A question about Cathodyne/split load/concertina PI's - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 16th October 2005, 12:16 PM   #21
diyAudio Member
 
Tubes4e4's Avatar
 
Join Date: Jul 2005
Location: Leverkusen
About 12DW7:

Quote:
Originally posted by Tubes4e4
JJ/Tesla too.
It is called ECC832, I think.

Tom
__________________
If in doubt, just measure.
  Reply With Quote
Old 17th October 2005, 12:57 AM   #22
diyAudio Member
 
Join Date: May 2005
Location: Pretoria, South Africa
Aletheian,

You have had so much good advice that you hardly need more, but since I seem to be writing nothing tonight....

I would echo PRR (post #15) and just use the circuit without NFB for your purpose. Otherwise a no go; you will need an input stage for anything proper. I have seen many attempts over a long time at making an over-simple circuit too prosperous - often not worth the effort because of minimal real advantage, if at all. As was said, straight output triodes are .... mmm ... not too bad on their virgin own.
  Reply With Quote
Old 19th October 2005, 01:23 AM   #23
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
I recently did a restoration on an AMPEG tube GIT Amp - The V9 preamp circuit.

I used new production 12DW7 from EI - the guy was VERY happy with the sound, said it was at least as good as when the amp was new.

Cheers,
Ian
  Reply With Quote
Old 19th October 2005, 05:41 AM   #24
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
While on the subject of Cathodyne Phase Splitters can anyone tell me what the effective source resistance (impedance) is in terms of tube parameters and RL = RK resistance value.

The 1960 article by Albert Priesman seems to suggest that the way to think of this is that the "effective" source resistance at both anode and cathode is rp + (1+mu)(RL+RK) so long as RL=RK and the effective source voltage is considered to be mu x Vgk.

Since we know that the gain of this thing is actually close to 1 which implies an effective source voltage approx. equal to vgk this requires some mental gymnastics to visualise.

Considering it as an approximately unity gain circuit and adjusting the above to suit would suggest that the effective source impedance is approx rp/mu + RL + RK OR 1/gm + RL + RK

Can anyone confirm the "maths interpretation" of what Priesman wrote and/or my interpretation of the same.

I have a Cathodyne driving 2 pairs of EL34 in Triode Mode. To improve the drive (at high frequencies in particular) I need a lower effective source impedance.

That would suggest either ECC99 (what I'm using now) or 6H30 at higher current setting and correspondingly reduced RL and RK is the way to go. This would seem to be right intuitively.

Does this seem right to you experts or have I missunderstood the Preisman article or screwed up the maths.

Should I perhaps just forget the B maths and analysis - stick on the bench, make the changes and see what happens?

Cheers,
Ian
  Reply With Quote
Old 19th October 2005, 06:26 AM   #25
diyAudio Member
 
Join Date: Jan 2003
Location: Sweden
Hi,

I have both measured and simulated the Cathodyne Phase Splitter and have confirmed that the Preisman calculations are correct, it is not that difficult to do.

However given that the gain to each output is normally given as u/(u+1) the output impedance will rather be (rp+Zk)/(u+1) i.e. much lower than you would expect, (it is described in the article, look at figure 3 and the surrounding text).

Do what I did, do some simulations in Spice or build one or 2 phase splitters and measure and you will find the results I did.

The reason I did this was because I earlier measured the open loop response of my OTL and didn't discover the poles I expected, the poles due to the phase splitter was much higher which of course was due to that the output impedance of the phase splitter was much lower than what I assumed earlier.

My measurementy method was to use different values of load impedances, (but equal for cathode and anode outputs) and comparing the voltage, from the voltage difference I calculated the output impedance.

The confusion around the cathodyne is probabaly due to that some want to measure the output impedance of cathode and anode independantly and then they are indeed very different, however when the load impedances are seen as independant it is wrong to talk about this circuit as a phase splitter, isn't it?

To your questions:

Quote:
I have a Cathodyne driving 2 pairs of EL34 in Triode Mode. To improve the drive (at high frequencies in particular) I need a lower effective source impedance.
Quote:
From the above - that would suggest
1) rp NOT that critical, it will be small compared to the other term
2) lower mu is an advantage
3) BUT mostly lower RL and RK values => higher tube current
You conclusions are correct but only if you not consider that that particular definition of output impedance also implies that the gain is u*Z/(rp+(1+u)Z+Z).

Given that the gain is most often written as u/(u+1) and the output impdance as I wrote above as (rp+Zk)/(u+1) you can draw the following conclusions:

rp is as critical as the anode or cathode resistors
High u is beneficial, (as long as the load impdances are equal but lower u keeps better balance if the loads are unbalanced, e.g class A2)

I use 12BH7 or 6SN7 as phase splitter as I think that fulfill the above requirements to a reasonable compromise.

Regards Hans
  Reply With Quote
Old 19th October 2005, 06:48 AM   #26
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
Hans,
THANK YOU !!!

So I want higher mu, lower RK and to a lesser extent lower rp (since for any sensible tube selection rp will be 1/4 off RK or less)

The higher mu will be handy since the whole amp is a bit short of gain and the other half of the tube is the input common cathode amp.

So I'll try
1) 6N1P (mu=35, rp = 8K)
2) 12AT7 (mu=70, rp =8K)
and drop the anode and cathode load resistors to the extent possible consistent with conservative power dissipation limits. Actually will try the 12AT7 first since that doesn't need any heater rewiring.

Have ONLY last night installed B2 Spice on my home machine so may try simulating as well.

Cheers,
Ian
  Reply With Quote
Old 19th October 2005, 07:13 AM   #27
diyAudio Member
 
Join Date: Jan 2003
Location: Sweden
Gingertube wrote:
Quote:
drop the anode and cathode load resistors to the extent possible consistent with conservative power dissipation limits.
For me the choice have always been dependant on the output level needed, higher anode resistors of course means higher available voltage swing, but as you say the lower the better for drive capability and frequency response.


Regards Hans
  Reply With Quote
Old 19th October 2005, 09:06 AM   #28
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
There's an argument to be made for low mu if there's a chance that the stage being driven will leave class A.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 20th October 2005, 12:21 AM   #29
diyAudio Member
 
Join Date: Jan 2003
Location: Sweden
Quote:
There's an argument to be made for low mu if there's a chance that the stage being driven will leave class A.
Yes that is correct and I also described that in an earlier post:

Quote:
High u is beneficial, (as long as the load impdances are equal but lower u keeps better balance if the loads are unbalanced, e.g class A2)
Morgan Jones have made a analysis of the Cathodyne in his book, (3rd edition) where he draw the same conclusion, i.e. that low mu is beneficial in the case there is risk of the output tubes drawing gridcurrent.

Regards Hans
  Reply With Quote
Old 20th October 2005, 12:32 AM   #30
diyAudio Member
 
Join Date: May 2005
Location: Pretoria, South Africa
After that good analysis by Hans perhaps just a practical point, i.e. it must be kept in mind that rp can change with the load resistors in the sense that it is dependant on Ia. I also like using the ECC81 here, but unfortunately do not have parameter/Ia graphs for that. Perhaps an illustration using ECC88 parameters might be interesting (mu about 29 and Gm about 3 mA/V here):

For a tube Va-k of 150V (where my graphs are valid) and h.t. of 400V, it is found that:
For RL=RK=33K, Ia=4.4 mA and rp=5.5K

For RL=RK=47K, Ia=2.6 mA and rp=8.5K

For RL=RK=100K (rather high), Ia=1.3 mA and rp=12K

This means that the (rp+Rk) term does not change all that much.

But Hans: You conclude that rp is as critical as RK (or RL), but according to the above it forms about 12% - 18% only of RK (RL). Do I misunderstand you here?

Also, Gingertube, the ECC81 has a mu of about 57 and rp of about 18K at the currents normally used, which make things a bit more critical - although, as said, I have often used it (I normally do not drive output triodes directly from a cathodyne, which eases matters somewhat for me).

And what about the E182CC? It can be used at quite a high current (mu about 22 and rp about 3K at 9 mA). One can of course go on suggesting tubes ad infinitum.....
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
How to figure gain on split-load with positive feedback? GordonW Tubes / Valves 4 21st May 2008 06:24 PM
Cathodyne Phase Inverter question woody Tubes / Valves 3 23rd July 2007 07:43 AM
Fourier transforms (split from 25W class A into 1 ohm resistive load) Steve Eddy Everything Else 20 20th April 2004 04:12 PM
Differential amp, Long tail, Split load - what difference in sonics? arnoldc Tubes / Valves 3 2nd February 2004 12:23 AM


New To Site? Need Help?

All times are GMT. The time now is 06:59 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2