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Dual Simple SE's in Push Pull w Differential Input

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My dac has a balanced output, and I have been recently messing around lately building balanced linestages.
My first one is the zen balanced line stage, by Mr. Pass:
http://www.passdiy.com/pdf/balzenpre.pdf

I plan on building some variations, and possibly a tube version after that.

My question is, if I already have a line stage with inverted outputs capable of around a 50Vpp swing, can I just plug the outputs right into the Simple PP circuit as seen in the attached, thereby bypassing the phase splitter?
 

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Assuming that your line stage could actually deliver the needed drive level with a reasonable amount of distortion, the method you describe should work, but I have not actually tried it. Most pentode P-P amps require some negative feedback to control the output impedance, and there would be no way to connect it with your proposed circuit. The amp could be operated in triode mode where it might be possible to operate without feedback, but the power output will be lower.

There is another possible solution, which I have done. The Simple SE board could be operated in P-P mode with your proposed drive system. Only normal drive levels are required. Use the two out of phase signals from your balanced line stage to drive each channel, and connect one P-P opt to the board with the red lead on B+ and a plate lead to the plate of each channel. In class A triode mode you can get 12 to 20 watts depending on tubes and load impedance. The "normal" 6600 ohm OPT nets about 35 watts leaving class A only on signal peaks. Again there is no provision for global negative feedback, but I have not found it necessary if you stick with triode mode operation. Of course two boards would be needed for a stereo amp.
 
Assuming that your line stage could actually deliver the needed drive level with a reasonable amount of distortion, the method you describe should work, but I have not actually tried it. Most pentode P-P amps require some negative feedback to control the output impedance, and there would be no way to connect it with your proposed circuit. The amp could be operated in triode mode where it might be possible to operate without feedback, but the power output will be lower.

There is another possible solution, which I have done. The Simple SE board could be operated in P-P mode with your proposed drive system. Only normal drive levels are required. Use the two out of phase signals from your balanced line stage to drive each channel, and connect one P-P opt to the board with the red lead on B+ and a plate lead to the plate of each channel. In class A triode mode you can get 12 to 20 watts depending on tubes and load impedance. The "normal" 6600 ohm OPT nets about 35 watts leaving class A only on signal peaks. Again there is no provision for global negative feedback, but I have not found it necessary if you stick with triode mode operation. Of course two boards would be needed for a stereo amp.

Ok. So if I use the 2 board SE solution, I could use the same OPT as you have spec'd on your site for a single SE, and only 1 OPT per channel, correct?
I understand I would need 2 boards.

Of course, I could leave well enough alone and use what looks like an awesome PP kit.

I am just wondering what a PP amp sounds like without the phase inverter stage- if it is somehow cleaner. What leads me to this question is that I have a really cool original Dynaco ST-35. It sounds great, but what bothers me about it is, with complex music, things get a little blurry, and there seems to be some phase issues. When I look at the schematic, the output stage is so damn simple, that I think something must be getting confused in the input stage. I am thinking the simpler the input stage, perhaps the better the phase information will be preserved. But my assessment maybe nonsense.

It seems like most decent DACs these days have balanced outs, yet most amps have single ended inputs. Seems like it might be nice to use balanced since I have them

Anyway, I love your site, and will be ordering something from you soon. Keep it up!
 
...or one could use a line transformer. See this post. Though, I do like the idea of using two Simple-SE boards to form a differential input Push-Pull amp...

~Tom

Tom, thanks for the link to that thread. There are so many threads on this site, even searching pretty hard its easy to miss. So what did you decide in the end? Did you go transformer input w your 300B?

Anyway, I am considering going with the dual simple SE route, but maybe I am making too big a deal out of using a differential input. Maybe its not that big a deal, and I go with the simple PP... I'll have to think on it a bit.
 
If I use your simple PP in triode mode, how many watts might you expect? And how about for pentode?

The power output depends on which tubes you use and what voltage you run them at. With good tubes (I use JJ's) and about 350 volts I get 8 watts in triode and 17 watts in pentode.

I could use the same OPT as you have spec'd on your site for a single SE, and only 1 OPT per channel, correct?

NO! You are essentially making a P-P amp, therefore you need to use a P-P transformer.

George: please check your email / forum PM

I know you (and a dozen others) have been trying to contact me. The place where I work has instituted a new policy that does not permit "any activity that supports a personal business" from company property which includes the phone, computer and internet connection. This makes it rather difficult to answer anything related to selling boards or components since I have been there for 10 to 12 hours per day almost every day. I had 2 days off in July. I should get to yours tonight or tomorrow.
 
Pentod push pull without phase inverter

Hi

I have worked on all sorts of tube amps, including Hammond and other organ amps, from the 1930's on up.

A lot of Hammond amps have no phase inverter.

The organ preamp drives a line out transformer, that gives a balanced out through the big cable to the Leslie amp. There are about seven different Leslie or external speaker amps all the way up to four #45 tubes in PP parallel

By having all the gain and drive levels worked out, you can balance everything so the amp never goes into distortation.

Also, a number of these amps run the input through a 12au7/6sn7 balanced stage, with the cathodes hooked together. I think they take a feed off the speaker out, sometimes this is balanced, and apply Neg feedback to the 12au7/6sny grids, or, if split cathodes, the cathodes.

So you could keep your speaker outs balanced, or use the 4 and 16 ohm taps and balance Neg back to the buffer stages.

You could also use a line in trans, and if there was another winding, drive it with speaker out. You would have to find a transformer with 3 windings, but very smooth sound that way.

Using a balanced input buffer to drive the pentodes, easier, just drive the cathodes from the 4 and 16 ohm taps in Negative

gEorge
 
NO! You are essentially making a P-P amp, therefore you need to use a P-P transformer.

I know you (and a dozen others) have been trying to contact me. The place where I work has instituted a new policy that does not permit "any activity that supports a personal business" from company property which includes the phone, computer and internet connection. This makes it rather difficult to answer anything related to selling boards or components since I have been there for 10 to 12 hours per day almost every day. I had 2 days off in July. I should get to yours tonight or tomorrow.

I realized my mistake with the OPT after I read your post more carefully, but thanks for clarifying.
Sorry 'bout that.

No problem about the response time. I understand you have a day job, and appreciate all your work on these projects.
 
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I think I am going to go ahead with this project. I plan to use (2) simple SE boards, so that I have a balanced input. This will essentially be creating a PP amp from 2 boards.

I plan on using KT-88s as output tubes. I will use a single power transformer and tube rectifier to power both boards.

The essential parts so far, may be:
Power transformer: Hammond P-T374BX
Output Transformers: Edcor CXPP25-8-5K
Filter Choke:Hammond P-T159Q

I do have a question about the tube rectifier as well as the final B+ voltage. I did find a few threads discussing the different tubes, but I am still unclear on which one I should use. Also, I am unclear on how much forward drop the tube rectifier will give me, and what my final B+ will end up being given a 375v transformer. Also, I am unclear on the power handling capabilities of the different tube rectifiers. Any comments on tube rectifier choice, and B+ for KT88's?

I'll use this thread to post my progress. Any comments are very welcome. I work as an electronics technician by day, but this is my first tube project, so I will no doubt have some very naive questions.
 
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Output Transformers

For this dual board project, and output transformers wired as George describes in post #2 of this thread, what transformers would be recommended? I plan to use KT88 tubes.

Looking at the Edcor push pull OPTs, I am thinking that an 7.6 or 8K transformer would work the best, since 1/2 of that value would be closest to the 5K OPT recommends with the single board single ended output.
EDCOR - Browse Series

How much headroom do I need in the wattage department? If I get 15W in triode mode, I would think a 25W OPT would be fine. Even if I run it in UL at 30W or so, I doubt I would ever really listen to louder than 20W. My speakers are 98db. Do I lose a lot of detail if I go up to 45W?

I suppose my choices are:
25W, 7.6K
25W, 8K
45W, 6.6K

Thanks
 
OPT transformers for this project

If I am using a 360-0-360 transformer, KT88's, and a 7600 PP OPT, what value of cathode resistor would I use, if any? If I look at a design like an ST-70, their plates are connected directly together, and then through a resistor. I could not tell from your response if the plates are tied directly together, or if each tube is using an individual cathode resistor. Could you clarify this point?

Also, as I understand it, a SE design dictates that there will be a DC offset at the output, thats why we need an air gapped transformer to handle the DC. But in the case of running 2 SE in PP mode, this is no longer the case, right? I believe the answer will have something to do with the value of the cathode resistor. Could you comment on this?


Assuming that your line stage could actually deliver the needed drive level with a reasonable amount of distortion, the method you describe should work, but I have not actually tried it. Most pentode P-P amps require some negative feedback to control the output impedance, and there would be no way to connect it with your proposed circuit. The amp could be operated in triode mode where it might be possible to operate without feedback, but the power output will be lower.

There is another possible solution, which I have done. The Simple SE board could be operated in P-P mode with your proposed drive system. Only normal drive levels are required. Use the two out of phase signals from your balanced line stage to drive each channel, and connect one P-P opt to the board with the red lead on B+ and a plate lead to the plate of each channel. In class A triode mode you can get 12 to 20 watts depending on tubes and load impedance. The "normal" 6600 ohm OPT nets about 35 watts leaving class A only on signal peaks. Again there is no provision for global negative feedback, but I have not found it necessary if you stick with triode mode operation. Of course two boards would be needed for a stereo amp.
 
The difference between SE and PP output trannies is PP trannies don't need the gap as there are two opposing DC currents flowing through it at idle condition. You need to think of the Simple SE as a PP amp, not an SE amp, for your application. You will find it very difficult if you don't know the purpose of the cathode resistor. You may need to read/learn some more theory to pull this off. Maybe start here then read PP and triode gain stage.
 
The difference between SE and PP output trannies is PP trannies don't need the gap as there are two opposing DC currents flowing through it at idle condition. You need to think of the Simple SE as a PP amp, not an SE amp, for your application. You will find it very difficult if you don't know the purpose of the cathode resistor. You may need to read/learn some more theory to pull this off. Maybe start here then read PP and triode gain stage.

Excellent reference. Thank you. I must have missed that one somehow.
 
If I am using a 360-0-360 transformer, KT88's, and a 7600 PP OPT, what value of cathode resistor would I use, if any?

The Simple SE is a cathode biased amp and thus requires a cathode resistor to control the current through the output tubes. This doesn't change when the amp is wired for P-P operation. I would start with a 560 ohm resistor in this case. In fact the PC board should be assembled exactly as if it was used in a SE configuration. The conversion to P-P is handled by the way the output transformer is connected to the board.

No DC offset is present in the transformer if the tubes are matched. Unmatched tubes will cause a small offset, but unless they are grossly mismatched it should not be a problem.

This thread has reminded me that my sound card has differential outputs. If I get some time this weekend I will try it.
 
Gapped OPT of SE amp has better linearity, so that'd be a plus (if you've already owned them). Tie the secondaries of 2 SE OPT in series or parallel to get the proper ratio/load is doable (just keep them floating from ground).

So, just feed differential signal into 2 SE channels and tie them together at the output of OPT, you're done.

One step further, make these 2 SE channels fed by a common HT supply, then you get a balanced and almost constant current draw throughout the whole operating range - a pure class A pp - that's one of the major philosophies of the Amity designed by Lynn Olson. :)

I've thought about doing that for quite some time, but don't have enough amps on hand.
 
Transformer Question

OK, so I have almost all the parts to put this together. I am waiting on the chassis, but I have a question about the power transformer. Remember I am using two SE boards in PP.

I have a dynakit PA060 transformer.
Product Detail: PA060 POWER TRANSFORMER
360-0-360 @ 300ma.

I was anticipating using KT88's. For this application I would need 4. It seems that the idle current biasing requirements are around 75-80mA. Already, I am exceeding the ratings of the transformer (80 X 4). Is this an absolute no-no?

In that case, I can go with (4) EL84's instead, which I believe I would bias around 50mA, so I think I would be in OK shape.

Also, I have C354 choke, which is rated at 200ma. But I figured since this is the stock choke in an ST-70, I would be alright.

Thanks in advance for your responses and comments.
 
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