Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dross on January 11, 2017, 09:42:09 am
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I'm sourcing parts for a Matamp GT120 build (based on the Hilbish board) and have hit a snag on the Power Transformer that I was hoping some people with more experience than I on these matters might be able to shed some light on.
The schematic shows a power transformer that has two 6.3V filament windings, one at 6A for the EL34 heaters, and the other at ~1A for the 12AX7 heaters. The PT (and overall circuit) are very similar to what is shown here: http://el34world.com/charts/Schematics/files/orange/orange_otr_120.pdf (http://el34world.com/charts/Schematics/files/orange/orange_otr_120.pdf)
Unfortunately I am having difficulty sourcing a PT that is reasonably in keeping with these specs. I know Mercury makes one, but I'm keeping Mercury as a last resort. Hammond has confirmed that that they do not make one, and I am still waiting on hearing back from Heyboer and Pennine. (I know Pennine has wound these in the past, but overseas shipping on transformers is a little unappealing.)
So I guess my first question is whether or not anybody has a good source for this sort of transformer that I have overlooked?
Barring that, what substitution could I possibly make here? I had thought that maybe I could use a transformer with a single higher current filament winding (~7A, like a 100W JCM 800 transformer, though the plate voltage would probably be too low) and then take the heaters for the 12AX7s and the lamp directly off of the EL34s, but I'm not sure if this is a good idea. Any input would be greatly appreciated!
Thanks much!
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Some alternatives to the MM $$$ PT...
I would think the 100W JCM 800 transformer would be a good choice. Unloaded B+ would be 500V. All filaments on the single 6.3 winding. You would need to modify the bias circuit slightly since the Marshall transformer does not have a separate bias winding.
Or, use a JCM900 100W PT. It has a separate 40v winding that could be used with a FWB for the bias circuit.
Or just buy a separate 12v/.5a or 6v/1a filament transformer to heat the little tubes. Such a transformer would be pretty small and cheap. You could even get one at RadioShack.
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http://www.classictone.net/40-18069.html (http://www.classictone.net/40-18069.html)
Is a project transformer I'm looking at for a 100 watt clone project, It has 10A for 6.3V heaters, plus 60V and 2x15V taps as well. Click the "Features" link to see the PDF of all the taps :)
-Brett
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Thanks much for the feedback guys! Looks like a substitution is in order. That ClassicTone looks like a good candidate, but I also found this JCM 800 replacement PT from Hammond that might work:
https://www.hammfg.com/files/parts/pdf/290HX.pdf (https://www.hammfg.com/files/parts/pdf/290HX.pdf)
It's got a 100V bias winding on it, so that solves that issue. I'm still a little wary of the ~360V coming off of the high voltage secondary. I guess that should translate to around ~500VDC on the other side of the rectifier which is in keeping with the target plate voltages.
Again, thanks much for your help!
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That Hammond 290HX is only 200mA, you need 400-500mA for 100w amp.
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The high voltage secondary is listed as 356 VCT @420 mA, which I figured would be OK - though now you mention it, and after looking at the EL34 spec sheet again, yes, something closer to 500 mA would be preferable.