What to do with this iron (originally from PP 7355)

[mfaughn]

Hi all,

I have some iron from a HK TA-260 receiver (same as A500 integrated). It used 7355 tubes. The amp was rated at 25 wpc. I suspect that was somewhat conservative as the iron isn't a whole lot smaller than what I have on my EL34 PP amp. The iron does not have UL taps with primaries around 4.4K. Pretty sure they were made by Magnetic Windings in Pennsylvania.

I originally was going to do a PP EL34 triode amp but have since decided I need more power than that will offer. I have looked into Le Renard and the Engineer's Amplifier. I could also more or less recreate the A500 power amp section.

I'm a relative novice. I've fixed a few amps, recapped some more, modded here and there. I do have experienced help available occasionally. Projects that are well supported by the community and/or are less fiddly to get working are probably best for me at this point. Any ideas or opinions greatly welcomed and appreciated.

[Eli Duttman]

Hmmm, pentode mode EL34s working in a near Class "A" operating point should get the job done. Use well regulated g2 B+, for max. open loop linearity.

Mullard 5-20 style topology, employing high gm small signal types, will (IMO) serve quite nicely.

[mfaughn]

If I understand correctly that is going to yield less than 20W. I'm interested in getting more than that out of them. I'd like to get 25W at least (which the HK A500 will do) and more if the iron will allow it.

I am driving Snell Aiii speakers now with a Magnavox 9303 (PP EL84). My friend brought over his Pilot SA-260 (PP EL34) and the difference was dramatic. The speakers opened up, soundstage and imaging improved, and tons more low end support. I know that power isn't the complete picture here but I am pretty sure that I am underpowered for these speakers now.

[Eli Duttman]

A PP pair of EL34s operating in true Class "A" will yield approx. 20 W. Move a little bit towards Class "B" operation and you'll get the "magic" 25 W. Except when producing peak power O/P, when the tubes alternately cut off, both tubes of the PP pair will be conducting 100% of the time. Call it "shallow" Class "A", near Class "A", or whatever tickles your fancy, that sort of setup sounds pretty darned good.

Squeezing 40% real world efficiency out of a true Class "A" pentode "final" is easy enough. You will have to reduce the "idle" current and raise the B+ rail from true Class "A" values somewhat to get a 50% real world efficiency yield. That makes things "officially" Class "AB", but you will never notice it.

[mfaughn]

Thanks. And thanks again as I have read many, many, many posts all over the place by you Mr. Duttman.

It looks like 6L6 would be just as happy here too, no?

As far as Mullard 5-20 goes...that is a UL design I think. So the change is simply that the screen voltage is supplied from its own B+ leg rather than from OPT taps, right? Other than that just go with a 5-20 variant and apply appropriate plate voltage and bias current for the target output power? In that case I might lean towards something similar to the Eico HF60 since I have all the tubes on hand (and not much $$).

[Eli Duttman]

It looks like 6L6 would be just as happy here too, no?

"Usual" for PP 6L6 "finals" is "iron" with a 6.6 KOhm primary. Maybe you can use comparatively low B+ and high "idle" current, AKA "hot and low".

The key things about Mullard style circuitry are a voltage amplifier DC coupled to a LTP phase splitter. How you bias the "finals" and the mode you use are your business.

The EICO HF60 uses Mullard style circuitry and self biased "finals". The low gm of the EF86 voltage amplifier doesn't impress me. Don't get me wrong. The EF86 is a great tube, but I like high gm inside a GNFB loop. IMO, you will do better with a 12AT7 section voltage amplifier and an ECC99 LTP, as the small signal complement. Don't forget a 10M45S CCS in the tail of the LTP.