Wow missed this last night! Jan stated exactly the same and Thank both of you for getting this through my thick skull lolol...
Really appreciate this place and the knowledgeable folks who help
Agree with you but you forgot the heating voltage. Tried running JJ tubes at 7v for a while, not sure if it ends very well.
Maybe surprisingly, no, in two different respects.
A load line can only be valid at one particular frequency, and will be different at every other frequency. The commonly used "load line" applies at middle frequencies, where we can make simplifying assumptions: the transformer is perfect and the load is a perfect resistor, the cathode biasing resistor is completely bypassed, etc.
So, at DC, the idling condition for your output valve, the valve's load line has a slope of the B+ across the sum of the cathode resistor (because the bypass capacitor has no effect at DC) plus the DC resistance of the output transformer (OPT) primary (because it's not a transformer yet at DC, just some copper wire).
And, at middle frequencies, the valve's loadline is just the reflected impedance from the speaker, because the cathode resistor is completely bypassed by a capacitor (close enough to zero Ohms).
Does that make sense? All good fortune,
Chris
They say 'Va(max)' just as a shortcut, and experienced guys know that it really is Va-k(max).
That is also the reason your multimeter has two leads: voltage is always against a reference.
If you think about it, saying 'Va = 300V' is meaningless. It is 300V referred to what? In the case of Va,(max) it is against the cathode.
Jan
That is also the reason your multimeter has two leads: voltage is always against a reference.
If you think about it, saying 'Va = 300V' is meaningless. It is 300V referred to what? In the case of Va,(max) it is against the cathode.
Jan
In the words of the late, great Leonard Cohen, "like a bird on a wire".
All good fortune,
Chris
All good fortune,
Chris
I second the idea to use a small and cheap transformer to keep especially the heater/filament voltages well within their ratings, if necessary at all. For example, a 115/6.3 Vac heater transformer with both windings wired correctly (in phase) and mains connected to the outer ends will yield to an output voltage of 120 * 115/121.3 = 114 V at the 115 Vac winding ends. It's power capability roughly calculates to 6.3 V times the load's maximum primary current.
This is the way I go when using my old 1970ies equipment, designed for our then usual 220 Vac mains voltage, at our actual >230 Vac if there's no mains voltage selector present. Of course my dropping transformers are 12 Vac instead of 6.3 Vac.
Best regards!
This is the way I go when using my old 1970ies equipment, designed for our then usual 220 Vac mains voltage, at our actual >230 Vac if there's no mains voltage selector present. Of course my dropping transformers are 12 Vac instead of 6.3 Vac.
Best regards!
"Whats your take, and is there anything Im missing ?"
the Plate-to-Cathode Voltage which is measured between the plate and cathode tube socket pins, not between the plate and ground. You measure it by placing the red + multimeter probe on the plate pin and the black - probe on the cathode pin. You can also calculate the Plate-to-Cathode Voltage by subtracting the voltage measured at the cathode pin from the voltage measured at the plate pin.
I am assuming you are familiar with safety guidelines when dealing with Lethal High Voltages!
the Plate-to-Cathode Voltage which is measured between the plate and cathode tube socket pins, not between the plate and ground. You measure it by placing the red + multimeter probe on the plate pin and the black - probe on the cathode pin. You can also calculate the Plate-to-Cathode Voltage by subtracting the voltage measured at the cathode pin from the voltage measured at the plate pin.
I am assuming you are familiar with safety guidelines when dealing with Lethal High Voltages!
Absolutely, and this one post i believe cleared up much! I knew that the ac was going through the cap, and without much resistance or reactance, i also knew the transformer was only seen as resistance or choke to dc but i never added it all together!!!
I also understood that the opt impedance varied as the reactance of the speaker changes with frequency, but never realized that at that point the cathode resistor had almost nothing to do with it because of the bypass cap!
One day im gonna write a book (not!) that you folks can proof read, labled tube amplifier basics for the learning impaired hahahah 🙂
Assume the voltage at 300B is fine but the 12AT7 and 6SN7 would have additional 20-30v. Is it within the range design?
Small update, The heaters at least for the 300B are at 5V on one tube and 5.2 (or so) on the other.. once i complete the next task, will circle back to see if i can find why there is any difference... started Checking the 12AU7 and 12AT7 but have to come up with a more methodical and repeatable Method
To be sure, i should be measuring dc when checking plate to cathode, Ac on heaters.. and both ac and dc on grids?
To be sure, i should be measuring dc when checking plate to cathode, Ac on heaters.. and both ac and dc on grids?
Well, the 300B is a triode, afaik. Are there any limitations wrt. the grid to cathode voltage? And, btw, what is it's nominal heater voltage?
Best regards!
Best regards!
Swap the tubes, see if the difference stays or goes with the tube. Tube heater currents vary, and that can lead to differences in heater voltage due to the heater circuit internal resistance.
If the tube heaters are supplied by seperate circuits, there may be a small difference in those. That can be checked by measuring the heater voltages with the tubes removed.
Diagnose!
Jan
The heater voltage difference could be from the transformer. Quality transformers would be the same, but with cheaper ones you never know. 5.2v is still within 5%, and it would be a tiny value resistor that you'd need to reduce it. Or you could split the difference and run the whole works at slightly lower voltage and get it at 4.9 and 5.1.
This is the danger of taking measurements. Now that you know it is slightly off it'll bother you. You otherwise would have been listening in bliss. I was using my amp for years and then measured and found 6.7v at the 6.3 volt tubes. Nothing ever happened. But once I knew I found the .27r resistors and now I'm at 6.25.
This is the danger of taking measurements. Now that you know it is slightly off it'll bother you. You otherwise would have been listening in bliss. I was using my amp for years and then measured and found 6.7v at the 6.3 volt tubes. Nothing ever happened. But once I knew I found the .27r resistors and now I'm at 6.25.
Heater voltage for the 6SN7 and 12AT7 is 6.7. The 300B heater voltage varies.. have not had hours to spend the time, been more like 20-45 min here and there.
Each time I let the amp warm up for at least 10 min.. Left channel 300B varies from 4.8-5.3 and right channel 5.2-5.6. Every time the reading shows left is lower than right... Did NOT switch tubes around yet, wanted to get as much as a snapshot as I could.
Im going to post another thread, as its a fork of this and probably better with another subject line....
Something to the effect of does anything look out of line with....
Each time I let the amp warm up for at least 10 min.. Left channel 300B varies from 4.8-5.3 and right channel 5.2-5.6. Every time the reading shows left is lower than right... Did NOT switch tubes around yet, wanted to get as much as a snapshot as I could.
Im going to post another thread, as its a fork of this and probably better with another subject line....
Something to the effect of does anything look out of line with....
Use an other ohm meter to monitor AC outlet, it would drop 3-5v if there is a heavy load in the house such as oven, HVAC, etc. Mine is stable at 120Vac and it would drop 3-5v when the HVAC kicks in, it takes around 30 seconds for AC back to 120v. So it's no surprised that you would see the different when you do the measurement.
I use an isolation transformer and variac, I dial as close to 120v as I can get every time. I also monitor it just before measuring and during....
But, that said I do see variations in B+ while at 120v in... Making me think the 274B might be a bit tired... But, even at that point, B+ feeds both 300B's
rectifier diodes (each heater circuit) and each and every time the left channels 300b is lower in voltage than the right..
Amp cooling off now. Going to swap the 300B's and see what role they play...
Also going to measure Vin and B+ in intervals to see how warmup might play a role.....