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Old 15th October 2007, 07:41 PM   #1
Fuling is offline Fuling  Sweden
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Default Need a serious PP driver stage

Hi,

Ive been contemplating a bit about power amps with the output tubes configured as cathode followers. The main disadvantage is the enormous input signal required to drive the followers, were talking 100Vrms and up.

The plan is to build an amp using triode-wired PL519 (40KG6) and Hammond 1650T output transformers (1,9k plate to plate, or should I say cathode to cathode).
To make life a little easier for the driver stage Im planning to run the output tubes "low and hot", 170V p-k @ 200mA and use the 16 ohm tap with an 8 ohm load to get 950 ohm c-c.
If Im reading the curves and loadlines correctly the driver stage would have to deliver 100Vrms (per phase) to drive this output stage into clipping.

The question is: What would a suitable driver stage?
Ive had some success using a choke loaded 12B4A to drive an SE cathode follower, but I dont have any centertapped plate chokes and Im on a budget here (as always...).
Can I use a pair of 12B4A with CCS plate loads (cascoded DN2540) as a differential gain stage with a common cathode resistor? I guess a third CCS in the "tail" would make a mess, but a simple resistor (for cathode bias) connected to ground should work, right?

I guess such a gain stage wouldnt be a very good phase splitter on its own, but thats not a problem. I would need a front stage anyway, so why not let that one handle the "phase splitting".

Would it be safe to use DN2540 as plate loads with 400V B+?
With about 200V/15mA across the chips at idle they would need heatsinks of course, but Im more concerned about the voltage at full signal. Theyre speced to 400V, can this be trusted?
(I distrust semiconductor specs in general...)
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Old 15th October 2007, 10:02 PM   #2
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Hi Fuling,

unless you are willing to invest in a separate driver stage B+ feed of considerable higher tension than the output stage, you always can consider choke-loaded drivers. Choke-loading will almost double the swing compared to simple resistive loading (where you drop much of the available B+ across the plate resistor).

Also, using pentodes (in pentode mode of course) for the drivers often helps a lot with such demands, because they usually can swing the plate much closer to zero volts across the tube than a triode can.

And there always is the path of engaging interstage transformers (even including doing the phase splitting job), but this usually will prevent the usage of global FB.

Nothing comes for free, eh.

Regards,

Tom
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Old 15th October 2007, 10:52 PM   #3
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In 95 or 96 there was an article in Glass Audio where the author was trying to stop ringing in his amp using 42KN6 output tubes. His design sounds similar to yours. I'll try and scan a copy if I still have it.
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Old 15th October 2007, 11:20 PM   #4
Fuling is offline Fuling  Sweden
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Quote:
I'll try and scan a copy if I still have it
Would be great, thanks!

Quote:
invest in a separate driver stage B+ feed of considerable higher tension than the output stage
No problem, I have several transformers with suitable HV windings available.

Quote:
you always can consider choke-loaded drivers
I love choke loaded gain stages, but unfortunately I have no suitable plate chokes. I could of course order a pair of Lundahl LL1667 or 1668 with a small airgap (just enough to allow some DC imbalance) but Im looking for a cheaper solution if possible.


Quote:
pentodes (in pentode mode of course)
Ive thought about that, but it seems that the 12B4A is capable of swingning even "closer to zero" than any pentode Ive seen.
When I was experimenting with an SE follower amp I got over 500Vp-p out of a choke loaded 12B4A with about 290V plate-cathode.

CCS plate loads should allow almost the same results, though limited by the 400V that the DN2540 can handle. With 400V B+ it should still be possible to get something like 330Vp-p out, which is quite a bit more than I need. IF the whole idea works as intended, of course...

A pair of LL1660S interstage transformers would be really cool in this application, serving as both plate loads, coupling elements and even phase splitters.
I actually have a pair of LL1671/50mA transformers already but they are not quite as versatile as the 1660S and the airgap is too big. To get an 1:1+1 ratio I would have to wire the primaries in // which would result in a very low primary inductance, it would probably take another PL519 to drive it...
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Old 16th October 2007, 12:54 AM   #5
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Quote:
Originally posted by Fuling
I love choke loaded gain stages, but unfortunately I have no suitable plate chokes. I could of course order a pair of Lundahl LL1667 or 1668 with a small airgap (just enough to allow some DC imbalance) but Im looking for a cheaper solution if possible.
Do you have any surplus/used PP output transformers? Just leave the secondaries unconnected. Might work well depending on your driver tubes.
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Old 16th October 2007, 03:27 AM   #6
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Cary Audio makes (or made) an 845 PP amp with 300B drivers. Dennis Had himself told me that he uses an output transformer to choke load the 300B's and leaves the secondary floating. Then capacitor couples the 300B plates to the 845's.

Victor
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Old 16th October 2007, 07:29 AM   #7
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If you're not going to do something like Lynn's Amity, maybe with 2A3's as drivers, then a pentode diff front end works superbly. I drove my 813PP triodes with 12HG7 and 12HL7. Superb.
http://www.pmillett.com/813_se_triode_amps.htm
http://www.pmillett.com/pentodes.htm
http://www.fortunecity.com/rivendell...cy/Legacy.html
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Old 16th October 2007, 04:21 PM   #8
Fuling is offline Fuling  Sweden
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Quote:
PP output transformers
I had a pair of decent quality 6BQ5 PP transformers but unfortunately I traded them for some rare tubes that I needed for another project.
If I can find anything suitable for a good price it would be worth a try. I guess mains transformers with 2x115V primaries cannot handle 2x100Vrms at 20Hz.
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Old 16th October 2007, 10:11 PM   #9
Fuling is offline Fuling  Sweden
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I decided to take another look at some interesting pentodes to see if I could find something suitable.
I found this one:
http://www.tubes.mynetcologne.de/roe...ntode_150v.pdf
I happen to have a whole box full of NOS Philips PL83...

With a 10k plate resistor and about 500V B+ this tube seems capable of swinging over 400Vpp, not bad!
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Old 17th October 2007, 05:50 AM   #10
WT is offline WT
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You can look at Tubecad for more idea of driver stage like this

http://www.tubecad.com/2006/09/blog0080.htm

or here

http://www.tubecad.com/2006/09/blog0079.htm
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