Kitty Hawk Junior buzzing in one channel

[Yoda]

Hello everyone,

I’m working on an interesting amp.

It’s a Kitty Hawk Junior. Some research says it is a copy of or based off a Mesa Mark 1?

There is buzzing/hum on the clean channel only and I am trying to diagnose. I ruled out filter caps because it’s only on the clean channel and somebody already went through and installed all F&T caps.

Any ideas? I’m thinking bad signal ground, but schematics are tough to find for this amp, for the version I have. Many are blurry or in German.

Thanks in advance.

[nuke]

Photos of the amp insides?

[Yoda]

Here are the only pics I have of it so far, along with what I believe to be the schematic (minus power section). Unfortunately I forgot to take a picture of the outside front before I took the amp apart:

[R.G.]

You're probably right - lost a ground on the channel.

We had another guy with a similar problem that turned out to be too high a resistance on one input's grid leak resistor.

For more ideas to track, you might like to read Guitar Amp Wiring Notes:
https://www.dropbox.com/scl/fi/vpijj3al ... qmbvz&dl=0

[nuke]

uggh. Good luck with that.

I think divide and conquer is in order, starting with the easy stuff. Does the volume/tone/master have control over the buzz? Swap some pre-amp tubes and see if you get lucky.

Looks like a headache to work on.

[Stevem]

Look like a real cold solder connection on the board right here at this filter.

[Yoda]

uggh. Good luck with that.

I think divide and conquer is in order, starting with the easy stuff. Does the volume/tone/master have control over the buzz? Swap some pre-amp tubes and see if you get lucky.

Looks like a headache to work on.

I will double-check if the controls had any effect on the hum but I want to say they didn’t. The hum remained pretty constant on the clean channel. Unfortunately I have to work some long days the rest of the week so I probably won’t take a real deep dive into this amp until the weekend but having ideas on where to start is super helpful.

I already started trying to trace out amp to see if I have the right schematic and see if any component values are wildly problematic.

I did swap out all four preamp tubes for new fresh ones and it made no difference.

[Yoda]

Well I messed around with the amp and after the customer stated the hum didn’t appear until the input jack was replaced. A Switchcraft jack was installed and it appears this amp uses Cliff style jacks. After removing the jack from the chassis the hum on the clean channel went away. However I had a diode removed from the circuit at the time as it wasn’t testing good until I lifted one leg and it tested fine but this diode was lifted when I did the input jack thing.

So to be sure I soldered the diode back where it was, left the input jack out and the hum returned. I’m unable to locate said diode on the schematic so this is a slow one to figure out. When I unsoldered one leg of the diode there was 3 holes in the PCB, almost like a transistor may have been there but without a good schematic I cannot tell what is supposed to be there.

But customer stated hum didn’t appear until the input jack broke and was replaced by another tech and making it go away this morning by removing the jack gives me hope, even if the hum came back.

[nuke]

Yeah, if the jack is supposed to be isolated from the panel, and grounded to a point in the circuit, that can indeed induce hum.

This is a common problem I've seen on Fender Blues Junior amps. The input jack is plastic garbage, and they fail all the time. You can put a regular Switchcraft jack in and wire it to the board. But, it should be installed with a shoulder washer and insulating washer to isolate it from the metal front.

I've seen work by prior techs who took a lot of time to run shielded wire directly to the first preamp tube on the tube board, cut traces and install their own 1M resistor, but fail to isolate it from the chassis panel.

Good find, I hope that's it. Don't know what you might have run into with the diode. Maybe trace out the board and figure out what it does, or whether it is likely original.

[R.G.]

When I unsoldered one leg of the diode there was 3 holes in the PCB, almost like a transistor may have been there but without a good schematic I cannot tell what is supposed to be there.

There are some dual diodes, three-leaded packages, in all of series, common anode and common cathode combinations. Generally these are power supply kinds of things, in TO-220 packages; or in SMD SOT-23 for signal-ish purposes.

What package was the diode you removed? Two-lead? Maybe the previous tech easter-egged diodes working on the broken jack problem.
Have all three pads on the PCB been soldered? Does it look like smooth factory wave soldering?
Does your ohmmeter say that any one of the three diode hole/pads is connected to ground? +V? Either power transformer wire?

[Yoda]

While probing to see if pinout voltages were normal I found that if I put my probe on pin 6 of V2 or pin 1 of V3 the hum disappears.


Unfortunately the schematic that I have been using doesn’t appear to be for this amp anymore so I am back to square one on that so I couldn’t tell you what V2 or V3 are doing at the moment.

[R.G.]

Is this the schematic?

kittyhawk-junior-amplifier-schematic.pdf

Are V2 and V3 12AX7?

If so, pin 6 and pin 1 are both plate pins. Having the hum stop when your probe contacts them could indicate that the thing is oscillating and the extra probe capacitance stops it. Or it could still be a grounding/wiring issue.

It would be very helpful if you would use your voltmeter and list the DC voltage on each pin of each tube. There are not that many.

[Yoda]

I was just coming back on here to mention that I recall reading somewhere that Mesa did some odd things in their early amps like soldering random pieces of wire to act as antennas to curb instability so I tried that and it made no difference again unless I touched them with insulated tools, so I started thinking the only difference between me touching them and just a piece of wire is some missing capacitance so this may be on the right track after seeing your comment. Unfortunately I don’t believe that’s the correct schematic for this amp. This amp only has a single input jack and there is no reverb driver transformer, there appears to be an IC on the reverb board.

The pin voltages are as follows:

V1:
Pin 1: 167v
Pin 3: 2.53v
Pin 6: 212v
Pin 8: 3.8v

V2:
Pin 1: 226v
Pin 3: 1.55v
Pin 6: 222v
Pin 8: 1.54v

V3:
Pin 1: 333v
Pin 3: 220v (could be suspect)
Pin 6: 214v
Pin 8: 1.74v

V4 (appears to be an LTP PI):
Pin 1: 310v
Pin 3: 50.4v
Pin 6: 291v
Pin 8: 50.4v

V5:
Pin 3: 467v
Pin 4: 461v
Pin 5: -41.9v

V6:
Pin 3: 467v
Pin 4: 461v
Pin 5: -41.9v

V1 through V4 are 12AX7s and V5/V6 are EL34s.

This amp indeed oscillates because when I try to play the lead channel to see if everything else is normal, everything else on the amp works as expected except the amp oscillates badly if you turn the gain up any reasonable amount so something isn’t right overall and oscillation may indeed be the culprit. There was even a 12AY7 installed by a previous owner I think to mitigate the oscillation so it comes on around 5-6 on the lead channel gain, whereas oscillation comes on around 2 or less with a 12AX7 in there.

That Geofex debugging page has a lot of good info so I’m gonna try what they say for oscillation and report back.

[R.G.]

Unfortunately I don’t believe that’s the correct schematic for this amp. This amp only has a single input jack and there is no reverb driver transformer, there appears to be an IC on the reverb board.

Hmm. OK, I'll take another look.

The pin voltages are as follows:
V1:
Pin 1: 167v
Pin 3: 2.53v
Pin 6: 212v
Pin 8: 3.8v

Are those DC voltages referred to the negative end of the first filter cap, or perhaps to chassis? I'm having a tough time interpreting them. If they are correct as stated, you may have found a clue to the problem.
The 12AX7 is a dual triode - two plates, two grids, and two cathodes.
P1 = plate 2
P2 = grid 2
P3 = cathode 2
P6 = plate 1
P7 = grid 1
P8 = cathode 1

In a normal self-biased circuit, the cathode sits at between +1 and +2V, as the 12AX7 cuts off with the grid much more than 2V negative compared to the cathode. One of the things I was hoping to find out was whether or not the grids were at 0V (pulled down by a grid leak resistor to ground) or not. These cathode voltages look high (in the absence of any other schematic info) and one of the issues you can run into is a grid resistor that is open or too-high a resistance. The plate voltages are high, but not hugely so.

That Geofex debugging page has a lot of good info so I’m gonna try what they say for oscillation and report back.

Yeah, I've used that stuff before and it worked well.

[Yoda]

All voltages were referenced to chassis ground.

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