Many questions about tubes

[AndyPanda]

I've got several USA Tungsol 6550 tubes from various Leslies etc. I've owned over the past 60 years. I'm curious about the different construction (shape and location of getters) and if they are OK to mix.

One pair dated 1958 with two D getters on same side and another on top and solid black plate.
a single from 1954 with all the getters on the top and solid plate
a pair with round getters, one on top and two on opposite sides and a single RCA branded that looks identical except the getter flash (am I saying that correctly?) has turned milky/cloudy.

For amps, I have a 122 with SS rectifier and a 22H with one 5U4GB rectifier and a 50C with two 5U4GB. All have fresh caps and resistors.

Sorry for asking so many questions in one post - maybe I should break it up into several posts? Anyway here's what prompted all this. The 50C has the highest plate voltage - higher than spec, about 410 between pins 3-8 (435 to ground - dropping 25 across bias resistor) and has the strongest 5u4GB tubes. The 122 has voltage a bit over spec but not quite so much as the 50C (lines voltage is extra high here where I live). Both those amps work really well with KT88 (I'm thinking of getting KT120 just to be safe - or is that silly of me?). And my 22H has a fairly weak 5u4gb tube and the plate voltage is lower than spec around 360v between pins 3-8 (about 385 to ground).

The pair of USA Tungsol with round getters - they both test very good and sound wonderful and match each other really well. But on either of the amps with higher voltage, one of the two tubes works great for 10 minutes and then the plate voltage starts to drop rapidly and the tube starts to heat up (and I shut the power off before it redplates). This happens to the same tube in either socket and in either amp. But in the 22H with the weak rectifier, that tube has no problem and runs perfectly. So I'm enjoying running those tubes in the 22H but wonder how long the rectifier will hold up and continue putting out the lower voltage.

So I guess my questions are:
Why did Tunsol make the different constructions and is one design superior to the other? Do they all have the same ratings? Can I mix them or best to keep similar construction as a pair? (in which case I need to find an orphaned "all top getter, solid plate" or sell my orphaned one.

How long will my 22H continue working with that weak 5U4? Does it indicate being worn out and will it continue to drop in voltage and die?

I was thinking of ordering KT120 tubes for the two amps with rather high voltages - though right now I'm running KT88 without any trouble and sounding very good.

I suppose I could just put all my Tungsols on eBay and run KT88 or KT120 in all my amps and a fresh rectifier in the 22H? Or should I try to drop the voltage on those amps that are high?

And finally (I sure am Mister Curious aren't I?) what would cause the condition of that one 6550 testing good and working/matching well with its twin for 10 minutes and then suddenly going bad (when the voltage is around 400-410)? But working perfectly when the voltage is 360-380?

Thanks in advance for edu-ma-cating me on this stuff!

[AndyPanda]

I also tried that same pair of Tungsol in the 22H using one of the stronger rectifier tubes giving 385 volts across pins 3-8 of the 6550 (right about spec per the schematic 410 to ground). And had no trouble with the tube. I guess in the 122 with the SS rectifier and the higher line voltage in my city it is just enough too high to take that tube over the edge. Anyone retro their 122 with a 5U4GB?

[luckydog]

My there's a lot of questions, I can have a go at some of them.

Faulty tubes: (a) the one with the cloudy/milky getter coating has lost its vacuum (b) the one that runs away after 10 mins prob has one of the grids sagging when it gets hot - or there's a chance that a 1st grid coupling capacitor has a fault and won't tolerate the higher dc levels with the new rectifier but prob not. (c) a weak rectifier tube is probably not long for this world so unless you love the sound it's best to change and avoid the small risk that it fails inelegantly such as stripping the cathode and shorting. These tubes are junk.

Different getter shapes and locations imply that the general internal layout of the tube changed, and that's unlikely just to be the getters. More likely changes to grid carriers, winding and spacing. So although various tubes will have 6550-like curves, they might not be well matched dynamically or even at static conditions. Whether this is sonically good or bad or not different is a matter of trial and is notoriously difficult to be objective about. So it's what you personally like that matters I suppose - they will all work in mix and match configs. It's elegant to have matched tubes though.

KT120 doesn't get you benefit in reliability or life, in principle, in these circuits. Whether they sound different, refer to the point above.

Those B+ voltages though on the high side for this circuit are OK and prob no benefit from changing it - it's normal to get mains variation depending on where you live. 6550 handles far more than that in other circuits.

HTH!

LD

[AndyPanda]

Thanks luckydog ... that is very, very helpful!

[AndyPanda]

I just noticed that the power transformer on the 22H is considerably larger than on the 122. I wondered why and then I realized that the 5U4GB rectifier draws 3 amps for the heater. So I think I just answered one of my questions about retrofitting a 122 with tube rectifier - the heater supply wouldn't be able to handle it.