Orange AD 200B Mk3 on bench

[xtian]

Weirdness going on with this amp. Date stamp inside says it was born on June 2014.

Ugly buzz with Master volume up at all, but no hum or buzz with MV at zero. See unofficial schemos:

[img713]http://www.prowessamplifiers.com/schema ... Preamp.jpg[/img]
[img582]http://www.prowessamplifiers.com/schema ... kIIIPI.jpg[/img]

First, the power section: 647v on plates. Can't adjust bias hotter than -41v, but datasheets suggest -32 is closer to target. With bias set to hottest, I'm seeing only 21mA per tube, where I should be seeing closer to 42mA per tube. While the circuit board has spots for 1R bias check resistors, they're factory jumpered, so I'm measuring resistance and voltage across legs of OT primary.

When I ground the output (center pin) of the MV, everything is silent. BUT when I ground the input of the MV pot, the buzz persists when turning up the MV. How is that possible? The center wiper is moving between two grounded terminals!

Last thing: there are three little blue MOVs (I think they're MOVs; see photo) strapped across the HV and heater coils. They're buzzing like fluroescent light ballasts. And they buzz even more angrily when I put light pressure on them with a chop stick. WTH?

[Stevem]

I do not think those are a varistor, I woukd think they would be a thermistor for slow turn on of the filaments?
When you pull out preamp tube V1 with the master wide open does the noise die?

[xtian]

Same buzzing noise EVEN with V1 pulled!

I mean, that should make my job easy, but I'm worried about the fact that the power section can't be biased properly, so wanting to start there.

I have emailed Orange tech support; hope I get a reply.

The buzzing varistors are extremely creepy.

[xtian]

Orange tech got back to me with schematics, and mentioned if the power tubes arc'd, they probably took out the 100R heater balance resistors. Sure enough, they fell apart in my hands. Replaced, and everything is good. No hum or buzz, just LOUD.

I love these Demeter transformers. Not just for their size, but the fit and finish is amazingly precise. Look and those shiny lams!

When replacing 6550s, Orange tech advised me:

And if replacing the 6550’s, you must add a couple millimeters of shrink-tubing on pins 2&3 on the new 6550 or KT88 power tubes (see attached jpg); because we've seen an arcing of electricity occur between pins 2&3. It's the high voltage running on those output tubes, and the closeness of the pin-out on the tube socket needs a little guarantee (ergo couple mils of shrink-tubing) to insure the arcing doesn't occur.

[Dr-Joned]

Those are some nice amps.

I think it's really cool that Orange is so in tune with the handicapped community, I mean, those amps are made for the hearing impaired right?

[Gaz]

Out of curiosity, what's the screen voltage on those?

I think the shrink wrap trick is neat, but I wonder if it's kind of a hack.

If the arc'ing happens due to voltage spikes on the primary I would think those MOVs would help. Did you ever confirm that's what they were? Man, it doesn't seem like 650v and 200W is that crazy. Maybe I misunderstood what the tech was saying.