SA 260 Pilot tube amplifier

[HollowState]

Stay away from metalized polypropylene caps. in the signal path.

Can you give me a good reason why, other then subjective hearing?

 

[Keit]

In the large company service departments I have worked in, bulletins would arrive from time to time about safety issues (eg beryllium oxide heatsink washers and beryllium oxide RF transistors, PCB oils leaking from old block capacitors, etc). I never saw any such bulletin about selenium rectifiers. Strange.

Nor in a quick search with Google "selenium rectifier hazard" could I find anything authoritive that confirms a significant risk, other than forums and ham radio sites that have limitted credibility.

Once somemeone however misinformed makes a claim about something in an internet forum, it then propagates like any rumour.

I did unearth a USAF document that assessed the risk of hazardous fumes - they concluded there was not a problem - however its' a 1955 report, so done when end of life was not near, and military equipment was usually built to higher standards anyway. Selenium rectifers were used a lot in USAF equipment in the B47 and B52 eras apparently.

The only medical reports I could find were about selenium exposure to workers in rectifier factories.

 

[trobbins]

It sounds like you are keen enough to spend time learning about basic amplifier restoration. Unless there is a rush, a lot can be learnt initially by checking and testing what you have got, without starting a major component swapout kneejerk reaction.

A lot of the existing parts can be tested and checked without the valves in place. Apart from using your multimeter for resistance testing, the best next step is to provide a safe AC mains connection and to beg/borrow/steal a variac, and temporarily solder in some 1N4007s across a 5U4G socket. Bringup up the power transformer with a variac allows testing of parts in a controlled 'safe' manner.

As well as a good AC mains cable, and removing AC wiring to other parts that are not needed (Aux power socket), I can suggest that a few basic additions are worthwhile, including fuses for the 430VAC red power transformer leads that go to the diodes (a fuse in the CT is not a good idea here because of the biasing).

As well as replacing electros and seleniums, the addition of 1N4007's in series with the 5U4G anodes is worthwhile in this amp, as well as adding some extra protection for the output transformers (because of their 'value'), and some failsafe modifications to the bias pots. You could also investigate how good the grounding layout is, by learning about good grounding practices and checking how it was done in your amp.

You may find you enjoy the whole process - there is a lot on the web to help assist, such as other similar restorations, and specific articles and pages on theory and performance. The following may also be of some help:
http://dalmura.com.au/projects/Renovating PA amps.pdf

Ciao, Tim

 

[henryr]

This has been very informative, thanks to all.
Keit, I chickened out and sold the unit as is on Ebay for $510.
As far as learning about tubes, I am going to follow the path in Bruce Rozenblit's book, "Tube Audio Design", starting with a single tube circuit. I think this will be safer.

 

[Keit]

I think you got a good price. considering the tatiness of the chassis. Well done.

I'm not familiar with the book, but I think you are taking a wise approach. Building tube amps can be an expensive hobby, but you now have plenty of cash to buy trustworthy parts to build one-tube circuit, and even go on to 3 or 4-tube circuits that can give you some seriously good audio if you choose and plan wisely.

 

[neali]

The subject of selenium rectifiers is interesting.

diyAudio and other tube forum websites are full of advice, mostly from American contributors but some English, to the effect that they will always eventually fail, and when they do they emit very obnoxious and harmful/dangerous fumes.

These comments are so common I assume there is a real basis behind them. There is also a lot of obvious rot about selenium rectifers on the Web - for example Wikipedia says they only last a few years. Well I have a Boonton Radio Corp Q-meter made in 1951. It still has the original selenium rectifier, and its still good. I have some Japanese tube based profesional radios with their original selenium rectifiers, and they are all good too.

I have decades of experience servicing professional equipment. Such equipment if made in Germany almost always had selenium rectifiers. Typically where in American equipment, consumer or professional, where you'd find a tube rectifier, in German equipment you would find a selenium rectifier. In servicing such equipment, made in the 1950's, thousands of times, I never once seen one fail with a bad smell, and only a few that had started to drop too much voltage.

Perhaps American companies never figured out how to make a good selenium rectifier. The German company Seimens obviously did.

In my limited personal experience, the failure mode of a selenium rectifier is increased forward resistance. When a selenium emits a ghastly cloud of sulphurous smoke, the usual cause is the electrolytic following it failing short. I saw this happen once in a working/playing radio. Never forget the smell.

Diodes don't make out much better with this failure mode. That is what fuses are for. I have restored a few late fifties early sixties German piano key radios. Wonderful radios and those Zippo lighter looking seleniums worked in all of them.