#26 pre amp

[bequerel]

mach1,
Thank you for a very informative reply!
The subject of glow tubes comes up from time to time on this forum, and frankly very few seem to know how they work.

[mach1]

Correction: The use of the term transconductance was totally inappropriate in my previous posting (serious caffeine deficiency). It should be replaced by the term shunt efficiency, where shunt efficiency = dI/dV , or change in current / change in voltage

[Salas]

Disco's gonna test a 1.5 Vout Mosfet CCS & BJT shunt reg set at 1.2A for #26. 1A available to the heater, 200mA to burn all time at the shunt transistor. I arranged it so it can utilize a 6V3 secondary tap for rectification, so it can work from 6.3-9V DC input. It has a trimmer too. It shows like working on the simulator, I just hope its gonna work properly in real life too, the output voltage you need here for heating up the 26 its just too low and it was a challenge to set it right and find the right transistor even in simulation. If we get good working order and good tone results will let you know. Don't know when he will do it.

[CFT]

Thanks Salas! Please update us when Disco has it done

[Richard]

Hi Sheldon & Rod,

Your circuit diagram is wrong. You show a capacitor in series with the supply output. This will block DC. A feedthrough capacitor would be represented by a small cap from the output to the chassis, at the point where the supply passes through the chassis. A feed through capacitor is a wire that is surrounded by a layer (one or more) of dielectric that shunts RF from the wire to the chassis. DC current passes through the wire from one side of the chassis to the other.

FEeedthough caps are very effective at frequencies that leaded parts can't do much with |(>10MHz), but without a metal enclosure the HF/VHF/UHF may couple electromagnetically from the source of noise (eg rectifier recovery pulses) to the low level audio circuits.

Running the power from the trafo/rec/caps to the preamp via shielded cable can help, when a metal enclosure is hard to do.

Sorry about the delay in reply. Thank you for the corrections and suggestions. I did some hunting in the shed and managed to find a metal enclosure to put the 2 transformers and 2 caps. At the moment they are all laid out on wood (Fig 1).

I’m still waiting for parts to build Rod’s filament psu, so went ahead and built a psu with 7805 regulator and 1086 ccs, sort of like Kevin’s suggestion on his website.

[Richard]

I now have a 3 chassis 26 preamp.

This weekend I finally managed to finish the pre and test it out. One can note the Hammond chokes on the pre under construction (Fig 2). I ended up putting the chokes in an Alu box to shield them, also put some Arcturus tubes in a finished version to make it look sexy (Fig 3).

Voltages were all good, no smoke but I do have this bloody low frequency hum and some hissing noise. I officially feel like I’m part of the 26 problems club. A view of the wiring is also attached (Fig 4).

The B+ psu chassis and filament psu chassis are about a meter away from the actual preamp
I plan on trying the following:
1) Shielding and grounding the umbilical cables a) joining mains 240V from main psu chassis to filament psu and b) from main B+ psu chassis to preamp chassis (Rod's suggestions)
2) shielding the filament wiring in the preamp chassis and grounding it
3) increasing the cathode bypass cap
4) ground the alu box with chokes
5) shield the tubes and ground that (Kevin's suggestions)

Any other suggestions would be most welcome.

Cheers,

Rich

[iko]

Originally Posted by Salas

Disco's gonna test a 1.5 Vout Mosfet CCS & BJT shunt reg set at 1.2A for #26. 1A available to the heater, 200mA to burn all time at the shunt transistor. I arranged it so it can utilize a 6V3 secondary tap for rectification, so it can work from 6.3-9V DC input. It has a trimmer too. It shows like working on the simulator, I just hope its gonna work properly in real life too, the output voltage you need here for heating up the 26 its just too low and it was a challenge to set it right and find the right transistor even in simulation. If we get good working order and good tone results will let you know. Don't know when he will do it.

Hey salas, good idea. Something to consider, can the bjt take the full 1.2A in case the filament dies?

[Richard]

A schematic of the preamp and filament supply is attached below. For those interested there is a description of the power supply in post 90.

Rich out

[Salas]

Originally Posted by ikoflexer

Hey salas, good idea. Something to consider, can the bjt take the full 1.2A in case the filament dies?

Yes, even 8A. Jaap he is on it, first clue is that it achieves 1.5V low indeed. He now checks the CCS part, still early breadboarding.

[kevinkr]

Originally Posted by Richard

A schematic of the preamp and filament supply is attached below. For those interested there is a description of the power supply in post 90.

Rich out

My one recommendation (unsolicited I'll admit) would be to change out that 7805 for a 7806 (Rohm amongst others) because the LT1086/5/4 etc perform significantly better with >3V across the device itself. These are low drop-out devices but that does not mean their performance does not degrade in subtle ways as you approach the drop out voltage, here the headroom is only 2.3V which is not necessarily enough to assure that all internal circuitry is functioning close to specification. Increasing the voltage across the LT1086 to 3.3V or so puts the device close to the point of diminishing returns as a CCS and is probably good enough - hence the recommendation for a 7806. (Alternately you could stick two 1N4148 in series with the 7805 gnd pin making it in effect a 6.2V regulator which perhaps would be slightly better from a theoretical standpoint. )