building a Matchless Spitfire clone, and was lucky enough to find an old transformer with very close to appropriate voltages! 6.3, 5, and a HT+ of about 265, close enough to the 285 on the schematic and a bit gentler on those super hot biased power tubes. what's really neat is it's got a dial on the side for different wall voltages, so it could be used in other countries, and it's got a 145VAC input setting... which is pretty strange, but basically constitutes a low power mode. i wonder what this came from...
i want to make sure i'm using best practices when trying to make this sucker work, so i wanted to run a few things by you guys.
first - the thing is almost 2x as big as the Hammond xfmr recommended to replace a Spitfire, so i figure i'm not pushing any power handling limits, especially with only 2x 6BQ5, 2x12AX7 and 1x GZ34. is this a safe assumption?
second, there's one mysterious green lead coming from the PT that doesn't read continuity or any kind of resistance to any other lead, and sits at about 82VAC... what the hell is this?? i was thinking electrostatic shield until i read the voltage... could it be a shield and the voltage indicates something is wrong internally? all the other windings tested as expected.
third, i want to understand proper mounting, combined with proper grounding of the outer shell. i've seen a lot of conflicting opinions on this. to avoid affecting the flux at all, i was planning on mounting it with brass bolts with nylon washers insulating it from the chassy at all points. but then i've seen others saying that it needs to be ground referenced at one point... and i'm not sure how this ties in with the 'air gap' that should exist. my attempt to combine these leaves me with this plan:
fourth, what do you recommend in terms of a combination of nuts and washers in order to mount this securely? my thought was internal toothed washer with the teeth facing the chassy, then nylon washer, then nut. is a nyloc nut any help here?
fifth, should the same general practices in the last two questions apply to mounting OT's and chokes? i already know about how they should be oriented in reference to each other, just wondering about grounding, insulation, mounting etc.
thanks in advance!!
i want to make sure i'm using best practices when trying to make this sucker work, so i wanted to run a few things by you guys.
first - the thing is almost 2x as big as the Hammond xfmr recommended to replace a Spitfire, so i figure i'm not pushing any power handling limits, especially with only 2x 6BQ5, 2x12AX7 and 1x GZ34. is this a safe assumption?
second, there's one mysterious green lead coming from the PT that doesn't read continuity or any kind of resistance to any other lead, and sits at about 82VAC... what the hell is this?? i was thinking electrostatic shield until i read the voltage... could it be a shield and the voltage indicates something is wrong internally? all the other windings tested as expected.
third, i want to understand proper mounting, combined with proper grounding of the outer shell. i've seen a lot of conflicting opinions on this. to avoid affecting the flux at all, i was planning on mounting it with brass bolts with nylon washers insulating it from the chassy at all points. but then i've seen others saying that it needs to be ground referenced at one point... and i'm not sure how this ties in with the 'air gap' that should exist. my attempt to combine these leaves me with this plan:
- brass bolts with nylon washers insulating from the chassy at all but one point
- for one of the bolts, use a conductive washer at the chassy attachment for a ground reference (also just considering a terminal and wire running to the star ground point at the neg. terminal of the filter cap)
- make sure that the rest of the transformer shell does not make contact with the chassy (this is what i'm assuming is the 'air gap') except at that one grounding point
fourth, what do you recommend in terms of a combination of nuts and washers in order to mount this securely? my thought was internal toothed washer with the teeth facing the chassy, then nylon washer, then nut. is a nyloc nut any help here?
fifth, should the same general practices in the last two questions apply to mounting OT's and chokes? i already know about how they should be oriented in reference to each other, just wondering about grounding, insulation, mounting etc.
thanks in advance!!
here's some pics of the beast... realized that since i found it with the bolts on it a bit loose, the laminates were slightly moveable. is that an issue? mainly thinking some dirt could possibly have gotten in there, wondering if that's a concern or a possible dealbreaker.
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my bad, i should have mentioned that. i used a chassy i had lying around for ground. didn't check it in reference to any other leads bc it didn't show any continuity to any of them.
If it's a shield, there will be some capacitance to the windings, which will pass a tiny amount of AC current. Voltmeter is likely a 10 megohm input, shows a voltage. This is often called "phantom" voltage - will disappear with any kind of load. Sometimes you can measure a voltage at a light socket, even though the switch is off - same thing, capacitance in the wiring. Sometime there's a core grounding wire, but it's usually bare, sandwiched under an end bell.
As far as transformer mounting, the bolts should be insulated from the core and the top end bell - they were likely sleeved originally, with fiber washers. I use shrink sleeving if I use new bolts. As long as they're all insulated at the top, they can all be grounded at the chassis side, though one would be enough.
Here's the reason: The bolts run through the core, and a tiny amount of voltage is induced in each one. Not a problem unless two of them make contact with some of the laminations or both end bells. This forms a shorted turn, and even a tiny voltage can produce a large current - current in the chassis will create hum, and the bolts may actually get hot (will waste power too).
As far as transformer mounting, the bolts should be insulated from the core and the top end bell - they were likely sleeved originally, with fiber washers. I use shrink sleeving if I use new bolts. As long as they're all insulated at the top, they can all be grounded at the chassis side, though one would be enough.
Here's the reason: The bolts run through the core, and a tiny amount of voltage is induced in each one. Not a problem unless two of them make contact with some of the laminations or both end bells. This forms a shorted turn, and even a tiny voltage can produce a large current - current in the chassis will create hum, and the bolts may actually get hot (will waste power too).