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6A3 / 6B4G Design
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Old 2nd June 2013, 04:08 PM   #41
yagoolar is offline yagoolar  Poland
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Originally Posted by TheGimp View Post
33 tube triode strapped

Ia = 6mA

Vak = 180Vak



Too measure rp at this bias point can I simply capacitavly couple a pot to the output and adjust thepot R value until Vo' = .707 Vo and then measure the pot?
I have built 6C4C parafeed amp with LED bias. One E180F triode strapped is used as a driver.
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Old 2nd June 2013, 04:17 PM   #42
TheGimp is offline TheGimp  United States
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While taking rp measurements I got to thinking about Ia vs Mu and distortion.

Increasing Ia at the same Vak results in increased distortion (although not much) and an increase in Mu ( again, not a lot) but a significant reduction in rp.

Witht the 33 I got rp values of :
ia= 6mA - rp = 10.25K
ia= 10mA - rp = 8.2K
ia=15mA - rp = 6.18K

If one were to have an IT with a current rating of say 10mA max based on gap, and one were to wish to use it with a tube with an Ia of 20mA (necessary to achieve low rp to drive the transformer) could one place a current source in parallel with the IT primary and use it to provide the additional current to bias the tube at it's optimum point while limiting the current thorugh the IT?

I suspect the problem would be on the positive swing of the anode as va could exceed B+ which would shut off the current source and abruptly shift the bias of the tube.

A solution to this would be to have the B+ of the current source set above the supply to the IT by at leasst the voltage swing expected at the anode.

For instance the 33 has a spec'd Vak of 180V. Add to that the bias voltage of 27V plus the IR drop of the IT(20V), and the expected output swing of 45V to drive a 6A3, and an additional 50V to keep the current source active, one reaches a B+ on the current source of 180 + 27 + 20 + 45 + 50 = 322V.

A 325V B+ driving the current source, with an RC filter to produce 210 to supply the IT to the tube should work.

This would allow the IT to be driven by it's specified current while operating the tube at it's lower rp point which should provide better matching.

or am I off in the weeds?

yagoolar, how does it sound?
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Old 27th June 2013, 05:17 PM   #43
Bigun is offline Bigun  Canada
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6A3 / 6B4G Design
I had a mono-block running once with 6C4C output though a Hammond OPT. The two driver choices were 6C45Pi and 6E5P (triode wired), driving through a Hammond inter-stage. The 6C45Pi sounded the more accurate/cleaner of the two. Filament of the 6C4C was AC heated with balance pot - hum was very low but if I were to build one of these from scratch I'd use Rod's heater or equivalant supply. I think I was using a higher impedance tap on the OPT than normally recommended

I didn't get the sense that pushing the tube hard for a bit more power would result in much of a noticeable increase in volume - but it will stress the tube and possibly the power transformer.

Also, just for interest: http://www.diyaudio.com/forums/tubes...drive-2a3.html
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

Last edited by Bigun; 27th June 2013 at 05:22 PM.
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Old 28th June 2013, 05:04 AM   #44
yagoolar is offline yagoolar  Poland
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My amp sounds great - quick bass, very detailed mids.
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