845SE : comments on this circuit?

Ziggy · 11th February 2009, 05:16 AM

Looking for comments or opinions on the attached schematic :

Ziggy · 11th February 2009, 05:38 AM

Of particular interest is the capacitor coupling to the output transformer, bypassing the H.T.?

Is this of any advantage?

Tweeker · 11th February 2009, 06:23 AM

No DC in the output transformer.

dsavitsk · 11th February 2009, 07:49 AM

Quote:

Originally posted by Ziggy
Is this of any advantage?

Search for "Parafeed" or "Parallel Feed"

Ziggy · 11th February 2009, 09:38 AM

Aha......so there is little chance of core saturation and almost any suitable impedance single ended output trannie can be used?
In other words, you don't need a high voltage capacity transformer?
I'll do a search as recommended.
Any other comments?

rcavictim · 11th February 2009, 10:01 AM

Quote:

Originally posted by Ziggy
Aha......so there is little chance of core saturation and almost any suitable impedance single ended output trannie can be used?
In other words, you don't need a high voltage capacity transformer?
I'll do a search as recommended.
Any other comments?

This technique allows one to use any push-pull xfmer as well! That REALLY opens up what kind of iron you can use. With a low enough plate resistance output tube (think a bunch of 6080/6AS7's in parallel), you can even use a toroidal power supply xfmer as the output transformer.

Ziggy · 12th February 2009, 01:03 AM

Thanks Guys.

Ziggy · 12th February 2009, 01:05 AM

Oh, one last question :

can I get away WITHOUT using the 20h choke inline with the 820v B+ or is this mandatory?

rcavictim · 12th February 2009, 01:27 AM

Quote:

Originally posted by Ziggy
Oh, one last question :

can I get away WITHOUT using the 20h choke inline with the 820v B+ or is this mandatory?

That is mandatory. A high wattage resistor can be substituted but circuit efficiency will plunge as will available ouput power. The choke will create higher than the B+ voltage available in output swing. A resistor cannot. The choke has to have enough inductance so as to not saturate at the lowest audio frequencies expected and at the output tube plate current.

Tweeker · 12th February 2009, 01:39 AM

It cant be replaced while keeping 820V B+.

It can be replaced with a constant current source (or lower performing resistor) running from over 1600V... You have doubled dissipation in order to have the same output (a sufficiently inductive choke can swing to twice B+).

You cant ignore OPT insulation requirements with parafeed. They are less as you dont have superimposed DC, but that amp will swing well more than 1,000V P-P (note the 3,000V feed capacitor) just in normal operation.

11th February 2009, 06:23 AM	#3
Tweeker diyAudio Member Join Date: Dec 2004 Location: Cincinnati, Ohio	No DC in the output transformer. __________________ Be sure your foil hat has a good low impedance ground.

12th February 2009, 01:39 AM	#10
Tweeker diyAudio Member Join Date: Dec 2004 Location: Cincinnati, Ohio	It cant be replaced while keeping 820V B+. It can be replaced with a constant current source (or lower performing resistor) running from over 1600V... You have doubled dissipation in order to have the same output (a sufficiently inductive choke can swing to twice B+). You cant ignore OPT insulation requirements with parafeed. They are less as you dont have superimposed DC, but that amp will swing well more than 1,000V P-P (note the 3,000V feed capacitor) just in normal operation. __________________ Be sure your foil hat has a good low impedance ground.

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11th February 2009, 05:38 AM	#2
Ziggy Account Disabled Join Date: Jul 2004 Location: West Australia	Of particular interest is the capacitor coupling to the output transformer, bypassing the H.T.? Is this of any advantage?

11th February 2009, 09:38 AM	#5
Ziggy Account Disabled Join Date: Jul 2004 Location: West Australia	Aha......so there is little chance of core saturation and almost any suitable impedance single ended output trannie can be used? In other words, you don't need a high voltage capacity transformer? I'll do a search as recommended. Any other comments?

12th February 2009, 01:03 AM	#7
Ziggy Account Disabled Join Date: Jul 2004 Location: West Australia	Thanks Guys.

12th February 2009, 01:05 AM	#8
Ziggy Account Disabled Join Date: Jul 2004 Location: West Australia	Oh, one last question : can I get away WITHOUT using the 20h choke inline with the 820v B+ or is this mandatory?

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