Well kind people, I have been benefitting from your excellent advise and been able to resolves my hiss problem, and 90% of my hum problem with an old ARC SP8. BUT now I am out of my depth I think....
I have a problem that I think is power supply related, but frankly what do I know. (Should a 12BH7A run way too hot to touch?)
The fault/problem has occurred before a few times (probably 3, and even possibly before I took it into my first repair guy). The sound is very concerning, like a wowwing and buzz, and I immediately mute it as it sounds kind of fatal. This time I kept it on (muted) and measured the DC voltages before turning it off on the HT and they were completely unstable as were the voltages at other familiar places on the circuit B+, anode etc.
I live in the south of England near the coast.
Thoughts?
I have a problem that I think is power supply related, but frankly what do I know. (Should a 12BH7A run way too hot to touch?)
The fault/problem has occurred before a few times (probably 3, and even possibly before I took it into my first repair guy). The sound is very concerning, like a wowwing and buzz, and I immediately mute it as it sounds kind of fatal. This time I kept it on (muted) and measured the DC voltages before turning it off on the HT and they were completely unstable as were the voltages at other familiar places on the circuit B+, anode etc.
I live in the south of England near the coast.
Thoughts?
The 12BH7 was at least in my experience always a bit on the toasty side of things. Keep in mind that compared to a 12AX7 or AU or AT the heater power here is signifficantly more than the required for those (600mA at 6,3V) which is double the power required for a 12AX7. It already shows on the tube that the heaters alone make the glass envelope quite toasty.
The 12BH7 is also a lot more powerfull tube. I have mostly seen it in service as a signal follower to drive bigger tubes (like KT88 and above) to improve drive capabillity. That said they can dissapate (each sections) 3,5W compared to a 12AX7 which does only 1W or 12AU7 which does 2,75W or the 12AT7 which does 2,5W. So yeah its one of the toastier double triodes of the entire noval family of tubes.
The 12BH7 is also a lot more powerfull tube. I have mostly seen it in service as a signal follower to drive bigger tubes (like KT88 and above) to improve drive capabillity. That said they can dissapate (each sections) 3,5W compared to a 12AX7 which does only 1W or 12AU7 which does 2,75W or the 12AT7 which does 2,5W. So yeah its one of the toastier double triodes of the entire noval family of tubes.
Well it has done it a few times today, and I have noticed the line voltage is also very low.....but just within allowable regulations.
218V and the HT should be 450 AC and it was 399, the HT which has always been low compared to the target 640 was also super low at less than 500. Then it all went unstable again.
Could new diodes help the HT up a bit, I have new PS caps?
Yeah I know. It'a annoying as it used to be 246V all the time. We had a few cuts recently and the voltage varies quite a lot.
Maybe I need a small step up to the mains transformer.....
I am pretty sure when it gets to near this low 218 value I get an AC on primary before regulation of 400 (instead of target 450) I get the problems, but this is tbh a bit of guess work.
Would new diodes provide any benefit do you think?
Maybe I need a small step up to the mains transformer.....
I am pretty sure when it gets to near this low 218 value I get an AC on primary before regulation of 400 (instead of target 450) I get the problems, but this is tbh a bit of guess work.
Would new diodes provide any benefit do you think?
I guess one of these to boost it a bit if it stays on the breadline....
RS PRO 1 Phase 720VA Variac, 1 Output, 240V, 3A | RS Components
RS PRO 1 Phase 720VA Variac, 1 Output, 240V, 3A | RS Components
If you can afford it that's one way but if you have a fully enclosed mains transformer with the same VA as your power transformer at a voltage of 10volts/15volts output you could put that in series with the live side of the transformer input .
Of course if you have kids in the house its a no-no but it will work and still be in the upper limits of the power supply .
The problem with a variac and I have a good Zenith (American ) bought from Stewarts of Reading ex UK military of 4 amps -two dials ( analogue ) - voltage and current reading built to military spec. is that what happens when you have it running at 230 volts output and the power company ups the supply voltage ?
Of course if you have kids in the house its a no-no but it will work and still be in the upper limits of the power supply .
The problem with a variac and I have a good Zenith (American ) bought from Stewarts of Reading ex UK military of 4 amps -two dials ( analogue ) - voltage and current reading built to military spec. is that what happens when you have it running at 230 volts output and the power company ups the supply voltage ?
well ....this morning it's 237V at the mains....
What I don't know yet is how it varies and when, and also IF this is my intermittent problem or not.
I also don't know why when I have 430V AC feeding my diodes with new PS caps I still only get 540V DC, then the circuit predicts 450V AC = 640 DC.
I guess the old 270k carbon comp resistors in parallel with the smoothing caps could throw this out? I am putting new diodes in so maybe I need new resistors here too?
thoughts
What I don't know yet is how it varies and when, and also IF this is my intermittent problem or not.
I also don't know why when I have 430V AC feeding my diodes with new PS caps I still only get 540V DC, then the circuit predicts 450V AC = 640 DC.
I guess the old 270k carbon comp resistors in parallel with the smoothing caps could throw this out? I am putting new diodes in so maybe I need new resistors here too?
thoughts
This was brought up on my old UK public help forum I was on.
As it stands UK utility companies have covered themselves with Legislation that allows this large range of "permissible " variations in power supply voltages .
OTOH-- at least one UK guy complained to his utility company and managed to get an electrical engineer out not to his house but the local step-down transformer who upped his voltage .
What I think your problem is is that you have local engineering /car repairers using welding gear ,even neighbours using their homes as businesses coming off the same step-down transformer .
One guy found his neighbour was doing exactly that it also caused spikes in the supply .
Put in a complaint to your supplier saying its causing instability in the function of your domestic equipment and are therefore not being provided with a fully functional service.
Back in my day working for BT we had the same problem but the electronic recorder we left at the engineering factory cost over £10,000 .
Times change--prices drop-dramatically here is just one UK company that sells small ones ( my old one was large and heavy ) for £132 upwards -
Voltage Data Loggers | Current & More at Loggershop
IF you have concrete evidence of this large supply variation it adds to your ability to convince the power company to take action.
As it stands UK utility companies have covered themselves with Legislation that allows this large range of "permissible " variations in power supply voltages .
OTOH-- at least one UK guy complained to his utility company and managed to get an electrical engineer out not to his house but the local step-down transformer who upped his voltage .
What I think your problem is is that you have local engineering /car repairers using welding gear ,even neighbours using their homes as businesses coming off the same step-down transformer .
One guy found his neighbour was doing exactly that it also caused spikes in the supply .
Put in a complaint to your supplier saying its causing instability in the function of your domestic equipment and are therefore not being provided with a fully functional service.
Back in my day working for BT we had the same problem but the electronic recorder we left at the engineering factory cost over £10,000 .
Times change--prices drop-dramatically here is just one UK company that sells small ones ( my old one was large and heavy ) for £132 upwards -
Voltage Data Loggers | Current & More at Loggershop
IF you have concrete evidence of this large supply variation it adds to your ability to convince the power company to take action.
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