design considerations,power tube overdrive and sustain without pre-amp distortion ???

[tonequester]

tonequester here.

This is my first visit to theory and design. I am "researching" the building of my first amp. I will probably try a kit build for experiences sake. I hope that
as a neophyte, my query will not be considered too ignorant for this forum, but nothing ventured....nothing gained. Is it possible to build an amp which could be overdriven
(output/power tubes), having good sustain, without inducing pre-amp distortion? When I say overdriven, I mean a "warm" overdrive suitable for playing blues, which would
as I understand it, emphasize 2nd order harmonics. If it is not possible, and I already have my doubts, are there any methods that can be employed to at least lessen pre-amp
distortion while retaining power tube overdrive. I realize that many amps excell at producing this kind of tone when played at full volume. Is this the only option? I am also at least familiar with the basics concerning emulators, power soaks, etc., and perhaps this is the only way to achieve this goal. Again, please pardon my lack of knowledge in
this area. I am primarily a player who out of necessity has had to work on my own equipment over the years. I've been fairly successful at troubleshooting and repairs, and have some experience in modification of guitars and tube amps, plus some effects builds. I am seriously studying electronics, as I continue to improve my own "sound".
Any comments/replies are greatly appreciated. I have already learned much from the music electronics, guitar tech, and the amp forum in a way that textbooks can't provide.
If Enzo and/or Steve Conner participate in this forum, I would certainly like to here from them if they can spare the time. tonequester.

[Austin]

The way I understand it, only dc coupled or transformer coupled amp can really push the output tubes into grid current. With a capacitor coupled amp like 99% the preamp section may try to drive the grid more positive than the cathode but the coupling capacitor will not let grid current flow and the result is a rounding of the up going peaks of the sine wave. This is the same thing as 2nd order distortion and is probably what it is when people think power tube distortion, blocking distortion from the capacitor. I don't think it sounds that great. If you were dc coupled and had a strong tube (probably as a cathode follower) driving the screen of your power tube into grid current then it would actually just keep getting louder and still be somewhat cleanish, until you were swinging so many volts that you hit cutoff on the low part of the wave.

There is another way to generate 2nd order harmonics, you could hook a 4 or even 2 ohm load instead of the normal 8 ohm load (by connecting all the speakers in a 4x12 cab in parallel for instance). This would reflect the lower impedance back to the tube and cause more distortion, mostly the 2nd order kind. It would also likely reduce the power transfer making it have less volume.

I know you posted about power tube distortion but 12av7 preamp tube is same pinout as 12ax7 and can be substituted, these tubes are not very linear and make lots of 2nd harmonics. One my favs for guitar, cheap too.

[Enzo]

This isn't my area, but overdriving means just that. The only way to get overdriven powr tubes is to max them. You wonlt overdrive the power tubes with polite little noodlings in the bedroom. That is why people sell attenuators, so guys can operate their amps at full powr without being all that loud out the speaker.

So you may find it warm, but it will be darn loud while warm.

Of course you can make a preamp that will be squeaky clean at full output. You may not like it or find it practical, but then you might love it too. Either way, overdriving the power tubes is just a matter of sending a large enough signal.

[Zozobra]

You can look into adding power scaling to the poweramp. I did this for a 20W cathode biased 6V6 amp I built for a friend and it works really quite well with the clean channel although I must say I'm not as fond of it with the dirty channel but that was more of a bolt on then something which was heavily optimised.

[NorCalTuna]

The OP doesn't actually state that he want's bedroom volume, just big huge warm tone, and this is probably a case where simplicity reigns. So if this were my project... I'd try something like a princeton or bandmaster front end- with a TMB stack and adjusted values (perhaps 500/.033/.033?), perhaps with 12ay7s, and a single ended output stage; that will have a higher proportion of even order harmonics. Then just adjust the caps until you get the bass you want and the volume pot (500k may be cleaner here) until you get your desired "preamp" drive, and perhaps add a voltage divider or another pot ("level" or "master") before the power tube to get all the levels right. The tone stack robs most of the gain from the first triode, iirc about 25db, so you just need that second triode to get the signal up to where your power tube of choice dictates it. Theres a number of other things to try (current source to the output tube grid comes to mind) but thats the framework I think I'd start with. Perhaps even less "pre."

I think the main question is how loud you need, and if you have a tube or a speaker or at least efficiency range in mind, barring power scaling tricks and attenuators- or if you care, i.e. no neighbors or always mic up at gigs anyway. If you're playing it though, say, an ev 12l, which is pretty dry but 100 db e and rock solid up to 200w, then you could use a 6v6 and still get about 106 db out of it at about 5w. If you used a se 6550, you could easily push 112db... I've always wanted to build this amp, btw, with, say, a 300b or an 807. The power tube is dictated by the application and then dictates the design, barring tricks and attenuators.

[tonequester]

tonequester here.

First off, thanks in all sincerity for the great replies to all who posted. Being a neophyte compared to most if not all, I truly appreciate the abundance of info that comes my
way on this forum. Since it will be a good year before I can possibly afford to do my first build, I am starting a notebook to gather as much info as I can from those who have "boldly gone
where THIS man has never gone before". Austin. Your revelation on the use of a lower Z load and it's reflectance back to the power tube is insightful to say the least. I'm glad to know about the 12av7"s non-linearity and harmonic characteristics, generally speaking. If one were to employ it in a multi-stage gain configuration, would it be best used in the last stage of gain ?
Great to make your "aquaintance" and thanks for the post ! Enzo. Your post was short and to the point(unlike my usual fare). Your comment on the "squeaky" clean pre-amp is duly noted.
I had suspected that there are design considerations that would at least greatly reduce pre-amp distortion. I would imagine that "tube hi-fi audiophiles" who design and build their own,
would have a few ideas on this subject. Thanks again for the reply, it's always good to "hear" your opinion. ZoZobra. Greetings to Manchester from Kansas City. Thanks for the tip on power scaling/pre-amp. I will "research" it thoroughly, as I am in total ignorance on the subject. I have always used the clean channel, almost exclusively on every tube amp that I've played.
I've never found the "dirty" channel to be anything other than "dirty". This is just my opinion ,of course. I appreciate your time used to reply to my query. NorCalTuna. WOW ! I have no doubt that you had already done some thinking about this one, before I ever posted. You've given me not only much to think about, but also have given me some info that I will no doubt have to ask further clarification about, in a future post. Thanks for your "recipe". Great correspondence gentlemen. Tonequester.

[Zozobra]

Merlin Blencowes book on power supply design covers power scaling/VVR. Its really easy to implement in sub 30W cathode biased amps and I'll certainly be adding that feature to all the amps of that ilk that I build. In a nut shell it adjusts the plate and screen voltage and thus the power output. You can apply the scaling to the full amp if you want. Some people include the phase inverter in the scaling too, just split the power supply as you see fit. It responds in a fairly power tube overdrive kind of way too and all you really need to add is a mosfet, a few diodes and a pot.

This the schematic for the amp I built for my bassists folk project. There are a few errors in the schematic and I forget what they are but you can see how the VVR is applied. There are a few things I'd do differently if I were to design it again today but such is life!
https://dl.dropbox.com/u/8802910/andy_rev_1.png

[J M Fahey]

Real *short* answer, build a Fender Blackface clone.
Which one?: depends on where you play.

That's what they excel at: killer smooth bluesy power tube distortion, with nil preamp distortion.
Choose your project according to venture size, from a Champ or Princeton up to a Twin Reverb.
A Super Reverb is an excellent Club amp, although too much for home use, if overdriven.
And, as Enzo said, you'll have to play it at least on 6 or 7 to get that liquid smooth sustain sound.

And you can Power Scale (or attenuate) any of them.

PD: consider an AX84 as a first build, lots of practical info, pictures, layouts, mods, and *very* good sound.

EDIT: hi zozobra, liked your amp very much

[Amp Kat]

All I'm going to say is there are more than one way to skin a cat and you will likely find that out after you do this build , ei you could have just taken JM's idea and put a distortion pedal in front and gotten the same tone. Of course the type of box you put in front can make all of the difference in the world just as other parameters of the amp built like tubes,transformers,caps and rectifiers. Just because a solid state device is in the circuit or in front of the circuit doesn't mean you can't get 2nd order or even 4th order THD from it if you use a tube preamp and output. There are many things that can happen and many different ways to achive it.

[Zozobra]

Thanks, never did get any decent clips of it before it went to its home. I'll get some soon though even if its only with my Zoom Q3.
This is the finished article:

[loudthud]

I mean a "warm" overdrive suitable for playing blues, which would as I understand it, emphasize 2nd order harmonics.

Don't try to overthink the design based on some tome you read on the internet. The only thing you need to know about harmonics and distortion is: Tubes!

Even order harmonics are produced by single ended amplifying stages (even before they clip), and by push pull stages that are overdriven to the point where they do not clip symmetrically. Most tube guitar amps do this automatically, no modification is required. Some odd order harmonics will be produced, but they will be balanced by even order harmonics. Negative feedback increases the concentration of higher order harmonics relative to the amplifier without feedback, so for the warmest tone, don't use feedback. A good dose of B+ sag induced compression and the rest is just voicing.
　
Push pull power amps are said to cancel even order harmonics. This is somewhat misleading. They do not cancel harmonics already present on the input. They do not tend to produce even order harmonics because deviations from absolute linearity tend to be symmetrical. This applies below clipping. When a tube power amp clips, there tends to be a baseline shift on the input due to grid current and that will produce even harmonics on the output because the squarewave will not have a 50/50 duty cycle. A 60/40 duty cycle is more common.
　
A Fender 6L6 blackface as suggested above is a good choice. A Champ is too limited on power although the tone is a little bluesier. If I was building an amp for blues, I would try parallel single ended 6L6s (2 or 3) and aim to get into the 20 to 30 Watt range.

Some reading that may interest you:

Discussion of how symmetrical distortion produces 3rd harmonic:
http://www.passdiy.com/pdf/distortion_feedback.pdf

Discussion of 2nd harmonic cancellation:
http://www.passdiy.com/pdf/zenlite.pdf

[Austin]

A Champ is too limited on power although the tone is a little bluesier. If I was building an amp for blues, I would try parallel single ended 6L6s (2 or 3) and aim to get into the 20 to 30 Watt range.

A parallel se 6l6 champ would be a little monster! You could sub in different tubes too, like one 6l6 and one el34, they wouldn't have to be the same. You could even run just one ouput tube for reduced power if you wanted. Only problem is that output transformers that can put out 30 single ended watts are going to cost a bit more and be large and heavy.

[pdf64]

Check out the Weber 'Lauren' kit https://taweber.powweb.com/store/kits_weber.htm#Lauren
It looks good and currently 10% off!
Pete.

[Austin]

Check out the Weber 'Lauren' kit https://taweber.powweb.com/store/kits_weber.htm#Lauren
It looks good and currently 10% off!
Pete.

I have never seen an 807 guitar amp before, I bet it might sound pretty good! The data sheet shows up to +30 grid volts and 600 milliamps of current! Wow if it were class a2 that could be alot of watts....

[tonequester]

tonequester here.

Thanks to all of my advisors. I greatly appreciate your input/output ! ZoZobra, I will check into getting my hands on a copy of Bleacowes book and will check out your schematic as well. I find that just about every schematic that I have viewed on this forum is quite worthwhile in furthering my continued education. By the way, the amp pic is most excellent. If it plays half as good as it looks, you must be quite proud. I most certainly would ! JM, I have seen some of the ads for Blackface clones. I know that I have heard several Blackfaces in my time, but I have never heard anyone play(or set one up for playing) blues. In the day, I could have picked up an original for less than I paid for my practice amp.
My first tube amp was a Princeton with tremelo instead of reverb, at a cost of $50.00. It looked brand new. I will check further into the clone kits, and I,m gathering up alot of info on
power scaling. The AX84 has been suggested as a first build for me, and if they supply as much info as you say, plus 'the sound" it's worth checking into further. Amp Kat. I agree with
the "more than one way to skin a cat" philosophy. I also agree with your point that tubes, is a magic word here all by itself. I have come to realize that what you said about the "hybridizing"
of amps these days, is correct. I have to admit that although I now know just how much S.S. is "blended" into my HT-1R, the fact remains that it's a great sounding amp(when it works) !
Glad to make your aquaintance Loud Thud. Thanks for giving me plenty to think about. I am especiall is they anxious to read the information that you have pointed to concerni would run
may very well have some misconceptions on the subject. I'm glad to here that you would run parallel, single ended 6L6's to get to the 20-30 Watt range. I have pretty much determined
that 20 Watts is my minimum. pdf64. Glad to make your aquaintance here as well. Correct me if I am wrong, but isn't the Weber you refer to the same company that makes hand-built speakers ? I will check out the 'Lauren' kit you suggested. if it IS the same Weber, I would bet that the quality would be there. I think that their speakers are great. Austin, thanks for the heads up on the use of differing tubes in a parallel configuration, and the warning about the transformer situation. I don't so much mind heavy, but "heavy bucks" is always a consideration for me, and usually a stumbling block as well. Once again, thanks to all for a plethora of information for me to consider. Best Wishes. tonequester.