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Old 11th October 2010, 09:49 PM   #11
athos56 is offline athos56  United States
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Quote:
Originally Posted by revintage View Post
Sorry, didnīt check my writing before posting. I ment the other way around. Off course the text should read:

"Move the coupling caps to between two drivers from between the second and the output tube"
Ahh that makes more sense . So maybe something like a 6SL7? I should also mention, that a preamp with a little gain is before this amp(56/76 preamp), so it doesn't have to be geared for a CD input.

Athos
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Old 12th October 2010, 06:11 PM   #12
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Default How about D3a?

here is some work I did last night. I lifted the bias scheme from Miles Prower's Vixen amp, Thanks Miles. The other thing I've been playing with is the E-linear verses Schade feedback. Thanks for looking, sorry about all the schematics, but I guess you can see the learning/thought process.
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Old 12th October 2010, 08:23 PM   #13
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Forget about Schade, it is for making pentodes work like triodes. You have nice linear DHT-triodes that need no feedback.

D3a is somewhat wrongly biased. It likes something like Ua=20mA and Ra 10k.
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Old 12th October 2010, 08:41 PM   #14
athos56 is offline athos56  United States
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Quote:
Originally Posted by revintage View Post
Forget about Schade, it is for making pentodes work like triodes. You have nice linear DHT-triodes that need no feedback.

D3a is somewhat wrongly biased. It likes something like Ua=20mA and Ra 10k.
Ahh, good point. Yeah I forgot change any of the values in the schematic, which are all really preliminary anyhow. So now the pentode/triode input question. Here's what I got on hand in enough numbers to populate the input position, 6SL7, D3a, 12AX7, 6SJ7, EF86, 5842, 5687, 6SN7.
Thanks

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Old 12th October 2010, 08:52 PM   #15
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Obviously I did not read what Revintage wrote either, I read what he meant

But that is OK now. However you need to move the biasing network to bias the CF's instead of the outputs.
The output tubes are biased by the cathode voltage of the CF. To bias the output tubes you adjust the CF grids.
As it is set up now the CF grid is at gnd potential and that will give you a few volts POSITIVE voltage at the CF cathode which is direct coupled to the output grids. I.e. you have a few volts positive bias on your output.

/Olof
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Old 12th October 2010, 08:58 PM   #16
athos56 is offline athos56  United States
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Quote:
Originally Posted by hemgjord View Post
Obviously I did not read what Revintage wrote either, I read what he meant

But that is OK now. However you need to move the biasing network to bias the CF's instead of the outputs.
The output tubes are biased by the cathode voltage of the CF. To bias the output tubes you adjust the CF grids.
As it is set up now the CF grid is at gnd potential and that will give you a few volts POSITIVE voltage at the CF cathode which is direct coupled to the output grids. I.e. you have a few volts positive bias on your output.

/Olof
Right, I was wondering about this when I was studying the Vixen. Looks like I might be lifting more from Miles.

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Old 12th October 2010, 09:54 PM   #17
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Default Revision

Ok, bias scheme revision. I'll have to Drop Miles a message of thanks...

Athos
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Old 14th October 2010, 08:40 PM   #18
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Here is the preliminary PSU.

Athos
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Old 16th October 2010, 12:04 AM   #19
athos56 is offline athos56  United States
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Now for more practical concerns, parts selection. Should I go for the cheapest option, the Russian 6S4S, which runs between $15 and $20 per tube or the Sovtek 6B4G, at a little over $60 bucks a tube. Or even go for the Sovtek 6A3, 2A3 or even the Sino 2A3. The output transformers are going to be Edcor 25w, 2.5k units, for $40 they are really economical.

Athos
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