Rebuild Orange OR120 - Questions about Voltages and PT

[nemoi]

Hi all, hope you're doing alright! New member here and already looking for advice 

I just finished restoring a '75 Orange OR120 that I bought earlier this year sight unseen. It arrived well and worked fine but still decided to check its guts and bias and discovered quite a mess, to say the least. Burnt goo around 2 output tubes, PCB burned through around the rectifier diodes, burnt wires, insulations with lots of soldering iron kisses, pretty sloppy lead dress... enough for me to take it apart completely and start with an empty chassis.
Stupidly, I didn't take any measurements before disassembly, but I did draw up a "layout" plan as I took it apart.
Fast forward, I finished all wiring, followed a startup procedure with my trusty light bulb limiter and everything seems to be quite alright - by that I mean no shorts, sparks, flames, fireworks, blown fuses, smoke etc. - which I'm happy about.

I set the bias at 63% max dissipation and took the following measurements:

B+ directly after rectification: 407V

V1:
P1 ---> 171 V
P3 ---> 1.117 V
P6 ---> 158 V
P8 ---> 1.232 V

Fil P4 to P9: 5.9V*

V2
P1 ---> 223 V
P2 ---> 97.9 V
P3 ---> 99.1 V
P6 ---> 97.9 V
P8 ---> 0.517V

Power Tubes
402-407V on all 4 Plates (P3)
401-405V on all 4 Screen Grids (P4)

Fil P2 to P7: 6.1V*

*I only took filament measurements at the end of each line - there's seperate PT windings for preamp and poweramp... bit lazy, but taking measurements inside a hot chassis is not something I enjoy all too much :p

So basically everything is working and the amp sounds good. It's getting quite hot, but I guess that's in these amps nature...

I mentioned quite alright earlier, because I'm only getting 407 Vdc B+ at the bridge rectifier and doing some research it should be north of 500V there. Even the only schematic of an OR120 with readable voltages (taken in '74) suggests 480V after the first filter caps... I'll attach a pic of the schematic at the end.

Which brings me to my first question: Am I risking damage to anything with the low B+ I'm having currently? Just asking out of curiosity and to help me learn and understand.

During the makeover I noticed another set of 3 cut & insulated wires coming out of the secondary of the power transformer. My guess is it could be another HT winding with another center tap. Stupid me listened to bad advice and didn't take measurements while I had the PT out... it's definitely a mistake I wouldn't do again but too late now.
Anyway, if I wanted to test this now that all is hooked up - can I just go ahead and measure Vdc between these 3 wires or do I need to lift all PT secondary wiring? I guess if I just measure resistance between these wires it wouldn't be enough to say that's definitely a hotter HT winding?

One last point: There's a noticable buzz coming from the PT - a tiny bit of that is also audible at the speakers if you listen real close, but mainly it's coming from the PT itself. The moment you start playing at any volume it's not audible anymore, but since I never heart a PT doing this I thought it's better to mention. I did some research on this subject and apparently tightening the screws directly on the transformer can sometimes solve this issue. Since I haven't powered the amp back on I can't say if it helped...


I'm really sorry to leave such a bible here, but I thought it's better to give you all the details I can think of at the moment. I'm sure I forgot something, so please let me know if you wanna see any photos, close ups or have some other questions.

So once again, hi to all from my side glad to be here and thanks for taking the time to read all that :p

[sluckey]

I noticed another set of 3 cut & insulated wires coming out of the secondary of the power transformer.

Most likely the three unused primary leads.

[nemoi]

I noticed another set of 3 cut & insulated wires coming out of the secondary of the power transformer.

Most likely the three unused primary leads.

Primary side is all hooked up the way it was when I received it - 0V /110V / 200V / 220V / 240V + 2 grounded wires... as I said it was stupid to not take any measurements while it was out so it's a gamble assuming it's correct. But since the PT has its wiring outlets on 2 sides, I think it's pretty safe to assume one is primary, one is secondary...
But many thanks for the fast reply!

[PRR]

> Power Tubes
> 402-407V on all 4 Plates (P3)
> 401-405V on all 4 Screen Grids (P4)
> I'm only getting 407 Vdc B+

I'm wondering where are the numbers for Signal Grid and for Cathode.

Yup, the cathode may be zero now but you SHOULD put a 1 Ohm resistor in series with each cathode when you remodel an amp. It is the only good way to be SURE each tube pulls a fair share (and no more).

The note says G1 bias should be -37V. Maybe with 1972 tubes, but it can vary a LOT. Anyway you don't report G1 voltage. If it is, say, -15V, each tube may suck 150mA, sag the B+ and the heater feeds, overheat both tubes and power transformer, a race to the death. (If it were zero volts, a very easy thing, you would probably be reporting smoke.)

[nemoi]

thanks for your reply, I probably should have mentioned:

I use an adapter that I put between socket and valve that has a 1ohm resistor wired internally on Pin8 to take the readings you need to calculate your desired bias setting. I did that on each output tube pair.

-35,9 - -36.1V on all 4 Signal Grids (P5)

so probably a bit on the hot side I guess?

ps.: tightening the bolts on the PT housing helped reducing the noise. There's still a little bit of noise audible but much better already...