The Kimmel Mu Stage

If they glow, this is the place to be
Post Reply
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8874
Joined: Mon May 21, 2007 9:42 pm

#1 The Kimmel Mu Stage

Post by Paul Barker »

Source Page

Because of the trouble I had with distortion of the VA driver valve in my present build, I almost stuck a depletion mosfet in it, but managed to resist the temptation. I decided I would use a pentode, but not use a solid state voltage reference, no solid state at ll was my ruling. So I opened up a very old page in the history of diy tube audio. Just at the time the SRPP was massively popular with the Loftin and White and the Ongaku circuits being poured over by all and sundry, Alan Kimmel put this page up. It goes back a long way to the time when Sound Practices was around. I am not sure that it got into SP, I don't recall so.

I always found Alan Kimmel's page a little hard to follow. So I am hoping that I have picked out the pertinent facts and emphesis and presented it in broken down form for others to use it as a tool with confidence.

The main thrust is to provide a low current high mu VA (V1 the bottom triode) it's perfect load, then provide that information to the driven valve with twice the current the triode can manage.

I have changed the pentode screen grid conditions to VR tube. Probably makes no difference in this application but I have the VR tubes so might aswell use them.

I hope the picture tells the story. I'll probably find a load of things I forgot to emphasise later and keep going back to alter it. That's all part of the wanderful world of the dyslexic wired brain. First you just bombard the paper with a load of words, and eventually you refine it.

Image
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8874
Joined: Mon May 21, 2007 9:42 pm

#2

Post by Paul Barker »

First mistake. I have new glasses but still can't see well. RD in Alen,s page I misread it says RP and that makes a lot more sense.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8874
Joined: Mon May 21, 2007 9:42 pm

#3

Post by Paul Barker »

The challenge will be the pair of resistors which take the current of the pentode that is not fed through the triode (remember current comes from ground and flows towards B+ so the triodes current flows to the pentode cathode and the rest of the cathode current plus the VR tube current has to come from this pair of resistors. The pentode's cathode current also has to send out the electrons which go through the screen grid, which the VR tube set resistors will also get rushing through it.

Alen has called these RK1 and RK2. They together must be exactly right to provide the current mentioned above, then have to voltage devide for the pentode's bias. It would be lovely to provide a pot here, but it would have to be high value resistance and high wattage. Not an easy monster to find. Probably we could use two fixed resistors of about the right values and place a low value pot in between them to fine tune.

Say your triode is a 6sl7 it may operate at 2 mA, so you need 2mA for the pentode plus 20% for the screen 2.4mA from these resistors then 10mA for the VR tube. Our voltage would be at least 150v say it is 150v?

150/.0124 = 12k Wattage 1.86 watts treble that for reliability especially if pot made in China I have bought some horrendous pots from China, which look like Allen Bradley's but have the quality feel and longevity of a paper aeroplane. So, 5 watts.

I don't know the bias of the pentode yet, say it might be -10v.

So you could stick a 500 ohm fixed resistor at the top, an 11k resistor at the bottom and a 500ohm pot between the two.

Top resistor sees 0.2 watts pot sees 0.2 watts and bottom resistor sees the bulk of the power, but now we are just shopping for a fixed resistor.

I would expect the story does not stop here. To start the project off you could devide the bottom resistor and pick off a level. This is because we are initially going to work from valve data, which is never accurate in these situations, so we must always start these projects with adjust on test (AOT) capability.

The VR tube load could be picked quite accurately and left alone as if we aim for 10mA we are far enough beyond the minimum 5mA, won't matter whether it is 7 or 12. The adjustment can be made on these cathode resistors.

Once values are established fixed resistors of right value and rating could be put into place for reliability.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
IslandPink
Amstrad Tower of Power
Posts: 10041
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#4

Post by IslandPink »

I'm no expert on the set-up details, but Mike Carrick ( in Essex, ex-Ford ) uses Mu-stages all the time, really rates them - you can't get him to use anything else for driver stages . He really likes the E83F in the top position, for tone & transparency - limited to about 10-12ma though .

If you like I can put you in e-mail contact with him if there are any bits you are bogged-down on ?
"Once you find out ... the Circumstances ; then you can go out"
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#5

Post by pre65 »

IslandPink wrote:Mike Carrick ( in Essex, ex-Ford )
We need more Essex men to balance out the nothern element. :wink:
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8874
Joined: Mon May 21, 2007 9:42 pm

#6

Post by Paul Barker »

Thank you Mark

I'll see if I can obtain E83F.

You can point him to here but if he wants to remain private do you still have my email address?

p.s. the filament hum cancellation is pretty ok even with just the 2k pot. Bigger things to worry about.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8874
Joined: Mon May 21, 2007 9:42 pm

#7

Post by Paul Barker »

Regarding hum. LEt's put it this way, it isn't that elephant in the room which everyone says to each other after they get out of your presence "Did you hear that hum?".
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Post Reply