Dual Simple SE's in Push Pull w Differential Input

[Anchan]

Originally Posted by tubelab.com

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.

[Anchan]

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.

[Anchan]

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

[Anchan]

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?

Originally Posted by tubelab.com

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.

[Ian444]

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.

[Anchan]

Originally Posted by Ian444

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.

[tubelab.com]

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.

[CLS]

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.

[Anchan]

Thanks for the replies. I'll order the OPT's on Monday, then I suppose I just kick back and wait. There is about a 4 week lead time on the Edcors.

I'll solder up the boards and prepare a chassis in the meantime.

[Anchan]

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.