New schematic for review

[Howlin' Mad Mac]

I've recently been asked by a friend to build a small SE amp for his home recording studio so after rumaging through my parts box I found a Tweed Champ style PT & Tweed Deluxe OT perfect for just such a project however he's looking for more of a British sound so I submit to you, my panel of experts, this "tweaked" Tweed Deluxe schematic. Please review for accuracy & submit any comments, suggestions, etc. you think I might find helpful & as always thank you for your input.

[tubeswell]

If its for a recording studio, you might want to use a CLC filter (with a ~50mA choke) between the rectifier and the OT/Plate supply node, otherwise you probably will get more hum than you want. Also you might want to consider NFB?

[redelephant]

They are probably typos, but the schem has some issues. The grid of tube no2 needs a DC path to gnd, that being usually a resistor (could be a choke). The screen grid of the 6V6 needs a connection, but that must be that 1kohm shown going to the plate. The OT wont do much good since it's made for pushpull and has no gap in the core. A trick could be to have a power resistor sink equal current down the one phase that has no tube connected.
What is 'british sound'? More bright as in Vox? Are there any SE jobs that are classic british? The SE output colors the tone a bit, warms it up, so may be hard to get that tone. I guess playing with response thru the coupling between stages will give it a basic sound. Coupling cap from 1nF to 10nF and resistor around 100kohms is a start.

[Howlin' Mad Mac]

Ok I see where you're going... For a choke like something in the 5 - 10mH range? Also I considered NFB like typically found on a Tweed Champ or Deluxe but the "tweaks" I've added were borrowed from a low watt SE Vox I think which had no NFB which is why I left it out but easy enough to incorporate to the design, maybe on a switch for with/without or I could add something like a Marshall style Presence control, eh?

[Howlin' Mad Mac]

Thanks for pointing out the boo boos... Regarding the grid of (12AX7b?) shouldn't the volume pot ground be sufficient? However the B+1/1K should go to the screen not the plate so kudos for catching that. As far as the OT it's wired exactly the same way Leo did it & I know they work fine that way as I've built Tweed Deluxes before & they sound good to my ears so I not really understanding what you're getting at there. Coupling cap values are a bit of a guess but taken from some Vox schematic I dug up somewhere... AC4 I think.

[MWJB]

The OT you have has no airgap but it will "work", another alternative could be to set up the output as a "self-split" push pull, when you switch in a power tube cathode bypass cap the amp will operate in SE, but you will still have current being drawn at both ends of the OT primary.

100uf first filter cap would be cheaper than a choke, often undesirable hum in SE amps comes from the heaters, so dc biasing the 6.3VAC is an idea if you have problems in that respect.

You want a 5W cathode resistor for the 6V6.

[MWJB]

Volume pot ground is isolated from the grid by the coupling cap, you need a resistor to ground after the coupling cap & before the grid @ V2B.

[J M Fahey]

I've built Tweed Deluxes before & they sound good to my ears so I not really understanding what you're getting at there.

No doubt you have, but they were Push Pull; this one is Single Ended ... with a PP designed transformer.
As posted above, it will work, sort of, but that's not the optimal solution.
May I respectfully suggest one of the AX84's ?
All you'll need is a new OT (which you really need anyway) and the sound is killer.
They have a huge and loyal following and a great Forum where all kind of tweaks (probably including what you need) are freely discussed.
Just my 8 Peso cents (around 2 U$ cents at current rates)

[redelephant]

Didn't mean to be a partypooper about the OT. Here's a simplified schem of how to pull symmetric DC current thru the tranny. But the resistor will dissipate as much as the tube and will be real hot. Not the most efficient way to do it. As mentioned by the others the tranny will work, and since guitar never goes too low in frequency I doubt you'll have problems with it, but a OT for SE should have a little airgap so the core doesn't saturate.
A 100uF first cap will shorten rectifier life, a 5-10H choke is a few bucks more, but gives better results.
A SE job is more noisy b/c of no out-of-phase b+ ripple cancelation we otherwise get from a PP output. Luckily high impedance tubes like pentodes and beam tubes give less output hum than low impedance triodes, but you'll still have noticable hum with only two 16uF caps and no choke. You can feed some out-of-phase ripple into the cathode and cancel hum that way, but that requires some experimentation in cap values. Not going into that, just confusing...
Tie the heaters to the 6V6s cathode, the positive bias will prevent any hum entering the circuit.

[mooreamps]

He's almost be just as well off putting in another power tube, instead of the 20K.

-g

[redelephant]

Yeah, agreed. Or a couple of 10watt light bulbs in series...after all, those are thermionic vacuum devices as well.

[MWJB]

A second power tube would give a swicthable push-pull option for no more outlay than a tube socket a couple of resistors and an SPST.

[redelephant]

Nah...stick to the original plan. Didn't mean to mess up your idea, the little 6V6 probably wont cause an issue even tho the OT is for PP. If not simply invest the 20bucks a new SE tranny sets u back.

[Howlin' Mad Mac]

Ok, I spent several hours over the weekend refining & revising so here's the final draft(?) ready for review. It's split into 2 schematics, the 1st being the main circuit, the 2nd is the power supply with 6.3v fillament supply shown below. Regarding my choice of output transformer I initially reported it as a "Tweed" Deluxe OT but it is in fact a "Tweed" Princeton OT. This transformer is listed as a SE transformer but I "ve questioned its primary center tap myself. I also have seen a similar version of this transformer with b suffix in a "Blonde" schematic with 2 6V6's wired in parallel. Anyway the OT in question is a Custom Magnetics 022913 made in Chicago & listed as a replacement for Princeton Reverb & Tweed Deluxe model amps. I've also got a Hammond Champ transformer about but I chose this one thinking it would give a bit more headroom. Take a look at the revisions & let me know but I think I've got it nailed this time. :-P

[MWJB]

Just a couple of points - the 2.2uf cap bypassing the 2nd triode will bleed some AC to ground, lessening the impact of the NFB loop, there will still be some effect, but perhaps if you want to keep the bypass cap in the circuit, check out the BF/SF Champ style loop?

any reason why you're going with 250K pots?

If you lift the ground reference on the OT common won't you get oscillation?