# How is it possible to run a 12AX7 vacuum tube at 415VDC?

1. Jun 27, 2016

### Planobilly

Hi,
Here is the schematic link: http://www.classictubeamps.com/schematics/Mesa/Mesa%20Boogie%20Solo%2050%20Rectoverb%20schematic.pdf [Broken]

The 12AX7 inverter tube has a measured plate voltage of 416 VDC just like the schematic indicates.

V1a rail voltage is 394VDC with a 200K plate resistor and a cathode resistor of 1K8 to ground.

I assume this amp design would eat preamp tubes on a regular basis!!

What do you think?

Cheers,

Billy

Last edited by a moderator: May 8, 2017
2. Jun 27, 2016

### jim hardy

3. Jun 27, 2016

### Planobilly

Hi Jim,
So I am asking having 415VDC from plate to ground is OK on V3b? I measured 416VDC from plate to ground and the schematic indicates 415VDC. If so what does the 300V max plate voltage on the data sheet under ratings Indicate? I assume one can and often does exceed the max.

The max plate dissipation is 1 watt and a typical plate current would be 1.2 mA. with a plate voltage of 415 V the plate current would be 1/415=2.4mA X 70% = 1.7mA. Most likely anything around 3mA would red plate the tube if not less.

So...I ASSUME the plate voltage is not so important as long as the cathode resistor keeps the current flowing through the tube within a certain range. I also ASSUME there must be some max plate voltage that will cause the tube to arc. One may get away with a certain level of increased plate voltage but you will not exceed the max current flow for long

NOTE: Plate voltage does have an effect on the sound and headroom...so I understand why Mesa would want higher plate voltage...I just wonder how close to the limit of a 12AX7 they are and what is the effect on tube life.

Also I made a mistake V3 is not the inverter tube, it is a preamp tube in the lead channel.

I don't often see these high voltages on 12AX7 preamp tubes. I assume it is being done in this way in this amp to cut cost as I can think of other ways to produce high gain preamp circuits with a lot less stress on the tubes.

Billy

EDIT: I understand now. The voltage is referenced plate to cathode not to ground as would be the case with a grid bias tube.

Last edited: Jun 27, 2016
4. Jun 27, 2016

### jim hardy

I knew you'd figure that out ...

look at the datasheet
http://www.drtube.com/datasheets/12ax7-sylvania1955.pdf

there's limits on heater to cathode voltage too. I never thought about that before, i suppose heater positive would steal cathode current away from plate ?

5. Jun 27, 2016

### davenn

yes, note that the cathode is lifted above gnd by that 100k resistor

6. Jun 27, 2016

### Planobilly

Thanks guys,

I also never gave any thought to heater to cathode voltages. I assume that would be the limiting factor as to how high the plate voltage could go. We are already at 215 volts on the cathode with the data sheet indicating a 200V max.

Billy

7. Jun 27, 2016

### Tom.G

The real limiting factor (danger) is insulation breakdown between heater and cathode. AFAIK it is only the outer surface of the cathode that is coated with the electron emitting material, often an oxide of Barium or Thorium.

8. Jun 28, 2016

### Planobilly

I think the reason for "pushing" these limits with 12AX7 tubes have several causes.

1.There are measurable differences in the "sound" a tube exhibit due to various relationships of plate voltage and current flow through the tube.

2. There is a need for higher gain in preamp stages to produce certain "sounds" associated with certain styles of "modern" guitar music such as "metal music" for example. For other styles of "modern" guitar music high gain is not useful or wanted.

3. The lack of availability and cost of other tubes that could be used as preamp tubes. The 12AX7 is the most common tube in use today for preamp tube and is widely available.
There are other options for preamp tubes as in the schematic below.

9. Jun 28, 2016

### Planobilly

Photos I took of a custom DC 30 I repaired a few days ago.

Cost was not an issue with this amp. A standard Matchless DC 30 cost in excess of $4000 dollars. This one was not standard and I have no idea what it cost. The clean channel used 12AX7 preamp tubes. The lead channel used the EF86. 10. Jun 28, 2016 ### Planobilly The above amp is an example of an amp used in a true commercial environment. As costly as it may be, there in fact three on hand. The cost represents on a very small fraction of the equipment used to produce a concert. The mixing board used for this band for example cost in excess of$250,000, The cost to put on a show in today's world is in the millions.

11. Jun 28, 2016

### jim hardy

EL84/6BQ5 !
Great tube . I used a pair of them for the low frequency center channel in my high school stereo amp project. When i got to college it was voted best system in the dorm.

Ahhhh nostalgia - thanks bill .

12. Jun 28, 2016

### Planobilly

Do you still have the amp?? If so, we want to see photos!!

One has to have been around for a while to have much of a real concept of "nostalgia".....lol

Vacuum tubes use in audio are actually on the rise as is the use and production of vinyl records. Some of us have never been much impressed with MP3...lol

Vacuum tube technology is old enough to be "new" again. Better than transistor amps? Perhaps not or perhaps yes. In many ways it is all a matter of perception. Because we don't have a machine to measure perception the debate will continue...lol

To me the question is meaningless to begin with. Tubes do stuff transistors can not and transistors do stuff tubes can not.

Cheers,

Billy

13. Jun 28, 2016

### jim hardy

You know those things you regret ? The output transformer i'd bought were bargain basement Olsen brand, plastic encased
they swelled, split and shorted after about ten years
i remember a twinge of sadness when i put "Old Paint" on the trashpile .
i replaced it with a Harmon-Kardon TDK3 .

i've had a pet theory since 1970's when solid state took over
and it's catching on now
Tube amps are a current source and that's why you need those 100 ohm resistors across output to protect the transformer.
remember your basics - a current source is high impedance and a voltage source is low impedance
loudspeaker drive by current source versus voltage source has to give different cone motion
it's the amplifier output impedance. It controls 'damping' .

https://www.passdiy.com/pdf/cs-amps-speakers.pdf

14. Jun 28, 2016

### Planobilly

I often real about the experiments and serious testing applied to stereo amplifiers/speakers. I seldom see anything near this level of testing and experimenting as it relates to guitar amps/speakers. Perhaps I just have not dug deep enough.

In my own mind I put these two types of amps into two categories. One, sound production amps and two, sound reproduction amps. ( I am not speaking about voltage/current here) The differences are huge.

The signal coming from an electric guitar has substantially less band width than the total audible band width. This being more or less in the range of a few Hz to around 5000 Hz including harmonics. Due to this fact, we need a speaker designed to produce those frequencies. The guitar amp also needs to amplify those frequencies.

While it is possible to plug a microphone into a guitar amp and sing or play a flute, that same set up in both cases produce very unacceptable results.

Trying to compare stereo audio designs to guitar audio designs does not work well because the two things are made for widely differing purposes.

Add in the complexities of purposely altering the guitar input signal to produce other sounds and effects the guitar amp moves even farther away from the typical stereo amp. To add insult to injury there is no real physical connection from the user to the stereo system but a very complex and real physical contact between the guitar player and the amp.

Obviously there are other musical instrument sound reinforcement amps other than guitar amps. Those amps are designed to accommodate the needs of the instrument in question, keyboards for example which again are vastly different from guitar amps.

Too many multisyllabic words in this post....lol...getting beyond my spelling abilities....lol

Cheers,

Billy

15. Jun 28, 2016

You guys touched on the debate of how one sounds vs the other.Frankly both from what I know and what I have heard while listening I think there is no audible difference between tubes vs transistors , the real audible difference comes from the difference in schematic , the layout itself and sometimes part quality.
I have heard differences between two transistor amps , and I don't think it's because of the transistors themselves rather the design and layout.
the most common difference I can hear between amps is the detail in reproduction, playing the same record one reproduces some small bits of drum and guitar while the other one makes them inaudible at that place etc.
I think that's all there is , everything else like talking about colors and warmth of an amplifier is slowly going into magic and in magic everything is possible.

16. Jun 28, 2016

### Tom.G

@Planobilly
Love those photos. The resistors actually have readable color codes! The Chinese ones these days make me think I'm going color blind.

17. Jun 29, 2016

### Planobilly

Hi Tom,

I assume Phil the designer got them new from the normal supply chain. Everything in the amp was well done. Carbon Comp resistors....not sure why anyone would use them in today's world. They are great if you like noise...lol

Well..I guess if you are important enough to have your name as the serial number on a amp then.............The good news is that Nick Jonas is a super nice kid who has not let his fame go to his head. He always says "thank you" and "yes sir" and is super nice to work with.

For me, I am going to Marathon now to go fishing!! No more amps this week!! Having said that, I am sure I will have to work on the auto pilot on the boat...it has been acting up a good bit lately.

See you guy in a day or two.

Billy

18. Jun 29, 2016

### jim hardy

i heard the hogfish are out

good luck !

19. Jul 1, 2016

### Planobilly

Well...back to the work bench!!

We left the dock at 5 am and finished cleaning up the mess around 9 pm last night. We caught the fish in the photo below in 830 foot on the bottom. Normally I would use a 10 pound lead weight but yesterday the current in the Gulf Stream was running close to 4 knots requiring 14 pounds of lead to get the bait on the bottom.

The amount of electronics needed to do this is pretty large. It has taken a year or two to learn how to use the equipment, some of which I designed myself. We now can catch these fish when ever we want to for the most part. Well...a little luck is also part of the deal..lol Running this little 28 foot boat 50 plus miles offshore is starting to kick my butt. Don't ever get old..!!..lol

Cheers,

Billy

20. Jul 1, 2016

### jim hardy

Wow.

When i was a kid we caught them a mile off Miami Beach.

Warsaw ?