SOLVED - Adjustable Bias mod on Tube Guitar Amplifier

[Whoops]

Hi,
I'm modifying my tube guitar amplifier with an adjustable Bias Pot instead of having fixed resistor values.

I used an CTS 10K bias Pot, and adjusted the resistor values on the circuit so that I got a voltage range with the Bias pot from -44V to -58VDC
I schematic of the Bias supply circuit with these mods is attached, new resistor values in RED.

The stock schematic of the amplifier is here:
http://bolt60.no.sapo.pt/documents/bolt60/Bolt60%20-%20Schematic.jpg

The amplifier was working before this mods,
but now although I have negative bias voltage I cant measure any Cathode current on the 6L6 tubes, it reads zero current. Also the theres no output sound going to the speaker. Tubes Light up and the plate voltage is 497V.

I'm using this device to measure the Cathode Current:
http://www.rig-talk.com/forum/download/file.php?id=49481&mode=view
the device works properly as Im able to use it perfectly with other amps, also used it with this amp before.

The 6L6GC tubes were working before the mods, I also replaced them for a fesh pair of new working 6L6 GC and there's still no cathode current reading or output sound.

Does anyone know what might be causing this?

thanks for your help

 

[PRR]

> new resistor values in RED

So why is the new 33K grounded at *both* ends??

You clearly aren't wired that way or your bias would go down to zero V.

The inserted resistors, as-drawn, will screw-up the "B1" supply. If B1 is sufficiently fouled, the whole preamp strip won't work, no-sound.

Something isn't as-described.

 

[Whoops]

Thanks PPR,
My drawing was wrong.
You were right the 33K is not grounded at both ends.

here it is the new drawing,
replaced it also in the first post

thanks

 

[john12ax7]

Did you measure the bias voltage before the mod? Off hand it seems like way too much for a typical 6L6 circuit. It's possible the tubes are simply off with that much negative bias voltage.

 

[Whoops]

yes, I measured it before the new values.

The amp was working with the Bias circuit already modified with an adjustable pot,
R7 was 470 ohms, the Pot as 10K and the resistor from the pot to ground was 10K

the voltage range was from -34v to -50V, this range was not flexible when Biasing the Tubes, I couldn't get the Current needed to bias the tubes at 70%.

The range that is advised in some places is a value between -45V and -60 V

 

[radardoug]

Why dont you look at a tube characteristics chart? A quick look at the RCA chart shows -20 volts on the grid with 400 on the plate.
So you have cut the tubes off completely.

 

[Whoops]

Not really, for this circuit the negative Bias voltage was fixed at -51v.

Although Tubes could be running Hot or Cold depending on the tubes themselves.

the range from -45v to -58V is not too far off from the stock -51v value.

Also there is no cathode current in any position of the standby Pot, that means at -51v also

thank you

 

[mjrippe]

Looking at your other thread for this amp, is it possible it is stuck in standby?  Measure voltages at all pins of the tubes and see what looks off.

 

[Whoops]

Looking at your other thread for this amp, is it possible it is stuck in standby?  Measure voltages at all pins of the tubes and see what looks off.

No, standby works properlly

 

[mjrippe]

Ok, wasn't trying to be funny there, just wondered if other voltages were normal.

 

[Whoops]

Ok, wasn't trying to be funny there, just wondered if other voltages were normal.

I know, you were trying to help.

In this amplifier plate voltage is around 500 VDC (450V AC transformer),
All the voltages seem fine.

Just seems the 2 output tubes are not consuming, I dont know why

 

[walter]

That looks like a simple mod, take it out. Get it to work as before and then put the bias pot back in.  I would also connect the pot wiper to one end as insurance in case the wiper ever does fail.

 

[Whoops]

That looks like a simple mod, take it out. Get it to work as before and then put the bias pot back in.  I would also connect the pot wiper to one end as insurance in case the wiper ever does fail.

I put the amp stock, no bias pot, R7 470r and R8 6,8K.
I still have the same problem, no consumption on the Power tubes.

Bias negative voltage is -48V now
Plate voltage 500V

So it seems its seems the problem is not related to the Bias modification, maybe something happened when I did the mod.
I checked all wiring to see if there was any loose wire, and it all seems tight.

I will measure all the voltages at the Tubes socket pins.

What could be related to have bias negative voltage but no bias current?

 

[Whoops]

The inserted resistors, as-drawn, will screw-up the "B1" supply. If B1 is sufficiently fouled, the whole preamp strip won't work, no-sound.

I measure the voltage, with the bias pot mods I showed in the first post the B1 voltage was still 47.7V, so preamp is receiving the right voltage.

Thanks PRR

 

[PRR]

> no consumption on the Power tubes.

I turn on the faucet. The sink does not consume water.

How to debug?

Water comes from the town tank, through faucet, down sink, to sewer.

While water sinks can overflow, electricity doesn't. So I may have either and inflow or an outflow problem.

So I go drilling holes in the water tower, street main, house main, faucet stub, drain, sewer, to see where there is or is not water.

Why would a tube draw current? "Bias" turns it OFF. What turns it on is voltage at the Attracting Electrode, G2 Screen on a pentode. And for that current to flow there must be a complete path from power supply + to power supply -.

Forgetting to connect screen grids is popular.

Snipping a cathode jumper to insert 1 Ohm resistors, then forgetting to connect cathode resistors (ALL the way to power -) is also popular.

Somewhere in there you should have Plates connected, because pentode with no plate voltage will throw huge current through screens.

 

[CJ]

check ohms from ground to cathode of power tubes,  pin 8, be sure to unplug and bleed caps,

check plate and screen voltage, pin 3 is plate, pin 4 is screen,  be careful on this one,

check bias voltage,  pin 5 is grid,

if all three are good, then the tubes have to work if the heaters are lit and the tubes are good,

 

[Whoops]

check ohms from ground to cathode of power tubes,  pin 8, be sure to unplug and bleed caps,

check plate and screen voltage, pin 3 is plate, pin 4 is screen,  be careful on this one,

check bias voltage,  pin 5 is grid,

if all three are good, then the tubes have to work if the heaters are lit and the tubes are good,

Thank you CJ and thank you all.

After I posted here, I didnt went back to the shop.
I asked my colleague to check the voltages and he told me everything was fine.

Today I went to the shop and measured the voltages, and it was clear something was wrong because I only had plate voltage.

R3 560R resistor was shot, replaced that resistor  and everything is fine now.

As for the Bias circuit, I changed R7 for 470R and for the Resistor from the outer lug of the 10K bias pot to ground I used 27K.
I took the values straight from the Fender AB763 schematic.
The Potentiometer Bias range is good and flexible, I was able to Bias the amp easily.

Thank you all for your help, problem solved.

 

[mjrippe]

R3 which goes to the standby switch, correct?  So your two problems were sort of related   Glad you got it sorted, I hear those amps are pretty cool.

 

[Whoops]

R3 which goes to the standby switch, correct?  So your two problems were sort of related   Glad you got it sorted, I hear those amps are pretty cool.

It could seem so, but I dont think thats the case, standby has been working properlly for the last 2 years. That doest mean it will work forever of course.

What I think happened was that the bias device we have has 2 socket probes. One of then has the guide pin broken, so you have to be careful when inserting it in the tube socket to make sure you are inserting it in the right pins. So first time I tried to measure bias I was not careful while inserting it into the socket and I only realized after that it was inserted in the wrong socket pins. There was no smoke out of the amp (R3 is 2 watt ceramic) so I just disconnected the amp re-inserted the probe in the right position in the tube socket and thought everything was alright, but not, I guess R3 blew in the process.

 

[mjrippe]

Ouch, I HATE those "self-induced" failures!  They can be the hardest to track down.  Had one recently where PSU caps were not discharged and I was removing a solid state amp PCB from a chassis - ZAP!  Now it blows fuses :-(