Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Gareth81 on December 31, 2011, 02:12:28 am
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Hi There
i have a silverface twin reverb that has had the usual mods done to make it blackface - the hgih frequency bleed caps have been removed, the phase inverter section and cap changed to bf specs, and the reverb circuit changed.
I also replaced the speakers with webers (alnico bluedogs, which I admit pretty un twin speakers, but I already had them lying around and they sound great!)
I have PPIMV in it for practical reasons, but when I played a genuine bf recently I found it was similar but different it some quite noticable ways. The bottom end was tighter and meatier for one.
Looking over the two schematics again, I have notice that the b+ voltages are higher on the BF and the resistors on the rail lower in value. Do people generally change these also?
What about the output transformer? Whats the general opinion of the mid 70's fender iron?
Regards
Gareth
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Yes, people usually change the resistors when Blackfacing. In other words, they go all the way with it.
Despite what the schematics say, it has been my experience that BF Twins usually run about 420VDC on the power tube plates (under load) and that is quite a bit lower than what I usually see on SF Twins.
The OT's..... well, I can't speak to technical differences inside the transformers, but they look the same and give the same readings on a meter and I haven't noticed any substantial differences in their sound when swapping them out.
The PT's are pretty different though. On the SF version, the voltages are higher, they got rid of the filament winding center tap, and the transformers are substantially larger.
Dave
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if you want to get exactly the same tone as a BF, you gotta respect all the original voltages, preamp tube, PI, CF, power tubes. By replacing the B+ rail resistors between the stages, you can get prettty much the same voltages everywhere. ( 5 to 10 V difference on the plates is not audible ). But remember that one blackface doesn't have the same tone as one another. Since they were all handmade, small differences could occur, depending on different factors ( caps make, tubes manufacturers, respect of the schems by the tech and so on ).
The important is that you really like it's tone.
Happy new year !
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Some folks also change the parts placement on the board, as well as doing away with some of the silverface-style wiring. But these don't really cause an audible tone difference.