preceeds the cathodyne PI, I envisaged Satamax as intending to do the same as me/Dano, by having an input/gain stage at the first triode of V1, then the PI as the 2nd triode. Gain wasn't an issue for me, I actually used a split load plate resistor at V1 triode A to make it manageable, the Dano (whilst not exactly a high gain amp) gets by with just a 6SN7 as the gain stage following the PI.

Well, perhaps I was being overly literal, but he said "a cathodyne as the first stage." The gain stage which typically proceeds a cathodyne has nothing to do with it per se, other than sharing a bottle if a dual triode is used- it's not part of a cathodyne. What you're suggesting (gain stage>cathodyne) would be a better idea though!

Nathan

[tubeswell]

I say again - Merlin's bootstrapped cathodyne (using two triodes) has gain.

[Wombaticus]

I say again - Merlin's bootstrapped cathodyne (using two triodes) has gain.

Properly speaking, it is not the cathodyne that's bootstrapped. It's one of the split load resistors in the preceding stage that gets bootstrapped BY the low impedance output of the cathodyne. Except, er, if you're using the cathodyne as an input stage, there is no preceding stage to bootstrap......

[tubeswell]

Well naturally... I was envisaging a preceding inverting stage (as per Merlin's circuit) when I mentioned it. Guess I shoulda made meself clear.

[kg]

MAX! how the hell are you, old friend?

-ken

[Satamax]

MAX! how the hell are you, old friend?

-ken

Hey Master! I'm cool! Self employed in roofing and general woodworking nowadays. About you? Still fiddling with imprezas? Do you still have the BAGA?

Keep yourself well.

Max.

Max Debelleix | Facebook