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john 807 7th April 2005 05:00 PM

811-A amp
Hi All
I need some help here in finding a suitable circuit for a 811-A But there is one catch can you try and find me one that does not use a interstage transformer I am at the end of my budget and need something like a cathode drive circuit any help
is very valued


Jhon807 :confused: :)

astouffer 7th April 2005 05:25 PM

2 Attachment(s)
This is about as simple as you can get. Its tough to get a hold of 6GB8s here in the US so I used a 6V6. Any pentode will work really, just watch how its biased because that directly controls the 811. I can post more schematics later if you are interested.


Fuling 7th April 2005 05:47 PM

I made a pair of monoblocks quite similar to the amp on the schematic a while ago.
I used different input and driver tubes, but the basic topology was identical.
Not bad at all actually!
You might want some negative feedback to tame the high output impedance though, since the 811A acts more like a pentode than a triode.

BTW, I dont know if 10000uF is enough for the filament supply.
I used an LCLC filter with some 100 000uF and 1,1mH chokes. Very quiet.

john 807 7th April 2005 06:06 PM

You are wonderfull
I have been looking for a circuit this good for days!!
thank you ever so much!!

astouffer 7th April 2005 10:03 PM

2 Attachment(s)
Fuling is right about the filament supply. The 811 is very sensitive to hum. I cheated and modified two $4 switching supplies to output 6.3v. It was cheaper than worrying about high current chokes and large filter caps.

astouffer 7th April 2005 10:06 PM

2 Attachment(s)
And here is the last one without using a transformer.

Fuling 7th April 2005 11:42 PM

I thought about switchmode PSUs too, but I didnīt find any suitable items back when I was working on that thing.
OTOH I was lucky enough to fins good iron cored chokes and cheap 27000uF 16 V lytics in a surplus store.

BTW, I used EL86 as cathode follower and E81L as input stage

john 807 7th April 2005 11:45 PM

811-a amp
There are loads of different idears there thank you for going to
all this troble it has made my life easyer it means I do not have to experement with different idears

The hum with the 811 is as expected with having the heater as the cathode I have got som massive filter caps here and a choke
that will quite it down a bit might have this built soon as i get the outputs that i ordered but still i need a UX4 base Should be able to get one from bluebell audio not really a problem

The driver valve, well there a few idears there My output-tran
is reated at 20w at about 5k so neg feedback would need to be used and there are a few different driver valves I had a look at those 6GB8 they are around $22.00 over there about Ģ11.00 here But i will use a 6v6 like you did seen as I have a Few Did you have to change any resistors ? Or was it just a case of changing the base connections.

Fuling 8th April 2005 12:08 AM

I ran my 811īs at about 430V anode-cathode voltage and 100mA anode current. My OPT was 5,6k if I remember correctly.
You can get local negative feedback around the output stage by connecting the OPT secondary between the cathode of the 811 and ground, much more elegant than global feedback in my opinion.

Remember that 811A draws quite a bit of current on the grid at the positive peaks, so choose a driver tube that can deliver some.
High Gm is also a good thing for tubes that is to be used as cathode followers, but beware of oscillations. (Use gridstoppers).

The resistor values in my amps were all different from the ones in the schematic here, since I designed my amps myself with completely different tubes.
Somewhere, i *might* have a schematic.

astouffer 8th April 2005 03:31 AM

I kept all the resistor values the same but my plate supply was a different voltage. Not sure what it was (my parents use the amp at their house) maybe 280. 811 plate voltage is around 700 and the grid is around +14. The output transformers are 6.5k Hammonds.

Now that the amp is built I would like to add some feedback to improve the sound a bit. Anyone know the best way to go about it? Add a 100 ohm resistor to the 1k coming off the bottom cathode and make that the feedback point? Calculating this is beyond my skill.


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