OR120 Pix Only Help!

[shiner555]

Hey All,

I recently picked up a 1972 Pix Only OR120. The amp had all of the original lydic's in it and most were blown. I have replaced all of the lydic caps in the system, and also did the AC input mod to run on 120V. Amp doesn't hum anymore, but it now does something that has me totally stumped. When you turn on the amp, and turn the volume up you may or may not hear a slight hum. If you don't hear a hum, you do not get any audio output of the amp. When you do hear a hum, you can plug in a signal and get an output. However, if you turn the volume up to about half way (pretty loud for this amp) and hit a big open chord on the guitar the audio quickly fades out, things go quiet for a second and then it comes back up. I can get the same thing to happen if I add a little volume and click through the bass boost selector. The switch "pops" when you click through the settings sending a transient into the circuit and you hear the audio quickly fade out for a min and then come back up.

I monitored the power supply voltages when this would occur, and they remained constant, no drop outs when the audio does. I checked this at each of the filter caps (485VDC on the main caps), the output tubes, and the mid point of the output xfmr (485VDC). Nothing drops out when the audio does. I also checked the bias output (-47VDC), and it remained constant when the audio would drop out.

Possibly I cracked a trace on the PCB when I was re-working the amp causing it to cut out?? Any other ideas of things to check out? This one has me stumped pretty good.

I didn't replace any of the DC blocking caps........

http://www.orangefieldguide.com/OFG_...20schem_72.gif

[Chuck H]

Sounds like a bad trace or a cold solder joint to me. Verify tube conduction by doing a proper bias current check.

[shiner555]

Yep, couldn't sleep last night so I woke up early and spent more time on it today. Basically re-did almost every solder joint on the PCB, and the wires attaching to it. Also re-did some of the joints on the output tubes. Cleaned all of the pots, jacks, and tube sockets. Checked all of the tubes on the tester, no problems. Checked bias on all of the tubes with no drive signal. All sitting at around 25mA. System is fine at low volume, but when you crank it up to about half way/noon, the audio will drop out/fade out quickly for about 1-2 seconds and then come back up. Power supply voltages are still reading fine when this occurs.

I'm going to try to start to hunt/monitor the AC signal in the pre-amp when this happens with a scope to hopefully find the smoking gun.....but this still has me puzzled big time.

I'm making a big assumption that there is nothing wrong with the iron in this amp........is that a bad call?

Any other ideas are greatly appreciated!

[Chuck H]

So, if you're testing the power supply when the problem occurs I assume the problem happens with the amp out of any cabinet and away from the speaker or even into a dummy load.?. That would pretty much indicate that it's an electrical or electronic issue rather than a mechanical failure.

I'll also guess that since the amp had bad caps when you got it that you never fired it up before any of the maintenance work you performed.?. That is, do we know if the amp always did this? or did this problem start after you worked on it?

Could be a funny tube.

[Jazz P Bass]

Yeah, go ahead with the AC signal tracing.

That will define which circuit is the issue.

[g1]

Another thing to check, if the fade in and out takes a few seconds, it almost sounds like one of the heaters could be dropping out. Have a look at it in the dark and see if they all stay lit when the sound fades out.

[shiner555]

Another thing to check, if the fade in and out takes a few seconds, it almost sounds like one of the heaters could be dropping out. Have a look at it in the dark and see if they all stay lit when the sound fades out.

Good suggestion! I'll be back at my bench in a few hours...so I'll report back.

If the heaters are going down for some reason, any ideas on a solution for that? They are direct off the AC xfmr.

[g1]

Rather than the whole heater supply, I was thinking more of any 1/2 of a preamp tube. If the sockets are board mounted, solder issues are often the cause. But as this is an old amp, I doubt the preamp sockets are board mounted. Still can be solder at socket, or dirty socket, or loose/broken socket pin.
Looking for a heater going out when it cuts out is just a quick check before doing the signal tracing the others mentioned.

[shiner555]

Rather than the whole heater supply, I was thinking more of any 1/2 of a preamp tube. If the sockets are board mounted, solder issues are often the cause. But as this is an old amp, I doubt the preamp sockets are board mounted. Still can be solder at socket, or dirty socket, or loose/broken socket pin.
Looking for a heater going out when it cuts out is just a quick check before doing the signal tracing the others mentioned.

I seem to have tracked the problem down to the PI circuit. When I send a burst signal into the amp, it goes quiet and the AC signal disappears coming out the top of V2 at pin 6. The waveform also looks all kind of distorted (this can be seen once the signal ramps back up). I did not see this distortion or loss of signal at V1 (for the most part). However, there was one time where I was able to get V1 to go quiet as well. Really strange thing is that when the amp is in it quiet state, if I take the scope probe and touch somewhere along the AC path I can get the audio to come back on. In the cases where I observed the audio dropping out at V2 I had the scope already clipped on.

I've gone and replaced all of the resistors in the PI circuit. Gonna get some new DC blocking caps on order as well. After putting it back together tonight I got a constant distorted sound at a low volume. I then noticed that I was not getting any DC voltage on pin 1 of V2. It then dawned on me that I had put the new 100k Plate resistor where I had removed the line out! DOH. I also noticed that the line out had a resistor across the jack, and that with me just removing it from the circuit, I need to put that resistor back in the circuit path. I can't find the line out anywhere in the schematics, and I didn't have time tonight to trace where it is coming from, but I wonder if this has anything to do with my issues.......I'll have to see tomorrow when I put things in their correct place.

Thanks for the suggestions to date!

[Chuck H]

I think tracking that line out and finding out what that circuit is doing/affecting is a good idea. Because the signal comes and goes, occasionally and on the first or second stage when the scope probe is applied, I'm thinking the scope probe may be stabilizing the amp enough to pass signal again. That is to say that you may have an oscillation. Have you checked current through the amp when the problem is occurring? If the current is indicative of conduction then an oscillation may be the problem. It could be in the mystery line out circuit or possibly a bad ground connection or failing decoupling capacitor.

[shiner555]

Yes!!! Finally got it figured out. The scope probe should have tipped me off like you said. When tracing back the line out, I realized that it was in series with the HF Drive. When I removed it from the circuit, I payed no attention to the resistor that was on the output jack. When I removed the line out feed, I broke the feedback loop from the cathode of V2, making V2 unstable as you mentioned. I also ended up replacing most of the resistors in the V2 circuit. Once put back together properly, the amp is now happy as a clam!

Many thanks to everyone for the help. I just wish the line out was labeled on the schematic..would've saved me a bunch of trouble....but hey I now know the circuit really well.

BTW, any suggestions on the proper bias point for this amp? I have the numbers off of the weber chart which call out 36mA for the plate voltage I'm seeing 460VDC. However, i was told to be real careful with running these things hot as they burn through tubes.

[g1]

36mA at 460V would be 16W. For a 25W EL34 that means 64%, so not terribly hot, should be ok I would think.
Watch for red-plating at over-driven full power, if you see any you can set it colder.

[The Dude]

I rebuilt an OR120 a couple of years ago for the local GC manager. He picked it up at an estate sale for peanuts. It was apparently the estate of either a tech or someone who liked to dabble- electronic doodads everywhere. Many of the parts in the amp had been removed- almost like the old guy was using it for parts. The OT was missing, several jacks, and other assorted parts. I got the thing running and I remember thinking, while testing it, that it was one of the loudest amps I'd ever played through. If you get to red plating that amp, you better have ear plugs in!

[Chuck H]

I have an amp in for ???? right now that's running a pair of Ruby EL34's at 460Vp biased to 20W gross dissipation. You can knock a couple of watts off assuming the screen is drawing some current. In the spec sheets it looks like screens at or around 450 will be drawing about three watts. So at 17W plate dissipation I'm having no trouble with the Ruby's. Under these same bias conditions (adjusted of course) I've also tried a pair of JJ EL34's and a pair of EH 6L6's. All tubes ran fine with no red plating at 20W gross dissipation. I chose the Ruby's because they seem to cooperate best with the bias wiggle trem in this amp.

My point is that 16W gross idle dissipation should almost certainly be fine. Watch the screens if the amp doesn't have individual 1k screen grid resistors. If it doesn't you might want to replace the existing value or add individual 1k screen resistors. The screens can often end up over dissipating on higher voltage EL34 amps under heavy clipping if not fitted with them. Especially with new production tubes.

[shiner555]

Cool, thanks for the comparison numbers. I'm going to have to see what sounds best between the 16W and 20W levels. I don't get how Orange says this amp is 120Watts.....maybe peak, but no way RMS with only 4 EL's.

Would I be insane to change the output to KT88's on this thing? I know for re-sale that would be suicide.....however, anyone know if the xfmrs can handle it?