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Old 6th March 2003, 01:50 PM   #21
Joel is offline Joel  United States
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Default too much?

Quote:
Originally posted by jim
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?
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Old 6th March 2003, 02:09 PM   #22
jim is offline jim  Belgium
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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
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Old 6th March 2003, 02:21 PM   #23
Joel is offline Joel  United States
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Ah, I see now Jim - it's a POSITIVE bias supply then? Ok, makes sense.

Joel
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Old 6th March 2003, 03:40 PM   #24
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Default Re: 811a Class A2

Quote:
Originally posted by jim
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.
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Old 6th March 2003, 03:59 PM   #25
jim is offline jim  Belgium
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Default IT Coupled

Brett,

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

Here's my IT-schematic :
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Old 7th March 2003, 02:35 AM   #26
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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!
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Old 7th March 2003, 02:59 AM   #27
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Quote:
Originally posted by 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.


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.

Quote:
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
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Old 7th March 2003, 03:43 PM   #28
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Quote:
Originally posted by Brett


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.
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Old 10th August 2003, 02:49 AM   #29
Fuling is offline Fuling  Sweden
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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?!?

Ive been looking at some designs using high 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 IITs.
After some failed experiments with DIY interstage transfomers Ive finally started looking for other alternatives (Couldn get them damned things to stop oscillating...)

Some kind of direct coupled cathode follower using a low Ra triode should work, I think. Ive 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 Ive found doesnt make much sense to me...
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Old 10th August 2003, 08:41 AM   #30
DSummer is online now DSummer  United States
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Quote:
Originally posted by Brett


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....
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