Class B fixed bias…

[elpico]

Sorry I, think my poor eyesight has tricked me there. Looks like the idle bias says "39" not 33.

The drive voltage is only 36.5 peak, so not true that it's larger than the idle bias.

 

[Helmholtz]

What you're talking about is efficiency, not class.

No, not talking about efficiency but about a missing conduction gap at zero crossing.

The 6CA7 was a copy of the older European EL34.
In fact I have a Siemens PA amp with EL34s from 1955 that uses these exact operating conditions.
Not invented by Tung Sol.

 

[elpico]

You are talking about efficiency. You're dismayed that the idle dissipation isn't zero watts, which has no effect on anything except the amount of power the amp consumes. As I've already said, amplifier class isn't determined by idle dissipation. Or anything dissipation. Dissipation doesn't figure into it all. Only conduction angle at rated output. Not sure how to explain that a clearer way.

Maybe I should say out loud that amps made of real parts never miror the behaviour of the theoretically "perfect" amplifier of that class? It sounds like you're maybe expecting them to? In the ideal class A amplifier the active device dissipation drops to 50% at rated output. A real implementation with tubes won't do that. They'll all dissipate much more than the ideal suggests they should...and that doesnt matter. If the active device conducts for 360 degrees at rated output, it's class A. People try to make it complicated in their minds, but that's all there is to think about.

My understanding is that 6CA7 was a beam power tetrode designed to safely work in EL34 circuits no? I think the story goes that pentodes were a patented technology initially, so the US companies had to come up with designs that would work in those circuits but were different enough internally to steer clear of the patent. I think they might've originally been more like a 6550 tweaked to bias up okay in EL34 amps.

I never claimed that Tung Sol invented the 6CA7. I said because Tung Sol seem to have been a company full of sharp engineering talent, I'd tend to trust they knew what Class B means. It's quite a simple thing after all, would be kind of wild if they didn't! but anything is possible I guess.

Are you maybe suggesting that this was EL34 data they simply relabled "6CA7"? I guess again, anything is possible.

Like I've said, I have no way of testing their published data myself and I'm not claiming to know how many degrees the tubes conducted for at that operating point. Lacking any evidence to the contrary, taking their word for it is about all I can do.

 

[peteb]

Pete, the 250vdc to the plates is in the reduced power mode. In full power, there will be something around 500vdc to the plates.

The screen grid voltage is very low in those MM amps.

The MM amps were designed by Tom Walker and refined by Bob Luly.

This brings up an interesting point. Thank you Wally.

The 250 V is the plate voltage of the Fender 75, an amp kind of similar to the MM amps, but not really.


The MM amps have a lot of versatility and can cover Fender ground.


Clean and distortion channels.
Both with gain and volume controls.
High and low power switch (class B or class AB) applies to both channels.
Bright switch.
Deep switch.
T,M,B Eq.
Reverb.


High power means 700 VDC and nearing class B.

Low power means 400-500 VDC and regular class AB operation.


The high power setting is clean and the low power setting is more normal.


One user said the high power (class B) setting had infinite headroom.

 

[Helmholtz]

You are talking about efficiency.

No, I don't want to talk about efficiency.
I mentioned dissipation just as a side-note.

My point was that the 25mA idle current causes a small range of class A operation around zero-crossing.
Class A means that both tubes are conducting simultaneously.
It follows that the tubes are conducting for more than 180°, which is the (modern?) definition of class AB.
I scoped the amp and it works as expected.

Here's a Philips EL34 datasheet from 1956 showing the "Class B" design examples:

https://frank.pocnet.net/sheets/030/e/EL34.pdf

I think Tung Sol just copied the EL34 datasheet with the 6CA7 as it was intended/designed as drop-in replacement for the EL34.

 

[peteb]

but I noted that MM claims a list of attributes about the amp, of which cleanness is not mentioned.

You need to read ‘thru’ the advertisement.

You should read the reviews of the amp.

A clean machine and a fender it is not.


The demos that sound like a fender are undoubtably on low power mode, which is not class B and not part of this discussion.

 

[peteb]

If they were trying to make the lowest distortion amp on the market they made a strange mistake by putting tubes in it.

They were going for the best of both worlds.


I think that any explanation for what they were striving for that does not include mention of the tone implications of 700 V on the plate is missing the point.

What makes the MM class B ? Low current, high voltage.

You have to talk about that.

What is the affect of high plate voltages on tone?

Clean, quick response and tight low end.

 

[peteb]

For context here's a 6CA7 datasheet that has some class B operating point data. Output power @ 5 or 6% THD is given for a range of supply voltages between 400 and 800Vdc.

That is great data.

I don’t think they mean a range of supply voltages between 400 and 800.


They mean 800 plate 400 screen. The same recipe as MM, 700/350.

 

[peteb]

This design example is also found on EL34 datasheets.
But note that the idle plate current is 25mA, meaning an idle dissipation of 20W or 80% PDmax.
IMO this is rather hot class AB.

My inside information says that the MM amps idle at 6.4 mA, 4.5 W plate dissipation for 15% of max plate dissipation, a cold class AB bordering on class B.


It makes sense. High voltage, low current (class B requirement) means low plate dissipation.

In the real world.

 

[peteb]

And does amplifier class mean "what do the tubes do at idle"? or "what do the tubes do at low signal level"? Not in the slightest.

https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/RCA-Books/RCA-Receiving-Tube-Manual-1975-RC-30.pdf

Class B is defined on page 25.


You are right that conduction angle during full output power completely defines classes A and AB.

Class B is defined differently than classes A and AB.


Please see class B on page 25.

 

[hepular]

You need to read ‘thru’ the advertisement.

You should read the reviews of the amp.

A clean machine and a fender it is not.


The demos that sound like a fender are undoubtably on low power mode, which is not class B and not part of this discussion.

you claimed in post 5 about Music Man amps

"clean output power was their primary goal.


cleaner than the Fender's of the day."

then when that was disproven you don't just shift goal-posts you head over to the cricket pitch.

 

[Bendyha]

You need to read ‘thru’ the advertisement. You should read the reviews of the amp. A clean machine and a fender it is not.

I have not stated that the MM 100RD is not capable of reproducing a sound that could be classified as "clean". Undoubtably it is! Only that your claim that the designer's primary goal in making the amp class B, was to achieve, above all else, a clean sounding amp. If that was what they were aiming for, they would have designed it differently.
Grounded-Grid tube amplifiers, like the 100RD is, were quite popular in the 1940 and 50s. They were used in transmitters, and there are many articles about grounded-grid linear amplifiers to be found. MM purposely departed from the more traditional tried and proven designs, not wishing to be as linear and clean sounding, and changed the output section somewhat, to introduce a warmer, less hifi-like tone.

I would like to be able to analyse the merits and any shortcomings of the grounded grid circuity, and why it is so well suited, if not predestined, to be used in class B operation, as it almost invariable seem to be, but it is a field of amplifier design that I feel unqualified to participate in a discussion about, other than to be able to clearly identify and callout irrelevant and uncircumspect loquacity.

The demos that sound like a fender are undoubtably on low power mode, which is not class B and not part of this discussion.

Then why did you introduce it?

 

[peteb]

then when that was disproven

Huh ?

Please read user reviews of the music man amps and this discussion does not take place.

I did want to add this however,


Leo only had one play in his playbook when designing a new line of amps.

1. Make the new amps cleaner than the previous amps.


See woodie > tweed> brown > black > silver > MM


It is a one way progression.

 

[peteb]

I can’t think of any examples?

You were right on post 1.


There are no commercially available class B tube guitar amps.

The 6CA7 tube specs for class B operation do not support class B operation either.


In these cases when they say class B, they mean class AB.




Q1: Why are the music man amps considered class B when they are not.

A1: The bias is considered cool.

Q2: if the bias current is 0.025 A, why is the bias considered cool?

0.025 A x 700 V = 17.5 W, 17.5W / 25 W = 70% of MPD.

A2: the bias is perfect for a class AB amp which the amp is.
The bias is considered cool because the current 0.025 A is lower than expected for a 6L6/EL34 type of tube. Usually a 6L6 gets biased at 30-33 mA.

 

[hepular]

Huh ?

Please read user reviews of the music man amps and this discussion does not take place.

I did want to add this however,


Leo only had one play in his playbook when designing a new line of amps.

1. Make the new amps cleaner than the previous amps.


See woodie > tweed> brown > black > silver > MM


It is a one way progression.

NO. If that were the case, the Master Volume switch and boost circuits would have never appeared.

 

[Helmholtz]

Point is that textbook class B would essentially mean zero idle current, resulting in severe crossover notch distortion.
This kind of distortion is special as it (as a percentage) reduces with increasing output, meaning that it shows most at low output.
Also very "cool" class AB operation can exhibit crossover distortion.
Using lots of global NFB can help.

I think the MM circuit with the tube stacked on a BJT lowers crossover distortion, as the transistor CE signal adds to the output and likely helps to fill the crossover gap.

 

[sds1]

The bias is considered cool because the current 0.025 A is lower than expected for a 6L6/EL34 type of tube. Usually a 6L6 gets biased at 30-33 mA.

????

 

[peteb]

????

Do you have a question ?

 

[peteb]

NO. If that were the case, the Master Volume switch and boost circuits would have never appeared.

I have read a lot about Leo and Fender. Leo is all about the clean sound.

And

Even considering MV and boosts etc.


There still is the clean sound.


It all starts with a good clean sound.

 

[hepular]

I have read a lot about Leo and Fender. Leo is all about the clean sound.

Uhhh. Leo did not design a single silver panel amplifier.

Ed Jahns, otoh . . .