Grateful Dead Music Forum

With Waldo around I hardly need to provide any specifics at all!

Perhaps not quite so black and white but essentially exactly what I meant.

waldo041 wrote:

bucketorain wrote:

Jon S. wrote:As a fan of really cheap fixes that cost little and sound good, an overlooked option for folks who want a bit more of the Jerry tone from what you already own is to actually lower the power tube bias a bit from its optimal level. Try it sometime, see what you think of its impact on your amp's edge/smoothness and dynamic range - you might be surprised at what you hear.

could you explain that a bit further please?

the power output tubes are either a fixed or adjustable bias. fixed bias a tech needs to physically change out a couple resistors to lower or raise the bias to the tubes. adjustable bias uses a potentoimeter to bias the tubes. the hotter a tube runs the faster it breaks up ie.. less headroom. the colder a tube is biased the later it breaks up ie.. more headroom.

peace,
waldo

Not entirely accurate....

Technically all output sections with tubes are either fixed bias or self(cathode) biased. All amps are adjustable in one way or another. It's just a matter of how labor intensive it is. The play on words came with the introduction of variable resistors, pots, trimmers, etc, on the bias circuit. It's a mistake to refer to an amp with a trimpot as "Adjustable Bias" but it has become so commonplace that it's become the acceptable term. It's still a "Fixed Bias" circuit. There are many amps that are cathode biased that have a trimmer in circuit as well and these amps are referred to as self biasing even though you can adjust the range in which the cathode swings.


As for how hot or cold you push a power tube, this does not follow any hard fast rule such as a hot tube equals less headroom, or a cold tube equals more headroom. It has more to do with the type and quality of the tube construction and type of bias scenario, fixed or cathode. Example being a GE 6550. If you were to run this tube at 45-55 percent of it's plate dissipation, it would be quieter and exhibit a harsh clipping. Pushing it up into the 60-70 range raises the perceived headroom and the tube operates much more efficiently.

FWIW, in my original suggestion that instigated this sidebar I was referring to fixed bias push-pull (i.e., Class AB) amps as personally I don't associate Garcia's tone with any commonly used cathode-biased circuit. But maybe that's just me.

As for headroom, it's a slippery concept. What an oscilliscope tells you is max headroom and what you perceive as such may or may not align due to psychoacoustical properties that include how humans perceive different frequencies differently.

In general - let me say that again, in general - my experience hearing push-pull circuits is that lowering the bias slightly but significantly from what your oscilliscope will tell you is optimal for the power tubes in question, while it usually affects the wattage only slightly, affects the tubes' dynamics and frequency response in a manner that, absent hard clipping, leads to a relatively smoother, less edgy, softer sound. (At and past clipping, the "hotter" tube may sound smoother.)

Here is a real life example. The amp in question is my Reverend Hellhound. The top setting is the "60" setting on the 60/40 switch, the bottom the "40" setting. Notice how little of a difference the bias change has on the output wattage. 3 watts difference is not even audible to me. But the psychoacoustical difference is quite audible.

That was what I meant in my original post which in retrospect I could have explained better the first time. But I sure do enjoy these discussions!

Reverend Hellhound
Electro-Harmonix 6L6EH
Vac = 120.8

"60W" power setting:

V+ = 475V
Vp = 473V
Vs = 459V
Vg = -52.0V
Ik = 31.0 mA (avg)
Po = 42W @ 8 ohms at onset of clipping
40W @ 4 ohms

"40W" power setting:

V+ = 477V
Vp = 475V
Vs = 462V
Vg = -52.0V
Ik = 26.4 mA (avg)
Po = 39W @ 8 ohms at onset of clipping
36W @ 4 ohms

Jon S. wrote:FWIW, in my original suggestion that instigated this sidebar I was referring to fixed bias push-pull (i.e., Class AB) amps as personally I don't associate Garcia's tone with any commonly used cathode-biased circuit. But maybe that's just me.

As for headroom, it's a slippery concept. What an oscilliscope tells you is max headroom and what you perceive as such may or may not align due to psychoacoustical properties that include how humans perceive different frequencies differently.

In general - let me say that again, in general - my experience hearing push-pull circuits is that lowering the bias slightly but significantly from what your oscilliscope will tell you is optimal for the power tubes in question, while it usually affects the wattage only slightly, affects the tubes' dynamics and frequency response in a manner that, absent hard clipping, leads to a relatively smoother, less edgy, softer sound. (At and past clipping, the "hotter" tube may sound smoother.)

Here is a real life example. The amp in question is my Reverend Hellhound. The top setting is the "60" setting on the 60/40 switch, the bottom the "40" setting. Notice how little of a difference the bias change has on the output wattage. 3 watts difference is not even audible to me. But the psychoacoustical difference is quite audible.

That was what I meant in my original post which in retrospect I could have explained better the first time. But I sure do enjoy these discussions!

Reverend Hellhound
Electro-Harmonix 6L6EH
Vac = 120.8

"60W" power setting:

V+ = 475V
Vp = 473V
Vs = 459V
Vg = -52.0V
Ik = 31.0 mA (avg)
Po = 42W @ 8 ohms at onset of clipping
40W @ 4 ohms

"40W" power setting:

V+ = 477V
Vp = 475V
Vs = 462V
Vg = -52.0V
Ik = 26.4 mA (avg)
Po = 39W @ 8 ohms at onset of clipping
36W @ 4 ohms

Hey now,

Again, just to clarify. The Hellhound ironically has a variable circuit on the cathode of the power tube. It has less to do with the bias circuit, and more to do with the cathode and overall transconductance of the tube. The slight change on the current draw of the power tube(Ik) is incidental of the real mojo that is taking place which is that the amp is a psuedo cathode(self) bias in the 40w setting. I've worked on many of these amps as well as other prototypes that Kager has unleashed on NYC. Your reading alone tells you that the control grid is fixed. Your B+ isn't changing on the plates. DING! Fender was so enamoured with a similar psuedo self biasing circuit back in the day that CBS era amps saw this type of circuit for all of what, 10 months in 1968? The 60/40 switch is tied to the cathodes lifting them from ground, and introducing a variable impedance. This was the magic of both very early Traynors, Ampegs and some Yorkvilles. Dennis Kager has about 20 or more patents on every variation of this circuit. I'm surprised the Reverend amps didn't last as long as they did. I really dig them and that very design. Take good care of that one. You probably want to get those EH power tubes out of there. You're selling yourself short on the tone in that 40 watt setting. The new Tung Sol 6L6-STR is about the sweetest current production tube going if you can't break the bank on some old American GE's or Sylvania's.


All of this is good conversation and I thoroughly enjoy it. I, in no way mean to one up anybody or knock someone else's amp and guitar expertise. I'm always looking for new tricks and tips myself. I repair amps, effects and guitars for a living so I'm pretty much knee deep in the tinkering on a daily basis.


Waldo, always dig your posts and I worship your unearthing of all the great info about Jer's rig, thanks! But man, if you're gonna run a 6550 at 90-95 percent plate dissipation, which is basically Class A, I don't want to be in that room.


Oh, and this just in. Hot off the wire: http://news.yahoo.com/s/nm/20080623/en_nm/boygeorge_dc

That's very helpful info., jonarobb. Would it be OK with you if I cut and pasted your response into a pair of ongoing threads on the Reverend Discussion Page and The Gear Page (with appropriate credit, of course)?

P.S. I love my Hellhound, I really do. Among the mods I've done to it (Pete Cage did the actual work, I just asked for 'em! ) are hooking the Schizo (US/UK) switch up to a footswitch and adding a trim pot to allow me to dial in as much or as little of the mid-boost (you'll probably tell me now it's more complicated than that but it's what I hear ) as desired. It's really nice, setting the mid-boost low gets me legitimately into the tweed zone with that amp which it certainly was never advertised as having the capability of (whether so designed or not).

I've never tried it with NOS tubes, primarily because I'm happy with it now with the EHs. According to Naylor, it was designed specifically around current production Russian tubes and that's what he advises sticking with. I did try and test a pair of Svets in it. Interestingly, I could coax a legit 50+W out of the amp with the Svets but it totally didn't sound as sweet with 'em (sounded quite harsh to me).

Jon S. wrote:That's very helpful info., jonarobb. Would it be OK with you if I cut and pasted your response into a pair of ongoing thread on the subject on the Reverend Discussion Page and The Gear Page (with appropriate credit, of course)?

That's absolutely cool. I dig The Gear Page myself. No real credit necessary. I learned it from opening up one too many amps and saying to myself, "What the @#$% is this?" And then being like, "Oh, cool, that's how that works!". The clarification was just a way to show you that although bias, and it's adjustment, obviously has an effect on tone, in this case, there is more than one way to milk a power tube for tonal variations and coloration. It's ironic that your amp has the circuit variation that it does. With the birth of the internet and the abundance of info out there sometimes it gets a little weird with everyone's opinion, etc. Example being how folks slam the Fenders from 1968 with the cathode lifted. If you get a chance to sit with one of those amps and really dig in you'll find it's actually very unique sounding and has a smooth clip to it, and a very violin sounding sustain/decay when pushed hard. Particularly the late 68'-early 69' Twins. It's also kind of telling that when Garcia stopped using that 4x10 Fender Concert right around that same era he moved over to a Twin Reverb full time. Before the whole McIntosh era he was using the tube output section from the amp. Maybe they were off the shelf cathode biased Fenders, who knows?

PS; Listen to Jerry's solo on any one of the 68-69 Death Don't Have No Mercy's. To my ear that is probably one of the best sounding guitar tones in music.