pentode aikido driver for 300b

[dave dove]

myself and jamesd have concocted this amp
it has been a few months in creation
any opinions would be appreciated.....

power supply is valve rectified LC or sometimes LCLC
(i can't fit the second choke onto the board)
i have tried CLC which gives @510V HT with droppers to give 460V on driver
but choke input sounds better in this case
the driver stage has further LC decoupling
i am considering some form of shunt regulation or minimal reactance type psu

6n6p operating point was actually derived for ecc99
so we may tweak this but it does sound substantially better with 6n6p

i would like to thank jamesd for his support in this project
it was he that led me through all the calculations etc
so without him it wouldn't have happened
hope you like it.....

dave dove

[James D.]

A couple of comments on Dave's amp.

In many ways this amp is a tribute to JRB of Tubecad fame. His ideas abound in it. The pentode Aikido is a super idea. It allows the nice low Gm audio pentodes to have a real chance of driving DHTs. ANd a very good job they do of it. I've used and like high Gm pentodes driving DHTs but had often wondered what an EF86 would sound like doing the same task - now I know .

The idea to use partial feedback around the 300BXLS came from Dave and is again inspired by JRB. John and Steve Bench's T-Rex amplifier design provided the template and Dave adapted it for this amp.

James

[IslandPink]

Nice work Dave,

Looking forward to hearing it at the next Eggborough meeting .

I see this as the logical extension of the WE91 circuit . The beauty of it is , you still have only one gain stage , with one coupling cap , you retain the high gain of the pentode , but with the extra current from the Aikido . As James has said , the pentode alone looks to be slew-limited into the 300B grid, but the Aikido should get around this .

James, do you know what the output impedance of the 300B with the feedback loop should be ? - Interested to gauge what the damping factor could be . Has the circuit lost much gain from this tweak ?

I'll be following the same path most likely, but first I'll built Thorten's WE91 variant, so we can get the max education out of this ..

Mark

[James D.]

Hi Mark,

Without going through the math to check how this circuit changes things, I would say about 150R based on JRBs example Tubecad Partial Feedback .

Hope that helps.

Looking forward to you joining the club I'm toying with the idea of adding it to my PT15 PP amp. Since these are driven by an IT phase inverter - it is a challenge to work out what values to use...

ciao

James

[Bas Horneman]

Thanks for sharing with us James and Dave! I'm afraid I'm going to have to ask the inevitable question and ask..."How does it compare to other amps you've heard?"

[dave dove]

bas

i've had this breadboard going for over a year now
kr300bxls output valves at loads of operating points
originally with lundahl output transformers
now with partridge output transformers

it has had the following driver stages:

6j5gt > 6ah4gt very good big dry sound k&k audio style
half ecc99 very polite, not enough energy for me
6em7 both cap coupled and direct coupled.... yuck
417a which i loved but had to try....
6c45pi which i didn't love even with some extreme battery biased operating points

then this which sounds like the music in my head
particularly since applying feedback around 300bxls
it has bandwidth, control, speed, transparency and loads of funk
and i haven't even started on the power supply yet

and all the caps, resistors and wire are cheap and nasty

we designed it to use ef86 and ecc99 which i had lying around
but changed to 6n6p because i really don't like ecc99

and it sounds wonderful

dave dove

[Bas Horneman]

Thanks Dave,

For that comprehensive answer! Tried anything choke loaded?

Regards,
Bas

[dave dove]

bas

yes
417a and 6c45pi
but only with hammond 157g

prefer this

dave dove

[Bas Horneman]

Mmm this schematic looks very interesting as a tube and a couple of resistors and a cap or two...is cheaper than some good choke.

[dave dove]

gotcha

dave dove