6973 in Place of 6V6

[HaroldBrooks]

Hello to all. Just received two 'Gold Plated' 6973 to 6V6 adapters yesterday, and nice results from the swap !

I experimented with running the 6973 pair in 2 of my vintage amps, namely a 1949 Vintage Supro supreme, an even older 1941 Supro Model 50. Both amps have already been modified by me to raise gain a bit, lower the lowest frequencies of Bass their character as a blues amps, but Both scream when driven hard up front with a parametric EQ as rock amps.

Here's my 'Qualitative' assessment of swapping in the 6973 pair : More upper treble, a bit more of the lowest bass, clearer distorted chords, and a type of tone phase shift on some bent notes that is very cool, and was there a lot less using the 6V6 power tubes. So all in all, it's a better setup for 70's - 90's rock, and easily reversible. The change is not huge, but present and audible, as well as the changes to the response.

I like having this option.

All comments Welcome !

[mozz]

Original 6973's are hard to find. I have a pair i used in a Supro style build and they do have a sound a bit different than regular old 6v6's.

[Chuck H]

The specs read like it's basically a different pinout 7189 tube. Which is basically a souped up el84 tube. And that means most modern OT's for a pair of 6v6 would have a lower than ideal primary impedance. But I couldn't say about those vintage Supros. Many vintage 6v6 amps did use a higher primary impedance than what we consider "normal" today.

[soundmasterg]

The specs read like it's basically a different pinout 7189 tube. Which is basically a souped up el84 tube. And that means most modern OT's for a pair of 6v6 would have a lower than ideal primary impedance. But I couldn't say about those vintage Supros. Many vintage 6v6 amps did use a higher primary impedance than what we consider "normal" today.

The 6973 was derived from the 6V6GT according to Vacuum Tube Valley # 15. It is certainly a beam tetrode and not a pentode. It has a higher plate voltage capability than the 6CM6 and the gm is a little bit higher too, but I'd say aside from the higher plate voltage capability and the smaller bottle size it is closer to 6V6 specs than that of the 7189/EL84.

Greg

[Chuck H]

Yes. interestingly it's described as a beam pentode.?. And the required bias voltage is certainly lower than 6v6 types at similar operating conditions, though higher than 7189 and el84 tubes. The example anode to anode test loads are also more similar to the 7189 and el84 type tubes. I do know that people liked them then and they like them now. Good that they can be had then.

[soundmasterg]

Wasn't EH making some modern ones?

I have a pair of NOS ones that I plan to use in something one of these days, but I haven't tried any new ones. I guess some of the new Supros use them also? Probably give a 6V6ish sound with more highs I'm guessing, but what do I know.

Greg

[HaroldBrooks]

EH is selling tubes that they call 6973s, but physically their tubes are shorter, fatter, and have obviously different internal components. May be they are the same, but use your own judgment to decide.

There are still plenty of vintage RCA and G&E 6973 tubes out there, and I scored a pair of RCAs that are pretty dang strong and work perfectly, for right around $40 for the pair on Ebay. The trick is not being too anxious, and wait for a good used set to come up on Ebay. I waited around a month, and then they appeared ! I took a slight risk because the pics were a bit fuzzy and the tubes had no boxes, but they were indeed tested as strong, and they indeed are as per comparison with my originals.

There are a few 'Substitute' tubes like the 6CZ5 and 6CM6 and the plate voltage maximum on the spec sheet is only 350vdc, but according to some, the rating method for the 6CZ5 is a "Design Center Value" and it's supposed to be more conservative, so running them up to 400v is OK, provided you don't exceed the Plate dissipation, and of course watch for any red plating. You need to Jumper pins 1 and 8 on the subs to ensure you have screen grid voltage at either point and this is easy to do right on the socket bottom.

[Chuck H]

How do they sound clipped? Reading the spec sheet they seem similar to the 7591 (another "beam pentode") in that the bias voltage for a given current is somewhere higher than el84 types but lower than others. And the ideal anode Z is a little higher too. I've read that 7591's have great clipping character. On par with the best old el34 types. Now I'm wondering about these 6973 tubes.?. I have a couple of amps that run about 365Vp on el84's. The Primary Z is right for the 6973 tubes and it would only require a rewire of the sockets to try them.

[Justin Thomas]

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

[Chuck H]

well, grain of salt and all that. Ampeg guitar amps were typically atypical of how other amps might do some things. I'll look at the schems for the amps you mentioned and see if anything gels...

[Justin Thomas]

Given the 7591 was conceived & born as a hi-fi tube anyway... I don't think there are many amps that used them that were expected to distort anyway (Gibson?). Except the GU12. They took the AC12 & renoved the negative feedback loop. That's it.
And that little beastie did period-correct Stones for DAYS.

[Chuck H]

Well, being as I've been deep in the realm of el84's for a long while I've recognized how different they are from almost any other tube I build with. And I've often thought that it might be interesting to have a sort of crossover design that was a hybrid with characteristics in between the standard big bottles and el84's. Looking at the specs I sort of thought the 7591 might be such a tube and I've read good things about it.

The amps you mentioned all use cathodyne type phase inverters. Not known for their refined clipped tone . I didn't get (or wasn't able to see) the primary impedance spec for any of the models. So I still wonder how the 5791's (or the 6973's) might sound in something like a Marshall or Fender circuit.

[Justin Thomas]

Refined shmefined... Boring! It's a guitar amp, not engine oil!



Justin

[Chuck H]

To be honest I've rarely come across a production TUBE amp that din't have something to offer when cranked up. Some more, some less. Exceptions might be new uber gainers and some SF Fender models (NOT THE BASSMANS!!! Easy now )

[HaroldBrooks]

Everything before the power section on just about every amp I own changes the tone to a greater degree than just swapping out power tubes, but there are some noticeable differences when using 6973 vs 6V6 or 6L6s... Let me see if I can enumerate what I have found :

1.) 6973 tubes clip nice, but... they get quite a bit nastier if you bias them hot. Valco biased 6973 tube in their amp using a 250 ohm common cathode resistor for a pair. With a wall voltage of 123acv, this works out to about 16 watts plate dissipation per tube ! quite a bit over the spec sheet 12 watts, to say the least. Increadibly, some of the tubes still don't red plate even given this high bias, but the sound get's nastier in this range. I backed off my tubes to approx. 12 - 13 watts each by using a 350 ohm cathode resistor, and they run cooler, but they sound a tad nicer and clearer IMHO.

2.) 6973 tubes seem to clip (in the Valco amp circuits) brighter and with more low base (a bit scooped) than 6V6's in the same circuit. Not sure exactly why, but if you dial back the volume control and hit the front end of the amp hard with a boost, the sound is 'Duller', and that's different that amps I own with a 6V6 or 6L6 pair.

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.

The fun starts at 3:31 https://www.youtube.com/watch?v=4JhldliZoLg


4.) In the balance 6973 tubes do clip differently than 6V6 or 6L6s (to name two others), but the entire amp circuit they are in, combined with the guitar and rig you use will be important to getting the 'Correct' clipped sound for that whole combination, and it could be clear and bright, or nasty and dirty. Not all amps using 6973 tubes sound the same, and that can be easily said for any other output tube as well, but some amps using 6973 tubes sound phenomenally Dirty if the power section is driven too hard, and the tubes are biased hot enough to cook bacon and eggs.