I'm rebuilding a cheap series-string heated stereo tube amp. Line up is 35W4, 12AX7, and two 35C5 tubes. I've been experimenting and I am using the schematic seen here:
Men Hardly Working: Archer StereoDyne/3 Amplifier Clone
Everything has been coming along rather well, but after breadboarding it, I got a lot of hum. So I built a simple power supply using an isolation transformer, fuse, power switch, rectifier, and a CRCRC filtering section consisting of 100uF -> 100 Ohm -> 100uF -> 100 Ohm -> 100uF. I've wired the heaters of all the tube in series and added 200 Ohms / 5 Watts to the string to drop the voltage and keep it steady at about 122 Volts.
I am running the heaters off the rectified DC after the smoothing caps.
The amp sounds pretty nice and I'm happy. The hum is almost completely gone.
But I am wondering if it's OK that I'm just using the 35W4 as a ballast tube to drop voltage in the series string? Will there be any problems with just the heaters wired and nothing else on this tube? Should I drop the tube and find another way to reduce my voltage?
Second question. I'd like to wire the 6.3 V pilot lamp to the power tap on pins 4 and 6 of the 35W4, but since I am not using it to rectify, I do not have the any pins currently hooked up except 3 and 4 (heater). The datasheet shows a circuit for the panel lamp that includes pin 5. I don't think that circuit would work under my circumstances. Thoughts?
http://www.r-type.org/pdfs/35w4.pdf
Thanks in advance!
Men Hardly Working: Archer StereoDyne/3 Amplifier Clone
Everything has been coming along rather well, but after breadboarding it, I got a lot of hum. So I built a simple power supply using an isolation transformer, fuse, power switch, rectifier, and a CRCRC filtering section consisting of 100uF -> 100 Ohm -> 100uF -> 100 Ohm -> 100uF. I've wired the heaters of all the tube in series and added 200 Ohms / 5 Watts to the string to drop the voltage and keep it steady at about 122 Volts.
I am running the heaters off the rectified DC after the smoothing caps.
The amp sounds pretty nice and I'm happy. The hum is almost completely gone.
But I am wondering if it's OK that I'm just using the 35W4 as a ballast tube to drop voltage in the series string? Will there be any problems with just the heaters wired and nothing else on this tube? Should I drop the tube and find another way to reduce my voltage?
Second question. I'd like to wire the 6.3 V pilot lamp to the power tap on pins 4 and 6 of the 35W4, but since I am not using it to rectify, I do not have the any pins currently hooked up except 3 and 4 (heater). The datasheet shows a circuit for the panel lamp that includes pin 5. I don't think that circuit would work under my circumstances. Thoughts?
http://www.r-type.org/pdfs/35w4.pdf
Thanks in advance!
first you got to have an isolation transformer, equipment working off the mains is a no-no in this board...
second, as long as the heater series string gets the right voltage, then no problem...
second, as long as the heater series string gets the right voltage, then no problem...
Yes, I am using an isolation transformer, a fuse, and a power switch. I understand the dangers of a hot chassis design.
So you are saying that it's OK for the 35W4 to be used only as a ballast in the chain, right?
How about my second question on the pilot light from pins 4 and 6 on the 35W4? Can I still use it?
Thanks!
about pilot lamps, today i will use LED's instead...
I can appreciate that. But I have this old pilot lamp, and I do not have an LED, so... 😱
At first, I had the pilot lamp in the series string with the heaters, but I noticed that when I switched on the amp, the light got really bright, then dimmed down as the heaters began to glow. I read a bit and found that isn't very good for the lamp. Then I found the reference to the pilot light tap on the 35W4 half-wave rectifier tube. I'm not using the tube as anything but a ballast at this point, but I thought maybe I'd better wire up my pilot light to that as intended.
yes...the cold resistance of the tube filaments was the cause of that...
you may want to put a series resistor with the lamp to limit current...
you may want to put a series resistor with the lamp to limit current...
The pilot light will be dim when connected across the tapped heater, but won't flare and burn out. In the typical series string radio you'd see it brighten after the tubes warm up, since it carries a portion of the heater current and the B+ current too, but still likely less than the rated 150 mA. The 35W4 data sheet may say that a resistor should be added if a pilot light isn't used. Pilot light may improve heater life.
35W4 may "wear out" (lose emission) faster in this usage, but who cares if it's not emitting...
35W4 may "wear out" (lose emission) faster in this usage, but who cares if it's not emitting...
I assume you've added some other diode(s)? A resistor could be used in place of a heater in the string. It would give a slightly longer warm up time.
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__/\_/\___~~~_____
....... [______∆______]
Putting the pilot light across the 35W4 tap and a 10 Ohm resistor might be about right. Crude graphic is the best I could do on this tablet...
....... [______∆______]
Putting the pilot light across the 35W4 tap and a 10 Ohm resistor might be about right. Crude graphic is the best I could do on this tablet...
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I'm using a standard solid state bridge rectifier with a CRCRC set of smoothing caps/resistors, which got rid of most of the hum I was experiencing using the half-wave 35W4 rectifier. Of course, this also left me with slightly more heater voltage to get rid of. So instead of a B+ (and heater voltage) of 117 volts, I am looking at about 122 volts with all the tubes and a 100 Ohm 5 watt resistor in series for the heaters.
I could replace the 35W4 with a big resistor of the appropriate wattage, I suppose, but the chassis I am repurposing to put the whole thing in happens to have 3 7-pin and 1 9-pin socket it it. Perfect for my uses. And I sincerely doubt I'd ever use the 35W4 for anything else, so it seems like a reasonable thing to do; just make it a ballast to suck up voltage and turn it to heat.
Another option would be to ditch the 35W4 and go with 50C5 tubes, but I am told the 50C5 isn't the same as a 35C5 with different heater ratings, it's actually a very different tube. So rather than get all crazy with what I have, I thought I'd just use it more-or-less as designed. I only went with a solid-state bridge rectifier because a) I have a bunch of them I got cheap from Allied, and b) I had a problem with hum on the amp as designed using the half-wave vacuum tube rectifier.
Thank you, I believe I understand. I will experiment with this tomorrow and report back.
I wanted to say thank you to everyone who assisted me. The amp is built.
I used a repurposed Blonder-Tongue Distribution amplifier that had originally had four 6J6 tubes in it (still have the tubes, just don't have a use for them at the moment).
I used the built-in power switch, fuse, and output terminals (repurposed as speaker terminals). I knocked out one of the 7-pin sockets and replaced it with a 9-pin socket for the 12AX7.
From left to right, the tube lineup is 35C5 / 35C5 / 35W4 / 12AX7. I am using a Triad N68-X isolation transformer and the output transformers come from the old hot-chassis Monarch tube amp that the tubes came from.
I drilled a hole on top for the 6 volt pilot light.
I have some extremely minor hum, but you have to put your ear right up to the speakers to hear it, so I'm not too concerned. At middle volume, there is some distortion; I may add some global negative feedback. Even at 1.5 watts per channel, I find it quite loud enough for what I intend to use it for. With those tiny little output transformers, it's never going to be Hi-Fi, but it should be quite pleasant enough for casual listening.
This is my third tube amplifier, and the first one that I put into a box and intend to keep intact. I will clean up the cabinet some and try to make it look a bit more presentable. All in all, though, I'm pretty pleased with myself.
I used a repurposed Blonder-Tongue Distribution amplifier that had originally had four 6J6 tubes in it (still have the tubes, just don't have a use for them at the moment).
I used the built-in power switch, fuse, and output terminals (repurposed as speaker terminals). I knocked out one of the 7-pin sockets and replaced it with a 9-pin socket for the 12AX7.
From left to right, the tube lineup is 35C5 / 35C5 / 35W4 / 12AX7. I am using a Triad N68-X isolation transformer and the output transformers come from the old hot-chassis Monarch tube amp that the tubes came from.
I drilled a hole on top for the 6 volt pilot light.
I have some extremely minor hum, but you have to put your ear right up to the speakers to hear it, so I'm not too concerned. At middle volume, there is some distortion; I may add some global negative feedback. Even at 1.5 watts per channel, I find it quite loud enough for what I intend to use it for. With those tiny little output transformers, it's never going to be Hi-Fi, but it should be quite pleasant enough for casual listening.
This is my third tube amplifier, and the first one that I put into a box and intend to keep intact. I will clean up the cabinet some and try to make it look a bit more presentable. All in all, though, I'm pretty pleased with myself.
Attachments
good job...
Thanks! I added the negative feedback today. Wired from positive speaker terminal to pins 3 and 8 on the 12AX7 which seemed to help. Also rolled in a 12AU7 instead of the 12AX7. Much better! I'm just letting it run now, make sure it doesn't catch fire or something. But I am basically done now.
What is U plate of new 12AU7? Im afraid You must make adjust of mode, because this is different tube and 100 kohm is too much. You can measure it current plate, that must be no more 10 ma /if Im right...I forgot it max current/. You can try disconnect feetback, after adjusting and measuring.
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