New Jolida 302b amp owner questions

[tolone]

I recently bought a used jolida 302b. I’m the third owner. I was told it has been modified but the last owner isn’t sure what was done to it. From searching this forum I read that there are quite a few mods for this amp. I was hoping someone could tell me the funtction of a couple switches that were added . I also noticed that an extra pair of RCA output jacks were added.

I’ve included some pictures of the outside of the amp to show switches and output jacks. Based on two little labels, I think the switches have to do with types of tubes. I can add an internal picture later.

I was told by the previous owner that the balance control had been bipassed and he didn’t know what the switches were for and just never used them.

I've never owned or heard a tube amp before so this is all new stuff to me.

[tinitus]

Originally Posted by tolone

I was hoping someone could tell me the funtction of a couple switches that were added .

Based on two little labels, I think the switches have to do with types of tubes.

might be different bias setting for using either EL34 or 6550 output
probably different cathode resistor

[tolone]

I've learned that this amp was modified at the factory. I emailed jolida the serial but haven't heard back yet.

It makes sense to me that the switches would be for different tubes. They should either both be up or down not one up and one down right? Sorry for the simple questions, i'm new to this stuff.

Where can i find a manual for the 302b? They are not posted on there website.

New pic of the inside.

[tinitus]

Originally Posted by tolone

It makes sense to me that the switches would be for different tubes.

They should either both be up or down ?

no doubt about that

man, there sure is a long run of cables between board and them switches
that can't be good
but please don't touch anything unless you know exactly what you are doing

[planet10]

This Jolida is a good candidate for modding -- one of the better prebuild kits. 2 of my biddies have them.

dave

[ronaldw441]

planet10,

I believe that there's an error on your schematic.

The instructions to convert to triode reads "Optional triode connection disconnect U/L tap from OPT (white & orange)".

On this schematic: http://drtube.com/schematics/jolida/sj-502a.gif and well as on both my 502's ("A" and "B" models) the UL taps are white and yellow.

If anyone wants to perform this mod they should double check the colors of the wires connected to pin 4 of the output tubes.

[planet10]

The schema i posted is one done by Chris of his personal amp.

dave

[vinylkid58]

Originally Posted by planet10

The schema i posted is one done by Chris of his personal amp.

That's actually the older "SJ" schem. which doesn't show the PS changes for the JD302/502 series. The circuit is basically the same, other than the bias resistor/pot values. I'll post the 302b schem and an older pic of my amp showing the taped-off UL leads and green resistor connecting G2 & plate for triode operation.

Interesting that the OP's amp has a newer board with 502/F1 designation, whereas my amp has the older "SJ" board.

BTW, I've never seen an owners manual for this amp.

[Davezack]

I recently bought the 302b which I am told has been
modded to a 502b allowing the use of 6550 tubes
and being able to swap the tubes without the
need for rewiring, but just a bias change with the
adjustment through the access holes in the
chassis.
Mine, however has no switches other than the 3
front panel knobs and the ac on/off toggle.
I love this amp!

[mgeldert1]

Hi, new member and I need to find a copy of the basic ops manual for the above unit-not the schematic (already located that in this forum).
Many thanks,
FrSkip