Mullard Amplifier Project

[simp2005]

Youre Doing a cracking job! I could call over one morning this week if you are about?

[Refugee]

Thursday is booked something else but the rest of the week should be fine.
We also have a spare part for another priority job due in on Tuesday so not too much time until that has arrived and been fitted.

[simp2005]

Thanks Simon. I will send you a text ealry in the week to srt something out with you

[Refugee]

I have not made a great deal of progress today as I had been having trouble with earth loops on the more sensitive inputs during testing of the first channel.
I do not have a battery powered high quality sine wave oscillator. I do have a very good quality mains powered unit but the earthing was giving me grief.

There were two choices.
Build a small high quality battery powered fixed frequency unit or make an isolating unit to add externally to the mains unit I already have.

The option with the lowest component count and quickest build time is in this case the isolating unit.
This consists of a small audio isolating transformer in a can that is earthed to the signal generator with an isolated output winding that connects to the sensitive inputs of the amplifier.
In my junk box I have the remains of a toy CB base station that I picked up from a pile of fly tipped waste for the small transformer and an old smoothing can as a substitute for a metal film can.
We are nearly there.
Then I will continue testing and finish off the other channel of the amplifier once I have put the top onto the can and fitted the output connector.
There may be some improvements to make to this amplifier too.

[simp2005]

What sort pf improvements were you thinking? I am all for anything you think will improve on the final qoulity of sound that i will eventually hear

[Refugee]

The sound quality will be good and there is little improvement to be made there.
The most important improvement is to deal with the speaker output sockets. These have a safety device to protect the output transformers if one of the cable becomes disconnected by accident. This can happen if there are children playing in the room where the amplifier is being used.
The type of sockets fitted have a very poor cut out and both sockets are faulty and they have a poor reputation anyway.
I do have spares of a more reliable type and could fit them. My own amplifiers are fitted with screw on connectors and the speakers with latching types so they are not likely to have a problem.
The other would be to fit input sockets to the rear of the cabinet to keep wires from cluttering the front up. The existing sockets would stay as they form part of the construction of the preamp section.

[simp2005]

Good thinking Simon as you know ive got a 3 year old running about so maybe screw type are a good idea! Also my speaker cables have spade connectors on them which woukd make connecting up easier

[Refugee]

The type of connectors my amplifiers have were there when I got them and I also had to grab a box of plugs to go with them when I got the amps.
They also fit ex military comms receivers and are very expensive on the web now. I don't have enough chassis sockets.
Latching sockets are still made and are a lot easier to get.
https://www.google.co.uk/?gws_rd=ssl#q=XLR+connector
The chassis fittings are normally sold at the places that sell the plugs.
The existing 2 pin DIN sockets have a built in switch that is very prone to failure and a protection resistor mounted behind them.
The best way I see of fixing the speaker sockets is to go for the 1/4 inch ones used on guitar amps as these have a much stronger switch and the existing protection resistor can be retained. The existing holes in the chassis also look closer to the 1/4 inch chassis fitting.
I have completed and tested my test set but got called away this afternoon before I could complete the second half of the preamp.

[Refugee]

A snapped belt has pushed in for attention today.
The good news is that it was penciled in for tomorrow and it is now out of the way.

[Refugee]

Once that snapped belt was in the bin I did indeed make some progress. My test set has proved its worth and I have identified all the inputs.
The tape input is just a low level line input and is not a tape head input as is printed on the front panel.
The magnetic gram input is indeed for a magnetic cartridge and has the correct tone correction for this. The adapter for my signal generator proved its worth for checking this out.
I have now replaced the diode on the power indicator and it is now working correctly with the EM84 giving full deflection just before the audio output begins to distort. I even managed to find an exact replacement for the diode in my box of vintage diodes.
The speaker connectors do indeed need to be replaced.
In my own stash of components I have some switched sockets that are stereo so they have three contacts that are all switched. This means that they can be wired to use two methods to stop the amplifier from running without a speaker. The switches on these sockets are much stronger than the ones on the two pin DIN sockets. Just ask anyone whom owns a radio with a two pin DIN socket for external speaker and they will tell you how much trouble they are.
Those DIN sockets are such a bad fit that you can see a little bit of the oscilloscope display through the gap by the connector even before we worry about reliability.
These switched sockets will do the job just as well as latching or screw in types with the output transformer protected in two different ways.
The main switch will put the load resistor into the circuit as a dummy speaker and the third connection that would not otherwise be used will be used to cut the input to the amplifier. This means that two things would have to go wrong for there to be a problem.