#26 pre amp

[jlevro]

Originally Posted by kevinkr

...other candidates would be the Hashimoto 20K part.

I suggest their 10K version, the HL-10K-6.

One thing about all of the Hashies that I've tried (and measured) is that they like to be driven from a source impedance very close to their rated impedance. In this case, the 7-8K from the 26 should match pretty well with the 10K primary. If you were to use the HL-20k-6, you'd likely experience a good deal of treble peaking.

But, as always, YMMV.

JL

[markusA]

Has anyone tried one of these in a PP variant?

[markusA]

Does anyone know where you can find a working spice model for the 26?

[iko]

Here you go.

* type26cun LTSpice model
.subckt type26cun P G K
CGK G K 3.5p
CPK P K 2.2p
CGP G P 8.1p
* koren 4
Bp P K I=(0.0169871083m)*uramp(V(P,K)*ln(1.0+exp((12.3990 4937)+(12.39904937)*(8.189852358)*V(G,K)/V(P,K)))/(12.39904937))**(1.40553086)
* koren 8
* Bp P K I=(0.01505035691m)*uramp(V(P,K)*ln(1.0+(0.04617378 061)+exp((13.71786498)+(13.71786498)*((8.563661196 )+(9.663362929m)*V(G,K))*V(G,K)/sqrt((-1.856325497e-005)**2+(V(P,K)-(-5.468070728))**2)))/(13.71786498))**(1.432783777)
.ends type26cun

[markusA]

Thanks, I'll try to make it work asap.

[mogliaa]

Hello,
Today I finished the 26 pre-amp, albeit still need to build the power supplies.
I did a listening test and was very surprised with the results. Great sound, very warm and natural and gave me the additional slight gain I needed.

I tried a pair of Sylvania 26, which could notice a bit of hum induced when I put my hand around the tube! Then switched to a pair of CX-326. Much better mid and treble.

Noticed that when HT was off had a distinct hum noise which is gone when I apply the HT. Any ideas why this could be?

I have the output floating to the amp i.e. the LL1660 secondary is not grounded...

Thanks
Ale

[pietjers]

Well, that looks great! And the (expandable) pictures are great also. Thanks for sharing.

Hum after switching off your HT is a strange phenomenon. Is this happening when filament supply is still on? Looks like the only source then.
Are there two filament resistors in series and then two parallel to get 10 Ohm? Looks like a little overkill to use an 80W (in total) resistor to dissipate only 10W.

Peter

[mogliaa]

Hi Peter,
Thanks, it's coming along after all the effort.
I'm suspecting is my HT bench supply as I turned down an output voltage setting potentiometer and not switching it off as I said on my previous post, so will do some tests to find out root cause. I did left the heater supply on by the way. It looks like is the HT supply as I can't explain otherwise why the hum is gone when both supplies are active...

Now need to crack on the power supply design. I will use an AZ1 with a pair of fast diodes for the bridge, an LCLC stage with 20H chokes and 47uF oil caps and a Salas HT shunt regulator for the HT supply...

[iko]

I'll be curious how you'll fare with the shunt regulator and output transformer. My #26 preamp prototype has an ugly 15mV RMS ripple with the shunt regulator and the Hammond 156C choke as load. I know for sure that my regulator has very low noise because when I replace the choke with a 4K resistor or a DN2540 CCS the output is very clean. The only way I can explain it is that there is some sort of weird interaction between the choke load and the shunt regulator. I will replace the shunt regulator with a VR150 tube regulator and see how that fares.

[kevinkr]

Originally Posted by jlevro

I suggest their 10K version, the HL-10K-6.

One thing about all of the Hashies that I've tried (and measured) is that they like to be driven from a source impedance very close to their rated impedance. In this case, the 7-8K from the 26 should match pretty well with the 10K primary. If you were to use the HL-20k-6, you'd likely experience a good deal of treble peaking.

But, as always, YMMV.

JL

Interesting, haven't tried the 20K transformer - something to note for the future. The additional benefit of the 10K is a couple of extra dB of gain where that is required.. Do you run these loaded into 600 ohms or into a higher impedance?