Ok, I measured the resistance between all the secondary pins, N/C is no continuity.
So the lower resistance numbers corrospond with the heater winding, pins 5-7. They have no continuity with any of the other pins.
4 doesn't have continuity with anything else, looking more closely at that one I don't think the pin is connected to a winding.
So I have 3 pins associated with the heater supply. Am I right to assume that 2 should give 6v+6v centre tapped?
I'm measured 5.78v between 7 and 6 and 7 and 5. This should be 6.3? Wondering if the transformer is knackered?
So the lower resistance numbers corrospond with the heater winding, pins 5-7. They have no continuity with any of the other pins.
4 doesn't have continuity with anything else, looking more closely at that one I don't think the pin is connected to a winding.
So I have 3 pins associated with the heater supply. Am I right to assume that 2 should give 6v+6v centre tapped?
I'm measured 5.78v between 7 and 6 and 7 and 5. This should be 6.3? Wondering if the transformer is knackered?
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Im wondering now if pin 7 of the second EL34 should have a connection to pin 5 on the MT? That would mean the EL34'S, EZ81's and the ECC81/ECC82 are be wired in parallel pairs?
Since it is meant to run in reverse aswell, have you tried 12VAC backwards to see if the voltages are ok then? Maybe it was meant to work with 245 or even 250 mains?
(Aside from the issue of what definitely seems to be the CT heater winding outputting 5.7 + 5.7v and not 6.3 + 6.3v) I'm not grasping how it works with pairs off valves in series. If only one side of the (let's say) 6.3v CT is connected to the circuit, each pair will only get 6.3 shared across it, when it should have 12?
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Ta. The resistance measurements in post #102 confirm the winding separations. The 375-0-375V winding on transformer pins 1-2-3 are settled imho. The heater windings related to transformer pins 5,6,7 are still a bit doubtful I think.
Measuring low values of resistance can require a different technique than simple meter resistance measurement. For example one method is to pass a dc current through those windings and use the DMM to measure the dcv across the taps - the DMM may then be sensitive enough to identify if pin 7 or pin 6 is the 'middle of winding' pin.
Alternatively, AC energising the primary and measuring the ACV across the pins is appropriate. Post #96 results don't give valid info, except for pins 1,2,3 as the meter was connected to pin 2, and so all measurements to pins 4 to 7 are phantom readings (due to stray capacitance).
In post #97, you identify pin 7 being connected to chassis via a series RC (60 ohm + 0.5uF), and 5.6V from between pins 7 and 6. In post #101 you measured 5.78V between 7-6, and between 7-5. Can you confirm that circa 11.6Vac is measured between pins 5 and 6 ?
5.8Vac is certainly a bit lower than anticipated. Post #25 indicates the primary has three tappings, with each tapping having a 10V span. Can you confirm you had a measured 240Vac being applied to the 240-250V tapping for those measurements ? If so then can you change the primary to connect to the 220-230V tapping and re-measure the heater pin voltages ? Many vintage PA amps have multiple taps to cope with a variety of mains voltage situations, and the appropriate primary tap to use is the one that generates about the correct nominal heater voltage (given that mains voltage can vary a bit for many of us). It may be that heater voltage (even with adjusted primary tap) is still at or a bit below 6.3V unloaded, and so the next test would be to load the output stage valves and see how much the heater voltage reduces at the valve base terminals as a final arbiter of what primary voltage tap to use. The complicating aspect is the series connection of heaters - as there can be some variance of voltage across each heater in the string - especially if valves from different manufacturers are used, and during the heat-up phase - so in general it may be better to operate with a pro-rata heater voltage that is at or under 6.3V, rather than above 6.3V - and the voltage to determine that would be the pin 5 to 6 reading as that is the 12V level being applied.
Post #89 schematic seems to be reasonable given the pin 5 is the 12V heater feed, and pin 6 is connected to ground. The 6BR7 heaters may be powered in a different manner for a few plausible reasons. One reason may be that they don't share heater voltage well in practise if connected in series to 12Vac. A better reason imho is that the heater voltage to each 6BR7 is only seen from 0V to about 6Vac (rather than one valve having a heater voltage from 6V to 12Vac relative to ground) - as that can minimise hum from 6BR7 circuitry, which may seem subtle, but hum was the bane of low signal level inputs in PA amps.
Measuring low values of resistance can require a different technique than simple meter resistance measurement. For example one method is to pass a dc current through those windings and use the DMM to measure the dcv across the taps - the DMM may then be sensitive enough to identify if pin 7 or pin 6 is the 'middle of winding' pin.
Alternatively, AC energising the primary and measuring the ACV across the pins is appropriate. Post #96 results don't give valid info, except for pins 1,2,3 as the meter was connected to pin 2, and so all measurements to pins 4 to 7 are phantom readings (due to stray capacitance).
In post #97, you identify pin 7 being connected to chassis via a series RC (60 ohm + 0.5uF), and 5.6V from between pins 7 and 6. In post #101 you measured 5.78V between 7-6, and between 7-5. Can you confirm that circa 11.6Vac is measured between pins 5 and 6 ?
5.8Vac is certainly a bit lower than anticipated. Post #25 indicates the primary has three tappings, with each tapping having a 10V span. Can you confirm you had a measured 240Vac being applied to the 240-250V tapping for those measurements ? If so then can you change the primary to connect to the 220-230V tapping and re-measure the heater pin voltages ? Many vintage PA amps have multiple taps to cope with a variety of mains voltage situations, and the appropriate primary tap to use is the one that generates about the correct nominal heater voltage (given that mains voltage can vary a bit for many of us). It may be that heater voltage (even with adjusted primary tap) is still at or a bit below 6.3V unloaded, and so the next test would be to load the output stage valves and see how much the heater voltage reduces at the valve base terminals as a final arbiter of what primary voltage tap to use. The complicating aspect is the series connection of heaters - as there can be some variance of voltage across each heater in the string - especially if valves from different manufacturers are used, and during the heat-up phase - so in general it may be better to operate with a pro-rata heater voltage that is at or under 6.3V, rather than above 6.3V - and the voltage to determine that would be the pin 5 to 6 reading as that is the 12V level being applied.
Post #89 schematic seems to be reasonable given the pin 5 is the 12V heater feed, and pin 6 is connected to ground. The 6BR7 heaters may be powered in a different manner for a few plausible reasons. One reason may be that they don't share heater voltage well in practise if connected in series to 12Vac. A better reason imho is that the heater voltage to each 6BR7 is only seen from 0V to about 6Vac (rather than one valve having a heater voltage from 6V to 12Vac relative to ground) - as that can minimise hum from 6BR7 circuitry, which may seem subtle, but hum was the bane of low signal level inputs in PA amps.
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Thanks, Trobbins.
Pin 5 has never been connected to anything. It doesn't even have solder on the tag so I'm not sure it ever was. Certainly not while I've been working on it...
And this is why I'm so confused. If it is a centre tapped 12v winding, if the whole Heater circuit appears to be supplied from that 6v half of it (pin 6). If each pair of series wired valves is only seeing 6v, shouldn't each heater in that pair only gets 3v? Each pair of series wired valves has a return to the star point ground via pin 7 on the second EL34.
To add to my confusion I measured 5(ish) volts at the valve base heater pins (leaving the 6BR7's out of the equation for now...). I don't get how that can be the case.
It's just not quite making sense to me. I feel as though I'm really close, I just need to join a few dots and the light will come on in my brain 😄
I'll confirm this tomorrow but I'm pretty sure this was the case, yes.
This didn't occur to me at all. I'll try it.
I've confused things here. To clarify, pin 5 has never been connected. It appears as so on this sketch but this is just a case of me not understanding how the windings worked.
Pin 5 has never been connected to anything. It doesn't even have solder on the tag so I'm not sure it ever was. Certainly not while I've been working on it...
And this is why I'm so confused. If it is a centre tapped 12v winding, if the whole Heater circuit appears to be supplied from that 6v half of it (pin 6). If each pair of series wired valves is only seeing 6v, shouldn't each heater in that pair only gets 3v? Each pair of series wired valves has a return to the star point ground via pin 7 on the second EL34.
To add to my confusion I measured 5(ish) volts at the valve base heater pins (leaving the 6BR7's out of the equation for now...). I don't get how that can be the case.
It's just not quite making sense to me. I feel as though I'm really close, I just need to join a few dots and the light will come on in my brain 😄
Yes that doesn't seem reasonable to supply 6V to what definitely should be 12V, given the series connection of heaters. Not sure if that is an original error (given no solder on pin 5 terminal), or there is another explanation related to the vibrator.
Can you identify that the 4-pin valve socket has a vibrator inserted (eg. marking on the vibrator or whatever that can is) ? Vibrators normally have more pins than 4, but I may have missed some info in amongst the photos/posts. And can you also sketch out where the 8 pins of the front panel 'Supply' socket go to, and the associated battery/standby switch ? There is a chance that 'standby' refers to powering heaters etc at 50% voltage - that is not an unexpected operation for PA gear that aims to reduce power consumption when not directly being used, but to minimise time to reach full volume, as per outside broadcast with a microphone.
Can you identify that the 4-pin valve socket has a vibrator inserted (eg. marking on the vibrator or whatever that can is) ? Vibrators normally have more pins than 4, but I may have missed some info in amongst the photos/posts. And can you also sketch out where the 8 pins of the front panel 'Supply' socket go to, and the associated battery/standby switch ? There is a chance that 'standby' refers to powering heaters etc at 50% voltage - that is not an unexpected operation for PA gear that aims to reduce power consumption when not directly being used, but to minimise time to reach full volume, as per outside broadcast with a microphone.
It feels like the capability to work with and without a vibrator is a key element here. Compromises on both sides, possibly.
What is the RMS output of a vibrator? Is it 12V (car battery) divided by root 2? So 8 - 9 volts?
What is the RMS output of a vibrator? Is it 12V (car battery) divided by root 2? So 8 - 9 volts?
A vibrator always put out a square wave voltage. Hence, it is 12 V effectively.
Btw, vibrators don't need more than four pins: Supply voltage, ground, and two outputs.
Best regards!
Btw, vibrators don't need more than four pins: Supply voltage, ground, and two outputs.
Best regards!
With the voltage selector now in 220-230 position, I did another unloaded voltage measurement:
I think we expected to see 12 volts between pin five and six here? 6.3 volts between 5 and 7 is obviously telling me something. This is even more odd given that five didn't appear to be connected. At least not as I found it. There's no way of knowing how many people have altered this over the years.
Not sure why the meter is overloading on the HT pins. I will need to check out the specs of my multimeter and see what voltage it's supposed to overload on. That's all I managed to do for today before having to leave the house.
Looking forward to getting back to it later.
I think we expected to see 12 volts between pin five and six here? 6.3 volts between 5 and 7 is obviously telling me something. This is even more odd given that five didn't appear to be connected. At least not as I found it. There's no way of knowing how many people have altered this over the years.
Not sure why the meter is overloading on the HT pins. I will need to check out the specs of my multimeter and see what voltage it's supposed to overload on. That's all I managed to do for today before having to leave the house.
Looking forward to getting back to it later.
Do we have any solid reason to believe that the vibrator was originally intended to run on 12 VDC? These were mostly in the 6 VDC era, the machine has been molested in the past, drawn schematics are shaky, etc. Do the heater dropping resistors look original? A close picture of them and their solder joints might help answer the question.
All good fortune,
Chris
All good fortune,
Chris
Has 4 pins.
The dropper resistors for the 6BR7's are definitely original I'd say.
I'm not in front of the amp at the moment but I'm pretty sure the rest of the heater circuit doesn't include any other dropper resistors.
There's no evidence of past 'molestation', I'm just surmising. There are a few wima caps which look newer that the rest of the amp but other than that we can't say for sure.
And the schematic might be 'shaky' but it's my first time having a go at drawing one, I'm learning. A few weeks ago I wouldn't have even attempted it! It's been a great learning exercise.
If I'd have had a schematic to go off in the first place it would have been easy but then I'd have learned nothing.
The dropper resistors for the 6BR7's are definitely original I'd say.
I'm not in front of the amp at the moment but I'm pretty sure the rest of the heater circuit doesn't include any other dropper resistors.
There's no evidence of past 'molestation', I'm just surmising. There are a few wima caps which look newer that the rest of the amp but other than that we can't say for sure.
And the schematic might be 'shaky' but it's my first time having a go at drawing one, I'm learning. A few weeks ago I wouldn't have even attempted it! It's been a great learning exercise.
If I'd have had a schematic to go off in the first place it would have been easy but then I'd have learned nothing.
Cool. The vibrator could have been incorrectly exchanged in the past, but the dropping resistors certainly look original. That removes one rabbit hole.
You know how to ask good questions, so you'll be fine.
All good fortune,
Chris
You know how to ask good questions, so you'll be fine.
All good fortune,
Chris
The schematic of the circuitry with vibrator is likely to be similar to the 'Astor car radios' section in https://www.cool386.com/msp/msp.html , as only three terminals of the four terminal vibrator appear to be in use. Each arm of the transformer push-pull winding is alternately connected to ground, and appears to have a parallel RC snubber to ground (to minimise switching transients).
I think I'm getting slightly left behind here. The focus seems to be on the vibrator circuitry. My understanding of the vibrator was that it was only used when the amp was running from battery power, is that incorrect? Is it relative to the heater voltage situation in some way?
Fwiw, my concern is that the circuitry is not yet known, and that may be related to why you seem to have valve heaters arranged in a 12V configuration, but have a transformer that seems to only have a 6V heater winding.