Single Ended Pentode Spud and Feedback

[ErikdeBest]

Dsavitsk

I think your feedback mode will work. I knew I had seen it before, employed by tubelab.com, and just to make sure I went to his website to the schematic.

http://www.tubelab.com/SimpleSE_schematic.htm

 

[dsavitsk]

Why not triodestrapping w/o NFB instead? You´ll still get 1W instead of pentode 2,5W with Ia=40mA.

That's how it is right now, but I've done that and thought I'd experiment with something else. Indeed, after building a lot of goofy amps lately, I did a straight cLCRC supply, transformer load, resistor/cap on the cathode just to remember what it sounds like -- sounds pretty good, actually. But, now it is time to do something different.

Dsavitsk

I think your feedback mode will work. I knew I had seen it before, employed by tubelab.com, and just to make sure I went to his website to the schematic.

http://www.tubelab.com/SimpleSE_schematic.htm

Look at that. :0

 

[revintage]

That's how it is right now, but I've done that and thought I'd experiment with something else. Indeed, after building a lot of goofy amps lately, I did a straight cLCRC supply, transformer load, resistor/cap on the cathode just to remember what it sounds like -- sounds pretty good, actually. But, now it is time to do something different.

Aha, nice to hear, not to hard to go back for comparision then. But I think you´ll get better sonic results taking feedback from anode to g1 despite the lowered Zin. Think feedback over transformers should be avoided as long as possible. Looking at the curves I would go for something like 200V, 150V, 40mA with 5k load.

 

[JLH]

I have had Edcor wind me some of their XSE transformers with a cathode feedback winding. It was basically their XSE15-8-3.5K model with an extra winding added. Its gapped for 50mA and measured out at 13Hy primary inductance. The tube I used is the 12GN7. Crazy good performance and the transformers only cost me $30 a piece. I would highly recommend you have Edcor make you new transformers.

Rgs, JLH

 

[revintage]

The tube I used is the 12GN7

Hey JLH,

See you use the same tube all over the signalchain. Wasn´t it you who used it at a RIAA LCR input?

 

[JLH]

Hey JLH,

See you use the same tube all over the signalchain. Wasn´t it you who used it at a RIAA LCR input?

Yes, I like high Gm pentodes. The one thing I do avoid is running them back-to-back. For example, I use the 12GN7 as the input tube for the LCR phono pre, then into the LCR and out to a 6HB6. I really don't like the sameness of sound you get by running the same tubes back-to-back in the signal chain.

I built a series of small power amps using cheap high Gm video pentodes. The goal was to see if I liked the sound of high Gm pentodes with CFB. I found they can sound good. However, you better like clinically accurate audiophile sound. I would say most of them sound like a very clean pure Class A solid state amp, but with the focus of tube SE. The biggest advantage to using high Gm tubes with CFB is the extremely low output impedance you can get. I built a small amp using the 6AH9 tube. I measured the output impedance to be 1.07 ohms. That's really good for a single ended amplifier.

 

[dsavitsk]

It works! I tried connecting Ck to the transformer secondary for 100% feedback, and while there was sound that technically was probably low distortion sound, it sounded terrible. Just flat and blah. With Ck removed, there was no bass. But, with Ck in place and no other feedback, it actually sounds quite good. A little bright perhaps, but not harsh at all. Really, very pleasant to listen to, and tight powerful bass. Maybe these phones don't need a big damping factor after all ... ?

(power supply is in another box)

 

[TheGimp]

Did you retain the VR tubes for the screens? I've seen motorboating if you use much (2uF) capacitance on the output.

Looks nice!

Steven

 

[dsavitsk]

Here's the schematic. You can't use more than 0.1u with the VR tubes or else they oscillate.

 

[ErikdeBest]

Regarding transconductance amplifiers... Some time ago I bought a 2nd hand couple of nice 10k:150R 50mA SE output transformers. My intention was (is) to compare the sound (and distortion) of different tubes under different operation points, using a sennheiser HD590. While I listened to it I put together a sort of Firstwatt F2 amp, using IRF820's for amplifiers duties, and 10M45S for CCS, fed from 24V. I hooked the HD590's to this amp and haven't gone back to the tubes since then... (I rewired the 6Y6's to operate as pentodes, but could not get good results, yet...)

 

[disco]

could not get good results, yet...)

What's the main trouble you're having, lacking bass?

 

[ErikdeBest]

Hi Jaap

thanks for the link! My transformers are from AE as well (amorphous core, potted...) and they perform well, bass is not lacking with either pentode or triode. So I was happy with the transformers, but the F2 clone brought more details in the music...

But surely I have not given up, and will keep experimenting and switching between both.

Erik

 

[dsavitsk]

Regarding transconductance amplifiers...

Does anyone have a primer on the how's and the why's here? Rp of this tube is 60K, which with a 5K:32 stepdown seems to me to be a Z out of ~400 ohms. So, if one were purposely designing such a thing and not accidentally backing into it, what would one look for in terms of specs? Is just Gm important, or plate dissipation, or rp? Does the stepdown ratio matter?