Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 404notfound on February 15, 2018, 05:12:25 pm
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Last night I plugged my Plush Royal Bass 1060s in to jam a little before bed and noticed that the light came on without me flipping the power switch. The power switch on this amp is a three-way toggle switch.
Up position = power on
Mid position = power off
Down position = reverse polarity
As it turns out, when I plugged the amp in, the switch was in the down position. I have done a little research about what reverse polarity is but, I admit, I still don't really understand it. Needless to say, when I saw the power light come on in what I thought was the off state, I panicked and immediately unplugged the unit. The amp was on for 10 seconds at most. I then flipped the switch to the off position and replugged the amp in. I then flipped the switch to the on position to warm up the tubes and ZERO. No power light, no tubes warming up, no sound, nothing.
Now I'm pissed thinking I may have just fried my amp. I checked the fuse and sure enough, it was blown out.
My questions are:
1.) Can I just replace the fuse and see if that fixes the problem, or could I do further damage to the unit by without any further research into the problem?
2.) Does anyone know of any resources where I can gain some further knowledge about the reverse polarity switch, what it's for, how it could potentially harm the unit etc.
Although tube amps are way before my time, I love the sound and hope to build one someday. I just don't know enough about them yet.
Any help with this would be greatly appreciated.
References:
http://www.plush-amps.com/
https://www.thegearpage.net/board/index.php?threads/plush-p1000s-amplifiers.809193/
https://www.talkbass.com/threads/plush-bass-amps.875225/
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The "reverse polarity" is something that has to do with the connection of the line to the PT
(https://i.imgur.com/moQJoDX.jpg)
on old amps there was a switch that acted connect a capacitor (.01 - 1400v AC on schematic - often called the death cap)
to one of the line wires (live or neutral)
with the purpose to avoid humm, this switch isn't so dangerous for the circuit (not more than an ON/OFF switch) but the cap,
if damaged, can be an issue for the player (so the name Death Cap), many people remove this cap nowaday, if you want to have
it on the circuit, use an Y class capacitor (specifically planned to be connected between line and ground, they blow open, never shorted)
Oh, I forgot, that switch was useful when the cord had only two conductors and was missing the earth conductor, nowaday, the best solution
is to put a 3 cord connector on the amp and connect the ground wire to the chassis
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About your blown fuse, is probably due to te fact that you acted the switch too quickly from ON to OFF an again ON
if you do that is possible that the PS e-capacitors are not discharged when you move again the switch to ON, but, at the same time,
the e-cap of the Bias Supply has no charge, so, for some instant, power tubes are without Bias voltage and then they conduct heavily
resulting in an inrush current that blows the fuse
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If the fuse has blow, I don't think you demaged the circuit
For safety you can use a Light Bulb Limiter the first time you put on a new fuse
Franco
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Okay. Thank you so much! I will put a three prong cord on and try a new fuse.
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slo-blo's don't blow easily. see the schematic where the power switch is, the .01/1400V capacitor (death cap). if is shorted, that would cause the fuse to blow. power switch could be broken as well.
Do you have a DMM or ohm meter? with power unplugged you can check to see if the short is on the 120V side of the the PT.
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The "reverse" does not affect power. It controls which-side of a 2-pin plug is capacitor-ed to the chassis. You flick it one way or the other for least buzz.
3-pin outlets should now be common. It is time to upgrade to the late 20th century with a 3-pin plug. The green wire straight to chassis. Remove the 0.01u cap at the switch.
If that cap was healthy, the switch position did not blow your fuse. Even if the cap was shorted (and you have been living on borrowed time), it would not blow fuse unless there is another fault in the system. I'm hoping the fuse was coincidence.