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 Post subject: Question
PostPosted: 10 Jan 2019, 11:11 
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Hi Everyone, Just playing around with "odd" thoughts....(so what's new) and thought the one in the image might have some interesting properties. Has anyone seen anything like it? Or have some thoughts on it.

Good listening
Bruce
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 Post subject: Re: Question
PostPosted: 12 Jan 2019, 07:43 
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Well now Mr Oddwatt you are the master at making things work. But from my limited understanding of totem pole or active load you have two loads. One in series with the other.

What if you took the load from the bottom tubes cathode resistor and just made it capacitive coupled. I know you were after OPT coupled but what if using power tubes and make it like a cathode follower. I think you would have to have a very low cathode resistor but is it possible?

And what of your oddwatt suggestion. Can you model it?

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 Post subject: Re: Question
PostPosted: 12 Jan 2019, 07:46 
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Oops, did you mean a driver stage? Possible?

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 Post subject: Re: Question
PostPosted: 12 Jan 2019, 13:46 
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gofar99 wrote:
Has anyone seen anything like it? Or have some thoughts on it.
Yes, I have seen this configuration. It is simply the use of an active load. However taking the output off a series fed transformer is a different kind of twist.

The goal in this situation is to define a stage with a much higher AC load without the attendant voltage drop in the load. This is generally done to increase amplification and reduce distortion. The upper tube is biased such that the DC current matches the desired lower tube's bias point, but the DC drop across the load triode can be made small. However, the AC resistance consists of the upper biasing resistance in series with the AC plate resistance of the upper tube. In this way, the AC load can be made very high and the amplification of the lower stage approaches the amplification factor µ very closely.

However, in general one uses a pentode for the upper tube because it's natural Rp is extremely high. You do however need to be careful because the inter-electrode capacitances of the tube can cause rolloff in the upper audio frequencies. This configuration is normally used when a high sensitivity and low distortion servo amplifier is required. It's usually not seen in audio amplifiers. due to bandwidth limitations.

By putting an unmonitored load in the B+ path, this is not a shunt regulated amplifier but simply a semi active load on the amplifier tube. In this configuration it is doubtful that this would provide any benefit. Also, I would think that putting this active load in series with the transformer would act as an AC voltage divider and seriously reduce the AC signal into the transformer. In general, a transformer load accomplishes the same thing as the active load in that it exhibits a very small DC load but a much larger AC load. Hence the reason that, on power stage load line designs, we usually only plot the AC load line and just assume the DC load line is perfectly vertical.

Overall, I'm not sure why one would take this approach. Were you trying to solve a particular problem or just tyring out new configuration?

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 Post subject: Re: Question
PostPosted: 12 Jan 2019, 16:54 
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What of a power amp with the output from the lower tubes Rk with capacive coupling to the speaker. I've build three of these designs as preamps and think them extremely good rivalling a 4S. What if you used power tubes for the same design to drive heavier loads?

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 Post subject: Re: Question
PostPosted: 12 Jan 2019, 20:32 
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Hi, Somehow my reply got lost.....Anyhow I was just pondering. The idea was to be able to use the positive aspects of a SRPP/active load (or whatever else you want to call it) to drive a push pull pair of output tubes without using any capacitors. Putting the trannie in the cathode of either triode upsets the benefits...so I wondered if you put it in the B+ feed what would happen. My suspicion was that it would not do what I wanted, but I thought I would inquire of other folks. I can do a long tail pair or split load stage ...but they have caps or a push pull first stage with trannie to drive the second one. I am not too fond of LTPs as they usually need a goodly amount of NFB to be linear enough for my tastes.

Good listening
Bruce

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 Post subject: Re: Question
PostPosted: 13 Jan 2019, 05:08 
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I'd say the srpp with transformer will allow higher votlage gain, swing and current to output valves.
It does somehow feel like a schematic with lower distortion but transformer may degrade sound quality way more compared to using hifi caps like ERSE pulse X.

You can always instead of a LTP go add extra triodes on top and you will get a LTP but with both outputs of SRPP charteristics.


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 Post subject: Re: Question
PostPosted: 14 Jan 2019, 16:53 
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Quote:
"I wondered if you put it in the B+ feed what would happen. My suspicion was that it would not do what I wanted"


I think it would do what you wanted, direct drive from transformer while having the power grid biased to ground of a refrence.


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 Post subject: Re: Question
PostPosted: 15 Jan 2019, 16:20 
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I thought the orignal scheamtic was correct and is the way you will use a SRPP by replacing the transfomer with resistor. Apparently not.
With the orignal schematic I'm in between it working and not working.
It may, maynot act like a common cathode with a upper valve to give it distortion canceling charteristics, I would recomend testing by building one replacing the transfomer with a resistor and observing the signal.

I still belive we can make a LTP SRPP if you connect the load as you would normally do in a standard SRPP.

EDIT: Since the SRPP reiles on the upper triode to output a inverse of the lower triodes delta grid to cathode voltage to cancel the distortion on sourcing current into load.
And the load connected between upper valve gride cathode sees a inverse coppy of the lower triode delta grid cathode voltage during sinking.
I don't think you will get any effect of distrtion reduction and the overall distortion would be that just of a single common cathode gain stage if it were to work as orginal configuration with b+ traine.

Anotherway to think about it is if it were to operate and work as a common cathode driving the b+ transfomer then the change current is purely set by the lower valve and the upper valve has little or nothing to do with it. This means the distortion should be similar or same of that single common cathode.


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