New LCR Phono : Pentode Aikido, Slagle 1.5k, C3g

[Paul Barker]

I must say however I'm still a bit surprised that I get loads of gain in my powers amps when running C3m or C3g un-bypassed driver ..

Well yes an Actual gain A of 71 is loads of gain.

[Paul Barker]

You might like to try a battery in the 1st stage as well - an AAA alkaline could work with a bit of an adjustment on the anode resistor. It's my preference over a cathode cap or LED. Some like LEDs though, even schottkys.
Stephen

Thanks for the advice Stephen, but I'm with Paul on this one I think . Anything other than high quality by-pass caps or grid battery bias sounds poor in the treble to me - and just less realistic . I could try grid battery bias on st. 1 of course...

Yes although I haven't visited other cathode bias methods for many years, having once learned I tend to not look back.

The biases I employ are valve diode, resistor, vr tube and fixed. If I have to I bypass the resistor.

[IslandPink]

Well yes an Actual gain A of 71 is loads of gain.

Sorry , to clarify, it was always C3m on the 300B amp, and that's what I did the comparison with . Bypassed cathode should give about 80 with that , but I never had the cap on there .

[izzy wizzy]

Thanks for the advice Stephen, but I'm with Paul on this one I think . Anything other than high quality by-pass caps or grid battery bias sounds poor in the treble to me - and just less realistic . I could try grid battery bias on st. 1 of course...

I wouldn't use anything else myself either. I don't like silicon any place It's the less realism thing that moves me too. Others have luck with SS stuff in the cathode but I never have. I wish I could get rid of the battery tho but they seem to last years anyway.

cheers,

Stephen

[Paul Barker]

Thanks for the advice Stephen, but I'm with Paul on this one I think . Anything other than high quality by-pass caps or grid battery bias sounds poor in the treble to me - and just less realistic . I could try grid battery bias on st. 1 of course...

I wouldn't use anything else myself either. I don't like silicon any place It's the less realism thing that moves me too. Others have luck with SS stuff in the cathode but I never have. I wish I could get rid of the battery tho but they seem to last years anyway.

cheers,

Stephen

I was using batteries like a fool for years, I was landed hook line and sinker when they suddenly became the thing to do. First I saw of it was Jeremy Epstaein's Free Lunch. I went forward believing batteries could do no harm, without ever evaluating properly.

One day I found out how batteries were degrading my sounds.

I do realise that Nick has turned up a paper which discredits VR tubes, that I now rely heavily upon. I suppose at some point I shall have to evaluate my use of those as instruments of cathode bias in case I am off the mark there too.

[izzy wizzy]

HI Paul,

Do you mean batteries in the cathode or batteries in the grid or batteries as HT? IME, I only like batteries in the grid. I don't like NiCads or NiMH in the cathode. Did loads of comparisons and always end up with the same thing. All this in phonos tho.

I've tried them under the grid resistor and in series with the grid and I can't tell the difference which surprised me.

cheers,

Stephen

[IslandPink]

For info , I spoke to John Atwood about this, and he said the key to using batteries is that there should be the absolute minimum of current modulation on the battery . This explains why grid battery bias sounds a lot better than cathode battery bias.

[Paul Barker]

HI Paul,

Do you mean batteries in the cathode

Stephen

Yes.

[Paul Barker]

For info , I spoke to John Atwood about this, and he said the key to using batteries is that there should be the absolute minimum of current modulation on the battery . This explains why grid battery bias sounds a lot better than cathode battery bias.

That explains it.

[IslandPink]

Ok, here's something to add into the mix regarding the pentode gain question ; I found two bits from James D emails :

"For pentode operation the cathode bypass capacitor is not needed as the cathode resistor doesn't affect the gain of the pentode and removing will result in more pure pentode operation and better sound - not necessarily more dynamic but it should remove the sonic signature of those big caps."

On C3m driver for power amp -
"Gm is taken from the published curves and so only accurate to 5% for the C3m (they are PTT spec valves so tight tolerance). At your op point Gm is 6mA/V so gain is a little less than your calc at 74 times ish. The cathode resistor on a pentode has only second or third order effects on the gain i.e. less than 5% and typically 1-2% so is generally ignored as other variables dominate."

I should have a look in Morgan Jones to see if he comments on this issue perhaps . Are you using the normal triode gain equation, Paul ?

I could try it ( adding caps) on the power amp, if I hadn't made it really awkward to get into that area under a sub-board on that breadboard . I can't face the fiddly work it will take to do it right now .

[Andrew]

"For pentode operation the cathode bypass capacitor is not needed as the cathode resistor doesn't affect the gain of the pentode and removing will result in more pure pentode operation and better sound - not necessarily more dynamic but it should remove the sonic signature of those big caps."

Sorry, James is wrong on that one; somewhere on here you find he agreed with me. The resistor is a from of negative feedback at the cathode, why wouldn't it reduce gain?

[Andrew]

This look about right (from here http://www.freewebs.com/valvewizard/pentode.html)

Gain: If both cathode and screen-grid are bypassed, the gain of the pentode is:
Av = gm * ra * Ra / (ra + Ra )
Since ra is very large, this can be approximated as:
Av = gm * Ra

If the cathode is not bypassed, the gain becomes:
Av(unbypassed) = gm *ra *Ra / [ ra + Ra + Rk (gm * ra + 1 )]
These formulae do not take into account the loading effect of the following stage, in which case Ra should be substituted for the value of Ra in parallel with Rl.

They are also very inaccurate unless very precise values for ra and gm are found.

[IslandPink]

OK, so I calc about 42 ( unbypassed ) and 74 ( bypassed ) for the C3m case, so I must have failed to notice this amount of change in between the two amp builds - I guess I had plenty of gain and both volume pot settings were on the low end of the range .
Cheers.

[Andrew]

Also to get the gm up you need to get the current up as gm varies with current.

Andrew

[Mike H]

See also:

Calculating Gain Of Cathode Resistor Feedback