Matchless/Bad cat new build questions

[norburybrook]

I'm trying to use up things I have around and have decided on this build.

Stray-Cat-15-Oct-2019.jpg

badcat_hotcat.jpg

Now, I have a Metro buffered FX loop that's sat idle. Could I fit this to this amp? due to the unusual pre amp I'm not sure where the send and returns to the loop would be and if it would be suitable.

M

[pdf64]

As is, a loop would have to be inserted into only one channel’s signal path, can’t serve both. If you want a shared loop, it would be a minor change to mix the channels with a couple of resistors and use their node to feed the loop. Connect the unused ltp input to 0V.

[norburybrook]

As is, a loop would have to be inserted into only one channel’s signal path, can’t serve both. If you want a shared loop, it would be a minor change to mix the channels with a couple of resistors and use their node to feed the loop. Connect the unused ltp input to 0V.

of course, yes that makes sense now looking at it.

OK I'll leave as is then for the sake of simplicity and squeezing things into a tight space with my chassis.

I'm quite intrigued to hear how this turns out. Just getting the bit's I need together at the moment. Going to use an AC 30 transformer as that's 180-0-180 so pretty close to this. Just wondering if it's worth doing the SS/tube recto or just doing the SS. Anyone have any experience with both? or thought's on the matter?


M

[martin manning]

The schematic shows 24V Vk through 270Ω cathode resistor, which is is 89 mA. At 379V Va that's 31.6W per output tube. You could knock 10% off for screen current, but it's still 28.4W for a nominal 25W Pa max tube. Hot Cat indeed!

Don't you mean 280-0-280? That will make something like 380V with a two phase SS rectifier.

Anybody know what the OT primary impedance is? 4k like an AC 30 would be more like 50W; 8k would be more like 30, and closer to Class A PP.

[10thTx]

If you want a shared loop, it would be a minor change to mix the channels with a couple of resistors and use their node to feed the loop.

IF you attempt this maybe 220k mixing resistors (& if not sufficient, up to 470k)

With respect, 10thtx

[norburybrook]

The schematic shows 24V Vk through 270Ω cathode resistor, which is is 89 mA. At 379V Va that's 33.7W per output tube. You could knock 10% off for screen current, but it's still 30W for a nominal 25W Pa max tube. Hot Cat indeed!

Don't you mean 280-0-280? That will make something like 380V with a two phase SS rectifier.

Anybody know what the OT primary impedance is? 4k like an AC 30 would be more like 50W; 8k would be more like 30, and closer to Class A PP.

Sorry yes 280-0-280 and I'm going to use 360R resistors for the EL34's so they're not running as hot!!!! I've got a 50w Marshall OT sat unused here so thought I'd use that.I'm not 100% sure what the primary is, I think I did measure it at one point. I think it's a Dagnal 50w plexi transformer.

I know these amps historically are HOT and eat output tubes hence people fitting larger cathode resistors to help. I think the original Matchless had even lower values!!


M

[norburybrook]

If you want a shared loop, it would be a minor change to mix the channels with a couple of resistors and use their node to feed the loop.

IF you attempt this maybe 220k mixing resistors (& if not sufficient, up to 470k)

With respect, 10thtx

thanks Jeff. I'm going to stick a line out on it and use wet/dry or have reverb /delay in front with pedals. Not sure how this amp sounds but I thought it might be different enough to warrant a build. I realise I have the Fender/Marshall thing more or less covered so decided something 'different' was the only way I could warrant a new build at the moment with little or no work for months.

M

[martin manning]

I've got a 50w Marshall OT sat unused here so thought I'd use that.I'm not 100% sure what the primary is, I think I did measure it at one point. I think it's a Dagnal 50w plexi transformer.

Probably 3k7 or something like that. That'll handle the power, and and place the load line pretty much like a JMP. I'd like to know what the Bad Cat OT spec is. To be PP Class A it would have to be up near 8k, but then the screen voltage seems much too high.

[turbofeedus]

I've got a 50w Marshall OT sat unused here so thought I'd use that.I'm not 100% sure what the primary is, I think I did measure it at one point. I think it's a Dagnal 50w plexi transformer.

Probably 3k7 or something like that. That'll handle the power, and and place the load line pretty much like a JMP. I'd like to know what the Bad Cat OT spec is. To be PP Class A it would have to be up near 8k, but then the screen voltage seems much too high.

I spoke to Bill at TDS (they make the transformers for Matchless and Bad Cat), he said for their 30W transformer set, the OT primary impedance is 4K
Here's the datasheet he sent me:

IMG_0824.jpg

EDIT: I can also confirm that running the output tubes hot seems to be intentional, I have a hot cat 100 (4x EL34), and they're biased at ~120% max dissipation.

[martin manning]

Great stuff, thanks! Must be 30W clean, and that won't be nearly all Class A, more like a typical cathode biased AB1.

Marcus, your Plexi OT should be fine, but I'd definitely lower the idle to 60-65 mA plate current, 65-70 mA cathode current at 375V, going for a maximum of 90-95% plate dissipation.

[norburybrook]

Great stuff, thanks! Must be 30W clean, and that won't be nearly all Class A, more like a typical cathode biased AB1.

Marcus, your Plexi OT should be fine, but I'd definitely lower the idle to 60-65 mA plate current, 65-70 mA cathode current at 375V, going for a maximum of 90-95% plate dissipation.

will the 360R 10W resistors take care of that?

while we're on this subject, how can you have a bias of 120% and not have valves red plating? I'm sure Ohms law will be in here somewhere in this answer


M

[martin manning]

...will the 360R 10W resistors take care of that?

while we're on this subject, how can you have a bias of 120% and not have valves red plating? I'm sure Ohms law will be in here somewhere in this answer
:mrgreen:

There may be some tweaking required, but yes, a larger cathode resistor will turn the wick down. And yes, you're right, higher resistance means more voltage drop per milliamp, so more negative bias Vg-k, and a lower current at the idle equilibrium.

I really don't know how the voltages on the schematic could be tolerated, unless the tubes are actually rated very conservatively.

[pompeiisneaks]

If the amp is setup with cathode bias and operating in Class A, it can take up to 59.8mA per tube, and with two EL34's that would be nearly 120mA at 90% diss, so that's considered safe for class A no? I'm not sure how to determine if it IS in class A, that's still always been confusing for me

~Phil

[norburybrook]

If the amp is setup with cathode bias and operating in Class A, it can take up to 59.8mA per tube, and with two EL34's that would be nearly 120mA at 90% diss, so that's considered safe for class A no? I'm not sure how to determine if it IS in class A, that's still always been confusing for me

~Phil

you and me both Phil


M

[turbofeedus]

Great stuff, thanks! Must be 30W clean, and that won't be nearly all Class A, more like a typical cathode biased AB1.

Marcus, your Plexi OT should be fine, but I'd definitely lower the idle to 60-65 mA plate current, 65-70 mA cathode current at 375V, going for a maximum of 90-95% plate dissipation.

will the 360R 10W resistors take care of that?

while we're on this subject, how can you have a bias of 120% and not have valves red plating? I'm sure Ohms law will be in here somewhere in this answer


M

In my hot cat, they don't red plate, at least it isn't visible to me. But replacing the cathode resistors is on my short list, along with the filter caps.
Here's a gut shot:

hotcat100-resized.jpg