Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dujuarez on March 07, 2016, 01:53:50 am
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I have a power transformer that I removed from a Crate 40 watt amp with solid state rectification. I have taken measurements but I'm having trouble determining all the wires. I've figured out the primaries and heaters and HT. I'm not sure of the other wires. I believe that one maybe for bias, (since the Crate 40 watt had bias pot) and I'm supposing that the other wire CT. I can't determine from readings. Here are the reading I'm getting:
Primary
Black and White - 3.3 Ohms
Secondaries
(1st)Red and (2nd)Red - 82.7 Ohms
Green and Green - 0.3 Ohms
(1st)Red and Red/Yellow - 40.0 Ohms
(1st)Red and Red/Blue - 48.5
Red/Yellow and Red/Blue - 5.8 Ohms
The following readings are when I change from the 1st red to the 2nd red
(2nd)Red and Red/Yellow - 40.0 Ohm
(2nd)Red and Red/Blue -34.4 Ohms
This is really throwing me off. Any help would be greatly appreciated
Thanks
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was the crate mostly solid state with a couple tubes?
look at the schematic linked, do the wires and colors match up? If it was mostly SS, those winding might not help much in all tube.
http://el34world.com/charts/Schematics/files/crate/crate%20bluevoodoo%2050w.pdf (http://el34world.com/charts/Schematics/files/crate/crate%20bluevoodoo%2050w.pdf)
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It's too late for this PT since you've already removed it.... But, next time, check the voltages and/or how the PT is connected in the original circuit and label the leads BEFORE removing it.
The two reds are the HT winding and will connect to the rectifier. Since the RED/YEL measures 40Ω to either red it must be the center tap and will likely connect to chassis ground. And since RED/BLU measures different resistances to each RED, it is not the center tap. It is the bias tap and will connect to the bias circuit or just tape the end if no bias circuit.
You can further verify by applying 120VAC to the primary. Connect one lead of your AC voltmeter to the RED/YEL center tap Now measure the voltage on each red lead. Voltage should be in the range of 200VAC to 400VAC and the reading should be about the same for each RED. Voltage on the RED/BLU should be much lower, maybe in the range of 20VAC to 60VAC.
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Shooter, it had 2 6L6s and 2 12AX7s. The colors are matching up, thanks.
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> label the leads BEFORE removing it.
+1.
> look at the schematic linked
Doesn't look like description.
If you are un-sure, put 6VAC to the assumed "120VAC" winding.
All other windings will be about 1/20 of normal. A "300V" winding gives 15V, a "6V" winding about 0.3V, etc.
If you utterly screw-up, put 6VAC into a 6V winding, nothing blows-up, but you have normal (and fatal) voltages everywhere. Try to get it right, and all voltages will be sub-lethal.
> (1st)Red and (2nd)Red - 82.7 Ohms
> Green and Green - 0.3 Ohms
> (1st)Red and Red/Yellow - 40.0 Ohms
> (1st)Red and Red/Blue - 48.5
> Red/Yellow and Red/Blue - 5.8 Ohms
Green is conventional for Heater; the low resistance agrees.
Red is conventional for high voltage, the higher resistance agrees.
Red with a tracer is HV CT.
You have an extra Red w/tracer. I think if you poke around, Red w/Yel is more nearly "centered", and the Red w/Blue is significantly offset. It is conventional to supply a 50V tap on a 300V winding for bias supply. When you have a fair guess, apply 6VAC to "120V", see if you get the same 15V RedYel to *either* Red, and around 3V RedYel to RedBlu.
Same as Sluckey said.
> mostly solid state with a couple tubes?
If it has a few Power tubes, then very likely 95% of total power is tube-suitable, and he can build a totally tube amp on it. The linked PDF shows a bluish 32VCT winding but by inspection the load here is like 2 Watts, so can't be any large part of a two 6L6 25-70W amp.
The linked PDF has bias from an Org-Org winding, then a complicated bias-dropper which would appear to force tubes cooler when tube current is high. This smells too-too-clever and modern to me. While the raw bias supply may be unexpectedly high voltage (two 63V caps in series??), a basic divider can get into a range that 6L6GC at 450V can enjoy.
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Thanks, I know I should have taken more time in dismantling the amplifier. I could kick myself now. I will test the voltages.
Thank you.