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Old 8th February 2011, 08:54 PM   #11
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eeyore - one more question. Do you have any data on the amplification factor of the F2a? Nothing in there on the data sheets I have. From what you are saying, it might be possible to drive it with an 801A...
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Old 8th February 2011, 10:07 PM   #12
mach1 is offline mach1  Australia
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Quote:
Do you have any data on the amplification factor of the F2a
It's very easy to work out form the plate curves. Mark your operating point and draw your loadline, then calculate change in operating point voltage over change in Vg1. This will work out to approx 15 -17, depending on how you operate the tube..

In triode mode, Siemens recommend an operating point of 330V, -13.7V, 98mA (combined anode + g2 current), with a transformer impedance of 1K5 and cathode resistor value 140R. this gives 5.5W @ 10% THD.
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Last edited by mach1; 8th February 2011 at 10:11 PM.
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Old 8th February 2011, 10:21 PM   #13
jlsem is offline jlsem  United States
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Quote:
Originally Posted by kevinkr View Post
Using a C3M in pentode connection with an inductive load may cause problems with LF response due to the rather high plate resistance of pentode connection. (You want low rp with choke loading) I've not seen monkey configuration used with anything but triodes I suspect for this reason.
I'm not sure that is necessarily the case with pentodes, as it is with triodes. For example, a 6L6 is loaded nicely with 2.5KΩ despite it's high plate resistance. The plate resistance/load ratio may be a problem with output transformers when considering damping factor, but suitable inductive loads for pentodes should be considered and tested individually rather than making a general statement like the one which is true in the case of triodes.

John
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Old 9th February 2011, 03:02 AM   #14
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Here is the first revision of the amplifier. Sticking with the C3m, but triode strapped. Also added a cap across the resistors to improve noise rejection from the power supply, as per Thorsten's recommendation on direct-coupled circuits.

Click the image to open in full size.
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Old 9th February 2011, 05:27 AM   #15
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Quote:
I'm not sure that is necessarily the case with pentodes, as it is with triodes. For example, a 6L6 is loaded nicely with 2.5KΩ despite it's high plate resistance. The plate resistance/load ratio may be a problem with output transformers when considering damping factor, but suitable inductive loads for pentodes should be considered and tested individually rather than making a general statement like the one which is true in the case of triodes.
Hi John,

Kevin is right with his general staement.

One can look upon a pentode like a AC currentsource.

A 6L6 or any pentode works properly with load but not without. You compare a 6L6 loaded with 2,5k(depending on secondary loading and winding ratio) to a C3m, as in this case, loaded with next to nothing.

Just like the 6L6, C3m will need a load of just a few kohm to work properly. Will not be doable here.

So as I said a few posts ago, a pentode without load does not work in any circuit.
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Old 9th February 2011, 11:29 AM   #16
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Hi,

Quote:
Originally Posted by EvilMose View Post
I am attempting to design an amplifier with a C3m wired as a pentode direct coupled to an F2a wired as a triode using the "Monkey" topology. Aware there are some non-standard resistor values in there. That aside, there are a few things I am struggling with:

I am having trouble figuring out how to nail down Vg1 for the C3m. Right now, I have a battery, but I am not sure it would work.

Click the image to open in full size.
Some notes.

First, the C3m as Pentode is not really suited to inductive load, unless you parallel a low Value resistor with the C3m. Normal Anode load would be around 12K, which means you will need around 240H inductance, at around 15mA for an 8Hz -3dB Point.

I think you are better off adapting the Pentode Loftin White Circuit with resistive load from +B instead.

Secondly, the best way to derive the screen voltage is of course by tapping it of the Cathode Resistor of the F2a (again, classic Loftin White).

Thirdly, I REALLY, REALLY think you are better off deriving the cathode voltage for the C3m also from the resistor chain on the F2a, I would not put an Electrolytic capacitor into the cathode of this circuit, much less a battery, honestly. There are also other benefits for this, discussed in the part of my DC Coupled Amp series that covered the pentode driven LW. I'd probably also try the F2a as pentode and use some NFB to get a sensible output impedance, probably F2A anode to C3m Cathode...

Ciao T
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Old 9th February 2011, 02:08 PM   #17
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Quote:
Originally Posted by ThorstenL View Post
Hi,

Secondly, the best way to derive the screen voltage is of course by tapping it of the Cathode Resistor of the F2a (again, classic Loftin White).

Thirdly, I REALLY, REALLY think you are better off deriving the cathode voltage for the C3m also from the resistor chain on the F2a, I would not put an Electrolytic capacitor into the cathode of this circuit, much less a battery, honestly. There are also other benefits for this, discussed in the part of my DC Coupled Amp series that covered the pentode driven LW. I'd probably also try the F2a as pentode and use some NFB to get a sensible output impedance, probably F2A anode to C3m Cathode...

Ciao T

I have yanked the battery in the new schematic in favor of the resistor chain approach. The 90 uf cap is based on the fourth post in this thread - figure this can be a motor run cap, assuming it belongs where I think it does. Will revisit the thread to see what I am missing and check out the pentode-driven LW.

Thanks!


Click the image to open in full size.

Last edited by EvilMose; 9th February 2011 at 02:10 PM. Reason: mistake with link
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Old 9th February 2011, 02:25 PM   #18
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Hey EvilM,

If you still want to go monkey, you should lower Ua and Ia of the C3m somewhat. In the ballpark of 150-175V/15mA would be better. Check tube data below:

http://www.scottbecker.net/tube/sheets/128/c/C3m.pdf

I this particular case the load line will be close to horizontal.
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Old 9th February 2011, 03:21 PM   #19
hooman is offline hooman  Iran
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Quote:
Originally Posted by ThorstenL View Post
Hi,



I'd probably also try the F2a as pentode and use some NFB to get a sensible output impedance, probably F2A anode to C3m Cathode...

Ciao T
nic my new project is 6aw8 +f2a in shindo circuit ..use nfb from f2a anode to 6aw8....i have expriance in se amp ...but this circuit is different .
f2a in pentod is powerfull and musical.... highend sound .
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Old 9th February 2011, 10:08 PM   #20
mach1 is offline mach1  Australia
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Quote:
nic my new project is 6aw8 +f2a in shindo circuit ..use nfb from f2a anode to 6aw8....i have expriance in se amp ...but this circuit is different .
f2a in pentod is powerfull and musical.... highend sound .
got a schematic ?
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