Anyone used interstage transformers for a 300B?

[duderduderini]

Hi
Just stumbled acrooss andrea ciufolli's 300b schematic using lundahl interstage tansformerstwixt the driver tube and 300b
What do people think of this (looks mighty simple)?
What is the general concensus regarding this topology..
Thanks for your time
Nick

[arnoldc]

This is actually my next project

I have a pair of Lundahl LL1660S/10mA that I'll use in SE input to SE output configuration. I've heard this IT in my previous project (6SN7 > IT > EL34 PP) and I wanted to use it in SE to drive a 300B (or maybe a 45).

I plan to use any of the following driver tubes-

10, 2C51, WE404A in triode

Sorry, I did not answer your question

[dejanm]

I do not think that there is a concensus about using transformer as interstage. Some believe that Transformers are bad because of their unlinearity and some believe that they are good - for various reasons.

Basically there are two most often used interstage concepts: transformer and capacitor. During transients, this capacitor discharges, which is not a big problem. The problem is that re-charging can take 5 times longer. And that means distorsion. If the transients are frequent (for example when you listen Metallica very loud), then this interstage can be a source of permanent distorsion. Transformer does not have that effect, if it is designed well. And these Lundahls are. The problem with interstage transformers is that they are rather expencive.

This is actually very old subject. Recommended reading about this particular subject:

http://www.audioxpress.com/resource/...ss/ga400ac.pdf

This is an article about PP but the statements about interstage are valid also for SE.

Another interesting reading is also:

http://www.tubelab.com/powerdrive.htm

This guy developed a completely new approach to this subject. He addresses also a very well known fact that some of the best amps (Kondo, Audio Note,...) are using capacitors as interstage. These amps are so expencieve that the cost of transformer doesn't play any role.

Andrea did a great job by adapting in reality Sakuma design and making it even more simple. About Sakuma you can read here:

http://www10.big.or.jp/~dh/index.html070810

Just go for it ...

[Shoog]

Its a very tough job for a driver to push both an interstage transformer and a 300B combined. If you are using it in PP then the current availble to each grid is halved (so using the LL1660 10mA, each grid only gets at best 5mA of drive). Most people tend to like to drive the 300B with a bit more than that. Also, even the best interstage transformer has capacitive loses which compound the 300B miller capacitance. Forget about stepup arrangements in the real world.


I would not like to put you off but there are other options;

1-parafeed the interstage. The transformers performance should dramatically improve and you can easily push 20-40mA of current through your driver tube. The question is does the improvement in transformer performance outweigh the penalty of the parafeed cap.

2-use an input transformer phase splitter, followed by a meaty driver, pushing a PP interstage transformer driving the 300B grids. (ala Raven amp).

Again I think the extra effort is well worth it. My current amp uses 6080's in PP with parallelled 5687 pushing a parafeed interstage transformer at 40mA. It sounds wonderful and wasn't as expensive to implement as might be expected. The 300B project would have similar drive requirements. Even in SE all of the above consideration apply.

Also remember that you are restricted to a none gNFB approach if you use an interstage transformer, so if you find that the top end is rolling off unacceptably, your only option is to push more current through your IT - which may not be an option.

Hope that helps.

Shoog

[duderduderini]

Hi everyone
I want to use the 300b;s in parallel set mode.. for reasons obvious and mysterious, I have my heart set on paralell single end.
SHoog in pse is there enough current to drive the 300b's?
Thanks
Ps the 417A seems a bit thin on the ground.. what octal alternatives can be used.. ?
ps number 2. Can say a 6sn7 in srpp be used (I believe that srpp can provide the swing necessary to drive the 300b)?
Thanks
Nick

[arnoldc]

The LL1660S/10mA is good for 20mA in ALT-T configuration.

I'll see how it goes.

[KSTR]

Lynn Olson's Karna (ECC99->45->300B) is one of the best amps with interstages I know of... it's all push-pull, though. And it's using Lundahls...

- Klaus

[Shoog]

I don't know enough specifically about the 300B to pass comment really. Using two in parallel doubles your problems though.

Take a look at some respected 300B designs and see what they are pushing through their drivers, then add a little to account for the loses through the IT. This should tell you whether 20mA of drive will be enough. I have a hunch you might struggle. If I was doing it I would build the Karna instead.

Another option would be to consider direct coupling, with possibly an interstage transformer between an initial preamp and a driver.

Shoog

[arnoldc]

Hi Shoog, what are those respected designs you mentioned? I know the works of Lynn Olson, and I don't think I can afford it

I've seen the designs of Andrea, Andy Grove, and they do drive the 300B with an interstage. Some use 5842 - IT - 300 and the Audio Note used Pentode (triode strapped) - IT - 300B...

Thanks for your insights, keep 'em coming

[Shoog]

Unfortunately I haven't looked into the 300B to much beyond knowing its hard to drive. Unfortunately i will probably never be in a position to build with one either

It was more by way of general advise.

Shoog