Matchless/Bad cat new build questions

[didit]

EL34 spec snap.jpg

With minor tweaks that schematic is quite close to this original Mullard spec for cathode-bias. With 4KΩ primary, 300Ω independent cathode and 1kΩ per screen it should be durable. Reducing primary voltage & increasing cathode resistor will bias quite cold. Afraid I'm uncertain following all of this thread. Regardless, neither set up will operate "pure" class A --- except of course at barely whisper over idle.

Best .. Ian

[xtian]

10W screen resistors!

Also, here, let me glue ALL the components together for you.

[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

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

thanks do you like the sound of the amp? can you describe it in some way?

100w...is it really loud?

M

[turbofeedus]

thanks do you like the sound of the amp? can you describe it in some way?

100w...is it really loud?

M

Trigger warning: toan words incoming.

I really like the HC100, more than some of the HC30's I've tried. The clean channels on both are great, but the gain channels are sonically quite different. The HC30 has a distinct thickness and rasp to it, kinda that vox grind but even more untamed. There may be a technical explanation, but the HC30 seems to fold up with all the gain available. The HC100 on the other hand is brutal, still grind-y but more punchy and refined. Now I can't recall, but this could very likely be a SS vs tube rectified thing, my HC100 is SS only. The transformers are also massive on the HT100, I'm pretty sure I don't even need to raise the chassis off my workbench even with the EL34 installed.

Very, very loud. I have a 2x12 with EVM12L clones, and when wound up I would not want to be on the business end without hearing protection.

Transformers:

hotcat100-resized2.jpg

[norburybrook]

thanks do you like the sound of the amp? can you describe it in some way?

100w...is it really loud?

M

Trigger warning: toan words incoming.

I really like the HC100, more than some of the HC30's I've tried. The clean channels on both are great, but the gain channels are sonically quite different. The HC30 has a distinct thickness and rasp to it, kinda that vox grind but even more untamed. There may be a technical explanation, but the HC30 seems to fold up with all the gain available. The HC100 on the other hand is brutal, still grind-y but more punchy and refined. Now I can't recall, but this could very likely be a SS vs tube rectified thing, my HC100 is SS only. The transformers are also massive on the HT100, I'm pretty sure I don't even need to raise the chassis off my workbench even with the EL34 installed.

Very, very loud. I have a 2x12 with EVM12L clones, and when wound up I would not want to be on the business end without hearing protection.

Transformers:


hotcat100-resized2.jpg

thanks Just been speaking to @ Dcribbs via PM and he said SS for the gain and Tube for the clean was his preferred tones. He's thinking about making another one again.

OK, thanks everyone. I've just got the artwork done for the chassis face plates so will be sending off my parts order soon.

I am definitely going to do tubes up build, just deciding mentally on the cab colours/design . I want to use up some of the bits of tolex I have left over from builds. That could mean snakeskin again as I have a lot left!!!

I'll do a full build thread as usual.

I wonder what my lessons will be on this build


M

[pdf64]

...how can you have a bias of 120% and not have valves red plating?...

Generally speaking, I think that red plating is way beyond the safe operating area; bear in mind that when metals get red hot their properties can change, surface treatments be affected, thin plate structures may become physically distorted. Very importantly for valves, gas atoms / molecules that were bound within the metal can be released, which can lead to (positive) grid current, such that plate current increases, things get hotter and spiral into meltdown.
So, it may be assumed that manufacturers ensure that red plating takes a fair bit more dissipation than the rated absolute max. And the absolute max is a fair bit higher than the design max. And the design max is a fair bit higher than the design centre, which is the system under which EL34 got their 25W rating.
RCA AN174 explains the relationship between those ratings http://www.one-electron.com/Archives/RC ... 0Tubes.pdf

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 ...

It can be tricky to get your head around what class A actually is and isn't.
Unless you're lucky or the amp has been designed especially for it, it doesn't equate to the power tubes idling at their plate dissipation limit; as AN174 shows, a plate dissipation limit isn't a definitive thing, different rating systems will elicit different limits.
The key thing about push-pull class A is that at the amp's max clean power output, the current in either 'side' doesn't cut off at any part of the signal cycle. That can be viewed on a scope across a small, unbypassed cathode resistor between each side's (or cathode's) return to 0V, eg the typical 1 ohm cathode current sensing resistor (not feasible with shared cathode bias, eg an AC30).
Give it a go on a real amp!

[norburybrook]

...how can you have a bias of 120% and not have valves red plating?...

Generally speaking, I think that red plating is way beyond the safe operating area; bear in mind that when metals get red hot their properties can change, surface treatments be affected, thin plate structures may become physically distorted. Very importantly for valves, gas atoms / molecules that were bound within the metal can be released, which can lead to (positive) grid current, such that plate current increases, things get hotter and spiral into meltdown.
So, it may be assumed that manufacturers ensure that red plating takes a fair bit more dissipation than the rated absolute max. And the absolute max is a fair bit higher than the design max. And the design max is a fair bit higher than the design centre, which is the system under which EL34 got their 25W rating.
RCA AN174 explains the relationship between those ratings http://www.one-electron.com/Archives/RC ... 0Tubes.pdf

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 ...

It can be tricky to get your head around what class A actually is and isn't.
Unless you're lucky or the amp has been designed especially for it, it doesn't equate to the power tubes idling at their plate dissipation limit; as AN174 shows, a plate dissipation limit isn't a definitive thing, different rating systems will elicit different limits.
The key thing about push-pull class A is that at the amp's max clean power output, the current in either 'side' doesn't cut off at any part of the signal cycle. That can be viewed on a scope across a small, unbypassed cathode resistor between each side's (or cathode's) return to 0V, eg the typical 1 ohm cathode current sensing resistor (not feasible with shared cathode bias, eg an AC30).
Give it a go on a real amp!

thanks pdf64

that RCA document is a real piece of history I'll try and read it and learn what i can, I do tend to start glazing over though when things get a bit 'dry' as I'm a musician at heart not an EE , however I learn something new and valuable every build

I'm even more intrigued to build this amp now...I'll probably hate it LOL....that's why I have to make it look nice in case i have to sell it.

M

[norburybrook]

OK,

I've made a start.

Been drilling and filing and sawing etc to get things to fit and and make holes for tubes/transformers etc.

had some nice face plates plates made.

Now trying to get everything to fit is proving difficult.

Is there enough room here between the output tubes and these switches?

the one on the left is the half power switch and the right is the SS/Tube recto switch.

Match switches.jpg

next i have to work out where to fit the cathode resistors and caps!!


M

[drew]

Here's a gut shot:


hotcat100-resized.jpg

That’s a production amp, not a home build?

[turbofeedus]

That’s a production amp, not a home build?

Production is a relative term, but yes, this not a clone.
I think the build is pretty OK. Not crazy about the flying 10W screen resistors or the filament wiring, but otherwise I think it's ok.
To what do you take exception?