Battery for Cathode Voltage?

[markw51]

I am considering building this amp:

http://www.the-planet.org/6EM7.html

because I need a small, low-power tube amp for limited space.

I am wondering why the designer would use a battery for the cathode voltage of the first 6EM7 stage. Why not just use the typical resistor and capacitor?

Mark

[plovati]

You can proide cathode bias in many manners:
I tried those



discussed in this Italian forum:

http://www.audiofaidate.it/forum/top...04&whichpage=3

of all, the best sounding was the IR LED (well an optocoupler in fact). Battery bias sounds bad. I do not why, the bias condition and dynamical impedance were comparable in every case.

The coiche for usying battery is dictated by the willings of avoid capacitor. Same results can be used with LEDs, less complication and better sound.

[heater]

That schematic seems a little dishonest to me "6EM7 DC Coupled Triode Amp"

Sure the is no coupling capacitor in the signal path. Or is there?

Let's try again. There is no coupling capacitor in the signal path between the first stage and the second, which looks good, but what about that 47uF bypass on the output stage bias resistors? Basically all your AC signal current in the output is pumping through that.

Is that better than the normal interstage coupling capacitor? I do not know. But if we use the normal interstage coupling we don't need to bias the output tube cathode up to 140 volts, therefor we don't need to waste 7 watts or power in those huge 10 watt resistors and we don't need that big ugly 47uF capacitor. And we probably can reduce the power supply by 140 volts which all sounds like a plus to me.

[markw51]

All very good observations I had not considered.

I think I will redesign it for self-biasing and capacitor coupling. That will make the design more conventional and waste less power and not require the ten watt resistors. Why in the heck require 10 watt resistors in an amp that puts out 2 watts at best?

Actually, if anyone has a good 6EM7 SE circuit they would share I would like to see it.

Mark

[Klimon]

of all, the best sounding was the IR LED (well an optocoupler in fact). Battery bias sounds bad.

What type of battery did you use? I hear that different types give very different results; Nimh getting good reviews in general. Another observation is that according to some people LED-biasing only works well if the tube draws >10mA. Is that also the case with your preamp or does led biasing in your example sound better than battery even when it draws less than 10mA?

Thanks,

Simon

[kevinkr]

Umm, guys, have any of you heard this design? I have and further know the fellow who designed it quite well. It does perform well, and sounds good, although I would recommend the use of a film cap /motor run in place of the electrolytic used in the cathode bypass.

DC coupling in this instance is generally understood to apply to the driver to power tube coupling scheme and in this context I would not consider it misrepresentative. Whether or not it is dc coupled, if cathode bias is used there is going to have to be a bypass cap on that cathode.

This is a loose variant of the loftin white amplifier theme and I have designed several myself, although I would admit to preferring fixed bias and no cathode caps for the reasons cited elsewhere. (Basically no cap is a good cap.)

I as much as anyone around here probably helped to popularize the battery bias idea locally speaking (Boston/North Shore/NH)back around 2001 - placing it in the cathode obviates the need for a coupling cap in the grid circuit. Note however that I now have serious sonic reservations about this approach due to battery issues and an inability to tune operating point except by varying plate voltage and also recommend a nicad over nimh due to generally lower internal impedance. Frankly I would stick a nice little film cap in parallel with that bias battery. Circuit operation will keep it charged and the battery chosen must be large enough such that the cathode current is < = to 1/10th the rated capacity of the battery to prevent overcharging. Generally a AA battery will be fine. I would buy a variety to audition, although panasonic would be high on my list. (Regardless of brand expect to replace in about 4yrs or rated float life of cell.)

Gary and I are fellow members in NNETG and as a group we tend to eschew the use of semiconductors and electrolytics in the signal path wherever possible. This design largely reflects that outlook. (The electrolytics are there to keep the thing to a specified budget.)

And yes the design does waste a little power, but given the low overall consumption I doubt it is an issue.

[plovati]

Originally posted by Klimon


What type of battery did you use? I hear that different types give very different results; Nimh getting good reviews in general. Another observation is that according to some people LED-biasing only works well if the tube draws >10mA. Is that also the case with your preamp or does led biasing in your example sound better than battery even when it draws less than 10mA?

Thanks,

Simon

Battery were a stack of two NiCd 280mAh (a big green button, for PCB mounting), no brands. The preamp draws 25mA (PL84 triode strapped).

I'm happy with this 6EM7 SE:
http://www.audiofaidate.it/materiale...rsione5000.pdf

This little tube is a real giant of Sound! Nice to see also :-)

[alexistheo]

Another thing about this design: it puts both sections at their optimal operating points. The high-mu section likes a higher B+ and the power section of the 6EM7 sounds better (most agree) at lower voltage and higher current. Direct-coupling allows them to share the same supply; quite an elegant solution in my opinion. If you have the space, use a film cap as kevin suggested - with the high value of Rk, you won't need much capacitance anyway.

[markw51]

Piergiorgio:

Thank you for sharing that schematic of the 6EM7 amp.

I am wondering: you have about 250 volts on the plate of the second stage whereas the RCA tube handbook specifies 150 volts on the plate for 50 ma plate current.

Mark

[plovati]

Originally posted by markw51
Piergiorgio:

Thank you for sharing that schematic of the 6EM7 amp.

I am wondering: you have about 250 volts on the plate of the second stage whereas the RCA tube handbook specifies 150 volts on the plate for 50 ma plate current.

Mark

Max rating of the tube is 330V, for 10W plate dissipation. The same couple of tubes (Ge) runs in these operating condition since 2001 without degradation in electrical parameters, after the initial settlement in the first months.