[radiorich]

Hello Tim,
That sounds good and I am sure you will hear a little hum !

Sincerely Richard

[TV MAN]

**UPDATE**  1-8-2025
With winter upon us, I've been spending some time in the radio room. After about 2 years of use, I threw this amp on the bench, determined to get the hum issue resolved, knowing what I learned from my other AMP experiment. I pulled the power trans and put washers and poly spacers sandwiching the chassis between the poly washers. That helped a lot! I had the PT directly on the chassis. It still didn't eliminate all of it. I tried to add some more space by putting the nuts between the PT and chassis with a washer and poly washers, but the screws weren't long enough. I noted static on the left channel and a lot of hum on the right channel, 60 cycle. I tracked it down to the output tubes by replacing with Russian made tubes. That quieted it down a lot. I tracked the left channel noise to the driver tube, replaced it with a common 6GH8A. I had to try several to get a quiet one. I had the best results by using the 6GH8 on the R channel and China tube on the L. It's pretty quiet now.

I also made a change with the power switch, moved it to the rear of the chassis, used a 1/4" mini switch mounted in the speaker binding post holes, then made a common negative for both channels. It goes the same place anyway. I added an indicator light to the front and powered it off one side of the filament (3.15V) to ground. Otherwise it's too bright. I really found that clunky power switch sticking out the front annoying, and had caught myself leaving the amp powered up without knowing for sometimes days at a time. Most people using a setup like this would use a terminal strip or preamp to power on and off. This was simply my choice. I also found that if the indicator light was still overwhelming, adding a 820R resistor in series puts it about right. Still quite visible, yet not so bright that you can't enjoy the glow of your tubes   It's a lot more enjoyable now.

*notes*
The last picture shows the light before I added the resistor. I used a 12mm LED indicator light (10mm is too small) which fits the 10mm (3/8) switch hole perfectly. 6-12v range. They are available on the auction site for <$15 for a 10 pack, shipped. Don't make the same mistake I did and get the 10mm. That's the OD, not the thread size and will fall through the hole. I reattached the terminal strip back to the PT screw, in which the ground lug only goes to the transformer shield and ground lug on the A/C plug. I tried connecting the two together (chassis ground to the screw lug and found the hum came back with a vengeance. So I clipped it back apart and it returned to normal.

 I don't want to really criticize China tubes, I know there are some good ones out there, but my personal experience with them is that they don't last long and are often noisy. I find the Russian made counterparts are better quality, and last longer, but sometimes not real consistent in how they are manufactured. Ex. Insides turned different directions, bulb height or plate heights different, getter locations different. But, I also know these were built for a purpose, not to look "pretty" reflecting off the chassis of your Mac or Dynaco. Also note that even tho these output tubes are kinda an industrial version of the 6BQ5, they don't interchange. The pinout is different. I also did a power measurement and found that even tho this amp is rated at 12WPC, I measured 17 WPC before clipping. Some possible explanations are that the amp is rated at 110v input, our line is 123v = higher B+, and I only tested one channel at a time, so I'm sure it would be a little less if I were testing both channels at the same time.

Edit: I forgot to mention that in post 20, I had reversed the ground and FB connections on the output trannys. My mistake was actually having the plate leads reversed. P1 & P2, blue & brown. If anyone builds this kit or another project, keep in mind that if you get an oscillation, reverse those leads. It reverses the phasing of the OP trannys. The hiss I mentioned was a lack of NFB (negative feedback).

   
   
   

[morzh]

We could kiss Russian tubes goodbye for a while now.

[TV MAN]

Yeah mike, sadly. I've still found some on the auction site but are getting hit or miss. Seems a lot of that stuff is getting harder to find and expensive.

[radiorich]

Hello Tim,
you , Said it man and those russian tubes were fair prices too all it will do is drive the others up like my General electric console uses 6V6's and I wanted to build a amp too .
Nice amp glad you fixed the issue .

Sincerely Richard

[MrFixr55]

I wonder if the fact that the bottom of the transformer does not have a metal cover contributes to the hum?

[TV MAN]

It could be affecting things. Not sure. I did note that in other amps that they use longer screws to hold the covers on and use the nuts as spacers, then another set of nuts to hold it to the chassis. Most do have covers on the bottom. Might have even seen the use of fiber washers on some. I'm doing a major clean in my attic and found a bunch of old HiFi stuff that I'm going to check and either sell or part out on the auction site. I'll look at how they are put together and take notes.

Rich, there are still plenty of 6V6 tubes on the auction site, some are outrageous (NOS), others,  if your patient or don't mind not having perfectly matched tubes, they can still be had reasonable. New ones - forget it. Well, there are still some available from the UK, and they are good tubes. I'm also experimenting with some other types of tubes in my other amp build.