Hammond interstage

[alexanderfa]

Hi
I am driving 2A3 with D3a (as triode) RC coupled and the result is good
I bought a pair of hammond interstage transformers 126B to try difference than RC coupling
I try them with EC8010 triode and have bad sound! Something like losing frequensies...
Did I make something wrong?
Should I try them with different tube?


Hammond Mfg. - Audio - Tube Driver Transformer - (124 - 126 Series)

 

[kaputt]

Inductance is a bit low so you might have earlier lf rolloff than before. I don't know about the interwinding capacitance of the Hammond IT but it could be high enough to affect HF as well.

 

[alexanderfa]

Inductance is a bit low so you might have earlier lf rolloff than before.

Looks like but:
Inductance of Hammond 126B is 44H
Tango NC-14 has 50H
Does not look to have low inductance compared to NC-14

 

[alexanderfa]

I measure them now with my LCR meter at 100Hz
Both primary and secondary test the same
Ls=15H
Cs=178nF
Rs=820ohm
DCR=176ohm

 

[andyjevans]

OK - I have some experience here:

1. Swap the secondary leads. The treble should come back again. It needs to be wired up right.

2. Try it out as a plate choke, with teflon FT-3 coupling caps. I use 0.1uF followed by 100K grid leak.

I prefer it as a plate choke where it's a great choice. You shouldn't have any treble issues there.

 

[alexanderfa]

OK - I have some experience here:

1. Swap the secondary leads. The treble should come back again. It needs to be wired up right.

2. Try it out as a plate choke, with teflon FT-3 coupling caps. I use 0.1uF followed by 100K grid leak.

I prefer it as a plate choke where it's a great choice. You shouldn't have any treble issues there.

Thanks Andy
I'll try tomorrow and tell you the result

 

[andyjevans]

Thanks Andy
I'll try tomorrow and tell you the result

One or the other should work. Plate choke should definitely solve the problem and the teflon FT-3 caps are by a long way the best I've tried. The 4P1L in filament bias is perfect with the 126B. Run it at around 25mA. Has a gain of 9 or 10. That's what I use. Long thread for "4P1L Line Stage". Should be in filament bias with Rod Coleman regs. I attach a schematic to give you the idea. As you can see I use PSE 4P1L instead of 2a3 so that with a gain of 9 or 10 I only need 2 stages. Can't do that with the 2a3. Sounds just as good or better to my ears.

 

[alexanderfa]

I do not have FT-3 caps. I'll try a MKP or OIL , and will order some FT-3.
I hear too much discussion for 4P1L maybe I should buy some and try.
I use AC for filament supply but I don't have any noise..
Does rod coleman regulator improve better sound or is only for filament noise?

 

[andyjevans]

The whole idea is to use DHTs. That's where the Rod Coleman regs come in. But the added bonus of filament bias is that the cathode resistor is small - typically 30 ohms or less. So no need for cathode bypass caps. That's one source of degradation out of the way. You do need a very clean filament supply since it also goes through the cathode resistor, so it's in the signal path. But Rod's regs with a good DC supply - preferably with a choke in it - gives a very clean supply. This arrangement was proposed by Thomas Mayer (vinylsavor) and has become popular in the last few years with 26, 01A and 4P1L tubes for which it's a perfect solution. Ale Moglia writes extensively on all this on his bartola valves website. Thomas has a series of very interesting blogs on his site too.

 

[alexanderfa]

Today I swap the secondary leads and the sound become normal
I also use the primary only as plate choke and capacitor coupling and I think Is better
I'll buy also some teflon capacitors to try
Thanks a lot Andy!

Does any have experience with EC8010 operation values?
I think datasheet isn't correct.. Ra=2.4K?? Rk=47 ohm??

 

[andyjevans]

Today I swap the secondary leads and the sound become normal.

I also use the primary only as plate choke and capacitor coupling and I think Is better. I'll buy also some teflon capacitors to try. Thanks a lot Andy!

You're welcome! I went through this exact situation myself, so that's how I learned.

The Russian FT-3 are a tiny bit better than the K72 teflons. There is also a FT-2 rated for 200v which is good.