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Rule of thumb for estimating positive feedback capacitor in pentode-DHT amp

208.85.78.1
Posted on November 4, 2009 at 16:36:08
Single_Ended
Audiophile

Posts: 45
Location: LA
Joined: November 13, 2004
 Hi gang,

I am hoping to improve the high frequency extension of a 6SJ7-CR-300B amp - based on Thorsten's 1nf positive feedback capacitors from the plate of the 300B to the screen of the 310A.

I guess one way would be to measure the frequency response with different cap values. But given a lack of test instrument, does anyone have a rule of thumb approach for determining the capacitor value using the circuit info and the tube operating point?

TIA.

RE: Rule of thumb for estimating positive feedback capacitor in pentode-DHT amp, posted on November 5, 2009 at 09:31:57
The Artful Bodger
 Yo Mon!

Sweet and simple, there ain't non.

The value of da cap depends on da ceercuat AND on da output transformer. Geet it wrong, you build an oscillator, not an amp and Dub does not ear good.

So without seeerius test-gear, just don't brova.

Greez TAB

Smoke dat sh*t mon!

PS, Allen - da article me mate chalky here refers to is of course posted in da tube diy azylum...

RE: Rule of thumb for estimating positive feedback capacitor in pentode-DHT amp, posted on November 5, 2009 at 05:58:59
Al Noakes
Dealer

Posts: 517
Location: London
Joined: November 24, 2008
 I would not bother . You would be far better off using a lower value of anode load and a far beefier , higher gm valve . I'm using E280F driving a 300B . This amp uses a grid choke for the 300B and the E280F has no HF issues driving this

Al

RE: Rule of thumb for estimating positive feedback capacitor in pentode-DHT amp, posted on November 5, 2009 at 03:36:27
Allen Wright
Manufacturer

Posts: 4729
Location: Schaffhausen
Joined: January 31, 2002
 I have no idea of what Thorsten proposed, and I gues most here also don't.

I suggest you start with the value he suggests and LISTEN to it - and vary it up and down in value untill it SOUNDS right to your ears. That's the only real test - measurements are not the real test.

Or post Thorsten's article so we can read it and then make suggestions.

Regards, Allen (Vacuum State)
Right On ! Cut & Try works well, if they can trust their ears..., posted on November 5, 2009 at 14:52:49
Interstage Tranny
Audiophile

Posts: 1403
Location: Eastern
Joined: October 4, 2006
 Very good advice, Allen. The feedback the OP is talking about is derived from the WE 91A amp. Since the screen of the driver tube is bypassed with a cap, the feedback network is more like a tone control. Get the series connected RC feedback network values right, and the bandwidth extension is admirable; or at least sounds better...

The bass will tighten and the highs will sound more natural, much less harsh. This can easily be heard when using low bandwidth opt trannies. Tweak that network and the sonics will please...

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