Cole lcr phono stage rebuild

What people are working on at the moment
Ant
Old Hand
Posts: 1371
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

Cole lcr phono stage rebuild

Post by Ant » Mon Dec 23, 2019 3:23 pm

I've acquired the Cole phono stage that Nigel wanted me to look at.
A lovely lovely sounding unit, but it does want some work.
As shown earlier, the s&b Mc transformers are completely fried, and when I spoke to them they said to send them down to them to see if they can be rescued. If not, I'll get some sowters or llundahls to replace them, but that's not a priority at the moment.

Speaking to Nigel, it was sent away for repair when it blew up, and I can see a burned out power resistor was replaced, and a pair of coupling caps look to have been replaced too. It should have had obbligatos by the looks of it, these have been replaced with yellow lcr caps. The repair work appears to have simply been to get it up and running again.

It does work, (mm only at the moment due to the buggered transformers) but when I plugged it into the bmu, after about an hour it tripped the breaker on the bmu. Suggests there is a problem with the mains TX on the primary side which doesn't show up when it's not plugged into a balanced mains supply.

The TX has a 300 0 300 100ma winding, a 10v and a 6.3v.
The 10v is for a 6.3v dc heater supply to the 2 ecc88s, the 6.3v is for the ez80 rectifier. The rectifier needs replacing as it is putting out way less than it should do.

The unit doesn't have a mains earth, everything is earthed via the phono sockets so I would rather change this. It's current earthing scheme is also a complete rats nest which irks me to look at..... I'm also going to change the 7 core umbilical cable between the box with the mains TX in it and the unit to an actual 7 core cable rather than the 7 cables inside some heatshrink that it has at the moment.

I'm wondering wether replacing the mains TX is the best way to start as I don't like the fact that there is an issue with it that shows up with balanced mains that doesn't show up without, and chances are the original blow up is related to this. It would mean making a new box for the transformers as the likelihood of getting hold of another one of the original transformers is low as it has a build date of May 2005 inside. So separate transformers would probably be needed.

Luckily the lcr transformers are ok, if they were buggered it wouldn't have been viable to rebuild it.

Suggestions are welcome on the best way to go forward

Cheers ant
Hey, wait, I got a new complaint
Forever in debt to your priceless advice

www.bte-designs.weebly.com

User avatar
Nick
Site Admin
Posts: 13080
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

Re: Cole lcr phono stage rebuild

Post by Nick » Mon Dec 23, 2019 5:29 pm

I would check what current is passing through the rectifier. It my be down on output voltage because of the load on it.
Resistance isn't futile it's V / I.

Ant
Old Hand
Posts: 1371
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

Re: Cole lcr phono stage rebuild

Post by Ant » Tue Oct 20, 2020 3:53 pm

Got the cole phono stage back from nicks workshop today.
I sent it up to nick because i wanted it fixed properly, not bodged and hacked about if id have done it myself.
It had to have a new mains transformer, and an actual earthing scheme. There was no mains earth connection in it, so everything was strapped to the phono socket earth.....
Its rather less dangerous now nick has given it a proper earthing scheme

I got toroidy to wind a new custom mains tx for it and sent that up to nick. Ive used sowter, and Danbury before for custom transformers, the toroidy is easily as well made as either, really nice quality. And a pretty good price too.

So, it now works, sans the dead mc transformers.

I presume that the first time it blew up, the mains tx shorted and put ht on the sockets which fried the mc transformers. There wouldnt have been anything between them and the voltage. Before, I couldnt work out how they had got hot enough to boil the potting compound. Its just luck that it didnt completely fry the lcr modules too. It had blown the coupling caps on the board and a power resistor in the psu, which must have saved the lcr modules

The second time it blew, the rcb on my bmu caught it before it could do any more damage.

Ive got it set up on the end of the linear lenco with the at33sa, using nicks ha1 step up.
And it sounds fantastic. It seems abit crackers that when we listened to it with simon and nigel at mi dads, and were rather astonished by it, that it was on its last legs. it was only used for about an hour after that when it blew up again....

So a very very big thankyou to nick for mending it
Hey, wait, I got a new complaint
Forever in debt to your priceless advice

www.bte-designs.weebly.com

User avatar
Nick
Site Admin
Posts: 13080
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

Re: Cole lcr phono stage rebuild

Post by Nick » Tue Oct 20, 2020 4:09 pm

put ht on the sockets
In fact, I think it put mains on the sockets. The transformer shorted between primary and secondary. The centre tap of the secondary was connected to the case of the phono stage, so the case of the phono stage was connected to the mains. The only route to earth would have been through the phono cables and to whatever it was connected to. The design was a disaster waiting to happen, and it happened.

There was no connection to safely ground in the phono stage itself. The original transformer was a 300-0-300 secondary for HT, I got Ant to get the replacement as a 0-300, so then used the previous 0v line in the connection between the boxes as safety ground. Then added a couple of UF4007 to the rectifier to replace the centre tap and floated the signal ground via a 100R resistor.

Just bad design.
Resistance isn't futile it's V / I.

simon
Eternally single
Posts: 4589
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

Re: Cole lcr phono stage rebuild

Post by simon » Tue Oct 20, 2020 4:50 pm

That's shocking. In both senses. Worth doing though Ant, glad it's up and running - does it sound better than before?

I really must get my finger out and knock one up. Too many jobs in the way sadly.

Ant
Old Hand
Posts: 1371
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

Re: Cole lcr phono stage rebuild

Post by Ant » Tue Oct 20, 2020 5:28 pm

If im honest simon i couldnt say if it sounds better or not, its that long since we sat down infront of it.

I would like to say that it does, because its doing the same thing now sat listening to jeff buckley as it did then with zakhir hussein.

But its in a completely different system. It still has that utterly completely natural presentation, that indefinable something which put it at the top of the pile on the day, and there was some rather stiff competition with the decware and the twin psu phono 2.

I put the presentation down to the lcr modules rather than the gain stages, they look relatively simple, nothing particularly special or out of the ordinary unless im missing something. The components in it are quite nice, its got some obbligatos on the output, it should have obbligato coupling caps but these went bang when it went bang the first time, and the replacements are those yellow lcrs that maplins used to stock. Also has black gates in the psu and kiwame resistors all over it, but nothing that would explain the naturalness it has.

I think those lcr modules are the voodoo. Dunno why, but i think they are doing something that straight rc equalisation doesn't, or they are not doing something that rc does.

It was definitely worth having it fixed properly
Hey, wait, I got a new complaint
Forever in debt to your priceless advice

www.bte-designs.weebly.com

simon
Eternally single
Posts: 4589
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

Re: Cole lcr phono stage rebuild

Post by simon » Tue Oct 20, 2020 6:48 pm

Result ant

izzy wizzy
Old Hand
Posts: 1218
Joined: Fri Nov 02, 2007 7:02 pm
Location: Folkestone
Contact:

Re: Cole lcr phono stage rebuild

Post by izzy wizzy » Wed Oct 21, 2020 10:45 am

In following phono stage design over the years, one constant seems to always come up. When someone has or hears an LCR phono, they all say there's something right about it; a natural presentation that they can't quite figure out but they know it when they hear it. Many DIYers seem to stop building phonos once they have one. I have not heard one.

Ant
Old Hand
Posts: 1371
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

Re: Cole lcr phono stage rebuild

Post by Ant » Thu Oct 22, 2020 12:37 pm

Just taken the brimar ecc88s out of it and put in a set of tesla pcc88s in instead, instantly more my taste. Alot more open than the brimars. Not as sweet, or as... err... valvey, but preferrable for me.
Also got a set of sylvania 7308s to try.
Supposedly, there are national matsushita pcc88s available that are supposed to be very good, might try some of those at some point.

It starts......
Hey, wait, I got a new complaint
Forever in debt to your priceless advice

www.bte-designs.weebly.com

izzy wizzy
Old Hand
Posts: 1218
Joined: Fri Nov 02, 2007 7:02 pm
Location: Folkestone
Contact:

Re: Cole lcr phono stage rebuild

Post by izzy wizzy » Thu Oct 22, 2020 1:44 pm

Ant wrote:
Thu Oct 22, 2020 12:37 pm
Just taken the brimar ecc88s out of it and put in a set of tesla pcc88s in instead, instantly more my taste. Alot more open than the brimars. Not as sweet, or as... err... valvey, but preferrable for me.
Also got a set of sylvania 7308s to try.
Supposedly, there are national matsushita pcc88s available that are supposed to be very good, might try some of those at some point.

It starts......
Don't know how the heater voltage is done but the PCC88 is 7V while the ECC88 is 6.3V

User avatar
Nick
Site Admin
Posts: 13080
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

Re: Cole lcr phono stage rebuild

Post by Nick » Thu Oct 22, 2020 1:48 pm

Don't know how the heater voltage is done but the PCC88 is 7V while the ECC88 is 6.3V
There is a three pin regulator in the stage so it will still be giving 6.3v
Resistance isn't futile it's V / I.

Ant
Old Hand
Posts: 1371
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

Re: Cole lcr phono stage rebuild

Post by Ant » Thu Oct 22, 2020 2:24 pm

Yep 7v heaters, they dont seem to be bothered by the heater voltage being down
I did once run some shuguang globe 300b's in a 2a3 amp at 2.5v, they didnt seem too bothered either. Mind you, the heater structure in those appeared identical to the shuaguang 2a3s i had at the time
Hey, wait, I got a new complaint
Forever in debt to your priceless advice

www.bte-designs.weebly.com

izzy wizzy
Old Hand
Posts: 1218
Joined: Fri Nov 02, 2007 7:02 pm
Location: Folkestone
Contact:

Re: Cole lcr phono stage rebuild

Post by izzy wizzy » Thu Oct 22, 2020 2:38 pm

If running near limits of anode current, then at lower heater voltages, you'll be stripping the cathode as it's not releasing electrons as freely as it should so valve life will be compromised. Not sure what happens if the valve is being run conservatively but I'd still expect lower life in the long run.The PCC88 is spec'd for current not voltage it being a TV valve run in a string so depends on how close you are to 300mA. +/- 5% is usually the spec.

User avatar
Nick
Site Admin
Posts: 13080
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

Re: Cole lcr phono stage rebuild

Post by Nick » Thu Oct 22, 2020 3:42 pm

It will be regulating the voltage and the heaters will be in parallel, so no telling what the individual currents are.
Resistance isn't futile it's V / I.

Ant
Old Hand
Posts: 1371
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

Re: Cole lcr phono stage rebuild

Post by Ant » Thu Oct 22, 2020 4:35 pm

Gone over to the sylvania 7308s to see what they are like, they are the usual 6.3v heaters
Hey, wait, I got a new complaint
Forever in debt to your priceless advice

www.bte-designs.weebly.com

Post Reply