dead output transformer

[serge1808]

I have an old selmer treble and bass reverb sv the other day when i was rehearsing the amp cut out and the mains fuse had blown. I found out at the end of the rehersal that the rehersal studio tech had re-wired the cabs to take 16ohms on all inputs and hadnt bothered telling anyone so basicly i was using my 8ohm output on a 16 ohm cab without knowing. Everytime i replace the fuse it blows i have tried removing all power valves but it still blows.

Im new to vintage electronics but im begining to think the output transformer has blown.
I put a multimeter on the input and one on the output of the transformer and im getting a constant reading of 40.2kohms on both the 8ohm and 16 ohm output. Im just confirming i take it that means its blown? cause i would assume it should give different readings on the 16ohm and 8 ohm output.

Also where would i get a modern replacement?(i dont want to start messing about with secont hand parts)
hope someone can help

[pdf64]

The output transformer may well be ok; did you isolate the transformer connections before making your measurements? If not then the results probably don't mean much.
Check out Tube Amplifier Debugging Page
and
My output or power transformer is dead, isn't it? How do I test it to find out?
Are you fitting slo-blo / 'T' rated fuses?
See http://www.webphix.com/schematic%20h...everbsv50w.pdf
Is a suitable fuse fitted for the HT / B+ (eg not 10A)?
Pete.

[R.G.]

What pdf64 said. Furthermore, running your output at 8 ohms setting with a 16 ohm cab is very, very unlikely to blow an output transformer that was not already about to die from something else. The cab/OT mismatch is almost certainly not your problem. Further:

Everytime i replace the fuse it blows i have tried removing all power valves but it still blows.

This is not one of the most obvious indications of a blown OT. I don't know why people like to think "my amp doesn't work. It's a dead OT isn't it?"

Blowing fuses is a good indication of anything that causes a lot of current flow from the mains: bad/shorted AC wiring, bad/shorted power transformer (again, not the most likely candidate), bad shorted wiring to rectifiers/filter caps, bad/shorted rectifier, going-bad filter caps (very likely if they have not been replaced in an amp this old, BTW), bad/shorted tubes; loss of bias to the output tubes can do it. I would bet on all of these except the bad PT before I'd bet on a bad OT.

I put a multimeter on the input and one on the output of the transformer and im getting a constant reading of 40.2kohms on both the 8ohm and 16 ohm output. Im just confirming i take it that means its blown? cause i would assume it should give different readings on the 16ohm and 8 ohm output.

What pdf said.

[Chuck H]

Ah, but it COULD be a bad OT based on the symptoms so far. And considering the cabs were all "rewired" that makes the possibility of an open load a consideration. If it were caps I would expect smoke or at least a funny smell.

Follow the testing regime at the site provided.

[Steve Conner]

Blowing fuses with the tubes removed would require the OT primary to be shorted to the core. Another possibility is an arced power tube socket.

Neither fault can be measured reliably with a meter. They tend to look OK under the low voltage from your meter's continuity test, only to break down when the HT voltage is applied. But to detect a short to the core, disconnect the primary leads from the circuit and check the resistance between any primary lead and the transformer's metal casing. It should be higher than your meter can measure.

As for getting a replacement, I assume you're in the UK since Selmers are pretty rare elsewhere. Bluebell Audio of Dundee stock the Hammond range of transformers, which includes some pretty good guitar amp OTs.

[serge1808]

Thanks for all your help guys ive got a lot to try.
I used a slow blow 1.6 amp fuse from maplin http://www.maplin.co.uk/1andquarteri...me-delay-t-462

on a visual inspection the capacitors look OK but i may as well try replacing these first to eliminate them

the o/t fuse hasn't blown as the main fuse blows first(not the fuse on the plug but the slow blow fuse on the chassis).

inspecting the valves they are all really old ( i think some of them may even be original) the rectifier valve was actually mis-shaped as though it had melted but as i said even with these removed the fuse still blows.

I never got any smoke from the amp just a loud buzzing noise (not through any speakers) and then the fuse blows

[Chuck H]

Without proper test gear the troubleshooting could put undue stress on the rest of the amp. The buzzing sound with no tubes in, and the melted rectifier are bad signs. You may want to take it to a repair shop for an estimate at this point.

[Tom Phillips]

...on a visual inspection the capacitors look OK but i may as well try replacing these first to eliminate them...

Replacing the parts is one approach. There are situations when a quick part substitution is part of the troubleshooting process. However, if you follow the troubleshooting guide you should be able to identify the problem first without rebuilding complete sections of the amp. The idea is to isolate the cause of the problem without just guessing and replacing parts. You may have old caps that you want to replace anyway but it’s still good to find the root cause problem up front.

It is probably not the case with your amp but sometimes a person would decide not to proceed with a repair when a serious problem was found. In one case I worked on a 50 W Marshall style copy that had both a bad power transformer and bad output transformer. Again, it may not apply to your exact situation but I think it is good practice not to start replacing significant parts before the problem is identified. I suspect that someone will chime in and say they always replace the filter caps if they are over XX years old. My point is, troubleshoot first then decide what to replace.


Regards,
Tom

[serge1808]

been tearing my hair out with this thing :-)
one of the diodes on the input was open so im just waiting on a replacement coming.
Is this quite a common problem on tube amps or does it more likely indicate a fault elsewhere in the amp?

[Steve Conner]

If one of the diodes was blown, I'd replace all four. Five if you count the bias diode, although it is very unlikely to need replacing.

1N5408 or 1N4007 will do, there's no need to use an exact replacement.

If you get stuck, try Flynn Amps in Glasgow, or me.

[big_teee]

If one of the diodes was blown, I'd replace all four. Five if you count the bias diode, although it is very unlikely to need replacing.

1N5408 or 1N4007 will do, there's no need to use an exact replacement.

If you get stuck, try Flynn Amps in Glasgow, or me.

You guys Know much better than I, but I would pull all the tubes, and check the caps for shorts.
Then without the tubes I would hook it to the dim bulb tester and go from there.
Start with the smaller bulb and see what all seems shorted.
Start the process of Illumination.
Good Luck,
Terry

[serge1808]

Its alive!!!!!!!
Thanks steve i replaced the diodes with 1n508's and it came to life again working perfectly.
I think i will need to restore the whole thing at one point that will be my summer job i think:-)
the re verb block is broken so ill need to try and source one any ideas?

[Chuck H]

Electronic restoration will include replacing old electrolytic caps, testing film caps for leakage, checking resistors for drifted values, cleaning contacts, etc., etc... If you just make sure it has good electrolytic caps, good tubes, proper bias and it sounds good, it's fine. Replace the rectifier that looks melted for starters.

[Steve Conner]

Chuck, the Selmer SV amps had silicon diodes, not a tube rectifier. I don't know where this talk of rectifier tubes came from.

I've ordered reverb tanks from TAD in Germany, but there are lots of different types. You need to know what impedance your drive circuit is designed for.

My T'n'B 50 SV had a bunch of drifted and noisy resistors. And no rectifier tube.

[pdf64]

Hotrox has good prices on reverb tanks, see- Reverb Tanks Hot Rox UK
Pete.

[Chuck H]

Chuck, the Selmer SV amps had silicon diodes, not a tube rectifier. I don't know where this talk of rectifier tubes came from.

Here:

inspecting the valves they are all really old ( i think some of them may even be original) the rectifier valve was actually mis-shaped as though it had melted but as i said even with these removed the fuse still blows.

A glass envelope or plastic base that got so hot as to warp and melt is a really bad sign.

[Steve Conner]

I ignored this since the SV amps don't have a rectifier tube. Not that I know of anyway.

But yes, if some tube has got hot enough to melt the glass, that is bad. It may be the reason for the blown diodes, and it may cause more expensive damage later if it's not fixed. Replacing the tube would be a good start. What kind of tube is it, can you see a number printed on it?