Rewiring a dual primary as a single? (Squirrel Monkey Amp) - diyAudio
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Old 15th December 2013, 01:44 PM   #1
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Default Rewiring a dual primary as a single? (Squirrel Monkey Amp)

In paragraph 6 of the Theory section for the squirrel monkey amp, he talks about rewiring the dual-primary transformer so that one of the primaries becomes a secondary and he implies that this is found in other amp designs. I haven't been able to find any details of how this is done and I can't find other examples. Can someone explain to me how you'd do this?
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Old 17th December 2013, 09:17 PM   #2
Spiny is offline Spiny  United Kingdom
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Provided the dual primaries are fully isolated from each other then this will work. The secondary primary is just treated as a normal 110 / 120Volt secondary along with the other secondaries.

the transformer needs to be 110 / 230volt device and be running on 110 volt mains.

The only problem with this method is the transformer needs de-rating I believe as the primary circuit has now only half the power capability. so a 100VA transformer would be capable of 50VA only - anyone with more knowledge please correct if this is wrong.

I would go with a correct transformer by preference
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Old 17th December 2013, 10:00 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

I wouldn't say that. Assuming the limit is core saturation,
the single primary can run more current than parallel dual
primary, so IMO your looking just at more resistive loss.

Generally you only derate a transformer by half if you
run a 230 primary at 115, by definition the same max
current can only transfer half the power for the latter.

rgds, sreten.
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Old 17th December 2013, 10:10 PM   #4
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From personal experience, if you try to get the rated VA using only one of two primaries, it will over heat.

Mike
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Old 17th December 2013, 10:15 PM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Michael Bean View Post
From personal experience, if you try to get the rated
VA using only one of two primaries, it will over heat.
Mike
Hi, but not a sensible scenario for an audio amplifier, rgds, sreten.
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Old 17th December 2013, 10:23 PM   #6
kevinkr is offline kevinkr  United States
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Each one of the primaries in a dual primary is designed and sized to carry 1/2 the rated overall current, my experience also is that running on a single primary and attempting to draw the rated VA will result in overheating. I would not run it much beyond half the nominal rating and I would probably look for a more appropriate transformer.

Depending on the quality of the insulation in the primary there is a low probability safety downside to doing this as well. Magnet wire often has voids in the insulation and should one line up between your primary and your repurposed secondary there could be an unintended leakage path between the two windings. Normally this would just blow the fuse, but in this usage it would result in line voltage on the secondary side of the transformer.
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Old 17th December 2013, 10:27 PM   #7
kevinkr is offline kevinkr  United States
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Wow, it's a guitar amplifier, the last place I would want to take this sort of short cut.

Find a power transformer with 150V and 6.3V secondaries or get Edcor to make one for you.

Also since this is a guitar amplifier it really ought to be in the Instrument & Amps forum where guitar amp mavens hang out. Moving it.
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Old 17th December 2013, 10:45 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

Whilst you will get more resistive losses the issues are
nowhere near as dire as some imply, derating it is true,
but it is nothing like down to half power, that's clueless.

rgds, sreten.

Its not a short cut or any compromise in the original design :
The Squirrel Monkey One Tube Guitar Amp
Its just a good way of getting the secondary voltage,
though the secondary tap will have far lower resistance than
is really needed to be put in series with one of the primaries.
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Last edited by sreten; 17th December 2013 at 11:07 PM.
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Old 17th December 2013, 10:59 PM   #9
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it would seem the original transformer was way over sized fro the job so wouldn't over heat.

But I hav to concur getting the rite transformer for the job would be not only safer but look a damn site tidier in your finished amp.
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Old 17th December 2013, 11:03 PM   #10
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Quote:
but it is nothing like down to half power, that's clueless
Who said that?

Mike
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