2A3 pre-amp

[Clive]

Once the gain structure of the system is setup I can't help thinking that with a DHT preamp the valve should see the full signal and the volume control (AVC?) Should be on the output of the preamp. Just my 2p.

Yep, but its the effect that will have on the output impedance that puts me off the idea.

I still use an AVC as a passive pre....it can't be worse than that can it? The trade-off is reducing any residual noise via the attenuator and amplifying the signal as much as possible in the first place. T'was just a thought.

[Nick]

Once the gain structure of the system is setup I can't help thinking that with a DHT preamp the valve should see the full signal and the volume control (AVC?) Should be on the output of the preamp. Just my 2p.

Yep, but its the effect that will have on the output impedance that puts me off the idea.

I still use an AVC as a passive pre....it can't be worse than that can it? The trade-off is reducing any residual noise via the attenuator and amplifying the signal as much as possible in the first place. T'was just a thought.

Yes, using an TVC is a entirly different matter. Using the 2a3 as a buffer to drive the TVC gives you the best of both worlds. Its just the use of a pot that has the problems.

To answer the question, yes, a pot after the 2a3 will be much worst than the TVC.

[Clive]

I thought that's what I said....ok I did suggest an AVC but it's the same difference isn't it? Or do I have that wrong. I was thinking Dave Slagle AVC.

[IDM]

Having seen the recent flurry of posts on different linestages I thought I would have another look a t my 2A3 pre built to Nicks circuit. Whilst the 2A3 sounds great almost magical, however whatever I do it is so microphonic. I was recently reminded by Phil that Nick rated the use of a 6080 in place of the 2A3. The only change to the circuit was to increase the cathode resistor from 1k to1.5K.
I didn't have any 6080's but a few mins on Ebay located some NOS National JAN tubes for £6.00 each so very cheap. So I lashed the pre-amp up with the 1.5 K on the cathode bypassed with 220uf. The cathode voltage is about 54V so each triode is drawing about 34mA. I am currently under-heating the filaments as I only had a 7V 50VA transformer with rectification (taken from the 2A3 Rod Colman circuit for the 2A3's) I put a 1R resistor in to drop the V down, however, the heater is currently only getting about 5.4V. so needs fiddling with. Output caps are 2.2uf Obbligatto's. Anode voltage about 160V.
When I was modifying the circuit I was a little concerned by htreadsI had read on DIY Audio that someone could never get the 6080 quiet enough, however, I have to say mine is absolutely silent. In fact when I first switched on I thought it wasnot working no hum no noise, nothing till I put a record on.
It's a little early to get a true impression of the sound however, initial thoughts are :
2A3 magic gone (but as you said this might be an artefact due to the microphonics). The 6080 has almost no microphonics (you have to rap the tube to get anything).
This linestage is very very detailed, I find myself being drawn to how plectrums are being used on strings. However, whilst very detailed it is not fatiguing it seems to just be very honest. No hint of sibilance on even some of my more demanding recordings. I also noticed that cello's sound very full with excellent drive and extension in bass. So overall very impressed.
I would really like to get the circuit truly optimised. At the moment I am thinking of fiddling with the heaters to get the voltage up to 6.3V (I suppose I could try regulation on the hearers, though they draw about 2.7A. I also thought I would try removing the cathode bypass to see what happens to the sound. The current bypass caps are nothing special just Maplin's finest. At some point I will rebuild with decent quality resistors such as Mills for the cathodes.
If anybody has any suggestions for improvements to the circuit I would be very keen to try them to try them.
Cheers
Ian

[andrew Ivimey]

If you still have the 2A3 pre up & running and want to substitute in 46 or 47 you might be interested with the sound.

Of course I don't know what Nick has done exactly with 2A3 as pre but am guessing its rather like Bottlehead's 300b pre and when faced with something like this I found that just plonking a 47 in where a 2A3 should go or a 300b with a little adjustment to the heater voltage (okay, I admit ooooops I do try dc heating sometimes though on the whole I like AC) then Bob rapidly becomes my uncle and makes for a very musical reproduction of the old vinyl or Squeezebox input where microphony or megahumnastiness (10Y), 30 or 31 has been the case.

[pre65]

Andrew, when you brought your 26 pre to mine I had a version of the 6080 pre in use, but not the proper Nick circuit with the EL84 load.

We both preferred your 26 on that occasion.

But, I might get a couple of EL84 and have another go.

[IDM]

Hi Phil,

I am using 6p14 the Russian equivalent to EL84 as they are cheap and I had them in the spares box.

Andrew, I agree it would be great to try some other DHT's the problem is that they are not that cheap and I reckon you need a few valves to find non or less microphonic pairs. There is also the consideration of the costs for valve heating. I personally find DC is the only way to go in a linestage and even using the Rod Colman boards, by the time you factor in transformers etc the experiment becomes expensive. I do have some Russian 4P1L's on there way to me. I hear they are supposed to be good.

Anyway at the moment I will fiddle on with the 6080 as it is impressive and I want to see how much I can squeeze out of it. When I was googling around I see there is a similar circuit to Nick's done by Pete Millet using an EL34 as the CCS. He was likewise very impressed.

Cheers
Ian

[pre65]

Anyway at the moment I will fiddle on with the 6080 as it is impressive and I want to see how much I can squeeze out of it. When I was googling around I see there is a similar circuit to Nick's done by Pete Millet using an EL34 as the CCS. He was likewise very impressed.

Cheers
Ian

Hi Ian, have you a link to that circuit (Pete Millet) as I have got EL34 and would prefer octal based valves.

EDIT. I found it.

http://www.pmillett.com/lowmu_preamp.htm

*

[Paul Barker]

For Octal Phil you aren't bound by EL34.

I used to find that NOS 6L6's and 6V6's were plentiful but EL34's have always been scarce.

You could just plug in 6V6 for a closer match to what you already built, and stand by to adjust the cathode resistor of the 6V6 to tune in the current.

As you said you had EL34's you will probably use those. But I mentioned this in case others reading have these other valves.

[pre65]

Thanks Paul.

I did use my 6V6 (Russian) for the original effort with 6B4-g but ended up using a 6AS7g and an Ixys 10M45s as the CCS load.

Obviously not as Nick suggested and as said before, not as good as Andrews 26 pre with my set of valves.

I think the GK-71 amp might be next on the project list.

[Paul Barker]

I am having a crisis of thinking about these high current preamps stages.

I took the trouble to read Pete Millet's downloadable article and I am offended by a couple of his remarks. He effectively claims that the traditional hi mu stages are distorted unless lashings of feedback is used, and then they don't sound nearly as good as his creation.

So what are we to do? Use multiple Pete Millet stages?

I don't buy it.

Sure if you have some diffecult interconnects to drive or a badly designed power amp which can't be driven without a sledge hammer, fair enough.

But we shouldn't be chucking out all our 26's 27's 76's 6j5's 6c5's ML4's MH41's and so on and so forth.

Fair enough if you want the tone control element of the extra triode, or if you genuinly need that much current.