Gunn amps 100w, faults, and transformers

[puddles]

we have a gunn 100w twin combo. It has blown its valves. they always do this; our last did it several times, and everyone else who has one. The shops say it is a mismatched transformer., does anyone know what the right type (or a batter one) would be? Basically, if you run it too loud for toolong the valves go (e.g. at a live gig without a big PA and DIs. Has anyone done this, rewired a Gunn not to blow?

[Rog]

Time to replace it, methinks?

Gunn were a very average amp in their time, certainly nothing wowrarecollectible about them.

Yes, they can be unreliable, although I had one which lasted for a year without troubles.

[ash]

Mine never blew valves, but it did blow a transformer, which I never replaced. I sold it to a guy who has rebuilt his before. He had a transformer rewound by a Russian fellow in West Auckland. I don't know any more about him than that unfortunately. The buyer may lurk here and be able to offer more info...

[puddles]

thanks for these replys already - the Gunn was free, the shop wouldn't buy it from previous owner because of Gunn reputation for meltdown.
basically my understanding (it was years ago) was that there was no transformer in NZ that perfectly matched the circuit, so they made do with the wrong one. A lot of Dunedin Sound bands used Gunns - the Clean, Snapper for sure. i like it, it has a really gronky sound - plangent - you know you're plucking a wire and that it's electric. It's unforgiving - it makes the most of string sounds and harmonics - I think that's why those guys used it. definitely not smooth, heaps of presence. Of course, fixing it may change that. My hope was, that if we can learn the real specs of the circuit, these days a more apt transformer may be available. but I literally know nothing about these things.

[B45-12]

we have a gunn 100w twin combo. It has blown its valves. they always do this; our last did it several times, and everyone else who has one. The shops say it is a mismatched transformer., does anyone know what the right type (or a batter one) would be? Basically, if you run it too loud for toolong the valves go (e.g. at a live gig without a big PA and DIs. Has anyone done this, rewired a Gunn not to blow?

When you say 'valves go' I assume you mean the output valves? If so, you mean the output transformer! Basically in that case the windings are too few to provide the proper ressitance for the anode output which means the valves overwork and blow up.

2 temporary fixes come to mind If this is centre tapped O/put transformer ground (push pull class B output which it most likely was) you could try increasing the size of the centre tap resistor or putting one in the pathway to ground - this would increase the anode load.

The other is putting a line dropper resistor in the power supply to the output valves and effectively working them on lower voltage but how good this is would depend on how much of the transformer mismatch there is - worst case would make the amp quiet and strangled as the valves would not be getting enough volts to do their job properly (though valves are usually fairly tolerant buggers - anywhere from 100v up will keep them happy).

Anyway those are some thoughts/mods you could show to the tech while hunting for the proper xformer - easily reversible and should not damage anything although you would probably loose about 10% volume. Hope they help.

[ash]

Its an ultralinear tapped tranny. A real pig-ugly unit at that. Another potential issue is that the power trannies can jam up to 700V into the plates in some circumstances. I can't remember what those circumstances are, but I measured that much on mine at one point.

They're a really complex circuit compared to the glorious simplicity of an old Marshall or Fender. I don't know what they were thinking. I speak of a Gunn Classic 100... other models may differ, of course.

[Rog]

For my tastes, its hard to go past the old Jansens if one wants a NZ bottle amp.

Like everyone of the time, I also used Holdens, but the Jansens seemed to have a slight edge in my view. The Holdens, to me, seemed more 'modern' sounding (maybe more sterile?) and the Jansens more early Fender.

[GrantB]

My Gunn had the coolest fuzz tone ever - you pulled the switch out to turn it on...real 70's. But, it was eternally unreliable. If remember correctly, it would work for an hour at a time, with 3 hour cool downs between such periods.

[B45-12]

My Gunn had the coolest fuzz tone ever - you pulled the switch out to turn it on...real 70's. But, it was eternally unreliable. If remember correctly, it would work for an hour at a time, with 3 hour cool downs between such periods.

That sounds like a really nasty intermittant - on some amps they used thermistors in vcarious configurations (resistance increses as they heat up) and when they go wonky, funny things exactly like you describe happen!

[Rog]

resistance increses as they heat up

Depending on whether PTC or NTC.

[sgt mukuzi]

take it to 34 audio man, he can fix it. gunn wired their own tranny back in the day

[GrantB]

My Gunn had the coolest fuzz tone ever - you pulled the switch out to turn it on...real 70's. But, it was eternally unreliable. If remember correctly, it would work for an hour at a time, with 3 hour cool downs between such periods.

That sounds like a really nasty intermittant - on some amps they used thermistors in vcarious configurations (resistance increses as they heat up) and when they go wonky, funny things exactly like you describe happen!

Believe me, you could almost set your watch by it!

[B45-12]

resistance increses as they heat up

Depending on whether PTC or NTC.

Speak not unto me of coefficients! negative or postive - but pray rather use 'inverse temperature/resistance relationships'.

Actually I'd forgotten about the negative one, but in that case it's like good old w/wound resistors!

It was the positive ones used, I seem to recall, some where in the anode circuit to stop things getting nasty if the valve got too warm/overdriven. Also some used to use in heater circuits to warm them up quickly then cut the current back to reasonable level again before things got nasty - they were the ones that could cut your amp off when they went open intermittantly (all the heating cand cooling was a great starter of cracks!).