Mullard Amplifier Project

[Refugee]

It looks very much like the ones in this amplifier have genuine Phillips/Mullard date codes so there should be no problem with them.
The first real fault I have found apart from leaky capacitors is a germanium point contact diode that only stops the indicator working.

[pamphonica]

According to a friend who worked at Mullard, the yellow print was indeed for the Jubilee year. They wanted gold but had to settle for yellow once the bean-counters found out the relative costs.
But they had a lot of the yellow paint left over afterwards, so ended up using it from time to time in subsequent years whenever stocks of standard white temporarily ran low.
There was never anything "special" about yellow print valves after all!

Jeremy

[Terry]

We found that the yellow print PC86's & PC88's weren't very reliable.

Considering that UHF tuners first appeared in 1964 for the opening of BBC2, that would be in the yellow print era, so that isn't surprising!

I didn't find these valves very reliable, either - perhaps being fully contained in tightly screwed screening cans didn't help there.

I don't ever recall seeing a PC86 or PC88 from any other manufacturer - were there Mazda branded versions, for example, and, if so, who made them?
I ask the latter question because it wasn't unusual to find the etched markings that all Philips group valves had on non Philips branded valves ...

Fortunately, the development of UHF transistorised tuners quickly stopped the spread of these terrible valves.

[Refugee]

There were some gold octal valves produced.
I guess these would be from the same time and would have been red in other years.

I have ordered the parts I was short of and they will be on the way now.

[Joe]

That's interesting. The only gold valves I'd seen before, in pictures rather than the flesh, have been in pre-war sets. The one in your picture obviously doesn't come into that category.

— Joe

[Terry]

I agree with Joe here.

What is the factory/date code on the EM34 - assuming it is  of Philips  origin ...

[AlanBeckett]

The Radio Museum has an Article on the history of the Philips 'Red' Valves, and the Gold ones. There are three 'Pages'.

http://www.radiomuseum.org/forum/the_phi...art_i.html

Alan

[simp2005]

Thats good news about the power supply capacitor. Will the other capacitors and diode be easy to source? Do you have any recommendations on good books to read about valve amplifier circuits?can pop over next week and see how you are getting on if thats okay with you? Would be good to try and understand what youre upto with it

[Refugee]

I have a spare diode but it is safe to go on with the rest of the work without me digging it out for now.
The second link in post 18 goes to the best reference for the circuit.
The preamp sections are like a mixture of the two valve and three valve preamps. Armed with these diagrams and with a magnifier I have worked out that I should be able to get to the capacitors to fit replacements. I keep them as stock items but was a bit low so have ordered more supplies and some specials for something else I am doing. They should be here in a day or two.
My supplier is doing free post on all orders but whom knows how long for though.
The gold EM34 does indeed have a date code.

[Terry]

The Km part of the type code is EM34 (6CD7), as we already knew - and D9A is Volvo, Hamburg, 1959, January.

As there is no Week number, it can't be 1969 and, as only the 'new' code introduced from 1955 onwards includes a number for the year, it can't be 1949 (or any earlier year).

That gold paint looks surprisingly fresh for 56 years old!