Bandmaster Reverb repair "mixed bias" question

[goldstache]

I have a repair for a friend in the shop and am a bit stumped on the "mixed" bias configuration I found it in.
Its a Fender Bandmaster Reverb AA1069
Normally this amp has a bias balance circuit for the fixed bias arrangement.


It has a replacement Bandmaster PT and has been almost entirely redone passive wise to stock.  All preamp stages have 10uF cathode bypass caps.  Other than that the preamp is identical.


Here is a brief description of Bias wiring.


Standard 54VAC tap off of secondary.
Reverse rectified and sent to lug 3 of 10K pot.  Off of that lug there is a 10K resistor going to the summing resistor (100K) as per schematic.  Now this is where I get lost........


Off of the junction of the 10K, 100K to the 6L6 grid there is another 10K back to lug 1 of the bias pot.  Then there is a 15K on lug 1 to ground. 
The wiper of the pot feeds the other 100K to other 6L6's grid. 
Im getting -48VDC on one grid and -44VDC on the other grid.


The power tube cathodes each have a 150E 7W resistor to ground.   With a rather large Ecap across both cathodes.  I don't know the value of the Ecap values are facing chassis and I haven't removed anything to check it.  I suspect a large uF low voltage value.  I am measuring 4VDC across one 150E resistor and 3VDC across the other. I have never seen this arrangement and I googled it, and became aware of "mixed" bias.


How do you dial in bias when this configuration is present? 
Thanks for your thoughts.

[sluckey]

This schematic should help...

http://el34world.com/charts/Schematics/files/fender/Fender_bandmaster_ac568_schem.pdf

[goldstache]

Boy did it ever!  Tube chart is ripped out of this one!  Didn't know it existed!
Thanks!

[sluckey]

I had intended to post a link to the layout too but just forgot.

http://el34world.com/charts/Schematics/files/fender/Fender_BANDMASTER_AC568.pdf

[goldstache]

Thanks Sluckey!
What's the most accurate way to calculate bias with this topology?

[sluckey]

I am measuring 4VDC across one 150E resistor and 3VDC across the other.

What's the most accurate way to calculate bias with this topology?

4v across 150Ω gives 26.67mA cathode current and 3v across 150Ω gives 20mA cathode current. Assume cathode current is same as plate current. This ignores the small screen current so actual plate dissipation will be a bit less than calculated plate dissipation. Now just multiply cathode current times measured plate voltage to find static plate dissipation.

[alerich]

What I would love to know is why the mix of cathode and fixed bias. I had a Marshall Studio 15 amp with a similar configuration but I never could find nay information as to why it wasn't just fixed or cathode bias.

[goldstache]

Thank you.  From what I've read it gives the best and worst of both topologies with regards to current draw.  My friend wants the amp to have a hotter biased sound.  Should I convert to fixed bias or tamper with cathode resistors to achieve this?  I've never seen a fender like it!

[sluckey]

If it were mine I'd convert the power amp and bias circuit to AB763.

[goldstache]

I rebuilt the Bias supply and surrounding power section parts to AB763 bandmaster spec.  Bias supply checks out and is sent to the power tube grids via 220K resistors.


I left the ceramics on the power tube grids to ground a la AC568 schematic.
I didn't change the plate resistors to the PI plates either, still AC568


I powered up and used the shunt method to bias.  In all acceptable dissipation ranges the amp is sputtery and distorted with barely any volume at all.
What am I missing here?  The tubes are showing a matched current reading within about 2 ma.

[Willabe]

Did you take out the power tube cathode resistors and cap?


                Brad   

[goldstache]

Yeah I took them out and used the legs of the 7w resistors as my attach points.  Cathodes straight to ground no cap

[Willabe]

Yeah I took them out and used the legs of the 7w resistors as my attach points. 

'attach points'?

You used the 7w R's leg as the ground wire from the K to ground? (Which is fine.)


                 Brad   

[goldstache]

Thanks Brad!  Yes that's it.  Cathodes directly to ground.

[sluckey]

In all acceptable dissipation ranges the amp is sputtery and distorted with barely any volume at all.

How did it sound before the conversion?

[goldstache]

Solid and quiet.  It just sounded biased cold.  Flat and lacking a bit of volume.  4 on the the volume sounded like 2 1/2 or 3 with barely any spank.
I've double checked what I rebuilt and took away and all voltages are spec.??????
Thanks everyone

[goldstache]

Should I convert the PI's plate resistors?  The AB763 is lower voltage at the anodes.




ehhhhh...... the irons still hot.  Im gonna try the 100K and 82K combo and see if that helps.  If not, maybe 100K's bias supply to grids instead of 220K

[goldstache]

Bingo!  Had to change PI plate resistors to AB763 spec.  I guess the lower current wouldn't drive the power tube grids????

Not sure about my assumption there, but I think I just learned a thing or two:)

[goldstache]

I ended up biasing it at about 19watts/tube.  Sounded better than the colder 12watt/tube it was at before.
Some thoughts.  It's a weird one!  Well especially in the way I found it. Thanks Sluckey for the nudge to convert to blackface. I also changed the bias resistor 15k to 6.8k for better adjustment with amps B+. It has a tube rectifier 5u4, and the bias supply of the solidstate rectifier version.  Replacement PT is for a bandmaster not a bandmaster reverb. I'm getting 283VDC at the 6l6 plates at approx 50ma.  I'm thinking it just needed biased around all the peripheral mods/design differences it has. 

I think I'm good thanks!

[sluckey]

I'm getting 283VDC at the 6l6 plates at approx 50ma.

That B+ is verrry low. Bandmasters typically run close to 450V on the plates. Even the 5E7 shows 410 at the OT center tap.

What is the AC voltage on pins 4 and 6 of the rectifier?

[goldstache]

If I remember correct 325 vac.  5u4 conversion 1.2 (390VDC) where as solid state rectifier would give closer voltages to ab763 (455VDC) I don't have it in front of me.  It was considerably lower than the ab763 schematic.
Should I bias a little cooler? 
I have other 5u4 but I'm thinking it won't make much of a difference as far as headroom.

[goldstache]

Hmmm.  Yeah 283VDC on the plates is really low.  I'm gonna have to recheck voltages tomorrow.

My AC secondary measurement must be off.

[goldstache]

Im getting around 383 volts on the 6l6 plates, not 283vdc!!!!
Sorry it was getting late. I guess since my B+ is low to begin with (410VDC after rectifier) running the bias a little hotter does bring it down even lower.


Is it better to pull more current and have less volts hitting the plates or the other way around, less volts more current. 
They both seem to have a "sound" within acceptable dissipation.
Thanks for helping me along.


-Joe

[Willabe]

The schemo Sluckey posted for your amp shows 455dcv at the 6L6 plates, you measure 383dcv at 6L6plates.

Something's not right. 455dcv - 383dcv = 72dcv. That seems a little too much of a drop for just a little hotter bias point.

What do pins 4 and 6 on the rectifier socket measure in AC volts? (With ALL tubes installed, measure pin 4 with red lead and black lead to ground on AC setting, then measure pin 6 with red lead and black lead to ground. Use your alligator clips on your meters leads for safety.)

It has a replacement Bandmaster PT and has been almost entirely redone passive wise to stock.

What's the number stamped on the PT's bell end?


                        Brad   

[goldstache]

The transformer is a classic tone bandmaster replacement,40-18005
325-0-325
Im getting 315Vac at the rect.

It is a Bandmaster reverb, so a few more stages might attribute to a little sag right? With the power tubes out my B+ is about 415VDC.  I think the 455vdc is for a stock amp with original transformer, so maybe it was spec'd a little higher to accommodate reverb circuit. Voltage is significantly lower for Tube rectified with a "blackface" era replacement power transformer, as it was solid state rectified.   I also ran the bias real cold to see what the plates would get at that current. (12ma)  It was 410VDC.
So I biased by ear to 383VDC  X   .05ma = 19Watts.  It sounded the best so far.  I definitely see why an extra bit of B+ would help with volume and tube life.   


Do you think that is where my low B+ is coming from?  I tried different rectifiers (5u4) and other sets of 6L6's with the same results.
Thanks, -joe

[Willabe]

I just looked at the Classic Tone bandmaster replacement, 40-18005 and it doesn't have a 5vwinding for a tube rectifier.

http://www.classictone.net/40-18005.pdf

So, your amp has an added 5v heater PT?


                Brad     

[goldstache]

Geez I'm sorry.  It's this part number!
p-tf22798
Too many windows on the computer open!

[Willabe]

Did a search, found this on the PT;

"Replacement for Fender Super Reverb, Pro Reverb, Bandmaster, and Tweed Bassman. Includes rectifier tap. Equivalent to #125P5D & #022798. Specifications Primary: 117 volt, 60 Hz Secondary: 325-0-325 V, 370VDC@200 mA(DC) with 50 V bias tap Filament winding 1: 6.3 V, 4 A Rectifier Filament: 5 V, 3 A Mounting Centers: 2.75 x 3.5 Weight: 7.5 lbs "

Still seems the B+ dcv your getting is a little low, even with a 5U4. PT's B+ wind is rated at 200mA, so why is yours sagging down to 385dcv?

You could try a GZ34/5AR4 to get a few more volts.


                       Brad   

[goldstache]

Great idea!  Then I will rebias and report back!
Thanks a ton!