I recently got this Jolida JD-302b. I briefly auditioned it and fell in love with it. Seemed like a great deal at $450. I've only recently begun auditioning tube amps and need help identifying the mods on it. It clearly defers from the schematic that I've found. I'm reluctant to take it back to baseline as it sounds sweet to me. I think it goes toe to toe with the MC-240 and the Dynaco St-35 that a friend brought over. I dig it. I'd like to know if it's going to eat tubes or is potentially dangerous. Any help for a newb would be greatly appreciated.
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Good Evening, Pedaler:
From what I can tell based on my memory and work at replacing the majority of parts and doing a few mods on my SJ502 (which is quite similar but with a few different spec'ed parts and higher output wattage, ~30W vs ~50W)) as per the suggested mods Jolida 302b Mods, it looks like the standard coupling caps were replaced with Soniccaps and the bias resistors were replaced with larger wattage ones. There wee some other resistors which were replaced with higher wattage ones. One of the power supply caps was replaced with a Sonicap. Neared to the top of your first picture/photo, it also looks like some resistors were wired/solder in series(?) with the black shielded cable - maybe to attenuate the signal voltage from the one or more of inputs? Of a note: Note: Prior to the start of my work on my SJ502, I had ordered four bias trimmer from Jolida USA and had asked if the KT120 could be used on this integrated. The response was yes as the output transformers could handle the higher output(?) from the tubes.
Um, that's about the extent of my two cents worth. 😎
Thanks.
Sincerely,
Kingsley.
From what I can tell based on my memory and work at replacing the majority of parts and doing a few mods on my SJ502 (which is quite similar but with a few different spec'ed parts and higher output wattage, ~30W vs ~50W)) as per the suggested mods Jolida 302b Mods, it looks like the standard coupling caps were replaced with Soniccaps and the bias resistors were replaced with larger wattage ones. There wee some other resistors which were replaced with higher wattage ones. One of the power supply caps was replaced with a Sonicap. Neared to the top of your first picture/photo, it also looks like some resistors were wired/solder in series(?) with the black shielded cable - maybe to attenuate the signal voltage from the one or more of inputs? Of a note: Note: Prior to the start of my work on my SJ502, I had ordered four bias trimmer from Jolida USA and had asked if the KT120 could be used on this integrated. The response was yes as the output transformers could handle the higher output(?) from the tubes.
Um, that's about the extent of my two cents worth. 😎
Thanks.
Sincerely,
Kingsley.
Wow, blast from the past. The 302b was my first tube amp, still have it. I see the usual coupling cap mods, and some bypass caps in the power supply. What I don't get are those 100R 5W sand cast resistors going from pin #4 to pin #6 (octal sockets), unless someone just put them there for future triode strapping.
I don't see anything that should cause concern. What tubes are in the amp?
jeff
I don't see anything that should cause concern. What tubes are in the amp?
jeff
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Yep. It does appear that it was prepped for triode but I don't see a switch. I asked the previous owner when I saw those but he tells me no. Also, when I read posts about triode strapping they show the resistors place between pins 3 and 4. I thought that a bit odd. I've also seen that the UL wire is cut. That's part of why I reached out to the collective. Thank you for having a look.
If the UL taps are cut from the tubes, it must be operating in triode mode. Otherwise the amp wouldn't work. Maybe you could look more closely at the output tube sockets. Or maybe the project was abandoned. Hard to say without more details.
In the first picture the connection of OPT shows that this is in UL mode.
The "white" and "yellow" wires are connected to EL34s g2 (4 pin).
EL34 socket
If you use it's as "trioded", the UL wires must be cut from pin 4 and isolate.
The 100R "sugar cube" resistors must be connect between g2 (4) and anode (3).
Sometimes this connected to (4) and (6) -NC- and (6) and (3) anode connected together via wire.
See @vinylkid58 post and picture here.
Thank you. This is all really helpful. I read the @vinylkid58 post and that's what got me prompted to ask about the cast resisters connected to (4) and (6). On mine the UL wire is connected as well. It just didn't fit with what limited knowledge I have. I'm happy with the sound in UL and will just leave the sand cast in place if they are not burdening anything. I'll focus my attention on changing the inputs, volume pot , selector switch and bypass the balance knob.
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If you resolder "white" and "yellow" wire from (4) -isolate its- and connect together (3) and (6), the result is "trioded" mode.
It's strongly loudspeaker dependent.
For UL to triode the power about halved and if you kill the NFB, the output impedance of amp is increasing.
If the speaker not really "tube friendly" (easy driving, relatively smooth impedance curve -mainly at LF-), the modification would be disappointing.
I used it (decades ago) with 89dB Acoustic Energy Aelite 3 loudspeaker with satisfying.
This speaker also fits to my 300B 10W amp.
For UL to triode the power about halved and if you kill the NFB, the output impedance of amp is increasing.
If the speaker not really "tube friendly" (easy driving, relatively smooth impedance curve -mainly at LF-), the modification would be disappointing.
I used it (decades ago) with 89dB Acoustic Energy Aelite 3 loudspeaker with satisfying.
This speaker also fits to my 300B 10W amp.