Inexpensive balanced push-pull amp with DHT driver.

Cressy Snr · Post by **Cressy Snr** » Sat May 25, 2013 2:55 pm

This little amplifier came into my head this morning and I am getting quite
excited about the possibilities.

The 6AQ5, 7 pin miniature output tetrode is a 6V6 in a little bottle.
Cheap as chips NOS at a fiver apiece and easy to come by. I've ordered eight.

This amp will use four per channel in a parallel tetrode push-pull, anode-follower output configuration, with gas tube regulated screens.

Drivers will be Mullard DCC90 miniature, directly heated, double triodes, lit with AC from a Hammond 2.5V heater transformer per channel. This little double DHT is ideal for a differential driver stage as it has a centre-tapped cathode.

It produced a magic sound as part of an all DHT phono stage a few years ago but was too microphonic in that application. As a DHT diff driver stage, it should be much less troublesome in that department.

3.5mA would probably not be sufficient drive current for a parallel push pull triode output stage, but should be fine driving a tetrode configured output.

Should get 20W per channel, in class AB1, no trouble.

I'll post a schematic proposal for comment later on.

Paul Barker · Post by **Paul Barker** » Sat May 25, 2013 3:27 pm

You are using the DCC90 balanced are you?

Cressy Snr · Post by **Cressy Snr** » Sat May 25, 2013 3:36 pm

Paul Barker wrote:You are using the DCC90 balanced are you?

Yes.

Paul Barker · Post by **Paul Barker** » Sat May 25, 2013 3:50 pm

Will be nice to hear them.

Cressy Snr · Post by **Cressy Snr** » Sat May 25, 2013 4:06 pm

Bollocks!

Forgot to take the Fb into account.
the DCC90 has a gain of 16, so with Fb applied I'm stuffed.

I'll have to have another think, or risk triode operation and hope it doesn't slew limit.

Could parallell up the front end if necessary, to increase drive current as the balanced drive from
the DAC has more than enough welly to cope with the input capacitance.

I have loads of DCC90s and 3A5s picked up NOS for a song some years ago.

Jeez! I'll have another think.

Paul Barker · Post by **Paul Barker** » Sat May 25, 2013 4:19 pm

Can't you then do two stages of DCC90?

Use stage one without feedback for initial voltage amplification and use shunt feedback between the output tube and the intermediate balanced drivers. Short feedback not including the OPT to reduce output impedance primarily.

R22 in the above schematic. He talks on it in the link to teach you how to calculate Fr, but you could just use a pot and adjust to taste.

Cressy Snr · Post by **Cressy Snr** » Sat May 25, 2013 5:02 pm

Paul Barker wrote:Can't you then do two stages of DCC90?

Use stage one without feedback for initial voltage amplification and use shunt feedback between the output tube and the intermediate balanced drivers. Short feedback not including the OPT to reduce output impedance primarily.

Aye, could do that. Cheers Paul.

JamesD · Post by **JamesD** » Sat May 25, 2013 5:54 pm

I'll be interested to see what power output you get from them - I did a design for a pair of 6005 in PP that got to 15Watts for the pair but needed to go heavily into A2 to do it - the 6AQ5 likes to go into A2 to the extent of about +20V in triode mode. It needs a good driver and either dc or IT connection to do it but really responds well.

J.

Cressy Snr · Post by **Cressy Snr** » Sat May 25, 2013 6:19 pm

Aye.. the limited power is why I chose to parallel them up as an output stage.
that, and the fact that by doing it I can also use Nick's push-pull 300B output transformers, he has at one of his external storage facilities, ie my house

According to the scuttlebutt you can get 10Wpc class AB1 out of a pair so maybe 20W out of four of the little blighters per channel.

Andrew · Post by **Andrew** » Sat May 25, 2013 7:17 pm

What about something like the ACF to drive them nicely into A2?

-- Andrew

JamesD · Post by **JamesD** » Sun May 26, 2013 9:59 am

ACF would work very nicely

I used an Aikido CF to save on valves! Its very easy to spend ten times as much on the drivers as on the outputs with 6AQ5!

I like Steve's idea of using inexpensive DHTs to drive them too. DCC90s are a neat idea

J

Cressy Snr · Post by **Cressy Snr** » Sun May 26, 2013 12:45 pm

OK here's the concept diagram.
1st and second stages are DC coupled, short plate to grid feedback loop around output stage, conventional screen supply at the moment.

JamesD · Post by **JamesD** » Sun May 26, 2013 1:09 pm

Just a note on the 'plate to plate' feedback. It will work, as you know, like it is but for the best results the driver should have a constant output impedance and either a very high impedance or a very low impedance.

So a pentode is a very good driver if you are using this feedback connection. A DHP would be fun - DHT into DHP into 6AQ5 for instance. If not a DHP then one of the twin pentodes would be nice too. Of course with the pentode driver the first stage is probably not needed...

Just a thought or two:)

ciao

James

Cressy Snr · Post by **Cressy Snr** » Sun May 26, 2013 1:25 pm

JamesD wrote:Just a note on the 'plate to plate' feedback. It will work, as you know, like it is but for the best results the driver should have a constant output impedance and either a very high impedance or a very low impedance.

So a pentode is a very good driver if you are using this feedback connection. A DHP would be fun - DHT into DHP into 6AQ5 for instance. If not a DHP then one of the twin pentodes would be nice too. Of course with the pentode driver the first stage is probably not needed...

Just a thought or two:)

ciao

James

Aye.... you need hi gm drivers and all that for this sort of feedback scheme. As you say, it'll work, but something working is a lot different to is sounding any good.

Paul Barker · Post by **Paul Barker** » Sun May 26, 2013 1:47 pm

At least you are getting loads of experinece with your cAd that you draw schematics in.

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Inexpensive balanced push-pull amp with DHT driver.

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