Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: markmalin on August 27, 2023, 10:04:00 am
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Hi all
I'm working on what appears to be an early Blackface Twin Reverb. I've pulled up the schematic for the AB763 and other BF fenders, but the filter caps on this one don't have the 2 reserve caps wired up in series. They are in parallel, 2 20uf/524V caps. I'm attaching a picture with the original caps. I replaced these old caps with F&T's, 22uf/500v, in the same arrangement as they were in the picture. I'm going to be transparent here and say that in adjusting the bias, as I let the amp run it started losing plate voltage on the power tubes. I investigated and I see one of my parallel pair of 20/500v filter caps was reading plate voltage, the other was like 2.8V, so I figured I have a cold solder joint on the one. But before I fix that I wanted to ask, does anyone have a schematic for a twin reverb that does NOT have the series pair of 70uf filter caps in the reserve filter section, but actually shows two 20uf's in parallel like on my amp?
humbly,
mark
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Ok, I'll chime in. So, as you know, two twenty's in parallel are not much different than two seventy's in series. I have not seen a "Twin" set up like yours, but I have seen plenty of instances where something seemed "out of place". The conclusion I have come to is that because of the sourcing of parts, or logistics issues, occasionally they had to make "on the fly" changes to keep production going.
Dave
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Look like a Vibrolux Reverb power supply witch use 16mfd
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Interesting. Yes, it's more like the Vibrolux schematic for sure.
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Since twins have a ss rectifier then reservoir capacitance isn't critical and they capacitance could have been doubled or more without harm and some benefit.
I've not seen a BF Twin with that ps arrangement. However the Twin 6G8 had two 20uf in parallel as the reservoir capacitance.
https://el34world.com/charts/Schematics/files/Fender/Fender_twin_6g8_schem.pdf
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I wonder if this is just a super early one
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I think that the early (AA763) Twin Reverbs actually had the 2 reservoir caps in parallel. Can't remember where I read it, but I was reading up on some stuff a couple of years ago.
The cathode resistor on the "roach side" of the tremolo tube - is it 56k or 100k? The early ones had a 56k.
/Max
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I wonder if this is just a super early one
The caps in your photo appear to have a 1963 date code: "6339" below "Made in U.S.A." indicate 39th week of 1963.
The 6G8-A Twin (https://el34world.com/charts/Schematics/files/Fender/Fender_twin_6g8a.pdf) also has all-parallel filter caps, with none in-series. I would think very early Twin Reverb ams might have a power supply configured like the late blonde-era Twins.
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The caps in your photo appear to have a 1963 date code: "6339" below "MAde in U.S.A." indicate 39th week of 1963.
The 6G8-A Twin (https://el34world.com/charts/Schematics/files/Fender/Fender_twin_6g8a.pdf) also has all-parallel filter caps, with none in-series. I would think very early Twin Reverb ams might have a power supply configured like the late blonde-era Twins.
Pretty crazy that those are still in there 60 years later... but amazing too
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Pretty crazy that those are still in there 60 years later... but amazing too
I took some filter caps out of a 1964 AC30. They must need replacing after 60 years, right?
However, checking the caps with a Sencore LC75 (http://ftb.ko4bb.com/manuals/76.205.207.216/Sencore_LC75_Z_Meter_2_Operation_Manual.pdf) with 400v applied to the cap revealed leakage current lower than some modern aluminum electrolytic caps.
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Pretty crazy that those are still in there 60 years later... but amazing too
I took some filter caps out of a 1964 AC30. They must need replacing after 60 years, right?
However, checking the caps with a Sencore LC75 (http://ftb.ko4bb.com/manuals/76.205.207.216/Sencore_LC75_Z_Meter_2_Operation_Manual.pdf) with 400v applied to the cap revealed leakage current lower than some modern aluminum electrolytic caps.
These guys were pretty bulged on one side. They all had like a pea sized "wart" protruding, so I swapped them. The rest of the amp is super clean for its age. I swear...to me the Fender Twin Reverb is THE Fender tone. I hate to send this one back once I'm done with it!
Mark
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Pretty crazy that those are still in there 60 years later... but amazing too
I often see that on many amps I fix.