KT120's and Fender (Sunn)300T- blows tube protection fuses

[pbman1953]

I installed 6- KT-120's in my Fender Pro300. The amp's stock tubes call for 6-6550's. After talking to some , I was told that the amp could handle the KT's. After playing it for about 5 minutes, with a moderate setting, one of the protection fuses blew. I wanted to see if the set the blew the fuse were ok, so I refused that set and removed a fuse from a previously used set. The set that made the fuse blew were fine. The amp continued on the balance of the night with 2 sets (4-tubes).


My question is if anyone has tried the KT120's in their 300T or Pro300?


I sent along the bias page from the manual to show how I used the builtin controls to bias.

[Ed_Chambley]

Fuses are designed for over-current protection.  If you had the proper fuse installed and it burned and installed another with 4 tubes and it ran fine, I would guess you had an over-current issue.


If you know the amperage the Power Transformer, HT, and Heaters, did you check to see if you exceeded the heaters with the 120's.  These take more heater current than 6550 and you added it times 6.  Also, did you get a large voltage drop on your plates of the 120's?


You can check for current easily in the amp.  Please post the specifications of your Power Transformer.

[pbman1953]

Thanks Ed, I'll see if Fender will divulge the specs. They can be kind of tight lipped at times




Thanks

[Ed_Chambley]

They can, but you can email Patrick at Mercury Magnetics for the spec for an exact replacement and it will be the same.


Go here:
http://www.mercurymagnetics.com/pages/mainframe.htm#_misc/contact.htm


Click the center email as it is Patrick.  Stock replacement specs only, not one of their overbuilt jobs.  It usualy takes him about 5 minutes to reply, but they are in Cali and we are east coast, so keep that in mind.

[pbman1953]

Sent,


Thanks

[pbman1953]

Here's the spec sheet from Patrick

[Ed_Chambley]

The attachment doesn't have amperage which is needed. Send him another email and tell him. Just say you have this PT and you are considering changing from 6550 to kt120. You want to insure you have enough filament current.

[pbman1953]

I'll have it Monday

[92Volts]

It looks like the tube protection fuses are 250ma for the combined cathodes of each pair of output tubes, if this is the right schematic: http://el34world.com/charts/Schematics/files/fender/Fender_Bassman_300.pdf

While the high heater current is a potential worry, it shouldn't be the cause of those fuses blowing.

If these tubes are properly biased, the output transformer impedance should limit the full-load power draw of KT120s tubes to about the same as the 6550s would draw, so this shouldn't be happening.

If the pairs are significantly mismatched, one set could be biasing too hot (and another too cold) even though the combination of all 6 tubes overall biases correctly.

Also, you need to let the amp warm up "fully" before biasing. I've been using some KT90s and was surprised by how long it takes them to heat up just enough to put out sound, let alone for stable/consistent bias, compared to EL34, 6L6, 6V6, etc. There's debate about how long tubes should be allowed to heat before bias "settles", but in my opinion I'd monitor bias until it's been on for about 15 minutes. If you turned it on, biased it as soon as it was hot enough to read the current, then played for 5 minutes... the problem was likely that bias current was continuing to increase throughout that time.

[pbman1953]

Can I do your steps and bias with the  back panel controls? I didn't plan to take the amp out of the case yet. It's heavy
Thanks

[jjasilli]

Yes, I looked for a proper schematic and couldn't find one with fuses shown.  It's frustrating when posters don't post the schematic.  It's unclear:  i) if the fuses are original to the amp; and, ii) if they're affecting filament, or cathode current. 


My question is if anyone has tried the KT120's in their 300T or Pro300?
Personal experience is nice, but unnecessary.  This is a general math problem that can be addressed by people of this Forum, but only with the basic info.

[pbman1953]

I'll find it and scan back

[pbman1953]

Here's a link to the service manual-


http://www.stevehouse.com/bucket/Fender_SUNN_300t-11x17.pdf


Pages 16-21 have it all.

They are listed as screen fuses.


It was too big to upload here

[jjasilli]

It appears that the fuses in question are F5, F6 & F7 @ 100mA ea.; and ea. serving the screens of 2 tubes.  It does not seem possible that 2 normal screens, whether 6550 or KT-120, could possibly draw anywhere near 100mA.  Looking at the tube specs, both tube type's G2 max out at about 13mA x 2= 26mA.   This suggests that something other than good KT-120's may be causing blown fuses.


You may wish to measure screen current @ idle & under max signal conditions. Run the amp into a suitable dummyload, and measure the voltage drop across the screen resistors. Use Ohm's Law to calculate the current flow.  This should verify screen current.


Always suspect tubes first.  Because this is happening after warm-up, a possible tube fault is that internal tube elements are heating-up, expanding, and shorting out.  I think only a short could cause enough current flow to blow these fuses.

[pbman1953]

I didn't get the PT specs yet but I was chatting with my tube supplier and he chimed in. He made some sense as far as chance taking but it's well beyond any warranty, age wise. Plus both transformers never seems to get hot , warm at best.





100mA for two big power tubes isn't that much actually. Putting in the KT120's might push past that. If you can find fuses that are a little bigger that might be fine. You are putting in larger power tubes, you have to expect there could be more current through the circuit. You could try 125mA fuses and see how it goes. I'm sure that will void any warranty, so be aware of that. And of course you are experimenting at your own expense. I can't take any responsibility for what happens. You should monitor the temps of the transformers to be sure they are safe.

[pbman1953]

Specs came in-




282 B+ 1.2 amps
250 ½ an amp
Filament 12 amps
40V 1 amp

[jjasilli]

100mA for two big power tubes isn't that much actually.
This is not for the power tubes; just for the screens. 

Putting in the KT120's might push past that.
No need to guess. I already did  the math from the tube specs.  I already showed how to measure screen current.  Note that with an LED in the screen circuit, thee might be as much as say, another 30mA through the 100mA fuse.  That would kick current through the fuse up to maybe 56mA; whereby a 100mA fuse makes sense.
 
If you can find fuses that are a little bigger that might be fine. You are putting in larger power tubes, you have to expect there could be more current through the circuit.
Once again, no need to guess.


You could try 125mA fuses and see how it goes. I'm sure that will void any warranty, so be aware of that. And of course you are experimenting at your own expense. I can't take any responsibility for what happens. You should monitor the temps of the transformers to be sure they are safe.
The screen + plate supplies are drawn from the same secondary winding.  That behemoth PT is likely to shrug-off another few hundred mA of current draw (which I don't believe is there anyway).  It's the tube screens that are delicate, but maybe a chance worth taking.  I still say there's a short or partial short somewhere.  Either inside a tube, or maybe somewhere in the complicated screen supply circuit.  Subject to actual measurement, it does not appear that the KT-120 screens are drawing more current than the 6550 screens; nor near enough current to blow a 100mA fuse.

BTW: are these slo-blo fuses?

[Ed_Chambley]

JJ, you may find this interesting. I have a push pull UL running a pair and just finished putting it on a dummyload to see how the screens react to cranking the amp up. Idle each is around 8.? Ma, just a little over 8.


Max clean is actually more than I previously remembered. Diode rectified and 565 plate HT current 320 ma. UL taps 40%. 4.2k  Max clean rocks the screen per tube 38ma. Consider the onset of distortion and may even see more.


I have read others measure 50ma at 100v which is rediculoous . If you must give up some wattage to lower the screen the kt120 will be happy.


Sure the datasheet gives the idea you may get 150 watts from a pair, but if treated Ampeg SVT 6550 these run well.

[Ritchie200]

As already been said, the math doesn't add up. Blaming the fuse is inviting trouble.  Also as already been said, always look at the tubes first. You said you ran four just fine for the  rest of the night.  Did you swap positions with the "bad" set or leave them in the same holes?  Unless the fuse was a no-name Chinese fuse I can tell you that the quality control of any U.S. Fuse company is flawless.  It's not the fuse' fault.  You had an over current event. Upping the fuse size is an invitation to let some magic smoke out.

Ed, what are you running for screen resistors?

Jim

[pbman1953]

I was waiting for 2 tubes and was running the amp with 2 sets.  Lets call the them slots 1 & 2. I added the 3rd set , in slot 3, and the fuse blew. I pulled the fuse on slot 2 and installed a new few to slot 3,with the new tubes. The amp ran fine .  Hence running with slot 1 & 3 and all was fine.

[Ritchie200]

Now that IS odd.  Something with the bias circuit?

It originally had 6550"s?  A full sextet?  Never had any issues with them?

Jim

[jjasilli]

I added the 3rd set , in slot 3, and the fuse blew.
Which specific fuse blew?


Max clean rocks the screen per tube 38ma.
Yes, measurement is key, (maybe I figured wrong?).  Ed's 38mA x 2 = 76mA screen current + LED current might blow a 100mA fuse. 


Still I thin k it's OK to up the value of these fuses.  The tranny should be fine.  Tube might blow, but what the heck it's for science!


BTW, the SS circuitry around the 3 fuses culminates in a 16V output.  Where does that go?  Does it trip a relay to shut down the amp in case of screen failure, or what?

[pbman1953]

The only blown fuse was the protection fuse that's  in front of each tube pair
.


There some internal fuses and the main fuse.

[Ed_Chambley]

As already been said, the math doesn't add up. Blaming the fuse is inviting trouble.  Also as already been said, always look at the tubes first. You said you ran four just fine for the  rest of the night.  Did you swap positions with the "bad" set or leave them in the same holes?  Unless the fuse was a no-name Chinese fuse I can tell you that the quality control of any U.S. Fuse company is flawless.  It's not the fuse' fault.  You had an over current event. Upping the fuse size is an invitation to let some magic smoke out.

Ed, what are you running for screen resistors?

Jim

1.5k right at the moment. Nothing like Hifi.


Screen current is variable or am I crazy? I mean I am watching the current through a meter. When I first started messing with these tubes everyone was really hyped as we had a new tube stronger with extended bass. And it is true to a point. They are not KT tubes. I have had them laying beside them real GEC kt88 and tungsol 6550 the old ones and the ge hotrodded 6550 and they look similar to the ge but just bigger.


I am not sure I have read anyone who doesn't just substitute with them to either hear them or in hopes they will run in old amps like SVT Ampegs. 

[jjasilli]

The only blown fuse was the protection fuse that's  in front of each tube pair
Did all three fuses blow?  If not, which one(s)?  (this ties-in to Ritchie200's suggestion to move tubes around)


Screen current is variable or am I crazy?
??? You already posted different screen currents under different signals.

[pbman1953]

The only fuse , and there's only been one is the protection fuse, outside and directly in front of a tube pair.


I mentioned before that I replaced the bad fuse and removed a fuse from the center pair. Hence running 4 tubes. The tubes that had the blown fuse worked fine with a new fuse the balance of the night.


I didn't have to move tubes, I made active tubes inactive.

[pbman1953]

FYI-


I may test the bias again.
 
When I first measured it, the bias wouldn’t get to the .1 (100mv) the manual asked for. The most I could get was .098mv and the pot was maxed.


Ed suggested to have the bias supply resistor tweaked to get more range.


 

[Ed_Chambley]

The only blown fuse was the protection fuse that's  in front of each tube pair
Did all three fuses blow?  If not, which one(s)?  (this ties-in to Ritchie200's suggestion to move tubes around)


Screen current is variable or am I crazy?
??? You already posted different screen currents under different signals.

Yes, I know.  Sorry rhetorical comment.  Should have stated such.  What I was gettig at Ritchie200, who plays with a pig, was inquiring about screen resistors.  I got the idea he was thinking peak 38ma was excessive, which it may be.  It is what it is.  I am finding more and more builders who are using these are regulating screen current.


For science, I agree.  Up those babys to 125ma.


Also, the poster emailed me with some info you guys should know.  With the 120's installed, he loses range of bias which means it is loading a bit more.  He said he could only get them biased by turning the trimmers all the way.  Since idealy the negative voltage should be near center.


I looked at the schematic for a couple of minutes, then lost my mind.  I would guess it is either a overcurrent or a bad tube.  Great conclusion, I know.

[Ed_Chambley]

FYI-


I may test the bias again.
 
When I first measured it, the bias wouldn’t get to the .1 (100mv) the manual asked for. The most I could get was .098mv and the pot was maxed.


Ed suggested to have the bias supply resistor tweaked to get more range.

I suggested if you plan to run these tubes you need to get a proper bias travel.


It is up to you if you want to to try running the tubes.  I said I would try it if I really wanted to use them.

[sluckey]

When you look at the output tube schematic, you will see each pair of tube screens connects to LED1, LED2, or LED3. In this case LED1 does not refer to a LIGHT EMITTING DIODE. It's just a label, although a confusing label. LED1 simply connects to point LED1 in the SS monitor circuit and of course, the fuse. So, if you been wasting time looking for light emitting diodes (LED1, LED2, LED3), stop. There ain't one!

BTW, the SS circuitry around the 3 fuses culminates in a 16V output.  Where does that go?  Does it trip a relay to shut down the amp in case of screen failure, or what?

That 16v is the B+ input to the SS circuit. It ain't an output. All the SS circuit does is lite a green LED if you have screen voltage, ie, the fuse ain't blown. If the fuse blows, the green LED turns off and the red LED turns on to indicate the fuse is blown. Green light good. Red light bad.

Under normal operation screen voltage is applied through a big voltage divider to the base of the transistor. The transistor is just a switch and will be turned on, which also turns on the green LED. When the transistor is on, current is shunted away from the red LED and it will be off. Now, if the screen fuse blows or you lose screen B+ for any other reason (including putting the amp in standby mode), the transistor will switch off, causing the green LED to turn off. Since the transistor is off it will no longer be shunting current away from the red LED and it will turn on.

This is a quote from the user manual...

On the top side of the chassis, at the rear of each pair (total = 3) of output tubes, is an LED and a fuse. The LED glows green
when the associated pair of tubes is operating properly, and glows red if a tube failure has caused the related fuse to blow. If a
fuse blows, replace it only with one of the same type and rating. NEVER use a fuse with a higher current rating, as this could
damage the equipment and present a serious safety hazard. Next, replace the tubes in their respective locations, check and
adjust the bias and balance.
NOTE: it is normal for the LEDs to glow red when the amp is in standby mode.

[jjasilli]

The only fuse , and there's only been one is the protection fuse, outside and directly in front of a tube pair.

I mentioned before that I replaced the bad fuse and removed a fuse from the center pair. Hence running 4 tubes. The tubes that had the blown fuse worked fine with a new fuse the balance of the night.

I didn't have to move tubes, I made active tubes inactive.

Yes, you did.  A bit hard to keep track.  This suggests maybe a fault in the section of SS circuitry where the fuse is blowing.


@ sluckey: thanks for clearing up those side issues.


@ed: 
What I was gettig at Ritchie200, who plays with a pig,
So that's the problem; he needs to get a puppy! 


Bias:  overall tube bias should be checked in a normal way -- not just per the Manual instructions by way of the the screen connection.

For science, I agree.  Up those babys to 125ma.
For added protection, a fuse could be temporarily added to ea power tube, say at the cathode.  This would definitely protect the OT and might save a tube from self-immolation.

[sluckey]

  Here's an idea... Put a 6-pack of 6550s in it and leave it alone.

[pbman1953]

If I upped the bias supply resistor I'd need some help to determine if its R333, 334 or 335

[Ed_Chambley]

If I upped the bias supply resistor I'd need some help to determine if its R333, 334 or 335

Like I said, I lost my mind looking at the schematic.  Opamps, transistors and microprocessors and a SS loop.  I can see a Tube Screamer in there tho.


I believe your Bias circuit is more complex than just changing one resistor.  I am guessing because I do not want to try to figure it out unless I had the amp in hand.  I would pull the chassis.  Install the 6550, make readings.  Install the 120 and take readings to see how much voltage drop you get.  Check the 16volt too.  Like I mentioned, I am not sure how the bias works, but I did look long enough to see collectors on transistors.


Anyway, to INCREASE negative voltage you want to reduce the resistor, if there is only one to reduce. 

[Ed_Chambley]

Here's an idea... Put a 6-pack of 6550s in it and leave it alone.

This is the easiest solution you will find.  Now back to work.

[sluckey]

You must have all six tubes installed and working with good fuses before you set the bias.

Your bias circuit is really just two identical bias circuits, one for each side of the push/pull triplets. Whatever you do to R333 and R334, you must also do to R353 and R369.

[PRR]

> In this case LED1 does not refer to a LIGHT EMITTING DIODE. It's just a label, although a confusing label.

Yes, confusing.

It is the feed from the fuse, one per pair. But voltage (implying fuse status good/blown) is indirectly monitored by a "LIGHT EMITTING DIODE" as in the quoted text. Here's an analysis.

The scheme is perhaps over-complex and has "flaws". They have to tell you Red is normal in Standby. (Logical if you know how it works and recognize they did not spend extra pennies to shut-down this branch of the 16V when the hi-voltage is switched off.) It would also show "bad" for wiring faults. But the LED color should tell you, usually, when a fuse has blown.

I have *never* seen *only* a screen fuse(s) with monitoring. This suggests the amp is volted and loaded way close to the edge of the cliff where the screens just vaporize.

[pbman1953]

Correct, the LED is red on stand by and green during operation.

[shooter]

the LED is red

the 1st time I installed and tested a Phillips MRI the table didn't work, table control board had a red LED lit, spent 4hrs figuring out Phillips used RED LEDS to indicate NORMAL power on

[jjasilli]

What color is an LED that ISN"T THERE? 


@ Shooter:  newfangled electronics are even worse.  Sometimes a red light means the appliance is ON; sometimes it means its OFF but plugged in.  Sometimes a blue light means its ON; or maybe an amber light means its ON.  Gives new meaning to:  The lights are on, but nobody's home!


In an old Woody Allen movie he's convinced that his appliances get together and meet in the living room to conspire against him when he's not home.  I know how he feels.

[sluckey]

Read all about it in the user/service manual. 

[shooter]

Read all about it

that's where the 4hrs was spent, have you ever read Dinglish

[Ritchie200]

So Ed, is it overcurrent or a bad tube.....
 

Ed brought up a good point that others are also alluding to.  Put the 6550's back in?  Fortunately Ed is uniquely qualified to answer this question.  Ed, you have spent a lot of time with this tube.  Do you think it is worth pursuing?  In this application they would be loafing along and would probably last forever.  Big bold clean tube with extended lows would seem to be ideal.  What do you think?  Worth the effort?

Jim

[Ed_Chambley]

So Ed, is it overcurrent or a bad tube.....
 

Ed brought up a good point that others are also alluding to.  Put the 6550's back in?  Fortunately Ed is uniquely qualified to answer this question.  Ed, you have spent a lot of time with this tube.  Do you think it is worth pursuing?  In this application they would be loafing along and would probably last forever.  Big bold clean tube with extended lows would seem to be ideal.  What do you think?  Worth the effort?

Jim

I really like the tubes.  I have not built anything with the that is not Ultra Linear.  I have also never used more than 2 on Push Pull.  I have built 3 amps very similar, just one blockes more bass.  Smalller caps.  All SS rectified and none with NFB.  Tried it and the amp gets stiff, very stiff.  Mine was first and the play time is not that great, but the other 2 are used by guys who run them harder.  One is a Steel Player and the other has a large house gig on Guitar.  The Guitar just get a retube and his were over 3 years old.


Most simply say provided your filiment current can handle the tha addtional current, you are good.  Well a Leslie 147 Iron has plenty, but if you install 120's in it and get the correct cathode resistor to get close to 90% dissipation the voltage drop is too much.


What I will say is I would never install any of these in an amp that has collector value and run the risk of trashing an original Transformer.  The problem is most that install these things never check operational conditions.  The simply add up the heater current and it is the same (or even if it is a little over some guy will ALWAYS say transformers are generally overbuilt) will put them in.


Here is where I will try and have had good results.  I have re-tubed an AMPEG SVT, 1972 with these.  Checked bias and current.  These things were a huge difference since the tubes he had were very old.


Use them and make proper mesurements, certaily.  Stick them in anythiing that has is supposed to have enough current, no way.


Amps are designed to a tube and not the other way around.  Sometimes we can substitute and it is fine, but even a simple substitution will cause a component change if it is just a bias resistor or adjust a bias pot.  Same thing.  The more the tube changes the circuit conditions, the more that is needed to be changed.


Just set on on a table beside a Real 6550 and ask yourself if you think adding 6 of thses things to my amp is going to chage the way the circuit operates.  In your Marshall, since I do not know the value of the amp this would guide me.  I would still   try them provided I had monitored the amp with the KT88's to compare to the 120's.  If conditions changed where I had to do more than change bias, I pull them.  If not, you will end up chasing more problems.

[Ed_Chambley]

Also, I think PRR may know more about these tubes than he has mentioned.  I have read a lot of his posts everywhere on 6550's.


Also, Dummyload had a plan to bake one or 2.  Maybe these guys can share their understanding of these.  I know the Data Sheet is crap.  Also, I only know 1 tube vendor who test over 500 volts.  Most use a Maximatcher at 400 volts max.  A tube can change a lot between 400 vdc and 550 vdc.


Yse the tubes would be loafing on plate voltage, but do we know if the transformers would be.  This is why I run 2 of them with 320ma, 400-0-400.

[PRR]

I know nothing about this new KT120. It may be a 1.5X slice of KT88 guts; or it may be less than it seems.

The amp is already a beast. I don't think anybody but the player (maybe) can hear what tube is in it. Unless some newfangled oversize bottle gives it troubles (and screen fuses should NOT be blowing). _I_ would fill it with Sovtek 6550 and go to work. The limited abuse I have done says they are/were as well-made as GE-Kentucky and did not creak (on dummy-load obviously!) like vintage Tung-Sol.

[pbman1953]

I've given some thought to this based on the post. What it really is that I'm really upset that SED is gone and their 6550's are triple the price for anyone who ha them.


That's what is really bugging me. 


So I'm thinking to go back to a set of Tung Sol reissue 6550 or possibly a set of JJ - KT-88. Or maybe a st of Gold Lion, I've never used them yet.


Another factor s that New Sensor has all these tubes and are they really different or are they same with different labels?   They bug me.


At this time, I've been using the KT120's in my Sunn 2000s and it's fine. Plus, I remember, using my Bias Rite on the 300T, months back. The OT transformer (with JJ 6550's) was putting out over 700 volts and with Sunn the KT's around 514- 520. Could this be the reason the amp has trouble with the KT-120's? With them the voltage has to be much higher right?


I guess I could get up the muscle to pull the amp out of the cab and check it.

[Ritchie200]

Over 700V?  On the plates?  With JJ's?  I know the 2000's were supposed to be conservatively rated at 120W but the old Super Leads were pushing that with 150V less.  Maybe the new KT88's are built crappier than the 6550's, but there are very few brands of KT88's that will last in my amp at 620V.  I realize how you beat the screens is also a factor as there is more to it than just plate voltage - but wow.

Jim

[pbman1953]

Jim


Just to be sure -




Over 700 V with the JJ's(6550) in the 300T




Low 500's with the KT120 in the Sunn 2000s

[66Strat]

Just my opinion, but it looks like the KT120s are drawing excessive current or possibly V4 my be causing problems with the bias voltage. B+ should be 742 bolts at idle per the schematic. Have you measured the DC bias voltage for the control grids? It should be about -50 volts (-49.4 per schematic). I would measure the bias voltage at each tube socket. In comparing the datasheets between the 6550 and the KT120, the current draw at idle shouldn't between the two tubes shouldn't be enough to cause that much of a difference in plate voltage. Operating screen grid currents between the tubes don't differ significantly per the datasheets.

Where did you get the tubes? Some vendors do a better job of screening to weed out culls than others.