Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Apexelectric on January 25, 2018, 09:17:55 am
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Does anyone have a schematic for utilizing two or three separate speaker outputs that allows the use of a dummy load in case someone turns on the amp without a load attached.
I’ve run into a couple situations with narrow panel chassis that don’t have enough room for a multi tap output selector switch an it is easier to use two jacks instead but I’d like to add a safety in case someone forgets to attach the speaker cable. To me this is especially important for head cabinets.
I thought of fusing the OT but it seems like if that fuse blows it might render the amp useless when you really need it rather than just realizing you forgot to hook up the speaker cab.
I’m sure there’s a way to accomplish this with switching jacks. Or a jack that I’m not considering. Also is there a more compact 3way selector switch that woul handle the speaker outputs well. The ones I’ve been using are rally bulky.
Thanks!
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Or a jack that I’m not considering
I typically just use a shorting jack like you would at the input
should keep the amp safe long enough for the player to smack themselves, then plug in the speaker
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Protecting the PT gets a little complicated when you use multiple speaker jacks. With a single jack (as mentioned) a shorting jack to ground or through a power resistor is typical. With multiple jacks the potential errors in hook up by the inexperienced are multiplied. On low wattage amps I typically install a safety resistor on the main output jack like on my 5E3s in the jpg attached. Of course, if someone plugged the on board speaker into the extension jack instead of the main jack you'd have a speaker and a resistor on the secondary.
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Vox was fond of using a 470Ω/3W resistor connected to the 16Ω tap for the AC-50 and AC-100 amps. I've seen another amp that used a 200Ω resistor connected to the highest impedance tap, but can't recall which amp. Neither amp used any switching jacks. The idea is that a 200Ω or 470Ω resistor has little effect on the speaker load but still provides protection against an open load which is dangerous.
A good quality selector switch must handle a lot of current, so it will have to be hefty.
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The no-load problem is over-Voltage. Fuse won't help.
Detecting that "none" of a row of non-parallel jacks is plugged will need more switch points on every jack. Not a standard jack.
For push-pull, adding two HV diodes on the OT primary will limit the spike voltage. This is done on many/most recent amps.
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So it seems that a impedance selector with a single or parallel jack setup would be best. In an effort to save space, since I like using narrow panel chassis’, what would be the lowest power rating I should consider for the selector switch and the dummy load resistor for a 50W output?
I’d probably prefer to find a two or three position toggle switch rather than the rotary 3 position.
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The switch Hoffman sells is suitable for a 50W amp. Don't turn the knob while playing your favorite power chord! You would want a 50W dummy load resistor for a 50W amp.
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Not sure what happened on the last post. Tried to indicate that I was looking for a 3 position toggle switch rather than a rotary for the sake of space. Sorry bout that.
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Look at the NKK M-series switches...
http://www.newark.com/c/switches-relays/switches/toggle-switches?contact-configuration=sp3t&CMP=KNC-GUSA-GEN-KWL&mckv=|pcrid|190658079764|&gclid=EAIaIQobChMIxYvCiqmC2QIV1xyBCh1sFA1XEAAYASAAEgIBk_D_BwE
Look at the data sheet to see how to connect the switch. It requires an external jumper to do what you want.
http://www.farnell.com/datasheets/1749445.pdf?_ga=2.175522272.356816462.1517406105-1563654323.1471433471&_gac=1.250923442.1517406316.EAIaIQobChMIxYvCiqmC2QIV1xyBCh1sFA1XEAAYASAAEgIBk_D_BwE
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Thanks, wasn’t confident that a 6A rating would cut it.
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50 watts into 8Ω would be 2.5 amps. 50 watts into 4Ω would be 3.5 amps
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> wasn’t confident that a 6A rating would cut it.
6 Amps in 4 Ohms is 6A^2*4r or 36*4 or 144 Watts. More in 8 or 16.
Switches are rated for lamps and motors which pull steady power for long time. Yeah, you can whaang your 50W amp to 70+W of gross distortion for a second, but the average over a bar is under 50 Watts. And breaks for drum solo. 20 minutes off every 40 minutes. You can't stress it like when I run my 6 Amp furnace (switch) all night and most of the day.
Switches are rated to BREAK under Full Load. Breaking a high current pulls an arc which burns the contacts. On guitar you need both hands to make a racket, so to flip the switch you have to stop playing and the sound will fade. (Actually you probably shouldn't be flicking this switch while there is any sound in the amplifier, because the switch will go open for some milliseconds and hard-driven amps hate that.)
I think a 3A switch would be fine for what you have. But use a 6A just to be generous.
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Thanks again; guys.
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For push-pull, adding two HV diodes on the OT primary will limit the spike voltage. This is done on many/most recent amps.
1000V 1A diodes in series for each phase?
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You need two 3KV/1A diodes, one from each OT primary plate lead to ground. It's common to connect directly to the tube socket. It's also common (and cheaper) to use three 1N4007s in series rather than a single high voltage diode.
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Thanks sluckey