KT88/6550 Bias 700ma per tube?

[justinsweber]

I was wondering if turning down the bias to closer to 600ma would be too problematic re: stability issues/sound?

I have read some posts on other sites where indiv turn the bias down by 10% or so and significantly extend the life of their tubes?

Any truth? Bad idea in this application? I can not versed enough in this design to know how hard it pushed the 6550 tube.

I run my tubes at 650ma - 680ma but wonder if turning down to 600ma woudl have any benifits or costs?

Any help?

Thanx everyone!

[Geek]

600mA... WHA?????

You mean 60mA, right?

Cheers!

[justinsweber]

yes completly 63-68ma

how low should a kt88/6550 be set to ext tube life but still sound good.

[TerrySmith]

I run mine at 50 - 55ma, they seem to be happy with that. IIRC the stock MK-3 bias ends up at 70ma per tube.

[EWBrown]

I know from experience (working on some) that VTL and Manley tend to run the 6550Cs in their big PPP amps, around 30 mA, and 807s at 35 mA, but then they also run these tubes at relatively high (550VDC and higher) plate voltages.

There is a point of diminishing returns as the current is increased, when shortened tube life trumps over any improvement in sound quality.

/ed B

[justinsweber]

Shannon sugests 70ma per tube right? but some run there down in the 50ma area? Maybe time for some fiddling.

[EWBrown]

Best thing is tp start at the low current , at 50mA, and then later turn up the current to 70 mA if you think it's necessary.

Given all that, when I build up an SE amp, I usually push the output tube close to its max PD limit .

YMMV, etc...

/ed B

[justinsweber]

Ill try turning them down to 50ma. I used a triode kit so they're tied together.... 100ma for both and see how it sounds. No stability issues with turning it down to 50m per tube?

[kheper]

According to the Dynaco manual, the MK3 was designed to run a KT88 tube at:

480V x.070A = 33.6W

The KT88 tube has a max anode wattage rating of 42. So, it was designed to run at about %80 of its max in the MK3.

Patrick Turner says that running a tube at %66 of its max anode wattage is optimal for PP amps in terms of both tube life and sound quality. (I cannot verify this, but I trust the dude.)

An EL-34 in the original st-70 design was really being stressed at %85 of its max anode wattage.

Backing off 10ma or so on the MK3 sounds like a good idea, especially since AC line voltages are higher than 110V now.

[justinsweber]

In the Poseidon PDF Shanon lits "Note: Set bias using pins 5 & 8
of preamp socket referenced to ground at pins 2 & 3. Set each to 700mV using bias pot on amp. Balance with R15 on driver.

You think backing it off to 640mV or so seems like a goot idea?

[EWBrown]

Just like bidding on e-bay, start low, then increase if necessary.

The perceptable audio difference between 64 mA and 70 mA (640 and 700 mV through 10 ohms) will be neglible, maybe some fancy expensive test equipment may show a slight increase in THD, but most mortal human ears won't notice . PP amps can be quite decent sounding with reduced plate / cathode current, as they are conducting only half of the entire voltage swing, and they can run nicely in Class AB1 or AB2, whereas SET rally has to be in the "hard" Class A operating parameters:

SE output stages are a bit more critical in order to hit the most linear part of the curve, and then I'll push the tube right against the outer limits of their " performanceenvelope" if necessary. Like taking a 12AV5GA and blithley ignoring the "11 watt" max plate dissipation and G2 voltage ratings, and blasting it up to 15 or even 18 watts PD (read George's commentary over at www.tubelab.com about this) If it glows red, just turn it down a bit ;-)

/ed B

[justinsweber]

Just put some Solvtek 5AR4s back in... Love the sound again. The Weber Rectifers are nice... but I do think tube rectifers offer something.