Ampeg R50H

[Rickfox2]

Hello, I just picked up a R50H half stack. The power tubes were different EL34's. Not a problem, but I wish to install NOS, but would like some guidance in the bias adjustment. Anyone here have any experience with this? Ex. voltages, location of bias adjust(I haven't opened it up yet), anything to prevent surprises when I venture inside. My Marshall bias is at 38. Can I expect something like this?
Any input would be appreciated!
Rickfox2

[Hangman]

Rickfox2,

The bias adjust is trimpot AP1. I don't know the location, but I don't believe there are any other trim pots in the R50H.

Bias voltage doesn't really tell the whole story. what you need is the Bias current. a good way to estimate the idle power dissipation is to measure the cathode current (which is approximately equal to the plate current at idle) then calculate the plate power dissipation using ohms law. IxV = P

ideally you want somewhere between 60-70% maximum plate dissipation at idle. So for an EL34 the max is 25W. so 25*.65 =16.25 W
we know V (its fixed) so we adjust the bias voltage to give us 16.25/390 = 41.66ma

unfortunately the R50H doesn't have an easy way to measure cathode current. so you can either make a bias probe or buy one. you don't have to buy one of those goofy "bias kings" or anything like that. If you already have a digital multi-meter, use something like this:
http://www.ebay.com/itm/8-Pin-Tube-Plate...Sw-0xYfSSm

they are pretty easy to make, but this is a pretty reasonable price. using the above method will allow you to set the bias for any fixed bias amplifier. although the bias probe will only work for the tubes listed in the ebay auction. thats an issue of pinout.

[Rickfox2]

Rickfox2,

The bias adjust is trimpot AP1. I don't know the location, but I don't believe there are any other trim pots in the R50H.

Bias voltage doesn't really tell the whole story. what you need is the Bias current. a good way to estimate the idle power dissipation is to measure the cathode current (which is approximately equal to the plate current at idle) then calculate the plate power dissipation using ohms law. IxV = P

ideally you want somewhere between 60-70% maximum plate dissipation at idle. So for an EL34 the max is 25W. so 25*.65 =16.25 W
we know V (its fixed) so we adjust the bias voltage to give us 16.25/390 = 41.66ma

unfortunately the R50H doesn't have an easy way to measure cathode current. so you can either make a bias probe or buy one. you don't have to buy one of those goofy "bias kings" or anything like that. If you already have a digital multi-meter, use something like this:
http://www.ebay.com/itm/8-Pin-Tube-Plate...Sw-0xYfSSm

they are pretty easy to make, but this is a pretty reasonable price. using the above method will allow you to set the bias for any fixed bias amplifier. although the bias probe will only work for the tubes listed in the ebay auction. thats an issue of pinout.

---

As I recall, with the Marshall I measured current at one of the pins of the EL34. Then did the math and adjusted accordingly. If I understand you correctly, this can't be done with a R50H?

[Hangman]

the JCM2000 ( and a few others) have 1 ohm resistors connected to the cathode of the power tubes.

if you want to know the current through a resistor it is I = V/R (in this case R = 1, so the voltage you measure is equivalent to the current.)

The same thing could be done if you added a 1 ohm resistor. I believe the R50H power tubes are PCB mount, which makes things more difficult

the bias probe I linked to has a 1 ohm resistor internally. So it would allow you to measure the bias current on any amp, without permanently installing a resistor.

There is also the possibility that you just measured the bias voltage, which is technically biasing the tube, but not to a known point. it could be exceeding the maximum plate dissipation, in which case you would start to red plate, or it could be biased too cold, in which case it would sound kinda blatty. Theres also the chance that it would fall within an acceptable plate dissipation range and be just fine. but as expensive at power tubes can be, its best to know.

[Rickfox2]

the JCM2000 ( and a few others) have 1 ohm resistors connected to the cathode of the power tubes.

if you want to know the current through a resistor it is I = V/R (in this case R = 1, so the voltage you measure is equivalent to the current.)

The same thing could be done if you added a 1 ohm resistor. I believe the R50H power tubes are PCB mount, which makes things more difficult

the bias probe I linked to has a 1 ohm resistor internally. So it would allow you to measure the bias current on any amp, without permanently installing a resistor.

There is also the possibility that you just measured the bias voltage, which is technically biasing the tube, but not to a known point. it could be exceeding the maximum plate dissipation, in which case you would start to red plate, or it could be biased too cold, in which case it would sound kinda blatty. Theres also the chance that it would fall within an acceptable plate dissipation range and be just fine. but as expensive at power tubes can be, its best to know.

---

Thanks! I forgot about the 1 ohm resistor. It occurred to me after my last post. I really appreciate your input and expertise.
Rickfox2