GK 800RB power transformer?

[Axtman]

I am currently working on an old Gallien Krueger 800RB bass head. To my dismay (and astonishment) the power transformer (not torodial type) is shorted. I have searched the usual places (Ebay, Talkbass, etc.) I sent an email to Mecury Magnetics to see if I could get it rewound. Does anyone have any other ideas? Thanks.

 

[Wasnex]

I am currently working on an old Gallien Krueger 800RB bass head. To my dismay (and astonishment) the power transformer (not torodial type) is shorted. I have searched the usual places (Ebay, Talkbass, etc.) I sent an email to Mecury Magnetics to see if I could get it rewound. Does anyone have any other ideas? Thanks.

Aside from finding a donor amp. I would try Heyboer. A custom wound transformer will not be cheap. I had Heyboer custom wind a PT for an early blueline SVT. $315 including shipping. I would find it hard to justify for an 800RB.

AFAIK there was once a kit to upgrade the original transformer to toroidal. So I wouldn't totally rule out a toroidal.

 

[agedhorse]

Donor unit. That's about your only (practical) option IMO.

 

[Slevage]

The original transformer was likely dipped in varnish. You won't be able to break it apart for rewind. Find a used amp for parts if you really want to fix it. A torroid would be a better choice if one were available. FYI, I had the transformer burn up in my 800 in the early 80's. The transformer was replaced under warranty when there were several repair shops that could do the work.

 

[Boneless]

A tech who knows what he's doing could/should be able to adapt another transformer. After all, it's all about power rating, inrush current, voltages at load and at no load (regulation of sorts), etc. (and, of course, available space inside the amp). And inrush current can be accounted for anyway.

 

[Kansas]

What causes these to fail? Heat? Hours of use? Filter caps? Curious because I have an old 400RB that I really like to gig with.

 

[agedhorse]

A tech who knows what he's doing could/should be able to adapt another transformer. After all, it's all about power rating, inrush current, voltages at load and at no load (regulation of sorts), etc. (and, of course, available space inside the amp). And inrush current can be accounted for anyway.

Not so easy with this amp, it uses dual higher voltage, higher current secondaries. Not at all common. (Open the image to see the reason for this).

 

[Boneless]

Not so easy with this amp, it uses dual higher voltage, higher current secondaries. Not at all common. (Open the image to see the reason for this).

...you'll probably find that the large majority of 800RB users will never use the bi-amping feature. (Some will, and they're shaged. But how many people actually take the trouble to use a double cabinet these days?). At that point, you could simply disconnect the "treble" power amp.

 

[agedhorse]

...you'll probably find that the large majority of 800RB users will never use the bi-amping feature. (Some will, and they're shaged. But how many people actually take the trouble to use a double cabinet these days?). At that point, you could simply disconnect the "treble" power amp.

Except that the low rails for the 100W amp also supply power to the low voltage rails for the preamp.

 

[Boneless]

Except that the low rails for the 100W amp also supply power to the low voltage rails for the preamp.

Yes, but that's a trivial modification. You can do that through the high rails (and a couple of component changes).

 

[agedhorse]

Yes, but that's a trivial modification. You can do that through the high rails (and a couple of component changes).

The dissipation will be a lot higher through the pass transistors, putting them in jeopardy.

It's not trivial, and this is how gear gets damaged, by poor advice resulting in equally poor modifications.

 

[Boneless]

The dissipation will be a lot higher through the pass transistors, putting them in jeopardy.

It's not trivial, and this is how gear gets damaged, by poor advice resulting in equally poor modifications.

Still trivial. It just amounts to setting up a decent regulator. When push comes to shove, you entirely redesign that bit up there in the top right corner.

 

[agedhorse]

Still trivial. It just amounts to setting up a decent regulator. When push comes to shove, you entirely redesign that bit up there in the top right corner.

On an amp that sells for <$500 used, really?

This is hardly a trivial or practical repair.

AZs suggested earlier, a donor unit is much more practical. Here's one that sold for $100, presumably the PT is good: Pardon Our Interruption...

 

[Axtman]

I checked with Mercury Magnetics and they said they would build me a new transformer per GK transformer specifications for $350.

 

[Wasnex]

I checked with Mercury Magnetics and they said they would build me a new transformer per GK transformer specifications for $350.

Heyboer is often less expensive.

 

[agedhorse]

I checked with Mercury Magnetics and they said they would build me a new transformer per GK transformer specifications for $350.

For that kind of cost, plus the labor and whatever else may be wrong, I would expect a good working unit will be less expensive in the long run (especially if that price is plus tax and shipping).

Here's an example of what you can find them for:
https://reverb.com/item/93213489-gk...9.WG0AiDif-G69F_dWQryNk9_oKDkVbfBS9KUNjlVymtU

 

[6thplanet]

I guess Mercury Magnetic has connections to get GK transformer specifications?

 

[Axtman]

6thplanet, Gallien Krueger sent me the specifications on the old (and new toroid) transformer and I sent it over to Mercury Magnetics.

 

[6thplanet]

Well that's mighty nice of them!

 

[Axtman]

Here are the specifications and drawings from Gallien Krueger on the old standard power transformer and new toroid power transformer for the GK 800RB. Maybe these will be useful for someone stumbling across this thread.