1963 Custom Kraft Model 500

Got this little amp to mess around with in the studio. Going to modify it to be safer, have more control, and potentially a bigger speaker as the cab can support it easily. Wondering if the community had any advice or recommendations.


Schematic attached.


Thank you,
Ryan

careful, mains directly connected to series filaments.

never seen or heard this but the series string is a budget design.the v3 doesn't make complete sense ;i am guessing it filled out the filament string adequately and the diode part is superfluous.

might sound good through a large cab

I noticed that the mains were connected directly as well. What is the point of the isolation transformer then? Seems like it could be after the transformer, no? I'm not going to do any voltage tests until after I properly ground the chassis and remove the death cap because I don't quite understand this thing just by looking at it. Also not a huge fan of the fact the switch is also the volume pot, seems not very ideal. Probably going to overhaul the entire power supply.



This amp actually sounds pretty cool. I'm up for this challenge.

Updates:


Amp chassis now has earth ground, IEC cable, cut death cap, and also added a fuse. I changed some of the electrolytic out as I happened to have the correct values on hand. But the original can cap is still in there. I just don't have the values on hand. It's the next thing for sure.


Questions:

Does anyone know anything to give the amp more volume? I feel like the amp isn't even getting loud enough to break-up. I figure this could be power related due to the old can cap, but I'm not sure. I know this amp wont be loud no matter what but I'd be surprised if I couldn't squeeze out 10db more tbh. The snr is dead quiet. This may be because I'm not getting enough volume to begin with.

The speaker doesn't say anything about what is rated. There is plenty of room in the chassis for a 10 or 12" speaker. What's the protocol for figuring out speaker rating?

The trem circuit isn't working but I think maybe I just need to turn it on via foot switch which I don't have one with a 1/2" jack on hand.


Ryan

FarisElek said:

I noticed that the mains were connected directly as well. What is the point of the isolation transformer then? 

Click to expand...

It's not an isolation xfmr. It is a step-up xfmr. Without it, B+ would be about 160V, which is not enough for producing decent power with tubes.

FarisElek said:

Does anyone know anything to give the amp more volume? I feel like the amp isn't even getting loud enough to break-up. I figure this could be power related due to the old can cap, but I'm not sure. I know this amp wont be loud no matter what but I'd be surprised if I couldn't squeeze out 10db more tbh.

Click to expand...

What is the B+ voltage and the tube complement?
I dont think you can expect more than 4-5W out of a single bottle.
And the two stage implementation does not cater for high-gain.

The speaker doesn't say anything about what is rated. There is plenty of room in the chassis for a 10 or 12" speaker. What's the protocol for figuring out speaker rating?

Click to expand...

The clean output power can be estimated by reding the output tube datasheet and measuring B+. Then you choose a loudspeaker with a rating about twice the power.
Regarding the impedance, you need to measure the CD resistance of the voice-coil (you must disconnect the speaker for that). The nominal impedance is about 1.2-1.5 times the DCR. Tube amps are quite tolerant regarding the actual load impedance (the impedance of a loudspeaker varies a lot with frequency).

The trem circuit isn't working but I think maybe I just need to turn it on via foot switch which I don't have one with a 1/2" jack on hand.

Click to expand...

The trem circuit operates when the jack is not shorted. The schemo a non-shorting jack so it should be on when no plug is in. It's probably faulty.

On a more general note, don't try to make it what it is not. It's a small low-power low-gain amp (really a student model) that can sound nice for clean tones. You may want to use an overdrive pedal but it won't be a Marshall.

abbey road d enfer said:

What is the B+ voltage and the tube complement?
I dont think you can expect more than 4-5W out of a single bottle.
And the two stage implementation does not cater for high-gain.
The clean output power can be estimated by reding the output tube datasheet and measuring B+. Then you choose a loudspeaker with a rating about twice the power.
Regarding the impedance, you need to measure the CD resistance of the voice-coil (you must disconnect the speaker for that). The nominal impedance is about 1.2-1.5 times the DCR. Tube amps are quite tolerant regarding the actual load impedance (the impedance of a loudspeaker varies a lot with frequency).
The trem circuit operates when the jack is not shorted. The schemo a non-shorting jack so it should be on when no plug is in. It's probably faulty.

On a more general note, don't try to make it what it is not. It's a small low-power low-gain amp (really a student model) that can sound nice for clean tones. You may want to use an overdrive pedal but it won't be a Marshall.

Click to expand...

Thanks for all the information!

I don't want to make it what it's not, I just want it be be as good as it can, and maybe a little better for fun. It just feels a little more quiet than it should. I actually am not looking for break up. I don't even use guitar pedals. It just seems suspicious that at fully clock-wise there isn't any break up and it's a little more quiet than the average practice amp.

I got the trem working. replaced some caps in the trem and it appears to have woken up. Makes me wonder if any of the caps are still within spec. They're red and orange caps labeled "STANDARD" on them. I'm not hearing any hum, but I doubt that can cap is any good. Surprised to see the the death cap was switched to a .01uf y-series ceramic cap and that a couple capacitors were already changed. Why not just remove the death cap completely and install a 3 prong? This little amp is crazy! I'm very into the tone though.

Below I've attached the schematic with the parts list.

I wouldn't be surprised if some caps needed replacing, particularly the cathode caps, when they dry out, the amp loses gain. Also the tubes themselves may have drifted significantly.
The schemo is hardly legible, I can't even figure out the tube's references, but it is quite possible to calaculate the gain of the various stages and compare to actual measurements, that would determine if the tubes are still within specs.

It’s a 50L6 tube, so probably about 3 Watts output.