Class A Trainwreck?

[mtheory]

I'd like to know if anyone has tried a cathode biased pair of 6ca7s in an express build, and if so, what was found along the way. I'm planning a new build, and would like to use a pair of 6ca7s, but I'm aiming for an output of 25-28 watts.

I believe, based upon what I've heard, that the Express has everything I'm looking for in terms of voicing, gain, and responsiveness, although I'm aware that dramatically altering the output section is going to create a different amp.

I'm hoping to keep intact some of the key parts of the original that I find so appealing, but with lower output. (I'm not at all into 6v6 grind, btw, and don't need another amp that I have to use an attenuator with).

My questions center around any particular design elements, including transformers, that will have to be altered in order to produce something SIMILAR to the Express, but with cathode bias, and pushing between 25-28 watts. Whatever insight can be contributed would be greatly appreciated!

Thanks.

[Zippy]

I'd like to know if anyone has tried a cathode biased pair of 6ca7s in an express build, and if so, what was found along the way. I'm planning a new build, and would like to use a pair of 6ca7s, but I'm aiming for an output of 25-28 watts.

My questions center around any particular design elements, including transformers, that will have to be altered in order to produce something SIMILAR to the Express, but with cathode bias, and pushing between 25-28 watts. Whatever insight can be contributed would be greatly appreciated!

Merely changing to cathode bias does NOT put you in Class A - regardless of what you read in the advertisements of the guitar magazines. You'll need to substantially drop the B+ voltage as well. Just changing the bias method does not have a profound effect on the output characteristics - witness those that have made their output bias switchable (fixed vs cathode). Most report that it's not worth it.

"Aiming for an output of 25-28 watts" - the Express is "only" rated at 30 watts. Going to 25 watts is a barely detectable difference.

[mtheory]

Merely changing to cathode bias does NOT put you in Class A

"Cathode-biased with no negative feedback" is a perfectly acceptable description of the term "Class A." Not that I personally give a rat's padoodle, mind you, but some do. My main goal is 25 or so watts, not someone's technically perfect definition of Class A.

witness those that have made their output bias switchable (fixed vs cathode). Most report that it's not worth it.

That's not at all what I'm proposing, but thanks for the heads up.

"Aiming for an output of 25-28 watts" - the Express is "only" rated at 30 watts.

Since when? I've only ever heard 35 watts.

Regardless, you're missing the point here as well. What I'm looking for is Express tone and gain sooner, through less headroom.

I've got amps of 7, 14, 18, and 50 watts that I currently gig with. I'm looking for something that's more than 18, but under 30. I'm focusing on starting with the Express at the moment because I really like the voicing, gain, and responsiveness of those amps.

If you've had specific, personal experience in converting an Express to cathode bias, I'd like to get some feedback on the specifics.

Thanks.

[stoo]

"Cathode-biased with no negative feedback" is a perfectly acceptable description of the term "Class A." Not that I personally give a rat's padoodle, mind you, but some do. My main goal is 25 or so watts, not someone's technically perfect definition of Class A.

Well then I guess my Marshallesque 18 watter is a class A!
Stew

[dave g]

Merely changing to cathode bias does NOT put you in Class A

"Cathode-biased with no negative feedback" is a perfectly acceptable description of the term "Class A."

You may want to retract that statement before the angry mob shows up

[mtheory]

Well then I guess my Marshallesque 18 watter is a class A!

Could be. The term isn't exactly defined with crystal clarity now, is it? Check the current draw at idle vs. cranked, and you'll know for sure.

It's really not at all relevant to this thread, however. I don't care if this amp ends up being Class A or not. My main focus is to have something with a pair of 6ca7s putting out around 25 watts. Any technical definition of what that actually is, from a class definition standpoint doesn't matter at all to me.

[mtheory]

You may want to retract that statement before the angry mob shows up

Seriously? That doesn't say much for the mob, does it?

ONCE AGAIN, the Class identifier of this build is a meaningless point to this thread. There simply is no reason to focus on clinical definitions of the term in this thread.

All I want is some information on making a cathode-biased Express that can clock in at around 25 watts or so.

[benoit]

"Cathode-biased with no negative feedback" is a perfectly acceptable description of the term "Class A." Not that I personally give a rat's padoodle, mind you, but some do. My main goal is 25 or so watts, not someone's technically perfect definition of Class A.

Class A operation has exactly nothing to do with cathode biasing or lack of negative feedback. The "perfectly acceptable" definition of class A operation requires that plate current flow in all output devices for the entire 360 degrees of the input cycle at the maximum undistorted output of the amplifier. This can happen with or without NFB, in fixed bias or cathode bias, push pull or single ended operation. I say this not to be argumentative or cause trouble, but simply to avoid the perpetuation of widespread myth mostly due (apparently) to the use of "class A" as a marketing buzzword in the guitar amp, hifi, and pro audio worlds. Randall Aiken has a great writeup on his site that explains class A in great detail. You can find it here: http://aikenamps.com/ClassA.htm

"Aiming for an output of 25-28 watts" - the Express is "only" rated at 30 watts.

Since when? I've only ever heard 35 watts.

30-35W is a difference of 10log(35/30) = .67 dB, which is just audible in a quite room and indistinguishable in a club or other live setting. Loudness will also depend on efficiency of speakers and enclosure so talking about the difference between 30 and 35W given the same cab is splitting hairs, and if different amps/cabs is more like apples and oranges.

However, I've heard people say they measured closer to 50W undistorted.

Regardless, you're missing the point here as well. What I'm looking for is Express tone and gain sooner, through less headroom. However, 50W to 30W is still only 2db difference which is still not much (all other things - cab/speakers - being equal).

If you've had specific, personal experience in converting an Express to cathode bias, I'd like to get some feedback on the specifics.

https://tubeamparchive.com/viewtopic.ph ... as+express
https://tubeamparchive.com/viewtopic.ph ... as+express
https://tubeamparchive.com/viewtopic.ph ... as+express

I know you're frustrated because you're not quite getting the answers you were looking for. I mean no harm here, but you shouldn't be surprised people reacted negatively when you made a grossly inaccurate statement about how tubeamps work on a very technically oriented forum. It may not be a big deal to you, and from a "how does it sound" perspective, whether you're a player or builder it shouldn't matter much what it is in terms of making a great amp. But you'll find people a lot more ready to help out if you ditch the sense of entitlement to answers and don't pooh-pooh the regulars who are just trying to set the record straight. The search function works great as well. I'm not the slightest bit upset about anything myself and this is meant as friendly advice so I hope it's taken in that spirit. This is a cool place, and you'll get as good as you give here. Good luck with your build!

Ben

[dartanion]

Class of operation IS well defined, but believe what you want to.

If you don't care about what class your proposed 25W Express operates in, you consider changing the title of this thread.

Dig out your EL34 datasheets and figure out what you need to do.

[John_P_WI]

OR put VVR or Power scaling on the Output tubes and drive the snot out 'em. That 'll get you less head room

[Firestorm]

I hate to let a good argument go unjoined ... but I will.

If the objective is to hold an Express to 25 watts (without regard to class or bias), you just cut the B+ voltage, or cut the screen voltage, or both. Get out your 6CA7 graphs and you'll find exactly where you want to be.

That said, you won't be able to hear the difference. Most folks run their volume pots close to 12 o'clock (I keep mine at 10:00 and that's with a 6072!), so maximum output power seldom comes into play.

[jjman]

I'm using 6v6's in cathode bias idling 12-13watts @ 345volts on the plates. (Not even close to class A per my scope.) Either with or without NFB I'm getting 9watts of max clean output.

Although most of my voltages are lower than the typical Express, the preamp and PI are essentially an Express. Optional breakup at reasonable volume was my goal and I’m getting that either with or without using the post PIMV I also have installed.

I recently changed the 6v6 grid leaks from 220ohm to 82ohm to get rid of the crossover distortion I was experiencing during near max to max volume. I tried higher value stoppers but switched to the leaks. My lower voltage on the 6v6s was allowing the PI’s output to overdrive the 6v6’s too much when the MV was maxed.

[funkmeblue]

I think the lowest wattage and close to class a design wreck would be half a liverpool or run an express at about 250 volts cathode bias or what the hell, run 'em ultra linear like alessandro does.

[mtheory]

I never intended to open an discussion over the clinical definition of Class A amps. I really don't care what class this ends up being, just so it gets me around 25 watts from a pair of 6ca7s. The first responder to this thread made the assumption that I had bought into some marketing gimmick, and that's why I was referencing Class A, but that's not the case at all.

The definition that I referred to is one that I got from a book I'd read. If it doesn't qualify as an acceptable definition, I apologize for my ignorance. From what little I DO understand about Class definition, I don't consider it to be something to obsess over, and it seems to be far less than crystal clear with anything but a single ended amp.

The "perfectly acceptable" definition of class A operation requires that plate current flow in all output devices for the entire 360 degrees of the input cycle at the maximum undistorted output of the amplifier.

As I understand, there are amps running multiple output tubes that border between Class A and AB. I've got one that, based upon current draw, is nearly full Class A. The difference between idle and full throttle is almost nothing.

I realize that there isn't much discernable volume difference between 25 and 35 watts, but I also realize that there IS a discernable difference in headroom, and that's why I'm aiming for 25 watts. I know what 7, 14, 18, and 50 watts sounds like, so I didn't come by this target without any foundation for what I'm looking for.

But you'll find people a lot more ready to help out if you ditch the sense of entitlement to answers and don't pooh-pooh the regulars who are just trying to set the record straight.

I wasn't trying to pooh-pooh anyone. I just wanted to point out that the clinical definition of the term Class A wasn't really relevant to my question. I meant no offense at all by stressing that. I just wanted to keep on topic.

[dartanion]

If you are using a TW style PT with the 300-260-0-260-300 HT secondaries, use the 260V taps for HT and this should get in about that ballpark as the plate voltage should be in around 340V. Chose the appropriate cathode resistor to bias it around 12W per tube and you should be good.