e180f/6688 pentode operting points

[danskercher]

I'm working out a schematic for a e180f/6688 --> 46 amp. I would like to try running the e180f in pentode mode. Can anyone recommend a good operating point for the e180f? I was thinking of something like 180V anode, 140V screen grid, and 8mA. But I really have no experience with pentode drivers.
Thanks,
Dan

[Jeb-D.]

If it will provide enough gain, triode wire it.

I played with pentode wired E180F's quite a bit a while back and find that they sound too soft regardless of operating point. They also clip really nasty (the peaks will invert) on the saturation side of the swing.

The latter observation can obviously be avoided by not pushing the tube to hard and biasing it with adequate voltage on the plate . And you may have better luck with the sound in your application than I did with mine. I don't mean to discourage you, just thought it would be worth sharing an outcome from a similar experience.

[danskercher]

Originally Posted by Jeb-D.

If it will provide enough gain, triode wire it.

I played with pentode wired E180F's quite a bit a while back and find that they sound too soft regardless of operating point. They also clip really nasty (the peaks will invert) on the saturation side of the swing.

The latter observation can obviously be avoided by not pushing the tube to hard and biasing it with adequate voltage on the plate . And you may have better luck with the sound in your application than I did with mine. I don't mean to discourage you, just thought it would be worth sharing an outcome from a similar experience.

So it sounds soft no matter what operating point you use. Hmm, I wasn't expecting that.

I was thinking of trying out a Loftin White circuit like Thorsten designed a while back:

Calculating DC coupled Single Ended Valve Amplifiers - Part #1

I've never built an amp with a pentode driver stage and thought I might give it a try.

Thanks for the input.
Dan

[Jeb-D.]

So it sounds soft no matter what operating point you use. Hmm, I wasn't expecting that.

That was my experience with it, though it could just be localized to application I was trying to use it in. If your going to breadboard the amp first, you might as well give it a shot. At the very least, you will still gain a deeper understanding of how to use pentodes and how they work. But I wouldn't suggest designing a permanent amp around it without hearing the design on the breadboard first.

[azazello]

Amazing tube! I used in my SE 2A3-RCA and SE 6S41S in mode like Yours.
But I think better tube is russian 6E5P /now it works with 6S41S with better sound!/.

[alexg]

Originally Posted by azazello

Amazing tube! I used in my SE 2A3-RCA and SE 6S41S in mode like Yours.
But I think better tube is russian 6E5P /now it works with 6S41S with better sound!/.

Can you please share your operating points of the E180F, I have a pair which I can use for my 2a3 amp.

Thanks.

[artosalo]

Can anyone recommend a good operating point for the e180f?

I have done some tests with soviet-made 6Ж9П, which is equivalent with E180F. It works very well and with low distortion when the cathode resistor is un-bypassed. Also I noticed that the minimum distortion "sweet spot" can be found by adjusting the screen resistor. The optimum value of Rg2 varies a little between individual tubes. So when fine-tuning the amplifier, just adjust the screen resistor to minimum distortion with a trimmer resistor and then replace it with fixed one.

My optimum circuit was as follows:

Ub = 300 V, Ua = 160 V, Ug2 = 160 V, Ik = 8,0 mA, Ig2 = 1,7 mA, Uk = 2,80 V.

Ra = 22 kΩ, Rg2 = 66 kΩ, Rk = 390 Ω.

G = 31,5 dB, THD = 0,09 % when Uo = 20 Vrms to 100 kΩ load.

The supply voltage is not critical at all. Some + 40 V is OK.
The screen supply must be well bypassed.

How are you going to use the 46 output tube, as triode or tetrode ?
With fixed or auto bias ?

[Tony]

Originally Posted by alexg

Can you please share your operating points of the E180F, I have a pair which I can use for my 2a3 amp.

Thanks.

if you are running the 6688 tube in pentode mode, then you need to know the peak dc grid voltage required by your 2A3....

the 6688 plate volts then needs to be that voltage plus 50 volts or so....
then pick a plate current, take note the you should start at 25% plate dissipation...
screen current is about 1/5 of plate current for pentodes,
pick your Rl based on available B+ and chosen plate current....
rk can then be determined once you plot the dc loadline on the plate characteristic curves......
these estimations are for dc operating points....

voltage gain is roughly gm x Rl.....

[azazello]

Originally Posted by alexg

Can you please share your operating points of the E180F, I have a pair which I can use for my 2a3 amp.

Thanks.

https://picasaweb.google.com/azazell...13993179171922
R plate E180F = 13 kohm /not 33 kohm/. Input poti - 2,2 kohm - RFT. Gain in triode mode is enough for driving 2A3-RCA/Ubias = -44V/. sound is "not repeatable!".
I used in my SE300B-JJ the sister of E180F - E186F-Miniwat-10000 Hrs /penthode mode/....amazing sound!!
/But I prefer listen my 2A3-RCA!/.