811a SET amp.

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pre65
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#286 Re: 811a SET amp.

Post by pre65 »

Cressy Snr wrote: Tue Nov 21, 2017 6:34 pm

Sorry Phil.
Nothing to be sorry about Steve.

I asked a question and you kindly gave me the answer I needed.

No offence intended to Nick, but I sometimes find his written replies enigmatic. :lol:
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#287 Re: 811a SET amp.

Post by Cressy Snr »

Glad to have been of help Phil.

However in future, the experts can answer questions of that nature.

I'll stick to the music threads and assorted stuff. Can't get into much trouble there.
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Dave the bass
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#288 Re: 811a SET amp.

Post by Dave the bass »

Cressy Snr wrote: Tue Nov 21, 2017 10:26 pm I'll stick to the music threads and assorted stuff. Can't get into much trouble there.
Don't you believe it! I know what you n Phil listen too! Loadsa ammo thrre! 😊
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pre65
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#289 Re: 811a SET amp.

Post by pre65 »

Today I have modified the baseplate to take B5 valve bases.

Only a small step, but a step none the less. :)
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#290 Re: 811a SET amp.

Post by pre65 »

I've now got a suitable transformer to build the grid bias circuit. :)

One thing I would like to know, what sort of current would a 24v grid bias circuit need to supply ?

I've been thinking about how to work out the figure, but my brain hurts. :oops:
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pre65
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#291 Re: 811a SET amp.

Post by pre65 »

I've found out that for an 811a the max grid current is 50ma, so that will help me with my calculations. :D
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pre65
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#292 Re: 811a SET amp.

Post by pre65 »

2 years have passed since the last post. :shock:

I'm getting interested in this project again, just got to find all the bits that I was playing with then. :lol:

I'll try and sort out the +/- 80v supply for the mosfet, and the +24v bias supply first.
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Paul Barker
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#293 Re: 811a SET amp.

Post by Paul Barker »

pre65 wrote: Sun Apr 26, 2020 7:43 pm +24v bias supply first.
You are direct coupling the source follower to the grid presumably so you level that at the gate using you’re -80v rail and earth voltage devided to shift the gate, the source follows with a step change so measure resulting level at source when adjusting output stage bias with youre pot which sets the gate level. You don’t require output tube to set this section dc conditions such that no danger. You can do this because you’re making dedicated power supply for the Nmosfet. Yes
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#294 Re: 811a SET amp.

Post by Paul Barker »

Paul Barker wrote: Mon Apr 27, 2020 6:43 pm
pre65 wrote: Sun Apr 26, 2020 7:43 pm +24v bias supply first.
You are direct coupling the source follower to the grid presumably so you level that at the gate using you’re -80v rail and earth voltage devided to shift the gate, the source follows with a step change so measure resulting level at source when adjusting output stage bias with youre pot which sets the gate level. You don’t require output tube to set this section dc conditions such that no danger. You can do this because you’re making dedicated power supply for the Nmosfet. Yes
Just looked at you’re diagram and see you’re bias arrangement would work. So maybe best you stay on you’re plan, which you’ll no doubt build in bias adjustment.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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