switch from cath. to fixed bias on Valco/Supro T-bolt

[smackoj]

I have a '67 thunderbolt amp that is labeled Gretsch Pro Bass. The scheme is the same as the Supro T-bolt model 6420 with ss rectifier. I would like to change it from cathode bias to adj. fixed bias but the way the signal goes from the PI to the power tubes is different than I am familiar with. Could one of you fellas lend me a hand on what I need to do to install the fixed bias? I have the sub-assembly built for the neg. voltage supply, I just need to know how to cut it in so both pwr tubes will have same neg. amount on the screens. I already have the 1 ohm R to grd. on both pin 8s of the pwr tubes also. 

Here's a link to one of the better quality schemes:  http://www.el34world.com/charts/Schematics/files/supro/Supro_6420.pdf

thx   

[jazbo8]

Simply connect the B- to R15 & R16 via two resistors (say 220-470K) at the junctions of the coupling caps and the grid stop resistors, and leave everything else the same - unless you want to have different bias on each tube, then you would need to add an balance pot.

[smackoj]

ok, so on the R15 I would use the 270 ohm and on the R16 the 470 ohm?  thx     

[sluckey]

Simply connect the B- to R15 & R16 via two resistors (say 220-470K) at the junctions of the coupling caps and the grid stop resistors, and leave everything else the same - unless you want to have different bias on each tube, then you would need to add an balance pot.

That wont work with this PI. You have to isolate the bias voltage from the grid of the bottom half of the PI. Gonna need a couple extra caps and resistors. I'll try to edit the schematic to show what I'm talking about.

Edit... Pic added

[smackoj]

"Excellent"  thx SL

jd   

[smackoj]

Oooops! I spoke too soon. I have the 'cut in' connections done the way SL showed me and that seems VG cuz I have neg volts on both pin 5s however, I can't seem to get more than -.21 volts dc at either pin 5? I think I'm missing something essential in this neg supply circuit? I've tried everything I can think of but can't get any useable amount of neg volts past the circuit and onto pin 5. I took a HV tap right from the tag strip where one leg of the HV secondary from the PT connects to the diode rect. I took that line straight to the neg bias supply circ and thru a 300 ohm R. I checked AC volts after this R and get 107 vac.....but, anywhere after that point, i.e. after the 1N4007 diode connecting to the 330R I only get -.20 volts?  I was originally using the description of how to do this mod from g. webers book. However, his description is for changing a tube rect. SE small 'practice' amp from cath. bias to adj. fixed bias. So, I realize I'm probably making a rooky mistake cuz I'm trying to change a cath bias PP amp with diode rect to adj. fixed bias.

I have access to schemes of fender adj. fixed bias circuits. Would copying the fender layout be the way to go here?

thx, jd

 

[sluckey]

I took a HV tap right from the tag strip where one leg of the HV secondary from the PT connects to the diode rect. I took that line straight to the neg bias supply circ and thru a 300 ohm R. I checked AC volts after this R and get 107 vac.....but, anywhere after that point, i.e. after the 1N4007 diode connecting to the 330R I only get -.20 volts?

You can't do that with a full wave bridge. You must connect to the HV tap through a cap.

Look at page 5, bottom left circuit...

    http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

Make your bias circuit look like that and it will work properly.

[smackoj]

thanks SL....working on this the right way now.

 

[jazbo8]

Simply connect the B- to R15 & R16 via two resistors (say 220-470K) at the junctions of the coupling caps and the grid stop resistors, and leave everything else the same - unless you want to have different bias on each tube, then you would need to add an balance pot.

That wont work with this PI. You have to isolate the bias voltage from the grid of the bottom half of the PI. Gonna need a couple extra caps and resistors. I'll try to edit the schematic to show what I'm talking about.

Edit... Pic added

Thanks for the correction.

Jaz

[JPK]

I took a HV tap right from the tag strip where one leg of the HV secondary from the PT connects to the diode rect. I took that line straight to the neg bias supply circ and thru a 300 ohm R. I checked AC volts after this R and get 107 vac.....but, anywhere after that point, i.e. after the 1N4007 diode connecting to the 330R I only get -.20 volts?

You can't do that with a full wave bridge. You must connect to the HV tap through a cap.

Look at page 5, bottom left circuit...

    http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

Make your bias circuit look like that and it will work properly.

This is a cool webpage full of great info, thanks for sharing.

[smackoj]

Update on this adventure in MODSland: unfortunately I still couldn't get it right and had to scrap the idea for now.....although I did learn some things, I am still uncertain about other things? What direction a diode is supposed to be wired as a 'for instance'. This is the 2nd time I have attempted to go from cath. bias to fixed adj. bias and I guess there is a gremlin somewhere in my shop. He's a sneaky little sombeee that eats the negative volts and crashes my toys....so, back to the status quo for the T-bolt.

thanks for the help and interaction.

 

[JPK]

Isn't the direction of the diode backwards from what you'd normally expect because it's a negative voltage half wave rectifier? I have this same circuit in my AX84 October. It's a 20 watt P-P and has the adjustabe cathode bias. It's almost the same circuit as the one on page 5. Just don't have a cap in series with the PT decondary feed. The adjustable bias is very handy.

PS: By the way I had to change that 33k tail resistor to a 47k just like the one on page 5 of the above link. My bias voltage was too low which made my bias current too high at its lowest adjustment.

[smackoj]

senor JPK; I see from your post that you have two VRs in your circuit. are you using a seperate pot for each tube? i thought the basic circ just uses one 50k pot?

thx     

[JPK]

You can do it either way. I can post the same view with only one pot if you're interested. I like having one for each power tube. If the tubes are matched I recon one will do just fine. Mine are matched but I still wanted an adjustment for both.

[sluckey]

unfortunately I still couldn't get it right and had to scrap the idea for now

The bias supply on page 5 works very well. Ask Marshall or Ampeg. The .047µF and 56K are absolutely necessary for proper operation. If you build your bias supply EXACTLY as shown on page 5 and cut it in to your T-bolt as shown above, it will work.

I once read that one fellow said the circuit did not work for him. But, then he connected the .047 cap to the other HT lead and it did work. I don't know if that is true or not. Maybe he inadvertently did something else while moving the cap. Or maybe I've just been lucky.  And I don't have a bias supply like that, so I can't confirm.

[smackoj]

SL: I understand what you are saying about the guy switching from one HT lead to the other and then it worked. I spent lots of hours trying to get the mod done and only managed to frag the PT. I think it popped the one side I had the tap taken off. Anyway, I was lucky to find another PT in an old build that fit and the amp sounds good again.....with cath bias however.....I don't think I'll try that mod again on the T-bolt. Probably try it on a more common PI setup like the fender and maybe I can get the neg. juice to act right?

 

[tubeswell]

Steve, the fixed-bias schematic you posted has a 200R cathode resistor on the output tubes. I don't wanna be mean - just sayin' ;-)

[sluckey]

Yes, I know. I probably should have removed that. But my partial schematic which is a hack of the schematic posted by smackoj was meant only to show him how to connect his bias supply into the grid circuit without causing problems back in the PI. Smackoj said early on "I already have the 1 ohm R to grd. on both pin 8s of the pwr tubes also." so I kinda figured the 200Ω was already gone.

BTW, you can run a negative bias voltage to the grids AND use a cathode resistor. Kinda like a cross between fixed and cathode bias. My Maggie M-10A does this with a 100Ω. The unbypassed cathode resistor also provides some local degenerative feedback that 'may' be part of the maggie sound.

[tubeswell]

Yep I 've seen hybrid biasing on some amps (AC50 and some fenders). Never tried it myself.

[smackoj]

<local degenerative feedback>  now you're talking guys

because I've already fragged one PT, I better wait for 'hybrids' a little while.

someone in the Stockcar Racing World (stockcar was before Nascar) complained to Richard Petty that stock cars didn't have 'motors' in them because a motor ran on elec. and Petty should stop calling it a motor and call it an engine. Petty told the guy, "if you don't blow up a motor once in a while, you ain't running fast enough to win!" So, I guess blowing up a PT puts me in good company?