Bias for SED Winged-C 6550s in Herbert

[Bevosss]

I've got a matched quartet of SED 6550s to put in my 2004 Herbert, leaving 2 EL-34Bs in there still.. The amp was previously biased (by a tech using a scope) for EL-34Bs.

I only have the Weber Bias Point. The manual gives 40-60mA as a reference point for 6550s.

So for the guys who have swapped SED 6550s in, what have you guys ended up biasing it to? And did you use a scope? (I believe that's the only way to tune it correctly under load).

Just trying to get some ballpark figures and/or specific info here.

Thanks!

 

[duesentrieb]

with the bias rite you need to adjust the current for a single tube. In case of 6550 I would suggest to set it at 40mA/tube.

You don't need to use a scope, biasing the idle current (master = 0, no signal) is common.

 

[Bevosss]

with the bias rite you need to adjust the current for a single tube. In case of 6550 I would suggest to set it at 40mA/tube.

You don't need to use a scope, biasing the idle current (master = 0, no signal) is common.

Hmm, well every tech I've spoken to about this (including the Australian Distributor for Diezel), has told me the only way to see what's happening at full power is with a scope.

One tech likened biasing using idle current to setting the idle on a sports car without running it on the track...he said 'sure the engine might sound great in the parking space, but that is not going to tell you much about how it performs under load.'

In any case, I figure it can't hurt to know what others have ended up setting the bias to with those particular tubes, hopefully there's not too much variation from one sed tube to another.

Btw my mistake, it's a Bias Rite I have and it accepts 4 tubes.

 

[duesentrieb]

you adjust the pressure in your tires to their proper value under idle conditions, not at full speed - because the manufacturer knows where to set them at - the gain of pressure compared between idle and full speed is well known. Same for tubes, the current goes up to 100-150mA per tube - and with the tubes well burnt in (manufacturer, reseller) and matched, they will have a good relation to their value idle vs. full load.

Your bias rite has four sockets, still you are reading the idle current for a single tube, which, recommended by Diezel, is max. 40mA per tube for 6550.

For further reading about biasing and such I suggest to go to
http://www.aikenamps.com

 

[PaulyPanacea]

listen to Olaf...he knows a thing or 2!

 

[Ventura]

Herr Peter recommends 35mA per tube, Herr Olaf 40mA.

I'm siding with the 40mA only because the cleans are better and the sound is just that much juicier.

Don't listen to Pauly though, he'll tell you to go 50+mA per tube He's a MADMAN!!!!

Peace,
V.

 

[duesentrieb]

@480V on the plates, 40mA is about 55% of the max. plate dissipation for the 35Watts of 6550 tubes. Thats a pretty reliable value even for crappy tubes, which the SEDs shouldn't be . . .

 

[Bevosss]

@480V on the plates, 40mA is about 55% of the max. plate dissipation for the 35Watts of 6550 tubes. Thats a pretty reliable value even for crappy tubes, which the SEDs shouldn't be . . .

Ahhh you are Olaf! My bad, I remember you from the Diezel forum...I'm only new here on rig-talk.

Thanks for the graph. Cool, so we are talking known values here. It's been a few years since I had to bias it myself, so I'm a little rusty.

(It had to get repaired since then because of a manufacturing error, some wrong tolerance caps kept blowing the fuse, so the distributor rebiased it at that time to a new set of EL34Bs I had).

Herr Peter recommends 35mA per tube, Herr Olaf 40mA.

I'm siding with the 40mA only because the cleans are better and the sound is just that much juicier.

Don't listen to Pauly though, he'll tell you to go 50+mA per tube He's a MADMAN!!!!

That is interesting, 70% was suggested to me too as a starting point, which is what, 56mA?

So, what happens at that level to the tone, is it even juicier? And at what point do you start getting shorter life out of your tubes?

Thanks for the help guys!!

 

[duesentrieb]

The higher the bias, the shorter the tube's life. After going thru lots of "trouble" with the current quality of tubes, Peter recommended first to set the bias not above the 60% line, and for tubes like 6550 or KT88 even lower. Also the very new Diezels have a lower plate voltage now, to keep the tubes healthier (with the lower plate voltage the screen voltage also goes down linear - and its the screen volts which limits the useage of many tubes of today).

Modern amps (like Diezels) don't need to be set to 70%, their sound is from the preamp, not so much the poweramp. 55-60% is perfect for tone and life.

 

[Bevosss]

The higher the bias, the shorter the tube's life. After going thru lots of "trouble" with the current quality of tubes, Peter recommended first to set the bias not above the 60% line, and for tubes like 6550 or KT88 even lower. Also the very new Diezels have a lower plate voltage now, to keep the tubes healthier (with the lower plate voltage the screen voltage also goes down linear - and its the screen volts which limits the useage of many tubes of today).

Modern amps (like Diezels) don't need to be set to 70%, their sound is from the preamp, not so much the poweramp. 55-60% is perfect for tone and life.

40mA it is then! SED tubes just went up in price, so I won't be in a hurry to replace them soon.

Thanks for all the info Olaf, and Happy New Year to you!

 

[racerevlon]

If I remember correctly I did 47 mA @ 480 with my 6550's and it sounds AWESOME!!!

 

[Bevosss]

If I remember correctly I did 47 mA @ 480 with my 6550's and it sounds AWESOME!!!

Did you go all 6550s, or 2-4?

 

[Ventura]

FWIW, I found my VH4S sounded better on Channel 1 (clean) with a slightly higher bias, 40mA. Everything else seemed the same more or less... As Olaf says, the preamp is where the money's at. Tube saturation is prevalent, but by the time I'd get to that point, my ears'd be bleeding.

Happy New Years to all Diezelers!!
V.

 

[PaulyPanacea]

Don't listen to me..im a rebel!! Still going strong : )