Aikido driver into an Interstage in Parafeed

[Andrew]

-- Gents

The original design for my power amp has a 417A/5842 into an interstage, I don't feel this has enough welly to really get a 300B going and was thinking that after Dave Dove's raring success with the Aikido front end to build something similar to Dave's amp.

Dave tried all sorts of drivers and came to the conclusion that you need to have plenty of gain and plenty of drive to get the best out of the 300B.

So, I have, on the drawing board, so to speak, an idea for an Aikido drive for my SE amp, but using parafeed into the existing interstage. I don't want to get rid of the interstage since it allows me to used fixed bias on the output stage, and well it was quite expensive too.

With an Aikido front end I can't use the interstage to load the driver valve then using it in parafeed seems reasonable to me.

However, that parafeed cap seems to need to be awfully big if it isn't going to cause bass roll off.

Is this a good idea? Or just a flight of fantasy.

thanks,

-- Andrew

[James D]

Hi Andrew,

It's a great idea! I'm using this arrangement in my PT15 PP amp with an IT phase splitter. It works great! Better than using a normal IT as the low output impedance of the Aikido really helps drive the IT well. The parafeed cap doesn't need to be larger than normal - 2-6uF is perfect. You might like to try lossy parafeed to tweak the sound, standard parafeed works well in anycase.

The only complication comes if you use partial feedback on the 300B, Then I think it ends up being series applied rather than shunt - as per RDH4. Still trying to get my head around this....

James

[Andrew]

Originally Posted by James D

Hi Andrew,

The parafeed cap doesn't need to be larger than normal - 2-6uF is perfect.

I got 3u3 in my Spice model (about 12Hz @ -3db) but I didn't want to believe my own sim, I thought "oh, that caps a bit bigger than I would expected", by about 3-4 times. Made me think I had the inductance of the IT wrong. I had assumed something around 1uF would have been OK.

Originally Posted by James D

The only complication comes if you use partial feedback on the 300B, Then I think it ends up being series applied rather than shunt - as per RDH4. Still trying to get my head around this....

I plan to go there, eventually, but I'm none the wiser than I was the last time we spoke. I knocked up a sim, but, to be honest, I was just hacking, I had no idea what the sim was telling me.

Anyway, at least it seems I finally got the parafeed bit right....maybe I'll get the partial feedback, eventually the light might switch on, just give me another couple of months....

cheers,

-- Andrew

[James D]

There has been some more discussion about parafeed on the MQ forum of AA and Voltsecond and Paul Joppa and Mickey all say that 2 - 6 uF works out as optimum. Nice work with the sim. to nail it

So I guess we both have to get the pf with IT sussed by EggFest 3 then?

Don't hold your breath!

James

[dave dove]

i've had problems with the partial feedback
and i still have no idea why...(&^$£%0

after some time i get high DC voltage on 300b grid
and the first i know of it is a cracking sound
which is the cathode bypass cap going
which suggests the PF cap is breaking down
but if i replace the cap the DC voltage remains
but weirder - if i swap the output valves
the DC voltage goes with it....&*$*£%^
if i remove PF altogether it's all normal!!

i could try just running 100K anode to anode
but i'm not sure this'll work so well
i guess it should go to the bottom triode
but not so elegant....

dave

[Ianm2]

I am giving single ended 300b a last gasp, fiddling with a parallel feed and an interstage driven next week. may try aikido linestage next, too. Its all happening!!

[Andrew]

Originally Posted by James D

There has been some more discussion about parafeed on the MQ forum of AA and Voltsecond and Paul Joppa and Mickey all say that 2 - 6 uF works out as optimum. Nice work with the sim. to nail it

So I guess we both have to get the pf with IT sussed by EggFest 3 then?

Don't hold your breath!

James

Quite, I'm not the builder in town...but...#2 son arrived yesterday, so precious little soldering time for me for a while......

cheers,

-- Andrew

[david counter]

congratulations Andrew,

[James D]

Congratulations!

Now that is what I call natural sound!!!

James

[NickG]

Congrats, hope mother and baby are doing well (no one cares how you are doing :-))