I've just finished building a pair of 300B SE monoblocks following the Walton Audio design (attached is a schematic). Both tubes seem to have a slight blue glow on the grid and one of them also exhibits a light blue glow around the glass. Swapping the tubes between the two monoblocks doesn't change anything, which assures me the circuits are in fact identical (no wiring errors).
B+ and bias seem fine to me, around the required voltages:
- B+ is at ~420V
- Bias across 880Ohm resistor is 71V (=80mA if my math is correct)
The tubes in question are a new matched pair of 300B from Electro Harmonix (non gold).
Filament voltage for both 300Bs is low at 4.3V but that's an issue with my DC regulator which I will address shortly.
I've attached pictures of both tubes:
- fine.jpg shows one of the tubes without blue glow on the glass which has some light coloring on the grid
- blue1.jpg and blue2.jpg show the other tube with some blue glow on the glass as well as on the grid
My question are:
- Is this normal? I've heard blue glow on the glass can be a bad vacuum which worsen performace, but the tubes have been matched (and tested) at factory so I guess they are still good for operation. Since the tubes are brand new with less than 30 minutes on them I can ask for a replacement hopefully.
- Are my voltages too far off from the schematic and will I need to change something in my power supply design?
- Could the low filament voltage be the culprit?
In the meantime I'll keep the amplifiers powered off to avoid any damage.
Please ask any additional data if necessary.
B+ and bias seem fine to me, around the required voltages:
- B+ is at ~420V
- Bias across 880Ohm resistor is 71V (=80mA if my math is correct)
The tubes in question are a new matched pair of 300B from Electro Harmonix (non gold).
Filament voltage for both 300Bs is low at 4.3V but that's an issue with my DC regulator which I will address shortly.
I've attached pictures of both tubes:
- fine.jpg shows one of the tubes without blue glow on the glass which has some light coloring on the grid
- blue1.jpg and blue2.jpg show the other tube with some blue glow on the glass as well as on the grid
My question are:
- Is this normal? I've heard blue glow on the glass can be a bad vacuum which worsen performace, but the tubes have been matched (and tested) at factory so I guess they are still good for operation. Since the tubes are brand new with less than 30 minutes on them I can ask for a replacement hopefully.
- Are my voltages too far off from the schematic and will I need to change something in my power supply design?
- Could the low filament voltage be the culprit?
In the meantime I'll keep the amplifiers powered off to avoid any damage.
Please ask any additional data if necessary.
Attachments
Have a read here:
Blue Glow in Tubes, Tube Getters, and other Info - www.thetubestore.com
similar content that shows the source of material:
Blue Glow In Tubes
I used to see this on PL509 line output stages years ago.
The filament volts should be corrected as imo are to low at 4.3 volts. There is under running but that is to much.
Blue Glow in Tubes, Tube Getters, and other Info - www.thetubestore.com
similar content that shows the source of material:
Blue Glow In Tubes
I used to see this on PL509 line output stages years ago.
The filament volts should be corrected as imo are to low at 4.3 volts. There is under running but that is to much.
Thank you for the reply. According to these documentations I'd identify my blue glow as fluorescence on the glass (which apparently is a sign of high vacuum so great) and fluorescence on the grid which is rare but harmless. This relieves me.
The filament voltages will be fixed as soon as I recieve the new (appropriate) regulators.
Voltages and bias looks good to you? I'm asking because I've read somewhere that blue glow on the grid could be the result of wrong biasing.
The filament voltages will be fixed as soon as I recieve the new (appropriate) regulators.
Voltages and bias looks good to you? I'm asking because I've read somewhere that blue glow on the grid could be the result of wrong biasing.
That's what I wondered. Its not easy working off pictures but yours did look to be more around the glass rather than around the internal structures and so more like fluorescence.
You are running well within the maximum anode current and overall dissipation so there should not be an issue there and that to me shows the biasing is essentially correct.
You might also find increasing the heater volts alters the glow you see.
You are running well within the maximum anode current and overall dissipation so there should not be an issue there and that to me shows the biasing is essentially correct.
You might also find increasing the heater volts alters the glow you see.
Yeah my glow is even more subtle than yours and in fact I had a hard time trying to picture it, so I wouldn't worry too much. BTW love your channel, the 300B filament supply is taken from your series, though I ordered the wrong regulator.
Alright I'll let them burn for a while and we'll see.
Thank you all for the replies.
The blue glow in tubes is the equivalent to the crema in an espresso .
Sure you can have just tubes or coffee but the blue crema makes life little moments special.
Sure you can have just tubes or coffee but the blue crema makes life little moments special.
It's normal. I believe a lot people request blue glow but sellers don't do that. 😀 Only 2 out of 4 of my set have blue glow. I wish all 4 do.
to lucat
Here :
Empty Page
there is a little section dedicated to 300B, come from W.E. (scroll down just 1/3)
Have you used the dc filaments for 300B?
In the circuit diagram there is a pot on cathode, iti is enough to lower the noise.
In my opinion the 300B must have the filaments in ac
Walter
Here :
Empty Page
there is a little section dedicated to 300B, come from W.E. (scroll down just 1/3)
Have you used the dc filaments for 300B?
In the circuit diagram there is a pot on cathode, iti is enough to lower the noise.
In my opinion the 300B must have the filaments in ac
Walter
Yes I have. I want to have as little noise as possible and there's already some coming from the two first stages of amplification which use ac filaments. I may later consider adding a separate 5v transformer to see what difference it makes.
You have 71V bias, and 880 ohms, 80 mA right?
The maximum DC current for the 300B is 100mA, but the tube requires 5V on the filament.
Of course, with signal, the peak current can be over 200mA, but the average DC current will still be about 100mA.
300B filament voltage is specified as 5V +/- 5% (+/- 0.25V)
4.3V is -14% (0,7V too low).
And the filament coating is not hot enough. I would not run the tube that way.
Fix the filament voltage first, then see what other problems you have, or do not have.
The maximum DC current for the 300B is 100mA, but the tube requires 5V on the filament.
Of course, with signal, the peak current can be over 200mA, but the average DC current will still be about 100mA.
300B filament voltage is specified as 5V +/- 5% (+/- 0.25V)
4.3V is -14% (0,7V too low).
And the filament coating is not hot enough. I would not run the tube that way.
Fix the filament voltage first, then see what other problems you have, or do not have.
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