Firing up an old valve amp of unknown provenance

[stuzl873213]

Depending on the amps age it may be a good idea to check the internal wiring.

Bend the wires, especially close to the valve bases and make sure the insulation doesn't crack.

Caps can be a pain, especially electrolytics. Larger caps usually require reforming regularly during storage but probably not applicable in this case.

Alternatively, take valves out, fit RCD breaker, turn on and cross fingers--3 outcomes : bang, burn or nothing

[Simon Barden]

Well, I finally took possession of the amp today. Checked lots of things first before applying power - mains and HT fuses were OK. Caps all looked fine. Took the valves out three nice Mullard ECC83 pre-amp valves (from 1974), I think the EL34s are Mullard as well by the date and type codes Xf3 and B5A4, so Jan 1975 (but any Mullard printing has either rubbed off them or never got put on top start with). Only the ECC81 PI seems to be another make.

But all is not well. Powered up neon came on, no bangs or anything. Waited a while and nothing happened. So started putting some pre-amp valves in. No glow at all. Put all the valves in. Not a single one glowed.

So I looked at the wiring diagram, measured the resistance across the '240v' primary winding on the mains transformer and it was open circuit to neutral. The '120v' tapping was a short to neutral. So it looks like the mains transformer is dead. Maybe someone tried it on the wrong tap setting.

This may already have been been a replacement transformer, as there were only two primary tappings, not 4 as shown on the schematics. However it could always have been a bodge job using an available transformer.

The 105v & 1150v positions were joined together on the voltage selector and the 245+225v positions were also linked (though the 220v position only had the wire passing through the eye and it wasn't actually soldered!).

Schematics here: (50 Plus Mk 4 head) http://soundcitysite.com/schems.shtml

So I'll need to find a new transformer if I can. There's a local amp tech, so I'll see what he says. But given that the primary side has gone wrong, does this indicate a problem on the secondary side?

[Marcel]

From what you have said Simon I get the initial impression that whoever replaced the transformer did a back yard job of it. Very bad practice to go through one tap to get to another.
As the transformer is a replacement it begs the question did they put a large enough replacement in? If for example the transformer was only rated at 140mA on the HT coil and at some time latter the load averaged 160mA then there is a reason for it to overheat and blow the primary circuit thermal fuse that is often fitted to newer transformers primary windings. Same applies if the heater windings need to supply more than the rated current, the transformer heats up and the 'you only have one chance' thermal fuse blows. Transformer thermal fuses are typically wired in series with the primary winding neutral connection point and are usually set to blow at either 75C or 80C.

A bit of digging into the back story to the amp may help to reveal if this is the case. Are EL34's original fitment? What possible reasons are there for the the first transformer swap? Does the current transformer look big enough to do the job (when you compare its size against other amps of similar wattage).?..

If nothing in the rest of the amp looks stressed or appears to have suffered unexpected heat damage then it would be worth looking into fitting a suitably rated replacement mains transformer.

[Simon Barden]

It looks about the same size as other transformers I've seen in photos, and it looks like it's using the original mounting holes. I'll take some pictures tomorrow.

It's certainly an EL34 amp design. I'm just guessing that the transformer may have been swapped due to the number of primary tappings, but it may have been factory fitted (badly) that way. But it did have a 'U.S.A.' paper label on the side of the transformer, so it may have come from a different amp (maybe a Fender), as there were and are still enough UK transformer manufacturers for Sound City to not to have to import any. It may even have gone abroad in its life, someone forgot to change the input voltage setting and the original transformer blew. I'll see if I can find anything about the part number on the transformer.

There may be someone on the Sound on Sound forum (my 'other' forum) who might know something (if he's still around) as they used to work for Roost, another small amp manufacturer who weren't too far from the Sound City factory and they got a lot of components from them.

[DrNomis_44]

If you are unable to source a replacement power transformer for the amp locally, you could try ordering one online from Evatco, since they stock both power and output transformers, here's a link to the Evatco website:


www.evatco.com.au


I'm guessing that the original power transformer was replaced (if that's actually what happened) because the amp probably developed a fault that pulled too much current from the power transformer, or the primary might have developed an internal short to ground, in which case the mains fuse should have blown first, we will probably never know for sure what actually happened, what I would do is go through every part of the amp's circuitry and check that everything is okay first before fitting a new power transformer, I would have to agree with what Marcel said in his posts.


Hope you manage to sort it all out and get it up and running again.

[Simon Barden]

Well, a quick search for the transformer part number seems to indicate that it is a proper Sound City transformer, probably by Partridge - though this only comes from one hit on an eBay seller in Spain. The part number is T.G. 9875-A265.



Here's the front of the amp chassis (some valves removed at the time):



And the back. The PI valve sits between the two EL34s. This is a later arrangement as the PI was originally the other side of the metal shielding plate with the other pre-amp valves:



Here's the only really suspect cap I could see with obvious cracking (though no leakage):



Here's the bottom of the two main smoothing caps. They both seem to have a small bubble in the rubber/plastic covering, so I'm not sure if that's normal or not.



Haven't had time to do any more real investigation as I need to tidy away all the stuff from the 'indoors workshop' (i.e. the dining room table), as we have friends coming over for a meal tomorrow.

[Marcel]

Nice photos Simon.
By the looks of it I doubt the transformer has ever been replaced. And the caps look good enough to give them a chance. The dimples on the caps appear normal and are often found on these type of caps, and they are often included as a designed weak point to allow any internal over pressure to escape.

Based on these photos my educated guess from the other side of the world is that the thermal fuse in the transformer has let go. Be it an actual over temperature event or simply age related fatigue there is nil way of telling and your only option is to replace it, which looks to be a simple enough job to do...

As a side note. personally I have never liked that style of voltage and speaker impedance selector switches. For those who are not familiar with them (and that includes most guitarists) it is often all too easy to select the wrong setting. There is the not too remote chance that incorrect voltage selection caused the primary to blow without taking out the mains fuse. One way of checking is to see if the heaters/filaments of all the tubes are actually okay by removing all tubes and testing their continuity with a multimeter or test if they light up in another amp. Provided that since the transformer failure event none of the tubes have been changed then If any are open circuit (more than a few hundred ohms) then wrong voltage selection (110V selected on 220V supplied mains) is a very likely original culprit.

[DrNomis_44]

Well, a quick search for the transformer part number seems to indicate that it is a proper Sound City transformer, probably by Partridge - though this only comes from one hit on an eBay seller in Spain. The part number is T.G. 9875-A265.



Here's the front of the amp chassis (some valves removed at the time):



And the back. The PI valve sits between the two EL34s. This is a later arrangement as the PI was originally the other side of the metal shielding plate with the other pre-amp valves:



Here's the only really suspect cap I could see with obvious cracking (though no leakage):



Here's the bottom of the two main smoothing caps. They both seem to have a small bubble in the rubber/plastic covering, so I'm not sure if that's normal or not.



Haven't had time to do any more real investigation as I need to tidy away all the stuff from the 'indoors workshop' (i.e. the dining room table), as we have friends coming over for a meal tomorrow.

Heard lots of good things about Partridge transformers, apparently they were used a lot in Hiwatt amps.

[Simon Barden]

Yes, Marcel, the voltage and impedance selectors aren't great. I'll need to keep the impedance selector but I plan to hardwire the voltage selector for the 245v setting (and add a label on the back to say so).

[wazkelly]

Looks like an old school Marshall copy.

My first amp was a Marshall JMP Super Bass 100, probably built in 60's or early 70's and it had the same voltage and speaker impedance plugs too. Dead set easy to knock them out without noticing.