Output Tubes

[leadtower]

Does anyone remmember if there was a post about using 6B4 triodes in the ST 70 output with the DIYT driver board?

[TomMcNally]

Try the "search" at the top of the page ... there are lots of 6B4 hits ... nothing specific to the ST-70 driver (that I saw) but lots of good info.

[leadtower]

Thanks Tom; You are right there is a good amount of info. Can I ask you a generic question? If you were to considor swapping in some kind of triode output circuit for the ST 70 driver; would you have a preference? Just wondering if anyone has considered this.

[TomMcNally]

I've messed around with the "triode mod" on an ST-70 before ... I can't honestly say I heard any sonic improvement.

Anyone else have any experiences ?

[Shannon Parks]

Search for: ned+carlson+6b4g+st70 on the Usenet (RAT posts).

[Blackburn Audio]

There is some information in on changing the ouput tubes to a 6B4 in our instructions we include with our ST70 driver boards. I don't know if it would help you guys or not, but if anyone is interested just shoot me an email at questions@triodestore.com and I'll send a copy of that section to you.



-Matt at Triodeelectronics.com

[Uncle Ned]

There is some information in on changing the ouput tubes to a 6B4 in our instructions we include with our ST70 driver boards.

Duh, yeah, I know. I wrote it.

Shannon's board is better suited to driving actual triodes than the Triode board is, due to long-tail phase inverter and the higher effective plate voltage on the phase splitter tubes. Put a 12BH7 or ECC99 in there, generating 60 volts RMS of drive for a 6B4G should be no problem at all.

With all due respect for the Mad Armenian (actually, I like Mr. Melkesethian), there's a reason he's been sitting on piles of those "6B4-G" tubes for years. Regular 6AV5 tubes aren't horribly expensive. If you're serious about triodes, get real triodes, if you aren't, simply triode strapping EL34's is dirt cheap. JMHO.

I would say that whether you are going to like triodes is going to be dependent on the speakers you are using. When I had an old Jensen knockoff corner horn, the best amp I ever heard through it was a Brook.
If that kind of speaker isn't your cup o' tea, you'll probably have different impressions. Triodes vs. pentodes has been a point for polemics for 70 years.

[leadtower]

Yes; I know if something sounds too good to be true it usually isn't. Those 6b4g's are obviously not the original triode tubes of that description or they would be selling for the going price. I am going to try them anyway for giggles. The triode strapped EL 34's have a little different sound to me than stock but I kind of like the stock sound with the DIYTube board. I use Pi Theater Speakers and Altecs so the triode mod may work well. Thanks all for the hints and tips.
I recieved the info on the mod; thanks Distorted skies.

[Kyle K]

Ned,

In the Triode Electronics ST70 manual and in your "Dusty Files" you make a point about the value of the grid leak resistors used by many vintage designs using fixed bias and how exceeding tube manufacturers recommended maximum values for these resistors could lead to instability with some tubes in output stages. It's rare to find an amp manufacturer, vintage or current production, who seems to respect the tube data sheets' limiting values. Do you find this really important? Why do the manufacturers consistently disregard these limits?

Thanks,

Kyle

[EWBrown]

Another interesting power triode is the 6A5 or 6A5G, think of it as a 6B4G with a cathode. They don't seem to be as available as 6B4Gs, and there are none that I know of being sourced in Russia or Eastern Europe.



Pinout seems to be "6L6 friendly" so it should be relatively easy to use in an ST70 or Eiclone. The B+ can be taken higher than the 250V shown here.

How useful are 6AV5Gs? I have a pile of used / pulled ones, from last year's "plinker pile" haul.

I also mentioned this tube under another posting, complete with a photo of the "broiling pan" chassis.

/ed B in NH

[mesherm]

I had considered 6AV5GAs since I had a supply at work but went with 6BG6s because of the "coolness" factor of the top plate caps. The 6AV5s are used as a horizontal deflection tube as are the 6BG6s. My amps with 6BG6s sound even better than my triode strapped KT88 amp so I would suspect that the 6AV5s might also sound good.

[EWBrown]

I've never seen much of anything about using 6AV5Gs in an amp (other than Bogen used them in some PA amplifier), and the RCA toob manual doesn't say too much about them in the "back section" of discontinued tubes. But I always suspected that they had some potential use. The ST bulb version also have that extra "coolness" factor. There are 12V, 17V, and 25V versions, for even cheaper than dirt pricing.

FWIW, AES has 6 and 12AV5GTs for $1.40 on sale.


Basic data here:



A better choice is the larger bulb 6AV5GA, the gm is 5900 and the rp is 14500, and the plate is of a larger and heftier construction, it should be able to exceed 11W PD in a triode strapped or pentode mode audio amp.

6CU6 is similar, as is the 6BQ6GTB.

/ed B in NH

[Blackburn Audio]

Distortedskies Wrote:

There is some information in on changing the ouput tubes to a 6B4 in our instructions we include with our ST70 driver boards.

Uncle Ned Wrote:

Duh, yeah, I know. I wrote it.

Ned, I'm not stupid I know you wrote the old instructions but I wasn't posting that for you, as obviously you already know, I thought it might be helpful to others on the forum.

-Matt at Triode

[Uncle Ned]

Ned, I'm not stupid I know you wrote the old instructions

Ok, I just wanted to let the other folks know that as well. Fair enough? Yellow_Light_Colorz_PDT_08

[Uncle Ned]

Ned,In the Triode Electronics ST70 manual and in your "Dusty Files" you make a point about the value of the grid leak resistors used by many vintage designs using fixed bias and how exceeding tube manufacturers recommended maximum values for these resistors could lead to instability with some tubes in output stages. It's rare to find an amp manufacturer, vintage or current production, who seems to respect the tube data sheets' limiting values. Do you find this really important?

Yeah, I do think it is. While there's plenty of good tubes being made, one can't count on tubes being as consistent as they were in the days of yore. I think a lot of manufacturers are lucky that people nowadays mostly make a practice of buying matched output tubes rather than, say, a collection of 3 different brands that came out of a bin under a tube tester at the Walgreens.

Also, I think using the lowest reasonable value of DC grid resistance probably makes amps sound better. There's always some DC offset across grid resistors, in some cases it's substantial, exaggerating this tendency can't help reduce distortion. The signal grid current of output tubes that aren't meant to be used as class B or C, can't be expected to be consistent between tubes or operating conditions.

When Fender went from using 220K resistors (with push-pull parallel 6L6's!) to 68K in 1969, people complained it made the amp sound worse rather than better, but Fender didn't substantially redo the drive circuit to compensate for the extra load. Fender also started having Sylvania make its 6L6GC's made to a Special Test Requirement in the 70's.

Why do the manufacturers consistently disregard these limits?
Thanks,Kyle

Well, a higher load it makes it easier to work up a drive circuit, and use cheaper tubes and fewer parts. I mean, just for the sake of finding out, I once bench tested a PAS3 and found I could get 34 volts AC into an open load (OK, an HP AC voltmeter, but that's pretty close to an open load). Back in the 50's and 60's, a few hundred bucks was the price of an audiophile amp like a McIntosh, with that kind of price pressure, it was real tempting to go Mad Man Muntz on the parts count. Real pentodes have fewer issues with signal grid current than beam tetrodes, which may explain why they became so popular for hi-fi gear.

Some experiments with excessively high grid resistor values have been real messes, the first thing that comes to mind is Jadis, but I remember seeing an old Bogen that fried a bunch of sockets by oscillating itself to death.