Kerr McCosh Monoblock Problem

[willbewill]

Thanks Scott - Will replace them tomorrow - but had a bit more of a prod and one of them came loose, dry joint? Might that have caused the failure?

[nigel]

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.

He is 

I see from the quote that you posted (Malcolm) that I sent some spare resistors with the amplifiers. I would agree with Scott's suggested course of action. Let me know what happens 

I'm a bit sceptical that a dry joint there would cause the failure, but you never know. Beware also that output valves which flashed over or redplated once will not necessarily do so again ...... straight away! (i.e. you may change the resistors & it will work OK again for a while ...)

[willbewill]

Thanks Nigel


It lives and breathes again - changed the resistors and put in some NOS Xaerix EL84s, not sure I'm happy using those used Russian tubes anymore (pity coz they sound good) - and is sounding good


Once again a HUGE thanks for rebuilding these so well and the most excellent improvement you made by putting those resistors in, they may have saved the amp

[rfgumby]

Yeah baby!   Let's keep our fingers crossed.

[nigel]

It lives and breathes again - changed the resistors and put in some NOS Xaerix EL84s, not sure I'm happy using those used Russian tubes anymore (pity coz they sound good) - and is sounding good

Phew! 

Hopefully, the fusible resistors have just done the job that we intended them to do! Actually, I think they were Scott's suggestion, IIRC, so Scott deserves a 

[willbewill]

Yeah baby!   Let's keep our fingers crossed.

Thanks Scott - and a special thanks for originally suggesting them

[willbewill]

Found some more info on Kerr McCosh

http://www.audio-kontakt.com/forum/viewtopic.php?p=73184

[rfgumby]

Oh that's helpful.

You had me worried they had a failure again!   

[willbewill]

Time to resurrect this thread with a question:

I now having to listen through headphones most of the time. I have a couple of nice rigs:

Bottlehead Crack and Original Little Dot which power 400 and 600 ohm headphones - AKG K240 monitors, K240 Sextetts & K340 Electrostatic/Dynamic

So if I got a couple of 600 to 8 ohm transformers I should be able to power the phones from the Kerrs?

Looking at http://www.hammondmfg.com/collins.htm

[nigel]

EDIT:

On your other thread, Niclas posted -

You could step down the voltage with a transformer as well.  But, impedance specifications on transformers can be a bit confusing.  A transformer does not really have a primary and a secondary impedance.  It only has a turns ratio.  It steps up/down the voltage and decreases/increases the current capability with that turns ratio.  If a transformer has a specification 8 ohms to 300 ohms, it only says that if you load the secondary with 300 ohm, the primary will be seen as an 8 ohm load to the driving circuit.  The transformer has fewer turns on the 8 ohms side and more turns on the 300 ohms side.  If you connect the 8 ohm side to the speaker output on the amplifier and the 300 ohms side to the headphones, you will step up the voltage, which is the opposite to what you want.  To step down the signal voltage, you need to connect the transformer the other way around.

He's right, of course. That won't work out too well at all. Use a resistive attenuator as detailed below or maybe don't do it ..

[niclaspa]

You could try making a headphone adaptor with a couple of resistors: http://sound.westhost.com/project100.htm  It will let you hear how suitable your amplifier is as headphone amplifier.  I use it on my amplifier and it sounds fine. 

[nigel]

You could try making a headphone adaptor with a couple of resistors: http://sound.westhost.com/project100.htm  It will let you hear how suitable your amplifier is as headphone amplifier.  I use it on my amplifier and it sounds fine. 

That's a much better idea - BUT - will leave the Kerrs with very little load. Maybe add a high wattage resistor of around 8 ohms (or 10 - 12 ohms, whatever you can get) directly across the output, before the attenuator described in that link. Some valve amplifiers are unstable with no load and catastrophic failure can result. I'm possibly being over-cautious here, but I wouldn't want to see them damaged (and it's the irreplaceable output transformers which could be at risk).

[willbewill]

Just to let folks know the Kerrs have now left the willbewill and their new abode is with ZZMoko

http://www.lencoheaven.net/forum/index.php?topic=22717.msg309319#msg309319

[ropie]

From full-time rock use to full-time jazz

[needlekiller]

pfew, if our zz"top"moko has checked the circuit, we got a new bunch of questions! but no matter we solve it!