The 300B interstage 300b amp

Using Lundahl latest LL1677/80ma SE it transformer.

Pictures still of old prototype , the idea is to make new monoblok amps , but i am slightly ( SLIGHTLY ) low on time .

NEW 300B / 300B actually comes as a monoblock

OK . This is a real Killer design.

The idea was a capacitor-free amp. With that in mind I started designing.
I also wanted something fancy that I knew would perform well.
I was thinking of using special tubes 845,211, 833, DA41, but reconsidered due to the high voltages involved (lethal no doubt) and the difficulty in
matching impedances (compromise in bandwidth and expensive as well).
The trend nowadays is a choke-loaded plates interstage.

So the challenge started here: how do I get rid of all capacitors?

Well actually the answer is quite simple: use interstages everywhere!
So what's the big deal you ask? Well it's just that I wanted an exclusive
design that hadn't yet been done (for sure someone, somewhere has
built it) but then again with easy to obtain tubes (and safe !!!
voltages), I decided on a 300B driving a 300B.
Now, where do I find an interstage that can handle 80 mA of
primary DC current? Well, since I am a dealer of Lundahl,
I had them custom made. They made them on the basis of the LL1660 interstage, 80mA and 25H primary inductance.
It's tempting to balance the magnetic fields. In theory it should work.
The idea is described below: After a few nights of sleep I came up with a brilliant idea: balance the transformer. Thus, if in the secondary I could get 80mA to flow, then I could pull the job off with a high quality interstage for 10 mA. The other problem is always hum. The only way to get rid
of this is by using DC on the filaments. It also helps to use all the
regulation and filtering possible on the power supply !
I am a fervent supporter of negative Bias and here it is needed in order
to balance the interstage and of course avoid having to go through a capacitor.
I must note however:
Auto bias has the advantage in that it slows down everything and exudes impressive warmth; whereas neg bias is more precise and also more analytical in nature.
For Xformer Zo I chose a 3K primary, because it gives off less distortion than
a lower plate resistance (which would give more power).
The HT is set at 500 volts.
If you use Krone Valves you can exceed 400 Volts.
Note the 10/630 cap is an MKP.
Anyway have fun if you intend to build this one;
It's a real killer.
As for the power supply:
The simplest way is a hybrid tube rectified choke loaded supply.
It's extremely musical and easy to build.
The sound quality changes dramatically with the rectifier tube chosen.
The 5U4G will work so will a GZ34,32,5R4
But the winners are GZ37 & 274b.The hybrid design uses the rectifier tubes as a slow turn-on device and by paralleling the diodes, we can double its current-delivering capacity.
Furthermore it is heavily filtered and uses chokes with enough rdc to lower voltage on driver tube.
2 nice mA meters tell you what is going on inside.
NOTE: you will need a powerful preamp that can swing at least 10V rms.
(Most tube preamps swing far more and most solid state will just reach that)

Copywright @ 2002 Benny Glass design