Sovtek 6B4-G

[Lowrider]

I would like to use this tubes in my PP EL34 amplifier, triode mode, no global NFB...

I have a very BIG constraint, I cannot eliminate the existing ground connection of the heater current lines, it is somewhere on the PCB, and I dont have enough experience to remove it...

Can I use them changing only the connection of the fixed bias from cathode to heater...

Of course I would like to change to Class A, but AB would be fine to start...

What else should I change ?

[Sweden]

interesting chart hope you get answers, Have you perhaps had a reply?

[Lowrider]

Nope...

[kmaier]

Hi Lowrider....

You basically need to remove the grounds on the 6.3VAC@3A lines to let them float. Depending on how things are wired... might be easier to lift the transformer lines off the PCB, unless it's a PCB mounted transformer.

However, you will have a few other problems to deal with using the 6B4G in this application:

1- Being a DHT, chances are you'll need a DC filament supply to get rid of the hum from AC filaments.

2- Input capacitance is higher on the 6B4G so you need to ensure your driver stage is capable of handling the additional load and having sufficient slew rate.

3- A 440+ volt B+ supply is too high for the typical operating specs. No idea if the Sovtek can handle such a high voltage, but the basic spec (for a NOS 6B4G) is 300V max (cathode to plate). Even with cathode biasing (55 volts) , you're still quite high.

What's the rest of the schematic look like?

Regards, KM

[Lowrider]

Thanks...

I moved to Angola, without my hifi (it is stored somewhere), so no mods for a long time...

[CharleyW]

With push-pull you will not need DC filaments.

With cathode bias you can (probably) get away with the higher voltage. I'm running Sovtek 6B4G's in an ST-70 (with a Curcio premium board), no NFB, and the tube life is starting to look impressive.

[Sweden]

Hello
But is there an advantage to DC, less risk of humming?

Has an original ST-70 that sounds really nice, and is building a 6B4G SE amplifier,
But to build on the ST-70 to 6B4G is true intresant, But the advantage of EL34 that it is scientifically cheap

[kmaier]

I think it depends on which 6B4G you have.... I think some of the new manufacture types are single plate construction and it could be that the filament arrangement works okay with AC. I have many NOS types from Amperex, Philco, CEI and others. These have a dual-section construction or dual triode construction internally. The plates and grids are paralleled but the filaments are in series. In this arrangement you won't have much luck with AC.... at least I never have.

As CW noted, he's got it running.... and the Sovteks seem okay with the higher voltage, nice.

Regards, KM

[Sweden]

Okey kmaier
I have double plates on both 6B4G and my Russian copies called 6S4S they also have bubble plates.

I am considering this
http://www2u.biglobe.ne.jp/~tossie/6B4G-E.html

The only thing I do not really
like the 12AX7, but it might be nice, is eager to try.


But you might have a good tip on Schedule SE to 6B4G

Regards
Sweden

[kmaier]

Agreed... nice amp design and all, but notice the HF response drops off rather quickly after 10KHz. Note that the paralleled 12AX7 current is only 1ma. By my calculations you should run ~3.0ma of current on the driver to overcome the input capacitance and get extended bandwidth beyond 20KHz.

You could also swap the 12AX7 with a 12BZ7 and double the current up by halving the resistor values, i.e., 75K plate load, 750 ohm cathode and 6K in the supply. The 12BZ7 is a larger tube (identical basing), each triode is like a paralleled 12AX7. This alone should extend the HF response out.

I did a DIY project last year that will uses a 12AT7 input/driver.... very simple circuit. 1st section is voltage gain direct-coupled to 2nd section as a cathode follower. Will drive a 45 or 2A3 easily with very good performance. PDFs are available if you like. You have choices... Tossie also has some other 6B4G amps on his various pages. He does really nice work.

Regards, KM