811a Class A2

[Joel]

too much?

Joel,
The current through the 33k resistor is approx. 1 mA.
The 280k resistor goes to the bias supply (adjustable between
5-30V)

Hi Jim,

So, you have the 33k resistor generating -33 volts of grid bias for the EL84, in addition to it's connection to a fixed bias source? Seems like you will be very far from class A with this tube, won't you?

 

[jim]

Brett,

I'll try as soon as I get the amplifier running.
It's still an "in the brain" amplifier
btw I've got NOS RCA-811A (for free)

thanks for the suggestions.

Joel,

The EL84 is biased by a separate bias-supply connected
to the 280k resistor.
This voltage biases the '84' to about -10,-12V at 25mA.

Jim

 

[Joel]

Ah, I see now Jim - it's a POSITIVE bias supply then? Ok, makes sense.

Joel

 

[Brett]

Does someone has some experience using the 811a in SE-A2 mode ?
I'd like to use it with the Lundahl LL1620 OPT and LL1660 IT.

Jim

I just found the schem of the WAVAC 805 (designed by the late Nobu Shishido) and thought I'd post it for some inspiration, especially as you mentioned possibly using an IT. The IT in the WAVAC is configured so that a large part if the DC bias flux will be cancelled (smaller gap, increased primary L), and IIRC has a step down to get better drive.

Enjoy.

 

[jim]

IT Coupled

Brett,

IT-coupling will be the future step......somewhere in 2008 or so

Here's my IT-schematic :

 

[apassgear]

I guess Brett's schematic came from Shishido's book "Sohshinkan Niyoru Single Amp Siesakushu" which I don't have. But on this book I undrstand there are a number of IIT including the 811.

BTW Brett, thanks for the schematic since I was looking for it, but never asked!!!

Now the problem is where to get a 437A. (at 1/10 the asking price I may add). The price of this tube will account for 1/2 the cost of the amp!

 

[Brett]

I guess Brett's schematic came from Shishido's book "Sohshinkan Niyoru Single Amp Siesakushu" which I don't have. But on this book I undrstand there are a number of IIT including the 811.

I don't actually have this book, I found the schematic on the web somewhere, and as is my habit, I saved it. One day I will buy a copy of Shishido-san's book but it's $US65.
He was a great designer.

I'm glad you liked the schematic. I also had the one for the WAVAC 833, but lost it when my HDD died.

Now the problem is where to get a 437A. (at 1/10 the asking price I may add). The price of this tube will account for 1/2 the cost of the amp!

Use a 6C45pi (possibly even a better tube) until you come a cross a stash of 437s.

Cheers

 

[apassgear]

Use a 6C45pi (possibly even a better tube) until you come a cross a stash of 437s.

Cheers [/B]

Thanks Brett, this might put me on the road for a IIT in the furure.

 

[Fuling]

What about using a choke instead of that 33k resistor??
I have a feeling that some extra current trough the driver tube would increase stability, and that current would "leak" away through the choke.
Or, you could keep the resistor in place but connect the "cold" side to a negative power supply?!?

I´ve been looking at some designs using high µ transmitter triodes lately (since I found out that I have the necessary PSU and output ironwares lying at no use arund the apartment), and most designs employs IIT´s.
After some failed experiments with DIY interstage transfomers I´ve finally started looking for other alternatives (Could´n get them damned things to stop oscillating...)

Some kind of direct coupled cathode follower using a low Ra triode should work, I think. I´ve been toying around with a 6EW7 (Ra 800 ohms Pa 10W) loaded by a couple of different chokes and a 1k resistor to simulate the 811A grid. Seems to work fairly well, but my chokes are just regular filter chokes with crappy HF response and I think a beefier driver would be of use too.

Next try will be a resistor loaded 6AS7 follower...

By the way, does anyone have some proper grid current curves for 811A??? The ones I´ve found doesn´t make much sense to me...

 

[DSummer]

I don't actually have this book, I found the schematic on the web somewhere, and as is my habit, I saved it. One day I will buy a copy of Shishido-san's book but it's $US65.
He was a great designer.

I'm glad you liked the schematic. I also had the one for the WAVAC 833, but lost it when my HDD died.



Use a 6C45pi (possibly even a better tube) until you come a cross a stash of 437s.

Cheers [/B]

I have this book, really $65??? I bought it from Hong Kong ~$35....

 

[Brett]

I have this book, really $65??? I bought it from Hong Kong ~$35....

D,

iirc I was quoting from the EIFL page. Do you have a source in HK you'd like to share where I could mail order? I'd love a copy of the book and $35 is more managable than $65.

Cheers
Brett

 

[Brett]

Whilst we're on the 811 A2 subject

http://www.one-electron.com/Randall/ganti_2.html

 

[DSummer]

D,

iirc I was quoting from the EIFL page. Do you have a source in HK you'd like to share where I could mail order? I'd love a copy of the book and $35 is more managable than $65.

Cheers
Brett

HK Bush Electronics in Central, another one located at Apliu St, Shamshuipo. I don't have their contact info on hand, but I don't think they will accept any mail order. I know there has some club members here from HK/China, may be they can help. Oh, how about ask those members from Japan, I'm sure it will be much cheaper....
PS. All those books are published from Japan with nice printing and packaging, they are worth to be collected in my bookcase even I can't read Japanese....

 

[jlsem]

I guess Brett's schematic came from Shishido's book "Sohshinkan Niyoru Single Amp Siesakushu" which I don't have. But on this book I undrstand there are a number of IIT including the 811.

The 811A Class A2 amplifier is not in this book. WAVAC sells a commercial design by Shishido called the "Dandy" (or something like that) that uses a Russian 811A (like the original RCA version).

I personally find these designs interesting. Has anyone used a Lundahl IT in reverse as the IIT? The Tangos originally specified by Shishido were special order items apparantly no longer available from new Tango. Also, is the current drawn by the grid only related to the DC resistance of the IIT secondary?

Thanks,
John

 

[jlsem]

Oops, I take that back. The 811A amp is in the book.

John

 

[ThorstenL]

811A Schematic #1

Konnichiwa,

I have aquite a few 811A Schematics on my HDD.... Here is one I like most....

Sayonara

 

[ThorstenL]

811A Schematic #2

Konnichiwa,

Here sadly rather low quality because of the stupid 800 pixel width limit of the board Shishido San's approach, similar to Wavac Musical Dandy 811A...

Sayonara

 

[ThorstenL]

811A Schematic #3

Konnichiwa,

Here another rather low quality scan....

Sayonara

 

[Fuling]

I sure like those direct coupled designs.
As I´ve said before I´m collecting parts for such an amp, and I´ll probably have a mono prototype up and running within a couple of weeks.

Tube setup will be something like:
Resistor loaded EC81 direct coupled to a resistor loaded 12B4A cap coupled to a choke loaded 6AS7 cathode follower which drives the 811A (puh!)

 

[dhaen]

Re: 811A Schematic #3

Konnichiwa,

Here another rather low quality scan....

Sayonara

Hi,

In this design without a path from G1 to ground, I'm intrigued as to what stops the 811 from "running away"...

Cheers,