Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: carausius on September 28, 2011, 02:42:54 am
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I've got a '73 Silverface Twin Reverb which has a weird reverb circuit installed, it appears to have once had a master volume
which has been removed at some stage.
After replacing all the electro's & adjacent resistors, some power tube plate resistors & a couple of disc caps, the amp sounds
great, apart from the reverb circuit with noises & interference when turned up.
This amp has the reverb driver 2.7K (Blackface value is 2.2K) cathode resistor & 25uf cap, , reverb recovery 820R cathode resistor
& 25uf cap, vibrato 2.7K cathode 25uf resistor cap all installed, the last pair go straight to the tubes & are grounded to the switch
jacks like some other silverface circuits.
However, there is a strange section of components, starting at the junction of the reverb recovery plate resistor & preamp coupling
cap. with a 300K resistor, a .01 cap, a 2.7 cap going to the reverb recovery grid resistor & a 1M resistor from there attached to the
vibrato resistor cap network.
The previous AA270 circuit to the MV looks almost as a Blackface reverb circuit apart from the addition of some
suppression caps & has no extra components installed like the MV version so I'm hoping I can rewire to that circuit!
I'm wondering what these extra components do, are they left over from the master volume? Can they be removed or are they still
needed as part of the reverb circuit?
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Those extra parts were connected to the master volume pull pot. I can't remember what the switch did but in one position it is a normal Fender reverb. I believe toy can remove those extra parts. Just check the circuit when done.
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Its a damn shame when people hack up amps. If I were you I would try to put it back to spec. It will work better. Its what it is. Its not a Blackface and it can be made to sound very nice.
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It was hacked up before I bought it, a hole drilled beside the back panel & some other strange "mods" I removed, also bias adjust has been added & MV cables taken out, suppression caps were all gone including the often still needed vibrato & reverb fixes fitted to the SF amps. Someone even stole the control knobs & Logo! I'm putting it back to near AA270 specs as much as possible & am considering fitting a Lar Mar or similar MV back in the amp later on. I might also change the PI back to stock values.
I myself have only replaced parts which needed replacing, all old electro's including the cap can, power tube resistors which hissed like mad, plus a couple of bad ceramic caps with silver mica's. The Brown caps were all ruptured, sticky & bubbling, I left the Blue tone caps in there as they look OK.
It's now in much better condition than when I bought it & sounds almost like new. Apart from the reverb section until I fix that!!
It has the original export multi tap 240V PT with plate voltage running at 440DC with tubes biased at 65% dissipation, maybe that's a bit low for a silverface as I thought they ran at higher voltages?
Unfortunately someone fitted a Marshall 100W choke in place of the original so I'll have to buy a new replacement
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Unfortunately someone fitted a Marshall 100W choke in place of the original so I'll have to buy a new replacement
Why buy a new one? It handles all the current, and likely has more inductance than the original (meaning better hum filtering). Unless it's just butt-ugly, why pay for a newer, lesser part?
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Some strange stuff going on this amp, there was a 10K / 93K instead of 100K / 82K on the PI plates :BangHead:, changed to 100K but no spare 82K so the 93K stays for now!!!
Got rid of all the extra MV stuff, replaced the Vibrato channel plate resistors, re-soldered all around the Vib preamp, had to remove loads of solder from the eyelets to ensure proper connections, took out some old stranded signal wires from eylets which were were broken up into pieces & replaced it with solid core cloth wire. It's all made a difference, just a slight hiss & once in a while feedback. Still gets worse with reverb level up full though!
Just noticed that the junction of the reverb mix /bypass resistor / cap / recovery grid resistor is not even soldered into the eyelet, it just hangs in mid air!!! That can get rewired properly next!! :icon_biggrin: Maybe it could be one of these components causing noise although they all test OK.
Almost there!! :d2: