My 2A3 is being left behind

[agent.5]

Nothing is absolute in the tube world. Of course, you can load a pentode with a CCS

http://www.tubecad.com/2006/06/14/va...ikido_cf_2.png

[arnoldc]

Hi James,

So, this thing will actually work I hope it sounds better than my 6SL7-2A3

I checked Thorsten's Legacy again and it seems the 12K was designed for some 400V B+ and with a little bit over 300V for the 2A3, I think the plate resistor should be calculated accordingly... or should I say I have to make a compromise because I only have 8k2 and 7k5 big Kiwame resistors

Can you tell me why you connected the VR to the cathode?

No improvement with grid choke? Hmmm, I have some 270K Riken I could put to use for the meantime. But I'll compare them anyway.

Zobel resistor + cap... I don't know how to compute for those I'll search the AA for that post.

WE connection! Yey! I have two huge Obbligato 30uF from Brian Cherry which I haven't used since I bought them

Is this close to what you described?

ps.

If the VR returns via the cathode resistor, then I can't use LED biasing, can I? I plan to use LED soon to bias the C3m...

agent.5, good to know that, but with a low B+ for this 2A3 amp, CCS load may not just make it, don't you think?

[arnoldc]

Hi James,

I cannot find Mark Kelly's post but I found yours, which answered (partially) some of my questions. And I quote:

I use a VR tube between screen grid and cathode (using a zobel network across it to neutralise the VR 'inductance') to keep the screen voltage static. This also allows for some 'current dumping' through the cathode resistor so I can use a smaller value resistor and drop the cathode bypass cap. I think this sounds better.

James

[hardcore]

I've done a 717 to px4 amp ala yamamoto
Some things I learned
- plate chokes don't work, better to use them on the screen.
Pentodes love resistive loads
-grid chokes don't work either. I transferred my S&B grid chokes to the input grid of the 717.
-WE connection is a matter of taste and takes some time to break-in
-I use separate filament trans both for driver and pwr tubes for low noise given my spkrs are 100db
-Overall, I'm quite satisfied with the sound a good mix of pentode aggresiveness and triode sweetness
-I've done a lot of iteration over the last four years on my amps. 6sl7-300b, 6s45-px25, 6sn7-2a3, 6j5-px4

Next would be dht drivers and dht output!

[Miles Prower]

Originally posted by arnoldc
I'm thinking of-

a) Pentode driver, I have a Western Electric 404A but it's just a pair!
b) DHT driver, I have 01A but I want to make a pre-amp from it. I have another 2A3 2A3 drives 2A3?
c) My friend bought about 80 pieces of C3m and I can get them really cheap. But I've read the C3m mostly for 300B applications...

Can the bright minds here suggest to this bored DIYer what to do with his 2A3?

How about driving the 2A3 with a Kimmel? This could either use a pentode on top or make it a hybrid with a power MOSFET on top. If you DC couple, you avoid all the problems that coupling with a capacitor can present. Add a few db's of gNFB and you'd really have something.

[arnoldc]

Sorry Miles, but I have no idea what a Kimmel driver is. Care to share some links?

Hardcore, thanks for sharing your experience. Since you're in the same country, is it possible to hear your creations?

[Ryssen]

I use a 6S4PEV and a CCS,it sounds goood..

[Miles Prower]

^^^^

Here you go: Kimmel Mu Stage.

It's like an SRPP or a regular Mu stage, but better and more flexible. It basically adds an external load resistor so that it may be used with completely different types of tubes since they don't have to run on the same currents. Thus, you can use a small signal, high gain triode for the voltage gain, and a high g(m) pentode as a follower for current sourcing capability. It also adapts quite well to solid state, making for an even beefier driver if you're going to push the 2A3 into Class A2. Even if you don't go that extreme, it will improve transient overdrive behaviour.

[James D.]

Hello Arnold,

I replaced a cascaded 6SN7 circuit with this and this one was more transparent and open with more extended bass response and much better bass tone and articulation, My son liked the amp so much he now has it and won't let me try any further mods in it

To your questions:

a) With the 300V B+ and value of 7k5 load will work nicely. I use a 7K there.

b) The pentode references the screen voltage to it's cathode so if you decouple to ground you have the low frequency pole that this generates distorting the bass response of the pentode (hopefully nelow the audio range but having phase effects through the bottom end of the audio range). You also have any cathode resistor/bypass variations imposed on the signal. This applies at the top end as well - again hopefully above the audio range but not necessarily. Referencing the sg to the cathode avoids these effects. So really it is just putting the sg reference where it should be anyway...

c) Grid choke is definitly a try it and see option - it add anything for us and was slightly worse - this might just be a reflection of pentodes not liking a reactive load (in general) i.e. producing a simpler distortion spectrum when resistively loaded.

d) The zobel I use is 100nF and a 390R - it is to neutralise the VR tubes inductance and reduces the small amount of rf hash that the tube produces.

e) WE with the 30uF Obbligatos would be very nice with a 100-140uF cathode bypass

f) Led biasing could still be used but you need to allow for the VR tube static current as well - so you would need a small LED array to handle that as the VR tube reaches minimum noise at about 14mA for a 40mA rated tube (It gets seriously noisy below about 3-4mA)

g) I wouldn't even think of ccs loading a pentode in this application - it works well for high loads on the system and give maximum voltage swing but it doesn't like lower loads and/or varying current draw (i.e. reactive load) - like a DHT tube with it's Millar cap on the input...

Something I'm increasing doing to get the cathode resistor heat above the chassis is to use a mains light bulb as the cathode resistor. You need to experiment with values as the resistance changes a bit with applied voltage - in Europe a 60W bulb makes a nice cathode load for a 2A3 for instance.

Hope that helps

James

[arnoldc]

Hi Miles, I'd do some reading about the topology and maybe give it a go when I understand it

James, thanks a bunch! That's a whole lot of info. I'll get my two pairs of C3m from my friend soon, so I'll be able to do something on my prototyping rig.

Ryssen, thanks for the vote on the 6S4PEV.