Matchless/Bad cat build thread

[martin manning]

Very nice, Marcus! Eagerly waiting to hear what happens when you power it up.

[dcribbs1412]

Nicely done Marcus!

D

[norburybrook]

Thanks fellas.


right then.

Powered up with no tubes on the limiter and all looked good, bulb flashed them dimmed as it should so no shorts. Realised I had to physically turn the on /off/standby switches 180deg as they didn't match the panel lettering!!

installed tubes, again the limiter showed no issues, so measured a few voltages knowing they'd be low but everything seemed OK, by that the heaters had power ands the plates to all tubes had voltages.


Plugged a guitar in and the clean input worked..great...I tried the gain input..nothing....also on the clean input nothing worked other than the clean volume, and the brilliance. Back to the bench I started going through connections with the continuity on the multi meter and found I'd missed a jumper early on the pre amp board. I thought that would solve the issues but alas no.

I've just been through all the connections to and from the pre amp board again and they all seem correct , so I'm scratching my head as it I'm sure it's a wiring error as only the clean volume works but everything down stream apart from the brilliance (which i think is like the vox cut control on the PI) is dead.?????


I'm just not sure what's happening as it's completely silent so not a scratchy/bad joint kind of thing.


The other big issue I found when I plugged in not on the limiter is the EHX EL34's were red plating badly, so I need to address that as well. I've got it on the limiter at the moment so they don't red plate trying to work out where I've gone wrong.

The topology of this amp is very different from the standard Fender/Dumble i'm used to so that's not helping either.

This part is always a bit frustrating after the amount of time and effort to build I'm sure I have a lessons to learn here again with this build so eventually I'll be happy.

scratching my head at the moment though.

all advice/help gratefully accepted


M

p.s. layout and schematic.

Stray-Cat-15-Oct-2019.jpg

badcat_hotcat.jpg

[xtian]

Happy to cast an eye, if you post clear photos of the pots' wiring and the schematic.

[norburybrook]

Happy to cast an eye, if you post clear photos of the pots' wiring and the schematic.

thanks a phot might not be helpful as I've used a single colour wiring but I'll go do one now.

I've just looked at the schematic and it would seem that the Clean channel goes straight to the phase inverter. Seems a bit strange no? also that would tally with what's happening, i.e. the master volume and the whole tone stack aren't in circuit only the brilliance control. Seems a weird way though. yes, the clean volume goes directly to one side of the PI and the gain volume the other and the master is on the other side???

pics.

Cat 1.jpg

Cat 2.jpg

M

[pompeiisneaks]

Also you'll get very weird behavior if any at all while on the limiter. Definitely get the bias set so no redplating, then remove the limiter to test for sure.

You can definitely use your DMM to test for ac signal at the anode post any coupling caps and follow it through the amp, to see if it's getting lost somewhere.

~Phil

[joeboo88]

nice job Marcus: i just finished mine (6v6 version) and I think i will put a 47pf snubber cap on the 2 100k resistors that feed V4.... kinda ragged overdrive, for now
i would post a picture but i dont want to hi-jack your thread. we need to get your gain channel up and working

Oh also you said = "also on the clean input nothing worked other than the clean volume, and the brilliance. "

the clean channel is correct and only the brilliance works with the clean. I found that out as well on my first build

is it possible to bypass the half power switch to see about the replating?
also check your volume pot pin 3 and gain pot pin 3 to make sure they aren't switched, its hard to see from the photo. But i had done that and my gain side wasnt working either until
I switched them..... good luck

[martin manning]

I think I'd try clipping something probe-like to the gain channel PI input (the cap will block DC), then plug something into the gain channel and probe the output of each stage, after the coupling cap (where there is no high voltage). That should identify where the signal is being dropped.

[norburybrook]

OK, went back to the studio and took it off the limiter and the gain channel and everything is working fine now.


The output tubes are red plating badly though. There is another unknown variable in this and that's the OT, a Dagnal I got from a friend I had to look up the connections and solder wires to the terminals , I think I did this OK. If I'd messed that up would it have any baring on the tubes red plating?

I have sound, but it's very humming/noisy, I presume that's the tubes glowing orange!!!!!

What do I need to do to sort this?


M

[martin manning]

So it fixed itself while you were out? That and the hum/noise suggests a bad connection somewhere.
The OT wouldn't cause red plating, but an ultrasonic oscillation might.

[norburybrook]

So it fixed itself while you were out? That and the hum/noise suggests a bad connection somewhere.
The OT wouldn't cause red plating, but an ultrasonic oscillation might.

I think perhaps as Phil said running it on the limiter was causing some issues.

As there is no negative feedback I presume the OT primaries are always correct?


So, the red plating? what should I check regarding sorting this out?

If I take the tubes out will the voltages give an idea?

Do I need to measure anything across the cathode caps/resistors or will the readings on the valve pins reveal all?


Only asking because they're buried under the filter caps board.


M

[martin manning]

So it fixed itself while you were out? That and the hum/noise suggests a bad connection somewhere.
The OT wouldn't cause red plating, but an ultrasonic oscillation might.

I think perhaps as Phil said running it on the limiter was causing some issues.

As there is no negative feedback I presume the OT primaries are always correct?

Correct. I'm still suspicious that there is some intermittent problem lurking.

So, the red plating? what should I check regarding sorting this out?

If I take the tubes out will the voltages give an idea?

Do I need to measure anything across the cathode caps/resistors or will the readings on the valve pins reveal all?

Only asking because they're buried under the filter caps board.

Readings on the pins should be fine, at least you can check the Rk values. Did you say you increased those a bit from the 270R specified? Other than that you can check the resistance to ground from the grids to be sure they have 0V reference (with power off). Also check that you have 0V on the grids when power is applied (with PI and power tubes removed).

[pompeiisneaks]

When using the voltages on that schematic diagram, and Robrob's bias calculator, I see 118% diss per tube, that is if the voltage drop across the cathode 270Ohm resistors is really 24VDC, and 352 voltage between anode and cathode. seems pretty hot to me for sure.

You can measure the exact voltage of the cathode resistors and measure the voltage drop across it and that will give you your plate plus grid current of the tube with ohm's law.
V = IR
I = V/R = 24 / 270
example that 24v of the schematic across 270 ohms = 0.08888... or 88 mA current on one tube. That's a lot for one that's supposed to be around 71 for 100% dissipation...

Seems like redplating territory to me.

Not sure how that amp didn't burn up tubes....

Unless that was by design.

Edit: also remember if cathode biased it can be up to 100% diss in theory and not be as problematic as the 70% for grid biased.

~Phil

[norburybrook]

So it fixed itself while you were out? That and the hum/noise suggests a bad connection somewhere.
The OT wouldn't cause red plating, but an ultrasonic oscillation might.

I think perhaps as Phil said running it on the limiter was causing some issues.

As there is no negative feedback I presume the OT primaries are always correct?

Correct. I'm still suspicious that there is some intermittent problem lurking.

So, the red plating? what should I check regarding sorting this out?

If I take the tubes out will the voltages give an idea?

Do I need to measure anything across the cathode caps/resistors or will the readings on the valve pins reveal all?

Only asking because they're buried under the filter caps board.

Readings on the pins should be fine, at least you can check the Rk values. Did you say you increased those a bit from the 270R specified? Other than that you can check the resistance to ground from the grids to be sure they have 0V reference (with power off). Also check that you have 0V on the grids when power is applied (with PI and power tubes removed).

Martin I'm using 360R resistors.

@Phil, yes hence me upping the resistors like the Ceriatone did.

I'm not sure I understand checking the resistance to ground to get a voltage rather than ohms with power off?

Time difference meant I've been asleep since the last post. It's morning here so will crack on with this.


M

[martin manning]

First check to see that the path to ground from the grids shows the right resistance, then see if any voltage builds at the grid lug with tubes out and power on to see if you are leaking DC through the coupling caps.