Since it's on both channels(?) it's unlikely that both tubes would go at the same time, in the exact same way. Even single parts in a single channel are unlikely to fail the same way at the same time. The first sine wave diagram shows that the asymmetry in the waveform was almost exact on both channels. So it's probably something in the bit of the amplifier that both tubes share, no?
Have you had a look at what your rectified AC power looks like coming out of the power supply? Maybe a bad diode or something? Or a bad smoothing cap?
Have you had a look at what your rectified AC power looks like coming out of the power supply? Maybe a bad diode or something? Or a bad smoothing cap?
It sounds very logical.
But I can only measure +-20v with my scope. 🙁, besides that I have a multimeter.
@rayma this is in both channels, so it’s unlikely to be both transformers.
Check D2 and D3 with the diode checker on your multimeter. If one of them has gone it might be passing AC to your bias supply which maybe could cause the asymmetrical waveform? Also flip your meter to AC mode with no signal and see if you read any voltage At the grid of the 300b.
Check D2 and D3 with the diode checker on your multimeter. If one of them has gone it might be passing AC to your bias supply which maybe could cause the asymmetrical waveform? Also flip your meter to AC mode with no signal and see if you read any voltage At the grid of the 300b.
It could be the transformers, if they were never adequate to begin with.
If the main filter capacitors were bad, there would be terrible visible and audible hum.
If the main filter capacitors were bad, there would be terrible visible and audible hum.
Both of them failing at the same time, in the exact same way, seems unlikely.
You’re right, it’s probably not the filter capacitors.
Starting from the power transformer and working towards the tubes, and compiling a list of suspects might be a good place to start:
It’s also worth verifying which of the sine plots in the first post are correct. Is it clipping when the tube swings positive or when it swings negative?
You’re right, it’s probably not the filter capacitors.
Starting from the power transformer and working towards the tubes, and compiling a list of suspects might be a good place to start:
- probably not the PT, since power seems to be getting everywhere
- maybe the HT rectifier
- maybe the bias diodes
- maybe the voltage regulator in the filaments
- worth checking the resistors and caps in the bias circuit
- etc.
It’s also worth verifying which of the sine plots in the first post are correct. Is it clipping when the tube swings positive or when it swings negative?
D2 and D3 is fin, but D1, D4 and D5 seems to be bad. Will have to order some new ones.
I have been traveling with limited internet access for the last two weeks and just saw this.
Your filament voltages are good, so D1, D4 and D5 as well as the regulator chip are good. They are all interconnected therefore hard to measure in the circuit.
The actual B- voltage is unimportant as long as there is enough to completely cut off the 300B tubes.
There is some conflicting information here, but the first important bit to know is the actual current in the output tubes. I have read "2 volts across the 10 ohm resistors", "50 or 80 mA" and "100 mA on both tubes" somewhere in this thread. If there IS 2 volts across the 10 ohm resistors, then there is 200 mA flowing through each 300B which is far too much. That much current will cause distortion, transformer saturation, and generally mess up voltage readings everywhere in the amp. The voltage on R10 or R21 is not important at this time since your drive waveform is good.
200 mA through both 300B's plus the driver tube and bias components will be pulling nearly half an amp through your 5AR4 and power transformer. This should make them very hot and unhappy which has not been mentioned here, so the actual tube current really needs to be verified before any good conclusions can be drawn.
We need the voltage reading across R18 and R29 which is in the plate circuit of each 300B tube. The voltage should be about 0.7 volts which corresponds to 70 mA of tube current. It should vary when the bias pots (R12 and R23) are turned. The voltage on the grid of the 300B's must change when this pot is turned in order for the current to change.
Your filament voltages are good, so D1, D4 and D5 as well as the regulator chip are good. They are all interconnected therefore hard to measure in the circuit.
The actual B- voltage is unimportant as long as there is enough to completely cut off the 300B tubes.
There is some conflicting information here, but the first important bit to know is the actual current in the output tubes. I have read "2 volts across the 10 ohm resistors", "50 or 80 mA" and "100 mA on both tubes" somewhere in this thread. If there IS 2 volts across the 10 ohm resistors, then there is 200 mA flowing through each 300B which is far too much. That much current will cause distortion, transformer saturation, and generally mess up voltage readings everywhere in the amp. The voltage on R10 or R21 is not important at this time since your drive waveform is good.
200 mA through both 300B's plus the driver tube and bias components will be pulling nearly half an amp through your 5AR4 and power transformer. This should make them very hot and unhappy which has not been mentioned here, so the actual tube current really needs to be verified before any good conclusions can be drawn.
We need the voltage reading across R18 and R29 which is in the plate circuit of each 300B tube. The voltage should be about 0.7 volts which corresponds to 70 mA of tube current. It should vary when the bias pots (R12 and R23) are turned. The voltage on the grid of the 300B's must change when this pot is turned in order for the current to change.
Voltage over R18 and R29 is 0,8 volts (80mA). I have no problems adjusting the Bias.
But my rectifier tube just died, so i have to replace it, so I'm waiting for a new one. I also replace C4, C5, C7 and C8 from 400V to 500V just to be safe.
I will write again when I have replaced the parts.
Hi Tubelab
I have installed a new rectifier tube, i also replace C4, C5, C7 and C8.
New checkout measurements:
Voltage reading across R18 = 81 mV
Voltage reading across R29 81 mV
Filament voltages = 4,938V
Grid 300B Tube 1 = -96,0V
Grid 300B Tube 2 = -96,8V
B- = 277V
B+ = 403V
I think I figured out why the output looks weird. When the signal measured on the grid of 300B is higher than about 12V peak to peak, the bias current starts to rise. see attached photos.
When the signal measured on the grid of 300B is below 12V peak to peak, the bias current is = 81mA
When the signal measured on the grid of 300B is 40V peak to peak, the bias current is = 120mA
My Output Transformers are lundahl ll1623 = 90mA
Red is signal measured on grid of 300B.
Blue is measured at the output of the output transfomer across an 8 ohm resistor.
At the input I use a signal generator which is set to 1KHz sine 1V
Help!!
When the signal measured on the grid of 300B is below 12V peak to peak, the bias current is = 81mA
When the signal measured on the grid of 300B is 40V peak to peak, the bias current is = 120mA
My Output Transformers are lundahl ll1623 = 90mA
Red is signal measured on grid of 300B.
Blue is measured at the output of the output transfomer across an 8 ohm resistor.
At the input I use a signal generator which is set to 1KHz sine 1V
Help!!
If you have 81 mV across a 10 ohm resistor your current is 8.1 mA which would give the results you are seeing. You need about 800 mv across that resistor to get 80 mA.