Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: capn_starsky on April 20, 2020, 09:34:03 am
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Good morning to everyone, I've got a transformer measurement question. I'm relatively new to tube amp building but have decent electronic experience and am perfectly comfortable working around high voltage...I'm just partly lacking some theory understanding. I got a cheapo Crate tube amp as a freebie for helping a buddy move. After a cap job, it works just fine, for a give away, but seeing as how I've got 30 bucks into it, I've started looking at gutting it and using the chassis and transformers for a build something along the lines of a JCM800 and it'd be nice to not have to shell out for new iron. I've not been able to find specs for the transformers online so I've just done a few measurements to figure some things out. Heater and Bias taps I'm fine with, but I've got a question regarding the high voltage winding. I measured each side with respect to GROUND and came out with around 160 VAC on each before getting to any filtering or rectification. There's no center tap (at least listed on the schematic attached to ground) so here's my question...with this measurement to ground, would I simply add the two together or would I need to open it back up and measure the voltage across both taps? Perhaps there's a calculation I'm missing also. Other than that, heater supply is fine as it's a 4x12ax7 and 2 EL34 and the bias supply also seems adequate.
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would I need to open it back up and measure the voltage across both taps?
do this.
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Is it a GT-50?
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Thanks for the replies! I was hoping I could just wing it and do some math because I already brought it back upstairs and don't wanna lug it back to the station. It's a VFX5112, schematic doesn't depict a center tap, but I'll verify that later when I open it back up. Super cheesy digital effects loop. But otherwise sounds alright. Kinda harsh and sterile, but since I'm not out much, I was looking for an excuse to build again. I was honestly thinking of getting a small parts kit from Mojo or just ordering the bits myself. I figure as long as it's not putting out something stupid, the transformers could be used as is for a 50 watt SS rectified Marshall build.
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Power transformer with no secondary C.T on high voltage ; You should not use ground to read AC output.
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In my experience, if you're guessing, you can expect to run into trouble later on, so it's best to know what you're dealing with right from the start. So, what Sluckey says - get the best measurements you can before you start modding. That way, there are fewer surprises later.
If you can, use a variac to measure the high voltage (all the while using caution - dangerous voltages!), rather than simply measuring ohms resistance with your meter. If you have a variac, you can attach your meter to the secondaries, then bring the variac up to 120v on the primary. Remove all tubes first, of course.
Your meter reading will give you the high voltage going into the rectifier, and from that, you can sorta extrapolate where your B+ might land. But be careful here because, aside from the high voltages, your meter might have limits on what it can handle, and a measurement across the full secondary (as opposed to the CT, which is only half the secondary winding) can get up to 700v or beyond, and not all meters are rated to handle that.
Uncle Doug has a few great vids on youtube dealing with the rudiments of transformers.
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> a cheapo Crate tube amp
HOW MANY "cheapo" amplifiers has Crate sold? Isn't there a name/number on it? (Has to be, so Crate dealers know what to charge.)
It is what it is. If you took DC readings and tube counts before you stripped it you know what can be done with the parts.
ac427v posted a plan of a GT-50 showing 420V DC. While that rectifier could be re-jimmyed to give 210V or 840V, why would you do that?
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It's a VFX5112.
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So 2x6L6GC? Schematic doesn’t actually state the power tube type but also doesn’t specify connection for power tube pin 1, which indicates against EL34..
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Bias voltage argues for EL34. OP said it had EL34's. Everybody that is selling one says it has EL34's.
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speaking of arguing bias;
what's the .8A max line :dontknow:
monitor primary to set bias?
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It says "line", so I would think 800mA at the power cord. Doesn't make sense, though, because this is the schematic for the 50W and the 100W.
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Doesn't make sense
:laugh:
ya got the line/plug part, then I drifted off to some poor 1st amp bench tester with flaming hair n burnt meter leads :icon_biggrin:
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The schematics posted by shooter and 2deaf are the ones I've found and they coincide with ALL parts and labels on the board itself. Found the user manual online as well, and it definitely lists the EL34 as the power tubes. No luck in finding a dealer that has the specs listed, though I've not searched too deeply for it. I'm definitely going to crack it open again today and just measure everything again. As far as the bias listed, I promptly ignored that and just biased the old fashioned way, seemed kinda sketchy to be measuring current at the wall if that's indeed what they meant by it. :laugh:
You all are a fantastic help by the way, I'm always impressed by the depth of knowledge here! For further reference, if anyone knows any other leads, the transformer itself is labeled SLME 94-270-02 with a part number of PT-070-10032-100-0.
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Alrighty, finally got to it! More posting for others to reference...on the SLME 94-270-02 power transformer in the Crate VFX5112, from the wall voltage here in the U.S...filament winding is standard with a center tap, bias supply winding is 15.5 VAC on either side of the center tap, and the high voltage with no center tap is 315VAC across the two leads. These are my measurements on my amp without any reference as to what the manufacturer says they'll do. Lastly, would this sucker make a good candidate for a Marshall conversion? I'd planned on doing a build anyway and if I can salvage some iron, that'd be nice but I'll shell out for better transformers if I absolutely need to. Thanks again to everyone for helping out so far!