4 output tube 1 ohm resistor

[michael]

hi, i have a bassman 100 im making into a twin reverb with reverb tranny extra tubes and circuit board. i have put a 1 ohm resistor on all 4 power tubes but now i think i need only 2    1 ohm resistors. 1  on each pair of tubes that are fed together. is this correct? 1 inner and 1 outer next to each other 1 tube with the 1 ohm resistor the other just grounded to chassis? and when i test 1 tube in this amp would it read at same ma as a 2 tube amp or will the ma be double? one more question please i ran my feedback wire from the speaker jack to my now not in use polarity switch so it goes to the feedback resistor as normal or switched in other position no feedback. will this hurt the amp? thank you and p.s i have ordered parts from doug twice now and am very happy with the qaulity. also a nice touch a hand written thank you to me for ordering said parts. your welcome sir

[HotBluePlates]

... i have put a 1 ohm resistor on all 4 power tubes but now i think i need only 2    1 ohm resistors. 1  on each pair of tubes that are fed together. is this correct? ...

You can do that, and know the total current for each side of the push-pull output stage. However, if it were my amp I'd put a 1Ω resistor for each output tube.

Pretend you see 1 tube out of the 4 redplating. You measure tube current to see if there is a problem, but you have but a single 1Ω resistor for each side of the output stage (2 tubes feed a single resistor). Both sides measure 80mV, which implies 80mA of current through the 1Ω resistor.

You wouldn't know that one the side without the redplating tube that each tube is passing 40mA (for 80mA total), while the other side has one tube passing 70mA and the other passing 10mA. In other words, both sides would look like they're doing the same thing when they're not.

So having a resistor per output tube gives you more opportunity for troubleshooting and more information.

... when i test 1 tube in this amp would it read at same ma as a 2 tube amp or will the ma be double? ...

If you had a single 1Ω resistor for a pair of tubes, then the measured current would be double that for a single tube. If you calculated 70% dissipation at 450v for a quartet of 6L6GC's under this scenario, you'd expect 46mA per tube (because 30w/450v = ~66mA, and 70% of 66mA = ~46mA). If the tubes pass exactly the same current through the shared 1Ω resistor, you'd measure 46mA * 2 = 92mA, which would show as 92mV across the 1Ω resistor.

You'll see you have no way to know how the individual tubes split up that total current that you measured.

... i ran my feedback wire from the speaker jack to my now not in use polarity switch so it goes to the feedback resistor as normal or switched in other position no feedback. will this hurt the amp? ...

It will not hurt the amp. Keep the feedback wire well clear of anything else except where it connects to the switch and the feedback resistor on the board; too close to high impedance, low-level circuits and you could get oscillation. However, I think you'll be fine with what you described.

[michael]

thank you sir i will leave the 4 -1 ohm resistors in place have a great day

[stratele52]

thank you sir i will leave the 4 -1 ohm resistors in place have a great day

Yes leave them .

[Colas LeGrippa]

You gotta use very precise resistors to do that. Dont forget that if you use 20 % , 10 % or 5% tolerance resistors, you will get false bias readings. Even if you measure your resistors with a meter before installing them. Measuring an exact 1 oHM resistor with a good meter can show .5 ohm or 1.5 ohm. All regular market meters are imprecise at very low resistances, like 1 ohm. You better buy 1 ohm 2w resistors ( online or through your electronic store ) at 1% tolerance. These are measured by very precise industrial meters. At 1 %, you can get 1.01ohm or .99 ohm. which will give you accurate bias reading.

Colas 

[stratele52]

You gotta use very precise resistors to do that. Dont forget that if you use 20 % , 10 % or 5% tolerance resistors, you will get false bias readings. Even if you measure your resistors with a meter before installing them. Measuring an exact 1 oHM resistor with a good meter can show .5 ohm or 1.5 ohm. All regular market meters are imprecise at very low resistances, like 1 ohm. You better buy 1 ohm 2w resistors ( online or through your electronic store ) at 1% tolerance. These are measured by very precise industrial meters. At 1 %, you can get 1.01ohm or .99 ohm. which will give you accurate bias reading.

Colas 

Easy to read in circuit how many ohms each resistor and calculate current

[sluckey]

Easy to read in circuit how many ohms each resistor and calculate current

I don't think so. I use a Fluke 87V and I still don't trust it to measure a 1Ω 1% resistor accurately. Even if I null the resistance of my meter leads, I still get varying resistance readings due to different pressure of the probes on the resistor leads, cleanliness of the resistor leads, etc. Besides, the whole purpose of using 1Ω 1% resistors is so we don't have to do any calculations other than 1 times X. I do trust the markings on a precision resistor.

[Willabe]

I would like to use 1000's  Will the larger resistors adversely affect the amp?

Sure they will. They will affect the bias.

Guys/companies did use 10 ohm R's for checking bias current.


                   Brad     

[SILVERGUN]

I know every one is suggesting 1 ohm resistors, why not 10's 100's or 1000's?  Given a choice, I would like to use 1000's  Will the larger resistors adversely affect the amp? 

It would create a combination of fixed and cathode bias....use the 1000's and now you have a 1K cathode resistor
You could do that to add some of the effect of cathode biasing ("compression") to a fixed bias amp.

From Merlin's website:
"...... there is no reason why we cannot use a little fixed bias and cathode bias simultaneously to achieve the desired mix of compression and 'bark'."

[stratele52]

I know every one is suggesting 1 ohm resistors, why not 10's 100's or 1000's?  Given a choice, I would like to use 1000's  Will the larger resistors adversely affect the amp? 

More ohm = more load on the tube = limitting the tube . This is the theory .

[Willabe]

Yes but it will change the bias.


           Brad   

[Colas LeGrippa]

you don't need a 1 ohm resistor when there is a resistor in the cathode path ( eg cathode bias ). Just measure the value of that resistor, read the voltage drop accross it, divide the voltage by the resitor value to get the bias. Example: 20V across a 240 ohm resistor is 83 mA . Ohm's law: E = RI . In other words:  current ( bias ) equals voltage divided by resistance.
 Colas

[stratele52]

Yes but it will change the bias.


           Brad   

Not if value ( ohms ) is low . You just can't have a warmer bias than the resistor value even if you decrease the negative grid bias voltage.

[Willabe]

Not if value ( ohms ) is low . You just can't have a warmer bias than the resistor value even if you decrease the negative grid bias voltage.

But that's not what you said.

I know every one is suggesting 1 ohm resistors, why not 10's 100's or 1000's?  Given a choice, I would like to use 1000's  Will the larger resistors adversely affect the amp?

You said 1000's.

I said 10ohm would be fine.


                Brad    

[Ed_Chambley]

I use DALE 1 watt, 1 ohm, 1%.  I have cross tested them with 10 ohm and the difference is nothing.  I was told to trust the Dale resistor and I have learned they work fine.

I never bias to a number when finished.  I use the number to insure I am in the ballpark then bias for tone.

[PRR]

> You gotta use very precise resistors to do that.

I don't think you can buy 20% resistors today?

10% is fine. If you are within 10% of trouble, you are just too close.

Nearly any 1-Ohm resistor you will find is 5% or better.

1-Ohm does not need any Watts. 100mA in 1 Ohm is 0.01 Watts, and you would not use a resistor that small (if you dropped it you would never find it again).

1 Ohm does presume you can read 0.050V well. With perfectly good 199mV DMMs selling under $10, that is not a problem. Back when meters went to 1.5V (1500mV), we used 10 Ohms to get a voltage we could see.

The internal cathode resistance of power tubes is around 50 Ohms. 1 Ohm is insignificant. 10 Ohms is still too small to fret about. 100 Ohms will really rob gain and headroom. 1K is over the top.

[stratele52]

Not if value ( ohms ) is low . You just can't have a warmer bias than the resistor value even if you decrease the negative grid bias voltage.

But that's not what you said.

                Brad    

Where do you read that ?

[Willabe]

My mistake, I got you confused with drgonzonm.


               Brad     

[stratele52]

My mistake, I got you confused with drgonzonm.


               Brad     

 

[G._Hoffman]

> You gotta use very precise resistors to do that.

1-Ohm does not need any Watts. 100mA in 1 Ohm is 0.01 Watts, and you would not use a resistor that small (if you dropped it you would never find it again).

Well, yes, but a 1 or 2 watt will typically have a better temperature coefficient, which probably isn't a big deal, but certainly won't hurt.

On the other hand, a 1/4 watt might blow up if your tube develops a short and save your output transformer.  I went this rout, on the one fixed bias amp I have built.


Gabriel

[Willabe]

Good points to think about Gabriel.


          Brad