[trem]

I have a Scott LK-48-B that I am trying to get running.   I replaced the can caps and any other e-cap in the amp.   Two of the three cans are not available so I just had to make those from individual caps.   So it powers up OK.   No smoke, no humming, nothing too weird, but my power tubes are red plating after about 20 seconds.   The schematic shows 410V to the OT center and 398V to the screens.
The problem is I have 460V at the screens and the OT center.   The spec power tubes are 7189, which are expensive for me.   I was told the Sovtek EL84M was a direct replacement.   They are a 300V (at the plate) tube, but to my knowledge the 7189 also has a max of 300V on the plates.   Am I all mixed up, or do I need to get the B+ down a lot?
Thank You
Pictures and schematic link below

http://i111.photobucket.com/albums/n143/zzmoore/ScottL-48001.jpg

http://i111.photobucket.com/albums/n143/zzmoore/ScottL-48002.jpg

http://hhscott.com/pdf/fs/LK48B_C1.JPG

[Niteshade]

Did you replace the coupling caps? Did you check the bias supply? (I didn't look at a schematic to see whether it has one...)

When you replaced the caps, were there voltage reducing resistors that you may not have put back in place?

[trem]

Did you replace the coupling caps? Did you check the bias supply? (I didn't look at a schematic to see whether it has one...)

When you replaced the caps, were there voltage reducing resistors that you may not have put back in place?

I did not think to replace any coupling caps.   Thought I would try to get it firing on all cylinders before I worried about that stuff.
I have -55VDC at the bias, supply, which is spec.
And yes, all the dropping resistors are in place.

It just seems to me that the specs on the schem exceed the limits of the power tubes.   But I could easily be wrong.

[JoshK]

400+v seems like a lot for the EL84.  I haven't done a load line at that voltage but I don't recall seeing any EL84 schematic that runs that high.  Can you determine current in the output stage, as in cathode bias resistor? 

Sure enough I look at the schematic and that is what it shows, but seems high to me.  Anyway, measure the voltage across R46 (the 10ohm resistor).  This will allow you to determine current, and then you can calculate dissipation to see if you are exceeding dissipation, which red plates suggests you are.

[JoshK]

Actually both 7189 and EL84 data suggests higher voltage is acceptable.  I am guessing you are just over dissipating the tubes.

http://www.nj7p.org/Tube4.php?tube=7189

http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EL84.pdf

[JoshK]

Hmmm....this may be a case where new stock is not up to the specs of some old stock tubes.

http://www.mif.pg.gda.pl/homepages/frank/sheets/163/e/EL84.pdf

This suggests JJ has a 300V limit, not what is shown in the older data sheets.  Probably many other new stocks tubes are similarly limited.  However, the dissipation is usually the harder limit.

[trem]

Hmmm....this may be a case where new stock is not up to the specs of some old stock tubes.

http://www.mif.pg.gda.pl/homepages/frank/sheets/163/e/EL84.pdf

This suggests JJ has a 300V limit, not what is shown in the older data sheets.  Probably many other new stocks tubes are similarly limited.  However, the dissipation is usually the harder limit.

So are you saying that possibly my modern day tubes can only take 300V on the plate, but NOS tubes could handle more like 400V ?
If that is the case, I think I have a problem.   If I try to bias the plates from 460V down to 300V, my current will go through the roof won't it?   The tubes would probably red plate faster at the 300V mark.
Am I going to have to save for real NOS 7189 tubes??
Thanks

[JoshK]

Drop the plate voltage won't make current go up.  Actually all else being equal it would make it go down (V = I*R).  If volts go down, for a constant R, current goes down too.  But things aren't usually that simple and rp changes a bit too, but most likely current still goes down.

[mikef]

Your amp is not spec'd for a standard EL84, which can handle a B of 300V  but need an EL84M or 7189, which can take the higher voltage (up to 440V). I have a 222C which uses the same tubes.

You can also find 6P14P Russian tubes on ebay which are cheap, and a direct substitute. Do a search on the Tubes Asylum of Audio Asylum for a bunch of recommendations and information:

http://www.audioasylum.com/audio/tubes/bbs.html

Mike Fox

[Niteshade]

I'd replace the coupling caps. Try reducing the voltage on Pin #9 of the El84's.

[trem]

Your amp is not spec'd for a standard EL84, which can handle a B of 300V  but need an EL84M

Mike Fox

I am using the Sovtek EL84M

Nightshade -
When you say "replace the coupling caps", which are you talking about ?   The two between the PI and the Power Tubes ?
If I drop the voltage to the screens with a higher value resistor, do I need the tubes in to check the voltage?
Thank You

[mikef]

I am using the Sovtek EL84M

The Sovtek EL84Ms are supposed to be able to take the higher voltage, though 460V does sound a little excessive. I haven't had any problems with them in my amp, though I haven't measured the B+ voltage.

Mike Fox

[ltr317]

460v is excessive.  I would measure all the caps and resistors, especially the resistors.  I think at least one of them failed. 

[Niteshade]

There is one coupling cap per power tube. The go between the power tubes and phase splitters. They're typically a small value- .01uf to .33uf at around 400V give or take a few volts. If one of these fails, the corresponding power tube will over heat. If the screen voltage is coming from a cap with a voltage divider on it, you don't need the tube in place for measurements. Reduce the voltage by 10-15 volts at the cap and see what happens.

[jrebman]

Does your amp have a selenium rectifier in the PS?  Did you replace that?  try to get the B+ down to about 420 or so and use the el84m or 6p14-ev tubes and you should be good.

You may also want to think about installing individual bias trim pots for each output tube.

-- Jim

[trem]

Does your amp have a selenium rectifier in the PS?  Did you replace that?  try to get the B+ down to about 420 or so and use the el84m or 6p14-ev tubes and you should be good.

You may also want to think about installing individual bias trim pots for each output tube.

-- Jim

I am far from being the worlds best amp tech.   But unless it is some where out of the ordinary, I would say no.   There is the tube rectifier for the B+.   Then there is a SS Bridge for the bias and heater supply.   But if you see one on the schematic, I would be in your debt if you pointed it out to me.
Thanks for the tip.

[jrebman]

Re: Help with Scott integrated ampWell, as I'm blind the schematic is not available to me but it's the bridge diode that is most likely the selenium one, but I honestly don't know if and when Scott stopped using them, and whether they shipped them with the kit amps or not.

I'm just recalling some of the plans I had for my 222c when I had one a couple years ago.

Do a google search on rebuilding the 222c and you'll find a page by a guy who did a very extensive rebuild and documented it all.  I do believe he also added bias trim pots for each output tube.  As I said though, it's been a while.

-- Jim

P.S. - when you get it all dialed in it's a nice amp.

[trem]

Re: Help with Scott integrated ampWell, as I'm blind the schematic is not available to me but it's the bridge diode that is most likely the selenium one, but I honestly don't know if and when Scott stopped using them, and whether they shipped them with the kit amps or not.

I'm just recalling some of the plans I had for my 222c when I had one a couple years ago.

Do a google search on rebuilding the 222c and you'll find a page by a guy who did a very extensive rebuild and documented it all.  I do believe he also added bias trim pots for each output tube.  As I said though, it's been a while.

-- Jim

P.S. - when you get it all dialed in it's a nice amp.

Are you sure I'm not blind ???
Not to mention stupid.
I have been staring at that selenium for a month (kind of).   All my experience has been with guitar amps.   In a guitar amp, rectifiers are either vacuum tube, or silicon (solid state).    So for some reason I have been thinking that nice, little, metal box on the side of the chassis had four silicon diodes in it (which you never see anywhere).   Even after your first post, it never occurred to me what was really in that little, metal, box of a rectifier.   Again, I just assumed silicon because that is what I am use to working with.   
I also have a Dynaco stereo kit that I am working on.   I was warned about the potential hazard of selenium when it was given to me.   I still need to replace that by the way.   OK, so I need to ditch this one in the Scott and rig up a silicon bridge somewhere.   I assume the IN4007 will work in this situation.
   jrebman - this has been a humbling experience.   Thanks for your help.   I owe you a favor.
Chip

[trem]

OK back to this nightmare.   Got rid of the selenium, and built a silicon bridge.   My main problem continues to be B+ that is about 60v too high.   I have tried different values of 10 watt resistors in between the tube rectifier and the filter caps.   I cannot seem to get any voltage drop at all.   Am I missing/doing something wrong?   Shouldn't a 1k 10 watt in series with the original 20 ohm 10 watt drop at least a little voltage off the rectifier?
Thank You

[jrebman]

Just curious, are you measuring with tubes in or out?  You generally won't see proper voltages unless the amp is fully loaded with tubes and ready to play.

Other than that, I have no real suggestions except for the fact that the power transformer was designed for 110 vac input, and your line voltage is probably 120 or higher -- sometimes as high as 130 or more in some places in the country.

HTH,

Jim