5F1 CHAMP SCRATCH BUILD - NEED HELP with 2 PROBLEMS

[Rnb716]

6v6X1 with 800 ohm bias resistor and your voltage readings provided, I come up with approx. 74%. This is pretty cold for a cathode bias circuit. The more you drive the 6v6 the sooner grid current starts and moves toward cutoff. Can you move more toward 500 ohm. Voltages at each pin of both tubes may help also.

[mauiboy81]

The pot connected in series with the cathode resistor is not bypassed by the cap.
800 ohms seems high. Are you biased too cold?
Are you using the plate to cathode voltage for your bias calculation?

[t7mackie]

Hi all. No luck.

Disconnected the NFB from the circuit and no change.

I originally had all grounds screwed to the same point recycling one of the original unused PCB risers. I separated them and laid them out exactly as in Rob Robinette's Champ layout including soldering the PT's center tap to the negative end turret of the 16uf filter cap, DC & 6.3 center tap grounds screwed to a different threaded hole and finally the EGC to a third treaded hole. - no change.

I don't think this is the problem but my next attempt will be to remove the switchable tube / SS rectification and put the amp to bone stock / tube rec. I'll also try replacing the coupling caps because the brand I used are China specials. The filter caps I used on this second build are generic but the ones I used on the first build that had the same problem were top quality tube doctor and F&T.

***********Another symptom I discovered - The signal loss and hum starts when I just pass 7 out of 12 on the volume knob. If I turn the volume up past 11 the amp sputters / motorboats loudly through the speaker.

Here's a link to a 30 second mp4 displaying the symptoms.

https://photos.google.com/share/AF1QipM ... VYXzQ0b29n

As always, thanks to everyone who takes the time to help and offer suggestions.

-mac

[mauiboy81]

Is the speaker jack right next to the input jack on the bottom of the chassis? If so I would move it. Also the power tube is at the input. And the pot connected to the cathode resistor is right next to the input 68k’s which are also very sensitive. I would find the right cathode resistor and lose the pot.

[Stevem]

Another thing you can do to better point towards if the speaker jack needs to be moved is to lift the output tube cathode bypass off of pin 8 and see if that helps.

I should have noticed it earlier, but yes that pot under the board in the preamp section is just begin to make issues and nightmares for you!!!

Remove that proto before you do or try anything else!!

[boots]

My first impression is that you may be picking up an oscillation due to the layout. Normally you would want to keep the inputs and outputs separated as much as possible on the chassis. It looks like your speaker connector and input jack are very close together? Also, the placement of the tubes vs. the board layout requires you to make awfully long runs of wire under the board to connect to the sockets. Too many wires, too close together, is asking for an oscillation problem. I agree with the earlier comment to get rid of all the mods and concentrate on getting the basic amp circuit under control.

[t7mackie]

HI guys,

I thought I posted an update yesterday but I guess I didn't hit, "submit." EDIT:: I did post it - for some reason I didn't see the updates y'all gave me until after posting this post. I will look at all the suggestions - thank you very much!!

Anywho - NFB removed from the circuit = no change.

==ALSO TRIED WITH NO CHANGE==
Separating the CT, EGC and DC chassis grounds to different & separate points on the chassis.
Replaced the coupling caps with poly films
Replaced the power tube bypass cap with a polarized one

I tried to brain this out. The problem is associated with a change in the volume knob. What does the volume knob affect? -The signal between the two preamp tube stages.

I removed the input bypass cap = the "point" changed from 7 on the volume knob to about 7.5 before getting the distortion / hum / loss of signal.

I doubled the resistance to the 1st stage preamp tube cathode to ground = the transition / hum sound is not quite as noticeable when turning the volume knob to 7.5. I also noticed after this change I can turn the volume all the way up and it no longer motorboats (was previously doing this at 11 out of 12 on the volume).

My bias resistance needed to be boosted to more than double to get proper plate dissipation on the power tube. I'm thinking the same may be needed for the preamp tubes. I will double the 2nd stage preamp cathode resistance next and see.

[t7mackie]

Is the speaker jack right next to the input jack on the bottom of the chassis? If so I would move it. Also the power tube is at the input. And the pot connected to the cathode resistor is right next to the input 68k’s which are also very sensitive. I would find the right cathode resistor and lose the pot.

My first impression is that you may be picking up an oscillation due to the layout. Normally you would want to keep the inputs and outputs separated as much as possible on the chassis. It looks like your speaker connector and input jack are very close together? Also, the placement of the tubes vs. the board layout requires you to make awfully long runs of wire under the board to connect to the sockets. Too many wires, too close together, is asking for an oscillation problem. I agree with the earlier comment to get rid of all the mods and concentrate on getting the basic amp circuit under control.

I had the problem before adding the bias pot but I'll keep this on the back burner. I will definitely do something about the speaker jack.

I reused the Champion 600 chassis thinking it would make things more convenient. To begin with, it's stainless steel... Not conducive to making two holes for tube sockets!

Thank you very much, all. We're getting there...

-mac

[t7mackie]

We have a bingo!

I moved the speaker jack. No change.

I thought the logic made sense so I disconnected the volume pot and lifted it up away from the circuit. Meh...

Finally, I disconnected the input jacks and lifted them away from the circuit - much better!!

It looks like the major issue, as pointed out, is the layout. It makes total sense, as hindsight usually does. The ONLY things in common to my 1st and 2nd build attempts are the transformers, the circuit itself, and the CHASSIS!

I'll tear it all down and try to do my best with the limited space available to clean the amp up. This will also give me the opportunity to do a much better wire dressing job and fix some other unnecessarily long runs like the PT red from the rec switch that's taking a rather circuitous route.

Hopefully in a couple days I'll have a positive update.

Thank you all for your suggestions and for keeping me from going down the rabbit hole of my own ignorance!

-mac

[Stevem]

This is a very common maylay when you go and added extra gain into the Orange tinnyterror amps and most any other amps that darn small with more the 2 or 3 stages of gain before the PI section.

The Orange amps have 3 dam speaker jacks in a row and my fix was to make Aluminum covered, light card board shield to encapsulate the input jack and then solder a ground wire onto the aluminum, or make the Sheild such that the threaded nose of jack passed thru and then grounded out once the jack was bolted down.

[mauiboy81]

Try pushing the mains fuse wire to the other side of the fuse holder to get it away from the grid wire coming off the volume pot wiper.


**EDIT**

It looks like the input jacks are not isolated and are grounding to the chassis and there is also a ground wire back to the cathode. Those jacks need to be isolated.

[t7mackie]

Hi guys,

Update:

I moved some stuff around and I was able to confirm the problem is the chassis layout. I relocated the pre & power tubes and moved the speaker jack. I had the speaker jack loose, turned the volume up to just below the problem spot, and moved the jack around getting it near certain components (tube sockets) and presto! Problem duplicated.

It seemed like an easy project but adding the tube rectification took away some much needed real estate. There's just not enough room in this little amp and the stainless steel chassis was a real bear to work with.

I ended up pulling the speaker jack off of the chassis and mounting it near the bottom of the cabinet on a little bracket. I'm getting a little 60Hz now but the volume knob is now usable up to 10 1/2, which is above it's useful volume anyway.

No more tiny amps!! A Harvard clone is next and I have a 15" x 7" x 3" blank Bud chassis. Plenty of breathing room!

Thanks to everyone for all the help, guidance & knowledge.

-mac

[angelodp]

Here is a version of this type amp that worked out pretty well, it might offer some insights.