27 and 37

[Paul Barker]

Interesting, I was pulling out my 27's for testing to try them as VA and also as driver, so looking for four. Found 7, all balloon, all mesh.

But wait a minute two of them are 37.

Quickly look them up, they are 6.3v. But also from the curves not exactly like the 27.

When you hold up the print off of each one in front of the other in the light, the 37 has greater 2nd harmonic distortion.

So I have decided to stick with 27.

Follishly as soon as I discovered I had a pair of 37 I went and ordered another two from eBay.

that's just the sort of impulsive valve buyer I am.

anyway I am sure the 37 is pretty much the same, and a lot better than most other valves. I suppose depends which filament supply you have on hand.

[IslandPink]

I know Jono and Mike Carrick are both using the 27 in pre-amps. I must try to hear one some time.

[Paul Barker]

I listen to it once before and it was nice. That is why I put some away for later. Later has arrived. They were a cheap mesh plate, still are actually.

We'll see what the sign wave shapes up like.

The curve tracer definitely earns it's keep planning at this stage in design, because you can see what your actual valves will behave like in the exact conditions you plan for. I offered the 27 charts up to the light with the 37 charts on top of them so you can see the curves of each in same plane. That revealed that the 37 on your left hand voltage swing stretches the gap greater than the 27. So 2nd harmonic for the 27 is superior. The gap at the right side is the same for both valves. It is something you would usually not see. Subtle but significant. Very stupid of me to order another two 37's as soon as I discovered they were more or less a 6.3v 27 prior to this discovery.

The distortion cancellation mechanism of cascading two distorted valves doesn't seem to hold water. I have never really fully got my head around it anyway. That because, the second valve has significantly greater voltage swing. The position on the curves of each valve influence the effectiveness of the technique. That means it is quite posiible that a 37 in the first stage may better cancel the distortion of a 27 in the second stage, or vice versa. So there is either an exercise to be had where the actuall portion of the load like for each valve with relative AC swing needs careful analysis. Or just switch positions and see what happens to distortion on the instrument.
In short the 37's may have their day.

I don't feel energetic enough to build the Kimmel stage at this moment.

Just need something that doesn't go to 8% distortion before 2 watts, but without having to resort to depletion mosfets for the VA and driver before they shape a sign wave accurately.

[Will]

I know Jono and Mike Carrick are both using the 27 in pre-amps. I must try to hear one some time.

Me too..I found them not bad a bit thin..fragile sounding on the other side the 26 is smooth creamy the battery valves I use are in the middle.
Thin custard

.

[Paul Barker]

Thank you Will, I know what you mean, I would prefer to call them gentle, delicate.