PPIMV and Fixed Biasing

[stingray_65]

Hey guys!

I'm having an issue with my TOS build. Not to leave things well alone, I changed Tubenits and Geezers cathode biased TOS build to Fixed bias. I had a fixed bias tap on the PT so why not?

I have it wired right now as shown below.

Thing is when I turn up past "6" on the PPIMV my bias current runs away with a hot signal put to it.

Lower level signal it behaves normally.

Have I wired this incorrectly?

Should I have changed the 250k A ganged pot to say 500k A or even 1M A?

This is my first fixed bias amp and also my 1st PPIMV

Any help is appreciated.

Ray

[tubenit]

I'm NOT positive but I don't think you can combine a PPIMV like that and fixed bias?

I think dialing the PPIMV will continually be changing the bias for you.

What's wrong with cathode biased?  Using the larger 147uf cathode cap gives it close to a fixed biased tone.

with respect, Tubenit

[stingray_65]

I was thinking the same thing Jeff, that as you dialed down the PPMIV the  parallel resistors would change value and change the bias.

BUT just 30 minutes ago  I set the bias at 30.0 mA and ran the PPIMV up and down and didn't see so much as a single digit raise.

I've got nothing against the tone of a cathode bias amp. It's very pleasant indeed! It's just this tranny has a bias tap and I've always shied away from doing a fixed bias.

I know it seems odd because this amp looks so nicely finished, but its a bit of a playground to me. I have a lot of firsts on this project. First combo cab, first time using piping, first laser etched faceplate, first build with all BRAND NEW parts no salvage stuff, not even a knob,cap or jack, first multi channel, first relay switched channels, first pentode gain stage, first fixed bias, first PPIMV, first high gain. the list goes on!

I'm convinced I will build this again 2 more times after I get the bugs out. I'll use a 24" chassis and a 2 x 12 cab, just so much going on that the 19" chassis is so crowded. I'll loose that Fattness knob, I can never see ever turning it down it sounds so FULL and LUSH! I also believe this may just be THE ULTIMATE HARP AMP. so I'll build it again with high impedance mic inputs and THAT will without a doubt be cathode biased. I'll also use a lot of NFB with filtering on the harp version.

I took on this project because my confidence in my skills has been growing. I'm doing more of the math and getting better at knowing when to apply the formulas. LOL you can not imagine how proud I am at the moment. I had mentioned before my node voltages were high, I measured the voltage drop across the resistor between nodes B and C, measured the actual resistance, determined the current draw at that point THEN with my desired voltage drop and current figured what size resistor I NEEDED. actually had a 5W ceramic and replaced the old resistor and now nodes C through F are within 3 volts of Geezers schematic!

For 30 yrs I've known Ohms law and always struggled with knowing just where to apply it and how. Today I got it, it clicked! I didn't have to ask anyone for help LOL I'm such a big boy now I might start dressing my own self in the morning LOL

I still have the 150/150 cap and 470R 10W and maybe when I done I'l P-P wire it in just to see, (likely) but I need to try this fixed bias thing. I have a BIG new 100W PT and OT sitting on a shelf with 4 6BG6GTA's calling me for over a year now. But I been scared to even look at a schemo or layout.

So any how :)
I'll beat my head against this as long as I have to. 

Ray

[LooseChange]

You need to add coupling caps between the volume pot and the grid leak (bias resistors).
The suggested method is to move those 47n caps to this position and replace with caps 10X those.
Make sense?

[stingray_65]

Thanks LC!

So you're suggesting it look more like this?

Do you think leaving the 47n in place and adding the .47uF's would hurt?

OH! more firsts on this build. chopsticks worked to find some parasitic oscillation and a slightly microphonic tube.

no more ringing when the OD is pushed over 4

[tubenit]

Ray,

I like your adventurous spirit, coloring outside the lines and your tenacity!  Go for it, man ...... I'm rooting for you!

with respect, Tubenit

[LooseChange]

Yes. But you are missing a coupling cap after the 82K plate resistor.

[PRR]

This should work.

The version 2 will NOT work: no DC path to 6L6 grids.

[LooseChange]

I missed that... The PPIMV control goes between the two sets of coupling caps (47n and 470n)... The bias supply grid resistors go after the second set of coupling caps.

Final answer.

[sluckey]

I agree with PRR. Your original circuit looks fine.

[CBD1964]

I have a traynor YBA-1 that has a MV installed, I noticed that the 2 resistors were removed that used to go between the 2 wires on the eylet board when it was installed.
If I take out the master mod, what resistor values should I put back in- can I experiment with their values ? I think they originally were 68K

[sluckey]

I see one schematic that shows 220K and another that shows 68K. Both schematics are in this pdf...

http://www.el34world.com/charts/Schematics/files/traynor/traynor_bassmaster_yba1.pdf

[CBD1964]

Thanks for that.
What advantage would there be by using one or the other?
Any noticeable sound difference?
I know that by turning up the MV to 10 I am taking the MV out of the circuit.....but then regular volume knobs only end up barely on at that point. If I in fact take out the MV, will I gain back better sweep of the Volume controls?

[sluckey]

What advantage would there be by using one or the other?

Try it and see. You won't hurt anything.

Any noticeable sound difference?

How it sounds is very subjective, especially for me. Bad ears.

I know that by turning up the MV to 10 I am taking the MV out of the circuit.....but then regular volume knobs only end up barely on at that point. If I in fact take out the MV, will I gain back better sweep of the Volume controls?

No. Removing the MV will be a lot like setting the MV on 10.

[PRR]

Stingray, I missed this the first go-round:

> my bias current runs away with a hot signal put to it.

"With a hot signal" is NOT "idle bias"!!

True, with Cathode Bias you are forced to run about the same current silent to slammed.

The main point of FIXed bias is that you can run large current on loud signals BUT run small current when silent.

Pair of 6L6 at 400V and 4KCT load will pull 200mA at FULL sine outut.

If we tried to idle them at 200mA, they would melt.

Instead we can bias as close to zero current as we like, and still get LOUD.

We can't reach, and do not want, zero idle current.

For battery-power and clean speech/music signals, we bias to about 10% of full-sine-output current. Clean speech/music has few peaks and 10:1 peak:average ratio. This will cover the peaks yet give good average battery ($$$) life.

Wall-power is cheap and stage amps usually play LOUD. There's no point in a cold bias. OTOH there is good reason to idle below maximum dissipation. Say 50%-90%. 70% is an often-used target.

For 6L6GC at 400V and 4K load, 70% works out to 50mA per tube, 100mA total idle. The FULL ROAR current is still 200mA.... this is controlled NOT by idle setting but by voltage and load.

Yes, idle bias is at _IDLE_, dead silent (at least very soft). It is like the idle screw on your car engine. It is what to do when there is nothing to do. Step on the gas, put a signal through, engine/amp energy will rise.

> so why not?

Lower reliability. Bias supplies DO fail. This can do major damage.