Orange AD 15 Combo ...

WAG A poor ground on one of the output bottles !

You could try checking the grounding arrangements. I've come across weird hum problems caused by someone having replaced an input socket with one that grounds to the chassis where it shouldn't, or vice versa, and also amps that rely partly on grounding through the control pot bodies making good contact with the chassis, where the nuts are loose or again a different pot has been fitted. Try running/cutting ground links to and from various points, particularly around the input.

Does it have a reverb tank? Fitted the right way round?

Gareth.

is

No

The grid has to have a DC path to ground, otherwise the coupling cap won't have a path to charge and discharge. Eventually , the tube will be cutoff from the stream of electrons charging the grid negative There has to be a resistor from grid to ground or perhaps the wiper is not coupled through a capacitor. In any case , I don't think think that this would be the cause of your feedback problem.

All four HT rails have their own R-C

the

The decoupling RC networks prevent audio from passing through the B+. The high voltage is common to several stages, if audio couples from one stage to the other through the B+, the amp will break into oscillation.The ripple might look low but when the amp breaks into oscillation, there is a good possibility that an audio signal is present on the high voltage. Many cap meters only test at power line frequencies. It is possible that the decoupling caps are not working as caps at higher audio frequencies, and it is also possible that the caps might be borderline. I would substitute those caps with any junk laying around just to rule them out.

Another possibility is that the ground for the preamp and the ground for the power amp might share a common path. That path might haved aged to a point where it is no longer well grounded. Tighten any hardware, like jacks , grounding screws etc... If possible, seperate the two grounds by running seperate wires from those stages back to the negative terminal of the power supply.

bg

Hi Gareth. All regular stuff, and what both of us come across all the time, I'm sure, so basically all already checked. Everything looks original. The input socket is on it's own little sub-pcb, and there is no evidence at all that it has ever been disturbed. This is connected to the main pcb by a short piece of screened cable, again undisturbed from factory original. All pot bodies are making firmly to the chassis, and no mounting hardware is loose. There aren't really any external ground links to cut, and adding grounds doesn't seem to make a lot of difference. Caveat. There is a difference when a mains protective ground is connected, against when it's not i.e. with it running on a standard bench isolation transformer. It is better when the ground is on - that is with the amp plugged into raw mains. Second, it is worse when the 'scope's probe ground is hooked onto the chassis. Both of these observations suggest a ground issue, but I'm still tending towards poor design, rather than a fault as such. No reverb tank fitted. Thanks for suggestions.

Arfa

Yes, absolutely true. I didn't put it very well. The grid has a DC return path, as you say, via the gain pot. The stage does not have additional AC coupling between the pot and the grid, with a separate fixed grid leak, as you might reasonably expect. So if the gain pot is at minimum, the grid will actually be DC grounded.

That is a possibility. I checked the caps with an ESR meter, which operates at an equivalent frequency of around 100kHz, so I was not just declaring them good by value, or the fact that there was little in the way of 100Hz ripple. Just as a matter of interest, none of the individually filtered HT rails are 'shared' as such by more than one stage. First filter nearest the reccy, feeds just the output anodes. Second, just the output screen grids. Third, just the phase splitter and fourth just the preamp valve, although I suppose that you could argue that represents two stages. I'm pretty sure that when it was howling, I did 'scope the four HT rails, and there was nothing significant on any of them, but perhaps not. It's just the sort of check that you make automatically in these cases without even thinking about it. I'll take a better look at this angle on Monday.

Not especially easy to do, as the power supply is all part of the same PCB with the whole of the rest of the amp. There are no signs that any hardware is loose, or that any mechanically made grounds, such as via pot bodies etc, are not good. I repair a lot of valve amps for a living, and have done for many many years, so these are all suggestions that are basic second nature to me, hence why I'm finding this an 'odd' problem. Still, thanks for all the suggestions anyway.

This is one of those situations where you would really like to have another one to compare by, which is why I asked if anyone had any direct experience of this amp.

Arfa

Well if I had a penny for all those pieces of equipment I'd tried to find the fault on that actually had no fault but was some sort of shitty/quirky/misunderstod design shennanigan, I would have several pennies in my pocket and lots more time on my hands to spend on finding faults that actutally weren't faults but some sort of shitty/quirky/misunderstood design shennanigan ...........

Gareth.

Where is the mains ground connected into the circuit ? checked for mains transformer P/Sec/ground leakage ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

LOL. Yeah, likewise ! This trade sure doesn't get any easier, does it ...? d;~}

Arfa

Good thought. I'll have a look tomorrow. Thanks.

Arfa

I'm inclined to agree that the problem is "bad design".

When J Gordon Holt reviewed the classic Futterman output-transformerless amplifier, he reported a hum problem when using a Decca pickup (which had a common L/R ground), despite the fact that the pickup was "isolated" from the amplifier by the preamp! Amazingly, Julius Futterman tracked down the problem and fixed it.

PS: I would love to hear the Futterman. I've heard systems with later versions, but never the original in my own system.

news:gml4st$f3k$ snipped-for-privacy@news.xmission.com...

won't

not

will

about

point

those

etc,

Oscillation means feedback. There has to be a feedback path, and the gain has to be high enough to sustain oscillation. I found a web page that mentioned something about a 1000p cap placed across a plate load resistor specifically to avoid oscillation. The schematic didn't appear to be from an ad 15 combo. Some people were removing that cap as a mod to get a brighter sounding amp. Another thought - the schematic I downloaded for this amp shows a connection from the 2nd stage grid back to the first stage input jack. It appears that the purpose of this is to ground the second grid when nothing is plugged in. If something is a bit bent in that jack, it might be possible for this 2nd grid to be connected to the 1st grid, which will make a feedback loop when the volume control is turned up. Positive feedback will also increase the noise. Something has to have changed because I don't think they would have been able to sell a crappy design and stay in business.

Good thoughts. I will check them out Monday. Interestingly, I did try hanging 1000pf across both of the load resistors, and they did produce a significant improvement, but not enough to actually totally stop it hooting. You could also definitely hear these caps starting to knock the 'brightness' on the head. As you can now see from the schematic, one or two aspects are a little 'unusual' compared to the norm - that additional connection for grounding the second stage grid for instance, or the dual gang master pot.

I must admit that if it is down to poor design, the effects do seem a little extreme, even given that the "Orange" name is just a badge, and it *is* a Trace Idiott design ... It's certainly not the best constructed item I've ever seen. Thanks all for the continued interest and suggestions. I'll post back if I get anywhere further with it as a result.

Arfa

brighter

hooting.

'brightness'

little

I tried to find the website again but had no luck. The cap appeared to be connected across r4 of the phase inverter. You could try yanking V2 out and see what happens with the gain cranked up.

Hmmm. Interesting. I'll give it a go, and let you know.

Arfa

m...

hat

ged

oop

ppy

are

ot.

I've

be

out

Hi, Sorry to butt in but did you ever find the problem? I have an AD30 with similar symptoms and would be very grateful if you could let me know what you discovered and what you had to do to rectify it.

Many Thanks

Thomas