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Old 23rd May 2013, 11:31 AM   #11
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Hi Thomas,

I've looked over your blog several times. Nice builds!

I do not see the PP class A amp you are referring to.i would probably go AB1 anyway. What plate load did you use for your PP amp?

I'd like to get 45-60W out of a pair.

Miles, I'm good with Pentode. Even screen drive at lower voltage than Tubelab uses. I have some old Fisher KX-100 (or at least access to) output iron to use as a Phase inverter for a screen drive type setup.

Thanks,

Blair
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Old 23rd May 2013, 11:58 AM   #12
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Hi

I did not publish the PP amps on my blog. I am mostly into SE.
For Class A PP you need twice the load impedance as for Class A SE. I used 8k plate to plate in my PP build. AB has different load requirements.

Do you really need that much power? Are your speakers that inefficient?

Best regards

Thomas
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Old 23rd May 2013, 12:13 PM   #13
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Hi Thomas,

My speakers are fairly inefficient. I use a 185W Emotiva most of the time to drive them. I have a ST-70 Dynaco, but it doesn't cut it at all during transients.

I'm not trying to go "Tubelab" here, but agood amount of power would be nice. I think I found your old schematic somewhere. Uses an interstate transformer for phase inverter duty? In a post I found, it said you used a 7K output transformer and the amp was good for ~15W class A. I cannot figure out how to get the image off my phone.

So, AB Pentode should have a realistic output of at least 40W I would think.

Blair
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Old 23rd May 2013, 12:30 PM   #14
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Hi!

I published it years ago on a german tube forum. But if you want to go Pentode AB, this schematic will not help much.

Thomas
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Old 9th June 2013, 04:00 PM   #15
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I think I am going to order some edcor output iron for this project. Too much going on this summer to swing the $400 for an octet of KT-120s for my other project.

So, is ~2.4K the proper load for a PP pentode pair also?

EDCOR - CXPP60-MS-2.4K

Maybe this transformer? I've also read that the 6CB5 and other horizontal sweeps are relatively easy to drive. Could I use a simple, single tube driver using something like the 6GF7A as a driver?

http://tubedata.milbert.com/sheets/049/6/6GF7A.pdf

Thanks!

Blair
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Old 10th June 2013, 02:39 AM   #16
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deicide67
I am testing power transformers for using 4X 6CB5 triode connected; I intend to use Choke input and set the voltage using AC caps; I intend to use 400 volts.
Luckily the power transformer has a 10 amp at 6.3 volts and 6.3 at 6 amps. If I don't like the amp I can always use 6550's at a higher voltage either UL or Triode.
I intend to use 2.2k A---A.{also siuts 4 X 6550's} and cathode drive using 6BL7's and 12BH7's or Russian 6N6 as drivers.
Phil
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Old 10th June 2013, 02:14 PM   #17
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Blair - Not sure if you've seen the 6CB5A thread linked below . . . I posted some PP schematics that I had breadboarded (that project is still together but dormant again - too many distractions!) There's also a pdf of Thomas Mayer's PP design that uses an interstage transformer and a few posts by Bandersnatch about his design that uses 6BQ5s as drivers. Maybe he'll see this and post a schematic and give us an update on how it all turned out, if it did. Anyway, check it out . . .

6CB5 AMP BUILD.

. . . Charlie
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Old 10th June 2013, 02:16 PM   #18
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Hi Charlie,

Blair wants to use the 6CB5A pentode connected, so this schematic won't help much. I did a Class A PP which gives twice the output power compared to a SE with a single tube.
I never tried the 6CB5A pentode connected.

Thomas
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Old 10th June 2013, 04:39 PM   #19
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For the PP pentode version I used a voltage regulator tube to supply the screen voltage. I don't think I drew up a schematic for it though.

. . . Charlie
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Old 10th June 2013, 06:49 PM   #20
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For Pentode, I would probably just use a separate small transformer with a choke input supply for the screens.

There is also a 1.7K @ 100W Edcor output transformer that I could use for PPP (4) 6CB5As.

The $40 difference in price is not too bad. I suspect around 40W per pair at 350V B+. Or will is be less?

Thanks!

Blair
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