Replaced power transformer, now my amp is really quiet

[kkatarn327]

Hey all, new to the forum, but I've been reading stuff on here for years, great forum.

Anyways, I have an Orange Rocker 30 head. Long story short, the power transformer fried a while back and I just got around to installing a new one (Axiom ORP-30 from Mercury). Everything seems to be working fine except that the output volume of the amp is significantly lower than it was before. Other than this, it works fine: it's not noisy, the EQ dials are responsive, the gain and volume controls are fine and work properly, the amp isn't overheating, even the tone and signal breakup are normal (despite the volume drop). I checked all the fuses, I tried using all the output jacks, tried different power outlets and even different locations, different speaker/power cables, and I even bought all new tubes for the preamp and poweramp but it is still very quiet.

I checked all the PT primaries and secondaries and even the power on the mains and everything tests out fine. No components show visual signs of damage (burned out resistors, bulging caps etc) and the ceramic resistors test out correctly. I'm beginning to think that whatever caused the PT to blow in the first place is now the culprit.

One more item potential importance... when I first opened up the amp after taking out the old PT, there was some visible burn damage. The black OT primary had a burn in it severing the lead entirely, and it left some charring on the other two primaries (yellow and white I believe, although the burns didn't break the insulation so they were still very much intact) and also a very noticeable burn mark on the inside of the chassis. I repaired the black lead and other than what I mentioned there was no further visible burn damage.

I suppose the next step would be to test all the caps but I'm waiting on that for a couple of reasons... first of all the way the amp is built it's kinda hard to access the bottom side of the PCB where I could test the caps. Also, I would think that if a cap was the problem there would be more issues. It's astonishing how perfectly the amp works despite the volume drop: no excess noise, no scratching or popping, no sudden drops or surges in volume, nothing. I'm starting to think it's something pretty simple that I'm just overlooking.

Can anyone offer any insight at all? I'm willing to try anything. The amp cost me a bundle and I've dumped a ton more money into it trying to fix it and I just want it to work. Any input at all would be greatly appreciated.

Thanks!

[Euthymia]

If all of your voltages look good, my first guess would be damaged OT.

It is possible for an output transformer to be damaged in such a way that it will result in the symptom you describe. Given that you witnessed a catastrophic failure of one of the OT primary leads, I'd say the chances are even better.

I don't know what equipment you have available, but you can test the turns ratio of a transformer fairly easily by applying 6.5VAC to the primary and measuring the AC voltage on the secondary.

I would first disconnect it from the circuit and measure the DC resistances between the primary leads and the center tap. If they are way different, you know something is wrong.

[kkatarn327]

If all of your voltages look good, my first guess would be damaged OT.

It is possible for an output transformer to be damaged in such a way that it will result in the symptom you describe. Given that you witnessed a catastrophic failure of one of the OT primary leads, I'd say the chances are even better.

I don't know what equipment you have available, but you can test the turns ratio of a transformer fairly easily by applying 6.5VAC to the primary and measuring the AC voltage on the secondary.

I would first disconnect it from the circuit and measure the DC resistances between the primary leads and the center tap. If they are way different, you know something is wrong.

Thanks for the reply. Unfortunately I don't have the means to do an AC check on the transformer. When checking the DC resistance however, am I looking for a certain range or just consistency between the taps?

EDIT: Just checked DC resistance across all the OT taps (3 per side). On the primary side, there is roughly 60 ohms of resistance across any two of the taps, except between the output of V5 (the second of two EL34s... thought I should mention this is also a class A amplifier) which shows about 3 ohms. On the secondary side, there is about 1 ohm of resistance across any two of the taps. There is no continuity across any primary to any secondary.

EDIT (again): Just realized that the primary from the output of V5, the one that tested out less resistance than the others, is also the lead that burned through. Case closed?

[Euthymia]

Just checked DC resistance across all the OT taps (3 per side). On the primary side, there is roughly 60 ohms of resistance across any two of the taps, except between the output of V5 (the second of two EL34s... thought I should mention this is also a class A amplifier) which shows about 3 ohms.

Need clarification: what is the DC resistance between the outer leads of the OT primary (the ones that connect to the power tube sockets)? What is the resistance between each of those leads and the center lead (the one that connects to the B+ supply)?

[kkatarn327]

DC resistance between

V4 output and center lead: roughly 60 ohms
V5 output and center lead: exactly 3.5 ohms
V4 output and V5 output: roughly 60 ohms

As I mentioned, I also just realized that the lead that burned through was the primary tap from V5 output.

[melvin]

Sorry to say it, but it sounds like your orange rocker 30 is has a push-pull output section and one side of the output transformer is indeed shorted. No one can argue that it's not class A now!

[kkatarn327]

Sorry to say it, but it sounds like your orange rocker 30 is has a push-pull output section and one side of the output transformer is indeed shorted. No one can argue that it's not class A now!

Yeah you're definitely right. It's weird though because the Orange website blatantly advertises this amp as class A operation. Cathode bias and class A operation are NOT the same thing