"The Tsar" GM70 single ended amplifier

[pieter t]

how % is a CFB?

4 %

 

[Profi]

Thank you. I'll count the whole transformer

 

[211audio]

The Tango OPT's also had CFB windings. They called it a KNFB coil.

f.e: TANGO 10887, 50W, 3.5K ohm load impedance with 36 ohm of KNFB coil resulting approx. 4.2K ohm.

 

[Profi]

It is easier for me to wind myself

 

[FullRangeMan]

These NFB OPT windings have what effect in sound quality?

 

[VladimirK]

I think the need in CFB winding depends on speakers one wants to drive. Those speakers with dips in the impedance curve will definitely benefit from CFB winding. At the same time, mids and highs with CFB could differ, one can not be 100% sure that they do not suffer.

 

[FullRangeMan]

Wow these are good news so.
Thanks, much appreciated.

 

[Profi]

These NFB OPT windings have what effect in sound quality?

with triodes have not yet tried, but with pentodes the results are excellent. CFB 14-16% allows you to completely abandon the general feedback. The sound is much better.

 

[211audio]

For me the primary goal of CFB is more control over the lower frequencies (increasing damping factor)

 

[FullRangeMan]

with triodes have not yet tried, but with pentodes the results are excellent. CFB 14-16% allows you to completely abandon the general feedback. The sound is much better.

Great news, thanks for inform.
Wonder what tech info I need to order an OPT with CFB 16% to 6C33 SE??

 

[pieter t]

Wonder what tech info I need to order an OPT with CFB 16% to 6C33 SE??

That's typically a tube that will not benefit so much from CFB.

 

[Profi]

For a triode for many sources of CFB no more than 7%. 6s33s requires the selection of triod inside the balloon, otherwise, when both heater is turned on, the sound is poor.

 

[FullRangeMan]

Great again, Iam updated so far w/this tube.
Thanks guys.

 

[deafbykhorns]

I have something similar
6E5P ccs-300b- IT - gm70
1075v 90ma
It sounds really nive, like a 300B on steroids

 

[FullRangeMan]

Always wondered how sound a GM70 with the minimum 800V aiming low output power??

 

[Flikoman]

I have something similar
6E5P ccs-300b- IT - gm70
1075v 90ma
It sounds really nive, like a 300B on steroids

Similar to mine. 3A5 -> 4P1L -> IT -> GM70. 1050V 93mA.

Next mod is higher B+, 1200VDC 80mA. I have gathered all the necessary components but lack of free time is a problem.

 

[deafbykhorns]

Next mod is higher B+, 1200VDC 80mA. I have gathered all the necessary components but lack of free time is a problem.

I had mine at 1200v to begin with and changed it to 1075 which seems to be a sweet spot with my OPT (Monolith Magnetics)

 

[Flikoman]

I had mine at 1200v to begin with and changed it to 1075 which seems to be a sweet spot with my OPT (Monolith Magnetics)

You never know if you don't try 🙂

You have better sound at 1075V B+ or? Lower B+ higher current. For OPT is better lower current because DC pre magnetization will be lower. Looking at the graph, GM-70 has even lower THD on 1200V 80mA operating point.

But, at the end, sound is what makes the magic and I'll see what's better for my ears. Good to share the experience.

Regards

 

[niksu]

It is an interesting design, and quite valid. I designed and built a much simpler GM70 amp that I completed about 7 years ago, class A1 fixed bias with interstage driven by a triode connected D3A, and all solid state supply with SiC rectifiers for the 1kV supply and CLCLC filtering with the chokes in the negative lead to minimize stress on the insulation.. I can get almost 20W in A1 with 7K primary 100mA/1kV. It remains in service to this point in time.

Hello, can you share your shematic?