December 2009 \ Tech Tips \ Ask Amp Man \ Turn The Amp OFF… Or Else

Turn The Amp OFF… Or Else

Jeff Bober

When a business traveler leaves his amp on at home for a month, can Jeff save his once-glorious tone?
Premier Guitar December 2009
Hi Jeff,

Last year I plugged into a Fender Vibro- King at my local used gear store and absolutely fell in love with the tone; the “fat” boost sounds playing magic with the P-90s on my Les Paul Jr. Anyway, I ended up trading three guitars and a handful of pedals for it, and walked out of that store lugging that behemoth of pure tone behind me.

For the next few months, I found that the sound of my Vibro-King inspired me to play every day. Sadly, I had to fly out of the country on business for a month, and when I returned home I realized that I had left my Vibro-King on the entire time (I knew I should have replaced that non-working jewel bulb right away). When I plugged into it, the sound fizzled and then stopped. I replaced the slow blow fuse with another and turned the amp back on. That glorious sound returned, but it was short-lived as the amp fizzled and then stopped working once again. I have tried many different types of fuses as prescribed by the Fender user manual, but the same scenario always occurs.

Can you help me? Can you me tell how I can get my beloved Vibro-King working dependably once again? I miss her so.

Thank you so much for any help you can provide.

Gregory Fischer

Hi Greg,

I’ve seen this situation before. It sounds like your amp is suffering from abandonment issues due to your extended travel abroad, and this is its attempt at payback. Oh wait … that theory was from my psych class!

Okay, I’m kidding of course. While there’s no way that I can be sure of what’s causing the fuse to blow, here are a couple of possibilities. The first—and probably only—user-repairable item I would suspect would be the output tubes. I’m going to break the following down into two steps, as it will make the explanations of multiple causes easier to define.

Step 1: Remove the output tubes (the 6L6/5881 tubes), replace the fuse and turn the power switch on. Let the amp idle in this state for as long as a typical time would be for the fuse to blow. If the fuse has not blown, move on to the next step.

Step 2: Turn the standby switch on, and again let the amp idle for a while. If the fuse has not blown in the typical amount of time then chances are that the output tubes are shorting after they’ve been on for a while. This is really not an atypical symptom of output tubes, and could easily have happened regardless of whether or not you left the amp on for all that time. Have the tubes replaced and biased and you should be back in business.

Now, if you did experience the fuse blowing at any point in the above procedure then the amp will need to be serviced by your local experienced tech, and here are the possible causes:

A fuse failure during Step 1 would more than likely point to either the mains transformer itself, or one or both of the primary capacitors (220uf 285V) in the power supply being electrically leaky, which would eventually cause an increase in current draw enough to cause the fuse to blow. In attempting to isolate one cause from the other, I would first disconnect the 220uf caps from the circuit. With the standby switch off, turn the amp on and let it idle for a reasonable amount of time. If the fuse continues to fail I would be forced to suspect the mains transformer. While the transformer in these amps is a pretty substantial unit, it is a possibility since transformers in most consumer goods such as these are manufactured for “intermittent use” and not “constant use.” I’m pretty sure that leaving the unit on for a month would fall into the latter category. If the fuse did not fail during this mains transformer test, I’d revert to suspecting the primary capacitors in the power supply. I would definitely replace both caps since the malfunction of one can place a serious overvoltage condition on the other.

If the fuse eventually failed during Step 2, I would suspect either the output transformer or any of the other six filter capacitors (22uf 500V) in the power supply. First, disconnect the primary center tap lead (red) of the output transformer from the circuit. If the fuse does not fail, this could point to an output transformer with a winding that is eventually arcing and shorting to another winding or to the case, which is at ground potential. If the fuse does still fail, we’re more than likely left with the diagnosis of failing secondary filter caps. Unless one shows distinct signs of being stressed (pressure relief vent protruding), or is physically blown (you can’t miss that one!), I would replace all of them.

Hopefully this will return the amp to being the “King of Your Tone.” If not, maybe you’ll have to sweet talk it and ask for forgiveness … and buy it a new light bulb!

… ‘Till next time.

Jeff Bober
Jeff Bober, Godfather of the low wattage amp revolution, co-founded and was the principal designer for Budda Amplification. He can be reached at pgampman@gmail.com.

     

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Comments

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Steve Errede
on 08/13/2010 		I have worked on several Vibroking amps since they first came out. Frankly speaking, there are some serious design flaws in this amp... The design flaw that is likely causing grief in the case Mr. Fischer's VibroKing is associated with the design of the VibroKing's monster power transformer. If you look carefully at the VibroKing schematic (available at www.fender.com/support/...) you will note that the power transformer has *two* primary windings - which were intended for "versatility" in terms of wiring the two primary windings in parallel for domestic use (here in the USA) for 120 Vac line vs. wiring the two primary windings in series for export use e.g. in the UK/Europe for 220/240 Vac. That's in principle all well and good, except, as usual, the devil is in the details, which in the case of the Fender VibroKing power transformer, the two primary windings that need to be connected in parallel with each other *don't have the same # of turns* - they are in fact quite different in their number of turns, as reflected in either measurement of the DC resistances of the two primary windings, or the inductances of the two primary windings - they are quite different - I have measured these myself. Because of this inequity in the number of turns of the two primary winding on the VibroKing power transformer, when 60 Hz 120Vac wall power is fed into the two primary windings of this power tranny connected in parallel, a significantly large, totally *internal* AC current flows between these two primary windings connected in parallel. Because the primary windings each have resistance, electrical power P = I_internal^2 * R is dissipated as heat, due solely to this internal current. Thus the VibroKing power transformer gets very hot, simply due to this current. In fact, if you completely pull all of the tubes in the VibroKing amp *and* leave the amp on *standby*, there will be no load on the power transformer, but after e.g. ~ 1/2 hour, it will be too hot on its surfac



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