Matchless/Bad cat build thread

[martin manning]

You can always play with different 1W resistors paralleled to see what a different bias point does.

Mismatched tubes will idle closer with cathode bias than they will with fixed bias, so you can be less concerned about matching. The nice thing is you can just plug them in and check the cathode voltage to see how close they are.

Switching inputs with a relay would be similar to the FET relay set up on your D amps. You'd just use one set of contacts to ground one input or the other, and the other set to direct the input jack tip to the other input or the one, but that's exactly what the two input jacks are doing now. To silence the gain channel you would need another ground close to the PI, like say grounding the MV wiper.

[norburybrook]

Great, Marcus, nice work and thanks for the report. Measure voltage on pins 3, 4, 6, and 8. I'll run the numbers. Voltages will be different in SS vs. vacuum rectifier mode, so if you have the time, get them both ways. That should be interesting in itself. Right now I'm estimating 93% based on Vk just above and the old plate voltage. The 1k2 resistors are running at V^2/R Watts, so 21.6^2/1200 = 0.39W (at idle), so yes you can leave them in.

OK,

Voltages.

with SS rectification

pin 3: 358vDC
pin 4: 346
pin 6: 359
pin 8: 24.1


Tube

pin 3: 333
pin 4: 321
pin 6: 336
pin 8: 22.4


I've also had a play around with a chopstick moving various wires and I've found the master volume wires were causing noise. I have the ground running straight to the star ground but it seems to be picking up noise from the transformer, so I'm going to shield the wiper cable and move the ground to the bus bar.

Also, I've found the are where noise and obviously gain is centered. My wooden chopstick is making noise when I move it around this resistor, to 470k

Matchless gain stage.png

it's almost like a noisy theramin I wonder if the resistor is bad(microphonic) or it's just how things are. I don't think I have another. perhaps a metal film would be better?

edit: swapped for another CF resistor much the same. that triode is obviously where the gain for the gain is coming from as everything connected there makes noise when chopsticking





M

[martin manning]

Here's the result. Looks like you are basically up to the limit with the SS rectifier, so the 277Ω Rk is probably a good place to stay. You could get some 270Ω resistors, but you are fine leaving the paralleled 1k2 in there. They aren't by chance some NOS Mojo bearing brand, are they? ;^)

I didn't notice the shield in the C-tone layout before. Obviously they found that area sensitive too. If you are getting tones, you are obviously causing some FB and oscillation just by bringing your chopstick close to the grids.

[norburybrook]

Here's the result. Looks like you are basically up to the limit with the SS rectifier, so the 277Ω Rk is probably a good place to stay. You could get some 270Ω resistors, but you are fine leaving the paralleled 1k2 in there. They aren't by chance some NOS Mojo bearing brand, are they? ;^)

I didn't notice the shield in the C-tone layout before. Obviously they found that area sensitive too. If you are getting tones, you are obviously causing some FB and oscillation just by bringing your chopstick close to the grids.

Thanks Martin, yes 98.8% will do nicely there's nothing NOS or MOJO in this amp. It's a Dumble Chassis and a Marshall OT I had lying around for ages, this seemed like an interesting build I could do with those bits, and it was/is just need to glue up the filter caps and have a general tidy up and make a cab. Oh and I need to make and AB box so i can switch channels.

I'm not getting tones, I didn't explain that well just noise that increases and decreases as i wave the chopstick close to the resistor.

It's actually OK, in practice as it's pretty loud and gainy on the gain channel so I suppose noise is to be expected . If I turn down the Master when in clean it's quiet.

M

[martin manning]

Marcus, I was going through the photos in this thread and I'm wondering where you put the 1k2 parallel cathode resistors. The 360Ω are buried under the filter board as you said, and hard to get at.

Another question: What did you use for the choke? An AC30 choke (Classic Tone's is 20H/100mA and 419Ω) would probably be the closest to the spec on the schematic (36H/450Ω) that would be commonly available, but whether you need that much inductance is an open question. edit: ok, it's in another thread on this project: https://ampgarage.com/forum/viewtopic.p ... 86#p413486

As we learned previously https://ampgarage.com/forum/viewtopic.p ... 55#p413655, the actual OT primary is 4k, but your Marshall OT is fine at (I assume) 3k6. As I said before, it's far from a true Class A in operation. A lot of people argue over that WRT the AC30, which isn't Class A either. The Class A portion of the load line is a bit larger than most PP amps, though, which comes with biasing up near 100% Pa max. You can get away with that in cathode biased amps.

Since you are liking this circuit so much, I'm fixing up the schematic to reflect the C-tone layout, which is probably based on an actual example and has been updated fairly recently. I see they have added a bleeder resistor on the reservoir, which is good, and they are showing the 5M1 across the extra SS reservoir cap same as in the schematic. If that's the way it was actually done, the resistor is way too big for a bleeder. Just reduce the 5M1 across the 100u to 220 or 270k 2W.

[joeboo88]

Martin. Can you explain the purpose of that 100uf cap/5m1 resistor?
You says it’s ss reservoir, I’m not sure what’s that is

Thanks Joe.

[martin manning]

In SS rectifier mode another 100u of reservoir capacitance is paralleled across the 33u that the vacuum rectifier sees, so the power supply is much stiffer. It needs its own bleeder, but a 5M1 will need over 40 minutes to fully discharge 100u. 510k would make more sense.

[joeboo88]

thanks for the explanation. It makes sense to me what that setup does.

I built a 6v6 version (my PT's current rating is 138ma that's why i went with 6v6) I only ss have rectifier, I probably wouldnt even need the extra filtering of that 100uf?

[martin manning]

The reservoir capacitance is limited for vacuum rectifiers. If you have solid-state only, you can increase the reservoir capacitance. I don't know what you have now, but I'd think 47uF would be sufficient for 2x 6V6 power section.

[norburybrook]

Martin,

Thanks for your diligence.

the Choke is a 10H choke so less than specified.

The 1.2K resistors are across the 360R ones, I left enough length in the B+ wiring form the board so I can unscrew it and lift it partially away to work underneath.


I never realised the 5M1 there is a bleeder resistor , normally they are around 270k as you say . I had to parallel 2x 10M to get that value, had I realsied I would have dropped something smaller in there. You are correct in the time to discharge. I'm cautious of residual voltage so generally measure voltage before doing any work and that cap stores a large charge for a long time.


So today as it was nice weather I started on the cabinet. I have some snakeskin and black tolex left over and as this amp is a 'leftovers' amp I thought that's what I should use.

So it will be snakeskin with a split black /snakeskin front with white piping . I'm going to get some wire mesh for the rear as well just to stop anyone putting their hand in there as the OT terminals are exposed.

IMG_20200712_165447.jpg

IMG_20200712_165425.jpg

Tomorrow is Tolex day.

M

[martin manning]

Nice looking hutch. I think the snakeskin and black combo will look great.

[Guy77]

Great looking cab Marcus! Yes take advantage of the nice warm weather for doing cabs. I like how the tolex grips to the glue and forms nicely around the edges when its warm out!

I love how the snakeskin and alligator tolex looks.
I just did an Alligator cab this week. They are interesting patterns. Its like a row of larger patterns and then a row of smaller patterns. I kept the larger pattern at the front edges and the smaller patterns further back.
The pic I attached when opened shows the pattern getting smaller as you look at the top of the cab and move towards the back.

You can also see the rows of pattern if you look into the amp opening and see the inside back wall.

Excited to hear clips of your amp!

Alligator.jpg

Cheers
G

[didit]

Fine looking furniture gents.

Best .. Ian

[norburybrook]

right then. Another fine day here in the UK so finished this off.

the tolex glue tacked up really quickly and really well...in fact too well I didn't get the snakeskin pattern completely straight, but you're working blind as you only see the white backing when you drop the amp on! ah well it is what it is.

I think the colour combination works well so I'm pleased with the end result

I glued up the filter caps and replaced the 5M1 bleeder with a 5w 220K and then dropped her into the cabinet.

Job done.

I'll do a video as soon as I get chance, or post sound files.

thanks for the support and advice from everyone.

IMG_20200713_151743.jpg

IMG_20200713_153354.jpg

IMG_20200713_153408.jpg

IMG_20200713_153539.jpg

M

[ChopSauce]

Looks good to me ...