# A little lite entertainment for your Wednesday morning.



## nonreverb (Sep 19, 2006)

Got an amp in for service yesterday with this in it.
Discuss at your leisure....


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## Chito (Feb 17, 2006)

How the heck did that happen?


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## nonreverb (Sep 19, 2006)

Chito said:


> How the heck did that happen?


It's a mystery right now Chito as I haven't ventured into the amp yet. I'll be sure to post what I find.


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## Always12AM (Sep 2, 2018)

Someone tried to smoke it


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## Alan Small (Dec 30, 2019)

wait, is it wednesday already???


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## Thunderboy1975 (Sep 12, 2013)

Its called Tube Screaming 
Guys used to do that back in the day to get a tremolo/Leslie speaker effect.

Am i the only one who knew this?


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## nonreverb (Sep 19, 2006)

Thunderboy1975 said:


> Tube screamer? 😱


It does have an Edvard Munch vibe to it.


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## BlueRocker (Jan 5, 2020)

Where do I get one - looks toneful!


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## Alan Small (Dec 30, 2019)

Thunderboy1975 said:


> Its called Tube Screaming
> Guys used to do that back in the day to get a tremolo/Leslie speaker effect.
> 
> Am i the only one who knew this?


please tell more...never heard of this


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## Thunderboy1975 (Sep 12, 2013)

Alan Small said:


> please tell more...never heard of this


Well what you gotta do is..just watch Spinal Tap its explained in the closing credits.


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## guitarman2 (Aug 25, 2006)

Thunderboy1975 said:


> Well what you gotta do is..just watch Spinal Tap its explained in the closing credits.


That tube went to 12.


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## oldjoat (Apr 4, 2019)

tube overheated in one spot and partially softened the glass... vacc pulled that side of the tube inward till internals were compromised.


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## gtrguy (Jul 6, 2006)

oldjoat said:


> tube overheated in one spot and partially softened the glass... vacc pulled that side of the tube inward till internals were compromised.


Yup, I've got a EH6V6 that suffered a similar fate.


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## StevieMac (Mar 4, 2006)

If you're prone to believe common notions, the amp sounded _really_ good just before it needed servicing.


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## tdotrob (Feb 24, 2019)

I bought a JCM2000 DSL50 that had bias drift issues(knowingly non working when I purchased) and that’s what two of the power tubes looked like.


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## guitarman2 (Aug 25, 2006)

gtrguy said:


> Yup, I've got a EH6V6 that suffered a similar fate.


I wonder if its a problem with these tubes. The only amp I ever owned with a power tube malfunction was a Tone King Metropolitan. They were EH6V6. Red plated blew and took out a resistor.


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## Milkman (Feb 2, 2006)

nonreverb said:


> Got an amp in for service yesterday with this in it.
> Discuss at your leisure....


I can't imagine I'm the first to say this but my first reaction was simply:

Holy Fuck

Then I thought I should temper that down to a Holy Shit.

But no, I think Holy Fuck is right.


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## nonreverb (Sep 19, 2006)

tdotrob said:


> I bought a JCM2000 DSL50 that had bias drift issues(knowingly non working when I purchased) and that’s what two of the power tubes looked like.


That version was nasty. Marshall quickly revised the circuit due to the withering blowback from customers and dealers.
I had one of those TSL122's from that era come into my shop with the B+ traces literally vaporized off the board.


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## gtrguy (Jul 6, 2006)

guitarman2 said:


> I wonder if its a problem with these tubes. The only amp I ever owned with a power tube malfunction was a Tone King Metropolitan. They were EH6V6. Red plated blew and took out a resistor.


Yes, I didn't have very good luck with the EH6V6 tubes. the JJ is much more reliable and rugged (can withstand much higher plate voltages) but I don't care for the tone- too much like a 6L6 so I went NOS and am happy.


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## tdotrob (Feb 24, 2019)

nonreverb said:


> That version was nasty. Marshall quickly revised the circuit due to the withering blowback from customers and dealers.
> I had one of those TSL122's from that era come into my shop with the B+ traces literally vaporized off the board.


Yep our local tech here had to resolder some burnt traces but he didn’t buy the replacement board. I’m not super amp technical so bear with me but from what I understood he just made his own little board for the bias section and lifted it off the main board and added some heat sinks and replaced the problem parts with good quality parts and it didn’t have one issue since.


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## nonreverb (Sep 19, 2006)

tdotrob said:


> Yep our local tech here had to resolder some burnt traces but he didn’t buy the replacement board. I’m not super amp technical so bear with me but from what I understood he just made his own little board for the bias section and lifted it off the main board and added some heat sinks and replaced the problem parts with good quality parts and it didn’t have one issue since.


There were two separate problems with the first gen boards. One was the type of resistors they had in the bias circuit. They drifted down in resistance as they warmed up. Of course the ones used for the voltage divider connected to ground would slowly drag the bias voltage down to the point where all the power tubes would start over conducting....basically a catastrophy in the making. I remember placing my volt meter on the bias circuit on one and watched the bias slowly disappear.

The other issue which was a real nightmare, was the boards themselves. There was a bad run of the main boards on the TSL122 where the user would hear constant crackling and frying egg sounds from the amp. I remember the first one of these I got, I spent a colossal amount of time trying to figure out what was wrong only to discover that it was the board all along.


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## tdotrob (Feb 24, 2019)

nonreverb said:


> There were two separate problems with the first gen boards. One was the type of resistors they had in the bias circuit. They drifted down in resistance as they warmed up. Of course the ones used for the voltage divider connected to ground would slowly drag the bias voltage down to the point where all the power tubes would start over conducting....basically a catastrophy in the making. I remember placing my volt meter on the bias circuit on one and watched the bias slowly disappear.
> 
> The other issue which was a real nightmare, was the boards themselves. There was a bad run of the main boards on the TSL122 where the user would hear constant crackling and frying egg sounds from the amp. I remember the first one of these I got, I spent a colossal amount of time trying to figure out what was wrong only to discover that it was the board all along.


Ahh thanks I appreciate that. That makes sense with what my tech told me then. He said the replacement board was a couple hundred from Marshall and he charged me like $30 to build it himself.


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## nonreverb (Sep 19, 2006)

tdotrob said:


> Ahh thanks I appreciate that. That makes sense with what my tech told me then. He said the replacement board was a couple hundred from Marshall and he charged me like $30 to build it himself.


Sounds like he was addressing the bias problem. The board problem cannot be repaired. It was a defect in the manufacturing process and the fault was located between the layers in the board itself. The only remedy for it was a new power board.


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## tdotrob (Feb 24, 2019)

nonreverb said:


> Sounds like he was addressing the bias problem. The board problem cannot be repaired. It was a defect in the manufacturing process and the fault was located between the layers in the board itself. The only remedy for it was a new power board.


Oh I see ok thanks I thought they were once in the same issue, I appreciate the clarification


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## Paul Running (Apr 12, 2020)

When operating an amp with valuable output tubes, in addition to the fuse protection, you could add a bias-protection circuit. Here's an example of one that was used in an early model amp. The red-dot connects to the grid-leak resistors of the output tubes and the green-dot to the cathodes. If the bias supply drifts too positive or completely fails, the relay powers-off and the cathode-bias is connected, preventing the tubes from running away. As noted, the bleeder resistor (100KΩ shown), should be selected low enough to just energize the relay.


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## jb welder (Sep 14, 2010)

I used to sometimes see glass meltdown like that in some of the old Ei Yugo EL34's.
I think some manufacturers may have used lower melt point glass so they were more prone to this.
Regardless, it only happens when there is a major problem, so the tube is toast anyway.



tdotrob said:


> from what I understood he just made his own little board for the bias section and lifted it off the main board and added some heat sinks and replaced the problem parts with good quality parts and it didn’t have one issue since.


Lifting the bias circuit out of the mainboard on the bad DSL/TSL units can sometimes be a long-term fix. Like @nonreverb said, there may be other issues that can show up later, or sometimes the bias isolation is all that is required.
Sounds like your tech just did his own version of this bias board: JCM2000 Stable Bias Mod


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## tdotrob (Feb 24, 2019)

jb welder said:


> I used to sometimes see glass meltdown like that in some of the old Ei Yugo EL34's.
> I think some manufacturers may have used lower melt point glass so they were more prone to this.
> Regardless, it only happens when there is a major problem, so the tube is toast anyway.
> 
> ...


Ahh ok, thanks for the link. It’s run 2 solid years since with lots of hours on it.


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## nonreverb (Sep 19, 2006)

For those who are interested as to why this happened, a cautionary tale.
Being a Marshall JCM900 100watt, it has two HT fuses. Tubes 1and 4, 2 and 3. They are accessible from the outside at the back of the amp.
They are clearly marked 500mA each. What I found was the one which governed the spot where our melted glass tube was located, had a 3.15 amp fuse in it. The other was a 1 amp. Further, it just so happens that the socket where this tube was mounted had the 2.2K screen resistor replaced....with 2 x 4700 5 watt paralleled resistors which to those of us who know the calculations, equals 10 watts dissipation across this resistor pair. Far more than should ever be there. So basically when it failed, it didn't draw enough current to blow the fuse or the screen resistor. It just melted away.....Unbelievably, the output transformer seems to be ok....guy won the lottery on that count at least.
The takeaway? Always use the proper rated fuse....always!


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## greco (Jul 15, 2007)

nonreverb said:


> ....had the 2.2K screen resistor replaced.


What is the typical power rating for a 2.2 K screen resistor in this application?


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## nonreverb (Sep 19, 2006)

greco said:


> What is the typical power rating for a 2.2 K screen resistor in this application?


5 watts is the OEM resistor value greco.


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## greco (Jul 15, 2007)

Thanks


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