• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Heater problem

nige838

Member
2009-12-12 9:07 pm
Hello, I've been working on the following schematic. I subbed the 35w4 for the 35z5 and the 12at6 for the 21sq7. I replaced the dropping resistor in the heater chain and used a 6h6 (only the heater's wired) to take up the extra 6 volts, leaving me with 115v. Anyways, the heater chain is as follows: AC directly to 35w4....50l6....12sq7....12at6....6h6.

THE PROBLEM:
The 35z5 gets REALLY BRIGHT, and the rest of the tubes don't light at all, leading me to believe that the 35z5 is absorbing all of the voltage somehow. given, the amp doesn't work at all.

I've triple checked the heater chain, all is ok. even double checked the pinout with the datasheet. I'm really stumped on this one. This is my second build of this circuit, the first worked flawlessly...but was built EXACTLY to spec...

Please help,
Nick G.
 

nige838

Member
2009-12-12 9:07 pm
sorry, forgot to post schematic...
 

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nige838

Member
2009-12-12 9:07 pm
ok, so this just got weirder...

I switched the power cord to this around and everything EXCEPT the 35z4 lit...what the heck is going on? I've got to be missing something, but for the life of me, I don't know what...

next, I'm going to try to find a 35w4 to sub back in it's place...
 

nige838

Member
2009-12-12 9:07 pm
thanks for the quick replies...

I used the 6H6 because I built this from 100% scavenged parts, which was a goal. Why would the different heater currents matter? Wouldn't they just take what they wanted?

Also, I'm a poor college student and can't afford the shipping for parts...


Thanks for the replies,
Nick
 
THE PROBLEM:
The 35z5 gets REALLY BRIGHT, and the rest of the tubes don't light at all, leading me to believe that the 35z5 is absorbing all of the voltage somehow. given, the amp doesn't work at all.

I've triple checked the heater chain, all is ok. even double checked the pinout with the datasheet. I'm really stumped on this one. This is my second build of this circuit, the first worked flawlessly...but was built EXACTLY to spec..

35Z5: 35V / 0.15A
12SQ7: 12.6V / 0.15A
50L6: 50V / 0.15A
12AT6: 12.6V / 0.15A
6H6: 6.3V / 0.3A

That, right there, is your problem. That 6H6 requires twice the heater current, and given that, you can't expect the other 0.15A VTs to split the voltage as they should. There is a reason why all those types that were designed to run with series connected heaters are almost universally designed to pull 0.15A of heater current. I'd ditch the 6H6, and install an appropriately sized ballast resistor.

It also looks a helluvalot like you're fooling around with a power transformerless design there. I would suggest getting a different design, as that's mega-dangerous :hot: Yeah, they used that a lot back in "the day" to save a few $$$$, but horror stories of electrocutions and near electrocutions also abound from those days. Besides, how many CD players, sound cards, iPods, etc can stand up to 120Vrms if you plug the durn thing in the wall socket the wrong way?

Better to get all 12V versions of the types listed here, a proper power xfmr, and a 12.6V xfmr to run the heaters.
 

nige838

Member
2009-12-12 9:07 pm
Hmmm you might want to use the correct resistor and get rid of the 6H6. It's heater draws 300ma while the rest are all 150ma.

35Z5: 35V / 0.15A
12SQ7: 12.6V / 0.15A
50L6: 50V / 0.15A
12AT6: 12.6V / 0.15A
6H6: 6.3V / 0.3A

That, right there, is your problem. That 6H6 requires twice the heater current, and given that, you can't expect the other 0.15A VTs to split the voltage as they should. There is a reason why all those types that were designed to run with series connected heaters are almost universally designed to pull 0.15A of heater current. I'd ditch the 6H6, and install an appropriately sized ballast resistor.

It also looks a helluvalot like you're fooling around with a power transformerless design there. I would suggest getting a different design, as that's mega-dangerous :hot: Yeah, they used that a lot back in "the day" to save a few $$$$, but horror stories of electrocutions and near electrocutions also abound from those days. Besides, how many CD players, sound cards, iPods, etc can stand up to 120Vrms if you plug the durn thing in the wall socket the wrong way?

Better to get all 12V versions of the types listed here, a proper power xfmr, and a 12.6V xfmr to run the heaters.

Thank you very much for your explaination. I see the error of my ways...
BTW i've got AC mains running into a 40v .5A transformer and then back into a twin transformer to isolate it from mains...but thanks for caring...

Thanks again,
Nick
 

nige838

Member
2009-12-12 9:07 pm
35Z5: 35V / 0.15A
12SQ7: 12.6V / 0.15A
50L6: 50V / 0.15A
12AT6: 12.6V / 0.15A
6H6: 6.3V / 0.3A

That, right there, is your problem. That 6H6 requires twice the heater current, and given that, you can't expect the other 0.15A VTs to split the voltage as they should. There is a reason why all those types that were designed to run with series connected heaters are almost universally designed to pull 0.15A of heater current. I'd ditch the 6H6, and install an appropriately sized ballast resistor.

It also looks a helluvalot like you're fooling around with a power transformerless design there. I would suggest getting a different design, as that's mega-dangerous :hot: Yeah, they used that a lot back in "the day" to save a few $$$$, but horror stories of electrocutions and near electrocutions also abound from those days. Besides, how many CD players, sound cards, iPods, etc can stand up to 120Vrms if you plug the durn thing in the wall socket the wrong way?

Better to get all 12V versions of the types listed here, a proper power xfmr, and a 12.6V xfmr to run the heaters.

Sorry to bother you again, but here is what I did....

I subbed the 6h6 for a 12sa7, 12.6v heater @.15A...
Same problem persisted...
the voltages add up to around 122 (I have 120 at the wall)
I still can't figure out what is going on, do you have any more ideas?

Thank you very much for your time,
Nick
 
Sorry to bother you again, but here is what I did....

I subbed the 6h6 for a 12sa7, 12.6v heater @.15A...
Same problem persisted...
the voltages add up to around 122 (I have 120 at the wall)
I still can't figure out what is going on, do you have any more ideas?

Thank you very much for your time,
Nick

No new ideas other than that something's not wired right. Either the heaters are miswired at the socket, or there's a cold joint in there somewhere. Other than that, no ideas as I've never done anything like that with any of my projects so far (all have been 6.3V VTs with heater power off a 6.3V xfmr).

However, a 35W4 isn't going to be very happy working into a 220uF capacitor. These VT diodes aren't made to source a high Isurge, and differ from silicon diodes in that regard.
 

nige838

Member
2009-12-12 9:07 pm
SOLUTION:

OK, so I've figured out the problem...

I connected the heaters directly to the mains with a new cord and they all lit:)

Checked the voltages and all are within a volt of desired voltage
all 12 volts read very close to 12
35w4 reads at 34
50l6 reads at 48.5

I think that it was because the heater on the 6H6 was a different amperage

NEXT UP:
Figure out why I'm only getting a buzz...
(but I'll work on it myself for a few days before I bother anybody else)

Thanks guys for helping me solve my problem,
Nick G.
 

Brit01

Member
2008-07-04 10:35 am
I think that it was because the heater on the 6H6 was a different amperage

I think that was explained to you before. If in series they all have to be of equal amperage. Otherwise wire in parallel.


I connected the heaters directly to the mains with a new cord and they all lit

You do not want to be doing this. Get some isolation between the mains and the heaters.

;););)

Good luck
 
Double-check the wiring around the rectifier. Don't forget that some of these rectifiers have a heater tap, meant for powering a panel bulb - this could add extra confusion. If simply swapping the polarity of the mains supply changes which heaters light up then you have a serious wiring error, and possibly DC mains too?

Do you understand why adding a 300mA valve to a 150mA chain doesn't work? It is not about current draw, but voltage drop and temperature effects.

You said you are a student. Hopefully not EE, or only just starting out?
 

nige838

Member
2009-12-12 9:07 pm
Double-check the wiring around the rectifier. Don't forget that some of these rectifiers have a heater tap, meant for powering a panel bulb - this could add extra confusion. If simply swapping the polarity of the mains supply changes which heaters light up then you have a serious wiring error, and possibly DC mains too?

Do you understand why adding a 300mA valve to a 150mA chain doesn't work? It is not about current draw, but voltage drop and temperature effects.

You said you are a student. Hopefully not EE, or only just starting out?

Yes, the polarity of the mains supply changes which heaters lit. Even off of the isolation transformers the polarity changed which ones lit.

No, I do not understand why the difference in mA doesn't work...

I study Sociology and History, nothing to do with engineering...

I thought that it was safe to run the heaters off of the mains, just not the B+ so long as a earthed the chassis? Is this not true?
 
It is not safe to run any part of a circuit directly off the mains, except where it is localised and isolated from the rest of the circuit. The live side of a PSU is OK (and unavoidable!), if done carefully. A heater circuit is not OK, as it snakes around the entire chassis. It was the practice in the 1960's and 70's, but we care more about safety nowadays. The only place it should be used is when repairing and restoring an item from that era - at least people will expect to see it and take precautions. It has no place in a new build, even if using old components. You should run the heaters from a transformer. The heater-cathode insulation is not designed to be a safety barrier, and can break down as valves age.

I suspect you have some clash between ground and neutral, but I don't know enough about American mains wiring practice to advise.

The original problem will have been complicated by the remaining wiring error, but even without this you can't put a 300mA valve in a 150mA chain. This is because every valve in the chain will take exactly the same current, whether that is right for it or not - that is what happens when things are connected in series. A 300mA valve will not drop enough voltage, as its heater will be too low resistance, and so it will not heat up very much. That in turn will mean that its heater will stay near its cold resistance (heaters, like most metals, increase resistance when hot). The small voltage drop will mean that the total current will be a bit greater than 150mA. That means that the 150mA valves will drop a bit too much voltage and so get hotter than they should. Their heaters will have a higher resistance than normal, so they will drop even more volts and get even hotter. Whether this develops into thermal runaway depends on the exact circuit details. Anyway, the 300mA valve will be too cold and the rest will be too hot.

I am pleased to hear you are not an EE major!