Fargen Blackbird on the bench

YouTube · Post by **xtian** » Tue Jun 25, 2013 2:16 am

This Fargen Blackbird is operable, but has a bad hum. I notice it has all vintage preamp tubes, but new JJ power tubes, and an amazing Cetron JAN 5R4WGB rectifier I've never seen before. Wow.

So I pull the power tubes. Still hums. On a dare (to myself) I pull ALL the tubes and the reverb cables. STILL HUMS! I pull the speaker cord, hum stops, and it's not a mechanical hum from the trannies. Plug speaker back in, hum returns.

Speakers measure OK. Same results with different cab.

Can this be anything other than hum induced in the OT by the PT?

Reeltarded · Post by **Reeltarded** » Tue Jun 25, 2013 2:27 am

I think it's all about the angles. WAG is that it sounds less hummy with the transformers both turned 90° from where they are now? Just a guess.

dobbhill · Post by **dobbhill** » Tue Jun 25, 2013 2:34 am

...or a very unbalanced primary on the OT. I found one once that had unmatched resistance on each leg of the primary.

D

YouTube · Post by **xtian** » Tue Jun 25, 2013 2:45 am

Well, I was able to remove one of the OT's bolts and pivot it. In this way, I can reduce the hum by half. In order to move the OT to where I think the null point is, I'll have to drill new holes.

Can't believe it left the factory like this.

YouTube · Post by **xtian** » Tue Jun 25, 2013 2:59 am

Well, this is where it wants to sit for best hum cancellation.

HTH am I going to mount it like that?

JazzGuitarGimp · Post by **JazzGuitarGimp** » Tue Jun 25, 2013 3:07 am

Does this amp have flyback diodes from the OT primary to GND?

Blackburn · Post by **Blackburn** » Tue Jun 25, 2013 3:22 am

Love that Cetron 5R4WGB! Think I'll get one for myself for that price.

Leo_Gnardo · Post by **Leo_Gnardo** » Tue Jun 25, 2013 3:44 am

Blackburn wrote:Love that Cetron 5R4WGB!

It's the ol' "potato masher." Tough as heck. Sometimes see one in a Leslie.

Reeltarded · Post by **Reeltarded** » Tue Jun 25, 2013 4:05 am

xtian wrote:Well, this is where it wants to sit for best hum cancellation.

HTH am I going to mount it like that?

Metal foam spray.

You're welcome.

YouTube · Post by **xtian** » Tue Jun 25, 2013 4:33 am

JazzGuitarGimp wrote:Does this amp have flyback diodes from the OT primary to GND?

No.

YouTube · Post by **xtian** » Tue Jun 25, 2013 4:50 am

Also must solve low reverb output. Tried replacing 12at7 with 12ax7, no change. Tank seems ok, makes loud crash when beaten. How to diagnose?

Reeltarded · Post by **Reeltarded** » Tue Jun 25, 2013 4:54 am

Sounds like low reverb input if it goes WHABOINGIONINGIOING when you hit it but not when you play it.

Post a gut shot of it near all the pretty reverb stuff and then consider that the transducer in the tank might be messed up anyhow.

Structo · Post by **Structo** » Tue Jun 25, 2013 12:32 pm

Did the customer recently buy this amp?

The lamination's were 90 degrees from each other which usually is pretty quiet.

But if turning them makes it more quiet, go for it.

Does the reverb tank check out?

Phil_S · Post by **Phil_S** » Tue Jun 25, 2013 1:27 pm

In addition to, or in place of, you might try a section of sheet metal mounted between the two transformers. You can probably mock up something without drilling any holes. Maybe use silicone as a temporary adhesive.

ampdoc1 · Post by **ampdoc1** » Tue Jun 25, 2013 2:31 pm

+++ for Phil's answer. The first amp I built from scratch, back in the late 80s, had a terrible hum when I first turned it on. I was using a screwdriver to loosen a part when I noticed that just most anywhere I held it between the output stage and preamp stages reduced the hum. I grabbed a piece of aluminum stock and mounted it with a couple of screws,....hum gone!!

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