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Old 11th March 2013, 08:57 PM   #11
GordonW is offline GordonW  United States
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Quote:
Originally Posted by artosalo View Post
To get 30 - 40 W with 6L6GB, 5881 or 807 you need 450 V (with 6k6 OPT).
Yep. I can vouch for that combination working, as I built just that, into a pair of old Dynaco Mk III chassis that I got without transformers.

I used 360-0-360 power transformers with solid-state rectification, and a pair of Hammond 1650P outputs (60w rated max power, 6.6K UL), with fixed bias (idled at between 50 and 55ma per tube). I used 6L6GCs, but as long as you kept the idle current at 50ma or less, you could also use 5881s. 807s would also be a drop-in, anywhere in that bias current range.

With the 6L6GCs (and I just used a set of plain-Jane Ruby Tubes 6L6GC-STR tubes) t'd go right to about 38 watts per channel, before starting to show the first signs of flattening at the top of the waveform...

Sounded fantastic. I just used the stock Mk III driver boards (if they'll drive 6550s, they'll absolutely drive 6L6s), with the feedback circuit tuned a bit for the different outputs... it took very little to make it work very well.

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Old 11th March 2013, 09:06 PM   #12
GordonW is offline GordonW  United States
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BTW: You could use the 6K transformers- but it'd probably be better to go pentode mode, fixed bias. Probably around 400-425v plates, and about 325-350 on the screens. That should give pretty low distortion (I'd expect somewhere between 1.5% and 2% open-loop, if the transformers are good). Probably about 35 watts.

I'm actually in process of building this exact combo now. Got a set of Scott 340B output transformers (6.2K, rated 35 watts full bandwidth. I'm going to run about 430v on the plates and 340v on the screens (already got a big honkin' PT from an old Fisher receiver that does just about that). The screens would need to be TIGHTLY regulated- I'm going to use a SS rectifier and big caps, then use a gas regulator tube in series (like the Westinghouse 7591 design published in hobbyist magazines, and like many of the Leslie organ speaker amps) to drop about 90v from the B+. Should regulate to within 5v, assuming the main B+ from the rectifier doesn't sag itself (and with SS diodes and a hefty PT, that should be pretty solid, too)...

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Old 11th March 2013, 09:15 PM   #13
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Quote:
Originally Posted by deicide67 View Post
Hi Guys,

I cannot find much for the 6L6 in UL. A few, but nothing substantial.
You won't. The 6L6 really isn't all that amenable to UL, due to the low screen voltage limitation. You'd be better off with parallel local NFB, or cathode lNFB if you have an OPT with a center tapped secondary (or a selection of different loads -- the 4.0R connection is a center tap to the common and 16R outputs). You will be requiring some sort of lNFB with the 6L6, as it likes to make nasty higher order harmonics.

See: Beam Power Tubes for more details on how to use this type, and the other 6L6-oids.

Quote:
Assuming a B+ of ~350V in UL into a 6K A-A load, what kind if power would one expect?

I'd like to run fixed bias and hope for around 30-40W.
Vpp= 360Vdc
V2k= 270Vdc
Vgk= -22.5Vdc, fixed
Rl= 6K6 (P-2-P)
THD= 1.8%

This is rated at 26.5W, however with adequate grid drive, such as 6SN7 cathode followers, I get 32W measured with Hammond OPTs (though I don't recommend them anymore, due to QC issues that have cropped up. Use Edcor if you're looking for budget OPTs, or go for more expensive types, though I'm not so sure they're all that much better than Edcor's) before the core saturates since the grid drivers allow for a small degree of unintended Class AB2 that's worth a few extra watts, and a lot less clipping distortion.

I wouldn't try to get much more power out of a PP pair of 6L6s. If you do, then you'd be better off with one of the other 6L6-oids:

807
1625 (the 12.6V heater version)
6BG6 (octal base version of the 807

All of these have plate cap connections that will allow for higher plate voltages, and higher power output. The A Number One problem with the 6L6 is that the plate is right next door to a heater pin, so you have the highest voltage next to the lowest. Much more than 300V is asking for a flash-over across the pins.

You could get 80W from a PP pair of 807s with Vpp= 600V, Class AB2, and source follower grid drivers with reasonable fidelity. 750V on the plates, and very deep Class AB2 can get 120W, but that'll sound pretty horrid, as this operation was intended for AM plate modulators where fidelity was neither necessary nor legal.
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Old 22nd November 2013, 09:59 PM   #14
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Jeepers Gents ....

Found this very late, so not sure whether of any relevance any longer. But as my application is very similar to the question by Deicide67, perhaps my use:

I am using 2 x 2 6L6GCs (Tungsol STR) in ppp in a 100W amplifier under the following conditions:

Plate-cathode voltage: 560V
Screen-cathode voltage: 460V (stabilised)
UL taps (separate winding): 25%
G1 fixed bias: -43V nominal (adjustable for cathode current)
Cathode current: 50mA per tube
Load resistance (p-p): 2,7K
Output into 8 ohms: 110W
OPT: C-core type

Flash-over danger inside tube? I doubt it. The EL34 has a similar structure in the base, and the Va for that is rated as 800V. The 7027A has the same internal structure as the 6L6GC and is rated for Va=600V, Vg2=500V.
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Old 23rd November 2013, 11:44 PM   #15
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Quote:
Originally Posted by Parafeed813 View Post
If you are willing to go AB2, you can get more, even in triode. I build a PP AB2 amp, with around 450 on the anodes. With 6L6GC's I get 40-45W (triode). The clipping is so soft it is hard to decide when to stop turning it up. But that is the whole idea about AB2: keep within AB1 90% of the time, with extra headroom to handle the transients.
Parafeed, by any chance were your transformers UL capable, and if so, did you hook up the UL windings to see what the result was?

Did you use mosfet followers?
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Old 24th November 2013, 12:38 AM   #16
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Have you seen this circuit? I have everything to build it, even the Acrosounds, except the tme. I have an article that claims 24 watts before clipping. Crank in more B+ and stouter output transformers and you should be able to get 30 or more watts.
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Last edited by mr2racer; 24th November 2013 at 12:43 AM.
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Old 24th November 2013, 02:26 AM   #17
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And of course fixed bias will get you a little more too.
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Old 8th December 2013, 10:51 PM   #18
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Quote:
Originally Posted by tomlang View Post
Parafeed, by any chance were your transformers UL capable, and if so, did you hook up the UL windings to see what the result was?

Did you use mosfet followers?
Yes, there are UL taps, but I didn't try them yet.
MOSFET followers indeed.
Unfortunately I can't make serious measurements as my scope broke and I moved my workspace. Lots of boxes and no space on the workbench.
I am a researcher, so there will be numbers, comparing triode to UL, different versions/brands, 6L6 vs 6550, KT88, EL34... Only when??
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