Need help with 6550's

jcm800

Member
Messages
10
Hello all,

Im new to this forum, it was recommended to me by another member.

I have done some reading and really dig this website. There are alot of knowledgable people on here, well on to my query.

I have a jcm 800 100 watt head and i wish to replace my 6550's with newer ones. I have been reding on line that the 6550's bias should be set at 90ma.... My question is..Do they mean 90ma per tube or per set? I was under the impression that the tube should be set to to 70% of max dissapation.......Am i off on this one?

Thank you for the help.
jcm800
 

Wakarusa

Member
Messages
1,459
The 70% value is a subject of some friendly debate, but the thing most agree on is that it is a maximum value, not a target.

So the right bias point is wherever the tubes sound best at or below 70% (if you're on the 70% side of the debate ;))

Also, you can't determine static dissipation without taking plate voltage into consideration. If we believe the rating of 35W per tube, then 90mA per tube would be 70% if plate voltage is 272V.

90mA per pair (45mA per tube) would be 70% at a plate voltage of 544V. The right answer is probably somewhere between the two.
 

jcm800

Member
Messages
10
Hi wakarusa

The plate voltage is 385v, so if i understood you correctly i would use this formula.

35w/385x.7 =63ma

Is this correct? Also are Tungsol 6550's rated at 35 watts?

Thanks
jcm800
 

Wakarusa

Member
Messages
1,459
Originally posted by jcm800
Hi wakarusa

The plate voltage is 385v, so if i understood you correctly i would use this formula.

35w/385x.7 =63ma

Is this correct? Also are Tungsol 6550's rated at 35 watts?

Thanks
jcm800
Yes on the 35 watts, yes on 63mA. I usually bias initially to 60%-65% and then tweak for tone not to exceed the 70% value.
 

jcm800

Member
Messages
10
Thank you very much, you have been a great help. The amp in question is a jcm800 and when i checked the bias with the old tubes they were 22ma...... I have them at 45ma right now but will bring it up to 60-65% of max dissapation.

Thanks alot i really appreciate the info.

jcm800
 

Reeek

Member
Messages
1,196
Originally posted by Wakarusa
Yes on the 35 watts, yes on 63mA. I usually bias initially to 60%-65% and then tweak for tone not to exceed the 70% value.
That's exactly what I do too . . . I use 65% most of the time so I have more upward adjustability then I plug in and go from there with tone tweaking. I try to use full chords and adjust for articulation and string definition, then I try to find the single notes with rogue harmonics and try to smooth them out with the bias trim pot if possible. Obviously, I'm looking for a compromise between single note sustain/harmonics and full chord string definition.
 

RevolutionMan

Member
Messages
414
JCM 800, See I told ya there would be some knowledgeable cats here who would point you in the right direction. Good to see you on the gear page, soon you'll be addicted to this place like the rest of us!
 

jcm800

Member
Messages
10
Hey revolutionman,

You are correct this is a great site. There are alot of knowledgable cats on here. Thanks for the heads up.

jcm800
 

bruce egnater

Member
Messages
413
385VDC is extrememly low for that amp. You should recheck. Normal range would be more like 430 to 500. Check your AC line voltage when making these measurments.
 




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