Tube Guitar amp, dummy load - Advice needed! - diyAudio
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Old 26th October 2011, 03:17 PM   #1
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Default Tube Guitar amp, dummy load - Advice needed!

Hi, I'm new to this but already chuffed with the amount of info I've found.

I apologise in advance if this is a very basic question, I've searched for an answer but my basic knowledge of electronics is (so far) very weak and I'm not sure I can formulate an answer based on other threads; I kinda need things explained in relation to my specific problem at this stage...if that makes any sense.

I'm building a clone of a Matchless Spitfire Head (15watts). I'm sticking to the Matchless schematic as closely as possible except for a couple of mods: only one input, and I'm replacing the line-out (which I won't ever use) with a 16ohm speaker output so I'll have 4, 8 and 16 ohm outputs.

Can someone explain how I go about installing a 'dummy load' to prevent blowing the output tranny (Hammond 1650E) if the amp is switched on with no speaker attached? It's unlikely I will ever do this in normal studio use but I'll have to initially in order to check all the voltages etc.

Any diagrams/photos/schematics would really help.

I understand the basic principle: appropriately valued resistor in parallel to the output on a switching socket, which disengages from the circuit when a speaker is plugged in, but what value resistors should I use? 15watt and 4/8/16ohm on each output socket, or do I just need one overall? And if so, how do I wire this in?

As you can see, my ability to make these decisions myself is poor so I spend a long time checking everything umpteen times as I hate getting it wrong. I'm pretty good at following a schematic, quick at soldering/building and I'm determined to learn this stuff but right now my understanding of how it all works is just rubbish.

Any help would be greatly appreciated.

Many thanks

Ben
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Old 26th October 2011, 04:22 PM   #2
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A 1000 ohm 1 watt resistor across one output transformer winding all the time should keep the transformer voltages down.This should go before the switch. I've left my speakers off from time to time on my dynakit ST70 and didn't blow anything, but maybe your O.T. is not as rugged as the dynakit. I installed one recently on each channel as my cotton insulation transformers enter their fiftieth year.
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Last edited by indianajo; 26th October 2011 at 04:24 PM.
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Old 26th October 2011, 05:16 PM   #3
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Ah wow, thanks for such a quick reply.

So maybe I'm misunderstanding the point of the dummy load? I thought (in my simplistic way) that the resistor was there to protect the OT from a backed up load? So how does a 1000ohm/1watt resistor on just one of the windings protect the OT (and thus tube) from say, an 'unloaded' 8ohm socket?

And if it's in all the time, does it not affect the sound/performance in any way when there is a load?

I'm not doubting your answer, I just really want to understand what's happening inside this stuff.

I'm not using a selector switch to set the output impedance I'm just having 3 separate sockets, one for each 'Ohm-age'...does this affect anything?

Many thanks again

Ben
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Old 26th October 2011, 06:09 PM   #4
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Three output connectors is fine as long as you only use one connector at a time.
You won't be able to connect an 8 ohm speaker and a 4 ohm speaker at the same time.
Most output transformers have a single secondary winding which is tapped at 0,4,8 and 16 ohms.
A 1k resistor across the full secondary winding (16 ohms) will help prevent the voltage on the primary from soaring to a level which could damage the transformer.
The 1k resistor will not affect the performance of the amplifier when a speaker is attached.
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Old 26th October 2011, 06:14 PM   #5
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1kohm wont help much if you get a signal into the preamp or a turn-on thump somewhere. Since it is only to shunt any accidental voltage spike and not continously absorp much power, any 1-5watt resistor from 5 to 50ohms will do. If R is < 12ohms place it on the 8ohm tap, if higher place it on the 16ohm tap.
The resistor is there to prevent an unloaded inductance (the transformer) from swinging a voltage high enough to destroy the output transformer. Dont sweat it too much though, I've forgot to load the output many times and so far no lightening flashes I don't think unloaded OTs are as self destructive as some say...
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Old 26th October 2011, 10:50 PM   #6
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Hi, thanks for your replies.

I would not, of course, attempt to use more than one of these outputs at once...I certainly know that much, I use these kinds of things all the time, I just haven't built one before.

So, a resistor placed between the OT and the 16ohm output socket will protect the OT without the need for a separate resistor on each output? I'm sorry, I'm such a novice at this side of things and only have a very simplistic idea of how these things work...which I'm sure is incorrect. I really must teach myself what's going on inside an OT, perhaps that would help me understand this idea better.

So, does it need to be 1Kohm, or 50ohms, or 16ohms? I don't yet feel equipped to know what's best, I just don't want to blow anything up before I've even heard the amp

And when you say 'across the winding' do you mean soldered between the Tip and Switching connections on the socket, or just in that line somewhere? If it's going to be in the circuit the whole time, do I even need switching sockets?

Sorry, so many questions.

Thanks again

Ben

P.S. Here's the 1650E OT schematic.
Click the image to open in full size.
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Old 27th October 2011, 02:46 AM   #7
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No, 1k, NO. Per your drawing the 1k goes between green and black. It is not involved with the socket at all, and never leaves the transformer circuit. My transformer windings go to a screw plate, where the speakers are wires to C (black) and any appropriate other. I realize guitar speakers use 1/4 phone plugs which have a switching function, but getting the protection resistor inside the speaker jack and directly on the output transformer secondary is your problem since you did not provide a schematic for that part. I might find a 50 ohm resistor audible- It would consume 1/6 of my amp power for no point. For sure a 16 ohm resistor would be audible, it would consume 1/2 the power of the amp. Like I say, my amp has not blown up in 50 years although I knocked the speaker wires off, and shorted them to case (black) many times. The 1k is just to be "safe", stay within the ordinary 600V wire rating. Your transformer wires might not be rated 600V. I'm pretty sure mine are rated 600v, the amp has taken several lightning strikes in the mains line with only the power switch damaged.
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Last edited by indianajo; 27th October 2011 at 02:52 AM.
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Old 27th October 2011, 03:05 AM   #8
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I have used a shorting phone jack, the kind that you would have on your input, and wired a 20 ohm resistor from the switch to ground. When the jack is plugged in the switch disengages the resistor.
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Old 27th October 2011, 04:17 AM   #9
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And don't confuse a dummy load and a protective resistor.

A "dummy load" is intended to act like a speaker load to an amp. It gives the amp something to work into, for example during servicing when you don;t want to hear all the sound. A dummy load would be selected to withstand the full output power of the amp. A basic power resistor is enough to work on the amp. If you want the load to also contribute frequency characteristics like a speaker - for example when sampling off the load for recording - then we can add reactive components to it, more or less like building a crossover.

The protective resistor is intended to be there to prevent the unloaded transformer from kicking back inductively and creationg a high voltage spsike that could arc within the windings. You don't want it to be so small it burns out at the drop of a hat, but it is not usually intended to match up and take the full power of the amp.

And since it only has to damp out the inductance of the transformer, it need not be as low an impedance as the speaker. So something like 100 ohms or even 1000 ohms should be large enough to work, and large enough that it won't affect the speaker in parallel with it much.

On most Fender tube amps, the main speaker output jack is a shorting type. If no speaker is plugged in, the output is shorted across. This is preferred to an open output jack. It usually does no harm to the amp, no one sits there playing full out into the shorted jack for any length of time.

Of course, if the open is at the speaker end of the cord, the jack won't help. A resistor would be insurance. Many if not most manufacturers of tube guitar amps these days add "flyback" diodes from the output plates to shunt off spikes.
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Old 27th October 2011, 09:06 AM   #10
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Ah wow, this is turning in to something more complicated than I thought.

Indianajo, thanks for your advice. How would I go about putting the resistor 'between green and black' without ever leaving the transformer circuit? My OT just has four wires coming out of the secondary side, and these are going directly to my outputs and ground, I have no access to the inside of the OT, I'm unsure what you mean.

Printer2 and Enzo, thanks for your advice. I now realize the difference between a true 'dummy load' and a protective resistor, it is the latter that I'm trying to install.

So if I get one switching jack socket, and solder a 100ohm (what wattage?) between the shorting tag (tip) and ground tag (sleeve) of the 16ohm output this would protect the OT regardless of which outputs are/aren't being used?

And I can then use open output jacks on the 4 and 8 ohm outputs?

Please explain how I'd implement a 'Flyback' diode in to this (if that's the best way to go?), where does the diode go and what type is it?

Thanks so much for all the advice.

I have some pics of my build so far if anyone's interested?

B
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