[Arran]

I didn't notice this before, but the diagram shows the Chinese 6P3P drawn as a power pentode, not as a beam power tube. So is the 6P3P actually the same as a 6P3P/6L6G, and the diagram drawn incorrectly, or is it a Chinese version of a 6F6? I also noticed that the 6P3P is self biased, sometimes called cathode biased, so there is some flexibility in terms of swapping tubes.
  As for a shorted Zener diode causing the output to clip, I don't really know why, if you look at the average tube amp they usually take the screen grid voltage right after the second filter cap in the power supply, and have one leg of the output transformer primary connected to the same, the other leg being connected to the plate side. So in those cases the screen grid voltage is usually higher then the plate voltage, and I never noticed anything odd about the audio in those cases. To me it would have made more sense building a fixed bias supply with Zeners then a screen grid supply with Zeners, but then again the amps I'm thinking of didn't use a twin triode for a 1st audio stage with that cathode follower setup. 
Regards
Arran

[BrendaAnnD]

Did some performance testing on the finished channel tonight. Interesting results.

Using my scope and 8 ohm speaker (steering clear of the resonant points of the speaker), I got 20V peak to peak whether I used program material or my HP audio generator, before clipping began on the bottom half of the waveform. This gives me 50W p-p. 

Using the same speaker and instead using a 1Mohm DMM, at the same point I got 5.4V RMS, which comes out to about 3.6W RMS.

Using the following specs for my output tubes, it says I SHOULD get about 6.5W out with the voltages in this amp, which match the highlighted section almost perfectly.

I'm confused. :/

[morzh]

Brenda.

P-P voltage is twice the aplitude. Forget it. 20V pp is 10V p is 7.1V RMS is app 6W RMS at 8 Ohm.

[BrendaAnnD]

So I am getting pretty much the rated power output of the tube.. wish I could get that to be symmetrical. The positive peaks hit about 15-16V before clipping, but I took the measurement at the point where the negative peaks began to clip (then backed the level off enough for the clipping to completely go away.)

[Mondial]

Brenda, its going to be real difficult to get the 6L6 to clip symmetrically. Typically, a single ended amp has a lot of second harmonic distortion because of the nonlinearity of the tube grid-plate characteristic. The tube gain is highest at high plate current and lowest in the region of cutoff, which is what you are seeing. So the amp always clips on the cutoff side first.

If you look at the distortion vs. power output graph of the link below, you can see the single ended distortion at 6.5 W output is about 10% and almost entirely second harmonic. This is the non-symmetrical clipping that you are seeing. The 6L6 is not really great as a single ended amp.

https://frank.pocnet.net/sheets/127/6/6L6G.pdf

That's precisely the reason the 6L6 is normally used in push-pull. In the P-P configuration, the second harmonic is cancelled out and the load can be optimized for minimum third harmonic, which becomes the major distortion component. Looking at the data, the THD at full output drops from 10% single ended to 2% in push-pull.

You may be able to get the amp to clip more symmetrically by playing with the bias and/or screen voltage. I just noticed that you mentioned that your screen voltage is higher than normal. This may contribute to the clipping if the grid bias is not adjusted accordingly. With higher screen voltage you may need more negative bias on the control grid to center the bias point in the middle of the tube input-output characteristic, making the output clipping more symmetrical.

[morzh]

I think this is the reason A amps are used as low power drivers for sensitive speakers. Class A is rarely used in hi-fi where you need mor than 1-2W to drive some 105dB speaker which will be very loud even at this power level.
Pushpull can be used where hi-fi output is needed for larger power, lower sensitivity speakers, like today's compression types, whose power is easily over 100W but the sensitivity is just above 80dB which pretty much means that one needs 100W to produce the same pressure as the 100dB speaker would produce at 1W.

So what you have will shine with old style paper cone wide band speaker or a speaker system like any legacy Klipsch jorn-loaded type.

[TV MAN]

Brenda, I took voltage and resistance readings right after I finished the wiring. G2 on the 6L6 is 211v, plate is 310v, cathode is 10v. This is on a 120v ac line. I am quite curious as to what amp you built (or are building)??? As far as power output, it would interesting to check mine, but I have never tried that before. I have a newly acquired audio generator and can connect the output to a scope across the load (speaker). I had also wondered what the frequency response was. Some instruction here would be appreciated.

FYI, I found a set of 'seasoned' Sylvania 6SN7GT tubes worked very well. There were all kinds of peculiarities with the China tubes. I have been using the amp about every opportunity I can before I start writing down my experiences, my likes, dislikes, etc.

I do note that the transformers run cool (warm to the touch, but not a "wow" followed by a violent retreat of my hand).

[TV MAN]

Well, seems this went dead. Still would like know what kit you have Brenda.

[TV MAN]

Ok here it is!   I have been playing around with this amp and doing some experiments. I bought a set of Bose Acoustimass 5 speakers and set it up on my shelf over my bench.
   
I chose this system for several reasons. I didn't have room (or room in my wallet) for a nice set of Bozak speakers. Maybe someday... I know these are really home theater speakers, but I have heard them play and thought they sounded pretty good. I found these for less than $100. The satellite speakers sit on my shelf nicely and take up little space, and the (passive) subwoofer fits perfect under my bench, again taking up very little space. I don't know for sure, but I am guessing it uses a dual voice coil sub (I guess it could have 2 drivers). It worked so I didn't mess with it. But, someday, curiosity may overcome me and I will get out the screwdriver

Ok, so how does it sound? Well, in a nutshell, GREAT! I have it connected to a cd/FM Walkman type player with no EQ or tone control of any kind. I really don't need it with this great little setup. How loud? Well, as I mentioned earlier, this little amp rocks! No, it doesn't bass things off the shelves, but it does thump! Way better than I would ever have expected. Clean? Yep, I don't detect any distortion at any volume level. No, I haven't cranked it to that point either and don't plan to. But, it is loud. I don't detect any hum either, even when I put my ear up to the speakers. I also noticed these Bose speakers don't seem to pull on the amp. It doesn't struggle a bit.

I may eventually get a real preamp, like a PAT4 Dyna or something small and affordable to have tone controls and inputs for testing tuners and such on the bench. I wanted to run it for awhile before doing a write up on it. I do have an audio generator now that I may connect along with a scope and run the ranges to see how linear this amp really is. I am rather curious.

I did axe the original China made 6SN7GT tubes for noise reasons. One simply liked to hum, and the other liked to make funny noises. I settled on a set of older Sylvania tubes that I think sound great. Quiet and clean.    
   
   

Here are my sentiments so far on this amp project: The good...
1. It is a nice kit, and with a good set of instructions, very easy to build and enjoyable
2. It sounds good and clean
3. Hard to beat for the price
4. Decent output tubes come with kit
5. Comes well packed
6. Simple design, no PC boards

Ok, now the bad...
1. Factory documentation is non existent.
2. Chinese preamp tubes poor quality. You'll want to put in some NOS 6SN7GTs.
3. I don't like how they assemble in pictures (which is why I redesigned it)
4. Lousy speaker binding posts. Get a good set before you even try to install.

[morzh]

Good project, Tim! I am glad it works well.