Burman 501 blows fuses

Fatfingers

Hi. I leant my Burman 501 combo to an impoverished but talented nephew and, a few years later, have been rewarded with a dead amp. He says it was working one minute, not the next. He took it to someone to get it fixed (whose abilities I know nothing of) who claims the output transformer is goosed (he has also nicked all the valves from it!).

It shows no signs of burning. The transformer certainly doesn't look like it's got hot and still looks fine in its red varnish. Measuring resistance across the primaries I get around 83ohms across the coil and 43 and 38 measuring from one end to the centre tap, which seems okay. Measuring from the primaries to ground shows open circuit. 

However, measuring from the secondaries to ground I get 0.1, 0.2, 0.3 and 0.4ohms, depending on which tapping I connect too. All of the above are with the transformer still in circuit, no valves fitted and no speaker connected (although connecting one makes no odds).

When I power it up like this (with no valves) the onboard fuse blows after a few seconds (there's only one slow blow, 3amp fuse on it and no standby switch).

Any ideas on how to start tracking down the fault? The next time I attempt to power it up I'll use a light bulb current limiter.

ICBM

If it had the original KT77s in it he owes you a substantial amount of money…

You won't be able to tell if the transformer has a short by measuring it, because just one shorted turn on the primary side would be enough, and that wouldn't show up as a substantial change in 40-ish ohms. The secondaries are meant to be that low.

But a blown OT is not very likely if it blows the fuse with no valves in - unless it has a short from HT to ground, which is possible but extremely rare because it would have to be through the interwinding insulation - more normally transformer shorts are from one wind to the next in the same layer. Meter from either of the primaries to ground and see if you get anything - it should be open circuit once the caps have charged to the meter voltage.

It could be a shorted rectifier diode - although that would most likely blow the fuse immediately, rather than after a few seconds - or a filter cap. Or something in the bias circuit possibly, although that tends not to draw enough to blow a fuse. Or (rare but possible) a shorted preamp valve filament. Did he nick the preamp valves as well?

This is one reason I prefer amps to have a standby switch! It makes troubleshooting easier.

Fatfingers

It didn't have the KT77s, no. It had 6L6s when I got it and I replaced all the valves with a new set of JJs before I leant it to him. And yes - the "tech" nicked all the valves - so my tests have been with none in it.

It could be less than a few seconds that the fuse tales to blow. It could just be the time it teas for the filter caps to charge. And the fuse is a slow blow.

ICBM

Fatfingers said:

It didn't have the KT77s, no. It had 6L6s when I got it and I replaced all the valves with a new set of JJs before I leant it to him. And yes - the "tech" nicked all the valves - so my tests have been with none in it.

This is not a good idea. While it is almost always safe to test an amp with no power valves in, (somewhat counterintuitively) it's very often not with no preamp valves. With no preamp valves in, the voltage at the far end of the B+ chain will rise to the full unloaded HT voltage and usually exceed the filter cap rating, since most old amps have only 450V caps and the unloaded B+ is usually around 500V - they're stacked in the first couple of positions to cope with it.

That won't be the cause of the fuse blowing though, because even if a cap did short it would burn out the B+ chain resistor feeding it before it could draw enough to blow the mains fuse.

I also don't think they have enough bias voltage to safely run 6L6s, although I can't remember for sure. KT77s are an EL34 equivalent, not a KT66/6L6-family one.

Fatfingers

Ah. Okay - thanks. I'll plug some preamp valves in before I try again. 

It had 6L6s when I got it and worked fine, so I assumed it had been rebiased to accept them. I'd like to refit KT77s.

I think I'll poke about in the power supply section and look for failed components.

Fatfingers

Fatfingers

I just went through the four rectifier diodes with a meter. Only two of them show infinity one way and around half an ohm the other. The other two are short circuited.

Fatfingers

They say 261–2 on them. Any idea what they are? Or what I should replace them with. I might as well change the lot.

Fatfingers

ICBM

1N4007s are the industry standard.

Fatfingers

Great. Thanks for your help, ICBM. Much appreciated.

Fatfingers

Assuming the new diodes get the thing up and running again, what do I need to do to run it with a pair of KT77s, other than adjusting the bias?

ICBM

Nothing, assuming it's working at that point.

Fatfingers

Cool. And do you approve of lifting the grounded power tube earths and replacing them with 1ohm resistors as a method of reading the bias voltage? Or would you suggest I buy or make a bias probe?

ICBM

Doesn't make a lot of difference. I fitted the resistors internally on my own amp.

Fatfingers

Okay. Thanks again.

ecc83

When! You get it all running sweetly, do consider changing that crap "TV" open pre set in the bias circuit!

I say "TV" because I saw a lot of them there and they have a habit of going "gappy" NOT what you want for bias adjustment!

Fit an enclosed type or, my fave, a multiturn jobbie.

Dave.

Fatfingers

Hi Dave. Yeah - it does look a tad fragile.

So EL34s or KT77s?

Fatfingers

Seems I was premature in blaming the diodes. With new ones in the circuit the resistance readings are the same, so it must be the way the circuit is configured - or that the fault is causing that reading.

Someone suggested disconnecting the B+ top the OT's primary and seeing whether it still blows fuses then. Well it does. I guess the good news is that that means it's not the OT that's causing the problem?

ICBM

Good news… it's not the OT.

Bad news… it may be the PT. If the secondary winding is shorted that would explain the diode measurements. Although, so could a shorted filter cap, so I would remove the new diodes and measure the PT winding.

Build a light-bulb tester before going any further if you can. Using a blowing fuse as a fault indicator isn't a good idea either really.

Fatfingers

Hi ICBM. I did, last night. After lifting the supply to there OT's centre tap I tried lifting the supply to the smoothing caps. Still lights up the current limiting bulb. So I lifted the supply to the rectifier diodes. Still lights up the bulb although not quite as brightly - I think, anyway. Does this indicate that the PT is knackered?