Not sure if the hum is from the MT placement, I assume so. I had to put it there, the layout was never intended for a MT as I was originally using a boost converter supplied by the power brick.
Congratulations for at least getting some musical sounds instead of the rubbish you were reporting earlier. Next hurdle is tackling the power fade issue and the hummmmm....
I dunno about coupling issues between the mains XFMR and the output transformers, as both of those look to be completely enclosed by steel, which you would think would prevent pickup from the mains XFMR. You may want to look into cleaning up your B+ supply a little more - splitting your dropper resistor and using a RCRC filter arrangement was a good suggestion. If you try that, be generous with the capacitors. If you set the time constant of each filter section to 1/10 the mains frequency or below, that may be sufficient. Another option might be to add a hum strap around the outside of your mains transformer using copper foil. Wrap the foil around the transformer coil and core outer legs, and solder the ends together to form a shorted turn. This works a treat to help cancel out stray flux, and is a common feature with a lot of mains transformers. If you are confused about how exactly to do this, I can draw a picture and upload it after I get back into work on Monday. I dunno about your fade problem - I would suggest monitoring both the B+ and filament voltages to see if either is sagging over time - I figure one of those voltages must be sagging to cause the fade you describe.
I dunno about coupling issues between the mains XFMR and the output transformers, as both of those look to be completely enclosed by steel, which you would think would prevent pickup from the mains XFMR. You may want to look into cleaning up your B+ supply a little more - splitting your dropper resistor and using a RCRC filter arrangement was a good suggestion. If you try that, be generous with the capacitors. If you set the time constant of each filter section to 1/10 the mains frequency or below, that may be sufficient. Another option might be to add a hum strap around the outside of your mains transformer using copper foil. Wrap the foil around the transformer coil and core outer legs, and solder the ends together to form a shorted turn. This works a treat to help cancel out stray flux, and is a common feature with a lot of mains transformers. If you are confused about how exactly to do this, I can draw a picture and upload it after I get back into work on Monday. I dunno about your fade problem - I would suggest monitoring both the B+ and filament voltages to see if either is sagging over time - I figure one of those voltages must be sagging to cause the fade you describe.
Last edited:
I went back and did a little math - you are dealing with mains ripple of 100 Hz. I would suggest using a separate dropper resistor string for each amplifier channel, as that gives you a higher resistance for the filter capacitors to work against. DA suggested some appropriate values to work with. If you split that dropper resistor to get an RCRC filter and use 47-100 uf, you should get substantial B+ ripple reduction. Since your B+ voltage is relatively low, the extra filter caps won't be too huge. However, you may need to be generous with the voltage ratings, as the caps will see the full peak B+ voltage until the tubes warm up and start to draw current. I figure the ripple might be getting in via the input triode stage, as that will pass along variations introduced by the B+ ripple. The pentode output stage is less sensitive in that regard as it is more resistant to B+ variation. Adding an RC filter between the pentode stage and the triode stage might be a good idea, and may be easier in terms of C values, as the triode 1st stage draws a lot lower current from the B+ than the pentode 2nd stage, so you can use a larger resistance and lower capacitance to cut down the B+ ripple.
Last edited:
Thanks to you both for all your help. I will take some time to go over all that and then have a go at doing it at a later point.
As for your fade issue, you might want to check the temp on your filament regulators, as they might be going into thermal protect - no filament, no sound... A heat sink would fix that pronto, and it might not even need to be very big.
I've just had another listen. This time I observed the amp more closely. I can't do any measurements or anything as I've brought it into the house and all my equipment is in my shed.
I noticed that the tubes glowed a strong amber right up until the amp lost volume. In fact, it still held after they faded for a short while before the sound output faded completely away.
I was hoping that watching the tubes would give some clues. Strangely the top and bottom of the tube both faded completely when the audio was lost. I'm sure I could see a blue glow in the centre of the left one once the audio had totally faded 😬.
Will have to do some Voltage measurements and see what going on.
If you are seeing a blue glow in the center of one of your tubes, it might be gassy. A bluish-white glow is the tell, that happens more in the interior of the tube. A deeper blue-violet glow that seems to stick to the outer glass envelope happens from stray electrons exciting fluorescent impurities in the outer envelope, and is harmless - it's a free light show that sometimes dances with the music. Having said that, the fluorescent blue glow generally happens at a higher B+ voltage than what you're currently using.
Tubes that are legitimately gassy can run away and cause problems - they will glow whether or not the filament is lit. The ionized gas in the tube forms connections between tube elements that shouldn't be there in a normal tube with good vacuum.
If your filament is getting choked off by the filament VR going into overtemperature protection, both the top and bottom of the filament will fade.
BTW, if you have an extra tube, try swapping it with the one that appears gassy. Rough treatment of the pins can sometimes open up micro-cracks that can let in enough air to cause problems - too much air will cause the getter (the silver coat on the inside of your tube, a lot of times near the top tip) to turn white, rendering the tube useless for nothing except making a noise when it gets tossed in the bin...
Tubes that are legitimately gassy can run away and cause problems - they will glow whether or not the filament is lit. The ionized gas in the tube forms connections between tube elements that shouldn't be there in a normal tube with good vacuum.
If your filament is getting choked off by the filament VR going into overtemperature protection, both the top and bottom of the filament will fade.
BTW, if you have an extra tube, try swapping it with the one that appears gassy. Rough treatment of the pins can sometimes open up micro-cracks that can let in enough air to cause problems - too much air will cause the getter (the silver coat on the inside of your tube, a lot of times near the top tip) to turn white, rendering the tube useless for nothing except making a noise when it gets tossed in the bin...
Last edited:
Depending on the positioning of the bundle of filament heating wires that are shoved inside the tubular cathode, you may see glow at both the top and bottom of the cathode. It is also common to be able to look down inside the glowing tubular cathode.
Quick post to show how the valves look.
The video shows it a few minutes after power on.
Then you can see how it gradually fades.
Didn't see any blue light in there this time, but it was darker in the room last night.
Interested to know what the volts are doing now. Will hopefully get time tomorrow to check things out
The video shows it a few minutes after power on.
Then you can see how it gradually fades.
Didn't see any blue light in there this time, but it was darker in the room last night.
Interested to know what the volts are doing now. Will hopefully get time tomorrow to check things out
Another quick question I forgot to ask if you don't mind? Can I shrink wrap in shrink tubing? 2 reasons I'd like to, for safety to cover the exposed terminals at the bottom, and to make it it match the OT's aesthetically. Was thinking of colouring the top black and wrapping it in a black shrink.
The gradual fade of the filaments seems to reinforce the notion of the filament regulators gradually overheating and shutting down...
I think so, too. I felt the tiny heatsink just after it faded (powered off), it was ridiculously hot!
Well, then, your mission clearly is to slap on a larger heat sink. Once you solve the fade, you can then work on reducing/eliminating the Hmmmmmm.
Ordered some uprated heatsinks for the LM317 👍. I've also cut some 12mm holes in the sides of the chassis and one directly underneath the LM317 board. I've ran the amp for about an hour without it going off so far so perhaps the improved airflow improved things, I will be upgrading the heatsink regardless. I used the heat probe on y multimeter to measure the temp of the existing sink, it was at 98 degrees C, so pretty hot!
I'm still trying to get my head around the voltages in the amp and what's required. I've had loads of great info but it's just not quite clicking into place....
I'm getting:
272V positive to negative at the rectifier output
241V after smoothing caps
230v anode (measured from pin 6 to HT ground)
I am looking at the table above taken from http://www.r-type.org/exhib/aai0120.htm
Diabolical Artificer has assured me it's not going to harm anything to run it at this voltage for a short while which is great. That means I've been able to use the amp a bit which I've enjoyed. However, I'd like to have a go of getting the voltages down to within the recommended range.
I can see 'Va' (Voltage Anode?) states 170. Is this measurement taken from pin 6 to ground? Or, am I getting that wrong?
Once I know I'm measuring it correctly and interpreting the info as its intended I can have a go at getting the voltage down.
Thanks again.
I'm still trying to get my head around the voltages in the amp and what's required. I've had loads of great info but it's just not quite clicking into place....
I'm getting:
272V positive to negative at the rectifier output
241V after smoothing caps
230v anode (measured from pin 6 to HT ground)
I am looking at the table above taken from http://www.r-type.org/exhib/aai0120.htm
Diabolical Artificer has assured me it's not going to harm anything to run it at this voltage for a short while which is great. That means I've been able to use the amp a bit which I've enjoyed. However, I'd like to have a go of getting the voltages down to within the recommended range.
I can see 'Va' (Voltage Anode?) states 170. Is this measurement taken from pin 6 to ground? Or, am I getting that wrong?
Once I know I'm measuring it correctly and interpreting the info as its intended I can have a go at getting the voltage down.
Thanks again.
The plate voltage is generally measured from anode to cathode, not anode to ground. The anode - to - cathode voltage is the actual voltage experienced by the tube. The voltage across the cathode resistor is also important, as it helps set the 1st grid bias, and along with the screen voltage, helps set the bias current for the output stage of the amplifier.
Your filament regulator was getting really hot.... When the temperature of the regulator starts getting above about 130C or so, that's when overtemperature protection starts to kick in and shut things down for the count... I wonder how hot things were getting before you added the ventilation holes - clearly enough such that the filament regulator decided to exit stage right...
If you're in an experimenting sort of mood, you can add a 100 ohm resistor from your raw supply to each amplifier L/R stage. You can then measure the voltage acros that resistor with a DVM and use the voltage measured to calculate the current drawn by each channel of the amplifier. You can use that current to calculate the value of dropper resistor needed to knock down the supply voltage to its proper value. Make sure to add some capacitance after the dropper resistor so that each stage has a reservoir to draw on for power peaks.
Your filament regulator was getting really hot.... When the temperature of the regulator starts getting above about 130C or so, that's when overtemperature protection starts to kick in and shut things down for the count... I wonder how hot things were getting before you added the ventilation holes - clearly enough such that the filament regulator decided to exit stage right...
If you're in an experimenting sort of mood, you can add a 100 ohm resistor from your raw supply to each amplifier L/R stage. You can then measure the voltage acros that resistor with a DVM and use the voltage measured to calculate the current drawn by each channel of the amplifier. You can use that current to calculate the value of dropper resistor needed to knock down the supply voltage to its proper value. Make sure to add some capacitance after the dropper resistor so that each stage has a reservoir to draw on for power peaks.
Last edited:
ok, thanks again for your help. When you say Cathode resistor, do you mean the Triode or pentode Cathode? These are pins 2 and 8, both have resistors.
The larger heatsinks came today. Pic of the puny old ones for comparison!
Hopefully this solves the problem of the shutting down.
Hopefully this solves the problem of the shutting down.