Bad Cat Hot Cat 15 Questions

[Fred G.]

Hi All,

I recently bought a used recent-model (2016, I think) Bad Cat Hot Cat 15R. I can't find the actual schematic, but the Hot Cat 30 schematic floating around is, I guess, close enough for the preamp, at least. I'm confused by V1a's lack of a cathode resistor or cathode bypass cap (the "gain" control in the gain channel). . The plate resistor is 220k. What is the net audio/sonic/circuit effect of this?

I also find this amp to be too "over the top" in the gain channel, although it certainly can be easily tamed by more sedate preamp settings. I'm wondering if I could also tame this beast by some strategic tube changes (i.e., 12AY7 subs).

But mostly, I'd like to understand this type of preamp tube configuration on v1a and what it's intended effect is. Any suggestions on component changes to back the gain down in the Gain channel would be appreciated, though, if only for discussion.

Man, this thing has a gorgeous tone in the clean channel, btw. Wow!

Thanks in advance!

Fred G.

[alexradium]

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
this amp is 4 stages plus CF so very high gain,whatever tube you use.
I have seen in a similar model they used a first stage cathode with 1k/22uF,you could probably tame that stage with 2,2k/4,7k alone and maybe inserting a resistor in series with the gain pot,like 470k.
Another thing you can try is lifting the cathode bypass in third stage.

[Enzo]

You can't just count gain stages. Looking at the 30 schematic, leaving V2b there is a 470k/150k voltage divider dropping signal level. Then leaving V2a, there is another voltage divider across the level control. That is 470k/270k.

[tubeswell]

In the Hot Cat 30, V1a in the gain channel is grid-leak biased, (but is it very hot biasing - most grid leak biased triodes use a bigger grid leak resistor to get the bias e.g. 5M,it it still ends up being a warmish bias. As it is, it might be easier on the tube with a small cathode resistor there - like 470R). Don't know if that's the same in the 15R. Anyway, hot biasing would provide nice sounding distortion.

In the Hot Cat 30, V3 (in the gain channel) is a DC-coupled pair, which would give good compression when driven hard. Don't know if that's the same in the 15R.

If you have Merlin Blencowe's 'designing tube pre-amps' book(s), he goes into some length on high gain pre-amp design and explains how some of the classic high gain circuits get a glassy pre-amp sound by using 4 to 6 alternately hot and cold biased stages, with the last stage being very cold biased (e.g using an unbypassed 39k cathode resistor). This gets rounded clipping on both sides of the wave-form. Many such amps also use one or more DC coupled pairs with CF stages to get more compression. As Enzo has pointed out, these amps also characteristically use plenty fo inter-stage attenuation. Don't know if that's the situation in the Hot Cat 15R. Perhaps you might have a go at tracing the schematic?

[Jazz P Bass]

Hot Cat 30 schematic for those reading along at home.

badcat_hotcat_30.zip

[Fred G.]

Thanks for the input, everyone!

I haven't yet opened this amp up to poke around it's innards, but rest assured, I will soon enough! I want to check it against the schematic for the Hot Cat 30.

This is a really great-sounding amp, in spite of the overkill gain channel. It's just weird to have to set most of the controls at very understated levels to get it to be a nice, crunchy blues-rock amp. It is also LOUDER N' HELL for 15 watts!

I got this amp for a ridiculous price from a co-worker. It's definitely a keeper, but it's hard to resist making a few tweaks to calm it down a bit. I have read these amps have a problem with eating up EL84's because they are biased so hot - that is also something I'm planning to look into - maybe run this amp a little cooler. Any insights, gossip, urban legends on that issue? I haven't opened it up yet, and can't find a schematic for the 15 watt version, so right now I'm just going by hearsay.

I gotta say again, that clean channel is absolutely gorgeous...I'd like to rein in that Gain channel a bit to show off a little bit of that lovely chime on the Gain side instead of smothering it with overdrive...

[tubeswell]

If those voltages in that Hot Cat 30 schematic are correct, I'm not surprised it eats tubes.

[Zozobra]

The hot cat 30 definitely eats tubes. I've read reports of people cooling the bias a good chunk without it having a huge impact on tone. Outside of very occasional studio use I'd opt for the cooler bias and not have to retube te damn thing every 5 minutes!

[Fred G.]

The hot cat 30 definitely eats tubes. I've read reports of people cooling the bias a good chunk without it having a huge impact on tone. Outside of very occasional studio use I'd opt for the cooler bias and not have to retube te damn thing every 5 minutes!

I still haven't had time to open this amp up and give it a good looking over (since a hot Cat 15 schematic cannot seem to be found). I've been thinking, though, perhaps a few tube substitutions may be a simple solution - maybe a 5V4 or 5Y3, to bring voltage down a little bit. I've also wondered about 12AY7s in the first couple of preamp tube spots. Opinions?