Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: olgoat on July 11, 2012, 08:18:16 pm
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I installed a new Magnetic Components multitap OT into my working 5E3 chassis using Dougs three position switch. I am getting sound and I think I have it wired correctly but I would like to verify my switch postions somehow at the speaker jack for the 4,8 and 16 ohm settings.
Is there a way to do this with a digital multimeter?
I searched around and found a lot of stuff on testing for dead OTs. I saw some posts on calculating turns and figuring out impedance and calculating the ohm output. Is that the only/correct way to verify my work?
Thanks
Tim
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You do know which color matches which impedance, right? You should be able to look at the switch and see which OT lead will be connected for each switch position. If not obviousat first, study it for a few minutes while rotating the switch. It'll make sense. You don't have to do any fancy calculations, injecting voltages, etc.
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Thanks. I did that first but I have gotten into the habit of not trusting anything I do. I double check it. I thought I could use continuity across the wipers of the switch but once it was in the circuit that didn't work.
It sounds good. I guess that's all that counts.
Not sure if it is the new Mag Component OT or the higher voltage PT (710V versus previous 650V) but the low end seems tighter than it was before. \
Waiting on a pair of Tone Tubby Low Watt hemp cone 10's now that I can set the output ohms to whatever I need.
Thanks
Tim
You do know which color matches which impedance, right? You should be able to look at the switch and see which OT lead will be connected for each switch position. If not obviousat first, study it for a few minutes while rotating the switch. It'll make sense. You don't have to do any fancy calculations, injecting voltages, etc.
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I thought I could use continuity across the wipers of the switch but once it was in the circuit that didn't work.
I would work if you had a super duper meter that could distinguish between 0Ω and 0.1Ω. There is not much dc resistance in the secondary of most OTs and there is even less of a difference between a 4Ω tap and the 8Ω tap (It's all just a piece of pretty big wire), so basically it just looks like zero ohms to many meters. Everything appears to be shorted together.
This is one of those cases where you can trust your eyes.
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> super duper meter that could distinguish between 0Ω and 0.1Ω
Get a spare 6V AC winding. Connect it direct to the known zero and 16 ohm taps. Connect an AC voltmeter to a speaker plug. Power-up the 6VAC (but not the amp!).
The selector switch should now give:
"16" = 6V
"8" = 4.5V
"4" = 3V
Exact values not important; general size and trend should be as indicated.
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Thanks for the tip.
Tim
> super duper meter that could distinguish between 0Ω and 0.1Ω
Get a spare 6V AC winding. Connect it direct to the known zero and 16 ohm taps. Connect an AC voltmeter to a speaker plug. Power-up the 6VAC (but not the amp!).
The selector switch should now give:
"16" = 6V
"8" = 4.5V
"4" = 3V
Exact values not important; general size and trend should be as indicated.