Think I've blown the preamp

Just swapped out the 6x5 rectifiers in the M3 and it switched on but after about a minute the power went off. One of the mains fuses in the back of the pre had blown so chucked the old 6x5s in with a new fuse and it powers on but I’ve got no heater on the left hand 6x5 and a constant buzz out of the left channel. Swapped the 6x5s over and the left one still has no heater but the right does so they are ok. Both the 5687s have got the heaters on ok.

Any ideas? praying the output transformer hasn’t gone :scream:

Ooooh dear , thats a bummer … lets hope someone has some ideas !

Just tried to play some music and getting nothing out of either channel, wondering now if one of the power supplies has gone?

I think you should turn it off rather than continue shoving 240V through it. Then talk to Graeme!

It is switched off, only switched it on to see if any signal was going through the line stage.

It was going in for a service next month but looks like that’ll be a repair and service now :sob:

Check both fuses at the back.
When I had an M3 It would sometimes power up but no music.
I doubt the rectifiers are an issue they are fairly bullet proof.
The OB2 is a weak link and should be checked.
I also had to replace the ELC82 on one side

It was one of the fuses at the back that blew and I replaced it.

Think one of the power transformer taps has gone.

If the heater is not lighing on on of the rectifiers, then it’s either a bad connection, or a heater winding. Could be a new mains transformer.

Think so, not an expert but it looks to be dual mono psu with two power transformers one has 14v going through a diode bridge and both have 420v going through a pair of chokes plus a 6v tap.

Assume the 6v is for the heaters but I can’t get in to check the output on one transformer as it’s blocked.

Checked the voltages on the valve bases and the knackered one has nothing at all but the other has 6v and 420v.

Think a complete transformer may have failed.

That would be a bummer. If you’ve got neither heater (6V) nor HT (420V) on the rectifier then that sounds like an issue on the transformer’s 240V mains primary side - either the primary winding itself or the circuitry feeding it. Are the power transformers separately fused ? If so it might be possible to measure the primary circuit resistance via a contact in its fuseholder. I’d expect it to be a few tens of ohms.

Feel free to bring it along this weekend and we can have a look at it if you like.


Kitchen table top repair, like last year - you’re supposed to be on holiday Graeme! :smiley:


Would love to but the car is packed to the roof including the passenger seat and footwell :frowning:

The transformers are separately fused so I’ll check the resistances tonight.

Both voltage transformers are measuring 12 Ohms on the input, I’ll have a chat with you at Lopwell to see if I can bring it to you.

What about the current transformers?

Have you tried raisin the voltage?


Finally got it over to Audio Note and there was nothing wrong with it.

Turns out that despite the new 6x5 rectifiers being described as “Brimar NOS 6x5” they were actually CV574 which draws much more current and was blowing the fuses.

As per usual AN were great, so for the grand sum of £50 he chucked some new rectifiers in, replaced the fuse holder (connection was a bit loose) and tidied it up with new screws etc.