Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Gretsch6132 on May 27, 2026, 04:39:40 pm
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I am working on a friend's 80's "The Twin" with the red knobs. It came from his dad in France so it was wired for 220/240V and had the euro plug. I replaced all the electrolytic caps with new F&T's and rewired the primary in parallel for 120V. It is working well and voltages through almost the entire amp make sense except for one spot where I am getting a reading on one of the big 220uF caps above the rated value. The spec sheet calls for 285V rated cap. I installed a 300V but am getting readings around 350V. The other 220uF is getting about half. I attached a schematic showing the location. I checked all of the resistors and the diodes in that area of the circuit and everything checks out. All the secondary voltages also look good. Anyone have any ideas of what could be going on? Is that cap going to blow?
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Are you measuring across the capacitor or to ground?
Looks like there's some sort of a ground lift when the amp's in stand by. In the spirit of not overthinking it, I would measure voltages across the cap in both positions.
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Hi Gretsch,
R304 and C301 are parallel, stacked in series with R305 and C302 (in parallel).
R304 and R305 form a voltage divider "forcing" reasonable load sharing for voltages across C301 and 302.
And yes Alnewman's idea of measure voltage across the caps individually is a good idea.
However measuring from Negative side of C302 (PS Earth/OV) to junction of C302-C301 should give you a nominal Half power supply voltage assuming R305 and R305 are good and no downstream fault. Likewise measuring positive of C301 should reveal nominal +470V, assuming you are in 100W/Full power mode.
My experience (in Aussie) is the "Lo Power" voltage is always a little higher than stated on Schematic.
Also please do check the Sockets on J02, J03, J04 as these a VERY prone to fractured joints.
Regards
Mirek
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Thanks for the replies. I should have mentioned that the measurement is when the amp is fully on. With standby on there is low voltage across both caps. That measurement is to ground. I will check it out.
For the mention of the jacks is that a general comment or is that related? Those are the input jacks you are referring to, right?
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No :l2:
The Pre out/In/Thru just before Power Amp.
But all the plastic Sockets which are PCB (horror) mounted need to be checked.
These have proven to be a major source of failure in this amp (and m any other Fenders)
I am not against sensible PCB esp for Turret use (ie FR4/Nema G10 baseboard) but 8/9pin cheap Sockets on single sided Phenolic Resin PCB is the Pits!! Then put a few 5 and 10Watt resistors on it :cussing: Problems that never go away!
Oh for Switchcraft ® Mono Type 12A Socket :laugh: :laugh:
Regards
Mirek