Latest 6B4G PP and some thoughts about signal return paths...

[yves]

regarding the caps

Hi James,
I was asking a question, I was not sure if it had
anything to do with resonances. To determine that,
we really need to know what inductance and series
resistance values are of the IT.

I had a look at the circuit again. The IT really likes to see
a low impedance to ground at the centre tap. This is a low
impedance relative to the inductances of the IT. If the
impedance were high relatively, then there would not be
good anti-phase at the inputs to the differential amp. The neutralising
caps would not function as intended when the phases
are not correct, cancelling instead of enhancing the
high frequencies. Again, we need to know the actual
inductances and resistances for this.

My 0.02 analysis, your inputs welcomed.
yv

 

[James D.]

Hi John and Yves,

Thanks for you thoughts. I will have to measure the IT (as it's a custom winding) and see what I have.

I did mean to measure them before putting them in circuit but I couldn't wait 🙂

ciao

James

 

[yves]

phase splitting

James,
I re-looked at the equivalent ckt you redrawn with the
22u cap in the IT centre tap.

The ground is now a series
resonant circuit with the cap, in addition to the
dropping impedance of the cap as freq increases.
yv

 

[darkmoebius]

Any news on this design?

It's been almost a year, but I was wondering how the amp turned out?

 

[James D.]

The amplifier turned out very well. It was one of my favourite amplifiers until I changed it into the power stages for two different guitar amplifiers for Christmas presents 😱 I found that substiting Lundahl transformers for my input and interstage transformers removed the need for the neutralising capacitors on the driver stage and let me change the 22uF and 2uF capacitors around to the "right way' round. I kept the neutralising capacitors on the output stage as they relaxed the sound from the amp.

At present I'm using a 2A3pp and PCL82 SE amp on my home system and working on a 833SE and 211 cathode follower PP amp in the workshop (both promise about 60+ Watts in A2) but the design of both is proving problematical! (in other words I'm stuck!)

ciao

James

 

[darkmoebius]

Thanks James...

I didn't realize until later that I was responding to multiple threads about the same amp.

Anyway, two years ago I planned on building Ralph Power's IT-PP6B4G amp and never got around to it because I didn't have very efficient speakers. Now, I have 97dB/8ohm single drivers and it'll rock like Woodstock

I've also been followingf Lynn Olson and Gary Dahl's work on the Aurora/Raven project even though I only understand about half of what's going on. Full PP just seems so appealing.

One thing I love the variations of this PP6B4G amp is the incredibly low parts count(and cost). Once you've got the OPT's, IT's, and Sovtek 6B4G's it is easy to swap the driver stage from SE to PP or different tubes with minimal effort and cost.

I've already got a pair of ~6k Vintage Sansui 1000A reciever output tranni's that have a reputation for being top performers even comapred to today's iron. (Tango or Tamura was originally part of Sansui)

I can't afford both the Lundahl IT's($100) and Sovtek 6b4g's yet at $100+ for a matched quad. So, I'll get one this month and the other next month. Although, I can pickup the driver tubes, caps and resistors now.