Vintage Tube Amp Modification

[Daven]

Hello all,

I'm new to the forum and not sure if this is the place to ask this, but here goes. I recently purchase a cheapo tube guitar amp called a Kustom Kraft Model 600. It is one those with a 6AU6, 50C5, and a 35W4 rectifier tube. I want to modify if it to make it safer by installing a 3 prong grounded power cord and a line fuse. The heaters of the tubes are connected in series with a resistor and the line voltage. The heaters are in parallel with the primary of the isolation transformer and the secondary of the isolation transformer is rectified to the supply voltages. My question is how to determine what fuse to use and if I need to include a thermistor in series with the heaters or just a slo blow fuse in order to compensate for the initial surge for the heaters. I don't want to use a fuse larger than necessary. Attached is the schematic as it is currently without the modification. Thank you.

[Daven]

Sorry, forgot the schematicKustom Kraft 600A.pdf

[Daven]

My mistake, the preamp tube is a 12AU6 not a 6AU6. I installed a .7 amp slo blow line fuse. Also its now putting out 1.6 W RMS of audio. Does this seem about right for an amplifier like this?...Thanks...Dave

[Leo_Gnardo]

My mistake, the preamp tube is a 12AU6 not a 6AU6. I installed a .7 amp slo blow line fuse. Also its now putting out 1.6 W RMS of audio. Does this seem about right for an amplifier like this?...Thanks...Dave

Yes, they're not terribly powerful are they. IF the filament circuit is isolated from chassis, on the schematic it looks like it is, you should be able to ground the chassis with a 3-wire AC cord set. I have a similar amp, LectroLab R200, in use as my test bench amp. The hum level used to bother me. I couldn't make proper decisions on gear I was repairing with that much hum. To improve it I bumped up the value of the hi voltage filter caps, also ran the filament so that the preamp tube was the end of the filament string near AC neutral, and rectifier at the hot side of the AC line. A lot better but the buzz level is still a hassle.

[Daven]

Yes, they're not terribly powerful are they. IF the filament circuit is isolated from chassis, on the schematic it looks like it is, you should be able to ground the chassis with a 3-wire AC cord set. I have a similar amp, LectroLab R200, in use as my test bench amp. The hum level used to bother me. I couldn't make proper decisions on gear I was repairing with that much hum. To improve it I bumped up the value of the hi voltage filter caps, also ran the filament so that the preamp tube was the end of the filament string near AC neutral, and rectifier at the hot side of the AC line. A lot better but the buzz level is still a hassle.

Thanks for the reply Leo. I installed a 3 prong grounded cord, replaced the death cap with a new safety cap, replaced the power supply filters, tightened and cleaned the tube sockets and the absolutely filthy controls. I didn't notice too much noise or hum after that but I've only ran it so far with the audio generator. I haven't plug the guitar in yet. Next I'm gonna check the bias on the output tube...Dave

[Daven]

After thinking about it a little, I think I want to go with an isolation transformer for the entire circuit. Currently the power supply is being powered by an isolation transformer, but the heaters are still connected to the line. No problem as long as there is no heater to cathode short. I don't think I want to chance it. Can anyone give some advice as to how to go about figuring out the power requirement for the isolation transformer? thanks...Dave