Musical instrument amp scenario reduce tube count

[jukingeo]

Hello all,

I am considering the Poseidon board for a musical instrument application primarily for an organ or even a bass guitar. Now I have noticed that the schematic shows a 12AX7 and a 12AU7. I am wondering if the first 12AX7 stage could be done away with and use a 12AX7 in place of the 12AU7 for the VA/splitter stage.

Considering that the amp is mono I would have to create another tube socket just for the use of half a 12AX7. In a hi-fi stereo amp I would leave the design as is, but here I would be wasting 1/2 a tube and would have to drill another hole for a tube socket. Now, I am wondering if these minor changes could be made and if it would still work properly as a driver for 6L6, KT66, or 6BG6G tubes. My concern is if I have to change anything in the LM334 area to compensate for the use of a different gain tube in that location.

Thanx,

Geo

[Ty_Bower]

Something like the schematic shown here?

http://www.audiokarma.org/forums/showpo ... stcount=16

[mesherm]

The 12AX7 plate current is too low to sub for the 12AU7.
Better idea, sub in a 12AT7 for the 12AU7, eliminate the 12AX7 stage entirely and input into the phase splitter stage directly. The circuit is so simple it should be easy to wire point to point.

[TerrySmith]

If you didn't want to waste 1/2 of a 12AX7 then use a 6AV6 in that spot. An all octal version could be a 6SF5 and a 6SN7!

[mesherm]

You can do it with a single tube. A compactron triple triode 6U10 will duplicate the 12AX7/12AU7 circuit close enough that you proly don't even need any component tweaking.

[jukingeo]

Something like the schematic shown here?

http://www.audiokarma.org/forums/showpo ... stcount=16

Yep! Exactly that. Thanx!

Edit: I just noticed on the schematic that the plate resistors for the 12AT7's are 16k...Am I seeing straight or is that a mis-print? Usually this resistor is around 100k.

The 12AX7 plate current is too low to sub for the 12AU7.
Better idea, sub in a 12AT7 for the 12AU7, eliminate the 12AX7 stage entirely and input into the phase splitter stage directly. The circuit is so simple it should be easy to wire point to point.

As in the circuit above, right? I probably will use that one then. However I did notice it is using an LM317 rather than a LM334. I wonder what are the differences.

You can do it with a single tube. A compactron triple triode 6U10 will duplicate the 12AX7/12AU7 circuit close enough that you proly don't even need any component tweaking.

I was thinking about that, but the 6U10 is an obsolete tube. I have seen it before in some Ampeg guitar amps and other hi-fi amps. The good thing is that AES lists the tube for under $6, so by right there wouldn't be an excuse not to stock up on it.

I have read up that many DIY'ers don't like the compactron tubes because small pins on these tubes do sometimes cause intermittent connection problems. I have witnessed this myself, BUT then again, I was servicing 50 year old amps. A new tube with a new socket probably will not have that problem.

While I don't think I would go this route for a 'permanent' rig, I surely might do so on a test board rig mainly because then I can go with the one tube setup, so thanx for the tip off.

Geo

[mesherm]

If the phase splitter plate resistors are too high in value then the CCS tail can't operate properly. I have used values of 22K in that circuit with a B+ of 250 volts so 16K doesn't seem totally unreasonable.

An LM317 would like to see more current that 4-9 ma so I personally wouldn't use it. I have used 3 terminal regulators as CCS in place of cathode bias resistors before in power stages with excellent results although I use TL783s because of their higher voltage (125v) rating.
You could simplify the circuit even further and combine the phase splitter and output section using a TL783 as a CCS tail for a matched pair of 6L6s. Then use a 12AX7 in SRPP as a VA.


The compactron pins are the same size as in 9 pin tubes. Its the Lockal tubes that have very small short pins although I have used 7F7s and 7N7s with no problems. The problem I DO have with compactrons is getting them in and out of the socket. One has to be very careful wigging one out. They are tight.

[jukingeo]

If the phase splitter plate resistors are too high in value then the CCS tail can't operate properly. I have used values of 22K in that circuit with a B+ of 250 volts so 16K doesn't seem totally unreasonable.

I had a funny feeling the constant current source had something to do with it, but I have seen CCS circuits before that pretty much used standard value plate resistors. So I was wondering why the change in this circuit.

An LM317 would like to see more current that 4-9 ma so I personally wouldn't use it. I have used 3 terminal regulators as CCS in place of cathode bias resistors before in power stages with excellent results although I use TL783s because of their higher voltage (125v) rating.
You could simplify the circuit even further and combine the phase splitter and output section using a TL783 as a CCS tail for a matched pair of 6L6s. Then use a 12AX7 in SRPP as a VA.

Yes, that is what I originally mentioned when I first started the post, but then the 6U10 came up and at first I thought it would be a far fetched idea as I thought the tube was expensive, BUT the tube is cheap! It is cheaper than a single 12AX7 or 12AU7. So it did get my gears turning to perhaps going with a poseidon 6U10 for a test bench set up.

The compactron pins are the same size as in 9 pin tubes. Its the Lockal tubes that have very small short pins although I have used 7F7s and 7N7s with no problems.

Yes, I read that the Loctals had the problem too. I had trouble finding sockets for them at one time and I sold a bunch of Loctal tubes off many moons ago. I had quite a few 7F7's and 7N7's.

At any rate, on old amps I DO have trouble with 9 pin sockets as well. But I am not concerned with 9 pin tubes as there are 9 pin sockets currently being produced.

The problem I DO have with compactrons is getting them in and out of the socket. One has to be very careful wigging one out. They are tight.

Well, I was worried about the availability of new compactron sockets. (I don't think I would trust NOS sockets). BUT as it turns out, they are making new sockets in China now and that could be enough to sway me to use the 6U10. Again, I might try it out for a bench project. Can't hurt right?

Thanx, Geo