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Harman Kardon Citation II PI question

Printer2

Member
2010-04-02 6:34 pm
I am trying to understand the PI circuit in this amp. I am a little confused about the output to the power tubes, one side is taken from the plate which has a 12k resistor on it while the other side is taken on the other side of a 4.7k resistor in series with a 8.2k resistor. Are they using these two resistors as a split load in order to balance the two outputs to the power tubes?

Also the two screens on the on the 12BY7A's are tied together and go through a 22k resistor to the supply. Are they running in triode mode? Or is there something funny that goes on when they are run through a common resistor? I do not know of any other amps that use pentodes in the PI section so I have not come across an explanation on this type of circuit before. Thanks in advance for your help.

[IMGDEAD]https://www.bassboy.com.au/getreel/classicamps/files/amps/harmkard/ctation2/circuits/cct.gif[/IMGDEAD]
 
Your first question is answered by your comment. The difference in resistor is to balance the output. This appears to be a hybrid of the LTP phase splitter.

I can't answer your second question as easily. I am not familiar enough with this circuit. It looks like it is just applying a screen voltage and is still a pentode connection.
 
I am trying to understand the PI circuit in this amp. I am a little confused about the output to the power tubes, one side is taken from the plate which has a 12k resistor on it while the other side is taken on the other side of a 4.7k resistor in series with a 8.2k resistor. Are they using these two resistors as a split load in order to balance the two outputs to the power tubes?

Yup, that's what they're doing. The phase splitter is an LTP with a passive tail load, and it looks to be a fairly small one at that. Given that VTs don't have the gain capability that transistors have, you end up with an inherent phase imbalance since the driven side sees: Vgk= Vi - Vtail

Whereas the passive side simply sees Vgk= -Vtail

If you don't have an active tail load, you won't see phase-to-phase balance, so you need to force it with unequal plate load resistors.

Also the two screens on the on the 12BY7A's are tied together and go through a 22k resistor to the supply. Are they running in triode mode?

No, pure pentode operation here.

Or is there something funny that goes on when they are run through a common resistor?

Produces nearly constant current through the screen dropping resistor, since as one side is pulling a heavier current, the other side is pulling less current. Since it's almost pure DC, you don't need to bypass the screens since the AC cancels out.

I do not know of any other amps that use pentodes in the PI section so I have not come across an explanation on this type of circuit before.

This comes close: Le Renard Main Schemo

Cascoding is quite similar to using pentodes in some respects.
 

Printer2

Member
2010-04-02 6:34 pm
Got to love this place, a lot of smart cookies.
Produces nearly constant current through the screen dropping resistor, since as one side is pulling a heavier current, the other side is pulling less current. Since it's almost pure DC, you don't need to bypass the screens since the AC cancels out.

I kind of thought the currents would sum together but did not know if that would be a good or bad thing. I wonder why everyone uses separate screen resistors in p-p amps when this would seem to be an advantage. Thanks for your answers.
 
If the splitter output signal gets big enough to drop the plate voltages below the screen voltage, it will start producing odd harmonic distortion big time (signal compression). For low plate voltage swings though, the un-bypassed common screen resistor probably acts similar to additional tail resistance. The screens will intercept a near constant fraction of the cathode currents up till the plate voltages drop down near the screen voltage. Any tendency to draw more than constant plate current (the sum) will drop the screen voltage, turning off the tubes somewhat, just like the tail resistor voltage rises to turn them off. Would help some to DC auto-bias the splitter tubes too, improving the constant current regulation.
 
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Not too concerned about distortion, actually part of the reason I am looking into this. Just for the heck of it I would like to do an all pentode amp.

There are 6 adjustments on a "Deuce": a bias trim pot. for each "final" and an AC balance trim pot. for each channel. Perfectly matched 12BY7s in the splitter positions are nice, but not essential.

It's very hard to better Stu Hegeman's effort in an all pentode amp, other than adding Jim McShane's modernizations. Jim takes advantage of parts Stu did not have access to.

Cloning the Cit. 2 is no easy matter. The O/P trafos are among the BEST ever made. Bandwidth is phenomenal. Edcor and Hammond stuff are dead out. The only off the shelf item that is remotely suitable is Plitron's 4004-01, which is somewhat shy in the power handling dept. Be sure to protect against infrasonic noise induced core saturation.
 

Printer2

Member
2010-04-02 6:34 pm
There are 6 adjustments on a "Deuce": a bias trim pot. for each "final" and an AC balance trim pot. for each channel. Perfectly matched 12BY7s in the splitter positions are nice, but not essential.

It's very hard to better Stu Hegeman's effort in an all pentode amp, other than adding Jim McShane's modernizations. Jim takes advantage of parts Stu did not have access to.

Cloning the Cit. 2 is no easy matter. The O/P trafos are among the BEST ever made. Bandwidth is phenomenal. Edcor and Hammond stuff are dead out. The only off the shelf item that is remotely suitable is Plitron's 4004-01, which is somewhat shy in the power handling dept. Be sure to protect against infrasonic noise induced core saturation.

Sorry, left out one important word. All pentode guitar amp. Bandwidth, what is that? :)
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Printer2,
Well, given that it's an HK tube amplifier, it would seem logical to do a reasonably good job on it. I have one here I'll be rebuilding for a very good friend. These are worth more nicely restored than as a different rebuild.

There is no shortage of amplifiers using real pentodes in the output stage. The less expensive units that use 6BQ5As would be a nice proof of concept type deal without ruining a larger amplifier. Not to mention that matching a new set of tubes is only a dollar or two (Canadian) extra for matching. May as well if you are replacing outputs.

-Chris
 
Sorry, left out one important word. All pentode guitar amp. Bandwidth, what is that? :)

The Cit. 2 is very much a HIFI design. Among his many talents, Stu Hegeman was a highly capable recording engineer. Stu KNEW what live music sounded like and he was determined to come close in his amps. That circuitry is EXTREMELY linear. You could intentionally unbalance the setup, but I don't know if a "picker" would like the tone.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Eli,
How do you find the Plitron output transformers? I have been toying with the idea of trying them out, but I was unsure about the quality. Plitron doesn't have a long history in tube amps.

Hi Printer2,
Guitar amp??
Surely you aren't going to tear down an HK II to make a guitar amp out of it! Please don't do that.

There are no shortage of EL34 amps out there for guitar, then there is the tone stack which is totally unlike anything for audio. Those outputs are very likely not what you want either. Most guitar amp output transformers are undersized by audio reproduction standards.

-Chris
 

Printer2

Member
2010-04-02 6:34 pm
Oh no! :eek: I would not butcher a perfectly good quality amp just to crank up to ten and feed some 12" that is willing to break up just because some hot pickup passes by. What kind of guy do you think I am?

Actually I have a Scott 299 that has not seen an electron in over twenty years but I could not bear to part it out even though the output transformers would have done just nicely in a 6V6 amp I want to build. Ended up buying a new Hammond to save the Scott.

No this is for my own build, I have a few nifty ideas I am chasing down (Won't delude myself that any are original, everything under the sun has already been done, more just an interesting packaging exercise.) and an all pentode amp is one of them.
 

Printer2

Member
2010-04-02 6:34 pm
The Cit. 2 is very much a HIFI design. Among his many talents, Stu Hegeman was a highly capable recording engineer. Stu KNEW what live music sounded like and he was determined to come close in his amps. That circuitry is EXTREMELY linear. You could intentionally unbalance the setup, but I don't know if a "picker" would like the tone.

Depends on the picker. Some like single ended amps because they give a strong second harmonic distortion when overdriven, others like push pull amps where the second harmonic is canceled out and you end up with more third. And then there is biasing preamp tubes in the center (Fender) which gives a more symmetrical clipping or biasing either hot (Marshall) or even another cold and then combining the signals to get a different tonal palette. Cathode bias, fixed bias, two more flavors. All kinds of fun.
 
Chris,

I have no practical experience with Plitron O/P "iron". All I know is that the spec's of the 4004-01 are fairly close to what Hegeman's design requires. OTOH, Menno Vanderveen, who designed the trafo, is nobody's fool.

FWIW, I don't think you are taking much of a risk trying Plitron O/P trafos out. There are enough successful projects on the WWW to allow that thought to occur.
 
Yup, that's what they're doing. The phase splitter is an LTP with a passive tail load, and it looks to be a fairly small one at that. Given that VTs don't have the gain capability that transistors have, you end up with an inherent phase imbalance since the driven side sees: Vgk= Vi - Vtail

Whereas the passive side simply sees Vgk= -Vtail

If you don't have an active tail load, you won't see phase-to-phase balance, so you need to force it with unequal plate load resistors.

Very good explanation Miles, thank you. A lot of people are so used to seeing a dual triode LTP that the two discrete pentodes is confusing for a while - especially when you add in the multiple NFB loops.

Also, the 12K and 8.2K /4.7K resistors are the starting point - there is a 5K AC balance pot you adjust for the optimum balance.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi LinuksGuru,
Resurrected Sansui AU-111
Cool. I have an original, untouched AU-111 that I'm planning to restore some day. It works, but is suffering from old age (just like me). This will be a ton of work.

Hi Eli,
Thank you, and a fair appraisal.
I have some power transformers from Plitron, but they have poor regulation and let too much line noise through. I think that the only use for a toroid transformer would be for audio coupling and switching power supplies. Common mode filters too I think. I am not happy with their power transformers though. These transformers are designed for a tube preamp.

-Chris