6V6GTA Tubes, H/K A300 Transformers, Now What? - diyAudio
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Old 22nd March 2010, 04:36 PM   #1
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Red face 6V6GTA Tubes, H/K A300 Transformers, Now What?

Hi, I'm new on this forum and new to building audio amps.

I have always loved rich warm open tube sound with it's roundness and effortless airy high frequency articulation but I've never owned one except a Citation II which I threw in the garbage
20 years ago because I was stupid.

So, I have a set of H/K A300 output transformers and 4 RCA 6V6GTA tubes NIB and want to make them into an amp.

What now?

I've read here and there about open ended, class A, class AB, triode, push-pull, etc., but I'm unknowledgeable in this field of expertise.

Should I turn and run, or buckle down and begin building?

The transformers are coming out of a semi-working A300 I own but I would rather build something different, better looking, & dedicated as an amp only, - perhaps even monobloks if I can locate another set of power and filament transformers, which I might be able to find at a local flea market.

The speakers I have I built using a set of EV-made 15" in
large ported enclosures with efficient 6" EV VMR mids and a set of T-350 tweeters, so power requirement is minimal, and ~5-15wpc would be fine.

Basically I want to have this all laid out for me as I don't have the inclination to learn all the finefinite details of electroacoustictranscondustionationismz
I just want to be bathed by sweet sound as I basque in the comfort of my listening chair.

Musical taste ranges:

Doobies, Metheny, Hedges, Rachmaninoff, Zep, Sheffield Labs, Nora, Krall, Airplane/ Starship, Starcastle, ELP, Paganini, Floyd, Jazz @ Pawnshop, Fleetwood Mac, Schnauss, Eno, etc.

Uhm... Ideas?


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Old 22nd March 2010, 05:20 PM   #2
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Location: So.Cal.
How sensitive are your speakers?

How comfortable are you working around high voltages?

I would consider Poindexter's 6GK5/6V6 music machine. About 6 watts triode. Well documented, modern implementation with fixed bias, a CCS and SS rectification.

His site is here: AudioTropic Projects; The DIY Pages.

The site still shows the older (5965) version of the MM; do a search here for the 6GK5 version.
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File Type: gif Poindexter 6GK5-6V6 mm.GIF (23.2 KB, 152 views)
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Old 23rd March 2010, 12:35 AM   #3
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Hey, thanks for the helpful info!

They are quite efficient, I would venture an inspired guess of ~95dB 1w/1m @ - 98dB/pair.

I will probably love the outcome so much I'll end up mortgaging the house for a 100wpc C.j., but for now I'd be elated with a mere 6w of true sonic purity, and I'll guess it should net about ~105-106dB/pair?

As far as being comfortable around HV, I'd say I'll be fine with it.
I use a pair of 4-400C tetrodes with 4600v on the plates in Grounded Grid configuration for RF amplification on the Amateur radio.

Voltage is voltage, and it should all be highly respected.

Good question, though.

I'm curious,, what sort of sound 'flavor' should the 6V6 tubes provide in comparison to other types of tubes?

How good / crappy are older RCA 6V6GTA compared to other brands?

These plates look really dark, basically black, I hope they're still good. They were a gift. I have no idea what tubes are in the A300, it's been buried in storage for a decade+.
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Old 23rd March 2010, 01:27 AM   #4
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Join Date: Apr 2004
Location: Monroe Township, NJ

Poindexter's design eschews NFB and that requires very high quality O/P trafos. I strongly suspect that the H/K O/P trafos will work best inside a GNFB loop. So, I will immodestly suggest you build an "El Cheapo" variant. Yes, Poinz's design and mine are "kissing cousins". His effort is an attack on SOTA. My effort is low budget, but highly competent.

Triode wired "finals" yield 6 WPC, which you say is enough. I need to get the A300 schematic to see what the power trafo can do.
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File Type: gif elCheapo-23jun06-map.gif (38.8 KB, 112 views)
Eli D.
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Old 23rd March 2010, 02:13 AM   #5
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Interesting info, Eli. O/P Trafos are all new to me, I know only so much (little) about RF tube amps, AF is a whole new ball game.

I'll try to find a scheme on it and add it here.

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Old 23rd March 2010, 02:21 AM   #6
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Default H/K A300 info & scheme

I found a tidbit of info, does this offer what you wanted?

Richard Sherman
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Old 23rd March 2010, 03:00 AM   #7
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Join Date: May 2005
Location: Victoria, B.C.
Default Output transformer ident. question?

Originally Posted by RennoVattio View Post
Trafos are all new to me, I know only so much (little) about RF tube amps, AF is a whole new ball game.
Would you be so kind to post the numbers from the top of the output transformers please. I have an older HK224 PP EL84 amp, and am curious to see if the tranies are similar.

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Old 23rd March 2010, 03:02 AM   #8
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OK, That's what I needed to inspire me to dig it out of storage tomorrow.

I'll post pics asap.

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Old 23rd March 2010, 05:49 AM   #9
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Location: Monroe Township, NJ

I've got the schematic. Thanks for the link.

Except for the B- supply, the H/K power trafo is near perfect. H/K used a Greinacher ("full wave") voltage doubler B+ supply. So did I. A 355 V. B+ rail is "ideal".

You have the makings of a very nice stereoblock "El Cheapo". A Triad C-24X filter choke and a AnTek AN-0130 toroid will complete the power "iron" set.
Eli D.
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