Hi !
I have in my hands a Beard P100 mk II , and even without any input signal, the 4 leds in the rear of the amp for bias ajust, are on and off more or less all the time, and i measure in the 10 ohm resistors at the cathode of the output tubes (6550) a variation in voltage between 0,4 v and 0,7 volts, is that normal ? what can cause that ? note that the negative bias at the grids is stable ( -70 v ).
thanks
I have in my hands a Beard P100 mk II , and even without any input signal, the 4 leds in the rear of the amp for bias ajust, are on and off more or less all the time, and i measure in the 10 ohm resistors at the cathode of the output tubes (6550) a variation in voltage between 0,4 v and 0,7 volts, is that normal ? what can cause that ? note that the negative bias at the grids is stable ( -70 v ).
thanks
I'm assuming that you are measuring a variation in bias current from 40 - 70mA with no signal applied and not as the consequence of adjusting bias settings.
If the above is the case it could be an indication of subsonic oscillation which can be caused by deteriorating power supply capacitors or an ill-advised modification.
I hope you are running the amplifier into a resistive or speaker load and not open circuit. If connected to speakers the problem should be obvious as you will see the woofers "breathing." (Moving in and out with no signal present)
If the above is the case it could be an indication of subsonic oscillation which can be caused by deteriorating power supply capacitors or an ill-advised modification.
I hope you are running the amplifier into a resistive or speaker load and not open circuit. If connected to speakers the problem should be obvious as you will see the woofers "breathing." (Moving in and out with no signal present)
To work out the correct bias, one needs the HT voltage value as this has a great effect on the current drawn. Every time output valves are changed, a re bias is usually required. If independent presets are used, ideal. If not the valves need to be matched in push pull amplifiers.
Say the maximum output from an 6550 is about 25 watts. So measure the anode voltage of the amp, for this exercise we will assume that to be 445 volts. Divide the 25 by the 445 then multiply by 0.7 the answer in ma. is the 70% plate load bias setting required for that amplifier.
The 0.7 means 70% Hot anode / plate dissipation. Therefore you can use 0.5 (50%) as a Cool AB setting, 0.6 (60%) as a Warmer AB setting the one I would prefer. Or go for the 0.7 or even 0.75 (70 - 75%) if you like it hot.
Example: 25 / 445 X 0.7 = 39.3 ma. Bias
The bias figure will depend on the anode voltage which will vary according to the current drawn by the valve. More current less volts, less current more volts. So you may need to do the calculation several times if large adjustments are made.
Example 2. 25 / 480 X 0.7 = 36.4 ma. Bias
Example 3. 25 / 410 X 0.7 = 42.6 ma. Bias
If looking for the Class A requirement, that's 100% plate dissipation. The figure to multiply by would be 1 but check the valve is operating within recommended plate voltages. Here is an 6550 example with 300 volts on the plate / anode.
25 watts max o/p of the valve divide by plate volts 300 multiply by 1 = bias current requirement for class A operation. Example. 25 / 300 X 1 = 0.083 = 83ma. or 25 / 350 X 1 = 0.071 = 71ma.
No rocket science - dead easy elementary stuff.
Say the maximum output from an 6550 is about 25 watts. So measure the anode voltage of the amp, for this exercise we will assume that to be 445 volts. Divide the 25 by the 445 then multiply by 0.7 the answer in ma. is the 70% plate load bias setting required for that amplifier.
The 0.7 means 70% Hot anode / plate dissipation. Therefore you can use 0.5 (50%) as a Cool AB setting, 0.6 (60%) as a Warmer AB setting the one I would prefer. Or go for the 0.7 or even 0.75 (70 - 75%) if you like it hot.
Example: 25 / 445 X 0.7 = 39.3 ma. Bias
The bias figure will depend on the anode voltage which will vary according to the current drawn by the valve. More current less volts, less current more volts. So you may need to do the calculation several times if large adjustments are made.
Example 2. 25 / 480 X 0.7 = 36.4 ma. Bias
Example 3. 25 / 410 X 0.7 = 42.6 ma. Bias
If looking for the Class A requirement, that's 100% plate dissipation. The figure to multiply by would be 1 but check the valve is operating within recommended plate voltages. Here is an 6550 example with 300 volts on the plate / anode.
25 watts max o/p of the valve divide by plate volts 300 multiply by 1 = bias current requirement for class A operation. Example. 25 / 300 X 1 = 0.083 = 83ma. or 25 / 350 X 1 = 0.071 = 71ma.
No rocket science - dead easy elementary stuff.
Thanks for the replies !
This amp has no modifications everything is original, the b+ is 590 V and i have at the grids of the output tubes about - 70 volts as this amp has individual ajustments for each output tubes, and in the back of the amp has instructions for bias ajustment...they say, rotate the pot until the LED lits, then turn it back a little just untill the led goes off, and i do this, but after 10-15 seconds the leds keep turning on and off, and i can measure in the 10 ohm cathode resistor variations in current between 40 and 70 ma, and i have the speakers conected to the output and can not see any movement in the speaker ( woofer ). Can this be a oscilation ( HF ) ? This amp has a normal Ultralinear configuration with 6550 output tubes, and both channels are doing the same problem, but the sound seems to be ok.
Many Thanks
This amp has no modifications everything is original, the b+ is 590 V and i have at the grids of the output tubes about - 70 volts as this amp has individual ajustments for each output tubes, and in the back of the amp has instructions for bias ajustment...they say, rotate the pot until the LED lits, then turn it back a little just untill the led goes off, and i do this, but after 10-15 seconds the leds keep turning on and off, and i can measure in the 10 ohm cathode resistor variations in current between 40 and 70 ma, and i have the speakers conected to the output and can not see any movement in the speaker ( woofer ). Can this be a oscilation ( HF ) ? This amp has a normal Ultralinear configuration with 6550 output tubes, and both channels are doing the same problem, but the sound seems to be ok.
Many Thanks
G2 is connected to the output transfo ( UL configuration ) via a 270 Ohms resistor, and the B+ is stable ( 590 V )
Thanks Kevin, since there were no modifications in this amp, i assume this problem can be caused by supply capacitors as you say, but if this is the case ,shouldnt i have na very audible hum in the speakers ? and there is no hum at all, i have to put my ears inside the speaker to hear very little hum.
Many thanks
If the output stage is capacitor coupled to the driver, try pulling the driver tube and see if its stable then..
Just make sure the bias is direct on the output before you pull the splitter/driver.(so the bias is still on the output with the driver disconnected)
Its an easy check to do..
NB watch for red plating..if there is a coupling cap problem the removal of the tube will put higher B+ on the cap..shouldn't be a problem...however if they are leaky..its just in the same state as power up before the splitter tube conducts.
Regards
M. Gregg
Just make sure the bias is direct on the output before you pull the splitter/driver.(so the bias is still on the output with the driver disconnected)
Its an easy check to do..
NB watch for red plating..if there is a coupling cap problem the removal of the tube will put higher B+ on the cap..shouldn't be a problem...however if they are leaky..its just in the same state as power up before the splitter tube conducts.
Regards
M. Gregg
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First and foremost, was it behaving like this before or this is new symptom that you notice lately?
How's the amp sounds?
Do you have a scope to look at the signal? Sounds like you have a DVM. You said you measure the grid. Put the DVM into ACV reading and see whether you get any reading with no input signal ( the speaker is quiet). I don't know whether the DVM can measure HF voltage. But it worth a try. A scope can spot the signal immediately.
If you say the woofer does not move, that pretty much rule low frequency "motor boating" or power supply problem as they are 50Hz/60Hz and it harmonics that you can definitely hear and see the speaker move.
Yes, it can be HF oscillation. Or it can be the LED/test point circuit go bunk also!!! I would not jump into any conclusion yet. You don't have nearly information for us here. Reply all the question above and we'll take it from there.
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