[TV MAN]

Mike, I found United and CDE for the big caps. I honestly dont remember if it came from Mouser or AES.

[TV MAN]

Progress from last night:

After attaching the PCB (hopefully for the last time), I pushed the tube sockets tight against the chassis, double checked to make sure all pins were in their place, then soldered. I soldered 2x just to make sure it was a good connection. I got the output transformer wires soldered on. I found lightly & carefully tinning the stranded wire helped it go in.

   

A comparison of the supplied electrolytics vs. the CDE.
   

They were a lot bigger and fit was tight, but they are in.
   

One mistake I made with my last build was not grounding one side of the filament winding. It produced an annoying hum among other things. It's NOT on the schematic, but this shows where I connected the ground for the output tubes. The voltage amp tubes have the filament center tap grounded. The current rating is the same for both filament windings, so I connected it like what appeared in the picture of the completed amp build shown on the auction site. One additional thing I did was to add a ground from the - on the PCB to connect to the ground on the terminal strip, just to insure if there's any resistance on the chassis it has a direct path to the B-
   

I did a no tube/no load partial power up and things look good so far. I did a few resistance checks 1st from the B+ to ground and didn't see anything unusual. I only powered up to about 60v and voltages looked about right.

[radiorich]

Hello TvMan nice job on the grounding and Yes, I like buying those CDE capacitors when I need bigger value .

Sincerely Richard

[TV MAN]

I did a full power up, slowly bringing it up on the variac and monitoring things. Initially I didn't make it over about 65v when I noted my 1st problem. I have a really nasty oscillation. I traced it to the feedback loop. Temporarally disconnected and the oscillation ceased, continued power on up to 120v. I note a low 60 cycle hum and "solid state" like hiss when the volume it turned up. I double checked things but so far haven't found anything amiss. So, I have some checks to make and try to find out what is wrong. Being tho it's a 60cycle hum, it's likely filament related. This amp uses a pentode preamp so it's gonna be more sensitive.

[TV MAN]

I think I found it! I reversed the output transformer ground and feedback and it eliminated the ocsillation and also eliminated the hum and hiss. It "appears" the ground and 8 ohm wires on the output transformer are connected backwards inside the transformer or the plate leads are reversed. So I'll reverse those permanently and do further testing. With the feedback disconnected the gain and distortion are a lot more along with other noise.

[TV MAN]

So, I put everything together and do a brief burn in. So far it is working "ok". I only had a couple DJ speakers to try it on handy, so thats what I used. I connected my cheap china tube preamp and my phone. It performs but I do have a low 60 cycle hum I'd like to eliminate and "some" occasional noise - suspect the preamp tubes. Sounds like tube noise anyway. Gonna pop a couple 6GH8A tubes in the socket and see if anything improves. Yes, both filament windings are grounded. I also added line caps since it seemed to pick up AC line interference. More to come later.

I swapped the ground and 8ohm leads on the output transformers. I may check a little more, I'm not 100% any of those leads match the diagram on the transformer.

   
   
   
   

[Mondial]

Regarding the hum, you might check the chassis grounding for possible ground loop currents. You mentioned previously that you added an additional ground wire from the circuit board ground to the chassis. Try disconnecting the additional ground and see if it makes any difference in the hum.

If you have multiple connections from the circuit board ground to the chassis, there may be induced AC currents flowing through the ground wiring caused by the magnetic field generated by the power transformer. Ideally there should only be a single connection between chassis ground and circuit ground. Also make sure that the audio input jack grounds are isolated from chassis and only connected to the circuit board ground at the input circuit.

[radiorich]

Hello TVman,
I agree with Mondial!

Sincerely Richard

[TV MAN]

Thanks for the advices, I'll give that a try. I have one going to the ground right off the bridge and the other goes to the actual chassis.

[Mondial]

The ground off the bridge rectifier may be a problem, as there is a lot of ripple current flowing at that point. Try experimenting with grounding at various locations, but try to stay away from grounds going between filter caps and rectifier. A connection right at the negative terminals of the big filters may be ok. The negative terminal of the bridge should go directly to the filter cap negatives not to chassis so that all the pulsating rectified dc goes directly to the filters without any other paths through the chassis.

[TV MAN]

I pulled it down last night and removed the extra ground, made things a little worse. I listened carefully with a set of headphones and have concluded it is a "grounding" type hum, present as soon as the power switch is turned on, worse when the tubes warm up, present even with the voltage amp tubes removed, no change when those are subbed. It sounds like an old PA amp with a flaky microphone connection. I've been prodding some with a ground lead and find by grounding the input to chassis, it makes the hum worse, not silencing like I expected. I can even ground the grids to the output tubes and it makes it hum louder, so I've got something going on. Probably something simple I missed.

[Mondial]

Tim, did you try removing the connection on the rectifier terminal strip between the bridge rectifier negative and the adjacent chassis ground lug? 

My suggestion would be to have the bridge negative be isolated from chassis at that point and connect bridge negative directly to the big filter caps negative. Then connect the big cap negative with another separate wire to chassis and circuit ground. Wired this way, all the ripple current flows directly to the filter caps without any alternate path through the chassis or PCB grounds.

[TV MAN]

Ill give that a try. Thanks for the ideas.

[radiorich]

Hello Tim,
Any progress on your amp ?
Sincerely Richard

[TV MAN]

I think I got it. I'm not 100% sure what I did, but it seems a lot quieter now. If I put my ear to the speaker or use headphones I can still hear a little hum, but it isn't notable now in a normal setting. I know any amp will have some low hum with your ear up to the speaker, so I'm not too concerned about this one. It doesn't have a choke or anything and it isn't a Fisher or McIntosh. It IS quieter now. I did remove that extra ground, didn't notice a huge difference either way, so I left it out. I did resolder the grounds near the bridge. I also tried clipping the jumper mentioned and grounding the cap neg to the chassis, but didn't notice any change so I put it back. Anyway, I'm going to put the cover back on and do a serious burn in and try some different preamps like a Dyna PAT, speakers and such and try to see how it really sounds. Other than a few bugs (thanks guys for the help) it may not be a bad little desktop or home theater amp. I'll update after pushing it it a little harder. Wife's asleep and I want to keep it that way for tonight.