#26 pre amp

[Rod Coleman]

Originally Posted by zigzagflux

Boy, after struggling with this same issue (and eventually resorting to standard LT1086 CCS heating), I really have my doubts that any BJT on the planet can accomplish this.

I recall my power electronics pseudo-professor (he was actually a design engineer in the real world) saying he always designs his BJT circuits assuming a beta of 10. Period, no exceptions. After much frustration, I am starting to agree with this formula. Especially when low capacitance is an important performance parameter.

That would be one heckuva transistor to accomplish this at even poor capacitance performance.

Forced beta of 10 is right for any circuit requiring saturation, or any circuit using old design BJTs. I had not appreciated how far behind the MJE15032 was compared to the Japanese, like the ROHM 2SC5511 I made my original heater with.

Since ROHM have withdrawn that one, we must turn to TOSHIBA. They have NPNs and PNPs with hugely improved performance, backed up by a specification that covers our area of interest. For PNP, just look at the 2SA1887 specs: over 5A for 40mA base current at 2V, specified.

In the circuit example, I show more like 80mA base current.

[Rod Coleman]

Originally Posted by NG

Hi Rod:

Thanks aaaahh lot ! I hope very soon I can finally start listening to some music after 2 years of building my PPP amp. The 26 preamp later on should be the "crown" !

Forgive me to ask this silly question, but I don't understand why there is only 1 drum cored choke L1 ? What should be the spec of the choke ?

Once again, thanks for your help.

Regards,
NG

NG, how many cathode resistors do you have? If there is only one shared, then one choke must be used. If PUSH and PULL have their own resistor,use 2x 6A chokes.

For 10A, Try the EPCOS B82725J2103N022 double toroidal ferrite choke. Wire the + and the -ve through it. This does a great job of sucking out HF noise.

EPCOS|B82725J2103N022|CHOKE, RING CORE, DOUBLE, 1.8MH | Farnell United Kingdom

Both chokes together drop 0,26V at 10A, so be sure you still have 5,0 .. 5,5V remaining at the CCS under full load.

[Rod Coleman]

Originally Posted by regal

Rod , do you know if your Gyrator+CCS has been tried with high sensitivity headphones (SPL lmV =100dB), I know the newer low current Tent labs module has been tried and had no hum. These #26 Preamp discoveries naturally lead to a headphone implementation.

I had lot of questions about the circuit, mostly for preamp or 300B/2A3 duty. Nobody mentioned headphones. But having heard good reports in this thread about applications using the 26, as well as 300B power amps, I would expect it to work very well.

Where hum is the most important measure, be sure to have enough Vce across each transistor, to keep the Hfe high and capacitance low. 2 to 5V is the working range dependant on your heatsink size and transistor choice.

[NG]

Hi Rod:

I really appreciate your willingness to share your knowledge. It's time to get the parts and work on in. I will post my findings and eventually a picture later on when it is finished.

Thanks again.

Regards,
NG

[NG]

Hi Rod :

One more question : heater 26 preamp gyrator/CCS application :
For the FET gyrator/CCS, since I will be using cathode bias, should I place the cathode resistor in between "filament supply 0V" and HT common ground ?

Thanks !

Regards,
NG

[Rod Coleman]

NG, Connect the cathode resistor (& cap if used) to the negative filament terminal on the valve base. Connect the other end of the resistor to HT 0V (Ground).

When you buy parts for the 2A3 build, get some 1 ohm 15..25W resistors to test it. build and adjust the current sense resistor till the 2A3 filament has 2.4..2.5V across it (no more!). Then, leave it for some hours on test, keep checking the voltage, and make sure the transistors don't get too hot. That way, you can be sure your heatsink is big enough.

Burning out your filaments because of a mistake in the circuit would be no fun!

[NG]

Hi Rod :

Something I still don't understand about the circuit for the 2A3 PPPdht : how I understand all the other circuits with gyrator, this 2A3 PPPdht circuit uses a 10A choke, right ? Shouldn't it be like this ?


Regards,
NG

[Rod Coleman]

NG, you're right, there's a mistake in it. This is how it should look!

[NG]

Hi Rod:

Thanks again ! Can't wait to hear how it sounds.

Regards,
NG

[xecluded]

Originally Posted by coolzero

Finally I have completed the pre-amp. Attached the schematic (sort of Kevin Kennedy, Jim D'Cort mixture). Thankful to Kevin for his great advices to come up with this final circuit which work nicely and worth all the efforts. Used a PSU with Hammond 372 X (which I had already otherwise an overkill I suppose) +5U4GB+10K(5W)+OA2/OB3 tube regulators+IXCY CCS (kit module K&K Audio -http://www.kandkaudio.com/accessories.html) to produce 250V hum free B+ which I built out of randomly picked up items I had already. Otherwise, any simple PS schematic would work I assume. For me, use of regulated B+ helped towards reducing the hum drastically. Hum with unregulated B+ was unacceptable to me (even though it sound great). For filament, I used a cheap China made external DVD SMPS which worked greatly with no hum at all.

Attached a picture of the final product (pardon for my poor craftsmanship).

Coolzero,

I don't have technical skill to build the 26 preamp that you've mentioned here but i am thinking about commision it to someone who knows how. What do i need to provide him other than the schematics that you've listed here (is this the final version by the way.) Do you have a parts list required to build this preamp. Also can you give some tips and tricks on how to eliminate the hum that i've heard so much associated with the 26 preamp. I really appreciate any help that you can provide. Thanks.

xecluded