FWIW I found a service manual for the amp here:
https://robrobinette.com/images/Guit...ice_Manual.pdf
Maybe one day you can replace it ;-)
The US version seems to be an LED based LDR. It is not at all clear to me what the PCB version even does...or why it needs the tube since it seems to have its own op amps... It's an order of magnitude more complicated than the US version.
Yes, I've had that service manual for some time now. The CdS element is the receiving photocell part of the optocoupler, the emitter seems to be a filament bulb, not an LED.
A quick search shows that I can still get a replacement optocoupler (£8.99, so affordable). What I'm not 100% sure about is exactly how the extra (EU fitting) board is wired in to the PCB, as the connections aren't clearly shown in that manual and if I fit the optocoupler and just desolder the flying leads to the board, whether it will work or not, or whether the board has had mods made that need reversing.
I'll probably ask my amp man about it, as I want him to give it a once over anyway, once our lockdown conditions permit.
The 7025s arrived and I fitted them today so it now has almost no hiss, but that now allows me to hear a bit of hum which was masked before. I probably should have ordered a matched/noise checked PI valve along with the 7025s, but it probably could do with new 6V6s as well.
Last edited by Simon Barden; 02-12-2020 at 12:54 AM.
We're currently operating under a WTF....and then I saw that Australia is working on an FTA with the EU.
Making the world a better place; one guitar at a time...
Well I've ordered the Optocoupler. Apparently you just remove the two extra PCBs (one is a power supply for the main vib/trem PCB) and the leads to them, solder the optocoupler in place and away you GO go GO go GO go.
Ordered new valves for the rest of the amp, plus some extra EH 7025s as they worked so well I'll try them in some other amps if they start to get noisy, plus always nice to have some spares.
Is this like one of those drug laws where it's legal to purchase but not to sell? I didn't know that the guys with the trenchcoats took orders ;-)Originally Posted by Simon Barden
Which ones did you get?Ordered new valves for the rest of the amp, plus some extra EH 7025s as they worked so well I'll try them in some other amps if they start to get noisy, plus always nice to have some spares.
Sold for replacements in vintage amps a sits the same neon +CdS cell used in all the amps that have that circuit. Like leaded solder, it's OK to use for repairs to vintage kit and for home use, just not new manufacturing.
I'm all for reducing the nasty stuff that doesn't need to be used in kit, especially as so much is throw-away, non-repairable kit these days. But in this instance, Fender really should have swapped to using one of their all-valve trem/vib circuits IMO.
I stuck with Electro Harmonics so I'll have them throughout; 6V6GTs and a 12AT7 for the PI valve. These will be rebranded Sovtek valves, supposedly a little better than JJs at the moment (though JJs are fine).
IMO2.
Just curious since you mentioned that the EH tubes were quieter. It was not a question about the human rights record of the mfg's country ;-)I stuck with Electro Harmonics so I'll have them throughout; 6V6GTs and a 12AT7 for the PI valve. These will be rebranded Sovtek valves, supposedly a little better than JJs at the moment (though JJs are fine).
Those old-style Fender Tremolo circuits used a component commonly called a roach in the US, if my memory serves me right, it consisted of a small neon bulb and a CdS cell (commonly known as an LDR, or Light Dependent Resistor) which were then encased in some heatshrink tubing, the neon bulb's glow modulated the resistance of the CdS cell.
I pulled one apart while servicing a blackface Fender Twin Reverb a few years ago, cause the CdS cell needed replacing, one of it's solder leads was actually loose and not making proper contact with the resistive element.
The valve stage that drove the neon bulb, is basically a 12AX7 configured as a low-frequency Phase-Shift Oscillator.
When they are working properly, those valve trem/vib circuits do sound great.
Last edited by DrNomis_44; 03-12-2020 at 02:42 PM.
So rather than start a new thread.
Obsessiveness with guitar sounds has me increasing my amp collection to 3.
cheers, Mark.
Nice little amp. As that makes N+1 amps you own, you now only need N+1 amps to complete your collection.
I'm halfway through the trem mods. I've removed the solid state board and wires, fitted the LDR and got nothing from the vibrato circuit when it's activated. Nice not to hear the clicking, but some trem action would also be nice. I'm not 100% sure about the fitted LDR assembly so have ordered another which should arrive any day now. I fitted some heat shrink over the legs which has now made it impossible to check voltages and resistance values when its in circuit.
The lead-free solder is of a very high melting point type and has been a real PITA to remove. Had to get the iron really hot and the moment you used thin-nosed pliers to pull at a lead, the solder solidified again. I've now got some different lead-free solder to my original reel, as that didn't seem to want to adhere that well, so my problem may just be a bad joint. But I'm not taking it all apart again until the new LDR arrives. I've also got the rest of the replacement valves to fit.
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