Kerr McCosh Monoblock Problem

[stevieg]

Malcolm yeah Brian is the chap that bought it ( Andra in Rab C ) and i have told him to name his price no matter how much he wants for it 

Respeck bruv. (Lenco Heaven's international community scratches its respective head.)

Cheers Steve

[Rogers Wah Wah Rabbits]

but Paul had problems with its function.  I know the phases are correct, but there are also phase switches for the channels as well, so you can come up with all sorts of funky arrangements, like guitar pickup phase switching.  Any Guru's out there know why that interaction occurs?  I've never had one fight quite like that before.

man and they say elephants never forget  in my defence i was probably confused 

[willbewill]

A little update. I have just received a couple of Mullard EZ81 valves from Brian's hoard via Mark:

Initial impressions are very good. I had only heard the Kerrs with Mullards prior to the rebuilds, after which I only had Tesla EZ81 (NOS) available. The sound was really good using GE EL84s and recently acquired Valvo ECC81s. But now swapping in the Mullard EZ81s they have become even more transparent, another veil lifted, I like them

[rfgumby]

Excellent, that mirrors my experience as well, both with EZ-81's and GZ-34's.

[willbewill]

Thanks Scott - I'm also waiting for a couple of Dario Miniwatt ECC81 to arrive, apparently made by Mullard and some folks reckon to be the best sounding ECC81. They have short grey half box plates and a large halo type getter.

[rfgumby]

I don't believe I've heard those, so I'll have to lean on your experience and perception about the "sound".

[willbewill]

I don't believe I've heard those, so I'll have to lean on your experience and perception about the "sound".

They arrived today and...


they are amazing The level of detail, clarity, separation and stunning bass - willbewill really likes

Oh and the midrange 

[rfgumby]

Again, c'mon Malcolm, you know rectifier tubes don't make any difference.     Mullards have been the Gold Standard for 81's and GZ34's over the years...

The funny part is, Solid State rectifiers make a notable difference too.  Hence, one of our side discussions a while back.  Not quite as obvious, but equally important in my opinion.

[willbewill]

Yeah, I need to put in an order for some to do SA rectifiers for the Genera psu.

[willbewill]

Aaaarrrgggghhhhhh!!!!!!

willbewill ain't a happy bunny tonight...

switched the Kerrs on to listen to some new music and...only one monoblock working

the other powers up, all valves light but no sound out of speaker, did all the normal checks - swapped cables, valves etc to no avail. Only sound I get is a slight buzz through the speaker as the phono lead is plugged in.

Any ideas folks?

So I'm now listening through one of my backup amps - a Pioneer SX-434 receiver like this one from 19734



doesn't sound too bad now it's warmed up a bit - but I miss my Kerrs...

[GP49]

If it were mine, I'd have the bottom off and be poking around with a voltmeter.  It could be something
as simple as an open dropping resistor in the power supply, resulting in a dead stage.

[willbewill]

Hi Gene

It hasn't got a bottom and valve amps are a mystery to me so don't know where to poke and those big caps scare me...


not a good week so far - reversed my car into a huge concrete column in Ikea and today had to call AA (UK car rescue service) because my car wouldn't start (needed a new battery - £102.50!!!)

[ian]

Bad luck Malcolm.  The only bright side is that these things tend to come in threes and you've had three.  I'll get you a nice cream cake to cheer you up next Saturday.

Cheers, Ian

[willbewill]

Right - I've done some prodding with a meter and the fusible resistors have both blown - should read 47r but I get nothing.

OK, protection resistors are installed

I went with 47R, 1/4W, fusible. No photo at the moment, someone else has my camera!

To test this out, the first thing was to check that they didn't fail at full power. I didn't fancy cranking the amps up quite that loud, so I used a dummy load and fed sine waves in. No problem here - I drove the amp to the onset of clipping & swept from about 50Hz to >20KHz. Tried it with square waves and with the amp driven into hard clipping as well. No problems.

As an aside - just before the onset of clipping, the amp develops around 30V pk-pk into an 8ohm dummy load. By my reckoning, that's about 14 watts RMS, which sounds about right.

Now, to try out the protection feature, I re-installed the dodgy JJ EL84 which was redplating. The hope here would be that the fusible resistor would open. However, the dodgy JJ now refuses to do its party trick & seems to work just fine This is rather irritating! Previously, it would start to lose the plot within a few minutes, at some random point, but now it refuses to do so.

I'm pretty sure it should work as expected, but I don't have anything to prove it with now. I'm not brave enough to try out that old Mullard with its own internal lightshow again - that was just too scary

Sonically, I don't notice any difference, so I think I'm going to pack these back off to Wales next week. I have a packet of spare resistors to go with them and I've positioned them such that they're easy to access.

So why did they blow - I assume a dodgy EL84, was using old Russian tubes.

Do I just replace the resistors and try with some known good EL84s??

[rfgumby]

I'd replace the resistors (get a handful of extras), and see if they blow again.  Often if one goes, the other may too due to an inrush from the other being open.

If they blow again, then swap out the output tubes and try again.  If it still happens, then we have to find another problem, or bother Nigel if he's still close enough to help.