Adjust BIAS on Dumble amps

[jazzbass]

Adjust BIAS on Dumble amps

hello everyone,

what I am making is an observation and my intention: I adjusted the bias of the final tubes in the amplifiers by adjusting the potentiometer placed on the chassis and checking how many volts are present across the 1 Ohm resistor installed between the pin n°8 and the earth .... and here the problems begin, it is not easy to fix the head of the resistance that goes to the ground with the head of the measuring bushing, very much for the quality of the plastic with which it is built and which melts and a lot because the contact (solder) between the metal and the resistance is not safe.
Then the value of the resistance comes into play which is never exactly 1 Ohm and it is not said that it is perfectly equal to the other resistances connected to the other bushings and to pins 8 of the other valves.
The result is an uncertain, complicated construction and voltage measurements are often unreliable, therefore useless, unless a certain tolerance is taken into account.
Let's not talk about the holes that decrease the bending resistance of the chassis, when it's good there are two if I have two power valves, otherwise four or more.

Measuring the BIAS is not necessarily an operation that must be repeated often except when changing the power tubes. Nor is it an operation that can be done correctly by anyone. From these arguments comes my distrust of the method and the results.

Ken Fiscer suggests for his amplifiers a much more precise even if a little uncomfortable method (but perhaps for this disincentive for anyone without the appropriate knowledge and correct manual skills): "The correct bias voltage for a Trainwreck Express amp in operating position using a digital multimeter is -30 volts on Pin 5 of the output tube, which may be adjusted by using the variable adjuster inside the chassis."

What do you think? what method and what values do you use with the 6L6, 6V6, 5881, EL34 power tubes?

a hug. Franco

[martin manning]

Measuring voltage across a known resistance is an accurate way to determine the current passing through it. Resistors of 1% tolerance are readily available, and you can take that to be true even if your DMM does not agree.

Test jacks connected to the current sense resistors with short leads and good soldering technique should not cause any significant error, but larger resistors, 10 ohms, say, could be used to reduce that error. Checking idle current periodically is a good way to catch any drift the might occur. It's both easy and safe.

Using grid voltage to set bias is not a good idea. There is too much variation in production tubes for this to be satisfactory.

Most people use an anode dissipation calculation to set idle bias, Ik x Va. Ik includes screen current, and so this is not true anode dissipation, which will be ~10% lower. Ignoring that, values in the range of 55% to 75% of the maximum anode dissipation listed in the tube data sheet are commonly used for power stages with fixed adjustable bias. Staying at the lower end of that range will maximize tube life. Note that this calculation requires the anode voltage, which requires access to the interior of the chassis and the expertise to measure it safely.

[jazzbass]

Thanks Martin,

finding 1 Ohm resistors with 1% accuracy is not possible through my regular suppliers in Europe, Tubetown and Tube Amp Doctor.
I found them at Mouser but I have to prepare a larger order.

Franco

[erwin_ve]

Thanks Martin,

finding 1 Ohm resistors with 1% accuracy is not possible through my regular suppliers in Europe, Tubetown and Tube Amp Doctor.
I found them at Mouser but I have to prepare a larger order.

Franco

Conrad have them. The link is in dutch, there is also a italian conrad. https://www.conrad.nl/nl/p/weltron-mfr1 ... 19206.html

[bepone]

who need 1% resistor in guitar amp?
if we take regular 5% res. what is the difference between 35mA and 36 mA can someone explain to me?

[bepone]

then, if we take cheap resistors 5% 10 pieces... is it possible to measure them with multimeter?? and take 2 of them what we like,
they will be for sure 1.02 ohm max.
but do we have 0.01 division on multimeter?
so , what is the question? aha, resistor of 1 ohm in 2 decimal spots.

ok next question this one is not important at all

[erwin_ve]

then, if we take cheap resistors 5% 10 pieces... is it possible to measure them with multimeter?? and take 2 of them what we like,
they will be for sure 1.02 ohm max.
but do we have 0.01 division on multimeter?
so , what is the question? aha, resistor of 1 ohm in 2 decimal spots.

ok next question this one is not important at all

A lot of the cheaper multimeters are bad in reading low resistance. I guess not everybody is chasing for a Fluke.
Next question:-)

[jazzbass]

Hi Bepone, Erwin_ve, Martin,

Thank you all, everything you wrote is very true.

I was caught up in the "mission" of doing the right things as much as possible that I forgot the reality.

There's a lot of talk on TAG about how the sound of an amplifier changes if I choose a type and brand of resistor or capacitor (and it's true) and I was angry because I couldn't make a decent bias measurement, with the voltage values /current between the various tubes, so much so that I decided to insert only two measuring sockets (plus earth of course) in a quartet, since the second tube in the row has similar bias values.
Using the method that Ken Fischer applied to the Express, the values were very, very similar and......I got lost in a reasoning.
Thanks and a hug.

Franco

[romberg]

who need 1% resistor in guitar amp?
if we take regular 5% res. what is the difference between 35mA and 36 mA can someone explain to me?

I agree! Even a 10% resistor can do an ok job here. The error in *not* measuring and including the screen current in this calculation is around 15%. And I don't see a whole lot of folks taking the screen current into account when calculating dissipation with 1ohm cathode resistors. In the end you just wanna make sure the tube is operating in a safe region. There is quite a bit of slop in what is considered a *correctly* biased amp .

Mike

[chaccmgr]

www.reichelt.de is a good source for e.g. resistors. They offer 0,1% tolerances (should be more than enough for this purpose)
I generally use 5%/2W Metal Oxide for safety margin. I always measure them before I solder and the tolerances are generally much tighter than 5%

their pricing is very reasonable and you can order small quantities. My favorite source here in Europe for electronics parts, their Website also offers several languages, even italian
limited assortments of high voltage "guitar-amplifier caps" though, their focus is broad general electronics
I'm not related with them in any way

[jazzbass]

www.reichelt.de is a good source for e.g. resistors. They offer 0,1% tolerances (should be more than enough for this purpose)
I generally use 5%/2W Metal Oxide for safety margin. I always measure them before I solder and the tolerances are generally much tighter than 5%

their pricing is very reasonable and you can order small quantities. My favorite source here in Europe for electronics parts, their Website also offers several languages, even italian
limited assortments of high voltage "guitar-amplifier caps" though, their focus is broad general electronics
I'm not related with them in any way

grazie chaccmgr, a hug

Franco

[sergio]

At mouser you can found vishay resistor 1ohm 1% 3w

[jazzbass]

At mouser you can found vishay resistor 1ohm 1% 3w

Grazie Sergio, Devo comperare altre cose e aggiungo nell'ordine queste resistenze.

Un abbraccio. Franco