• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

811-A amp

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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

thanks

Jhon807 :confused: :)
 
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.


Adam
 

Attachments

  • 811a_single_12ay7.jpg
    811a_single_12ay7.jpg
    91.6 KB · Views: 4,216
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.
 
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.
 

Attachments

  • 811a_single_12ax7 !!.jpg
    811a_single_12ax7 !!.jpg
    68.4 KB · Views: 3,336
I thought about switchmode PSUs too, but I didn´t 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
 
811-a amp

Thanks
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 £11.00 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.
 
I ran my 811´s 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.
 
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.


Adam
 
811

I have a book here that has all the valve type calculations I will
find the feedback calculation for you

I had to do this on my 807 push pull amp thats a beast of a thing !! the feedback was set for a 600 ohm out put inpedence
where as the out put tran I was using is at 8ohms so I had to change the feedback resistor I think it ended at about 2k

You are right it is better to calculate it rather then use the feed back of another circuit then you know that it is right.

It is not hard math really I will have a look

:)

John807
 
811 feedback

Done it worked it out for you !!

you need a 1k8 resistor coming from your out put transformer

then you take the ground of your pre amp valve's cathode bias cap and resistor not the driver because it is copuled to the 811
solder the 1k8 to it and a 100ohem resistor to ground dead simple!!!
 
astouffer said:
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.


Adam

Ack! +14 on the grid?

Seems a bit high... how much current at idle are you drawing?
It's a zero bias class B tube, iirc... are you trying to run in Class A??

Anyway, check out the Altec 1570b schematic. Removing the feedback network's cap and a few other minor mods, like a better output transformer and it is a very nice amp design...

_-_-bear :Pawprint:

Oh... wait a sec, single ended 811? Hmmm... I'd expect that the B+ needs to be dropped some to keep the quiescent current down?

The two schematics show <500v and ~300v respectively...
interesting idea to run the cathode of the driver to the grid of the 811 with no ground reference at all...
 
Just took some measurements last night. The 811 grid was +18 and exceeding the PD. I added a 150 ohm 5w resistor from the wiper on the hum pot to ground and this dropped the PD down to 40 watts. I must have forgotten that 65 watts PD is intermittent only, 45 is continuous.

The grid must have positive bias otherwise there would be no current draw. Its the reverse of most other tubes. There is also a resistor going bad. It causes some static after a while and seems to stop the second I reach for the meter. I hope it dies a firey death ;)


Adam
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.