E810F / 7788 pentode micpre ?

[clintrubber]

Found a few E810F tubes (pentodes) and the high mu (50mA/V) and the 110 Ohm equivalent noise resistance hints at a mic-preamp, either in some transformerless balanced way, or with just one tube and 1:1 input-TX for balancing.

http://www.mif.pg.gda.pl/homepages/frank/sheets/009/e/E810F.pdf

I guess I could start from EF86-like circuits and adapt.

Or starting from scratch is fine as well, but just wondering, any existing mic-pre circuits that happened to use the E810F / 7788 ?

What I saw so far are various phono-preamp stages with this, but was wondering if anybody knows or has used this tube for mic-pre duties, either with or without input-transformer.

Thanks,

Peter

 

[ruffrecords]

I would take that equivalent noise resistance figure of 110 ohms with a very large pinch of salt as it is specified at 45MHz. At audio 1/f and partition noise will be much more significant in a tube of this type. The specified hum voltage of 150uV looks pretty grim too. Even so, when wired as a triode and fed with dc heaters it should still have a decent gm which should lead to a good equivalent input noise in the audio band.

Cheers

Ian

 

[clintrubber]

[quote author="ruffrecords"]I would take that equivalent noise resistance figure of 110 ohms with a very large pinch of salt as it is specified at 45MHz. At audio 1/f and partition noise will be much more significant in a tube of this type. The specified hum voltage of 150uV looks pretty grim too. Even so, when wired as a triode and fed with dc heaters it should still have a decent gm which should lead to a good equivalent input noise in the audio band.

Cheers

Ian[/quote]
Thanks Ian for responding,

You're right, the ENR is at a way different frequency. Still its use for phono-stages sounds like it'll be usable for at least somewhat decent low-noise audio, but too bad there's no further info on that.
Looks like there's no real reason to take this type over the usual suspects then (unless an enclosure is used that manages to show off the 'SQ' marking on the tube :wink: )

Regards,

Peter

 

[PRR]

> high mu (50mA/V) and the 110 Ohm equivalent noise resistance...

At 45MHz and at 35mA!

Oh: in american data, "mu" is usually Amplification Factor. 57 for this tube as triode. Gm is transconductance, current per volts.

0.050A/V is 20 ohms. Use a conservative 3X multiplier because that cathode resistance is HOT (red-heat is about 3X room temp), the low frequency noise resistance may be 60 ohms.

Plus 1/f noise, which depends on cathode processing. And this tube was not produced for low 1/f noise. So it is a crap-shoot.

At 2mA the Gm is more like 10, or Rn up around 300 ohms. Gm and Rn scale about as square-root of current.

At 35mA, plate resistance is about 1K2, a nice low value. We can get there with G2 near 100V. Assume a 250V supply, the plate resistor can be 150V/35mA= 4K3. Gain as triode would be 40, plenty to swamp next-stage noise.

Or with 5K:600 transformer, gain about 23dB, output over +30dBm.

Interesting, but you need a SERIOUS power supply, or it isn't spectacularly great.

> I could start from EF86-like circuits and adapt

I guess I could start from an econo-car and build a race car. (I guess people DO.)

I'd approach it as a hi-Mu triode with very high possible cathode current. Similar to 10 sections of 12AT7 in parallel, but much lower heater demand and just one socket.

 

[clintrubber]

Thanks for responding, I'll print & re-read and get the various figures in line.

FWIW, here' s a circuit using small plate resistors as well:
http://www.arduman.com/aa/Sayfalar/thorsten/thorsten4/phono_schem.htm


> I'd approach it as a hi-Mu triode with very high possible cathode current. Similar to 10 sections of 12AT7 in parallel, but much lower heater demand and just one socket.

Hmm, so the only advantage seems space-saving, but the hefty power supply would take that back again...


Regards,

Peter

 

[bcarso]

Check out microphonics too. Some of the high-gm tubes with very tight grid-cathode spacing can ring and ring once excited with an impulse---I think I mentioned this somewhere in connection with the 6C45.

 

[clintrubber]

Apologies to those who answered, still haven't thrown some brainactivity & the calculator to this - but I will. Appreciating the responses so far.

Thanks,

Peter