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Old 1st August 2008, 12:56 PM   #1
GordonW is offline GordonW  United States
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Default 6V6 PP-based Baby Huey/Mullard inspired design needed...

OK. Got parts, looking for a plan.

Here's what I have:

Chassis with power transformer and power supply (about 300V B+ or so), and four 6V6 output tubes (already installed). Was a PPP mono amp... I'm wanting to convert over to PP stereo.

Output transformers- from Hammond organ amps- part # A024158. As used in the AO-44 PP 6GW8 and AO-35 PP 6BQ5 power amps. About 8K ohms CT primary: 8 ohms secondary. Probably capable of 12-15 watts.

I've built several Magnavox clone amps... looking for something different. Especially as these output transformers were originally intended for 8 ohm speaker loads, while the Magnavox outputs were made for 4 ohm loads.

I would like, if possible, to make this amp where it could TOLERATE 4 ohm loads on the outputs... which implies, to me, that it needs to, somehow, manage to decrease the effective plate impedance of the 6V6 output tubes- but without ultralinear operation (no taps on these transformers).

To that end, I was looking, briefly, at the feedback mechanism of Baby Huey... from the output plates, to the plates of the phase splitter. However, I know that it's been stated that Baby Huey probably wouldn't have enough gain to drive 6V6s in that application... my proposed solution would be to add one additional 12AT7/6201 tube as an input amp, for both channels of the amp (one side of the 12AT7 as a grounded cathode input stage, feeding into the Baby Huey-style phase splitter with feedback).

Now, I know these output transformers are NOT ultralinear... so that's another difference I've gotta deal with. But, with the amount of gain I can get from TWO gain stages (the grounded cathode input AND the modified-LTP phase splitter), I think I should be able to get back the low output impedance, fairly well...

In essence, what I'd really like to do, is try a non-ultralinear Mullard-style circuit, with triode front-end gain stage, but with Baby Huey-style feedback from the outputs to the phase splitter... anyone got any WAGs on whether this sounds like a feasible idea? Or, given the parts I listed above, is there an easier plan that would work well (to get low output impedance, with decent overall gain)?

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Gordon.
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Old 1st August 2008, 01:23 PM   #2
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hey-Hey!!!,
I've come to question the 'lots of gain needed' philosophy.

There are a few means to deliver local FB. The Gary Pimm 47 amp is one, the RCA SP20( or single pair version http://mysite.du.edu/~etuttle/electron/elect35.htm ) work really well. With U-L taps there is E-Linear, though that does best with pentodes up front, or better still Cascode.

Short story IMO is that if you build with a low gain single stage up front, it is quite easy to modify with a higher gain element later if you've guessed badly wrong( not likely ).
cheers,
Douglas
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Old 1st August 2008, 01:58 PM   #3
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Quote:
6V6 PP-based Baby Huey/Mullard inspired design needed...
Gordon,

Did you ever do the HK 250 output stage amp, (the one with the ECC81, not the 6CG7 version)?


Andrew
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Old 1st August 2008, 02:30 PM   #4
GordonW is offline GordonW  United States
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Quote:
Originally posted by Andrewbee


Gordon,

Did you ever do the HK 250 output stage amp, (the one with the ECC81, not the 6CG7 version)?


Andrew
Had thought about that. MIght be a way to go.

I would wonder which one would wind up with the lowest output impedance, though... the HK design or the Mullard with output anode/phase splitter anode feedback...

Also, Doug... as I said, I don't have ultralinear capability,with the parts available. Hence, the search for alternative "impedance reducing" strategies...

Regards,
Gordon.
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Old 1st August 2008, 02:33 PM   #5
GordonW is offline GordonW  United States
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Quote:
Originally posted by Bandersnatch
hey-Hey!!!,
I've come to question the 'lots of gain needed' philosophy.

There are a few means to deliver local FB. The Gary Pimm 47 amp is one, the RCA SP20( or single pair version http://mysite.du.edu/~etuttle/electron/elect35.htm ) work really well.
Looked at that RCA design... only problem, with those pentodes, it would require more "bottles" than I have room for.

Hence, the idea of modifying a triode-front-end Mullard... that could be done in stereo, with just three twin-triodes...

Though, the basic idea of the feedback scheme on the RCA might be a good option. I'd think going from plate-to-plate might be easier to implement, however... Bricktop over at AK certainly got that to work easily enough... and it seems to have the added benefit of helping stability and bandwidth on the top end, too...

Regards,
Gordon.
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Old 1st August 2008, 05:03 PM   #6
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hey-Hey!!!,
You can also cross couple final's plates to opposite driver grids, similar to what was used in the Citation II. I've done this once, and I am going to rig my Mk.III's this way. Principle difference being that I have room for two tubes per channel in the monoblock amps. They'll use 6NS7 LTP running open loop, feeding the LTP with cross-coupled FB from the final's plates.
cheers,
Douglas
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Old 1st August 2008, 05:25 PM   #7
GordonW is offline GordonW  United States
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What I like about the plate-to-plate approach, is its simplicity... no caps required. Just a single resistor.

Can't beat that, in parts count!

Regards,
Gordon.
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Old 1st August 2008, 11:30 PM   #8
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Gordon,

With a "twist", "Baby Huey" may be just fine. You fear that the 12AX7 will not provide sufficient gain. Buffer the 'X7 sections with DC coupled ZVN0545A source followers. Now, the 'X7 is working into just its anode load resistor. An added benefit of buffering is the option to use "fixed" bias. The 6V6 data sheet gives 100 KOhms as the max. allowable grid leak resistance, when "fixed" bias is employed. You could go down to 22 KOhms and the FET will not "complain".
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Old 2nd August 2008, 07:15 AM   #9
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The 6AU6 is almost the ideal driver for a Partial feedback amplifier. It can handle respectable current of at least 5mA with a gain of about 90x and a suitable high output impedance. Also its cheap and sounds good.
Any triode front end in a partial feedback amp imposes great limitations on the design as current is always the loser to impedance and gain.
Another option would be to use something like the ECL82 which would give you huge amounts of gain.

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Old 2nd August 2008, 12:13 PM   #10
GordonW is offline GordonW  United States
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Well, I did some calculations...

Looks like with my original idea (12AT7 front end/12AU7 LTP), I can get, before feedback, a gain of about 32 from the first tube (no cathode bypass cap) and about 7 from the LTP.

Comparing that to the Magnavox AMP175 clone I built before (also with 6V6 output)... it had a SINGLE gain stage (12AX7) of about 50 or so (the phase splitter is a paraphase- NO gain).

So, that gives a total preamp gain (first stage times LTP) of about 224. If you divide 50 into that, I've got about 4.5 times the gain of the Magnavox design... which had PLENTY of gain to run from any preamp I hooked up to it!

That means, I have a gain factor of about 4.5, to throw at the internal feedback loop.. about 13.5 dB. Which leaves a gain there, after internal feedback, of about 1.5... not a bad place to be, since the first stage can swing plenty of voltage (probably 40-50 volts, where I only need 25 or so, MAX). Or, I can go down to 10dB feedback to the LTP and up the feedback on the global loop 3dB over what Magnavox used... whichever sounds better...

And, the nice thing, is that with the lower gain on the first stage (12AT7 without cathode cap opposed to 12AX7 with cathode cap), I've got better bandwidth than the Maggie designs too... might not need the "crowbar" compensation cap to ground that Magnavox used after the first stage...

Regards,
Gordon.
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