So a client of mine has this amp for his office use. It's a Bad Cat 5R. Don't know much about them but have heard they are related somehow to Matchless. This is my first venture into a Bad Cat and having worked on many Matchless amps I can say they certainly are similar.
The job: Modify the crap out of this amp!
My client has recently become hooked on old amps, Marshall, Tweed Fender, Supro, 50's Gibson, Ampeg. This amp just isn't doing it for him any longer. It's stiff and rather dull.
Now, it's a very simple circuit with only 2 tubes. A 6V6 driven by (interesting choice that I like!) a 6SL7. There is only so much modifying one can do. When I fired it up the first thing I noticed was what a lack of distortion there was. I like my little amps, distortion (Gibson GA5 from the 50's) or no distortion (Magnatone Varsity) to have a warm singing quality to them. My client is into warmth more than over the top blast so the goal is to get it to do both. I found the amp to lack warmth.
I have a back to front approach with amps whether I'm trouble shooting or modifying. Example: first thing to look at with a dead or dull sounding amp is the speaker and/or the power supply. So I started there. First thing: Why use a Celestion Vintage 30 on a 5 watt amp? If you want to go for cleaner tones that's great and I'm not objecting their speaker choice, it's purely subjective. But for this project we went with a 20 watt Weber 12A125A the one that says "woody and reedy".
https://www.tedweber.com/12a125-a
I love this speaker. My favorite amp that I built uses a 30 watt version. A bit on the darker/warmer side of the spectrum, nice smooth breakup.
The second thing is the power supply. Stock it uses 2 1N4007 diodes loading into a 100uf 450V capacitor. I decided if it's an easy fix to add a tube rectifier let's go for it. I like my little amps to compress! You can see diodes right under that blue power resistor.
So once I looked around I noticed 2 things right away that made me happy. Chassis is pre-punched for more octal sockets and there are 2 yellow wires coming off the power transformer. That means there is a 5 volt filament tap! YES! This job just got easier.
So I took out the plastic socket plug and installed a socket. Removed the 2 diodes, put the red wires to pins 4&6, the yellow to pins 2&8 and replaced the 100uf with a 47uf cap. Why replace that cap? The 5 volt filament line is at risk with anything too high. With diodes the sky is the limit, but with a tube rectifier you don't want to go too much above 50uf.
I opted to use a 5V4 for the rectifier. For squishiest results go with a 5Y3. I figured this would be a dramatic shift as is and didn't care to go too dramatic. Fun thing is, they are interchangeable and my client likes to fiddle with tubes.
The circuit itself:
I won't go too much into detail on the circuit mods cause I encourage you to use your own creativity. What I did start with was the cathode resistor on the output tube. It's a 270 ohm. I replaced it with a 500 ohm. It's just more loose. I also replaced all those horrible brick drop polypropylene caps with warmer sounding Mojo Dijon caps and any resistors in the signal path with carbon composition. Also the first gain stage cathode cap is a 6.8k. I just copied Fender and used a 1.5k bypassed by a 22uf cap. The whole amp seemed to come to life in a good vintage fun way.
So now this amp sings at any volume. It has a master volume which didn't really serve too much of a purpose before but now you can bring it down and blast the preamp. With my S.G. it's like a mini AC/DC concert.
I think these are good amps even stock. Very well made and built to last. The reverb is transistor driven but that doesn't offend me, it's quiet and sounds nice. But if you aren't opposed to making your Bad Cat a Badder Cat, try some of these fun mods, use your creativity and you can tailor it to suit your needs.
Disclaimer: if you are a novice, don't try this at home. Pay someone like me to do this kind of work. Tube amps are no joke inside, they can kill you with the amount of juice running around in there! So tell your favorite tech what you hear in your head and let the magician work his or her magic.
Thanks! JB
The job: Modify the crap out of this amp!
My client has recently become hooked on old amps, Marshall, Tweed Fender, Supro, 50's Gibson, Ampeg. This amp just isn't doing it for him any longer. It's stiff and rather dull.
Now, it's a very simple circuit with only 2 tubes. A 6V6 driven by (interesting choice that I like!) a 6SL7. There is only so much modifying one can do. When I fired it up the first thing I noticed was what a lack of distortion there was. I like my little amps, distortion (Gibson GA5 from the 50's) or no distortion (Magnatone Varsity) to have a warm singing quality to them. My client is into warmth more than over the top blast so the goal is to get it to do both. I found the amp to lack warmth.
I have a back to front approach with amps whether I'm trouble shooting or modifying. Example: first thing to look at with a dead or dull sounding amp is the speaker and/or the power supply. So I started there. First thing: Why use a Celestion Vintage 30 on a 5 watt amp? If you want to go for cleaner tones that's great and I'm not objecting their speaker choice, it's purely subjective. But for this project we went with a 20 watt Weber 12A125A the one that says "woody and reedy".
https://www.tedweber.com/12a125-a
I love this speaker. My favorite amp that I built uses a 30 watt version. A bit on the darker/warmer side of the spectrum, nice smooth breakup.
The second thing is the power supply. Stock it uses 2 1N4007 diodes loading into a 100uf 450V capacitor. I decided if it's an easy fix to add a tube rectifier let's go for it. I like my little amps to compress! You can see diodes right under that blue power resistor.
So once I looked around I noticed 2 things right away that made me happy. Chassis is pre-punched for more octal sockets and there are 2 yellow wires coming off the power transformer. That means there is a 5 volt filament tap! YES! This job just got easier.
So I took out the plastic socket plug and installed a socket. Removed the 2 diodes, put the red wires to pins 4&6, the yellow to pins 2&8 and replaced the 100uf with a 47uf cap. Why replace that cap? The 5 volt filament line is at risk with anything too high. With diodes the sky is the limit, but with a tube rectifier you don't want to go too much above 50uf.
I opted to use a 5V4 for the rectifier. For squishiest results go with a 5Y3. I figured this would be a dramatic shift as is and didn't care to go too dramatic. Fun thing is, they are interchangeable and my client likes to fiddle with tubes.
The circuit itself:
I won't go too much into detail on the circuit mods cause I encourage you to use your own creativity. What I did start with was the cathode resistor on the output tube. It's a 270 ohm. I replaced it with a 500 ohm. It's just more loose. I also replaced all those horrible brick drop polypropylene caps with warmer sounding Mojo Dijon caps and any resistors in the signal path with carbon composition. Also the first gain stage cathode cap is a 6.8k. I just copied Fender and used a 1.5k bypassed by a 22uf cap. The whole amp seemed to come to life in a good vintage fun way.
So now this amp sings at any volume. It has a master volume which didn't really serve too much of a purpose before but now you can bring it down and blast the preamp. With my S.G. it's like a mini AC/DC concert.
I think these are good amps even stock. Very well made and built to last. The reverb is transistor driven but that doesn't offend me, it's quiet and sounds nice. But if you aren't opposed to making your Bad Cat a Badder Cat, try some of these fun mods, use your creativity and you can tailor it to suit your needs.
Disclaimer: if you are a novice, don't try this at home. Pay someone like me to do this kind of work. Tube amps are no joke inside, they can kill you with the amount of juice running around in there! So tell your favorite tech what you hear in your head and let the magician work his or her magic.
Thanks! JB
Everything sounds like an ideal upgrade. How do I get more gain out of it?
ReplyDeleteDid you attempt any of what I wrote? First start with the cathode bypass resistor on the 6SL7. The 6.8k one. Replace with anything from 820 ohms to 1.5k and bypass with a 22uf cap. That makes a difference right there. Generally speaking the lower the resistor value the more gain you get out of the tube. I would start there.
DeleteThe second gain stage cathode resistor you can play with too. These are very simple amps, not terribly removed from a tweed Champ or GA5. So they require a simple approach.
You can try a 6SL7 to 12AX7 adapter. They're inexpensive and fun.
While I love the 6SL7 for it's roundness, it's gain factor is 70 whereas the 12AX7 is 100. That's not a small difference. I've used them for different tones, more or less gain etc. I have a friend who is hooked on them.
If that's not enough then maybe install a universal gain stage if you feel you have the skills. These amps have plenty of room. But me, I prefer to keep it simple.
Hope this helps.