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Old 4th July 2013, 03:28 PM   #621
Abszero is offline Abszero  United States
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Hi Andy,
Thanks for your input. Where did you get the Onetics and what price would be?
I tried a quad with UBT-1 1.6k and sounds good with RkCK elna( got a a THDof 1% at 3.7W right before all the higher order harmonics start to appear on the FFT), but not close to filament bias. I love the sound with filament bias. I am using the same UBT-1 forced to 3.2k ( by connecting 8ohm speaker at 4ohm output), and despite the fact that Lprimary is low (8H) sounds excellent. In this case I measured THD of 1.4% at 2W and 1.83 at 3W before clipping)
I am also trying to find a best match of the amp with a backloaded horn full range driver. I am in the process of building the enclosure for a Fostex FE166En and looking forward to use it. Right now I have a 2way Klipsch speaker 93dB which works great. However, I am sure that with full range driver there is a different experience.
Best,
Radu
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Old 22nd August 2013, 09:05 AM   #622
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Here's my latest schematic, using the LL1660/18mA instead of the Hammond 126C. I might change the operating point of the driver 4P1L if I used two glow tubes - could then increase the 150v voltage reg to say 180v or 210v. But it sounds very nice as it is. It's my daily amp. I actually have enough gain with a 2v DAC (ES9023) not to need a preamp, so just this. Speakers are Mark Audio Alpair 10s.

It's useful to have output tubes with a mu of around 10 or 11, after having used 2a3 and 300b previously which required 3 stages of DHTs if one wanted to avoid 2 stages with an indirectly heated driver. So now I have just the 2 stages of DHTs. Yippee! Should work fine with Fostex speakers.
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Last edited by andyjevans; 22nd August 2013 at 09:17 AM.
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Old 22nd August 2013, 10:03 AM   #623
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I'm interested to know wich other VR to increase to 210V and if I have to change or use same value of paralleled cap.
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Old 22nd August 2013, 10:20 AM   #624
stajo is online now stajo  Sweden
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Andy. How important is the matching of the output tubes?
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Old 22nd August 2013, 10:33 AM   #625
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180v = 0A3 + 0C3
210v = 0C3 + 0C3
225v = 0A3 + 0D3
255v = 0C3 + 0D3
300v = 0D3 + 0D3

Matching the output tubes is probably quite significant. I'll have to rig up some way of doing this - Ale has a circuit. If anyone else does, please post it!
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Old 22nd August 2013, 10:52 AM   #626
Ceglar is offline Ceglar  Australia
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If you're wanting to keep the common filament arrangement as it is, you could try feeding each tube with separate and in phase secondary windings from the IT. This would give some control as to idle current balance in the output pair, depending on how you reference the respective secondary grid returns. Some small value of positive voltage derived from the filament circuit itself via a pot could work.

Last edited by Ceglar; 22nd August 2013 at 10:54 AM.
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Old 22nd August 2013, 04:23 PM   #627
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Quote:
Originally Posted by andyjevans View Post
180v = 0A3 + 0C3
210v = 0C3 + 0C3
225v = 0A3 + 0D3
255v = 0C3 + 0D3
300v = 0D3 + 0D3

Matching the output tubes is probably quite significant. I'll have to rig up some way of doing this - Ale has a circuit. If anyone else does, please post it!
The same value of paralleled capacitor for all combos?
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Old 22nd August 2013, 04:29 PM   #628
stajo is online now stajo  Sweden
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Quote:
Originally Posted by merlin el mago View Post
The same value of paralleled capacitor for all combos?
Yes, just put the cap over both tubes.
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Old 22nd August 2013, 04:36 PM   #629
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Old 22nd August 2013, 04:59 PM   #630
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Yes - .056. Don't use larger than that. And as said, it goes round both tubes
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