Turn your Express into a Class A amp?

[rooster]

OK, I wonder if anyone else here has done this? I have one that I was not using that much because it is too loud, so I thought about this. (I have a smaller one, 25 watts that I use all the time and love.) Also a friend of mine has a Class A amp with reverb built by a pretty big name builder locally. It runs 6L6s and has a grindy tone that I really don't like that much - however - given his hard picking stype and LP, it seems to cut a room pretty well.

I have built a couple of Rockets and have an AC30 clone and I have worked on a few Chieftains over the years that I documented. So I thought, WTF, I had nothing to lose. The job was simple, convert the back end to Chieftain specs, change the 'Presence' to a cut control, and play the amp.

OK, it worked but the front end hiss was exaggerated even more in a Class A format. Hmm. And then the preamp was too powerful on the backend.

So what to do. After looking at things I realized I didn't have a lot of options with the hiss - it had to go. And that was the third gain stage I am talking about. So I looked the the connection from stage 2 to the PI and came up with a very simple coupling arrangemant after a few experiments. I pulled the .002 cap and replaced it with a .01. Likewise I pulled the .1 and replaced it with a .0047. Building a 'bridge' then of these two caps with a 270K resistor between them, I connected the three combined (in series) components to the tail of stage 2 and the input of the PI. I put heat shrink over the parts and just 'flew' it in place.

OK, so now the V2 tube is not hooked up, you can pull it out and the amp will still run.

And I also gave myself a switchable option of the V1 first stage cathode cap from stock (1.5K/25uf) to a single 1K resistor, no cap. Matchless uses the whole tube and a single resistor of 1.5K value. When you do the math, for just 1/2 a tube then, you would use a 750 ohm resistor. Eh, 1K is close enough. (FWIW, between the two options, I prefer the 1K value.)

Alright, I know, to some of you here it must seem like I really bastardized a good amp. Well, its not like I can't go back. That said, I doubt I will. This amp is very fun to play at all volumes and has a very smooth breakup. It loves pedals, too. The plates are seeing 370VDC or so and - SURPRISINGLY - where I use the Cut control on the AC30 to get most of my treble, with the Express tone stack and where it sits, I can turn both the Treble and the Cut control fairly high (12+ o'clock range without any icepick). Also I can really use the bright switch to good advantage at lower volumes, it behaves better here than with the Express, actually. Also note: I replaced my 1M volume pot with a 500K version, like the AC30. I have plenty of power but it also sounds great at lower volumes, too. This was a good decision for me overall.

OK, I know Rawnster is doing the Normster kind of build and using EL34s in Class A, who else here is doing this or has done this? Did you like what you ended up with?

[dartanion]

Explain why you believe this would be operating in class A.

Both the Express and the Chieftain are class AB.

Interesting experiments. Sounds like you found something you really like. That's always satisfying and feeds the amp building desire.

[rooster]

Hm, well I read Aikens, too. Is this really the time and place for an Aikens Rally?

And I think, like Aikens, you are saying that the Rocket is also Class AB, as well?

OK. Well I guess I don't mind being called ignorant or stupid if that is going to happen because I am in good company. The attached flyer is from KF years ago. It is printed on red paper so the black ink is hard to read. But you can read it. Clearly KF refers to the Rocket as Class A and I totally get what he is saying. Is he wrong? Eh, I'm too busy with other things to be a cop about such Aikens things, you know?

I do want to answer your question, but see this link first? http://www.badcatamps.com/badcat_amps.html#section
(Liars in the year 2010, right? Even with the enlightenment of Aikens articles. How can these people be so stupid, you know? Bad Cat Amps is just a bunch of hillbillys. Shame on them!)

So I am saying that if you talk (had talked in the case of Ken) to amp guys like Mark Sampson, Ken Fischer, Dave Funk, Gereald Weber, etc., they don't have a problem calling an AC30 'Class A' even though Aikens does. And I'm like them on this subject.

Well, so my answer: I am calling my modified Express, my Rocket, my AC30, and my Fender Champ, all Class A amps because this is a language I have learned from people whom I put more stock in than folks like Aikens. That's why.

[dartanion]

The point is that the definition of class of operation is often ignored and lore takes over. It's easier to just let the myth continue than it is to indoctrinate the masses in the truth. I get the question all the time, "are your amps class A?", and I explain the truth to them whether they understand what I am saying or not. It interesting that even well skilled builders still argue over things like this. The definition is pretty clear, yet you get folks who will tell you there are 13 inches in a foot. WTF?

Randall Aiken is an extremely intelligent guy who has been kind enough to share his wisdom in his tech pages. Nothing is more enlightening than objective evidence and his AC30 white paper is brilliant. There are so many misconceptions about "class A" that it's a breath of fresh air to see Randall dispel the myths with hard evidence.

Don't take this personally as I am only pointing this out.

[FYL]

So I am saying that if you talk (had talked in the case of Ken) to amp guys like Mark Sampson, Ken Fischer, Dave Funk, Gereald Weber, etc., they don't have a problem calling an AC30 'Class A' even though Aikens does.

Well, Aiken is right: the AC30 isn't a class A amp. Neither are the Liverpool, the Rocket and most of the others.

Some people consider that any cathode-biased amp is class A. They're just plain wrong.

[martin manning]

I do want to answer your question, but see this link first? http://www.badcatamps.com/badcat_amps.html#section
(Liars in the year 2010, right? Even with the enlightenment of Aiken's articles. How can these people be so stupid, you know? Bad Cat Amps is just a bunch of hillbillys. Shame on them!)

From the Bad Cat link:

"Ideal for low-volume studio recording, the Mini II is a Class A, point-to-point wired, tube powered brute. Experience the sweet tone of a dual overdriven 12AX7 pre-amp section and a single EL-84 power tube in your bedroom without disturbing your neighbors."

Note that they say this amp has a single-ended power section. I don't think anyone will deny that a single-ended amp is class-A, at least until it starts to clip ;^). With the Trem Cat write-up, however, it's another story. In that case they are using the AC30 mystique and dogma regarding class of operation which, in my opinion, is pure marketing. Caveat emptor.

MPM

[Merlinb]

Well, so my answer: I am calling my modified Express, my Rocket, my AC30, and my Fender Champ, all Class A amps because this is a language I have learned from people whom I put more stock in than folks like Aikens. That's why.

Why bother to mention its class at all though, if the description is simply wrong?
If you tell someone your amp is class A they will say "so what?".
If you tell them it is class A they will (should) say "Oh, class A like the AC15, so not class A at all. So what?".
And if you still tell them it's class A they will say "oh, so it's single ended then. So what?".
And if you persist in saying it is in fact push-pull and class A they will say "oooh, so it's true class A. What's class A anyway?"

[marcoloco961]

I have an Adobe file that explains classes of amps well. I don't pretend to understand it, but it's way of determining the class of amp is pretty straight forward. Attached is a file detailing their explanation of a class A amp. The theory I can understand. But how one would figure if an amp is true class A without an 'O'-scope test is beyond me.

Could anyone briefly explain the best way to determine which classification the amp falls under? Or how one would do an 'O'-scope test to determine classification?

[FYL]

But how one would figure if an amp is true class A without an 'O'-scope test is beyond me.

Class A = max current flowing at all times; no difference between idle and full blast. In practice there can be a few mA diff.

Class AB = idle current flowing at all times; one tube drawing max current while the other stays idle for half an alternance, then the reverse for the other half.

[rooster]

With this explanation then I get your intent, dartanion. No problem. Too, I see that there is a place for people like Aikens, why not, and yes, as you say, he is a very prolific technical writer. Certainly he has posted things that he must have imagined would polarize some people. But maybe he didn't. Either way, at the end of the day it is a very unimportant truth in regard to amp building. ....And I say 'unimportant' because you can't do anything with it, its just looking at something old, seeing that it may have been mislabeled and renaming it. Who cares? Do any of you think Ken Fischer did? Finally, it doesn't mean that Aikens invented the Class A that BECAME Class AB - he invented nothing new, only made an observation and posted it on his website.

Also I am not one of those people who is that concerned about his postings to want to 'meet him after school', eh, you know? However, I do want to say that in the world of guitar playing, and yes, amp building, there are guys who have done a lot of wonderful things without the benifit of any real scientific knowledge whatsoever. Frankly, the fact that Dick Denny designed and built an incredible sounding amp - and all with the mindset that it was operating in Class A - speaks more to me than a voice who comes along 30 years later saying that this not the case exactly. Eh, but maybe that's just me.

In a way, to me, its like someone declaring, after the painting has been enjoyed by many people for many years, that a color that Van Gogh called 'blue' is really teal. Why should this information matter? Will knowing this change the color or what the artist did with it? Will a new artist paint the same sky, call it 'teal', and will it, because it now has the correct name, have more of an impact on the world?

I was presenting an alteration of a circuit on the ampgarage, asking for some input from others who have done something similar. What I get is a a handful of people who are so overwhelmed with the color classification that they cannot discuss the painting. Never mind then.

[Jana]

Oh, I like the analogy of the colors teal and blue!

Okay, I'm going to do something that I don't usually do--moderate. LOL, I am usually the one to "instigate!"

I think the underlying question from Rooster, which got lost in the the class discussion, is what is it about the the different amps that changed the hiss level or other characteristics of the pre-amp?

Is this correct?

[rooster]

Jana - Ha! But yeah, sure, that was of interest to me.

I can report that the Rocket preamp, all 3 stages, are quieter (less hiss) than the modified Express. Which is pretty interesting to me. It suggests to me that - if you had the interest - that you could build a Rocket preamp stock, and then add the Express 3rd gain stage on the end of this (with the 2nd half of V2) and end up with less hiss than a stock Express. ?? Yes, I think this would be the case, however the overall gain might have to be reduced. But maybe not because the Rocket uses a cathod follower for stage 3 that doesn't really put out a lot of gain. See what I mean? Well, this would really be bastardizing the Express circuit however.......

I am trying to think of an existing amp that uses this topology but I can't think of one.

But I am convinced that the 'hiss' of the Express is generated by more than one stage now, and I always thought that all of it came from stage 3. And I have clearly observed that the Vox/Rocket/Marshall/Bassman preamp - what with using the cathode follower for stage 3 to feed the tone stack - is quieter than what the V1 tube (tone stack after stage 1) in the Express is feeding stage 3. ...Well, right now I feel this is correct. My tubes are good ones and as quiet as they can be I think.

OK, but Jana, since you asked, I was also curious if there were others out here who just kinda gave up on the Express and maybe considered altering it a bit. Especially considering that the Rocket exists as it does and Rawnster is enjoying the Normster Reverb circuit so much - which is a *Class A* type circuit, creating a little less output and a bit more grind.

[Jana]

Substitute "cathode biased" for "Class A" and you might not have to wear the armor plated undies.

I haven't built a TW so I can't comment on specifics. What I do know from my experience with other amps is that hiss comes from all stages. Usually it is the first stage that is the biggest contributor. Not because the first stage hisses more but because all the following stages have a chance to amplify the hiss from that stage. Take a standard 4 hole Marshall and crank everything up and there will be hiss. Take a 2203/2204 style amp and turn up the master and the gain and there will be a lot of hiss.

The TW circuit doesn't generate any more hiss than other amps, it's just that there aren't any attenuation circuits in there to cut it down.

Just my meager ponderings.

[martin manning]

Someone says: "I want to run my Express in class A." People who are sticklers for precise language reply, "You can't do that, it's not possible." The counter argument is, "But other people have been doing it for years."

If it turns out they weren't, or maybe they just thought they were, or maybe some other people who didn't know any better said they were, and it caught on, it really doesn't matter. The point is that the question "how can I make my Express run better now that I've converted it to class-A," strictly speaking, doesn't compute.

It's like me saying "I want to drive my car under water, and it's not working very well. I've heard a lot of other people say it can be done so I should be able to do it." People who are very knowledgeable about cars don't know what to say because, well, cars just don't run under water without taking some very extraordinary measures, and they just can't get past that. Now suppose it becomes clear to all that the people who say they are driving their cars under water are really only driving in the rain. Now that everyone understands the problem, they can move on and discuss the requirements for operating under those conditions.

What is needed here is an accurate description of what is being referred to as "class-A format." If it's some particular topology that is being referred to, then describe the problem that way, and avoid the "C" word.

MPM

PS, Jana, I tried to think of an analogy using colors, but I couldn't come up with one.

[rooster]

Jana - Ok, I'm going to bail here but it has been interesting.

Right now I am thinking of a car mechanic that might post on this site. A customer calls him up and says, "I was backing out of my driveway this morning and released the clutch too quickly, killing the engine. What should I do?" To which the mechanic replies, "Well, I will send out a tow truck to pick up your car. We will replace the motor and get you up and running again as quickly as we can."