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

[shooter]

Is higher than 100 a hotter or cooler bias?

Higher is HOTTER, 100 is current.

what's good for 6550's is also good for the 120's in your amp since it was designed for 6550.  If your PT is taking skin, you have maxed out the amps capability.

[pbman1953]

Ouch, def, no skin just stinging hot.
Thee's so much power maybe 'll try a lower number to cool things off a bit.


I'm sure there's no harm . right.

[shooter]

there's no harm . right.

Your amp was designed to dissipate 100mA per side, the amp doesn't care whether that "LOAD" is from 6550s, 120's, or a BIG A S resistor.  the "extra" heat is probably from filaments.
play it til I breaks

[66Strat]

Treat it like a Harley Sportster. Fix it till it's broke.

Seriously, the amp circuit was designed to idle at 100 milliamps with 6550s. IMO, you have the amp set up as close you're going to get to optimum using KT120s.

[Ritchie200]

pbman, I know you have used this tube before and apparently liked the result.  Now that you have had a chance to wring them out in this amp, what do you think?  How does it differ from the 6550's?  Give us your sonic assessment!


Jim

[jjasilli]

1.  How hot is the tranny with 6550's compared to KT-120's? IOW you're not giving a baseline for comparison. 


2.  If the tranny is hotter with KT-120's don't automatically assume it's HT draw.  A KT-120 heater is rated to draw up to 1.95A; 6550: 1.6A.  6X KT-120 heaters could be drawing about 2A more in heater current.  This point has been made more than once.  That alone could account for more heat.  It would be helpful to measure these things rather than speculate.


Tubes should not be re-biased in accordance with tranny temperature!  This is absurd.


Hot trannies are no problem unless so hot as to cause failure.  Your tubes are too hot to touch too.  In the unlikely event that the tranny really is is too hot to run KT-120 tubes, then either: i) don't use those tubes; ii) install a fan to cool the tranny; iii) remove the bell covers for better heat dissipation; or iv) install a supplemental filament tranny.

[pbman1953]

I don't have any 6550 vs. KT120 , tranny heat tests. I didn't pay as close attention back then. I do now because the KT was not the stock tube.  One thing is the ambiant heat around the the 120's compared o the 65's is more.


A fan would probably be a very good idea. I'll look for one to face it because, I won't mount one inside. 

[Ed_Chambley]

If I cannot keep my hand on a transformer without it burning me,  I would be concerned. JJ mentioned you not having a comparison. Next time you play, put the 6550s back in. It should not take you but a few minutes now that you know how.


This amp is really not the same as an old SUNN or even the Traynor you have. I wouldn't simply expect it to easily run these tubes unless at the time of the design it was considered.


If you do this, you will know for sure. I play these tubes, I get it. They are thunderous even with guitar.


I would just hate to see a transformer burned when it is so easy to swap tubes and check. There are other ways to test, but this one is easy.


Doesn't mean you cannot run them. You know how to install a filament transformer. A fan is not a solution to stressing a transformer, but is a good thing.

[pbman1953]

I will Ed, either over the weekend or the next time we play here, which is in 2 weeks








Thanks

[jjasilli]

You can use an infrared thermometer.  I got one for about $17 at Harbor Freight. 


This is a rare occurrence where I disagree with Ed.  1.  I don't believe a Tranny that's hot enough to burn you is necessarily a problem.  450 degrees is hot enough to burn you real bad, but not hot enough to ignite paper, much less harm metal.  Vintage bargain basement amps had trannies that ran hot enough to burn you.  Core temp is the real problem, but basically unknowable.  Insulation failure is the biggest threat.  2. I do believe a fan is an effective cooling solution.

[6G6]

I vote for a filament transformer and a fan.
There is no such thing as being to reliable. 

[Ed_Chambley]

You can use an infrared thermometer.  I got one for about $17 at Harbor Freight. 


This is a rare occurrence where I disagree with Ed.  1.  I don't believe a Tranny that's hot enough to burn you is necessarily a problem.  450 degrees is hot enough to burn you real bad, but not hot enough to ignite paper, much less harm metal.  Vintage bargain basement amps had trannies that ran hot enough to burn you.  Core temp is the real problem, but basically unknowable.  Insulation failure is the biggest threat.  2. I do believe a fan is an effective cooling solution.

JJ, I do not see a disagreement at all.  Fans INSTALLED in a head are really nice and I put them somewhere in every amp.  I was not writing to you either as I know you know the difference between an amp that has a hot transformer caused by overcurrent and one getting hot from ambiant surroundings.


I have a few radios the transformers will blister you hands and play fine.  Yep, they get hot.  They live in Georgia and 2 of them always have, even when we did not have air conditining.  Old Radios.  Now if I were to hotrod one of these radios to run a power tube that took more current, I just took the hand of the Devil for a Dance.  Didn't I just ask more from the transformer than the engineer of the radio had planned?


It is the core tempature, but aren't we speaking of an increase of 6 times over 6550 to KT120?  This is a tube bass amp and with modern electronics I just do not see the engineers planning for the additional needed current.


So let me ask you directly.  Without you checking the amp are you absolutely certain this Power Transformer will run these tubes full throttle, outside in 95 degrees with a fan blowing 95 degree air on the transformer and it will hold up without a problem?  The OP certainly will push this amp hard.


Well I guess if you put it that way.

[Willabe]

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.

90% current draw at idol in AB PP?

With a Leslie (and most amps?) wouldn't you want a more conservative/cleaner 70% at idol in AB PP?   

[Ed_Chambley]

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.

90% current draw at idol in AB PP?

With a Leslie (and most amps?) wouldn't you want a more conservative/cleaner 70% at idol in AB PP?   

Yes, if you maintain the UL setup, but a Leslie sooulds much better with Guitar if you Cathode Bias.  At least it does to me.  The heavy lows leslie cabs can make will stick around.  I should have said this is cathode bias.


I do not know anyone who uses a Leslie in the guitar rig and maintains the stock output section.  Sort of want it to blend so some headroom has to go.

[Willabe]

Ahh, ok. Thanks Ed.