• 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.

Lamp in HV Centre Tap - why?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I am working on 2 1950's amp restorations at the moment.
One a SE 6V6 and one a PP 807 Output.
Both use Tube rectifiers and both have a standby switch between the HV centretap and 0V AND a 6.3V 300mA lamp in series with the switch to 0V.

These lamps are not front panel indications but are "buried" inside.

I have been unable to formulate any reason why the lamps are included except perhaps that they provide a fuse function and an indication of when the High Voltage is ON.

Is there any other reason why this might have been done?

Thanks,
Ian
 
Thinking about it some more I think you are probably right - it was probably the best "cheap" solution to inrush current limiting available in those days before negative Temperature Coefficient (NTC) Thermistors were invented. It would help a bit with rectifier tube life.
Cheers,
Ian

I like your tag line - I do a fair bit of time and budget estimation for designs in the day job and I always tell folk that the first half of the project takes 90% of the time and the other half takes the other 90% of the time.
 
Last edited:
Yes, a simple inrush limiter seems like a reasonable explanation, though resistors in series with rectifier plates would be a better alternative, spec sheet for the rectifier (min series resistance incl resistance of HT winding) is kinder to rectifier.
Aardvarkash, agree with sentiment in your sig, I keep trying to explain to friends that there is no such thing as a "finished" amp, it is always a moving target!
Built a new amp during holiday on a scruffy used chassis, strictly proof-of-concept, sounds decent, but I am not ready to post it for comment until I am over the initial "I made it so it must be OK" moment - sorry if I am threadjacking:)
 
Haha, not this week, hope you have wool carpet, when splattered it just smells bad, pick the splats out with tweezers, synthetic not so good...(it melts) I try to keep soldering activities in spare room with a mat to catch the splats, mostly works, most days...
 
Last edited:
How about a... well, "outrush" limiter? (I made it up) If draw gets excessive on the supply, the lamp's current heats it, increasing its resistance. That tends to help soft limit B+ current.

If the lamps are not readily visible from the outside, that pretty much eliminates the lamp as indicator. The 1950s gear wasn't usually designed with diagnostic internal circuits, so we'd assume the lamp served the circuit in some other way than that.
 
Thinking about it some more I think you are probably right - it was probably the best "cheap" solution to inrush current limiting available in those days before negative Temperature Coefficient (NTC) Thermistors were invented. It would help a bit with rectifier tube life.

I've got a TV set, made in 1949 which has an NTC in the heater chain. It's actually indicated as a "Brimistor" on the schematic.
 
I've got a TV set, made in 1949 which has an NTC in the heater chain. It's actually indicated as a "Brimistor" on the schematic.

Unsurprisingly Brimistor was a trademark for Brimar - of valve fame NTC's.
The bulb though, is a PTC...

Tangentially, we've had a few funny descriptors for these devices:
NTC's
PTC's
Brimistors,
Thermistors - could mean anything...
Posistors - never heard of "negistors" though...

Forgotten the name for the combined PTC+NTC that was used in the degauss circuit of some tv's.
 
Unsurprisingly Brimistor was a trademark for Brimar - of valve fame NTC's.
..

Forgotten the name for the combined PTC+NTC that was used in the degauss circuit of some tv's.

It's interesting it's a Brimistor in this set, as all the valves and the crt are mullard ! (it's a Pye set, so not that's surprising)
The degaussing "Posistor" is normally indicated as a "dual PTC"
 
It's interesting it's a Brimistor in this set, as all the valves and the crt are mullard ! (it's a Pye set, so not that's surprising)
The degaussing "Posistor" is normally indicated as a "dual PTC"


Not from my recollection. The dual device had a PTC element in series with the coil and an NTC in parallel with the coil. They were thermally coupled.

Agree with Toprepairman's view.
 
Indeed, PTC very much like a lamp but even more so. A constant current supply for the filaments. Don't bother considering it as a constant current generator for audio though, (others have suggested using lamps, erroneously) as you'd end up with a frequency response of DC to a couple of Hertz.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.