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Old 8th July 2008, 03:21 PM   #1
regal is offline regal  United States
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Default Tube line stageoutput impedance

Would it be reasonable to use a tube line output stage as a totem pole with an output impedance of 3500 ohms?

I know there are better options but I am trying to get away from the existing SRPP and stay in the confines of the PS.

The head amp followin this has an input impedance of 47k.

Are there other issues doing this beside the 8% drop in Vrms?
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Old 8th July 2008, 03:53 PM   #2
kevinkr is offline kevinkr  United States
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I'm not totally sure I understand your question, what topologies do you define as a totem pole. (a term commonly used in ss technology, and generally not used with tubes.)

Stacked topologies (your totem pole) would include SRPP, Mu follower, and the White cathode follower, as well as a standard CF with a tube based CCS for the load. Of these only the white CF and the SRPP (for specific load conditions) would be considered pushpull and hence somewhat analogous to a ss totem pole output.

So, what exactly do you have in mind?

FWIW you would have to work pretty hard to get such a high source impedance out of any of these topologies, you could easily reach 1K or less with commonly available tubes.

You haven't provided any information on available supply voltage, what's driving this "totem pole" stage or any particular reason for rejecting the SRPP which is simple and works fairly well - where it may not work so well is with a load that presents a varying load impedance.
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Old 8th July 2008, 04:06 PM   #3
regal is offline regal  United States
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A totem pole is where the output is taken at the plate of the lower triode instead of the cathode of the upper triode (SRPP.)
Zout=Ra of the tube.

A totem pole is the same as the Aikido input stage.

Broskie goes into great depth as to why an SRPP is the wrong circuit for a line stage. It is push-pull and has a certain ideal load, which I calculated as only 7k with this operating point. Deviate from this load and distortion goes up which I measured to actually occur.

My best bet may be a Mu Follower, I think the 188V B+ with the 6DJ8's may be enough to pull off a mu-follower.
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Old 8th July 2008, 04:51 PM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by regal
A totem pole is where the output is taken at the plate of the lower triode instead of the cathode of the upper triode (SRPP.)
Zout=Ra of the tube.

A totem pole is the same as the Aikido input stage.

Broskie goes into great depth as to why an SRPP is the wrong circuit for a line stage. It is push-pull and has a certain ideal load, which I calculated as only 7k with this operating point. Deviate from this load and distortion goes up which I measured to actually occur.

My best bet may be a Mu Follower, I think the 188V B+ with the 6DJ8's may be enough to pull off a mu-follower.
A mu follower should work given 6DJ8 and the supply voltage you mention. One thought would be to do a choke loaded mu follower. (Basically the SRPP topology with the upper cathode resistor replaced with a choke of both appropriate inductance and dcr - works well with 6SN7/7N7, but have not tried this with the 6DJ8 - the low dcr required might make this impractical.)

I don't read the tubecad blogs much so I should have suspected that the usage you gave for "totem pole" came from there, maybe it's just me, but I don't much like the term..
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Old 8th July 2008, 05:38 PM   #5
regal is offline regal  United States
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With the Mu-Follower high much of B+ should I consume with the two resistors connecting the tubes (top cathode+bottom plate.)

I don't have enough B+ to dedicate the typical 1/3 B+. I was thinking 10V-15V's would be enough since I am only outputing 1.1 Vrms.
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Old 8th July 2008, 07:05 PM   #6
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This is the "ultimate" mu-follower that kevinkr suggests.
Zout ca 200ohm and gain over 30dB. It will surely put out more than 30Vrms into a HiZ load.

http://www.eflatjump.se/CHOKEMU.PNG
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Old 8th July 2008, 08:44 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by regal
With the Mu-Follower high much of B+ should I consume with the two resistors connecting the tubes (top cathode+bottom plate.)

I don't have enough B+ to dedicate the typical 1/3 B+. I was thinking 10V-15V's would be enough since I am only outputing 1.1 Vrms.

Should work fine, remember it is a floating circuit node and the actual ac voltage across that resistor is very close to 0. Into your stated load I expect you could swing at least half the available supply voltage if not more..
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Old 8th July 2008, 09:08 PM   #8
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Old 9th July 2008, 01:40 PM   #9
regal is offline regal  United States
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Quote:
Originally posted by revintage
This is the "ultimate" mu-follower that kevinkr suggests.
Zout ca 200ohm and gain over 30dB. It will surely put out more than 30Vrms into a HiZ load.

http://www.eflatjump.se/CHOKEMU.PNG

They must have changed the LL1668. It is now 680 ohm DCR and 100H. I don't see how you could run that with the bottom tube at 330 ohm Rk, would seem the balance would be off quite a bit.
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Old 9th July 2008, 01:57 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by regal



They must have changed the LL1668. It is now 680 ohm DCR and 100H. I don't see how you could run that with the bottom tube at 330 ohm Rk, would seem the balance would be off quite a bit.

You might have to get something wound. Electra-Print could probably provide you with something suitable. I don't think you need more than 60H with a 6DJ8, but the choke would be bulky.

A regular mu follower based on the 6DJ8 would be the first thing to try.
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