Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Widgetmjl on March 17, 2019, 11:05:39 am
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Hey guys - thanks in advance for any insight.
Alas the elusive (correct, detailed) schematic for this amp - I haven't been able to find a fully detailed one. No bueno. There is a PDF version floating around that is of very little help due to inconsistencies. (Attached is the one that's "out there")
So I'm a bit out of my depth - having brought my GA amp to a tech in town (actually the second tech to take a stab at this beast) Things appeared to be moving along - He replaced the caps, blown fuse, swapped out the old two prong plug etc...The tech identified the culprit preventing the amp from working as the power transformer (T1). The original was toast - so he had me purchase this one: https://www.mojotone.com/amp-parts/Power-Transformers/GA15-15RV-Power-Transformer
This is where he got flummoxed - it was clear after several weeks that the transformer we got from Mojotone was not going to work - simply couldn’t figure out how to mount the thing. . This is what the tech said "the transformer we got from Mojotone is a muti-tap transformer and isn't specific to the amp (Mojo does say on their website that it'll work for Gibson GA type amps - but alas it appears not mine). It'll fit many other amps as well...It has 2 more legs on the secondary side than what the original transformer had. I need to match up voltages from the transformer to the amplifier. This needs to be correct or damage to other components will happen - if it works at all..."
I've also attached a photo of the original transformer as well.
Any help or insight would be much appreciated as to EXACTLY what power transformer (T1) would work for this amp. Some of the suggestions from other forums and techs are below - but they all require some level of retro fitting and/or modifications to mount correctly.
HAMMOND: 270DAZ 76 117 60 520V C.T. @ 104ma. 6.3V @ 3.5A
http://www.hammondmfg.com/pdf/EDB270DAZ.pdf
EDCOR: Power transformer for a 120V, 60Hz. or 240V, 50/60Hz. line to 510V (255-0-255) at 125mA center tapped and 6.3V (3.15-0-3.15) at 4A center tapped.
https://www.edcorusa.com/xpwr013
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I can’t find a 6.3V heater winding on the Mojo PT!?
That seems something of a major omission for a tube amp.
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I can’t find a 6.3V heater winding on the Mojo PT!?
That seems something of a major omission for a tube amp.
Apologies if I appear woefully ignorant (I am ! :w2:) - as mentioned I am way out of my depth and trying to gather as much info for the tech trying to fix this beast. It appears that he purchased a Mojo transformer that was clearly marked as for the RV series of Gibson amps. Mine amp is from the RVT series. So it was impossible to mount and connect.
Not sure what exactly you mean - is this major omission on the part of Gibson or Mojo?
What transformer do you think might be a better or best option?
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> I can’t find a 6.3V heater winding
They call it 3.15V-0V-3.15V. It has a center-tap. Gibson does not use the CT.
The Violet 24VAC winding, the sheet clearly says "not used" on non-reverb amps.
The high voltage winding is single 400VAC. Gibson used probably 250-0-250 (500VCT). You need a different rectifier configuration. You get a little too much voltage for 6BQ5/EL84. I suggest you move the OT feed to the screen feed and change R32 to get a good voltage drop (and added filtering).
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The heater current is a little short with the Mojo transformer.
Do you have a choke coil in your amp? Some versions seem to have those and some don't.
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Thanks for responding PRR!
So it appears that there is no simple "drop in" transformer solution out there? I have no issue with returning the incorrect Mojo TF - all I am hoping to find is a replacement power transformer - That's simply not an option?
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does it "physically" fit in the space?
Is this a "keep it original" restoration, or I just wanna play?
If it's a just wanna play, JB weld, rivet, screw, a piece of aluminum over the hole, redrill new mounts. If it's period correct, send it out to a re-winder n PAY. buy another amp with known good PT
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does it "physically" fit in the space?
Is this a "keep it original" restoration, or I just wanna play?
If it's a just wanna play, JB weld, rivet, screw, a piece of aluminum over the hole, redrill new mounts. If it's period correct, send it out to a re-winder n PAY. buy another amp with known good PT
Thanks again for your suggestions - If its a guitar mod, where I'm a lot more comfortable discussing, the order in which I prioritize things is 1. Playability 2. Period correct 3. Cosmetic.
As for this amp? Keeping in mind I have already RETURNED the Mojotone and am now looking for a better (best) replacement solution, that is the optimal replacement choice for this style amp. Being as period correct as sending the TF out for a rewind is not gonna happen and frankly the stock parts on this are by and large crap. (I do like the tone a LOT more than I did my Fender Blues Jr)
If you have any suggestions on whether a Mercury, Hammond, or EDCOR, or Tube Depot: https://www.hdsugarloaf.com/ would offer the best solution? Apologies if this is redundant.
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> I can’t find a 6.3V heater winding
They call it 3.15V-0V-3.15V...
Doh, sorry, trying to make out the pale font on a smartphone screen, I thought it was 315-0-315; can't believe that didn't seem suspicious :laugh:
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what are the measurements from center to center of the mounting bolts. measuring the center to center of chassis holes may be easier. what is the window size for the endbell.
for clarification - the D & E dimensions in the attached drawing.
--pete
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I may have this transform your amp has vibro and reverb correct?
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If Scsoul or someone else does not have the correct transform you may want to look at some of the Classic tone models. The ones spec'd for Valco amps tend to match up well for Gibson amps; lower B+ voltage than typical Fender amps. I agree with others that unless Scsoul has that transformer you are not likely to find the perfect physical fit, but its the electrical specs that are the issue.
BTW that transformer from Mojo is spec for the more recent Trace Elliot Gibsons - not the vintage amps.
Your comment about the crap components - when you get a transformer (and you will) make sure that all the capacitors that are cardboard tube type, and waxed paper, are replaced. They will all be bad in my experience.