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Old 26th April 2012, 06:36 PM   #21
rongon is offline rongon  United States
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I've been grappling with gain staging calculation a lot lately. I think I'm finally getting the hang of it, so please take whatever I write here with a grain of salt. ANYBODY LOOKING AT THIS -- Please correct me on anything you see below!

---

So here's my proposal...

With only 300V of B+ available, you won't get much power out of a pair of 6L6's. You'll definitely want to run them as pentodes (or in ultralinear if your output transmformers have screen taps). But then again, with that low a B+ available and with the current limitations of your power transformer, you'll get little more power from a pair of 6L6's than you will from a pair of 6V6's. So why use the bigger tubes?

The good thing about using 6V6's is that they will be really easy to drive. You won't need a current-hungry, high voltage driver stage to get them to full power. You'd actually be able to use a single long-tailed pair phase splitter like in Eli's El Cheapo amp (which basically uses a 12V, 7-pin version of the 6V6 in its output stage). Since you already have four small signal tube sockets, you could use a pentode for the constant current source in the LTP phase splitter cathodes. That would be a very high performance option.

With 300V of B+, I'd expect the bias on a pair of 6V6's to be about -15 volts or so (not sure exactly). I got 21x gain from a 12AT7 LTP phase splitter, so if you can get the 12AT7 running with a bias of about -1.7V or higher, you will have all the headroom necessary to put a little feedback around the circuit. (6dB of NFB requires double the drive voltage).

So, working from the output tubes to the input of the circuit:

- 6V6 with -15V bias means 15V peak (30V peak-to-peak) will drive the output stage to full power

- the 12AT7 will need to deliver at least 15V peak, and more for negative feedback (which reduces the input sensitivity of the stage with the feedback entering it).

- The 12AT7 has a published gain spec (mu) of about 60. In an LTP with single-ended input, you'll get only half that, so 30. There will be losses with resistors as plate loads, so figure on a final gain of 21 or so.

- The input voltage to the 12AT7 cannot exceed its Vgk (bias voltage). Figuring on a Vgk of 1.5V, that means you can get about 1.25V peak signal into it before it starts distorting. 21 x 1.25 = 26.25 -- Which is enough for about 5dB of NFB.


If you use 6L6 with these same conditions, you'll need a few more volts of grid bias (probably -18V) and you won't get hardly any more power. That means you won't have the voltage gain available from the 12AT7 for feedback. Also, 6V6GT's sound really nice.

If you use the "Mullard" style circuit you've posted, you'll have waaaay too much gain unless you put a lot of negative feedback around the circuit to reduce the gain to something usable. Then you'll need to work on getting the circuit stable, make sure the OPT's don't ring, etc. But that's good too.

I like Eli's idea of using a two-stage, LTP to PP, and use the extra tube sockets for pentode CCS's for the LTP's. You might even be able to use 6DJ8's for that.

--

Again, please check all that, and if anybody has corrections, please post 'em. I'm hoping to learn as I go along.

Thanks, and I hope this helps.

--
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Old 26th April 2012, 07:13 PM   #22
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My OPTs are 3,600 primary. The 6V6 does not look very happy with this.

I like the pentode CCS option. I was reading an article the other day about CCS loading a Phase inverter with a pentode and thought it looked pretty nice!

Blair
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Old 26th April 2012, 08:38 PM   #23
rongon is offline rongon  United States
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Quote:
Originally Posted by deicide67 View Post
My OPTs are 3,600 primary. The 6V6 does not look very happy with this.
Oh, I missed that. You're right about that. 3k6 is low even for 6L6.

Do you know what the max current capability is for your power transformer? One thing that works well is to run the 6L6's (or even EL34's) at a low plate voltage but high plate current. You don't get much power, but you can get the distortion low.

The Russian 6P3S (what Sovtek sells as their cheapest "6L6GC") is only rated for 20 watts max plate dissipation. You could run it at 300V on the plate, 65mA plate current per 6L6. You could run the screens from a lower voltage regulated supply (like 250V), again to lower distortion.

But all that assumes that your power transformer has enough current capacity.

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Old 26th April 2012, 08:55 PM   #24
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Hi Rongon,

That was my plan. I'm not exactly sure what the tranny is rated for, but the receiver had 23 tubes. 4 7355s, and an assortment of other 7-9pin variants. So, I'm looking at 4 X 6l6s, and 4 tubes of my selection.

Assuming I do not choose any current hogs, I would assume I should have decent amount of current.

How about this?

You mentioned the Mullard topology having too much gain. What if I use a 12AU7 as the driver, and either a 6CG7 or 12AU7 as the PI? That should tame a little of the gain right?

I found a data sheet with the 6l6 running at 270v into 5k at 17W. I figure I can get around the same power, but possibly bias it down a bit due to the lower transformer load.

Thanks!

Blair
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Old 26th April 2012, 09:20 PM   #25
rmyauck is offline rmyauck  Canada
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Maybe triode strap output tubes (6V6GT or better EL84/6BQ5 ) run in parallel for a 7.2K primary impedance. Also EL34 triode strapped too may take 3.6K better. Fixed bias should be used for best results too!

Randy
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Old 26th April 2012, 09:35 PM   #26
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Hi Randy,

I do not have enough heater current to go EL34 triode, or that is exactly where I would be I love EL34 in triode.

I really expected a different PP primary out of these closer to 5-6k.

Blair
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Old 26th April 2012, 09:57 PM   #27
rmyauck is offline rmyauck  Canada
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Get Heater supply transformer as they can be had for low cost. Antek may have something suitable.

6CW5/EL86 is another choice, but PS trans would have to be changed.

Tubes & Transformers

Randy
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Old 26th April 2012, 10:36 PM   #28
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Hi Randy,

I would do that if I had the real estate to add another transformer. I'm kind of stuck on this one.

Blair

Last edited by deicide67; 26th April 2012 at 10:38 PM.
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Old 26th April 2012, 11:19 PM   #29
rmyauck is offline rmyauck  Canada
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Maybe put the PS transformers (and choke if used or leave room for a good one later ) on a small separate chassis or make mono-blocks for better stereo separation, sound stage etc.

Last edited by rmyauck; 26th April 2012 at 11:30 PM.
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Old 27th April 2012, 12:10 AM   #30
rongon is offline rongon  United States
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deicide67,

I expect that since you have four octal sockets for output tubes and four 9-pin mini sockets for driver tubes already mounted in the chassis, that you want to use those?

I don't see where you could stuff four more octal sockets for push-pull-parallel 6V6's (which is what I think rmyauck was suggesting). Pity, that would be awesome... Triode-wired 6V6's are supposed to sound very nice, and since you know you like triode-wired EL34's, I'll bet you'd like those too.

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