Questions about a Bad Cat Hot Cat?

[turbofeedus]

Hey all.
I've had a hot cat for a while now, the 100W version. I went searching for schematics, just trying to wrap my head around how it works, and found a forum (can't find it now) discussing some of the drawbacks of the design on this amp. Schematic is attached.

From what I remember, people had issues with how the grid leak bias was setup on input stage of the "gain" channel. From what I've been reading it seems like 1M is quite low for grid leak. There is a 5.1M pulldown, but it seems like maybe those positions should be switched? I thought this might be a schematic error, but sure enough I checked my amp and it's wired as shown. How is this affecting the operation of the stage? Is it something worth messing with?

There were also some questions marks around the MV, that it wouldn't really work well as shown. Again, I checked my amp and the schematic is correct. What's the issue here? The master volume does seem operational on my amp.

Any other issues you see with the schematic? My amp seems to match the preamp exactly, but my power amp is just 4x EL34.

[sluckey]

The 5.1M and 1M need to swap positions. The three signs of a grid leak bias stage are... cathode grounded, grid has high resistance to ground, and the grid must have an input cap to insure the negative charge on the grid has to leak through that high value grid resistor rather than through some lower input resistance such as a guitar pickup.

At least that's what I was taught. Having said that, that's not the first grid leak bias circuit I've seen with a 1M grid leak resistor. I figured it was an engineering/schematic error in that case also, but the amp worked.

[Tony Bones]

If it works, and it you like the way it sounds, then who cares what the armchair engineers think?

Having said that, it's possible that the schematic is wrong. Between the two of us, you're the only one with an actual example of the amp to open up and compare to the drawing ...

[dcribbs1412]

I've built amps based on the schem posted along with the Ceriatone layout.
Nice sounding circuit, great simple clean channel. OD channel was monstrous and MV worked fine, I switched the 1 and 5M resistors. Cant remember if there was any noticeable difference.
Builds went out the door to new owners.
On the list of future "gonna keep one for myself" list of builds.
Like Tony says if it sounds good play it, Or if you feel like tinkering, change it and report any finding back.
I would like to know if it changes the tone.

[turbofeedus]

The 5.1M and 1M need to swap positions. The three signs of a grid leak bias stage are... cathode grounded, grid has high resistance to ground, and the grid must have an input cap to insure the negative charge on the grid has to leak through that high value grid resistor rather than through some lower input resistance such as a guitar pickup.

At least that's what I was taught. Having said that, that's not the first grid leak bias circuit I've seen with a 1M grid leak resistor. I figured it was an engineering/schematic error in that case also, but the amp worked.

Yeah, works in my case too. My understanding is if the input swings too positive (from a hot pickup or a pedal or something) it can cause problems. I've definitely put high output pedals in the front and got some unpleasant "collapsing", I'm thinking this might be related?

If it works, and it you like the way it sounds, then who cares what the armchair engineers think?

Having said that, it's possible that the schematic is wrong. Between the two of us, you're the only one with an actual example of the amp to open up and compare to the drawing ...

IIRC, there was discussion that it may actually be causing unnecessary wear to V1. In most cases I agree with you, the sound is the ultimate arbiter, but I don't need to be smoking tubes needlessly.

My amp does in fact match the schematic.

I've built amps based on the schem posted along with the Ceriatone layout.
Nice sounding circuit, great simple clean channel. OD channel was monstrous and MV worked fine, I switched the 1 and 5M resistors. Cant remember if there was any noticeable difference.
Builds went out the door to new owners.
On the list of future "gonna keep one for myself" list of builds.
Like Tony says if it sounds good play it, Or if you feel like tinkering, change it and report any finding back.
I would like to know if it changes the tone.

I may just, I've actually been trying to sell it locally for unrelated reasons. It's a real monster, just seems like people aren't really going for the 100W.
We'll see.

[chief mushroom cloud]

god...if you only knew Heidrich and Sampson......a TV repair tech and a desperate wannabe. The wild west coast is full of tin earz and foghorn leghorn builders and so called 'goorooz'. Why, hell, where do you think the goop came from? Kangaroo Jizz?

[turbofeedus]

god...if you only knew Heidrich and Sampson......a TV repair tech and a desperate wannabe. The wild west coast is full of tin earz and foghorn leghorn builders and so called 'goorooz'. Why, hell, where do you think the goop came from? Kangaroo Jizz?

Why did you capitalize Kangaroo Jizz? Is this some band I haven't heard yet?

[chief mushroom cloud]

god...if you only knew Heidrich and Sampson......a TV repair tech and a desperate wannabe. The wild west coast is full of tin earz and foghorn leghorn builders and so called 'goorooz'. Why, hell, where do you think the goop came from? Kangaroo Jizz?

Why did you capitalize Kangaroo Jizz? Is this some band I haven't heard yet?

like a brand name for a circuit-hiding goop....
A couple of my fav players use Badcats...Steve Wilson being one. And Dave Gregory occasionally uses them. And John Thompson is a personal friend of mine.

[pdf64]

The standby switching arrangement looks to be of the bubba approved 'hot switching' type, so to avoid damage / catastrophic failure of the tube rectifier, I suggest to avoid the use of standby when switched to the tube rectifier mode.
The benefits of grid leak biasing of input stages is that it ensures that the tube's heater has a solid ac 0V reference, thereby eliminating hum due to h-k leakage.
And it saves on the expense of an ecap, which was probably a significant consideration back in the 50s.
For modern guitar amp usage, it will help to promote asymmetric amplification of the signal, and may offer a degree of signal compression, as larger signal levels, ie whose positive peaks will be rectified by the g-k diode, will charge the input coupling cap and temporarily increase the bias voltage at the grid. The use of a 1M grid leak resistor (over the 5M1) may tend to promote these effects.

[turbofeedus]

The standby switching arrangement looks to be of the bubba approved 'hot switching' type, so to avoid damage / catastrophic failure of the tube rectifier, I suggest to avoid the use of standby when switched to the tube rectifier mode.
The benefits of grid leak biasing of input stages is that it ensures that the tube's heater has a solid ac 0V reference, thereby eliminating hum due to h-k leakage.
And it saves on the expense of an ecap, which was probably a significant consideration back in the 50s.
For modern guitar amp usage, it will help to promote asymmetric amplification of the signal, and may offer a degree of signal compression, as larger signal levels, ie whose positive peaks will be rectified by the g-k diode, will charge the input coupling cap and temporarily increase the bias voltage at the grid. The use of a 1M grid leak resistor (over the 5M1) may tend to promote these effects.

Isn't that how most people do standby switches? Just lifting the HT center tap? I thought that is how Fender does it, or at least did...
Mine in fact is SS rectification only, however I've made a habit of just not using or building with standby switches anyway.

[pompeiisneaks]

The standby switching arrangement looks to be of the bubba approved 'hot switching' type, so to avoid damage / catastrophic failure of the tube rectifier, I suggest to avoid the use of standby when switched to the tube rectifier mode.
The benefits of grid leak biasing of input stages is that it ensures that the tube's heater has a solid ac 0V reference, thereby eliminating hum due to h-k leakage.
And it saves on the expense of an ecap, which was probably a significant consideration back in the 50s.
For modern guitar amp usage, it will help to promote asymmetric amplification of the signal, and may offer a degree of signal compression, as larger signal levels, ie whose positive peaks will be rectified by the g-k diode, will charge the input coupling cap and temporarily increase the bias voltage at the grid. The use of a 1M grid leak resistor (over the 5M1) may tend to promote these effects.

Isn't that how most people do standby switches? Just lifting the HT center tap? I thought that is how Fender does it, or at least did...
Mine in fact is SS rectification only, however I've made a habit of just not using or building with standby switches anyway.

Most fender schematics I've seen have a spst switch inline with the power rail. It's on the B+ side before the first cap, or just after it. Not on the center tap.

~Phil

[turbofeedus]

The standby switching arrangement looks to be of the bubba approved 'hot switching' type, so to avoid damage / catastrophic failure of the tube rectifier, I suggest to avoid the use of standby when switched to the tube rectifier mode.
The benefits of grid leak biasing of input stages is that it ensures that the tube's heater has a solid ac 0V reference, thereby eliminating hum due to h-k leakage.
And it saves on the expense of an ecap, which was probably a significant consideration back in the 50s.
For modern guitar amp usage, it will help to promote asymmetric amplification of the signal, and may offer a degree of signal compression, as larger signal levels, ie whose positive peaks will be rectified by the g-k diode, will charge the input coupling cap and temporarily increase the bias voltage at the grid. The use of a 1M grid leak resistor (over the 5M1) may tend to promote these effects.

Isn't that how most people do standby switches? Just lifting the HT center tap? I thought that is how Fender does it, or at least did...
Mine in fact is SS rectification only, however I've made a habit of just not using or building with standby switches anyway.

Most fender schematics I've seen have a spst switch inline with the power rail. It's on the B+ side before the first cap, or just after it. Not on the center tap.

~Phil

Gotcha, I must be confused.

[Tobyk]

The 5.1M and 1M need to swap positions. The three signs of a grid leak bias stage are... cathode grounded, grid has high resistance to ground, and the grid must have an input cap to insure the negative charge on the grid has to leak through that high value grid resistor rather than through some lower input resistance such as a guitar pickup.

At least that's what I was taught. Having said that, that's not the first grid leak bias circuit I've seen with a 1M grid leak resistor. I figured it was an engineering/schematic error in that case also, but the amp worked.

Gibson had some old schematics (some of the plenty GA-20 versions) with grid leak values like 470K or 1M for a 12ax7 or 12ay7. Apparently they worked. So what’s up with that?
Why use up to 15M or more if 470K us enough? I think someone should dig deeper into this.

Edit: I think these 5M–15M grid leak bias resistors might be a case of safety margins. A signal hot enough and a 470K grid leak won’t be able to keep the negative grid voltage.

[sluckey]

Read page 12 of this pdf from Merlin... http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

Magnatone used very large resistors (22M) in their grid leak stages. Look at V2 and V3 in this schematic...

https://el34world.com/charts/Schematics ... ne_m20.pdf

In tech school in the '60s we were taught that grid leak biasing used very large resistors for the same reasons that Merlin stated. I saw a lot of 4.7M and 5.6M and 10M in consumer electronics during that time. Look at the All American 5 schematic...

https://www.angelfire.com/electronic/fu ... AA5-5.html

Magnatone had some sharp engineers. Not so much with Gibson.

[Tobyk]

Read page 12 of this pdf from Merlin... http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

Magnatone used very large resistors (22M) in their grid leak stages. Look at V2 and V3 in this schematic...

https://el34world.com/charts/Schematics ... ne_m20.pdf

In tech school in the '60s we were taught that grid leak biasing used very large resistors for the same reasons that Merlin stated. I saw a lot of 4.7M and 5.6M and 10M in consumer electronics during that time. Look at the All American 5 schematic...

https://www.angelfire.com/electronic/fu ... AA5-5.html

Magnatone had some sharp engineers. Not so much with Gibson.

Yes, I know the workings of it. But apparently it works with smaller resistors as well, which is interesting and hasn’t been discussed much, if at all, I believe.