revisited- Bias the Fender 300 Pro

[pbman1953]

Ok, now that I have an understanding to the layout and "sides" of the bias system.  I took my Bias Rite out and wanted to see , after the manual settings, what the result are.


The results are on the cold side for this sextet of KT-120's-




All in Watts


v5- 25 
v7- 26.6 
v9- 23.4




v6-  24
v8-  24.5 
v10-  22.5


Max dissipation for the KT 120 is 60 watts


Should I leave it or go for at least 50-60% of dissipation?


We've been through this before about the PT and it does get hot to touch after playing for an hour or so. Plus I haven't had playing time to switch back to the 6550's to see if the transformer is cooler with them instead of the KT's.

[shooter]

Should I leave it or go for

bias is about 2 things;
1 the care and feeding of the equipment.  Since you "might" be on the edge there based on your heat test, peta (people for the ethical treatment of AMPS) might take it away, or protest.

2 sound, here only you can be the arbiter.

personally I'd play it, while playing I would learn to measure current with confidence, then prove to yourself the PT operation and ability can take more abuse, or already being abused

[pdf64]

Is there any info about this amp available, schematic, other threads etc?
It isn't a PS300, right?
Whatever, I think that any rationale for the 'bias to idle at X% of limiting plate dissipation' guideline gets blown out of the water when a tube type other than that for which the amp was originally designed are fitted.

[pbman1953]

No, The amp is  a Fender (Sunn) 300T

[PRR]

>  sunn300T.png 

Clearly says "set for 100mV" across 1 Ohm common to each trio (average 33mA/each).

I'd say your 22-27mA each is a shade cool but plenty close to rock with. This idle goal is more about the output stage conditions (tube general type, B+, load) than the plate dissipation rating. You could try to bring it to 100mA-120mA per trio (the 120 cuz the bigger plate won't mind) and see if very-soft sounds are cleaner.

However you don't drag out a 300 Watt amp to play "very soft". Or to sit "idle". So I don't see the idle point being real critical. Warm enough so it plays soft without hoarseness. Cool enough to get a rest when the drummer steals an extended solo. On this VERY BIG amp with BIG TUBES, that may well be under half Pdiss.

I've had amps biased nearer 30% Pdiss. Yes, that one sounded better hotter, but it wasn't a "real" improvement until I dialed it up to 150% Pdiss. Which won't last.

[pbman1953]

A little more info-




At the 100mv setting, current draw was 774 volts, that's when the PT was hot to touch. At 138mv (35-36 watts) the PT  current came to 762 volts and noticeably cooler to the touch.




Between the sides, there's a 2mv different to achieve 35-36 watts.


I'd have to take time for listening tests.

[pdf64]

...At the 100mv setting, current draw was 774 volts, that's when the PT was hot to touch. At 138mv (35-36 watts) the PT  current came to 762 volts and noticeably cooler to the touch...

On the face of it, that doesn't seem to make sense? ie turn it up to 200mV and the PT would be icey cold
Logically, as the load on the PT increases (ie power tube plate current) it should get hotter.
Perhaps it's time to get a cheap IR temperature meter, maybe $20?
Also, it's always good practice to note the mains voltage at the time of taking any measurements inside the amp.

[pbman1953]

Harbor Freight is fun place, maybe they'd have something.

[pbman1953]

I made the sound tests-




100 mv- was a little mushy but very warm


140 mv- was tight but a bit harsh , but detailed


I'm settling on 118-120 mv- best of both words with rounds and flats. Anything else I can take care of with tone controls.


Plus the PT could be held and wasn't as hot as playing it at 140 mv. The bias cover is going back on.


Thanks to all and to PPR for the numbers to start with.

[shooter]