The Unofficial Orange Overdrive Page
Last update: 22/12/2002: Clean up, addition of link to OTR schematic
Overdrive versus Graphic
Overdrive amps were basically Graphic amps equipped with a master volume. This was considered a big improvement, especially for players that liked play with overdriven Orange amps without having the deafening volumes of the non-master Graphics.
Orange Overdrive amps have no additional gain stages and no extra ECC83 tube. This is mentioned wrongly in some sources (cfr. The Tube AMP Book 4.1 by Aspen Pittman p64 and p189)
Another post on Master Volumes in Oranges made by Amperex on the Plexipalace BBS:
"The overdrive heads are indeed graphics with the dual master added. The unfortunate side effect of the 1 meg dual pot is that even cranked all the way up it messes with the overall volume and tonal response a little bit. I change that master to Ken Fischer's dual master that simply replaces the 220K bias resistors, and IMO it not only sounds better, but you get the fully cranked sound and it really improves the fidelity of the clean sound.
One more important point in reference to the orange circuits is that they changed the circuit in '74, and rewired the phase inverter. I suggest to anyone that has a '74 graphic or later, including the overdrive, to switch to the old circuit. It only takes a about a half hour to rewire it, and incidently Mike Soldano use to offer this mod when he was in L.A. and obsessed with Oranges. (he actually still offers it). It really makes the amp more punchy and responsive. The post '72 amps IMO can sound kind of mushy, not as dynamic as the earlier ones. Orange continued to make some more little circuit changes around '76 and IMO those amps are the worst they ever made.
For anyone whose wondering, the new Oranges don't sound at all like the old ones. I talked to the guys at the NAMM show last year and they told me that they are totally new circuits. Bummer. Get an old one!"
The Overdrives use basically the same circuit as the Graphic amps with an additional Master Volume. The Master Volume is put behind the phase inverter, so it means that a double pot has to be used.
Brian K. Wallace has been so kind to draw a schematic "like" the Orange Overdrive. This means that he and I derived a schematic by the data we found. This is only a first revision of the schematic, we hope to update it by comments from Overdrive owners.
For more details on master volumes look here.
Halfway '90s Orange launched the OTR. The difference with the original overdrives is that some circuit changes were made that are called "O.T.R." or "Oscillatory Transition Return".
"...the OTR new version simply has extra grid bias and >22uf decoupling caps."
Resources: a very friendly tech at Matamp
Pedro Lourenco was so kind to draw up an OTR schematic based on his 2002 OTR80.
This schematic is not verified, so use at your own risk.