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Old 18th March 2005, 08:34 AM   #1
diyAudio Member
Join Date: Dec 2004
Location: Roma
Default Hybrid Srpp


I need to have a vacuum tubes pre-pilot for a current BJT.

I need 40 volt rms maximum at minimum THD+N (0,5% at max) ad very low output impedence (max. 100 Ohm)

So I'm looking for a scheme of an Hybrid SRPP i think the best solution and very simple.

The idea is to use a paralled 6922 as the lower tube and a transistor (Bjt) like MJE 350 or 340 as upper dispositive.

The power supply voltage shold be around 300 volt to have the swing I need.

Any suggestion??

Thank you very much.

Gian Maria. Rome. Italy.
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Old 18th March 2005, 11:05 AM   #2
jeapel is offline jeapel  Canada
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Join Date: Sep 2003
Location: quebec
One solution would be a 6as7 cathode follower and
no need mje350 because it s very linear.


zout (open loop)=100ohm
zout (close loop 6db feedback)=50ohm


Or 6as7 with mje350(+ heatsink) but 100ohm zout
with tube is not a easy thing without a big
feedback and i dont like big feedback 10 db feedback
would be for me a max for a good amp.
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Old 18th March 2005, 04:30 PM   #3
jeapel is offline jeapel  Canada
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Location: quebec
I just do a very very quick check maybe i m wrong :-(

1-ccs with mje 350 + heatsink ( ~60v @ 70ma)
2-6as7 (~125v @ 70ma best is around 60% of pa max)
3-R cathode + bypass cap (bias ~50v)

for a supply of ~ 235v

test with 6db or 10db feedback to get 100ohm zout

good test it s a interesting project
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Old 18th April 2005, 10:48 AM   #4
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Join Date: Dec 2004
Location: Roma
Hi and thank you.

I don't want to use feedback.

I just need a simple schematic of Hybrid SRPP (no mu follower), using a 6h30 or 6922 paralled or ecc99, with a MJE 350 or MJE 340.

The schematic reference for the ccs is onhttp://www.machmat.com/sales/kits/ccs.htm, but I don't need the simple ccs, because the output impedence will be the Rp of the tube, to high for my purpose.

I need an SRPP, like Diego Nardi uses, and not mu follower that needs, by other, one more capacitor.

Thank you so much to everybody. G.M.

p.s. Maybe mark_titano already has a solution...?
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Old 18th April 2005, 11:37 AM   #5
wkopacz is offline wkopacz  Poland
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Join Date: Jan 2002
Location: Świdnica
Have U seen G. Pimm's Pages ?


Regards, wk.
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Old 18th April 2005, 11:56 AM   #6
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Join Date: Dec 2004
Location: Roma

First of all thanks.

Yes I know this page, but still we have complex schematic with feedback, mosfets, pentodes or mu follower.

So I repeat I would need an easier schematic SRPP with a BJT (transistor) not mosfet, strating from the CCS on the site I mentioned before.

But as I repeat, the CCS above is excellent circuit, but lack, for my needs, of a very low output impedence.

Thanks again, G.M.
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Old 18th April 2005, 12:22 PM   #7
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Location: Melbourne, Victoria
Relatively complex, yes, high output impedance, no. Have you noticed there are two outputs - including a "MU" output (from the source of one of the MOSFETs)?
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Old 18th April 2005, 12:38 PM   #8
Previously: Kuei Yang Wang
Join Date: Nov 2002

Originally posted by gianmaria
So I repeat I would need an easier schematic SRPP with a BJT (transistor) not mosfet, strating from the CCS on the site I mentioned before.
The short answer is "You cannot do that".

Here is why:

1) Your BJT is an enhancement Device (meaning for the NPN device it's control electrode [base - which is analogue to the valve's grid] needs to be positive with respect to the common electrode [emitter - which is analogue to the valve's cathode]).

2) Further not only are you asking to use an enhancement device, but also one where the control electrode draws significant levels of static and signal dependent current.

The "long" answer is: "You can achieve something that is close to what you want, BUT it requires great complexity and is unlikely to perform well."

Here is what would be needed:

1) You need a "ring of 2" CCS using NPN Transistors (or an NPN Transistor and a TL431 for better DC precision).

2) The "Pass" transistor needs to be able to withstand the voltages involved AND must be able to provide a high current amplification at the kind of currents flowing, implying probably a darlington device or arrangement, or indeed (dread of dreads) a Mosfet.

3) You need to feed the "error amplifier" transistor collector in the ring of 2 Circuit from a high voltage, high impedance current source, be aware that this current flowing in the error amplifier transistor is added to the current flowing in the pass transistor and that the impedance of this CCS will be in parallel with the impedance of the main circuit.

4) You need to bypass the error amplifier transistor's emitter/collector connection for AC using a suitable capacitor.

Now, all that effort to void using a single and inexpensive depletion mode Mosfet which will provide materially better performance strikes me as irrational.

Any particular reason you "MUST HAVE a BJT Hybrid SRPP" and the refusal to use a Mosfet?

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Old 18th April 2005, 01:13 PM   #9
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Join Date: Oct 2004
I think that gianmaria is looking for something like Diego Nardi's circuits:



He uses an hybrid srpp with an mje340 and to polarize the transistor he used a special Hewlett Packard AlGaAs (low current) led.
This one, HLMP-K150:


Only with this one is the design works well ( Nardi's words). I suspect that the reason is because it works as coltage ref even if only 1mA pass through it.
In this way the current through the reference chain does not disturbs the tube.
Kuei, how does ti looks?

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Old 18th April 2005, 05:12 PM   #10
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Join Date: Feb 2005
Location: Florence
Cool Hybrid SRPP

Ciao GianMaria.

Ok I understand that you want a scheme of an Hybrid SRPP, but what you want and what you ask it's different. Diego Nardi makes his hybrid, because he's a professionist, ( he cannot makes or give a scheme coping the work of another )The best choice is mu follewer with mosfet see AUDION N 11 or Alan Kimmel works It's all you need. Of course mje 340 with ALgaas led is more COOL
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