Originally posted by boroman
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Originally posted by boroman View PostBy the way, just a free 2 cents... that's a serious desing failure with the OR200 - if you all suggest not to "drive the amp hard" and worry about the screen resistors/fuse pop/tube blow, how this amp was even constructed that way? I'm really disappointed, because I wanted to gig this amp pretty serious way.
Re-cap:
Thanks for the clear gut shot.
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I have re-capped the beast, changed all the screen resistors to 330 ohm / 2W (original: 270 ohm / 1W). I just had a chance to plug it in and it works, but I haven't hooked it to a bass cab yet. If you want me to make some close up shots, I'll do that. Seems that not many of these are around. Also, near the 16+16uf multicap there were two resistors across it 150k and 100k. Schermatic calls for 2x100k... I left the factory ones.
By the way, just a free 2 cents... that's a serious desing failure with the OR200 - if you all suggest not to "drive the amp hard" and worry about the screen resistors/fuse pop/tube blow, how this amp was even constructed that way? I'm really disappointed, because I wanted to gig this amp pretty serious way.
Re-cap:
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Originally posted by SoulFetish View Post
Also, nickb, a few posts ago you wrote:
Since the average and RMS methods were used to provide the last two measurements, how were you calculating screen dissipation for the first measurements?
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Originally posted by Helmholtz View Post
I second this wish and would include SoulFetish.
I am especially interested in the value of the cathode resistors.
I just looked through some of my photo archive to see if I had any other pictures of the chassis, in addition to the transformer wiring. No such luck. I'll have to take some photos next time I have it open.
Been getting up to speed on this thread, but is there any area of the circuit specifically that needs some clarification?
Also, nickb, a few posts ago you wrote:
Originally posted by nickb View PostThe screen dissipation at full power is 3.38W ( 3.17W with a 1K instead of 270)
Using the average method ( average screen x average screen current) gives 3.6W
Using the RMS gives 6.14W. The problem is the voltage and current are almost 180 degrees out of phase so that is a really bad method.
Driven hard,
The screen dissipation at full power is 4.41W
Using the average method 5.6W
Using the RMS gives 8.4W.
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Originally posted by Helmholtz View Post
Thanks, missed the second attachment of post #6. Would reduce effective plate-cathode voltage by around 5V at full output.
Now we need confirmation, as all schematics are said to be "approximate" only.
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Originally posted by catalin gramada View Post
There are 30 ohm as seen in Boro pics
Now we need confirmation, as all schematics are said to be "approximate" only.
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Originally posted by Helmholtz View Post
I second this wish and would include SoulFetish.
I am especially interested in the value of the cathode resistors.
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Originally posted by nickb View PostAs Catalin and Boroman have actual amps maybe they can help make this thread have a definitive schematic?
I am especially interested in the value of the cathode resistors.Last edited by Helmholtz; 06-23-2020, 06:34 PM.
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I switched to UL , still using the 6L6's and a 'real' speaker.
The screen dissipation at full power is 3.38W ( 3.17W with a 1K instead of 270)
Using the average method ( average screen x average screen current) gives 3.6W
Using the RMS gives 6.14W. The problem is the voltage and current are almost 180 degrees out of phase so that is a really bad method.
Driven hard,
The screen dissipation at full power is 4.41W
Using the average method 5.6W
Using the RMS gives 8.4W.
Conclusion:
1) Using the RMS method is plain wrong and gives a very misleading (about x 2) high value. Average does pretty well for an easy to do measurement.
2) Bigger screen grid resistor of a reasonable size help but don't make a huge difference.
Yes, HH, thanks. Most meters do remove the DC component so I guess you could say they are not true RMS after all.
As Catalin and Boroman have actual amps maybe they can help make this thread have a definitive schematic?
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Originally posted by Helmholtz View Post
The DC average method might work satisfactorily with standard screen supplies, but I have some doubts with UL configuration, where screen voltage varies heavily with signal.Last edited by catalin gramada; 06-23-2020, 03:14 PM.
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Originally posted by Dave H View Post
I don't think that works with screen current and screen voltage being out of phase. Didn't we decide in that other thread a while back that using DC average values for current and voltage gave a better approximation for screen dissipation?
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Originally posted by Dave H View PostDidn't we decide in that other thread a while back that using DC average values for current and voltage gave a better approximation for screen dissipation?
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Originally posted by nickb View Post
Is your meter true RMS? If you measure the RMS screen current and RMS screen voltage then you can simply multiply them to get the screen dissipation.
What you say 'lower value', what was it you lowered the value of?
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