[RodB]

Tim,

I have some bakelite sockets in my work shop that I can dig up. I believe quite a few are NOS. I will take photos tomorrow and if you're interested you can have them for the cost of shipping.

[RodB]

Tim - Photos of 8 octal tube sockets. (Top and bottom)
   
   
Send me a PM if you're interested.

[TV MAN]

Thanks Rod for the offer but I don't think those would work. There's some differences in the sizing and mounting holes between China (slotted screw holes) and NOS USA tube sockets. I've tried some others and nothing lines up. I'd have to pull parts and grind my tube socket holes larger. I do have some ceramic ones coming in from over seas that measure correctly. I ordered several different types so hopefully I'll get something that will work.

[TV MAN]

I got some new ceramic octal tube sockets installed and also tried some different coupling caps. 
The results: 
The good - both tubes light up   
The bad - sounds awful!

Ok, so just why? Maybe my coupling caps. I tried some generic yellow .33 caps that came with a kit. I wasn't super pleased with the brown drop caps I had in there previously, so I tried those. So, I'll try something else. Maybe some of those highly sought after leaky bumble bees I'll also use some socket adapters and check voltages so I can see if the new sockets are working. Voltages at the pins check ok. May be time to break out the scope again. It'd be great for a guitar player - just the right distortion, but for HiFi - not so much.

[RodB]

You sound guys are never satisfied.

[TV MAN]

Well, maybe - maybe not  It's been an interesting project so far! Something so simple, yet so cranky. The 1st one worked perfect, aside from some static from those lousy tube sockets, a little wiggling and it clears up. THIS ONE, been a challenge. Identical, but not identical. The 1st one worked so good that I don't want to touch it, then there's this one. Probably some dumb simple thing that I've overlooked. Well, it's like this - only a real car guy can understand! You never really finish a classic car! There's always something else

[TV MAN]

Well, found muh culprit. After using socket test adapters and finding voltages all ok, swapping 6V6s, no avail, swapped the 6GH8a with a good Phillips 6U8a and bingo! Clean as can be! Junk Jap made 6GH8 tubes! Well, I kinda think they may all be junk. Anyway, the couplers are fine. It hadn't been sounding very good and with one output tube having to be flipped or moved to get it to light up, figured the sockets were the main problem. Wrong! Stupid tube failed! Same with the china made ones. Don't last. That said, I'd observed some weird effects putting my meter/oscilloscope probes on the lower 6V6 G1 (cathode side of the phaser), so I swapped in a 10k stopper (left the 1K on the other tube) and that stopped the weird effects/oscillations. Read that needed to be done on some circuits using the 6U8/6GH8 driver instead of 7199 in AK. Well, must be something to that since it seemed necessary in this adaptation. I can elaborate if anyone would like the reason for this.

[RodB]

So, here's Prince on stage halfway through Purple Rain when he stoops down to his tech and says "Rod, pull out that 6GH8 and replace it with a 6U8." That's genius. Good find, Tim.

Television circuits would use ferrite chokes to reduce parasitic oscillation. Is that a possible solution?

[TV MAN]

OK, I've done some more tweaks and experiments and learned some more. I found that those .33 uf coupling caps from a kit are junk! I replaced them with some IC (Illinois) caps from AES and it really woke it up. I went back to 1k stoppers after some signal checks. Below is a pic of the drive to the 6V6 grids just before it distorts. This is using the 6U8a driver. It is really cleaning up and sounding much better and putting out power very close to the 1st build. I couldn't get the same coupler caps for this build as the 1st so I tried some brown dips. Nope - go with the tubulars. I'm still tweaking so stay tuned.

   

Signal from 6U8a

   

Junk parts! Well, the dips might be ok for something else, just weak couplers. 

   

[TV MAN]

Bump. Still experimenting.

[morzh]

Tim

What exactly did the caps do to the signal (I see some blimps at the tops on the yellow trace, but there is something also at the bottoms of the blue one).
And the brown dips - are they also junk or do you hear something, or does the scope shows something?
I am not sure I can understand how a cap, (if it is a cap and not some Chinese housewife buying some aluminum foil wrap in a supermarket and then twisting wires to it's product), can do something like it. At some frequencies there are resonances, anti-resonances etc, but those are not exactly audio frequencies. And the losses etc, unless they are egregious, shouldn't do this either.

I'd like to hear about your findings.

PS. Yes, for a high quality Hi-Fi I would use nothing less than a legit company, even if it builds the caps in China (all do), but no-names simply cannot be guaranteed to perform. This is exactly why I do not buy caps from the "he is a good guy and he sells caps" because, well, he might be a good guy, but if I need a cap to withstand 400V and I see no company logo, no marks, no dates, no nothing except "0.01uF 600V" on this yellow job, I do not really know what's inside.

[TV MAN]

In my 1st build of the same project I used couplers that I got from AES - some white ones, and that amp sounded pretty good. The 2nd one has been difficult from the start. For some reason I only ordered 2 of the white caps to "try", then couldn't get more when I tried to order, so I stuck the yellow caps in that came with the kit - generic China ones. Thought cap was cap. Noope! I had all sorts of weird problems and didn't sound that great either. I ordered those brown ones, thinking they were like Orange drops from AES, but still 'lacked something' in sound quality. I don't have any scope comparisons, but I ordered IC caps from AES for my EL34 builds and tried those and - yes, there was a difference. Much cleaner more transparent sound. So, there is a difference and I can sense it. Worth going out and buying $100 x2 caps? Naaaah! But def something quality like Solen, Sprague or IC. And value matters too, go too big and they sound muddy, too small and they sound tinny and lack bass. Seems .1 to .25 is about right.

It's kinda weird how things work out sometimes, I just threw my 1st amp together doing about what seemed right at the time figuring I'd end up redoing about half or all of it for tweaks and changes. That one worked flawlessly! Aside from a few tube swaps, I haven't done anything to it. The 2nd amp I revised and neated up. Figured I'd redo the 1st the same way. Yes, it looks much neater - but I've had that thing apart and together more times than I care to mention, changing everything from caps to tube sockets! I finally did get it working good, but they still don't sound quite the same. I am going to try some different output trannys and feedback settings.

On the scope reading, I'd found a bad NOS 6GH8a tube that was also causing weirdness, so I switched to a JAN 6U8a and that solved a lot of oscillations and intermittent stuff going on. Honestly, once I learn a little more about my scope and how things interact, I can do some tweaking in the driver stage to improve. I also have a new signal gen since that above post which helps. 

One thing I noted that might or might not be contributing to the weirdness is the power transformers. I ordered the kits at different times, and while the specs are the same on both transformers, the 2nd trans has slightly lower voltage out, by about 10-15v B+ I recall. As a result, I cannot get quite the same power output from both amps, the 1st is 15w and the 2nd is 14w. Anyway, I'm no super expert and cannot go into theory like some of those guys over at AK, but I can cut and paste, tweak and usually get something sounding about right. Anyway, stay tuned. I am still learning.

[morzh]

Were the voltage specs on both xfmrs the same? Then they used a thinner wire at the secondary, i guess, and this is what results in the drop. ALso your transformer probably would run hotter.


As for the caps, I only bought them occasionally from AES, when ordering something else.

I mostly buy from Mouser. Mostly IC/Vishay for the tubulars. And, if buying for an audioamp (not an antique radio), I would not go to AES, except where they sell Solen and such.

[Arran]

Tim;
I have known TV repairmen who cursed those 6GH8 tubes, mid to late 60s RCA colour TVs used fistfuls of them, and whenever a CTC 17 would act up they would bring a sleave of 6GH8s with them. Apparently RCA made ones were the worst, but the RCA sets were hard on them to start with. I did not know that these amp kits from the "middle kingdom" used them, they are a triode-pentode which is sort of unusual for a 1st audio/driver tube.
Regards
Arran

[TV MAN]

I did an experiment with these amps a month or so ago and I think they sound a lot better. I had been using a Douk preamp (tube), and had also used a PAT4 (different speakers tho), and really thought they sounded a little lame. Not bad by any means, but, could be better. I'd improved one with better coupling caps, and have been playing them through some old RCA speakers. I was too the point of trying some different output transformers, but hadn't gotten around to it yet. Those could still make a difference, ... But, what I found that really brought them to life was the Pas3 preamp I brought back from the dead! I truly truly wouldn't have thought that pre would have made that much difference, but it did! It sounds so much more crisp and smooth bass.  I wouldn't have believed it if I hadn't heard it for myself. I don't know if it was an impedance mismatch or just quality. 

Another big improvement was getting a better (commercial) CD player. I had been using a cheap portable (Onn), lame lame lame! I tried a Sharp (Japan) CD player, and it sounded a lot better, but when I plugged in the Tascam, it really brought things to life! So much cleaner, crisper and depth. So I guess the point is before tearing into a dead set, make sure it's plugged in!

I missed the mention by Arran about the 6GH8s. Originally the circuit design used 7199, which aren't cheap or easy to find. So I used the 6GH8. In my EL34 build, I used 6U8a. It originally called for a 6AN8a, but with the layout, the 6U8 worked better. I'd been crisscrossing about everything like spaghetti.