300B SE Design

[Monta]

Hi all,
I'm just registered, I'm building my second Tube amp.
this time, as a starting point I choose the well known JE Labs 300b SE schematic.
I found all components accordingly to the original schematic, but I have a copuple of 3.5K output trannys instead of 3K, and the measured power supply voltage is slightly different than expected.
So I made an excel file to re-calculate the output stage bias (changed the 880ohm resistor with a 1k) , but i'd need your help on the 6sn7...
Do someone knows about a tutorial or a book which explains how to calculate the input stage?
Or even better can someone explain it to me??

Thanks a lot!

Cheers

 

[tomchr]

Monta,

First of all, you'll probably get more (or higher quality) help if you post a schematic. Yeah, I could google it and maybe even find it, but I'm lazy like most and would really just prefer to have my schematics served on a silver platter... 🙂 It also makes it easier to figure out what exactly we're talking about.

I wouldn't imagine that a change in load impedance from 3k to 3.5k would result in any major changes. I would start with the original circuit without any mods other than the change in OPT. Tweak from there once I got the amp working. But on the other hand, it doesn't sound like you're making any dramatic changes so it's probably fine as you've designed it already. You'll get ever so slightly lower output power with the 3.5k tranny as you would have had with the 3k. But you should get ever so slightly better THD also. Trade-offs, trade-offs...

References:
Morgan Jones, "Valve Amplifiers, 3rd Ed".
Radiotron Designer's Handbook, 4th Ed - aka RDH4.
Many of the older ARRL handbooks can be handy too.
Valve Wizard.

~Tom

 

[Richard Ellis]

Here it is...
__________________________________________________________Rick....

 

[Nikon1975]

There is a JE Labs builder threads, I am sure you can post there your questions.

Davide

 

[Ciscokid]

Monta,

I've built these amps using 2.5k hammonds. Loved them! Later on changed out the OPTs for some Hashimoto 3.5ks. Sound is great! I swap these amps in and out of my system with an HK Citation II when I am looking for something different!!! Go ahead and use the 3.5ks you have.. Worst case, if you don't like the sound, you can always upgrade!

Here's my web site if interested..

http://www.tubesrit.com

Later,

ck

 

[Monta]

Thanks! a lot of interesting stuff to study!
By the way, I wrote down the measurements and changes made.
Do you think I need to change something in the input stage with those voltage values?

Thanks again!

 

[DHT112A]

Just my .02cents but I would use something other than a 6SN7. I built that circuit as my first project many years ago. Stuck rigidly to the schematic and parts list and found the 6SN7 to be, for lack of a better word, junk. Tried several different 6SN7's. Weak, anemic.

 

[Monta]

Junk seems quite a strong negative word! Anyway at the first "power on" the sound was not so warm and clean as expected... how can I explain... I seems it runs too high on bias point, losing ambience and warmness.
This is the reason why I tried to re-calculate the bias point to see where I were...
Anyway, do you think there could be a better choice for the input stage tubes?

 

[DHT112A]

I agree, it was rather strong but at the time it was all I could think of. I just did not find it to work out at all. I went on to experiment with other drivers and found that virtually everything I tried seemed to make the amp sound fuller, like it had more power.

 

[Monta]

Wow! it is not very good to know at that point!
Anyway I will try re biasing first and then if it is still unsatisfactory I will change driver stage.
Thanks!