mosfet follies question are you around RG Keen?

[swingarm]

Greetings!  Forgive me if i have posted this in the wrong area.

I was looking to try this method of driving the power tubes in a bass amp project. the tubes are 4x s11e12's and apparently these tubes do not like fixed bias if you go by forum scuttlebutt .

I bought these locally quite cheap and was told they where quite similar to 6550's and in some ways that seems true but they have a plate dis of 28w max 800v and max screen voltage of 300v. After doing some reading it seems everyone who tried to run these fixed bias failed. The tube data refers to fixed bias so this tube must run successfully fixed bias somehow  . I assume there could be issues for many reasons that the posters did not recognize or did not disclose. However I have no experience with this tube

Since I have no data to suggest how "hard" the 300Vs rating is, the safest thing is to use a voltage-doubler supply. This will provide the 300Vs and 600Va. Bias is then based on the 300Vs and might require a CF to achieve maximum output power. I thought maybe your Mosfet follower would be a good fit here. I was going to bias each tube  independently so i guess i would need to use 4 devices.


     I have a large power transformer with two secondaries  one is 438 -0- 438 with a resistance of 22ohms from one leg to the center tap the other is 330-0-330 with a resistance of 147ohms.

    From page 25 of Gar Gillies book it says the mil rating can be estimated up to 200mil with fair accuracy.

    I get 438/22= 19.90 x 25 = 498ma for the red wire secondary
    and  330/147= 2.25 x 25 = 56 ma for the brown wire secondary

   So another option  Is use the brown secondary for  the screens and preamp, and the red for Va.

Any advice would be appreciated. I've never used mosfets like this before.

[swingarm]

Here are some tube specs of the s11e12

RATINGS

Heater Voltage                                                       6.3           V              Vh

Heater Current                                                      1.6           A             Ih

Maximum Anode Voltage                          800       V          Va(max

Maximum Screen grid voltage                    300       V          Vg2(max)

Maximum Control Grid Voltage                 -100      V          Vg1(max)

Maximum Voltage between Grids 1&2         400       V          Vg1-g2(max)

Mutual conductance                                 13.5      mA/V    gm        *

Inner µ                                                  5.5                    µg1-g2   *

Maximum Anode Dissipation                     28         W         Pa(max)

Maximum Screen Dissipation                     5          W         Pg2(max)

Maximum Cathode Current                       300       mA       Ik(max)

Maximum Heater to Cathode Voltage         350       V          Vh-k(max)

(DC Heater negative)

Maximum Heater to Cathode Voltage         150       V          Vh-k(max)

(DC Heater positive)

Maximum Resistance Grid1 to Cathode       100,000 Ω          Rg1-k(max)

                        Fixed Bias

Maximum Resistance Grid 1 to Cathode      1          MΩ       Rg1-k(max)

                        -Cathode Follower

Maximum Acceleration                             2          g

(Continuous Operation)

Maximum Shock                                      500       g

(Short Duration)

Maximum Peak Anode Voltage                   1,500    V          Va(pk)max†

* Measured at Va=Vg2=150V; Ia=200mA; Ig2=12mA; Vg1= - 8.5V.

† For duty cycle of 1/25 and maximum pulse duration 200µ seconds

All Maximum ratings are Absolute Values not Design Centres.
INTER-ELECTRODE CAPACITANCES (Pf)

Anode/Grid1                                          1.8        ca-g1

Grid1/Earth                                            19.5      c in

Anode/Earth                                           16.5      c out

"Earth" denotes the remaining earthly potential electrodes, heater and shields connected to cathode
DIMENSIONS

Maximum overall length                                        98                     mm

Maximum seated height                                        83                     mm

Maximum diameter                                              44                     mm

Base                                                                  B8G

Net weight                                                         71                     g

Mounting Position – Vertical. If run horizontally then it is recommended that the axis AB be on a vertical plane

Diagram showing
CONNECTIONS

Pin 1                 Internal Connection         IC

Pin 2                 Heater                           h

Pin 3                 Anode                           a

Pin 4                 Grid 2                           g2

Pin 5                 Grid1                            g1

Pin 6                 Beam Plates                    bp

Pin 7                 Heater                           h

Pin 8                 Cathode                         k

Note. – Pins 6 and 8 should be connected at the valve holder

[Phoenix]

Hi Swingarm.

I'm not sure exactly what your question is, you seem to have a few, and to maybe be getting ahead of yourself.
I'd suggest you first try getting your poweramp working in class AB1 before adding mosfet source followers to the mix for class AB2.
You should decide on a some voltages first, then use the datasheet to choose a loadline. If you want to use a higher screen grid voltage than is shown on the datasheet then you can estimate new curves based on the data that you do have and choose an appropriate loadline for the new curves.

One thing to note, on your second post you've highlighted the Rgk (grid-to-cathode resistance) for cathode bias, not for fixed bias, which is the value that you should use. We regularly exceed this value (and almost every other value on the datasheets) in guitar amps, but when playing with unfamiliar tubes it's definitely simpler to stick to the maximum datasheet values to avoid potential problems. Regarding that value (Rgk) specifically, exceeding it may cause bias instability and thermal runaway. So stick to the 100k value for fixed bias. I think you're confusing this value because of your intention to drive the power amp with source followers (cathode follower equivalent), but if the power stage itself is still fixed bias, the driving stage has no effect on the requirement.

Also, the Vg2 (screen grid) voltage ratings on beam tetrodes is a bit more of a hard limit than it is on true pentodes like the EL34, if you've ever played around with 6550's or KT88's with high screen grid voltages you'll know that you get a marked increase in failures without any tangible benefits. As you increase screen grid voltage, you simply increase the drive requirements, and increase screen grid dissipation. These things are usually considered bad. Stick with a voltage doubler scheme, it works really well for beam tetrodes.

[swingarm]

Thanks or your reply phoenix. i am getting ahead of myself you are right. Its an obscure tube and I'm wondering i its worthwhile pursuing.
Its just that 6550's are so expensive and these cost me 75 canadian for 4. I'e built several guitar amps using the usual octal tubes but have never built a bass amp. Im using the svt preamp and noticed the svt's power amp used the cathode followers to drive the output tubes. This is quite a complicated project and i will make the multi taped inductor with instructions I found elsewhere. http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampInductor.html so yeah I'm ust in the planning stage and saw mr Keens discussion on mosfet drivers and thought it might be a good fit. I wonder what kind of neg voltage i might need for the screens?  (at the mosfet)any idea?

[swingarm]

You should decide on a some voltages first, then use the datasheet to choose a loadline. If you want to use a higher screen grid voltage than is shown on the datasheet then you can estimate new curves based on the data that you do have and choose an appropriate loadline for the new curves.
Thanks I'll look into that.

here is a link to one of a few discussions I've seen talking about bias issues with these tubes. I'm no electrical engineer just a tinkerer that likes to build guitar amps and learn a little something each time and keep my heart beating till completion.

http://www.diyaudio.com/forums/tubes-valves/135370-12e1-s11e12-kt88-substitution.html

[Phoenix]

Well if you're basing the poweramp off of the SVT, you'd be best to start by copying it (and making appropriate adjustments for the different tube type). Read through Mosfet Follies again, if you've understood it, you'll see that you can pretty much drop an appropriately rated mosfet in place of an existing cathode follower, which is what you'd do here. It also goes on to suggest places where you could ADD a mosfet, like in DC coupled poweramp grid-drive, in which case you need to add the supporting circuitry, but in this case all that supporting circuitry is already designed, you're just substituting sand-state for hollow-state.

Anyway, to actually answer your question - what voltage you need for screens (I think you meant negative rail/C-/bias voltage based on context)? You need to read through Mosfet Follies again. All the information you need is right there, you just need to wrap your head around it a bit better.

I'd still recommend you just start with a conventional class AB1 poweramp with AC coupled power tubes before complicating things with direct coupling. You seem very unfamiliar with these techniques, and they can be very tricky for even seasoned designers to get working exactly as expected.

[swingarm]

You seem very unfamiliar with these techniques

Indeed I am.My theory is weak. I own a DMM and a primitive signal tracer thats it

I will try to "wrap my head around this"

Thanks for your reply.

[R.G.]

I'll watch. But for the moment, what Phoenix said is the best way to go. Get your basics sorted out first, then worry about the subtleties.

[PRR]

This isn't a MOSFET question.

S11E12 is kinda-sorta a 6550 with a MAJOR difference.

The "28W" is insignificant. The S11E12 is rated for *pulse* operation in a TV projector. When pulse circuits go wrong, they go a lot more wrong than audio circuits typically do. The Pdiss number has to leave a lot of headroom.

The deal-breaker difference is "Inner Mu ug1-g2"; 5.5 for S11E12.

Mug2 is about 10 for 6550 and nearly all "audio" power tubes (EL84 is 18).

This is a fundamental difference in the tube's conductivity. ALL of the design changes.

While the general direction of change is clear, the exact values are not. And one thing I have learned from messing with HIGH power amplifiers is that small errors become BIG SMOKE and major crying for burnt parts.

One thing I am sure of: your Vg2 won't be anywhere near the 450V of that PT or the 300V of a happy 600Vp 6550 amp, but VERY much lower than any audio-tube builder would expect.

And cheap tubes do NOT mean a cheap amplifier! Transformer cost (with shipping!) overwhelms tube cost. Even with transformers in-hand, because you must expect failures and replacement transformers.

I'm also thinking that 99.44% of bass players will be happier with a pint-size 1,000 Watt SS amplifier than a 100W crate full of bottles.
_____________
Clarification: S11E12 is the pulse-rated small-glass version of 12E1, Mu 5.3, rated 35W in linear regulator work.

KT88/6550 is clearly a similar chassis but very different grid-control parameters.

A low-THD amplifier needs a high gain so the driver does not strain.

A regulator pass-tube's driver can usually swing a wide range and "THD" is not directly relevant.

A low Mu directly implies a higher conductance, which is important in linear regulator work, not so much for transformer-coupled amplifiers.

[swingarm]

Thank you for your response PRR. I can see I'm over my head with this tube. I have never calculated load lines etc. However I just started reading about them and thank Phoenix for pointing this out to me. I think I will abandon this tube and go with more traditional options. Thanks to all that responded.

*One last thing, would someone be able to articulate  what  Cathode Bias with a Constant Current Source is and how you could use it with partial fixed bias as well? Is it a complicated approach to using the subject tube?

[Phoenix]

One last thing, would someone be able to articulate  what  Cathode Bias with a Constant Current Source is and how you could use it with partial fixed bias as well? Is it a complicated approach to using the subject tube?

It's a very complicated approach to using any tube. Here's some reading for you. Scroll down the page a little to find the relevant info.

EDIT:
Oh, and while you're reading up on loadlines, check out Merlin's page here. Start with common cathode gain stage, then read single ended output stages, and finally push pull output stages.

Good luck!

[swingarm]

Merlin's book looks like it might be worth purchasing, Thank you Phoenix.

*I'm always so lazy when it comes to theory, guess I'm going to have to actually try and study and learn something. Blind emulation can only take you so far LOL.
Thanks again!

[Phoenix]

They're the best books on tube guitar amplifier design available, in my opinion. I've bought all three he's published so far (1st and 2nd preamp editions, and power supply). He wasn't happy with his power supply book due to some errors, and has withdrawn it from sale, but if you can find one secondhand, I'd still highly recommend it, it's an excellent reference tool. He's promised another book to be released sometime this year, I look forward to buying it, regardless of the topic (which is still subject to speculation).

[PRR]

> lazy when it comes to theory

This is an excellent plan. Do you design your own car? Your own well-pump? Would you really design a house? (Not just a floor-plan, but specify every stick without reference to joist/rafter tables?)

PLAGIARIZE!! There ARE big tube amps designs and nobody can stop you from copying them. The commercial products obviously used tubes which were "economical"; both suitable, inexpensive, and readily available(*). I suggest you may burn-up more parts testing an unusual/novel plan than you will with a known-good plan.

[swingarm]

I agree PRR.  In other tube amps i built i used Kevin Oconnors project books . In fact for this amp I thought i might be able to use this tube without a whole lot of trouble. Now i know i was wrong so i'm going to use the power amp in the setorian proect in tut5 and use the pre from the original svt in tut 3 (not his revised simplified design) I will only run 4  power tubes no s11e12 tubes anywhere . Kinda disappointed i spent the cash on those s11e12's but maybe I'll use them or something someday. Thanks for your help. BTW looking at that svt schem i plan on only using one channel. Can i omit the 6c4?

http://www.drtube.com/schematics/ampeg/svtpre-jp.gif