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Yellow Paper Mod - VTA M125s - did it help my situation?

KT88Lvr

KT88Lvr

Active Member
Hey Folks,

Two Tubes4HiFi - M125 Monoblocks.

Last year I performed the Yellow Paper mod- to protect my Rectifiers (Mullard GZ37s) as mentioned over at the VTA site) which includes diodes.

It is my understanding that the diodes act like a fuse.

On Saturday my wife was listening to the system and I was in the other room - I heard - "hey come quick - something smells really bad" - I ran in and found one of the amps - the rectifier tube completely red and likely the output tubes as well - didn't get a good look as I was panicked. I immediately shut off the amp.

Later in the day I pulled the Rectifier out - and move it over to the good amp and powered it up - it powered up just fine and the bias remained steady - so hopefully the rectifier has survived.

Thinking one of the output tubes went bad (fairly new Gold Lions) - I then placed the good rectifier from the good amp into the "bad" amp - and powered it back on.

It came on - and the bias seemed to increase over time - then it reached a point and stopped increasing. I then set the bias as I normally would and kept an eye on it. The amp remained just fine for a couple of hours.

I'm scratching my head as to what went wrong - but in the meantime - I've ordered new tube sockets and some resisters etc so that I can do a clean partial rebuild of it. I'm wondering did the mod help? Shouldn't the rectifier have not red plated like it did?

Thanks for any help or ideas - weird that it suffered so bad - but great that it came back to life. Troubling though for sure.
 
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the diodes are there to help prevent failures caused by excessive reverse voltage. They do nothing to protect a rectifier that is flowing too much current, which is whats happening if it glows red.

Last time I had a rectifier do that it was working into a shorted capacitor. If it happens for any length of time it can seriously weaken the tube. I'd be concerned about what caused it to happen in the first place though. Don't be too surprised if that same amp acts up again. I have had bum output tubes flow tons of current, and I've also had issues where a tube lost bias voltage for whatever reason and went to full flow mode. usually that one is a bad contact in the tube socket or a bad solder joint on the socket.
 
Also, its recommended to use a SS rectifier in these amplifiers. I'm using coper tops in mine with the diode mod and have had zero problems. The one I'm using has a thermistor and a couple resistors inside. Isn't using a GZ-37 rectifier pushing it a bit with a quad of KT-88's? You can add the oprional delay board when using the SS rectifier also. It should do well in a ST-70 though. I've thought about trying one in my ST-70.
 
gadget73 said:
the diodes are there to help prevent failures caused by excessive reverse voltage. They do nothing to protect a rectifier that is flowing too much current, which is whats happening if it glows red.

Last time I had a rectifier do that it was working into a shorted capacitor. If it happens for any length of time it can seriously weaken the tube. I'd be concerned about what caused it to happen in the first place though. Don't be too surprised if that same amp acts up again. I have had bum output tubes flow tons of current, and I've also had issues where a tube lost bias voltage for whatever reason and went to full flow mode. usually that one is a bad contact in the tube socket or a bad solder joint on the socket.
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Thanks very much that now makes sense.

I did have a weird I think "motorboating' type sound a long ways back when I first did the yellow paper mod on this amp. It was this amp that did this - the other amp came up fine after the mod. So I took this amp out of the system and hooked up a cheap speaker to it - and a source - I then took off the bottom of the amp and powered it on - no motor boating - just came on. So I took chop sticks and pushed at all the various wires etc to try to see of something was shorting. Amp worked just fine - nothing happened. I then moved the amp back into the main system and it has been fine ever since - until last Saturday.

The tube sockets look pretty bad - some melting etc (too hot soldering) - so I plan on replacing all of them and cleaning up the wiring as best I can. I've even ordered some spaghetti clear tubing to cover some of the exposed leads (like you would see in a P2P type amp.)

Thanks again.
 
Lavane said:
Also, its recommended to use a SS rectifier in these amplifiers. I'm using coper tops in mine with the diode mod and have had zero problems. The one I'm using has a thermistor and a couple resistors inside. Isn't using a GZ-37 rectifier pushing it a bit with a quad of KT-88's? You can add the oprional delay board when using the SS rectifier also. It should do well in a ST-70 though. I've thought about trying one in my ST-70.
Click to expand...

Thanks. Yes I've used the Weber SS rect when I first built the amps - then one completely fried.

My experience is to keep the bias down to about 1.0 (vs the old setting of 1.2 which did have me going through rectifiers) with 4 tubes and .5 with two tubes.

I've had good luck with rectifiers keeping the bias at the above numbers. And I do have some new Webers packed away but fortunately haven't needed to use them.

They have been fairly trouble free for the last 5 years.
 
KT88Lvr said:
Thanks very much that now makes sense.

I did have a weird I think "motorboating' type sound a long ways back when I first did the yellow paper mod on this amp. It was this amp that did this - the other amp came up fine after the mod. So I took this amp out of the system and hooked up a cheap speaker to it - and a source - I then took off the bottom of the amp and powered it on - no motor boating - just came on. So I took chop sticks and pushed at all the various wires etc to try to see of something was shorting. Amp worked just fine - nothing happened. I then moved the amp back into the main system and it has been fine ever since - until last Saturday.

The tube sockets look pretty bad - some melting etc (too hot soldering) - so I plan on replacing all of them and cleaning up the wiring as best I can. I've even ordered some spaghetti clear tubing to cover some of the exposed leads (like you would see in a P2P type amp.)

Thanks again.
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The sockets sent with these aren't the best. It doesn't take much to ruin one. I did have one connector that got too hot and pushed up through the socket. I really wasn't on it that long with the iron, but long enough I guess. Luckily I had another socket I pulled from the front of my ST-70 when I replaced it for test jacks. A problem I read about the copper top suggested for these amps is that the internal soldering isn't the same as it used to be and the thermistor used is too small. The resistors should be against the inside wall of the cover also.
One fix is to remove the cover and drill some holes around the bottom and top to help cool them. I haven't done it yet, but I do plan to.
 
motorboating is often bad decoupling between stages. Most of the time that translates to a bad filter cap or bad grounds to the filter cap. The fact that moving wires made it vanish makes me extra suspicious of bad connections. May honestly be worth just doing a detailed inspection of every connection in here and re-doing anything thats shaky.
 
There is one lug screwed to the chassis with a lot of ground wires attached. Honestly, that solder lug could have been bigger to fit all those ground wires to it. That point needs some heat to get the solder to flow through all those wire ends at that lug.
 
You can use the modified "yellow sheet mod" to double the current capacity of the rectifier by using two rectifier diodes in series to increase piv, with the positive ends at tab 4 and 6 jumpered together to parallel the tube sections.upload_2021-3-19_11-56-49.png
 
Lavane said:
The sockets sent with these aren't the best. It doesn't take much to ruin one. I did have one connector that got too hot and pushed up through the socket. I really wasn't on it that long with the iron, but long enough I guess. Luckily I had another socket I pulled from the front of my ST-70 when I replaced it for test jacks. A problem I read about the copper top suggested for these amps is that the internal soldering isn't the same as it used to be and the thermistor used is too small. The resistors should be against the inside wall of the cover also.
One fix is to remove the cover and drill some holes around the bottom and top to help cool them. I haven't done it yet, but I do plan to.
Click to expand...
Thanks - yeah - I was thinking of upgrading the sockets- but played it safe and ordered all new ones of the same brand. I'll definitely be more careful with the iron. I'm going to replace all 5 sockets on both amps - I've ordered the resistors etc so I can start fresh.
 
gadget73 said:
motorboating is often bad decoupling between stages. Most of the time that translates to a bad filter cap or bad grounds to the filter cap. The fact that moving wires made it vanish makes me extra suspicious of bad connections. May honestly be worth just doing a detailed inspection of every connection in here and re-doing anything thats shaky.
Click to expand...
Thanks - that is exactly my plan - I'm going to take out all 5 tube sockets and the wiring and see what I have - and start re-doing anything questionable.
 
Lavane said:
There is one lug screwed to the chassis with a lot of ground wires attached. Honestly, that solder lug could have been bigger to fit all those ground wires to it. That point needs some heat to get the solder to flow through all those wire ends at that lug.
Click to expand...

Yep - I remember that - I wish it was done better - I'll likely redo it.

Last year I ordered a bunch of desoldering stuff - a fancy Japanese solder sucker along with a cheaper one and a bunch of desoldering wick. My magnifying head set and my Hakko station - I should be in good shape. :)
 
Pio1980 said:
Did you jumper 4 to 6?
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No - I didn't - I did just as the picture above has. I read the section at the VTA site to get a good understanding - the only change suggested was by one of my favorite Tube folks - Jim McShane - but I kept to the standard plan.
 
KT88Lvr said:
No - I didn't - I did just as the picture above has. I read the section at the VTA site to get a good understanding - the only change suggested was by one of my favorite Tube folks - Jim McShane - but I kept to the standard plan.
Click to expand...
Use two diodes in series on each leg, and jumper 4 to 6 to get the best protection for the tube and best performance from the amp.
 
KT88Lvr said:
Thanks - now it is very clear - and I have a bunch of those diodes. Is there a link or something - with some history on this version? Looks wild. I can solder and follow directions - that is about it though.
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There is, can't remember offhand. Let me look.
 
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