Voltage dividing HT for Submini Cathode follower

[goldstache]

I have been getting to know the venerable 6021 sub mini dual triode.  I have a power supply built for noval style preamps giving me about 360VDC unloaded.


My design is just a preamp stage direct coupled to the next triode stage set up as a cathode follower.  I have the first stage biased and sounding nice.  But since my B+ is so High, I am having trouble making a 150VDC drop to the Cathode follower.  I know I should just redesign the power supply but wondered if there are other options. 


Here is a block diagram of my HT


Rectified AC > Reservoir cap (22uF) > RC filter (3.9k 3W, 22uF) > Tap to V1A plate 330VDC > 47K plate dropping resistor.  This gives me about 130VDC on the plate of the first stage. 


Now, How can I get the supply down to about 150VDC for my cathode follower.  Using a voltage divider seems like just poor design and will also effect my first stage B+, Im a newb so any thoughts are appreciated. 
THanks!!!!!!!!!!!

[kagliostro]

This gives me about 130VDC on the plate of the first stage

130V DC or 310V DC ?


K

[goldstache]

130VDC
Max plate voltage for 6021 is 150V

[DummyLoad]

Sylvania datasheets show 165v.

--pete

[DummyLoad]

best thing you can do is drop B+ to 250V or thereabouts.

neat tube - designed tolerant to positive and up to 5.5mA of g1 current.

what i come up with 330V supply: 68K kathode R, 1.8K bias R in a bootstrapped configuration. this puts Vtube right at 165V (max) and the output swing within Vmax.

heater-kathode V limit is of concern. on 6021 it's 200V peak. you will exceed that with this scenario. it may be advantageous to elevate heaters to 50V or thereabouts.   

-pete

[HotBluePlates]

... I have the first stage biased and sounding nice.  But since my B+ is so High, I am having trouble making a 150VDC drop to the Cathode follower.  ... Tap to V1A plate 330VDC > 47K plate dropping resistor.  This gives me about 130VDC on the plate of the first stage. 


Now, How can I get the supply down to about 150VDC for my cathode follower.

Did you try using a 47kΩ resistor between the cathode & ground?

For the 1st stage, there's is a 47kΩ resistor between B+ (330v) and plate (130v), so there is 330v-130v = 200v dropped across the resistor (4.25mA? Why so hot?). If you biased the cathode follower the same way (same 1MΩ from grid to bottom side of same-value cathode bias resistor, with a 47kΩ from the "ground side" of that to actual ground), you'd also drop ~200v across the cathode follower's 47kΩ, and have 200v at the 6021 cathode. 330v - 200v = 130v plate-to-cathode.

That stays under the 165v plate (-to-cathode) voltage limit.

Less B+ volts to start with would be better than trying to find ways to make a too-high B+ work. Alternatively, get a tube without such a low plate-to-cathode voltage limit.

[goldstache]

I attached a rough schematic.  I am still at the beginning of tube audio design. Thank you for your help and ideas, please bare with me.


I currently have this breadboarded up without the cathode follower V1B.  It's just the first stage into the EQ out.  Its at unity, unless I go soft on the EQ controls.  I would like to have the cathode follower before the EQ to get a bit of gain back from the insertion loss and smooth out the usefulness of the EQ. 


The above Posts are Awesome.  I am not familiar with Bootstrapping, though I read up a bit and think that its a neat solution and I may have to tinker with it


I tried out the CF by putting in a voltage divider, with large Wattage handling, after my initial RC filter and it gave me a useful supply voltage at the expense of lowering my V1A plate supply.  I knew it would happen, got down to about 75V on V1A's plate and over 100V for the CF stage plate supply.  Actually sounded pretty good, especially the lower impedance into the EQ really smoothed out the controls.
So my thinking, if I dare to call it that, was to bias V1A as per schematic and add the cathode follower after to iron out most of my dislikes with the performance thus far. 


Just not sure I know how.


Can I use a voltage divider in the supply to V1A's plates:


         B+
         l
         l
         l
         Resistor
         l
         l________________To V1B
         l
         Resistor
         l
         l   
         l
         V1a plate


So not only do I need to calculate V1A finished load but also the Voltage Divider getting my V1B plate voltage????????  OH BOY


As for floating the Heaters, I may fiddle with it if I can't get the cathode to heater voltage in safe tolerance.
Again thanks for any input!
 

[goldstache]

Also ignore the 1.14 volts on the cathode.  I forgot to erase it! the other reading is correct.

[DummyLoad]

280V Ebb


gain triode:
91K Ranode
1.8K Rkathode
47uF bypass
1.77mA
bias should be around 3-3.5V. seems to have best linearity.


cahode follower:
91K Rkathode
2.7K bootstrap R
1.5mA


add another filter stage with a 15K/22uF to shave 50V off B+


--pete

[DummyLoad]

use 6948 - if you can find them - somewhat rare.  Va is 250V is submini designed for audio.


http://tubedata.milbert.com/sheets/084/6/6948.pdf



http://www.ebay.co.uk/itm/6112WA-6948-JAN-PHILIPS-ECG-VALVE-TUBE-NOS-/261248945571


an excellent tube though... 


--pete

[goldstache]

Thanks for help!

[goldstache]

Okay so I amended the original schematic and attached it for clarity, well yours at least.  Is this even close to what you're describing above. 


1. In all the schematics I've looked at, usually the Bootstrapping seems to split the value of V1A's plate resistor in half, and then tap off that intersection with a CAP to V1B's cathode. Is the 2K7 resistor so low to give me the voltage division required to operate the CF's plate???????? Or do i have the parts designated to the wrong places??????


2. Also, the additional filtering (15k/22uF).  Am I tapping off separate points for each Anode?
 Like:


                           V1A                V1B
                             l                     l
                             l                     l
Ebb > 22uF > 3.9K > 22uF > 15K > 22uF
            l                       l                   l
            l                       l                   l
          GND                 GND             GND


Or am I just tapping off the last point for both stages' Anodes.


3. And do I have to have a cap to the cathode V1B?





In short................
Im confused


Digging the Bootstrapping concept though!!!!!!!!!!!!!!!!!

[DummyLoad]

it is said that a picture is worth 1000 words. IMO a schematic exponentially more...

tube-cad can misbehave with user defined tubes such as this one. hopefully it'll get you within reasonable operating parameters.

--pete

[goldstache]

Dummyload, thanks for taking the time to illustrate!  Boy was I off!  I will try it this afternoon and report back.

[goldstache]

I've attached the schematic with the revisions you suggested. 
It sounds great!
Couple of things...........


Based on what Im seeing voltage wise, I was under the impression that under the Max Ratings section of the data sheet for the 6021 it says "Plate Voltage 165Vdc" Meant that the plate voltage should not exceed 165v.  The CF sections plate is 290V???????  Should I interpret that relationship of anode vs. cathode voltage should not exceed 165v?  Since there is about 150V across the valve? (plate to Cath) Is that what they mean by saying Plate Voltage Max?????


If I audition the first stage only it has whopping gain and sounds huge.  Then when I apply the CF stage with bootstrapping, I get Barely enough volume out of the circuit to recover to unity after the EQ.  Not what I was expecting?  In my mind, I know the input impedance of the second stage is lower and it's set up to "Cathode Follow" effectively giving me a 1:1 amp factor with lower Z to drive my EQ.  I get the same audible insertion loss whether I drive it Hi-Z (first triode driving tonestack only)  or when I drive the tonestack after the CF.  I know Im missing a lot here. 


To Calculate current of V1A
I divide 290V by the load resistor 100,000? OR...... do I divide the Vdrop across the 100K by 100,000??? I am under the assumption the Latter. Giving me about 1.8ma.  When i look at the data sheet curves i get confused on how this was picked.  The plate resistance for the tube is 6.5K, I read somewhere that to bias it, pick a value 10X or more of its internal plate resistance.  So that explains the 100K.
The Grid voltage is likes close to zero for a guitar circuit.  The guitars input may swing to about 500mv or so when playing.  The cathode resistor is picked to keep the Cathode more negative than the grid to keep the valve open?
I've been reading everyone's text books and I still don't understand how to plot on the curves exactly.  Especially the Grid voltage selection.  I just got a scope but haven't used it yet.  Any pointers based on above design as a reference point?

I mean it sounds biased linear.  And cathode voltage reads around the range you proposed DummyLoad.





I am just getting my feet wet here and am thankful for the input!!!!




Thanks for the help.  What's that feeling, I feel....????????  oh, Its just my brain actually having to work!

[DummyLoad]

the 6021 it says "Plate Voltage 165Vdc" Meant that the plate voltage should not exceed 165v.165V measured from anode to cathode.

165V measured from anode to cathode. IOW, the voltage across the tube shall not exceed 165V.

you want the CATHODE sitting at 1/2 of the SUPPLY voltage which is 290V.

To Calculate current of V1A

measure the DC voltage across the cathode resistor. V/R=I    EX. assume the reading is 3.6V. divide 3.6V by 1800ohm and you get .002A or 2mA.   

BTW, what's the anode voltage of V1A? measure DC volts from anode pin to ground.

same thing with the cathode follower.   EX. assume we hit B+ dead center; 290V/2 = 154V so if we have 154 at the cathode/91K + 2.7K (cathode resistor + bias resistor) V/R=I so then 154V divided by 93700 ohms = .00164A or 1.64mA

--pete

[goldstache]

Thanks Pete.  That clarifies the cathode Current draw measurement? So the Measurement I took above, is the current across the plate resistor??


Also, I have about 108 Volts at V1A plate to ground.


I have another 6021 on the breadboard next to the one above.  Hmmmmmmm. 
I put in a resistor substitution box in place of the 1.5k on V1a's cathode, as I moved around (higher/lower)I got very little noticeable difference sonically.
Got any ideas to make this just a two stage eq style preamp/pedal/effect?  Should I just redesign with a toroidal I have sitting around that gives a B+ of around 260V?  I'd like to make something similar to the Matchless HotBox or Dougs Preamp Pedal,  just thought I'd start simple stage by stage and get a feel for it.


I'd like to keep it to one tube for now.  Just a preamp/EQ.  I know the tube has it's amplification limits, but I'm just using it in pedal format to have a secondary channel/boost with EQ.  Wouldn't mind some overdrive with additional stages down the road. 
Thanks

[PRR]

Written last night but the innernet went-out AGAIN (argh)------

WHY are you only dropping 20V in the 3.9K dropper?

For good filtering action you aim for 20%-40% drop, 275V-200V. And if that "343V" is raw, you really need more filtering for an input stage.

However with a hi-gain first stage and a hot guitar you *may* run into overload too easy unless the supply voltage is high.

Ah... this is NOT a high-gain tube. Mu is just 35. Expect stage gain of 20..... duh! Sylvania 6021 data has amplifier table on page 6. When in doubt, assume Fenderish conditions: 250V  supply, 100K "Rb" plate load, 220K "Rcf" next-stage load. Here we must shift to 200V data and assume Rcf is midway between 0.1M and 0.47M. Gain will be 24, Rk is about 2K, current like 1.1mA at 200V supply. Input overload is over 1Vrms so plenty for guitar.

If 100K plate resistor and 1.1mA current, then at 200V supply 110V is dropped in Rb and 90V across the tube. This is well below 150V-165V rating. So extrapolate to 250V supply. 250V/200V is 1.25 times. 113V across tube is still plenty-safe. First stage plate current is 1.4mA.

Cathode follower load.... if V1a plate voltage is 113V, and V1b cathode follows 2V up, we have 115V across CF's resistor. Assuming the same 1.4mA we get 82K cathode resistor. Bah, I'd make it 100K like the plate resistor; why stock odd values?

I think you can even go 300V of B+ and only put 165V plate-cathode. Barely-legal. But why push it? What point in a 1.5V input overload when hot pickups rarely beat 0.5V?

We have 343V raw. We want 250V filtered. We have 1.4mA+1.4mA= 2.8mA current. 343V-250V= 92V to lose. 92V/2.8mA= 33K dropper resistor. Power is 92V*2.8mA= 0.258 Watts. However you usually want at least two R-C filters from a raw B+ to a preamp. So a couple 15K or 20K resistor and 22uFd caps. Each stage gives >40dB cleaning of 120Hz buzz, so power to preamp will be very-clean. Power in these resistors is 0.129 Watts each, 1/2W parts will do fine.

> do I have to have a cap to the cathode V1B?

In most cases, yes.

But when driving a Fender tonestack, *all* paths from in to out are blocked with caps. So no blocking cap needed.

[DummyLoad]

why stock odd values?

why not?   

--pete

[goldstache]

Thanks for replies.  My 340 volt pwr supply has some filtering within it already.  So I think a reservoir and 1 RC filter should do! 
However I did build up at 260 V supply with a toroidal I have laying around.  I will tackle a redesign tonight.  For the record I was using 100k instead of 91k.  They were handy!

[goldstache]

Well here is what I ended up with.  I built up a supply using a toroid and some rectifiers.  Im running the heaters 12VDC now.  B+ is about 291VDC at the reservoir cap.  Sounded really nice, with loads of output, for about a day or so.  Then I lost about 20V on my HT supply and crackles started appearing at the EQ and Gain controls when in use??????  There is no measurable DC in that part of the circuit however.  The problem really occurs when I lower the gain control.  The input impedance must get screwy on the grid of V1B and signal cuts out replaced by hummmmmm!  Worked great for a minute there.


I have the "hot" portion of the circuit on barrier strips, and the Passive parts (controls, tone stack, etc.) on a breadboard section.  Might just be a bad Breadboard. 


What do you think?  Any other Ideas?  How about a Cathode Biased PWR Section?!!!!!!!!
Im getting about 1.5ma per stage. 


Thanks for all the input! 

[DummyLoad]

maybe you fried the filament in one of the triode. these are 6.3V tubes. not 12V. if you had a series string with 2 tubes or a single tube with a series load resistor to drop 6V across it, then that's OK. looking at the schematic, that's not what is shown: what is shown is a single 6V tube with 12V applied.


--pete

[goldstache]

Duh!!!!

Sorry long day at work.  I had the second 6021 in series with it. I removed it when I simplified the breadboard to 1 tube.  No wonder she was so hot to the touch!!!!! Fried a 6021!!!!!
Shame on me.

Can I use an lm317 variable regulator off of my 12volt rail to obtain a 6.3?  Since there is just one tube I won't need more than 300 ma.  And I think the 317 is rated at 1 Amp max current.
Thanks for stating the obvious Dummyload!  You were right!
Also B+B is 260VDC not 290
Thanks.

[goldstache]

Just looked at your attachment!  I'll try the series resistor.  Brilliant!

[DummyLoad]

Fried a 6021!!!!! Shame on me.

it happens...just try not to get shocked too many times.    OTOH, it used to be a form of treatment. 

Can I use an lm317 variable regulator off of my 12volt rail to obtain a 6.3

yes.

use a 7805, 340-5 etc., and fewer parts:
connect two diodes in series that connect from ground to the ground pin of regulator - orient so that both cathodes are towards ground and the anode of the string is connected to the ground pin of the regulator. should net you 6.2-6.5V depending on diode drops. you are lifting the ground pin of the 7805 off ground with two diode forward voltage drops. each diode forward voltage drop is approx. 0.6V.

use a 317:
connect a 120R from adj. pin to output pin, then connect a 470R from adj pin to ground. bypass the 470R with a 10uF =>10V  electrolytic cap. observe polarity of the bypass cap: the (-) lead goes to ground. connect a 47uF to 220uF =>10V from regulator output pin to ground. you can use 500R or 1K pot in place of the 470R for adjustment.

depending on the PT 12V rating, you're going to have 18VDC filtered. the regulator will be sloughing off about 11.7V the load will be about 300mA. the regulator will be dissipating Vin - Vout  * load current, or 11.7V * 300mA = 3.5W. the regulator is going to get warm. use a heatsink rat shack and fry's have them if you don't have one. in pinch use a 1"x2" peice of aluminum. mount with insulator washer and use thermal grease (aka, heatsink compound, thermal compound).

http://www.radioshack.com/to-220-heat-sink/2761363.html

http://www.radioshack.com/to-220-mounting-hardware/2761373.html

http://www.radioshack.com/arctic-silver-ceramique-thermal-compound/2801098.html   <<< ouchhhh 8 bux???

--pete

[goldstache]

Thanks for the Detailed description!  I used the 317 and its dialed in.  Im seeing exactly 6.3v on the heaters.  Now that I have it up and stable and sounding good, any suggestions for biasing additional stages (6021) for a light to medium overdrive.  Don't worry I will go back to 12V series Heaters if I do !!!!!!  I have more 6021's and would like to make an Overdrive still.  Im just not sure what topology to go with.


As it stands:


IN > V1A Preamp > EQ > Gain pot > V1B make up stage..........


As i study and build and play tube amps, I realize a lot of the beautiful breakup of an amp, resides in overdriving the Output section.


I see a lot of different staging arrangements in the "Tube Overdrive" schematics I've seen.  To my eyes, it seems that most just drive each stage until finally a voltage divider is placed before a grid of later stage to make it usable overdrive.  Sometimes there is a cathode follower on the end which is followed by some filtering(EQ). 


With what I have going on currently, It's more like the front end of a full amp.  So the more stages I tried to add the worse it sounded. 
Just want to go a little further with it before I just build it and start another idea!!!
Thanks so much!

[DummyLoad]

any suggestions for biasing additional stages

duplicate what you have now. split the new CF to lo-Z out for effects. tap off the plate of the gain stage to a third gain stage to sum the effects return and dry signal to feed power amp. don't forget coupling caps!! 
build an LTPI or gain/cathodyne and and drive a P-P pair of submini tubes.

turn it into a subminiature tube herzog? you'll need another two gain stages and power stage to do that. add switchable cathode bypass cap or put 5k-10k pot under bypass cap to dial in variance. you've lots of options. explore. copy other plans. it's on a breadboard for reason. right? ;-)

build a subminiature stand alone reverb? 

endless possibilities...   

--pete

[goldstache]

Here is what I ended up with.  Sounds real nice.  Really shines in stage/recording environments.  It allows for a completely tonal transparency with a good boost, or you can use the EQ to fake some other guitar sounds. 


I would like to know everyone's thoughts on a couple things.


1. Sounds best at the end of the my Effects chain just before the amp.  I don't use a lot of effects but I noticed it would drive some effects, especially those like Verb or OD, too hard. Causing them to sound a bit blown out!  Do you think a bit of series resistance would help tame the issue?  It really only is capable of this scenario when both gain and volume controls are above 80%.  So i can live with it if the added resistance is gonna "dull" the sound or take away headroom.


2.  I have a 3PDT switch in the enclosure to be able to bypass.  It has a pretty significant electronic POP when switching from Effect ON to Bypass.  I didn't ground the input or the output signals on the switch when it is bypassed, and I didn't add any Pulldown 1M to the output either.  Wondering if you think any of the above would help.  I can deal with a slight amplified mechanical POP from the switch being in there with a tube hot, but i feel this is Electronic and I may be able to soften the blow a bit with your help.


Thanks for all the help.  Im really pleased with the way it sounds and what I learned along the way!


Also, on the schematic it says the first resistor after the reservoir cap is 10K.  The voltages in the schematic are with an 11K resistor there.  Didn't have a 10K on hand when I built it.
I also added a 10K:600R audio transformer to the output for a line out option.

[DummyLoad]

put a 10K pot under the 100K pot. wiper of 10K pot to line-level out.


--pete

[goldstache]

Cool DummyLoad!!! 
I'm trying maintain balanced 600 ohm out.  I'd settle for unbalanced if the Z is close to line level.
How did you go about this calculation? 
Thanks!