Kitty Hawk Junior buzzing in one channel

[nuke]

If the above linked schematic is correct, you cannot replace the input jack with a standard switchcraft or even a typical Cliff jack.

The jacks in that schematic have extensive sensing and switching functions to determine which input is active with a plug inserted. This mess of wiring alters the functions of all sorts of stages in the amp, such as muting the OD channel if the plug is in the normal jack. Whole lot of other stuff is affected too that I haven’t complete sorted out.

Have a look at the original jacks, if still present.

[R.G.]

All voltages were referenced to chassis ground.

Yes I had a feeling some of these voltages were pretty warm on the preamp tubes.

Did you happen to measure the grid voltages???

[R.G.]

If the above linked schematic is correct, you cannot replace the input jack with a standard switchcraft or even a typical Cliff jack.

The jacks in that schematic have extensive sensing and switching functions to determine which input is active with a plug inserted. This mess of wiring alters the functions of all sorts of stages in the amp, such as muting the OD channel if the plug is in the normal jack. Whole lot of other stuff is affected too that I haven’t complete sorted out.

Have a look at the original jacks, if still present.

Yeah, I would have been after that, excepting he says it only has one input jack.

[nuke]

Yeah, I would have been after that, excepting he says it only has one input jack.

I saw a second cliff jack under the Switchcraft jack, but on closer examination I think it is for a pedal, since there isn’t a pedal input on the back panel.

Two relays and four or so vactrols on the board. It also looks like unmasked copper board with the parts stuffed on the copper side. Maybe another layer underneath or not. Would have been nice to tin the copper like they did in the old days.

In the spirit of “the last thing touched is frequently the problem” maybe look at putting a cliff jack if Ithat was the original setup and isolate from the chassis .

[Yoda]

Guys I have been trying to my wits end to post a picture of the front of the amp but I cannot locate the link ending in JPG to embed a photo here from my Imgur account, whether from the app or the browser. I don’t know how I got it to work for the other three pics but I will post it as soon as I can I will keep trying.

[Yoda]

Didn’t realize I can attach them through the forum itself so here they are:

IMG_1764.jpeg

E4E235DF-2048-4CC3-9A98-7F5947056EDD.jpeg

[nuke]

Ahh, that makes more sense. Some missing pages to the schematic, no power supply, no switching and no reverb circuits, but at least it appears to match up with the gist of the amp.

The schematic shows two relays and one of the vactrols. There's 3 more vactrols on your board, doing I'm not sure what. The effects loop and reverb are probably on the board near the input jack side, the foot-pedal logic on the small board on the rear panel.

The schematic depicts a single channel amp with a defeatable distortion stage. The relays simply ground it out of the circuit and pass the clean signal through the preamp stage, and the other relay changes the treble capacitance in the tone stack and bypasses the master volume.

Since the clean "channel" is also in use for the lead function, that would implicate the relay function most likely. My luck with relays is not good. I think relay 1 is a likely source of trouble.

The schematic shows the relays in the "clean" setting. If one of the sections of relay were not reliably making the contact with ground rail, that would be a likely source of hum/noise.

Set the amp to clean (which is most likely with the power off), check the resistance across the overdrive pot pin 1 and 3 and find the junction of the 470nf and 1M resistor leading into V1b grid. Should show 0-ohms to ground in both points.

[Yoda]

I checked those points and the resistances indeed read about 0.

The area I was talking about before when I said the hum went away if I touched pin 6 of V2 or pin 1 of V3, that’s this area of the schematic so I checked all the plate and grid resistors and even removed the 22nF cap and tested it and everything was in spec.

IMG_1874.jpeg

I suppose the next thing is to check the relays and see if they’re switching correctly.

[R.G.]

With the schematic shown, there's a good chance of local oscillation. The 220pF across the 100K plate resistor and the 1nF across the 100K cathode resistor is a clue that this may have been a design issue. They're limiting the bandwidth of the stage. The 220pF/100K is a cutoff of 7.2kHz, and 100K/1nF is down around 1.5kHz. Those are pretty drastic.

I wonder why the designer didn't put a grid stopper between the gain stage plate and the grid of the follower, right at the socket lug of the grid of V3a. That may be worth trying first. Follower stages are fairly notorious for wanting to oscillate all on their own.

I think it would be interesting to see what happens if you break any coupling between this gain/follower stage and the rest of the amp by:
1. Tie the three ground connections in the stage together at the stage and run a dedicated ground wire to the first filter cap One True Ground to eliminate any ground feedback.
2. Insert a 4.7k to 10K resistor between the power to this stage and the B+ to V1A&b and V2A plate resistors and decouple that with a 10uF of so electrolytic and a 0.01uF ceramic
3. Decouple the B+ to the gain/follower stage with a 0.01 ceramic from the V3a plate to the local star you made in step 1. This needs to be literally from the plate lug on V3A to the closest possible ground connection in the local star.

The point of all this is to break any ground interactions between the gain/follower stage and to RF decouple the stage from power and ground. None of these will hurt the otherwise normal action of the preamp. This stuff represents a shotgun approach to killing self oscillation in the stage. It's a guess...

[nuke]

Good call RG. That stage caught my eye with a little, "wtf is this all about".

Wanted to eliminate the relays as a cause. I've not had good luck with relays in the past.

[Yoda]

I will take all that into consideration when I poke around in the amp again in the next day or so.

It may be a design flaw but I find it interesting that it didn’t hum until the previous tech replaced the input jack, supposedly.

Very interesting amp.

[Yoda]

Well now the hum went away again after lifting one leg of two capacitors, the 22nF over the plate resistor and the 1nF over the cathode resistor. I hope I found the culprit in these bizarre bandpass filters. I was able to put a 12AX7 in V1 and turn the gain all the way up without oscillation in the lead channel.

[R.G.]

I think you have definitely found the stage causing it, and a plausible explanation - it's going into oscillation and that gets detected as hum.

If you're happy with the sound with one or both caps out of circuit, you're fixed. If you want to keep both caps, you could do some or all of the things I suggested.

The grid stopper on the follower section might well fix it too. Followers, especially capacitively loaded like this one have a tendency to oscillate. It's especially an issue for MOSFET stages with their large bandwidth; it's a little unusual for triodes, but everything that can happen does, somewhere.