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Old 28th January 2007, 06:58 PM   #1
Klimon is offline Klimon  Belgium
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Default el34PP circuit evaluation!!

Hello,

I've been looking around for a 'good' el34pp schematic now my hammond 1645 opts have come free from another project (won't need the UL taps for Miles Prowers 807 beast); my preference goes out to modern variations on the Mullard 5-20 circuit and I'm doubting between two candidates. Hopefully someone with a sharper sight can help me make the better choice.. Main difference between both circuits are the input and phase splitter tubes: 6sj7+6sl7 vs. ef86+ecc83

Mapletree audio design, schematic on page 7:
http://hollowstate.netfirms.com/Stea...s%20manual.pdf

The well-known circuit from the Lundahl website:
My new project, 2 x KT88 60W amp

Power supply quality is not an issue as I will design a different using vintage iron. The mapletree design has shared cathode resistor for each el34 pair which I'll possibly change.

I welcome your observations

Simon
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Old 28th January 2007, 07:46 PM   #2
Klimon is offline Klimon  Belgium
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Too late to edit but I've been wanting to ask why the grid stoppers at the input tubes' grid (4k7 in one circuit, 11k in the other) are so immensely high in value; are they only there to prevent oscillation?

Also forgot to tell that I'll be adding a volume pot; the mapletree circuit specifies a tiny 0.7v input for full output so probably the other circuit won't need a preamp either.

Cheers,

Simon
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Old 28th January 2007, 10:14 PM   #3
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Hi,

Quote:
Originally posted by Klimon
Too late to edit but I've been wanting to ask why the grid stoppers at the input tubes' grid (4k7 in one circuit, 11k in the other) are so immensely high in value; are they only there to prevent oscillation?
Pretty much.

That's not immensly high either, it depends on input and Miller capacitance. Typical power tube snubbers are 10K and 470 to 4.7K for preamp tubes. They aren't critical at all.
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