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Old 12th September 2008, 12:28 PM   #1
GordonW is offline GordonW  United States
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Default Designing a "cheap" highish-power PP amp...

I've been thinking... I think I may modify my screen-drive amp project.

I see potential, for a VERY LOW COST PP screen-drive amp, with maybe 50w/ch or so output.

Here's my thoughts so far:

CRITICAL PART #1: Output transformers- since it's screen-drive, the bias currents in the output tubes will be VERY LOW. Hence, any DC imbalances between the tubes will also be very low, in proportion. This leads me to toroidal output transformers... I was looking at various options, and there's an Antek power transformer... 230 (2x 115V) to 9V (two parallel secondary windings), 100VA. That gives, if my math is right, about 5.2K to 8 ohms. Best part, is price is only about $18 each.

Critical part 2: PowerDrive module, courtesy of TubeLab. Yes, I'm going over to the dark side... but to do this in any sort of economical manner, seems to warrant solid-state for the current driver (source follower) driving the G2s of the output tubes...

Other than that, the amp will probably be a fairly "conventional" Williamson design... input amp, cathodyne phase splitter, diff amp. I'm thinking something like a 12AT7 or 6DJ8 as an input tube (maybe a 6N1P?), and a 6SN7GTB-class tube (something that can handle at least 450V on the plate) as the diff amp. Should be able to swing enough voltage, especially driving a MOSFET follower.

I am thinking of using plate-to-plate feedback from the output tubes to the diff amp, as a way of reducing the necessary loop feedback over the whole amp. Should improve stability, I would think.

As for output tubes- I'm looking for something that's STILL in production somewhere... which kind of limits me to tubes like the EL590S (expensive) or maybe the 6146B (in production, and MUCH cheaper).

Anyone care to look over the specs of the 6146B and let me know if it would be ostensibly suitable? I'm thinking B+ of somewhere around 475-500V or so... that's a bit low for a 6146, but I guess I could bump it up a bit if needed. Would like to keep it under 500V if possible...

So, any thoughts? I'm thinking that this MIGHT be do-able for less than $400 total in parts...

Thanks for any musings of any sort, as I descend into potential madness...

Regards,
Gordon.
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Old 12th September 2008, 01:18 PM   #2
SY is offline SY  United States
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Why not use sweep tubes? I last paid maybe $6 for 6JN6, and there's about a million other tubes just like it. Much higher perveance than a 6146, probably a higher mu.
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Old 12th September 2008, 04:29 PM   #3
kevinkr is offline kevinkr  United States
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I'm completely with SY on the sweep tube recommendation, and for his stated reasons. Now I'll add some more of my own:

Don't know of anyone using screen drive with the 6146B, but this is quite commonly done with sweep tubes.

Using sweep tubes will also allow you to stay comfortably under your 500V desired supply limit.

I would think tubelab's powerdrive topology would be pretty ideally suited to screen drive applications as well. The mosfet driver can do all the heavy lifting required to drive the screens leaving you to choose any tube you like for the voltage amplification chores.

Quiescent current can be quite low easing concerns about dc imbalance induced core saturation. (power toroid opts) You can always servo the bias voltage to the control grid later if this proves to be an issue, just make sure that the servo is really good at ignoring signal induced fluctuations in cathode voltage.

Matching the output tubes for transconductance at several different operating currents would probably help the ac balance, further improving linearity. (Probably doing something like fixing the control grid bias and varying the screen grid voltage and comparing those results tube to tube.)

I'm assuming that you will use at least some limited overall loop feedback to lower output impedance and further improve linearity.

If I were still a pushpull dude I would be pretty excited at the opportunity this project provides for learning..

I am sure George (tubelab) will find this thread shortly and weigh in with some really good recommendations on readily obtainable and inexpensive sweep tubes for your amplifier. The real issue is not whether or not the tube you want to use is still in production, but whether or not it is currently affordable and likely to be available in the future - just because it is currently available does not mean the preceding criteria are met..
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Old 12th September 2008, 05:02 PM   #4
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Look at this thread for the low down on cheap tubes. I think the 6BQ6 for 98 cents qualifies as cheap, and 50 watts per pair in screen drive is possible.

Tube sale at AES

I have a pair of genuine Plitron toroidal OPT's. These OPT's are specified for use at 400 watts and 20 Hz! They are huge.

Yes the DC offset is very low at idle because the current is low. That is only half of the problem. As the output power and hence the tube current increases the average current through the output tubes is unequal because no two tubes are ever perfectly matched. I have observed some low frequency saturation effects at reasonable power levels and frequencies. (30 watts at 30 Hz).

I will eventually figure out how to tame these offsets, and I will post the results. For now I am still figuring out some unique ways to connect the sweep tubes that I got on sale.
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Old 12th September 2008, 05:43 PM   #5
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I bought a bunch of 12bq6 to try them in a "both grids drive" mode.
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Old 12th September 2008, 05:48 PM   #6
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"CRITICAL PART #1: Output transformers......100VA"

You better check the primary inductance on those xfmrs. The larger the VA rating, the lower the inductance:
2X core area -> 1/2 the turns for a given voltage
2X core area and 1/2 the turns -> L = k* Area* turns^2 -> 1/2 the inductance. 10X core area -> 1/10 inductance

Nice 100VA tweeter amp.

I'm waiting to see the Antek tube output xfmr. pricing.

By the way, the 6146B has g2/g1 Mu of 4.5, surprisingly similar to the horiz. output tubes (usually 3.0 to 4.2 range). So g2 drive should be workable for this tube, but at $15 to $20 a tube it appears.



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Old 12th September 2008, 06:06 PM   #7
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The Antek 100VA power toroids are maybe 64 Hy, the 50 VA possibly twice that according to their data sheets. But since they'll saturate at about 300V at 60 Hz, or 100V at 20 Hz, they won't support much low-frequency power.

I've been thinking about Crowhurst's twin-coupled circuit with toroids...
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Old 12th September 2008, 09:02 PM   #8
GordonW is offline GordonW  United States
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Quote:
Originally posted by tubelab.com
Look at this thread for the low down on cheap tubes. I think the 6BQ6 for 98 cents qualifies as cheap, and 50 watts per pair in screen drive is possible.

Tube sale at AES

I have a pair of genuine Plitron toroidal OPT's. These OPT's are specified for use at 400 watts and 20 Hz! They are huge.

Yes the DC offset is very low at idle because the current is low. That is only half of the problem. As the output power and hence the tube current increases the average current through the output tubes is unequal because no two tubes are ever perfectly matched. I have observed some low frequency saturation effects at reasonable power levels and frequencies. (30 watts at 30 Hz).

I will eventually figure out how to tame these offsets, and I will post the results. For now I am still figuring out some unique ways to connect the sweep tubes that I got on sale.
I'm thinking that plate-to-plate (output to diff amp) feedback might help this... but I don't know if it would reduce the effects you mention to a low enough level for it not to be an issue. Seemed to help on both ends of the spectrum on a Heathkit W4 amp (which also seems to have marginal output transformer capacity, in some respects), according to what I've seen in a few places...

Apparently, Bob Carver is working on this type of thing, if I correctly read what he's writing recently. He's talked about a "DC restorer" circuit... sounds suspiciously like what we're talking about here...

Regards,
Gordon.
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Old 12th September 2008, 09:10 PM   #9
GordonW is offline GordonW  United States
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Quote:
Originally posted by smoking-amp
By the way, the 6146B has g2/g1 Mu of 4.5, surprisingly similar to the horiz. output tubes (usually 3.0 to 4.2 range). So g2 drive should be workable for this tube, but at $15 to $20 a tube it appears.
Don [/B]
That's what I was looking at. The 200V max screen voltage rating led me to think that the gain of G2 might be in the right ballpark...

And, $15-20 per tube is OK, for a "production" amp of 50w/ch, I'd think. Not much different than some EL34s or 6L6s, and less than most KT88s...

Now, to get this transformer thing licked...

Regards,
Gordon.
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Old 12th September 2008, 09:15 PM   #10
JoshK is offline JoshK  Canada
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What about EL509's? Svetlana makes them I think. Lots of russian ones on ebay (6P45S).
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