Single ended 2 x 6V6 in parallel schematic ideas?

[tubeswell]

Hi y'all

I would like to try building a SE 2 x 6V6GT - cathode biased with a 'verb. It would have a parallel triode Voltage Amplifer stage and separate driver (apart from the reverb) similar to the attached schematic (but with 2 x 6V6GT in parallel)

I am trying to work out the size of the OT - I realise it needs a 2k5 primary - Is that going to be 10W output? or can I get even more power out of paralleling the two 6V6s?

[tubeswell]

I've just found the Angela Super SE amp :-)

http://www.angela.com/catalog/how-to/Super.SE.6V6.html


Which sort of answers my questions.

Anyhow if I was wanting to run separate (say 470R) cathode resistors/bypass caps for each 6V6, and have the capability of subbing one 6L6/5881 (as per MWJB's suggestion) in this thread:

http://music-electronics-forum.com/s...ad.php?p=46347


Do I need to have a different cathode resistor/bypass cap for the 6L6/5881 if it is running about 370VDC on the plate? I figure (i.e.; I guess wildly) that if I have 370V Plate and maybe about 20V on a 470R cathode resistor, that will give me 42mA tube current and 14.7W - Is that too cold for a 6L6?

Or do I need a higher Plate/Screen supply for the 6L6? (I was thinking of having a 300-0-300 PT running a single 5U4GB to get the B+ of about 374 - instead of a 325-0-325 running two 5Y3GTs - I am thinking to run 3 or 4 preamp tubes to get 3 triodes for reverb and three for parallel triode VA and Driver)

(Incidentally I understand that the 2k5 OT primary applicable to the 2 x 6V6s would serve both applications, and I've found a 15W SE OT with 2k5 and 5k primaries)

[tubeswell]

Yet another question on this - related to matching

If I ran matched 6V6s in parallel for this SE design, would they be quieter hum-wise, than unmatched ones? (i.e.; does matching only reduce hum in push-pull output stages?)

[Synaptic Amps]

I've been working on similar single ended designs. I've found it difficult to source PTs that can supply the juice needed to run two 6V6s without vastly exceeding their plate voltage. Your tube rectifier should help you in this regard. However, in your schematic you show 320-0-320 through a pair of 1N4007s yielding 382VDC... I'd expect to see closer to 450V. 270-0-270 would put you more in range of 382 which I think would be much better with your 6V6s.

Bypass cap switching on your preamp stage cathodes can be cool. I saw you labelled your stage 1 cathode switch FAT. Unless you have another smaller cap like a 1uF or 0.68uF fixed in place the effect will seem more like a BOOST switch then a FAT/BRIGHT switch. Either way can be useful. You're going to get a nasty pop when you engage those switches. Rather than leave the cap hanging free in the open switch position wire it up to ground through a large value resistor like 100K. Wire the switch to bypass this resistor. Same switch effect. No pop.

Per your output stage cathode. I don't know how 470 ohms became "the standard" 6V6 cathode. Fender used it so it's the law, I guess... Granted I'm not using Fender transformers or Fender circuits, but I've never obtained optimal bias with a 470 ohm resistor. Not even close sometimes... totally depends on the circuit. You're smart to be anticipating experimenting with other values later on but as you noted in your schematic you won't know what value is best until you've got the amp wired up and take some measurements and do some listening. In preparing yourself for this you've got at least two options: One is to collect a library of cathode resistor values so you can swap 'em in and out until you zero in on the bias you want. I prefer to use adjustable tap wirewounds. If you think 470 is a good target then get a couple of 1K adjustable WWs in there and you'll be able to precisely set the bias current you need no matter what voltages you end up looking at and no matter what tubes you end up using.

Running "matched" tubes in a parallel SE output stage is going to have ZERO effect on hum. This being the case, you can take advantage of the tonal differences between tubes and mismatch them on purpose. You could even go so far as to run an EL84 with a 6V6 in parallel SE.

I'd be interested to learn where you found a 15W SE OT with 2k5 and 5k primaries unless of course your talking about the 125ESE.

[tubeswell]

I've been working on similar single ended designs. I've found it difficult to source PTs that can supply the juice needed to run two 6V6s without vastly exceeding their plate voltage. Your tube rectifier should help you in this regard. However, in your schematic you show 320-0-320 through a pair of 1N4007s yielding 382VDC... I'd expect to see closer to 450V. 270-0-270 would put you more in range of 382 which I think would be much better with your 6V6s.

Bypass cap switching on your preamp stage cathodes can be cool. I saw you labelled your stage 1 cathode switch FAT. Unless you have another smaller cap like a 1uF or 0.68uF fixed in place the effect will seem more like a BOOST switch then a FAT/BRIGHT switch. Either way can be useful. You're going to get a nasty pop when you engage those switches. Rather than leave the cap hanging free in the open switch position wire it up to ground through a large value resistor like 100K. Wire the switch to bypass this resistor. Same switch effect. No pop.

Per your output stage cathode. I don't know how 470 ohms became "the standard" 6V6 cathode. Fender used it so it's the law, I guess... Granted I'm not using Fender transformers or Fender circuits, but I've never obtained optimal bias with a 470 ohm resistor. Not even close sometimes... totally depends on the circuit. You're smart to be anticipating experimenting with other values later on but as you noted in your schematic you won't know what value is best until you've got the amp wired up and take some measurements and do some listening. In preparing yourself for this you've got at least two options: One is to collect a library of cathode resistor values so you can swap 'em in and out until you zero in on the bias you want. I prefer to use adjustable tap wirewounds. If you think 470 is a good target then get a couple of 1K adjustable WWs in there and you'll be able to precisely set the bias current you need no matter what voltages you end up looking at and no matter what tubes you end up using.

Running "matched" tubes in a parallel SE output stage is going to have ZERO effect on hum. This being the case, you can take advantage of the tonal differences between tubes and mismatch them on purpose. You could even go so far as to run an EL84 with a 6V6 in parallel SE.

I'd be interested to learn where you found a 15W SE OT with 2k5 and 5k primaries unless of course your talking about the 125ESE.

Thanks for your reply Synaptic Amps - I can't take credit for the schematic - I only borrowed it from psychonoodler over at the Dumble forum. Although I did wonder about switch popping for the cathode resistors - having tried those and other mods on my PV Classic 30 ad-nauseum.

I am thinking to build it up with the 2x 6V6s. I suspected that matching them wouldn't affect hum in this case, but I wanted to check. I am really interested in someone being able to verify my guesses about biasing for subbing in a single 6L6 (instead of both 6V6s) in either one of the sockets if I set them up with separate cathode resistors. I don't want to have to re-bias it each time I do the sub if that can be got away with

I'm not woried about finding the right sized stock PT, So far in my builds I've had all my PTs (and OTs) made up to what I want - There's a guy in Auckland who still knows how to wind PTs and OTs for cheap. I'm thinking I'm not going to use SS rectification - I'm going to go with a 5U4GB and the 5G9 I built recently get a B+ of 374 with this type of recto paired to a 300-0-300 HT winding.

I found the 15W OT on trademe - Apparently the guy sells a few of them. I'll let you know if it works.

[Synaptic Amps]

That sounds like a real good iron connection you've got there. I'm pretty unimpressed by the Mercury Magnetics I've been using. I wouldn't mind if you could share some contact info for this gentleman in Auckland.

I think your best bet if you want to be able to swap a pair of 6V6s or a single 6L6 without adjusting would be to use a single cathode resistor for both tubes closer to 200-300 ohms. A 12W 500 Ohm Adjustable Wirewound would be more than adequate and would enable you to tune in the best compromise bias point to allow you to just plug and play. If you end up favoring one configuration over the other you would be able to quickly and easily adjust the cathode optimizing the bias for that specific arrangement.

[Synaptic Amps]

More specifically to your question... If you anticipate seeing about 355 volts from cathode to plate (375-20) then 42ma is much too cold for a 6L6. 75ma would be closer to the target bias current. Under the same conditions the target 6V6 idle current would be about 35ma. For two 6V6s in parallel you might aim for a total current draw of 70ma. If you setup you 6V6s to run slightly hot at idle, say 72.5ma through a single cathode resistor then you'll be able to swap in a 6L6GC and be just about right on.

The target idle current draw for either a single 5881 or KT66 with a 355V plate-to-cathode voltage would be about 65ma. Swapping either of these into your circuit they would run on the hot side. The current production KT66s I've tried seem to welcome this. There's no reason you couldn't try an EL34 as well giving you lots of different sound options all achievable by direct tube swapping without any cathode adjustment required.

[tubeswell]

That sounds like a real good iron connection you've got there. I'm pretty unimpressed by the Mercury Magnetics I've been using. I wouldn't mind if you could share some contact info for this gentleman in Auckland.

I think your best bet if you want to be able to swap a pair of 6V6s or a single 6L6 without adjusting would be to use a single cathode resistor for both tubes closer to 200-300 ohms. A 12W 500 Ohm Adjustable Wirewound would be more than adequate and would enable you to tune in the best compromise bias point to allow you to just plug and play. If you end up favoring one configuration over the other you would be able to quickly and easily adjust the cathode optimizing the bias for that specific arrangement.

Thanks again Synaptic amps

I sent you a private message about teh tranny - hope you get it - I have tried to configure my options to receive private messages lately, but it seems to keep telling me I can't - anyway - I sent an RFS to tboy via a private message - hope he receives it - even if he can't send one back

[tubeswell]

More specifically to your question... If you anticipate seeing about 355 volts from cathode to plate (375-20) then 42ma is much too cold for a 6L6. 75ma would be closer to the target bias current. Under the same conditions the target 6V6 idle current would be about 35ma. For two 6V6s in parallel you might aim for a total current draw of 70ma. If you setup you 6V6s to run slightly hot at idle, say 72.5ma through a single cathode resistor then you'll be able to swap in a 6L6GC and be just about right on.

The idle current draws of a either single 5881 or KT66 (each about 65ma in your conditions) look like they would all match up very well with a pair of 6V6s giving your amp lots of different sound options all achievable by direct swapping without any cathode adjustment required.

I've been using those new sensor 'tung-sol reissue' 6V6s - got one running at 37 (or so) mA in a 5F2A - seems to be holding out (touch wood), so I'll do what you suggest. I suspect a 270-330R common resistor oughta do it.

[Synaptic Amps]

I did receive your private message but was unable to send you a reply to it.
I didn't realize that we're worlds apart. I'll have to look around here in the states, surely there must be other folks out there that make custom wound transformers. No doubt they're all as you say, however, approaching retirement.

I'm a big fan of all the New Sensor output tubes. In a way it's too bad they're not offering a 6L6 model but their 5881s and KT66s have a very pleasing sound to my ears.

[Paul P]

I have a Heyboer output transformer from Allen Amplification that I use with
parallel 6v6s and it can be used with a single 6V6, EL34 or 6L6. Model
is TO11C, quote :

8,000 ohm primary to 8 or 16 ohm secondary for use with a single 6V6. 4,000 ohm to 4 or 8 ohm for use with a single 6L6, EL34 or two 6V6s in parallel. Rated 95ma continuous.

Paul P

[Wakculloch]

Brimar tube manual gives a cathode resistor of 240R, with a HT of 280v.
When i did a SE 6V6 amp with a HT of 350V i used a 220R that gave good results. Two 6V6's could then share a 110R cathode resistor.
Have also used 6L6's in this amp without mods.

[Sir Cuitous]

Wow - I would have posted sooner but I didn't see this thread. Tubeswell, I just finished making an Angela straight to the schematic (with one major twist). I used a Weber PT (for Fender Deluxe) and a Hammond 125ESE for the OT. I had a spare Mercury choke from a Champ project which also worked fine. It turned out to have a stunning tone IMHO. The twist was that I put in two big switches to change the triodes of the pre-amp tubes. I felt that if I was going to wire two sockets, I might as well do some more wiring. The switches work well and I can run triodes from each tube as the first or second stage. I used a board for a Fender Deluxe which was similar enough. I built it in the form of a Deluxe Reverb so that I could mount the switches on the back panel. I used only one 5Y3 rectifier and it still pushes the 6V6s hard. I believe two 5Y3s are overkill. I have some pics at this link:

http://s286.photobucket.com/albums/l...Amp%20Project/

[Synaptic Amps]

Wakculloch, I'm curious to know what bias current you're getting with your 220 ohm cathode resistor. Can you provide a cathode voltage reading?

[tubeswell]

Hey Circuitous

Those are nice lookin' geetars.

I couldn't see the valve compliment on your photobucket link. What sort of bottles are you running on it? I'm thinking of more Tung Sol 6V6 reissues, as I've got these in two amps so far and (touch wood) no problems, although I've read various damming reports about the size of their screen grid support rods.