Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: richierick on October 08, 2013, 05:47:31 pm
-
I'd be really grateful if 'The Amp Gurus' would help me. I got a SF Twin Reverb with MV chassis, and I have refurbished it using AB763 specs. All the 'Irons' were fine, so I cleaned it up and did the caps. The Amp plays beautifully, is LOUD and clean, no hums, etc. I can't seem to get the reverb to work. The return works fine, amplifying any signal fed there. The send circuit however is barely audible. I checked it by playing the guitar, and taking a feed from the jack and putting it into a practice amp. The plate voltage (Pin 1 & 6) is present (438V), the Cathode voltage (Pins 3 & 8) should be 6V, but I'm getting approx. 1.15 V. Also, all the other triodes light up really well, but the 12AT7 reverb tube has a really feeble glow. Could it be gunk, in the tube sockets, or the pins are in need of re-tensioning? Also AB763 has 2.2K/25uF while the SF has a 470 Ohm resistor between the Cathode (Pins 3 & 8) and ground. Thanks so much for this forum, and all the help you guys give on a daily basis. The forum has been my tube amp school.
-
I got a SF Twin Reverb with MV chassis ... I can't seem to get the reverb to work. ...
First thing, make sure you try the push-pull master volume in both positions. "Pull" is supposed to through a switch which puts the reverb signal through a resistor instead of sending it into the reverb pan, and from there into the reverb recovery circuit as a gain boost.
Second, make sure someone didn't monkey the wiring on that switch to have the boost on full time instead of only when the knob is pulled.
Third, check the value of the 470Ω cathode resistor. The 6v spec seems a little strange, as the idle current in that circuit would have to be a lot higher than in the earlier amps. 10v/2200Ω = 4.55mA for the non-master Twins, 6v/470Ω = 12.8mA. You might even simply swap a new 12AT7 in there to be sure the old one isn't spent.
Fourth, look at the RCA jacks for the reverb send. The wire running to the master volume switch connects here. I wonder about a silly mod like a resistor from hot to ground on the *send* jack (a 220k from hot to ground is normal on the return from the reverb tank).
-
Thanks, ever so much Blue Plates, I will be checking those things when I get home. Will post the results tomorrow.
-
Sorry I mangled your name, as I'm reading this on my phone. It should read 'Thanks, ever so much HotBluePlates, I will be checking those things when I get home. Will post the results tomorrow'.
-
No worries... the handle has been much more mangled before, and it really doesn't matter to me. :icon_biggrin: