[TV MAN]

Ok, so I tried an experiment tonight. I put a switch on to kill the feedback or keep it in the circuit. Result? FAIL! That's a no-go. I thought it was great for a moment as the gain was tremendous and the highs and mids came alive, but so did the distortion, hum and all kinds of noise. Probably OK for a guitar amp, but on this HiFi amp, noooop. I also added a really cool Johnson Cinch jack to fill in an extra hole over the bias pot that I can use to check the bias voltage right on top of the chassis. So that experiment is a success. Once the current is set, you can check the voltage too. I have the voltage at -26.4 and current is about 30ma idle each tube. It's weird how the tone changes when you adjust this. This seems to be the sweet spot. It runs cool and had plenty of punch. It's really almost to the "leave it alone" point, and start on the next one. It's working really good right now. I almost hate to mess with it any more.

EDIT: The switch I added to turn on or off the NFB might actually not be a total failure. It could be used to help weed out marginal or noisy driver tubes. I note this in that some of the common 6GH8a's are noisy.

   

Sooo - at this point, time to start building it's twin sister. Now that I have the build down, I will try some more experiments as I go together with this one. Gonna try a better step bit and see if I can enlarge a couple tube holes for the octal sockets. I might attempt using a chassis punch, but not holding my breath that it'll work. If not, then I'll grab the die grinder and do a little more "redneck engineering"  
   
   

[TV MAN]

I finally had a chance to do a test power up on my twin amp build, which is a twin, but not identical twin. With this being my second crack at this, I revamped some things in the layout to simplify construction, clean up & be easier to service. I separated the FB network and put it on it's own terminal strip (upper RH corner) which cleaned up the terminal strip near the driver tube. I also rerouted some of the wiring around the driver to simplify. I pretty much left the output area as it was, except I am trying different screen resistors to see the effect on the G2. I used a 100r resistor on the original, tried a 4.7k and it only dropped it 10v. Just an experiment. I'll probably go back to 100 on the final, although I see a lot of those connected directly to B+. I personally feel there should be something there. I also moved the bias filter cap to the control. Plenty of room, and makes more sense to filter it there right before it's sent to the tubes, and neats up the bias terminal strip. I used different coupling caps, since I didn't order enough of the bigger ones. I'll see if it really makes a difference. They weren't cheap ones either, so I really don't expect anything super different. Same value.

You'll also note that I revamped the power supply. My initial was just that - an initial. I found that my choke fit the existing holes already in the rear of the chassis, and it fit perfectly in that area, which neats up the top of the chassis. I moved the filter caps between the PT and A/C plug, mounted to a couple stickys and glued in place, Stays put, yet not hard to snap off and replace when that day comes. I also used a 5w power resistor instead of that huge 10w. I let it play for 20 minutes and could lay my finger on it with out withdrawing forcefully. I'm using a plain ole 6GH8A driver, Japan, and a couple Russian made 6V6s.

It powered up beautifully without any setbacks other than the fact I must have put the line control in backwards. Simple fix. I did reverse the plate leads on the output tranny and had no feedback issues, so this DOES make a difference! It makes sense as this would reverse the phasing of the transformer. I found the blue lead (at least with this tranny) goes to the low side of the inverter, and the other (brown?) to the high side. I've made this mistake more than once on other amp builds, so learned something. I also had the SAME hum issues from the PT, so moved the trans nuts around and used washers and poly washers as before and made it totally quiet. Not quite as attractive as tying the transformer directly to the chassis, but I'll sacrifice some cosmos for performance. I'm still planning to play around with some other tube types and try a couple different output trannys but I need to set a standard for comparison. Next up is going to be a power, freq response and distortion test. So far my main tester (those 2 things on each side of my head) say is sounds pretty good!

Edit: Mike, I know you mentioned that large yellow cap by the driver tube. It has some pretty thick leads and feels solid. I ran the lead through the terminal strip to the socket. It's not going anywhere.


   

[morzh]

Looks good.

Judging by the value of the caps, your rectifier is solid state?
If so, a small NTC to keep the inrush down could come handy.

[TV MAN]

Mike, I have a CL90 I could put on. Honestly, I question if it really needs those big caps. Probably 47s would be fine. It's what came with the kit and they seem to work pretty good. I did reverse my level control. Ugh! Awful! You just TRY an desolder a 6 pin something without destroying it. I ended up cutting one of the traces and soldering direct to the control pins to make this thing work right. I have it set up so you can connect a stereo something or other and have it sound good. You can run it either way. Mono or stereo (mono).

Make SURE you solder that control on the side of that little PC board that has the square, otherwise - welcome to my world.
   

[TV MAN]

I did some testing, but I don't think my results are accurate. I think my scope is off, so I deleted until I can verify. I do know the amps sound pretty good, but I don't think I am getting the full output that I originally thought. So far, they sound really good on some 10" speakers I have, but they seem to lack a little on bigger speakers. That's about all I know at the moment.

[TV MAN]

I just finished some more power testing, and also used those testers on the sides of my head. Soooo - I tried several different sets of speakers and my PAT preamp. They both sounded about the same, pretty quiet and clean ... than is, until I really cranked it. Now bear in mind, even tho I'm an old timer (kinda), I like to feel my music as well as really hear it. That said, this should explain why I've blown a lot of foam speaker surrounds and tweeters. Sooo - back to the test. The 1st amp built sounded pretty OK up to clipping. That's where I backed off just a bit. The 2nd build, well, it lacked after just a shade over what I call 'cranked'. In fact, it sounded awful and fell totally flat!

Soooo - back on the bench! On my scope, I could only get about 21vPP before it clipped. I checked the other and it didn't clip until about 31v. I also tested my kit amp using the 6P1 version of the 6BQ5/6V6. and it peaked at about 33v before clipping. Woah! What?

So, then I remembered I'd left the 4.7K screen resistors on there, which is about the only difference I could think of. So I swapped for 100R. Powered back up and the output raised to about 28vPP before clipping. Then I jumped them, and that brought it up to about 29.5VPP before clipping. So, those screen resistors DO make a difference, even if idle voltages aren't much lower. A couple things that could also attribute to slightly different power output (very minimal) is that the tubes in the 2nd amp are very slightly lower emission, and a little farther apart concerning matching, where the 1st amp tubes are dead on matched. The other thing is for some reason my B+ is very slightly lower on the 2nd amp. The 1st is 330v on the 6V6 plates, and the 2nd is about 318 on the plates. The 1st kit is a little older, purchased in '22, and the 2nd is purchased within the last couple months. The Power trans might be slightly different. I'll do some more checking.

I also checked the signal at the grid of the inverter tube just to see how much and where it clips. Well, it is solid up to about the 90% setting, which is well below the setting where it clips on the output. I didn't check the output grids tho. Later.

I'm not sure how accurate my PP readings are on my scope, but I did note that the freq response was pretty flat from about -25hz to 15k (which is the highest I went), so as of right now, I'm not sure about the power output. I do know they sound good now.

[morzh]

Tim

Those volts you measured: across the primary or across the load? If the latter...that is total too much.
If the primarey, OK, understood, though I do not know your turns ratio.

[TV MAN]

I finally have some numbers. It took watching a few YouTube videos and measurements to get there, but the tally is in. My scope IS accurate, which I confirmed with my Fluke. With my 1st tests on the 2nd amp, nothing was adding up or making sense. Now I know why. Never would have imagined the G2 resistors would have made that much difference, but they did! So, I corrected that and did some more tests and this time things made sense. My scope showed right around 30VPP, which I divided in half, then X .707, which gave me about 10.6v, then squared and divided by 8. and that gave my right at 14w. I confirmed the RMS voltage with my Fluke at 10.65 and used the same formula. This makes sense since I built the amp to push about 15w. So, it's doing close to what it should. With the 4.7k screen resistors, it was only pushing about 8w! So, no wonder it sounded so lousy when I pushed it. It sounds much better now. The 1st unit pushes 31v, which puts at at 15w. It's kinda hard to get an exact reading because the level control is stepped, but I can also adj the signal generator too. But, it's close, and about what a pair of 6V6s are suppose to put out.

 A note: The PP kit amp put's out about 17w (33vPP). It DOES sound just a little stronger when I push it. It's actually rated at 12w, but I think since our line voltage is higher than 110 that it raises the B+. It could also be under rated.

My scope is just a little hand held unit and it's kinda hard to see things very clear, but I can see when the signal starts clipping. . I've got my eye on a new one with a bigger screen  that I may pull the trigger on. This is still an experiment. More to come.

Edit: Mike, they are on the primary of the OP tranny. I think the lower B+ on the 2nd amp build is cutting my wattage slightly. The output across the load is in the 10.6-11v range A/C or 30-31v PP