Roadking series 1 bias mod

[i_want_the_chair]

Hi everyone, I am trying to mod my Roadking series 1 head so I can bias the tubes, I have removed the fixed biasing resistors attached to pin 5 on all the output tubes (4 6l6's, 2 el34's) and replaced with 2k trim pots. the R's were 1.5k for the 6l6's and 5.6k for the el34's - I added a 4.7k resistor in series for the el's. I made a tube adaptor so I can measure (in series) pin 8 for the mA bias current and just add a 1ohm resistor across the leads to read mV bias. Sounds right?? PROBLEM! No matter which way i turn the trim pots it doesn't change the values I'm getting on my DMM's. I don't have the schematics so have done all this by following the 'obvious' and what I have read on this forum and other sites. Any ideas what I'm missing here??? The Roadking has progressive linkage and the values change with the number of tubes on the playing field, lowering with the more tubes in play, am I missing something here? The readings I'm getting are a frosty 22.6mV (el's on their own) down to a gelid 17.4mV with all 6 tubes selected.

PLEASE HELP!!!

 

[domct203]

Hi everyone, I am trying to mod my Roadking series 1 head so I can bias the tubes, I have removed the fixed biasing resistors attached to pin 5 on all the output tubes (4 6l6's, 2 el34's) and replaced with 2k trim pots. the R's were 1.5k for the 6l6's and 5.6k for the el34's - I added a 4.7k resistor in series for the el's. I made a tube adaptor so I can measure (in series) pin 8 for the mA bias current and just add a 1ohm resistor across the leads to read mV bias. Sounds right?? PROBLEM! No matter which way i turn the trim pots it doesn't change the values I'm getting on my DMM's. I don't have the schematics so have done all this by following the 'obvious' and what I have read on this forum and other sites. Any ideas what I'm missing here??? The Roadking has progressive linkage and the values change with the number of tubes on the playing field, lowering with the more tubes in play, am I missing something here? The readings I'm getting are a frosty 22.6mV (el's on their own) down to a gelid 17.4mV with all 6 tubes selected.

PLEASE HELP!!!

Welcome to the board.

The resistors you changed out are most likely your screen grid resistors. The bias resistors should be located somewhere on the mainboard near the PS Caps.

Dom

 

[i_want_the_chair]

Hi Dom, thanks for the welcome The Screen grid resistors are connected to pin 4 and are a decent sized chalky black, mounted in a gap in the PCB, for heat dissipation I'm assuming. Please correct me if I am wrong.

The Roadking has a breakaway PCB with the tube sockets mounted to it, as well as the relays for the progressive linkage switching. I will trace the circuit back to the main PCB as you suggest and see what I find.
Any chance anyone has a schematic for this gain beast?

 

[domct203]

Ahh, I totally missed that you mentioned "pin 5". I saw that you changed 1K5 resistors and immediately thought "screen grid".

I found the schem for the RKII, I'm sure it's the same bias supply. There is already a trim pot for the EL34's.

Here is the Power Supply including Bias Supply:



The entire RKII schem was found here:
http://www.fileden.com/files/2008/12/25/2237836/Mesa Boogie Road King II.pdf

Here is the thread I posted when I modded my Roadster for adjustable fixed bias, I would think the PCB layout for the bias supply should be similar:
http://www.forum.grailtone.com/viewtopic.php?f=24&t=54589

I hope this helps,
Dom

 

[i_want_the_chair]

I may be going about this wrong...
I literally removed the resistors directly connected to pin 5 (see page 10 of the manual you linked). I thought this would give me individual bias control on each tube. Are you saying I need to control the overall bias to the pairs?? Should I return the original resistors and just have a main bias, 6L's and EL's respectively?
Thank you very much for helping me.

 

[domct203]

I may be going about this wrong...
I literally removed the resistors directly connected to pin 5 (see page 10 of the manual you linked). I thought this would give me individual bias control on each tube. Are you saying I need to control the overall bias to the pairs?? Should I return the original resistors and just have a main bias, 6L's and EL's respectively?
Thank you very much for helping me.

Yes, you should return those resistors.

For the standard Recto 100W output section bias supply you would swap the 22K resistor (R95) with a trim pot, using it as a variable resistor. I would start there, however I don't know how that will effect the EL34 bias. You might end up having to tweak both trim pots together to get the values you need.

Dom

 

[i_want_the_chair]

Shall do Dom, I'll swap it out with a 22K trim pot and post some photos
thanks heaps!!

 

[domct203]

Shall do Dom, I'll swap it out with a 22K trim pot and post some photos
thanks heaps!!

I would use a larger value pot to give you more adjustment range.

Read here for a lot of good info on this:
http://forum.grailtone.com/viewtopic.php?f=24&t=35849

Dom

 

[i_want_the_chair]

So, no photo's yet, but I've looked over the schematics for the RK II. If you look at the power amp schem. pg 10. If I swap out the 220k resistors (directly off the 6L6 input) with trim pots will that give me independant control to the pairs of tubes?

 

[domct203]

So, no photo's yet, but I've looked over the schematics for the RK II. If you look at the power amp schem. pg 10. If I swap out the 220k resistors (directly off the 6L6 input) with trim pots will that give me independant control to the pairs of tubes?

While yes it will give you a voltage drop as you increase the resistance, I don't know how much adjustment will be available, or how touchy it would be to adjust.

The mod I did to my amp, as well as stated in the post I linked is the standard mod done for adjustable bias. The resistor in the bias supply is used differently in the circuit, IIRC it is more of a voltage divider (someone correct me if wrong) than 220K of series resistance.

Why are you against modding the bias supply?

Dom

 

[i_want_the_chair]

It's just that I wish to be able to bias individual pairs (each individual tube would be better) due to the switching the roadking does, not all 4 6L6's are in operation in all selected power amp linkages. I wish to 'fine tune' the 2 pairs separately and eliminate not having an exact matching quad of tubes.

 

[domct203]

It's just that I wish to be able to bias individual pairs (each individual tube would be better) due to the switching the roadking does, not all 4 6L6's are in operation in all selected power amp linkages. I wish to 'fine tune' the 2 pairs separately and eliminate not having an exact matching quad of tubes.

Well that makes sense. I say if it's an easy mod you should try one of the 220K resistors. Just be careful with large adjustments, your Phase Inverter (V6) plates are on the other side of that 220K. I'm no amp builder so I can't say for sure if it would be an issue.

Good luck, please post some pics and your results!

Dom

 

[jupablkxten]

What you are doing and what is being suggested to you is like eight different kinds of wrong.

Return the value of the resistor on the input grids of the output tubes to their original value (1.5k or whatever was there.) Do not install a trimpot in their position.

Leave the 220k bias feed resistors alone. Lowering this value will increase the negative voltage to the input grids. Lower to 150k if you wish to install KT88/6550 tubes.

You already have adjustable bias for the EL34 tubes (as shown on the schematic). If you want to individually bias the 6L6 pairs, install 2 trimpots in place of the 68k resistor, wired as voltage dividers. The output of one divider should be connected to one 220k bias feed resistor and the other divider to the other resistor. Make sure that the one end of the resistor is connected to the bias supply and the other to the signal path - not to each other.

Honestly, if you want something like this done you should take it to a tech.

 

[i_want_the_chair]

Cheers Bottle Rocket, thanks for the advice I have a roadking 1, the schematic is for a RK2. I don't have a V6. So this schematic is only a very loose reference. I thought the idea of these bulletins was to do your own tech work or at least consult with techs, PLUS the closest MESA tech is 1400 Km away = $$$$$$$$$$$$$$$$$$$$$$ in freight + whatever they charge me to do a modification.