[gregb]

Looking at another picture I think Mondial is correct, a schematic would seal the deal.

Gregb

[TV MAN]

The input filter is a 47uf, which isnt too far off and the big cap is after the choke on the b+ to the output x-formers. The other 2 filter caps are 22mf and are for the G2 for the 6L6s and plate supply for the pre amp tubes. The diodes you see are 47v zeners in series and supply the G2 for the output tubes. This is the original design of the circuit.

[TV MAN]

I finally had a chance to snap a pic of the schematic. This should clear up any questions and help the curious. It shows the power supply and one channel. The other is identical, just not shown. I may redraw to include both along with my labeling for the final.
   

[morzh]

So they use Soviet or Russian tubes, possibly of Chinese manufacture?

[TV MAN]

Yea, they are definitely an overseas type and from all that I have read, they are the same as our American types. They test the same on my testers, although I noted the 5Z4 did test stronger than a NOS 5Y3gt. The 6P3P tested like a 6L6GC. These appear to be all China made. I have had issues in the past with Chinese made tubes as far as life goes, but on the Russian made tubes - Zero problems. Here's a close up of the 6N8P tube, in which you can note that I am not using. I might later try a set of USA made 6L6GCs just to see if there be any difference in performance.
   

[morzh]

Russian 6П3С (6P3S) and 6Н8С (6N8S) is what this is supposed to be.

[Arran]

Mike;
  I've been looking into the Chinese radio industry, at least from the tube era, and it seems that they either used North American style tube types or Soviet tube types. What I don't remember is whether they used Cyrillic letters on some of the Soviet types or all. The tube shown looks like it has a number 75 on it as well, maybe a manufacturing date like 1975, followed by XI, 11th month maybe? An interesting arrangement of the two triodes, cathode follower I think. Is that a 6SN7 or a 6SL7?
Regards
Arran

[morzh]

Arran

This is 6Н8С (I use cyrillics here).
   

Compare to the pic above.

Chinese used Russian transliterated names, with the Russian "С" (S) at the end (stands for "steklo" - glass) replaced by "P" (not sure why, could be Chinese translit for their version of the Russian designation). All Russian octals had "С" at the end (Steklyannye Lampy - Glass tubes)

[BrendaAnnD]

Question for Mike:

One of the outputs in my amp apparently was fubar from the word go. With the outputs of both channels in circuit, I was getting only 265V on the plates. The amp sounded pretty good, especially after I finished wiring in one complete channel, but it seemed it was clipping way earlier than it should (lower half of the waveform at the speaker output). That, and both the power transformer and choke were running very hot.

Then, suddenly the working channel went very low. I checked my wiring but found nothing. Then I tapped on the output tube, snap snap snap, then it came back up for a few seconds before going out again.

I pulled that tube, and swapped in the one from the other channel. Audio came back up, crisp and clean and I could run the volume a lot higher than before (never even heard any clipping up to the level I got to.) And the plate voltage bounced up almost 100 volts... Oh, and I think that at least one of my 47V zeners is shorted, as the screens are running around 217V or so.

So, as well as any observations any of you can make, I need to pick Mike's brain about replacements for those output tubes. Pretty sure I have a couple of Sovtek 6L6's in my shop stock. Would these be suitable?

[Arran]

Brenda;
  As far as I know Sovtek 6L6 tubes are electrically the same as the U.S ones (the Sovteks are re-badged 6P3Cs made in the Reflektor plant I think) , I think the same goes with the China 6P3, other then in build quality. What I wonder about though is how the Soviets denoted the differences between different types of 6P3Cs, such as between a G and a GB, etc., I don't know if they had metal ones once or not.
Regards
Arran

[morzh]

Brenda

Russian 6П3С (6P3S) if you could still find a good old one or an NOS tube, are well-regarded and were very reliable, and is a Soviet-made 6L6, same as 6П6С is the 6V6. They are exactly the same, and inserted instead of US or Chinese counterpart will work just fine.

Actually the Russian tube appeared when RCA equipment was delivered according to Lend-Lease program during WWII and the tube was licensed and copied.

[BrendaAnnD]

"I feel so happy tonight, so happy tonight.."

I put in my pair of Sovtek 6L6's, and all the voltages are now spot on, except for the screen voltages. I've ordered new zeners to take the place of the originals that failed. Transformers are all running cool now. I'll order some new 6P3S tubes for it after my pay hits.

[morzh]

6P3S and 6L6 are the same. If your pair of 6L6 is good, stick with it. Especially if 6P3S is Chinese and not Russian.

[BrendaAnnD]

I like these Sovtek 6L6's. Seem to be quite good tubes.

Haven't played a lot with this type of amp before. Would I be correct that the high G2 voltage would cause the tube to clip early on negative peaks? It seems to have a lot of headroom on positive peaks. Never really seen a single ended amp that didn't clip more or less equally.

[BrendaAnnD]

Yea, they are definitely an overseas type and from all that I have read, they are the same as our American types. They test the same on my testers, although I noted the 5Z4 did test stronger than a NOS 5Y3gt. The 6P3P tested like a 6L6GC. These appear to be all China made. I have had issues in the past with Chinese made tubes as far as life goes, but on the Russian made tubes - Zero problems. Here's a close up of the 6N8P tube, in which you can note that I am not using. I might later try a set of USA made 6L6GCs just to see if there be any difference in performance.

Have you had the opportunity to do a run up on specs? How much power do you get before clipping starts? What is your actual G2 volts (mine is giving me almost exactly the same as the plate)? Looking at it on a scope, it clips early (and badly) on negative peaks. It does get good power and sound before it clips, I just think it should be more symmetrical.