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Old 29th September 2009, 11:59 AM   #1
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Default PP question - best driver stage!

I'm currently building a PP 300b amp with O-netics OPTs. I have a chance to build something really good here. It will be all-DHT so that's a final design decision - non-negotiable I'm afraid! I have loads of DHT driver tubes of all types - 71A, 10Y, 31, 46, 1624, etc and also 26 which might be usable. There's little I don't have in DHTs, even the European ones like RE134.

I also have several Lundahl interstages, like LL1660/PP, LL1671PP, plus LL1660/5mA, LL1660/20mA which could be used as phase splitters. I also have loads of Russian teflon coupling caps.

I've had great sound with a SE input stage with IXYS 10M45 cascode active load and red LED in the cathode. Love it and want to keep it. Currently 1G4GT but could be 26 or 01A.

So, cooks, there are your ingredients. I have the chassis built ready. Which of the above would you use? Your mission should you choose to accept it is to negotiate your way from a SE input to 300b outputs! I have about 320v available for the driver stage at up to 200mA.

andy

Last edited by andyjevans; 29th September 2009 at 12:02 PM.
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Old 29th September 2009, 12:23 PM   #2
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hey-Hey!!!,
This is not enough voltage for what I have in mind. Probably better suited to a pair of 845... In any case, look at type HY40. I figured a B+ around 750V would do well( for resistive load ). Now if you got a 1+1:1+1 IT, the '40 might be able to run from that low voltage. Consider raising B+, perhaps use the Blumlein Garter to bias/balance them( and add ~150V to B+).
cheers,
Douglas
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Old 29th September 2009, 12:30 PM   #3
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Personnally I would go with the LL1660/PP and drive it with a LTP at the front. This should give you the maximum drive potential with no chance of blocking distortion, and no caps in the signal path. If you need lots of gain then consider one of the other Lundahls up front to drive both branches of the LTP and maintaining all the gain of the driver rather than halving it.

Shoog
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Old 29th September 2009, 12:37 PM   #4
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Quote:
Originally Posted by Bandersnatch View Post
hey-Hey!!!,
This is not enough voltage for what I have in mind. Probably better suited to a pair of 845... In any case, look at type HY40. I figured a B+ around 750V would do well( for resistive load ). Now if you got a 1+1:1+1 IT, the '40 might be able to run from that low voltage. Consider raising B+, perhaps use the Blumlein Garter to bias/balance them( and add ~150V to B+).
cheers,
Douglas

The Garter arrangement works extremely well for balancing current and should be considered for the output stage. There is a version with transistors which gets over the issue of burning up twice the voltage in bias (a big issue with 300B's).
In a LTP driver stage the Garter would undermine the LTP behaviour and could only be considered if both input of the LTP were driven. Consider putting a CCS below the garter and referencing the bypass caps to its anode, this would create DC current balance whilst maintaining LTP behaviour.

Shoog
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Old 29th September 2009, 12:39 PM   #5
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Hi guys1

I'm ruling out high voltages - 450v or so for the 300b is quite enough. I could use that voltage for the driver, but I have a separate transformer for that giving about 320v - less if I use choke input which is an option.

So, yes, LL1660/PP is an option - could even use 10Y as the LTP or 71A, 31 etc.

If I cap couple from the input stage, I ground the second grid of the LTP, yes? that's what I did in my current amp.

If I direct couple can I do the same or should I use a 1M resistor from first grid to second grid and a cap to ground like the Mullard circuit?

andy
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Old 29th September 2009, 12:43 PM   #6
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Does the maths work out to make this a two stage design if all of the gain of the driver is available to you?

Shoog
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Old 29th September 2009, 12:53 PM   #7
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Hi Shoog,

I really need three stages with DHTs. Nothing over mu=15 here. Plus driver will be anything from 3 to 10, closer to 3 probably. In any case I'll probably need a preamp, but that's OK - have a nice 26 preamp. So gain isn't critical, but it does need to be more than 2 stages with DHTs.

andy
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Old 29th September 2009, 12:56 PM   #8
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I should add I'm driving Apogee ribbons - Caliper Sigs. In a smallish room at low volumes so I already know PP 300b works. But of course, sensitivity is around 84db, so do need 4 stages in all. Right now I have 1G4GT (mu=15) into 6SN7 into 807 and that's enough gain for three stages, but I won't get a driver stage with as much gain as 6SN7s and as said, this is going to be strictly DHT!

andy
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Old 29th September 2009, 01:00 PM   #9
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have you looked into the Karna. Seems as if you have the iron to pull it off.

http://www.nutshellhifi.com/triode2.html

Shoog
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Old 29th September 2009, 01:01 PM   #10
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Addendum - i might possibly, possibly (don't know and don't really want to) use a pair of 2C22 (6J5 type with 2 top-caps) for the LTP. That would give me the same gain as a 6SN7. It's about the only indirectly heated valve I could imagine using. That could work without a preamp. But more likely 3 stages of DHTs

andy
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