Orange OR200 the definitive maintenance thread

[catalin gramada]

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:

Limit the drive as nick suggest. Marshall get the most notorious failure rate in 200w Majors known as 'smoke on the water' effect. The same circuit as you OR. It well depend how hard is drived. You can use it for decades if don't push it over limit.

[nickb]

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:

That doesn't seem fair when it's been in use for fifty years with the original screen grid resistors. No-one else blew them. Any chance the cab used was too high an impedance? Or maybe you're just evil!

Thanks for the clear gut shot.

[boroman]

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:

[nickb]

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?

Power = integral of voltage x current with time.

[SoulFetish]

I second this wish and would include SoulFetish.
I am especially interested in the value of the cathode resistors.

Alright. I'll jump in.
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:

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.

Since the average and RMS methods were used to provide the last two measurements, how were you calculating screen dissipation for the first measurements?

[catalin gramada]

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.

Very sorry can not help. I don't have an Orange but a 200w 4xkt88 UL Simms Watts instead, pretty close circuit.

[Helmholtz]

There are 30 ohm as seen in Boro pics

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.

[catalin gramada]

I second this wish and would include SoulFetish.
I am especially interested in the value of the cathode resistors.

There are 30 ohm as seen in Boro pics

[Helmholtz]

As Catalin and Boroman have actual amps maybe they can help make this thread have a definitive schematic?

I second this wish and would include SoulFetish.
I am especially interested in the value of the cathode resistors.

[nickb]

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?

[Helmholtz]

it seems to me the dc don't vary as much as in pentode mode

You should scope the screen voltage with a high voltage probe.

[catalin gramada]

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.

it seems to me the screens dc don't vary as much as in pentode mode

[nickb]

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?

That's true. I was testing with a resistive load so they were in phase. I'll try it with a speaker and see what happens. Another caveat is that this wasn't a UL arrangement. I had forgot about that and grabbed what I had. The average method approx is OK but I think it falls apart when the amp really pushed as is the case here as the waveforms get more extreme. I have a gut feeling that it (i.e. screen power measuremeny) really needs to be tested under real conditions rather then test signals to get a truly meaningful number.

[Helmholtz]

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?

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.

[catalin gramada]

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?

My first test was done with 1K screens resistors. The meter is a Fluke 187, claimed as true rms