MT and rectifiers work. Started with 50V on the variac and took a few measurements. With 50AC in I was getting 96-0-96V out.
Couldn't understand why I was seeing 45V on the heater pins when measuring from each of them separately to the chassis ground but when measuring between them, its 1.39V. So good there.
Ramped up the voltage slowly until at 247V input. Now seeing 385-0-385 on the secondary.
Rectifiers are putting out 512V DC.
I can't quite work out why, but measuring the primary MT mains input live to the chassis ground gives me a lower reading than 247V, I think it was about 220ish. Measuring the same applies to the heater pins on the valve sockets which are at 5.5v measured from the pin to ground?
Tomorrow I'll do some more testing on the blocking caps to measure the DC voltage either side as advised, hopefully they'll be fine.
Was really nice to see the power indicator light illuminate for the first time. The lamp was still good after all those years.
I was thinking about replacing the shared cathode bias resistor for 2 separate ones. Would this be better for biasing the vales? I was wondering about variable resistors for this?
Next step, sort out a permanent mains wiring solution and get the chassis earthed.
Change the speaker output posts to for trs type sockets. Make safe the 100v output.
Then, I might put the valves in see what happens.
Couldn't understand why I was seeing 45V on the heater pins when measuring from each of them separately to the chassis ground but when measuring between them, its 1.39V. So good there.
Ramped up the voltage slowly until at 247V input. Now seeing 385-0-385 on the secondary.
Rectifiers are putting out 512V DC.
I can't quite work out why, but measuring the primary MT mains input live to the chassis ground gives me a lower reading than 247V, I think it was about 220ish. Measuring the same applies to the heater pins on the valve sockets which are at 5.5v measured from the pin to ground?
Tomorrow I'll do some more testing on the blocking caps to measure the DC voltage either side as advised, hopefully they'll be fine.
Was really nice to see the power indicator light illuminate for the first time. The lamp was still good after all those years.
I was thinking about replacing the shared cathode bias resistor for 2 separate ones. Would this be better for biasing the vales? I was wondering about variable resistors for this?
Next step, sort out a permanent mains wiring solution and get the chassis earthed.
Change the speaker output posts to for trs type sockets. Make safe the 100v output.
Then, I might put the valves in see what happens.
Maybe your mains neutral is not at protective earth level, or you haven't connected chassis to mains protective earth (which sounds like what you are missing from your latter comment). If the chassis was a floating piece of metal, then a DMM will read a voltage that depends on the capacitive coupling between active wiring to chassis and neutral wiring to chassis, as well as depending on the meters input resistance (10 Meg).
Same goes for the heater voltage measurements. Your schematic doesn't show the heater circuitry - one common form of circuit uses a heater winding with a CT tap that is connected to chassis (0V) - another common circuit when there is no CT tep is to use two 50 ohm resistors, and referred to as a 'humdinger' - and then there are many variations on a theme for lowering residual hum, but they are perhaps best left for latter.
Using separate cathode resistors for each output stage valve certainly makes it easier to balance the idle currents if you only have used or unmatched valves to hand. Perhaps still use a power resistor for passing the majority of cathode current, and a trimpot or pot to bypass one side to 'tweak' the two sides to be the same (ie. balanced) at idle. Often the original cathode resistors were designed for good new valves, whereas nowadays some prefer to operate at a lower current to reduce the plate power consumption, which often means adding some additional resistance in series.
Same goes for the heater voltage measurements. Your schematic doesn't show the heater circuitry - one common form of circuit uses a heater winding with a CT tap that is connected to chassis (0V) - another common circuit when there is no CT tep is to use two 50 ohm resistors, and referred to as a 'humdinger' - and then there are many variations on a theme for lowering residual hum, but they are perhaps best left for latter.
Using separate cathode resistors for each output stage valve certainly makes it easier to balance the idle currents if you only have used or unmatched valves to hand. Perhaps still use a power resistor for passing the majority of cathode current, and a trimpot or pot to bypass one side to 'tweak' the two sides to be the same (ie. balanced) at idle. Often the original cathode resistors were designed for good new valves, whereas nowadays some prefer to operate at a lower current to reduce the plate power consumption, which often means adding some additional resistance in series.
More like to ring the most snot out of the poor tubes as physically possible. Competition you know; every fraction of a watt counts.
If the original single cathode resistor is ok, then an option is to then insert a separate extra resistor to each cathode. That may assist if you wanted to reduce the idle cathode current a bit, and can provide separate current sensing resistors for checking idle current of each. If the added resistors are not too much in value then there may be no need to bypass them. As an aside, I typically add 10 ohm resistors in this manner to give current sensing, and 10 ohm doesn't need bypassing and doesn't really reduce cathode current by much.
The HT circuit did have me scratching my head, it's compilated by the connections at the old 10 pin power socket and connections to and from the vibrator.
I'l draw out what's there as it is supposed to look. It was omitted from the previous sketch to make things less cluttered.
I got the rectifiers conducting yesterday by taking each of the HT pins to pins 2 of the first EL34 and pin 7 of the other EL34, with a connection between the other 2 and 7 pin.
Pin 2 on the first EL34 valve also connects to pin 5 on the first EZ81, with a connection between pin 4 and pin 5 on the other EZ81. Then this pin connects back to pin 7 on the second EL34.
One of the secondary Heater winding pins originally went to the 12v vibrator socket where it was grounded to the chassis via a resistor and a cap by the looks of it. I disconnected this wire wire and fed that to pin 7 of the second EL34, and took the other MT 6v pin to pin 2 of the first EL34 with a temporary connections to get the EZ81's to conduct so I could do a bit of testing.
I'l draw out what's there as it is supposed to look. It was omitted from the previous sketch to make things less cluttered.
I got the rectifiers conducting yesterday by taking each of the HT pins to pins 2 of the first EL34 and pin 7 of the other EL34, with a connection between the other 2 and 7 pin.
Pin 2 on the first EL34 valve also connects to pin 5 on the first EZ81, with a connection between pin 4 and pin 5 on the other EZ81. Then this pin connects back to pin 7 on the second EL34.
One of the secondary Heater winding pins originally went to the 12v vibrator socket where it was grounded to the chassis via a resistor and a cap by the looks of it. I disconnected this wire wire and fed that to pin 7 of the second EL34, and took the other MT 6v pin to pin 2 of the first EL34 with a temporary connections to get the EZ81's to conduct so I could do a bit of testing.
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Will you aim to keep the 12V option + vibrator for powering the unit? There is a case for removing the stuff you don't need so the circuit is as simple as it needs to be, and there are no added features that could affect reliability.
Be a cool thing to see working I suppose but I've no real practical use for it.
Done a quick sketch of how I think the heater wiring works.
I was going to draw out the connections to the socket pins but seems a bit pointless, might add them in later, I've drawn that part out separately already and took loads of pics. It's not going to be kept anyway.
What I'm not quite understanding is this; the drawing shows each pair of valves wired in series. That require 12v from the MT secondary to give 6v for each valve? The CT is grounded.
I thought each pin of the HT winding was 6v relative to the 0v CT.
But there's a 6v bulb wired between each of the pins?
Done a quick sketch of how I think the heater wiring works.
I was going to draw out the connections to the socket pins but seems a bit pointless, might add them in later, I've drawn that part out separately already and took loads of pics. It's not going to be kept anyway.
What I'm not quite understanding is this; the drawing shows each pair of valves wired in series. That require 12v from the MT secondary to give 6v for each valve? The CT is grounded.
I thought each pin of the HT winding was 6v relative to the 0v CT.
But there's a 6v bulb wired between each of the pins?
Did a bit more today. Fitted an IEC socket in the original power socket location after removing the old one and the redundant 12v wiring. Not sure if I'll keep it here but at least I've got something solid to power it up and it's properly earthed connected to the chassis
It's now been on full mains voltage with valves in. Hooked up a speaker to the 8ohm winding so the OT has a load.
Noticed none of the tubes are glowing. Except the EL34's, which I can see a tiny bit of glow from.
So some further investigation is needed....
I haven't managed to do much yet but I've done a few voltage measurement. HT is around 350v and I'm Seeing High DC voltage on all the anodes.
I'm still only seeing 5v heater voltage at the MT. Checked the heater pins on the EZ81's and EL34's which show 5v, the ECC81 and ECC82 also.
The 6BR7's are only showing 1.5v??
Thinking it's must be heater related given non of the valves aren't glowing?
The rectifiers seem to be doing their thing so the heaters must be ok for that section, although there's no visible glow from these whatsoever.
The 6BR7's are wired parallel, there's a feed from the positive heater pin of the first EL34 which then forks off between 2 resistors. I assume these are to reduce the 12v down to 2 lots of 6v as they're not wired in series pairs like the other valves. Used a voltage drop calculator and it came out at 38ohms resistance to drop 12v to 6.3v with a 0.15a load. There's no markings on them unfortunately as it's peeled off!
Bit of head scratching, probably something obvious but it's all good learning.
My previous heater circuit sketch was wrong and didn't show the preamp stage valves so I've done another one.
It's now been on full mains voltage with valves in. Hooked up a speaker to the 8ohm winding so the OT has a load.
Noticed none of the tubes are glowing. Except the EL34's, which I can see a tiny bit of glow from.
So some further investigation is needed....
I haven't managed to do much yet but I've done a few voltage measurement. HT is around 350v and I'm Seeing High DC voltage on all the anodes.
I'm still only seeing 5v heater voltage at the MT. Checked the heater pins on the EZ81's and EL34's which show 5v, the ECC81 and ECC82 also.
The 6BR7's are only showing 1.5v??
Thinking it's must be heater related given non of the valves aren't glowing?
The rectifiers seem to be doing their thing so the heaters must be ok for that section, although there's no visible glow from these whatsoever.
The 6BR7's are wired parallel, there's a feed from the positive heater pin of the first EL34 which then forks off between 2 resistors. I assume these are to reduce the 12v down to 2 lots of 6v as they're not wired in series pairs like the other valves. Used a voltage drop calculator and it came out at 38ohms resistance to drop 12v to 6.3v with a 0.15a load. There's no markings on them unfortunately as it's peeled off!
Bit of head scratching, probably something obvious but it's all good learning.
My previous heater circuit sketch was wrong and didn't show the preamp stage valves so I've done another one.
I often don’t see any heater glow at all on small signal tubes unless the lights are out in the room. If you see a voltage on the cathode resistor you know it’s conducting. Or wait till they heat enough to feel.
You may need to confirm the power transformer winding sections with some unloaded measurements. That initially requires confirming winding sections/taps based on resistance measurements. Having a 12Vdc supply option with vibrator may add additional windings or taps or 12Vac with CT - it could be a bit tricky. There may also be an internal shield (identified from no resistance to anything else, and also capacitance to primary and secondary windings).
With measured 240V on primary, then measured secondary voltages may be a bit higher than spec or nominal level (eg. a 6.3V nominal winding may have circa 6.9Vrms unloaded).
If possible, confirm adequate insulation resistance between primary winding and core, and to secondary windings - I typically test insulation resistance at 1kVdc, but you may only have a 500V tester?
With measured 240V on primary, then measured secondary voltages may be a bit higher than spec or nominal level (eg. a 6.3V nominal winding may have circa 6.9Vrms unloaded).
If possible, confirm adequate insulation resistance between primary winding and core, and to secondary windings - I typically test insulation resistance at 1kVdc, but you may only have a 500V tester?
You are definitely measuring AC on the heaters, and not DC? You mentioned 'positive heater pin' above?
It is not optimal leaving the EL34s with series heaters. They were not designed for that and it could cause one to hog more current than the other, maybe related to the issue you have?
It is not so clear in the schematic, but there are 5 parallel lines of series heaters; EL34s on one, EZ81s on the other, ECC81s on another and 6BR7's on the other two, with dropping resistors.
You are paying for the compromise of being able to power from 12V or the mains. I suppose that 12V tap is the one that is used to boost the voltage to 350V when a battery is being used. Is there a source of 6.3V in there? Another tap on the transformer?
It is not optimal leaving the EL34s with series heaters. They were not designed for that and it could cause one to hog more current than the other, maybe related to the issue you have?
It is not so clear in the schematic, but there are 5 parallel lines of series heaters; EL34s on one, EZ81s on the other, ECC81s on another and 6BR7's on the other two, with dropping resistors.
You are paying for the compromise of being able to power from 12V or the mains. I suppose that 12V tap is the one that is used to boost the voltage to 350V when a battery is being used. Is there a source of 6.3V in there? Another tap on the transformer?
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The whole heater circuit is AC, the only rectified voltage is HT. Ignore 'positive pin', I just meant this is where the 6BR7 pair appears to be supplied from.
I will concentrate my efforts on MT today.
Just to be clear on what I've done so far with the heater wiring, the red drawn line shows one pin goes to ground via a vibrator pin and some resistors, this is untouched and was originally like that. The purple cross shows a blue wire which went to a power input socket pin. I've removed this and made a connection from the neighbouring pin at the MT to the heater pin EL34, which in turn feeds the other heater lines.
This shows the wiring to and from the vibrator socket a bit more clearly
I will concentrate my efforts on MT today.
Just to be clear on what I've done so far with the heater wiring, the red drawn line shows one pin goes to ground via a vibrator pin and some resistors, this is untouched and was originally like that. The purple cross shows a blue wire which went to a power input socket pin. I've removed this and made a connection from the neighbouring pin at the MT to the heater pin EL34, which in turn feeds the other heater lines.
This shows the wiring to and from the vibrator socket a bit more clearly
I think I'll start today by desoldering all the MT secondary wires and testing the voltage at all the pins off load.
Trobbins, I don't have an IR tester at home unfortunately. The did have one untill recently and it was indeed 500v.
Trobbins, I don't have an IR tester at home unfortunately. The did have one untill recently and it was indeed 500v.
Yeah, these definitely aren't, wg_ski. The 6BR7's wouldn't anyway as their only getting 1.5v. But I'd expect to see the others glowing in low room lighting. It was just the first thing I noticed on powering it up for the first time. Obviously something amiss with the heater circuit which I'm hopefully going to get to the bottom of.
I measured the voltages on the secondary side off load, these are all relative to the 0v CT pin.
I guess those 161 V terminals are symmetric wrt CT. Where are they connected to? What's the voltages from 157V to both 161V terminals? Finally, where are the heaters connected to? Is there another winding?
Best regards?
Best regards?
From 157 (I'll call this pin 7) to pin 6, 5.3v. pin 7 to 6 is 5.6v.
7 was grounded via a 60 ohm resistor (measured) and a 0.5uf capacitor in series.
6 went to the original 10 pin socket, there was a wire going from the socket to the heater pin of the first EL34 where it branched off to the rectifiers heater pins and the input stage valves
7 was grounded via a 60 ohm resistor (measured) and a 0.5uf capacitor in series.
6 went to the original 10 pin socket, there was a wire going from the socket to the heater pin of the first EL34 where it branched off to the rectifiers heater pins and the input stage valves
The measured transformer secondary voltage are ambiguous as shown. It may all become clearer if we can sort out the transformer secondary winding configurations 🙂
Can you confirm all those secondary taps are galvanically connected by DC resistance measurements ? If so, then can you determine if the 0V tap has a similar dc resistance to each of the 375V taps, and similarly to each of the 161V taps? Additional energised measurements would then be needed to confirm if the 375V taps had 750Vac from tap to tap (ie. a 375-0-375V centre tapped winding configuration for full bridge rectification), and similarly if the 161V taps had 322Vac from tap to tap. Measurement of the 157V tap to other taps may indicate if it is part of a winding with other taps (eg. 375-0-157-375) or perhaps just a separate winding with only one end connected to anything else (eg. the 0V tap). Similarly for the 7V tap.
Can you confirm all those secondary taps are galvanically connected by DC resistance measurements ? If so, then can you determine if the 0V tap has a similar dc resistance to each of the 375V taps, and similarly to each of the 161V taps? Additional energised measurements would then be needed to confirm if the 375V taps had 750Vac from tap to tap (ie. a 375-0-375V centre tapped winding configuration for full bridge rectification), and similarly if the 161V taps had 322Vac from tap to tap. Measurement of the 157V tap to other taps may indicate if it is part of a winding with other taps (eg. 375-0-157-375) or perhaps just a separate winding with only one end connected to anything else (eg. the 0V tap). Similarly for the 7V tap.
