Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: beckspeed on August 01, 2016, 06:59:06 am
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I have a Silverface MV Twin with no vibrato.
I have checked with footpedal and with rca grounded out - no vib but I do hear an audible click and there is a slight volume change.
I have good ground at the RCA for the pedal and at the INT pot
also The 4 wires for push/pull gain have been terminated - done by a previous owner - I just cleaned up their work so they were not shorting out.
attached are the V5 voltages as seen from inside of the chassis. Everything was powered up, connected to guitar, connected to cabinet and Vib engaged. Reverb tank was not connected though.
(http://el34world.com/Forum/index.php?action=dlattach;topic=20645.0;attach=59980;image)
Any other checks I should do to determine the culprit?
Thanks
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The changing plate voltages indicate the tremolo circuit is working. Probably just need to change the trem roach (optocoupler). This assumes it is wired correctly.
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just checked the diagram and it seems like the terminated wires for the push/pull gain may be part of the issue. I have 0v @ pin 2 and 7 on V5 - the both run to the master volume switch. should those two wires be connected rather than just individually terminated?
(http://i74.photobucket.com/albums/i279/adambeck1979/100W_Mast_Vol_SF_Twin_Rev-2_zpss9lcrmue.png)
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I have 0v @ pin 2 and 7 on V5 - the both run to the master volume switch.
That mvs has nothing to do with the tremolo. V5 pins 2 and 7 should be 0V when the tremolo is on. Both pins will be approx. -50V when the tremolo is turned off with the footswitch or if the footswitch is not plugged in. The trem circuit appears to be working. Change the roach.
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ok! Thanks Sluckey!
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Sluckey! You were right. New roach and it's vib-ing like a champ!
I'm still familiarizing myself with this amp, I've only had it a few weeks and I'm noticing odd quirks.
As I said in a previous post, the push / pull gain has been disabled (I'm fine with this)
I noticed I have Reverb on the "normal" channel - but no vibrato? How is that possible? Was this modded? Can I hook up Vib on normal channel too? has a much warmer sound.
Thanks!
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just in case you can see any creative wiring from this:
(http://i74.photobucket.com/albums/i279/adambeck1979/Fender%20Twin%20Reverb/IMAG0490_zpsracovsoi.jpg)
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It has been modded to have reverb on both channels:
http://el34world.com/Forum/index.php?topic=19718.msg205525#msg205525 (http://el34world.com/Forum/index.php?topic=19718.msg205525#msg205525)
To have vibrato on both channels you would have to change the circuit to bias type:
http://el34world.com/schematics.htm#Hoffman_AB763_Two_Channel_ (http://el34world.com/schematics.htm#Hoffman_AB763_Two_Channel_)
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Can I hook up Vib on normal channel too?
Yes. Look at the wire on the outside lug of the INT pot (not the grounded lug). Follow it back to the board and you will see it connects to a 220K resistor. Unsolder the wire and resolder it to the other end of that 220K resistor. That's all.
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wow that sounds almost too easy...
ok here we go!
Thakns
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Sluckey You are the man! I finally got around to cracking the TR open it took 10 minutes to disassemble and two minutes to trace the wire and switch it from one position to the next. Works great. I now have Vib and REV on both channels.
One question though: It seemed to take a few minutes to actually work? Is that because the Vibrato tube had to warm up? I thought i did somehting wrong and then all the sudden I gave the pedal another stomp and it was working?
Thanks
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Slow starting trem can usually be fixed by replacing the tube with a good strong tube. You may need to try several tubes to find one that reliably starts quickly. There can be other causes for this but the tube is the easiest thing to do.
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right again Sluckey! moved some tubes around until I had a good / quick Trem start. some really pulse - sound like a heartbeat in the background. The one I got in there is quiet and fast.
Thanks again