Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: blueznet on September 26, 2010, 06:04:12 pm
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I recieved 2 Fender Dual Showman Reverb 100 watt heads to completely rebuild. They were both victims of hurricare Katrina and were completely submerged in water. I am having to start with chassis only build cause everything else was in the chassis is being thrown away.
I cannot find a 1974 push/pull master volume Fender layout no where. So I building it off a schematic and a 74 Twin Reverb (100 watt) layout. Lot of people tell me the twin and dual showman are the same. They are not the same as far as resistor values and some caps.
Question: On the first cathode bias cap (from right to left) there is only one. It calls for a 22uf/35vdc with a 820 ohm resistor soldered across it. There is also a 470 ohm resistor where normally the other cathode bypass cap should be. Why did Fender use a 22uf/35vdc cathode bypass cap instead of the normal 25uf/25vdc cap? I am having problems finding a Sprague Atom 22uf/35vdc cap. If I used a 25uf/25vdc as a sub, how would this affect the tone?
I will never take a build like this on in my life. Thanks for any feedback. Good to be back here as well :)
Darrell
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Hi Darrell,
Use anything close... 20uf 16v will do. The 470ohm is probably the reverb driver cathode resistor.
I would rebuild using the AB763 Hoffman turret board. And if you must ad the master do the basic voltage driver into the phase inverter.
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Lot of people tell me the twin and dual showman are the same. They are not the same as far as resistor values and some caps.
The TR and the DS used the same everything, except cab and plates.
Fender regularly revised the designs, applied a lot of undocumented mods and substituted parts.
The amps could have been modified or creatively repaired by a third party.
If I used a 25uf/25vdc as a sub, how would this affect the tone?
It won't, the difference between marked values - 22 vs. 25µF - is less than what you can find between two supposedly identical caps from the same batch!
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Lot of people tell me the twin and dual showman are the same. They are not the same as far as resistor values and some caps.
Lot of people tell me the twin and dual showman are the same. They are not the same as far as resistor values and some caps.
The TR and the DS used the same everything, except cab and plates.
Fender regularly revised the designs, applied a lot of undocumented mods and substituted parts.
The amps could have been modified or creatively repaired by a third party.
If I used a 25uf/25vdc as a sub, how would this affect the tone?
It won't, the difference between marked values - 22 vs. 25µF - is less than what you can find between two supposedly identical caps from the same batch!
The TR and the DS used the same everything, except cab and plates.
Fender regularly revised the designs, applied a lot of undocumented mods and substituted parts.
The amps could have been modified or creatively repaired by a third party.
The push pull mv tr and dual showman reverb are differant. These are all stock from Fender. differant resistor values, wire dress is the same. They both use differant resistor values on the rect board. they use differant 1 watt resister on the filter cap board. The customer wants them both back to stock. I have many close board pics of both the tr and dsr.
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Fender used what was on sale and cheap at the time
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Fender used what was on sale and cheap at the time
Yea I know. I am still trying to figure out why Fender wired it up the way they did so it would boost with a pull of the MV. Thanks Doug.Lot of people tell me the twin and dual showman are the same. They are not the same as far as resistor values and some caps.
Lot of people tell me the twin and dual showman are the same. They are not the same as far as resistor values and some caps.
The TR and the DS used the same everything, except cab and plates.
Fender regularly revised the designs, applied a lot of undocumented mods and substituted parts.
The amps could have been modified or creatively repaired by a third party.
If I used a 25uf/25vdc as a sub, how would this affect the tone?
It won't, the difference between marked values - 22 vs. 25µF - is less than what you can find between two supposedly identical caps from the same batch!
The TR and the DS used the same everything, except cab and plates.
Fender regularly revised the designs, applied a lot of undocumented mods and substituted parts.
The amps could have been modified or creatively repaired by a third party.
The push pull mv tr and dual showman reverb are differant. These are all stock from Fender. differant resistor values, wire dress is the same. They both use differant resistor values on the rect board. they use differant 1 watt resister on the filter cap board. The customer wants them both back to stock. I have many close board pics of both the tr and dsr.