5E3 vs. 5E3 clone PI

[Rhodesplyr]

Last year, I helped a customer debug a 5E3 clone he'd bought, built by someone else, that had really fizzy/blatty distortion related to the phase inverter. Someone one here recommended I put a 1Meg grid stopper in front of the PI, which calmed it down effectively

Now, I have a vintage '59 5E3 in the shop for minor repairs that has previously been almost completely rebuilt with new parts. It does not have the same PI distortion problem even though it has no grid stopper on the PI.

So, I have two amps, built 4-5 decades apart with the same circuit, similar parts, and similar layout, but one needed a grid stopper to tame the PI, while the other didn't. Both are populated with old-stock tubes.

What do you think would account for the difference? Transformers?

[Enzo]

I account for the difference by saying the added resistor never actually cured the problem, it just masked it enough to stop the symptom. In the same way that enough morphine will "cure" a broken arm. If you had some sort of instability or parasitics, adding that resistor may have pushed them to where they wouldn't happen, but the wiring dress or leaky part or whatever it may have been was still there. In the other amp that worked without the extra resistor, the conditions do not exist in the first place. In the first amp, we never found out what was wrong, we just found a way to make the problem stop.

If that resistor was necessary, then ALL 5E3s would need it.

[Rhodesplyr]

Enzo,

The clone was built with reasonably high-quality parts that I checked carefully, and I played with lead dress without any effect. Looks to have been a Ceriatone kit. Unlike the original, the clone's circuit is built on a fiberglass eyelet board that sits slightly higher off the chassis than tag board. Might that be enough of a difference to create an oscillation problem? If so, then "masking" it was likely the most cost-effective solution.

[Gingertube]

The Wiz has addressed this:
The Valve Wizard

He goes into additional detail in his book (RECOMMENDED) but the recommendations (based upon good theory and some practical experimental results) are

To make the cathodyne sound better when overdriving the output tubes and the cathodyne splitter itself:
1) Use a large grid stop (500K to 1M) on the cathodyne triode
2) Up the grid stops on the power tubes to say 47K (or even 100K).

It sounds like the respective overdrive levels of the cathodyne triode itself may be different:
Check to see if the 2 amps have the same output tube grid stops, that determines max loading on the PI outputs when driving the output tubes into grid current .
Check that they have the same global feedback arrangemet, that determines how much overdrive is demanded from the PI.

Cheers,
Ian

[JoeM]

I'm probably out in left field on this - but did you try reversing the OT leads? The 5E3 doesnt have NFB, but I'm wondering if some kind of coupling effect might still be happening and caused some oscillation that was damped by the grid stopper you put in.

[Wombaticus]

I account for the difference by saying the added resistor never actually cured the problem, it just masked it enough to stop the symptom. In the same way that enough morphine will "cure" a broken arm. If you had some sort of instability or parasitics, adding that resistor may have pushed them to where they wouldn't happen, but the wiring dress or leaky part or whatever it may have been was still there. In the other amp that worked without the extra resistor, the conditions do not exist in the first place. In the first amp, we never found out what was wrong, we just found a way to make the problem stop.

If that resistor was necessary, then ALL 5E3s would need it.

The issues with the traditional cathodyne going squirrely when overdriven don't have anything to do with instability or parasitics. In fact, it's exactly what theory predicts should happen when either the cathodyne's cathode output causes that output tube to pull grid current, or the cathodyne itself is overdriven hard enough that grid current is induced in the cathodyne itself. As Gingertube pointed out Merlin has covered this ground comprehensively.

Personally, I haven't yet encountered a cathodyne that didn't benefit from Merlin's suggested mods (humungous grid stop and larger power tube grid stoppers), and I think that with these in place many of the objections to the cathodyne really go away and make it worthy of serious consideration even for bigger bottle amps.

Incidentally, it's worth reinforcing that the input impedance of the cathodyne is so high to begin with, and the gain is so low (unity), that you don't really get any significant attenuation of signal generally, or of trebles specifically when you employ a humongous grid stop.

As to to why the two amps are behaving differently, it may be that the tubes have aged differently and the better behaving one has tubes with a bit lower transconductance, so it's a bit mellower and the cathodyne isn't being smacked as hard. On a related note, I certainly notice a difference in the overdrive character between using the original lower-gain 12AY7 in V1 vs. a 12AX7 as many people nowadays employ, with the 12AX7 I think resulting in the PI from just being hit too hard and getting a bit yucky, especially when the PI has not been modified.