5301/5501 Query

[Gareth Jones]

I've just picked up a 5301 Keyboard 12 for next to nothing. My thinking was that the G10L-35 is worth way more than I paid for the amp and it comes in a nice cabinet to boot. So maybe I could just use it as a small portable cab or build another amp into the cab.

So I removed the amp chassis today and found that the PCB is pretty interesting. If you look at the photograph you can see component markings for K/B and missing ones marked for Bass. So it seems that early versions of the Keyboard 12 5301 and Bass 12 5501 used the same PCB with some different components and presumably quite a lot of different component values.


Now I've heard a bass 12 used quite effectively as a guitar amp, certainly a different sound from the Lead 12. I also don't own a small bass amp, one would be kind of useful as I recently made a 25.5" scale bass.

All of which got me thinking, rather than build a whole new amp for the cab I could convert the existing amp into a Bass 12.

I have a schematic for a Lead 12 which I would say uses a different PCB. I also have a schematic showing 5301 and 5502 pre-amps but the trouble with that is that many of the component values are not legible, plus of course the bass amp shown is the 20 watt 5502 and while the pre-amp might be the same between the two models it also might not. There don't seem to be any other 5501/5301 schematics online. I also note that later Bass 12 models use a different PCB rather than the shared one, so I've found photographs on line but they won't be much use.

So the question is this. Does anybody have a Bass 12 5501 of a similar age to my Keyboard 12 5301 (1986) so they could photograph the PCB to allow me to figure out the component values? Long shot I know, but worth a try.

Thanks in advance

Gareth

 

[XTRXTR]

I've been looking at the schematic Last updated 06/06/86 for the K/B 5301 and the Bass 5502 and they share the same PCB JM84 and the same preamp and power section. The page says 20 watt combo series but the notes on the page say the 5301, 5302, and the 5502 share the same output section and a note at the bottom saying 5301 is a 12 watt Keyboard combo. I don't know this for a fact but it is my guess that the K/B is listed as 12 watts because it is biased or set for lower gain so no keyboard could distort at all. Where as a Bass has more boom boom at lower frequency carries more energy so the RMS output is higher but the components are the same in the output. Also K/Bs have a different impedance matching than Guitar or Bass at the inputs so a difference is likely in the input preamp section and then the tone section which is what I see on the schematic as well. e.g. Mid is 4k7 pot and Bass version is 22k pot, and some notes in the power amp calling for different values 1kR(5301) instead of 2k7 in two places (maybe the biasing difference). The K/B link would need to be removed and the bass link restored, that changes the two K/B input Volume pots to one preamp and one MV. K/B 5301 connects directly to power amp at point "A" and the Bass 5502 has a MV just before the same point. I think with the schematic and some careful continuity tracing you could figure this out. I hope this helps.

 

[Gareth Jones]

Thanks that's certainly a good starting point.

I still have as hankering to convert it to a Lead 12. I wish I'd never sold my 3005. I'd particularly like to hear my home built Esquire thing through one of those.

Thanks

Gareth