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I had our Bad Cat Hot Cat 30R Combo Amp sent over for repairs last week, and finally got started on it late yesterday. The note on it only stated Screeching when Brilliance Knob turned up. What I found, though, was just switching it out of S/B, it was growling loudly, as well as screeching. Turning down all of the volume controls (Master Volume, Level, Gain, Edge, Volume) had no effect on the loud growling, so shut it down, and set it down on it's face to extract the chassis.
I don't have the matching schematic for the 30R....not sure if one was ever produced for distribution, though I'd have to believe engineering drawings exist (existed). This one has seven preamp tubes. The only Hot Cat 30 schematic I have has 3 preamp tubes. The Tube Chart diagram I have shows seven preamp tubes. This one has six on the top surface, and one hidden away inside.
badcat-hotcat-30-amplifier-schematic.pdf
After chasing this with the amp connected to a dummy load, as well as my audio analyzer/scope, where it began with a large knurly-looking 120Hz hum on the scope, I finally found by unplugging the inside tube, it went away. I don't have the schematic for the Reverb circuit, nor the Brilliance circuit, nor the Effects Send/Return jack circuit, nor where the Aux Return jack comes in. Navigating thru their forest of components placed over the tube sockets below, use of teflon sleeving everywhere, along with the same White Teflon hook-up wire, nicely bundled together, tracing out the circuits in the land of the unknowns became tedious. I did draw out the Reverb Send & Return circuit, finding the Reverb Return comes into the system at the Feedback side of the LTPI driver circuit. I haven't found where the Send originates from. In the past, I used to get nasty growling when the Return RCA cable didn't make solid connection to the RCA jacks on the chassis, nestled just above the distribution of their Logo Lighting.
I did discover the tube inside the chassis is in control of the Effects Return circuit, which appears to connect to the Driver circuit, though didn't yet determine whether it's on the signal side or feedback side of the LTPI. What I did finally find was, the Effects Return jack was involved with the screeching & oscillation, as was the Return level control, appearing to be a shunt-based control (top of pot not used on either the Send nor Return level pots). If I patched the Send to the Return, I could make it oscillate using the Brilliance control at least half way up, and turning up the Return level pot. Remove the patch, it would be ok as long as the Return jack remained grounded (oxidation on the Normal of that jack was part of the problem, which I cured). Patching in a 100 k termination plug definitely yielded oscillation, though 50 ohm termination didn't).
I finally changed that inside tube to a 12AY7, and got much better results. At least as long as I didn't unmute that Return Jack, or patch in a non-low-Z source impedance.
The EL-34 tubes are running at 80mA thru the cathodes. I didn't measure the screen current. Supplies for the plates are 376VDC, with Cathodes at 21.5VDC 270 ohm/10W cathode resistors (with 220uF bypass caps). Their schematic shows 382V at the top of the screen resistor, 370V at the plates, 24V at the cathodes, 367V at the screen, 1k resistor, which is 15V across it or 15mA, so 80mA - 15mA = 65mA Plate current & 23W plate dissipation....near max rating of the tube (using the plate & cathode voltages I measured).
We only have a few Bad Cat combo amps in our inventory....this one is by far my favorite. I'll post some more images when I get this back together. Does anyone have additional documentation on this Hot Cat 30R? I'll have to draw up when I sketched out today and post that.
I don't have the matching schematic for the 30R....not sure if one was ever produced for distribution, though I'd have to believe engineering drawings exist (existed). This one has seven preamp tubes. The only Hot Cat 30 schematic I have has 3 preamp tubes. The Tube Chart diagram I have shows seven preamp tubes. This one has six on the top surface, and one hidden away inside.
badcat-hotcat-30-amplifier-schematic.pdf
After chasing this with the amp connected to a dummy load, as well as my audio analyzer/scope, where it began with a large knurly-looking 120Hz hum on the scope, I finally found by unplugging the inside tube, it went away. I don't have the schematic for the Reverb circuit, nor the Brilliance circuit, nor the Effects Send/Return jack circuit, nor where the Aux Return jack comes in. Navigating thru their forest of components placed over the tube sockets below, use of teflon sleeving everywhere, along with the same White Teflon hook-up wire, nicely bundled together, tracing out the circuits in the land of the unknowns became tedious. I did draw out the Reverb Send & Return circuit, finding the Reverb Return comes into the system at the Feedback side of the LTPI driver circuit. I haven't found where the Send originates from. In the past, I used to get nasty growling when the Return RCA cable didn't make solid connection to the RCA jacks on the chassis, nestled just above the distribution of their Logo Lighting.
I did discover the tube inside the chassis is in control of the Effects Return circuit, which appears to connect to the Driver circuit, though didn't yet determine whether it's on the signal side or feedback side of the LTPI. What I did finally find was, the Effects Return jack was involved with the screeching & oscillation, as was the Return level control, appearing to be a shunt-based control (top of pot not used on either the Send nor Return level pots). If I patched the Send to the Return, I could make it oscillate using the Brilliance control at least half way up, and turning up the Return level pot. Remove the patch, it would be ok as long as the Return jack remained grounded (oxidation on the Normal of that jack was part of the problem, which I cured). Patching in a 100 k termination plug definitely yielded oscillation, though 50 ohm termination didn't).
I finally changed that inside tube to a 12AY7, and got much better results. At least as long as I didn't unmute that Return Jack, or patch in a non-low-Z source impedance.
The EL-34 tubes are running at 80mA thru the cathodes. I didn't measure the screen current. Supplies for the plates are 376VDC, with Cathodes at 21.5VDC 270 ohm/10W cathode resistors (with 220uF bypass caps). Their schematic shows 382V at the top of the screen resistor, 370V at the plates, 24V at the cathodes, 367V at the screen, 1k resistor, which is 15V across it or 15mA, so 80mA - 15mA = 65mA Plate current & 23W plate dissipation....near max rating of the tube (using the plate & cathode voltages I measured).
We only have a few Bad Cat combo amps in our inventory....this one is by far my favorite. I'll post some more images when I get this back together. Does anyone have additional documentation on this Hot Cat 30R? I'll have to draw up when I sketched out today and post that.
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