Tube swapping and preamp plate voltages.

[stelligan]

Discussion of this was occurring in a thread on another topic. Here are some snippets and some questions. What are considered optimal preamp voltages at V1 and V2? And what brands of 12ax7 are people finding that routinely impact these voltages up or down? Is it consistent by brand, or variable in a wide or narrow range? Here is some of the discussion to start. I hope folks don't mind being quoted. I thought it would tidy up the 124 thread and provide for a more focused discussion of this.

Thanks,
Dave

YMMV - I've come to learn over time that there's a significant variation in current draw from one 12AX7 to the next, which will impact your plate voltage and the B+ voltages, all else being the same. IMHO, this is one reason why tube swapping can have such an impact on the amp's tone.

B.

Well I'll be damned, I had never thought about the effects of tube swaping from that standpoint, but it sounds like curent draw could certainly be a factor.

Cheers,

Gil

Huh, I never thought about that... Is there a simple way to test 12AX7's for current draw?

Thanks Brian.
And yep. I discovered the current draw thing in a rather brash sounding D'Lite build I had. In my limited swapping, I found it interesting that a certain JJ drew the voltages down about 9 volts lower and tamed that amp nicely. The JJ's I tried consistently drew more current than the EH's I had on hand. Has anyone done any extensive testing of this? Derail #2 - sorry.....

I'm not sure current draw is the correct term, I'm thinking the whole current draw thing is more or less the gain of a particular tube. While the gain of a 12AX7 is quoted as being 100, in reality the gain of a 12AX7 must fall within a certain range.

That is the way I see it. If I'm incorrect please explain why I am incorrect.

Of course the experiment to do now is to replace the cathode resistors with a 2K linear pot in series with a 1K resistor and adjust each tube you put into the amp for the same current draw and plate voltage and see what they sound like. Maybe there are people blowing a lot of cash on NOS tubes when a $2 pot would solve their woes.

Regarding the above mentioned experiment. I think there possibly would be no audible difference in tone at all, of course I could be wrong.

It's a good derail. How 'bout measuring the plate voltage on V1 instead of focusing on the dropping string? As you've both noted, it's the voltage on the tube that matters. What is the voltage when it sounds good?

Zippy,
On the build I was referring to - I was focusing on the preamp tube plates. I had been swapping OD 12ax7 tubes. EH I had in V2 was giving 212v on V2a and 219v on V2b. The JJ I ended up with brought V2a to 203v which sounded real good to me. It was a Bassman PT "D'Lite" HRM with 220k/150k on V1, 180k/150k on V2 . Voltages on V1(a & b) were: 190v & 196v. I haven't messed with tubes and trimmer since and am very happy with that amp. Anyone with absolutely ideal voltage values for V1 & V2? I'll search some more.

Thanks,
Dave

Can't say that these are 'ideal' but I like to shoot for 188-195V range on V1 and maybe 195-202V range on V2.

JJ's short plate 12ax7's draw more current...yes Mark, current draw is the correct term here, just measure across the plate resistors and you'll see, so the JJ will drop the voltage a bit more.

Not a bad tube either, IMHO.

Jelle

[greiswig]

I was really surprised that a noob like me had stumbled upon this before someone with Gil's experience, and I guess I thought this would be common knowledge. I still bet it is, just that not many people have shared it.

I found it out when trying to balance my PI initially: I found a 12AX7 that I preferred in that slot, and set the trimmer. Then I started to try to find another tube to use as my "PI Spare." So I was looking at first to match the voltage and the voltage differential as closely as possible, so that I wouldn't have to worry about that trimmer if I had a tube failure. I found two JJ long-plate 803S's that were pretty close on the voltage differential, but about 10V different overall. Between those two same-brand tubes, it wasn't so much a change in sound as a change in feel, particularly at higher volumes.

So I started to look for similar things in my V1 slot in particular: I had a favorite tube that I liked there, and had adjusted my voltage string to put that tube at about 190V. I started looking for tubes that matched that voltage, and sure enough, if I had a tube that was 8-10V higher on the plates, it sounded a bit sterile and stiff. And if I had tubes even in the low to mid 180's, they seemed to lack sparkle.

So I do think this is worth exploring. It's useful for finding specific spares for specific slots, if nothing else.

[talbany]

I've played around with this a bit and found if your swapping out different tubes old and new the voltages could swing up to 10 volts in either direction.... This is one of the reasons I've pretty much given up on recommending a certain tube that is the beat all end all...especially the older ones....As for the tubes current draw that would be an interesting experiment....Not sure how it translates into tone but maybe kind of cool for quality control....



Tony VVT

[stelligan]

You can buy tubes with matched and balanced triodes for the PI and it does cut down the swapping at the PI. I too like JJ 803s at the PI and have ordered them with matched triodes with good result. I guess it may also be helpful in V1 & V2. Still am curious to find a pattern among tube manufacturers regarding this "current draw" thing......

[odourboy]

Good move Dave - we did a fine job at totally derailing the other thread!

I don't really have much to add but I'd be curious to see if there's a correlation between gain and the how much current a 12AX7 can sink. Alas, I've got too much on the go to spend the time satisfying said curiosity!