6an8 discussion.

[Boy Howdy]

Since tweaking my Sceptre a couple years ago (http://music-electronics-forum.com/t18973/), I'm totally in love with it. But is it as good as it can be? I have no idea. There's so much about electronics that I don't know.

I've had a few revelations though lately. I do like the ultra-linear output tranny. I always said that I liked the big sound of Sunn amps, and come to find out that is a characteristic of ultra-linearity. Also atypical of guitar amp desirability is the clean Sunn sound, again due to the ultra-linear OT, but also due to the fact that the preamp has only one tube.

I love how no matter how loud you play it, you can't get rid of the twang factor. And my motto has long been "It don't mean a thang if it ain't got that twang." I get plenty of sustain (I don't like too much, I like to work for it.), but I like how it retains its tone as it's turned up. The bass doesn't get muddy/farty because of the OT and the twang hangs around because it's not Boogie-fied with too many gain stages. Long before I knew anything about electronics, I figured out that I didn't like amps with more than two preamp tubes. It's like Robben Ford once said (can anyone point me to the quote? I've been trying to find it forever.), paraphrase: tone and sustain are at odds with one another. The more of one you get the less of the other.

Anyway, I love how my Sceptre does what I've always asked of my amps. I want a nice bright, clean sound and then just add gain to it. Sort of a heavy metal country guitar sound ... like my hero, Ritchie Blackmore ... who, of course, used Marshall Majors (also with an ultra-linear OT).

Sunns aren't for everybody, that's for sure, but with a few tweaks (see the linked discussion) they can be pretty damned awesome ... if they suit your style.

Okay, so this has turned into an "in praise of Sunn amps" post. Enough from me.

I'm curious about the 6an8. I know some replace these with 12ax7 (or similar) tubes. Why? What is gained or lost by the pentode half of the 6an8? Is it a tone thing? Just trying to learn a thing or two. Any thoughts appreciated.

[sgunn]

You can't replace a 6an8 with a 12ax7.

[Boy Howdy]

I read somewhere where someone *modded* a Sunn to accept a 12ax7 (or 12au7) or something).

Nobody has anything to say about this tube? Can someone point me a link or something?

Does the pentode half have more gain than a triode? Is that maybe why they use it, to make up for the fact that there's only one preamp tube? I mean, obviously it's a direct lift from the Dynaco hi-fi circuit, but maybe that's why Dynaco went with it? Don't know.

[loudthud]

Some years ago I measured the gain of the 6AN8 power amp front end. Going from single ended input to differential output (adding both outputs together) the gain was 400. This is more than twice what you could get from a 12AX7. The Dynaco circuit operates with a closed loop gain of about 20 to the 16 ohm tap. The Sunn Model T 1st and 2nd generation use a 12AX7 to drive the power tubes (4x6550) and there isn't much gain after feedback, about 3 to the 16 ohm tap. The high screen voltage of the power tubes in the ultra linear configuration requires lots of drive and therefore, lower closed loop gain.

The pentode of the 6AN8 creates quite a bit of even harmonic distortion because it operates with such a low plate voltage. Even so, the Dynaco design gets the closed loop distortion down to the 1% range with lots of feedback. Pretty good for an amp that came out in (I think) 1959.

[Bruce / Mission Amps]

Hmmm a gain of 400... did you mean a total of 400 times the input signal?
If the gain of a 12AX7 triode was say 30, and you ran that to the input of the other triode.... would the final "gain" be:
30 times 30 = 900?

[Boy Howdy]

Yeah, LT, you got some 'splainin' to do.

[Boy Howdy]

So this additional gain is there to compensate for the fact that there's only one preamp tube? Or rather, they only included one preamp tube because they knew there would be enough gain because of the extra gain in the pentode section of the 6an8?

[Boy Howdy]

So, would ya'll say that the inclusion of the 6an8 is a plus over say a 12ax/u/t7? Since this power amp design (as well at least some aspects of the preamp) was lifted straight from the Dynaco hi-fi design, one wonders if it is an optimal design choice for a guitar amp. I mean that is certainly subjective as all get out, but is this a good, albeit alternative, design choice? Why would Sunn have then gone for a 12ax7 in the Model T? I won't be changing out the 6an8; again, just trying to learn something.

And what about the high plate voltage? Which presumably applies only to the PI and power tubes - I haven't measured or seen any voltages published on the Sceptre, but on other similar guitar models the preamp plate voltage appears rather low (~160 volts, if I recall correctly). I've seen where some say that this higher voltage makes for a harder/harsher distorted sound. Opinions?

I think I actually prefer higher plate voltage - but maybe I'm kidding myself. I like an amp with a bright, clangy, dynamic sound, as opposed to the warm, sustainey thing (both are nice, of course). I had a Vox AC-50 once - also said to have high plate voltage - I loved that amp.

I haven't had much of a chance to play my Sunns loud, haven't played a gig in too long. All I know is that is sounds amazing for clean and distorted when run through an attenuator. But of course it's hard to really know what you're dealing with with an attenuator in the mix.

[loudthud]

A Leader signal generator was connected to the center terminal of the Treble pot in GZ34 rectified 60W Sunn Studio PA (goes right to pin 8 of the 6AN8). The 6550s were removed, the line voltage was set to 110V. Main B+ was 540V. One channel of the scope was connected to the gerenator output set to 10mV per division. The other channel of the scope was connected Differentially to the grids of the 6550 power tubes. The deflection factor was 5 volts per division.

Note that with this type connection a Concertina phase inverter will have a gain of 2 because it is being treated as a differential amplifier. The scope inverts one signal and adds them.

[Boy Howdy]

When I modded my Sceptre, I changed out all of that business at the inputs and put in the regular 68k/1Meg deal. But I only had one 68K so I put a 47k on one of the inputs (I will eventually convert the other two inputs to a preamp in/out). I was comparing the two yesterday and the 47k sounds a little better to my ears. It's a little sweeter, brighter, spankier. Why is that?

[soundmasterg]

So, would ya'll say that the inclusion of the 6an8 is a plus over say a 12ax/u/t7? Since this power amp design (as well at least some aspects of the preamp) was lifted straight from the Dynaco hi-fi design, one wonders if it is an optimal design choice for a guitar amp. I mean that is certainly subjective as all get out, but is this a good, albeit alternative, design choice? Why would Sunn have then gone for a 12ax7 in the Model T? I won't be changing out the 6an8; again, just trying to learn something.

And what about the high plate voltage? Which presumably applies only to the PI and power tubes - I haven't measured or seen any voltages published on the Sceptre, but on other similar guitar models the preamp plate voltage appears rather low (~160 volts, if I recall correctly). I've seen where some say that this higher voltage makes for a harder/harsher distorted sound. Opinions?

I think I actually prefer higher plate voltage - but maybe I'm kidding myself. I like an amp with a bright, clangy, dynamic sound, as opposed to the warm, sustainey thing (both are nice, of course). I had a Vox AC-50 once - also said to have high plate voltage - I loved that amp.

I haven't had much of a chance to play my Sunns loud, haven't played a gig in too long. All I know is that is sounds amazing for clean and distorted when run through an attenuator. But of course it's hard to really know what you're dealing with with an attenuator in the mix.

The Sunns were initially Dynaco circuits lifted from the Dynaco amps of the time....in fact the early ones were Dynaco kits, stuck in a Sunn box, and the price was greatly inflated. They sold well. The amps were changed with different features here and there to cater to different power levels and different applications. (ie bass, guitar, keyboard, etc) An engineer was brought in to redesign the preamp(s) to be a little better suited to musical instruments, and any Sunn that was in one chassis rather than two would benefit from these changes, but keep in mind that the design goals of the time were loud and clean. Having the 6AN8 in the circuit as compared to a 12AX7 or similar was due to economic reasons entirely. One tube that could do the job gainwise is a lot cheaper to purchase and to wire up than multiple tubes. A triode - pentode will have more gain in most cases than a triode - triode, even when considering a high gain dual triode like a 12AX7. The addition of the UL and the negative feedback linearize the amplifier, again...loud and clean. UL by itself already has negative feedback, so adding more makes things sterile to many people's ears. Back in the day though, they measured it on test equipment for distortion levels and called it good, especially for hi-fi. There was no consideration made for tone in many cases, and certainly no catering to the guitar amp market as far as designing the amp. I've spoken to Conrad in the past about this as he and I are friends and he has said that if he knew then what he knows now he wouldn't have used the 6AN8, but hindsight is 20/20! You can always rewire the socket and redesign the stage to use a 12AX7 if you want, but realistically you would need to add another gain stage to get decent tone.

The high voltage is on the power amp and phase inverter, but also on the tubes before that in the preamp. The amp could be redesigned to sound different, but then it isn't a Sunn anymore. In general with tubes, a higher voltage generates more gain, and more headroom. There is a sweet spot though and if you go outside of that, then the tube ends up sounding cold and hard or harsh as you described it. If you go with a lower voltage to an extreme then the tube will have no headroom and distort on pretty much any signal. The tweed Fender circuits are around 160v in the first preamp stage, wheras a blackface fender design is more like 180v to 220v depending on the amp. Voxes are on the lower side of things too, as are Marshalls since they copied the tweed bassman. To my ears, the Sunn amps sound great for bass, but lousy for guitar. They sound better for guitar with the negative feedback disconnected, but they still have a hard or harsh sound. Part of that is the 6550/KT88 power tubes as that is their character. A vintage Vox AC50 is going to have less than 500v on the power tubes and the preamp voltages are more like a Fender rather than the Sunn style higher voltages. They also have no negative feedback so they have a thick and harmonically rich sound for that reason.

Greg

[Boy Howdy]

Hey Greg, thanks for the reply. I've been trying for what seems like weeks to access this site, but kept getting blocked, something about viruses.

I was just looking at a schematic for a Solarus that appears to be virtually identical except for the power tubes, which are el-34s. That schematics shows v1 and v2 at 165 volts. I wasn't able on just a quick glance to tell if it has a different PT though. And I haven't measured the voltages in my Sceptre or Sentura II.

I have done a fair number of mods to mine, and I do really like it, and I have compared it to other amps. Maybe I'm just a weirdo.

I do find though that as I turn the amp up the PI overloads sooner than I would like. Just about the time I'm getting a nice crunch from the preamp, the PI is starting to get overly compressed. I would like to figure out how to tone down the gain of that pentode a bit. Imaradiostar (Jamie) tried to help me with that, but I couldn't get what he was suggesting to work.

[soundmasterg]

Hey Greg, thanks for the reply. I've been trying for what seems like weeks to access this site, but kept getting blocked, something about viruses.

I was just looking at a schematic for a Solarus that appears to be virtually identical except for the power tubes, which are el-34s. That schematics shows v1 and v2 at 165 volts. I wasn't able on just a quick glance to tell if it has a different PT though. And I haven't measured the voltages in my Sceptre or Sentura II.

I have done a fair number of mods to mine, and I do really like it, and I have compared it to other amps. Maybe I'm just a weirdo.

I do find though that as I turn the amp up the PI overloads sooner than I would like. Just about the time I'm getting a nice crunch from the preamp, the PI is starting to get overly compressed. I would like to figure out how to tone down the gain of that pentode a bit. Imaradiostar (Jamie) tried to help me with that, but I couldn't get what he was suggesting to work.

You're welcome.

The ads that are on this site (as well as others) sometimes do infect computers with garbage. Run a good browser such as Google Chrome, and some antivirus protection, and you should be ok. I got the google warning when I would open this site, and just continued on and didn't have any issues, but then I have good protection on my computer and know what I am doing to avoid getting infected since I used to work in IT.

The old schematic voltages aren't necessarily accurate these days with the higher AC wall voltages. The voltage in the preamp can also change from tube to tube since they all bias up differently. You can experiment with preamp voltages by changing the dropping resistor that supplies that stage, or stages before that too if you want. Go up in resistor value, you get a lower voltage to the stage that node supplies. This gives you less headroom and quicker distortion out of that stage. Go down in resistor value, this gives more gain and more headroom. Gain is not the same as distortion or overdrive by the way. The phase inverter design is a cathodyne, and if you haven't read it yet, you should really get Merlin's preamp book and read his website about the subject. His site is the valve wizard site. THe cathodyne stays clean for the most part until it starts to distort, and then it distorts suddenly. When it distorts, the quality of its distortion is often objectionable. This can be modified to almost reduce the effect. The other thing that affects the sudden onset of distortion is the fact that there is a lot of negative feedback in Sunn amps. They are already a UL circuit, which introduces its own negative feedback, so they don't really need even more NFB. Put in a switch to disconnect the NFB and you may find you like the distortion better. The amp will be more raw sounding and more aggressive, but it will also hum more.

I don't think toning down the gain of the pentode is necessarily the answer...though it might be. You need high enough gain out of the PI to drive the 6550/KT88's to full power. The pentode is technically not part of the phase inverter anyway...you could probably call it a driver stage. Due to the nature of the cathodyne, the triode side of the 6AN8 doesn't really overload at all until you get to a certain point, and then it will with nasty results. Merlin's suggestions can reduce most of that quite well. I have also found that you can increase the voltage supply to the phase inverter with a cathodyne type, so that it will drive the power tubes better, and then reduce the gain of those stages by biasing and plate and/or screen resistor changes. You have to make sure any changes don't exceed the max ratings for the tube in question though.

You may try posting here >> The sunn Discussion Page - Index as this is the "real" Sunn forum. I am not sure why Tboy hasn't fixed the links to there yet as when you click here it should go there. You will probably get more help there that is more Sunn specific.

Greg

[Boy Howdy]

As per Greg's suggestion, I relocated to the other forum. Questions about Sentura II circuitry. But I'm not at all sure that makes sense, as I've only had 12 views and no replies there, and 462 I think it was here. I may have to move back here. Anyway, the link to the other posts is above.

[soundmasterg]

As per Greg's suggestion, I relocated to the other forum. Questions about Sentura II circuitry. But I'm not at all sure that makes sense, as I've only had 12 views and no replies there, and 462 I think it was here. I may have to move back here. Anyway, the link to the other posts is above.

The other location is where most of the Sunn guys hang but Tboy doesn't have the links to it here anymore so it doesn't get the same traffic as previously.

Greg