Supro build, more breakup.

[Pryde]

Grid-leak bias is kind like Bigfoot from what I've read. Everybody has an opinion, but nobody's ever seen it. I applaud your experiment and the clear results you returned. So we have your confirmation that this kind of bias is hugely dependent on the tube. More so than I would have ever guessed. Thanks

It really is a remarkable difference between those preamp tubes but I suppose being that it is a simple circuit the tubes probably should make a big impact. The power tubes did as well: I first used new production EH 6973 and they really didn't sound nice at all, very harsh. Then some RCA NOS 6973 really made a great difference, warmer and better all around. Great sounding circuit but definitely picky with tubes.

[DRH1958]

Westinghouse didn't have a factory in England as AFAIK. This is probably a Mullard rebrand. There was a lot of that going on back then, still is. The fact that you got the bias UP to -.76 helped as the -.49 was too low for the tube to get going. And you also found that NOS 6973 older tubes sound better. I have found this to be true in all my older amps I have rebuilt/modded. Just something about those '60 vintage tubes. I have found they don't make quite as much difference in newer, higher gain amps though. You might want to find some more Mullards. I have found used tubes are just as good as NOS most of the time. Those old tubes just seem to keep going and going.... Here is the way to tell if you have a Mullard. The "B" stands for Blackburn factory; Blackburn, England. That is the key. Tube enthusiast: know your tube codes. | Effectrode Glad the amp is giving it up for you.

[uneumann]

Well I think I will call this one resolved

I tried bypassing input resistor, it helped marginally with a bit more volume.
swapped in 1M input resistor to ground. Got some more volume and breakup but it was pretty harsh sounding so went back to 100k.

Got into my pile of NOS 12ax7s. Threw in another Amperex bugleboy and hey instantly some more breakup. On the right track I swapped in different ones with varying results until I put in an old Westinghouse (England made) and WOW there it is just like that. Actually it is getting probably a bit more breakup even than the video. Stays clean til 4/10 then starts breaking up great with fantastic compression and growl at 8/10. Sounds so nice I will leave it here.

The westinghouse is measuring -0.76v on the grid compared to -0.49v with the amperex. Apparently this is the sweet spot. Crazy I tried new production and a few NOS tubes with not great results then this old beat up one is a gold nugget

I appreciate all the great suggestions, the important thing is that I learned a good deal more about these funky grid bias preamps so for that I thank you.

You're Welcome! I've swapped tubes in a grid-leak bias input stage in the THD Flexi-50 for some time. Don't know why these stages get such a bad rap. They are tube-sensitive, but we swap tubes all the time trying to get tone variation in other circuits that barely respond. What's wrong with a stage that's virtually designed to produce tube-related tone variations?

[Enzo]

When the amp was designed, they were not into all this tube swapping for subtle tone nuance. They expected you could go to the drug store for a new tube, plug it in, and it should work. It ought NOT be tube sensitive. We swap preamp tubes all the time looking for the just so overdrive or whatever we so crave. But the amp still works reasonably with most any 12AX7.

[Pryde]

Westinghouse didn't have a factory in England as AFAIK. This is probably a Mullard rebrand. There was a lot of that going on back then, still is. The fact that you got the bias UP to -.76 helped as the -.49 was too low for the tube to get going. And you also found that NOS 6973 older tubes sound better. I have found this to be true in all my older amps I have rebuilt/modded. Just something about those '60 vintage tubes. I have found they don't make quite as much difference in newer, higher gain amps though. You might want to find some more Mullards. I have found used tubes are just as good as NOS most of the time. Those old tubes just seem to keep going and going.... Here is the way to tell if you have a Mullard. The "B" stands for Blackburn factory; Blackburn, England. That is the key. Tube enthusiast: know your tube codes. | Effectrode Glad the amp is giving it up for you.

Thanks for the link. I cant make out the etching as it has mostly worn off unfortunately. The tube also has print on it that says "Westinghouse" and "England". Well it sounds damn nice so we will run with it.

[Pryde]

When the amp was designed, they were not into all this tube swapping for subtle tone nuance. They expected you could go to the drug store for a new tube, plug it in, and it should work. It ought NOT be tube sensitive. We swap preamp tubes all the time looking for the just so overdrive or whatever we so crave. But the amp still works reasonably with most any 12AX7.

Indeed it does work reasonably with any 12Ax7 I put in there but some much better than others. I can only judge by my ears but the Amperex sounded pretty tame with "some" breakup occurring at 8 while this Westinghouse branded tube sounds like a pissed-off Marshall at 4-5 volume. I will try to record the difference cause it is without question different. Maybe it is just driving the PI harder, etc. not sure but I like it.