Cornell power shaping

[mikeydee77]

Hi
Anyone have a picture of the output power switch wiring on a cornell romany Pro?

I have a very early one that appears to have been modded and wires removed messed with judging by poor soldering. I have gone back to the builder Denis Cornell but he has lost his computer files so I didn't want to add to his pain.

It's a two pole rotary switch.

[glebert]

What is the switch supposed to do? Does the builder not even have a schematic? If I was a builder and couldn't answer basic stuff like that I would offer to fix it for free.

[Jon Snell]

Sounds like a bit of reverse engineering is requied.
If you don't fancy it, you can send it to my workshop and I will take a look for you. Please pm me if I can help out.
What model is it? 10, 20 or 50Watt?

It's only an amplifier so there is not much it can be to get it wired correctly. It must follow certain rules of design and that won't be difficult to sort out. Especially as he has an Arbiter background.


It appears that Denis is only available through his linkedin account.

[mikeydee77]

Denis was going to take care of it but that would involve couriers or travel. He is great to deal with his email address from the website is operational. I think he has the info but in a big pile of hard copy. He has given me a diagram of the switch but it does not match what I am looking at.

The switch in question is one of those plastic rotary ones with 2 poles. 1 pole selects between different power transformer windings and thus gives four output levels. I think some one else has tried to modify it further to change the bias on the second pole as well but since they have left parts disconnected I can't be sure.

I am happy enough to figure it out but I thought someone might have a pic and be happy to share it out as would ensure that the amp is wired correctly.

These are well made boutique amps and well worth a look if you ever get a chance.

[Jon Snell]

This is all I can find at present.

The switch should have fibreglass spacers, not a plastic Lorlin type because they contacts have to deal with loudpeaker loads, ( 4, 8, 16R) by the look of it and the Lorlin type are only rated at 100mA which is not sufficient.
I would think the other pole is for negative feedback loop settings.

Have fun.

[mikeydee77]

Nice find on the pic..

But yes they are the lorlin or alpha rotary switches. It is switching windings of rectified dc so I suspect around 50v per contact though not measured as yet. Other cornell amps I have worked on did need the switch replacing so presumably operating on the edge of specs or perhaps relies on user switching to standby.

[Mick Bailey]

Does the preamp run at a fixed voltage? If so maybe the switch just drops the voltage to the output tubes' plates and screens.

[mikeydee77]

Hi Mick
Just been having a play... the contol just alters the power valves 6l6 supply only. Bias and pre amp all stay put.

418, 345, 341 and 339 in positions marked as A B C D on front panel. manual says that this is 50w, 5w, 1w, 0.5w but don't think this correct spec for this amp. It does work but is more like 20% full power on max reduction.

I think this is an early model given it has prototype marked on it.

Mike

[glebert]

I would suggest you post the diagram you were given, even if it doesn't match what you think your amp is built to, and some pictures of your amp. Your description is a little confusing, you said switching between multiple windings of rectified DC. I doubt there is more than one DC rectifier, so I am assuming these are different secondary windings off the power transformer and the switch chooses which one to go to the rectifier.If so, what AC voltages do you have off the power transformer? To get down to 0.5watts by reducing plate voltage on a pair of 6L6 seems like you are in cold clipper territory or lower. I think I've seen more amps scale down the plate voltage on the PI tube to get down to very low output power.

[g1]

Hi Mick
418, 345, 341 and 339 in positions marked as A B C D on front panel. manual says that this is 50w, 5w, 1w, 0.5w but don't think this correct spec for this amp. It does work but is more like 20% full power on max reduction.

Any time I see that kind of wattage labelling, there is usually a stepped PPIMV involved (laney cub head for example). I wonder if that could have been on the second pole?

[mikeydee77]

Fair point on the pic glebert...

Here is what I have a photograph of the switch as is... and the info from Cornell. The rectifier is a little 1n4007 on the end of the winding as it joins onto the rotary switch.

[mikeydee77]

Trouble uploading pics

[mikeydee77]

Here is the photo of the actual switch.

Pin 1 and 2 are the rectified windings
pins 2 to 3 are bridged with the 20R resistor
pins 3 and 4 are bridged with 11R (2x22R)

Feed from A pole heads off to the standby switch and then the AC input of the rectifier.

[Mick Bailey]

The diagram makes more sense than the picture. From the diagram the B+ is;

P4 100v tap dropped through 270R
P3 100v tap
P2 200v tap
P1 300v tap

Are the additional bridging resistors original? It seems to me that they're an afterthought or modification. The voltages in post #8 don't correlate with what you'd get if the amp was wired to the diagram and explains why there's not much power output change. I don't see the blue 100v connection in the picture - is it missing or has it been clipped?

[mikeydee77]

Hi and happy new year...

The resistors fitted don't look original in that the soldering, forming of the resistor legs is a little rough plus the wires insulation is a little melted.

The 20R resistor that looks like a regulator does look original though in that it sits neatly alongside the yellow caps. Subjective stuff I know. Rest of the amp is very neatly built.

Nope the blue wire is not there at all. so it doesn't match the diagram.

I think this model is an early prototype and someone has had a go at modding the output switch and kind of taken it away from whatever original was. looks like they have up part way though as the other pole does nothing just looks like a potential divider with nothing at the top of it either to adjust NFB as suggested by Jon snell or perhaps bias I was thinking.

It might be worth seeing how increasing the 20R and 11R resistor to get a wider range of output power levels. I was also thinking VVR might be a good solution.