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Old 3rd October 2014, 10:35 PM   #1
famousmockingbird is offline famousmockingbird  United States
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Default Mosfet driver

I have a 5E3 clone that I built and use it for band practice and some gigging depending on the music we are playing. I usually have the amp's volume maxed and I use my guitar volume as a distortion control. The tinkerer in me got the idea to drive the output tubes with some mosfets seeing how I drive them so hard most of the time. I am hoping somebody has tried this (most likely tubelab) and is there a noticeable difference in distortion characteristics (audibly)? I am hoping it tightens up the distortion a bit. I have reduced the size of the coupling caps which helps with the flabby low end. I have a schematic worked up but I don't have much room in the chassis so I would like to make a small PCB to do this. I am new to making PCB's so I will post one when I am done along with the schematic so people can double check my work for me (if they are so inclined of course).

Another question, don't mosfets have a large input capacitance and will this be a problem because the 5E3 PI is only drawing 800uA?
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Old 6th October 2014, 11:07 PM   #2
gingertube is offline gingertube  Australia
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Yes MOSFETs have large input capacitance but that is irrelevant since adding MOSFET Source followers to a cathodyne phase splitter is a waste of time. The cathodyne already has incredibly low output ompedance (Zout approx. = 1/gm)
If you want to improve the overdrive characteristics of a cathodyne phase splitter amp, like the 5E3, then read this:
The Valve Wizard
The "trick" is to run large gridstop resistors on the output tubes, try 47K.
Cheers,
Ian
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Old 7th October 2014, 01:14 AM   #3
JMFahey is offline JMFahey  Argentina
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Quote:
The cathodyne already has incredibly low output ompedance (Zout approx. = 1/gm)
You mean the cathode out does ... the plate out has the usual high impedance, or Rload//Rplate
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Old 7th October 2014, 01:25 AM   #4
gingertube is offline gingertube  Australia
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Juan,
OK lets have a go at that OLD arguement - here is something "I prepared earlier". I used "u" for mu in the algebra below..

The cathodyne has equal output impedances at the anode and cathode SO LONG as the loads on the anode and cathode are equal. When overdriving the amp then the loads will become un-equal when the output tubes start to draw grid current (positive peaks of the grid drive).

Maths (skip to the summary below if maths "gets on your wick".
For equal loads on Anode and cathode

Zout = RL.ra/RL(u+2)+ra

The ra term on the bottom line is insignificant compared to RL(u+2) term so drop it. Then the RL terms top and bottom lines cancel leaving

Zout approx = ra/u+2

At typical values of u (>=20) u+2 approx = u, so simplify again

Zout approx = ra/u = 1/gm

If driving output stage directly The equal loads on Anode and Cathode will NOT be guaranteed if:
Output Stage strays into grid current.


If the Anode load drops significantly then:

Zout cathode = RL+ra/(u+2) x ra/RL, the ra/RL term is insignigicant so

Zout cathode approx = RL+ra/u+2, at usual values of u

Zout cathode approx = RL/u + ra/u = RL/u + 1/gm

That is it increases by RL/u

If the cathode load drops significantly then:

Zout anode = RLxRL(u+1)+RL.ra / RL(u+2)+ra

RL squared (u+1) is much larger than RL.ra and RL(u+2) is much larger than ra so

Zout anode approx = RLxRL(u+1)/RL(u+2)

and at reasonable values of u

Zout anode approx = RL

Summary:
As the loads on Anode and cathode become unbalanced then

Zout anode increases from 1/gm toward RL
Zout cathode increase from 1/gm by maximum factor of RL/u

Thats why the HIFI guys suggest that low u is better when driving other than Class A Output Stage.

AND

That is why folk like the Wiz recommend (for guitar amps) increasing the grid stops on the output tubes to a value like 47K. This minimises the load change when output tubes swing into grid current. You can "tune" the amount of distortion in the power amp in overdrive by adjusting those grid stop resistors. I've seen up to 100K used. I use 47K routinely on 6V6 and 6DQ6A amps with cathodyne splitters.

Cheers,
Ian

Last edited by gingertube; 7th October 2014 at 01:36 AM.
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Old 7th October 2014, 02:06 AM   #5
JMFahey is offline JMFahey  Argentina
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Dear gingertube
Quote:
The cathodyne has equal output impedances at the anode and cathode SO LONG as the loads on the anode and cathode are equal.
I never said that but quite the contrary
And as far as Math is concerned, it's me the one who usually tries to introduce it in DIY threads

So don't try to convince me of what I'm already convinced

By the way, my field is Guitar Amps (well over 10000 built by 2004/2005 where I lost count) so overdriven amps are my bread and butter.

And I particularly dislike cathodynes for their unsymmetry when heavily overdriven

Take care
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Old 7th October 2014, 02:38 AM   #6
gingertube is offline gingertube  Australia
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Quote:
Originally Posted by JMFahey View Post
Dear gingertube

By the way, my field is Guitar Amps (well over 10000 built by 2004/2005 where I lost count) so overdriven amps are my bread and butter.

And I particularly dislike cathodynes for their unsymmetry when heavily overdriven

Take care
Which is why you are valued so highly here (by me and others) and why I always take particular note of what you have to say.

I agree that the cathodyne is not the splitter of choice for heavily overdriven amps (the differential splitter is a much better option for them) and why I sometimes wonder why so many folk want to build 5E3 clones.

If you do have an existing amp with a cathodyne splitter (which was the case for the 6V6 and 6DQ6A Amps I mentioned), or feel that you really do want to build something like the 5E3 then the Wiz's recommended mods will go a long way toward adressing the cathodynes weakness in overdriven amps.

The algebra I posted just shows why large grid stops on the output tubes does work to improve to situation.

To be clear to other readers - if you are building an amp that you want to give you the best overdriven sounds then Juan and I are both saying DO NOT use the cathodyne phase splitter.

If you already have an amp with a cathodyne splitter and you want to improve its overdrive performance than do the Wiz's mods.

The original analysis comes from Priesman
http://www.aikenamps.com/images/Documents/cathodyne.pdf

More recently this analysis has been documented by Morgan Jones (Valve Amplifiers) and simulation results from SY (see the Red Light District Amp on the HiFi Forum here) have confirmed that the Anode and Cathode source impedances are equal when the loads are equal and also confirmed that using high value grid stops on the output tubes to minimize the amount that the loads become UN equal (when in overdrive) helps.

Best Regards,
Ian
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Old 7th October 2014, 05:02 AM   #7
famousmockingbird is offline famousmockingbird  United States
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I am going to sit down and run some simulations and also do the maths of course but I am still unsure of what you guys are saying. I agree that the the cathodyne distorts badly and why I wish to fix it. I do have a large grid resistor feeding the PI as the wiz suggests, I have not tried 100k on the 6V6 grids yet. The problem in my opinion is blocking distortion which even thought the output impedance of the cathodyne is low it doesn't fix the problem of the coupling capacitor.

Like I said I haven't done the maths yet but I don't think I am going to get into class AB2 with just a cathodyne PI, or even fix the overload issues with just basic resistors at the grids of the 6V6's. Another part of the issue of imbalanced loading of the PI is that I plan on maybe trying class A2 by using a LM317 to auto bias the power tubes, from what I am thinking this will make matter worse for the cathodyne PI because the the tube with the distortion on grid current onset will not be in cutoff like in AB operation, this also is leading me to try to drive the 6V6's with mosfets.

I highly appreciate both of your inputs so thank you very much. I am also a hobbiest and am still learning.

I would really like to hear what you have to say JMFahey because like you said over driven amps are your "bread and butter" I will take any advice you give me in my quest for low power overdriven guitar nirvana. I do not wish to build a whole different amp for I have been out of work so I would like to simply modify my existing build. If I had the choice I would have one amp of each that does just one thing well but I can't. So I am thinking of changing my 5E3 into a class A2 power house, or maybe just class AB2 but I really don't need too much power. The 5E3 is bias so hot towards class A I don't think changing to CCS for bias will be any worse in eating away at my stash of grey glass RCA VT-107-A's.

Thanks
-bird

Last edited by famousmockingbird; 7th October 2014 at 05:27 AM.
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Old 7th October 2014, 05:13 AM   #8
gingertube is offline gingertube  Australia
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Hi Bird,
I am going to reverse what I said originally. MOSFET Source Followers on the Cathodyne Phase Splitter will help because it puts a Class A operating load onto the Cathodyne so that it is never subjected to the very low load impedance of the an output tube with grid current and therefore the Anode/Cathode loads remain balanced.

Here is a 200Watt AM Transmitter Modulator circuit for reference - note that the Cathodyne phase splitter has cathode followers on it. Note the output tube grids are direct coupled from the followers and the bias is applied at the follower grids. You could do this with MOSFET Source followers. Ignore the 6AU6 and 6AQ5 which are just a screen supply regulator.

Regards,
Ian
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File Type: gif Vike_Val_Mod.gif (29.9 KB, 195 views)

Last edited by gingertube; 7th October 2014 at 05:32 AM.
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Old 7th October 2014, 07:03 AM   #9
gingertube is offline gingertube  Australia
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Actually that circuit has a diff amp after the Cathodyne splitter (the 2nd 12AX7 triode) and then the cathode followers but the point of keeping the anode and cathode loads on the splitter equal, still applies.
Regards,
Ian
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Old 7th October 2014, 09:40 PM   #10
famousmockingbird is offline famousmockingbird  United States
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Hi Ian,
Thanks for the reference, it confirms the way I was going to implement the source follower. I will post a schematic once I figure this out some more, for I can't seem to get it to work correctly in a simulation

I omitted the cathode bias scheme for the 6V6's and tied the cathodes to ground through 1R resistors. For mosfets I used IRF630's and tied the drain to 200v source, the source to a -200v source through 10k resistors, and the gate was tied to a -100 through a voltage divider for the bias. No matter what I change the -bias to I get 200mA through each 6V6 The IRF630's are AC coupled to the cathodyne PI. What am I missing?


-bird
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File Type: gif sourcefollower.gif (6.8 KB, 170 views)
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