6973 in Place of 6V6

[Chuck H]

Funny, I never realized how clumsy the stereo mix was for that track before now because this is the first time I've listened to it through headphones. But yeah, there's a noticeable modulation in the high bass/low mids. I guess I just figured it was a dirt box and/or a stationary wah. Page did a lot of things in the studio so I never tried to "emulate" his tone. If your amps are doing this, well, I don't think it sounds unmusical at all. Maybe it's a context thing and it does sound unmusical in the wrong key or with different EQ and gain settings?

[HaroldBrooks]

Perhaps unmusical is an improper choice of words. It's a modulation that corrupts the higher fundamental tones, but yes, it sounds fantastically GREAT and still gives me goose bumps !

But if I was playing something that required a smooth distortion-(the type that sounds like a smooth buttery cake as opposed to a broken up piece of toasted bread being coughed up), or if I was playing dissonant chords and I didn't want to create too much musical tension, that might be the wrong distortion... But it's a qualitative evaluation, and as everyone knows, beauty is in the eye of the beholder, and for that song, for that time the guitar sounds awesomely beautiful !

That's why it's a good practice to use switches and pots to change values, so you can ride a spectrum of settings and easily get back to stock if you need to, and never say never !

Some awfully ratty and small sounding amps sound huge if you get the right mics on them, the right reverb, and the right post amp EQ. It's a complex dish, that one thing is for certain and there seems to be a place for any amp or tone if the time is right.

[g1]

There was some talk here recently about the musicality of the IM distortion being dependent on whether created by 50Hz vs 60Hz mains. Stands to reason that it would be important.
Sorry I can't remember who here mentioned it, maybe someone from 'across the pond' ?

[Chuck H]

I remember that. And I also don't remember just where it is here. But IIRC it was said that 50Hz was key and 60Hz just didn't do it. Of course there may have been some proprietary hedging on that opinion

[soundmasterg]

Hmmm... Disclaimer: I don't have a scope.

My only experience with 7591s was in my Ampegs: a 63 Reverberocket, a 65 R-Rocket 2, a 66 R-Rocket 2, a 68 Rocket II (coincidentally exactly like a Reverberocket 2 in a different box & on a PCB), & a 72 GU12.

The only one of those amps I would say had any great clipping character was the GU12. Reason I mentioned no scope was because I don't know where any of that clipping was coming from. But I wish my friend hadn't blown my GU12 before I could get the right speaker; I'd blast that thing on 10 all day if I could. Not so any of the others...

Justin

Everitt Hull, the founder of Ampeg HATED distortion and rock and roll and designed his amps to not distort. So basically most Ampeg guitar amps are designed with too little gain by most guitarists standards. 7591's and 7868's are electrically equivalent at least with old ones, not sure about modern, and they are similar to EL84/6BQ5 in that they have high gain and high sensitivity for a power tube. This means that you can drive them easily with a preamp with low to moderate gain, and a decent phase inverter design that can drive the tubes such as a LTP. The cathodyne doesn't quite cut it as well as the LTP or floating paraphase. Anyway, the 7591/7868's breakup smoothly with a lot of harmonics and touch sensitivity and a more balanced sound compared to a 6L6 which has lots of bass and highs but is sort of mid scooped, or an EL84 which is often lacking in the bass but has really nice highs and mids. The 7591 is closer to an EL34 that way, but since it is a beam tetrode and not a pentode it doesn't compress and clod up like the EL34 can do sometimes. They can distort nicely if that is what you desire but it would take more than just grabbing a preamp design and sticking it in front of a 7591 output section. Dial it in so the power amp distorts first, then the phase inverter next, then the preamp stage before the phase inverter, etc., and the amp will have tons of harmonics and a really nice touch response. I often think the only reasons the 7591 didn't become more popular were that they were at the end of the tube era, and the unique pinout meant people couldn't sub them into the same socket as a 6L6 or a 6V6 without rewiring. Power-wise they are in between both and have a sound that in some ways is better than both.

Greg

[soundmasterg]

3.) 6973 tubes also clip with some type of low sub-harmonic that is non-musical in it's character (not an even harmonic). It's very, very subtle at first, just barely noticeable... but if you really hit the power section hard (based on the way your amp is setup), you will hear the sub-harmonic discordance more and more, progressively. It's an intermodulation distortion that in the extreme, might be considered undesirable and too gritty, but may also sound really cool ! Think the last chords in Led Zepplin's " What is and what should never be"... Page may have been using a Thunderbolt with 6l6's, but from the Valco amps I own at least, the ones using 6973 tubes sound the closest to his early recordings, regardless of what was actually done at the time.

.

Page used a Supro with 6973's. Many people thought he used a Thunderbolt, and it will give a similar sound, but it was a different model that he used and he finally said in an interview within the last year or two which actual amp it was that he used on the earlier Zeppelin stuff.

Greg

[Chuck H]

The 7591 is closer to an EL34 that way, but since it is a beam tetrode and not a pentode...

Sylvania, Tung Sol and others are calling that tube a "power pentode" or a "beam pentode". I found no reference from anyone that made that tube designating it a tetrode. Of course what they called it and what it is may just represent sales trends at the time.?. It happens.

[Leo_Gnardo]

Sylvania, Tung Sol and others are calling that tube a "power pentode" or a "beam pentode". I found no reference from anyone that made that tube designating it a tetrode. Of course what they called it and what it is may just represent sales trends at the time.?. It happens.

Someone oughta ... dissect a vintage 7591 and report on it. Next time I have one that's packed up, I'll give it the hammer. Unless someone beats me to it. Some tubes, you can see inside the glass & there's the beam forming plates. Or not. Not so easy to see in a 7591.

[Helmholtz]

Sorry I can't remember who here mentioned it, maybe someone from 'across the pond' ?

I remember joking that the 50Hz might make sound Marshalls better over here in Europe.

[Chuck H]

Someone oughta ... dissect a vintage 7591 and report on it. Next time I have one that's packed up, I'll give it the hammer. Unless someone beats me to it. Some tubes, you can see inside the glass & there's the beam forming plates. Or not. Not so easy to see in a 7591.

Well if it's a "beam pentode" I suppose it'll have beam forming plate. So the question would be whether or not there's a suppressor grid? Which is of course the element that the beam forming plate was intended to eliminate, but maybe.?.

[Justin Thomas]

Blast & barbecue your infernal potato, I'm about to go open an RCA 7591A just to satisfy y'alls curiosity!

Jusrin

[Justin Thomas]

Okay. I took my phone flashlight to a vibtage 7591A, RCA brand. There <IS> a beam plate in there, even though it has round holes that line up with the ones in the plate. So, yeah. Beam Pentode. No suppressor grid.

Justin

[soundmasterg]

Okay. I took my phone flashlight to a vibtage 7591A, RCA brand. There <IS> a beam plate in there, even though it has round holes that line up with the ones in the plate. So, yeah. Beam Pentode. No suppressor grid.

Justin

There is no such thing as a 'beam pentode' even though many manuals termed it as such. It is a beam tetrode since one of the 5 elements of the pentode is replaced with beam forming plates, thus leaving you with a tetrode.

https://en.wikipedia.org/wiki/Beam_tetrode

VTV in their last issue talked about the 7591 and said for awhile they were made by RCA with the same exact cathode as the 6BQ5/EL84 in it. The EL84 and EL34 were also made in some places as a true pentode and in others as a beam tetrode. The 6CA7 is similar to the EL34 but is a beam tetrode. Same with the KT77. The 6BQ5 is often an American made beam tetrode that is similar to the EL84 pentode that was often made in Europe. I personally like NOS GE 6BQ5's in my AC30 better than any EL84 type I've used, but they're getting harder to find and expensive.

EDIT: Here's another link that I just found that is apt to the subject.

https://www.youtube.com/watch?v=35cdWTQ0GI4

Greg

[Chuck H]

Another for Justin because he sacrificed a tube. I almost did it because I do have a pair of pulls... But it IS a pair and I've never tested them. If they're good I'd have hated to waste them.

EDIT: Actually, Justin said he used his phone flashlight.?. But I guess I'm uncertain if he just used it to look into the tube or smash it open

[Tony Bones]

There is no such thing as a 'beam pentode' even though many manuals termed it as such. It is a beam tetrode since one of the 5 elements of the pentode is replaced with beam forming plates, thus leaving you with a tetrode.

Yeah, OK, but who decided that the beam forming plate doesn't count as an electrode? It's connected to a specific potential (same as cathode, just like suppressor grid.) It influences the trajectory of the electrons due to it being at cathode potential. Why is it not considered an electrode?