I wasn't necessarily talking about an IEC socket in reference to the article, just generally musing over options.
I think it's down to personal preference at the end of the day especially at a hobbyist level. If it was intended to be sold then I suppose that would be another matter entirely.
It is a convenient way to terminate the electrical connections within the amplifier, and it also solves the issue of someone moving the amplifier when the cable is snagged, which could break the connections internally.
How about something like this with a short length of cable outside the amp, so you can access it for servicing? It would still need the short bit of cable to be anchored though.
Lots of old equipment has a hard wired plug lead, though, so I don't think it has to be top of the list of enhancements. An earthed lead is a much more important addition. The earth wire just has to be attached to the chassis close to the point it enters the amplifier, using a minimum M4 bolt and star washers (I read somewhere) and should not be used for other connections (so not part of a shared ground scheme).
Do you have a dim lightbulb tester? If you have that ( it just needs a 60W bulb, I get mine from secondhand shops) then you can bring it up with some power without too many concerns. There did not look to be too many horror stories with the pictures you provided. Tubes don't need to be in, and clamp some meter leads onto the secondary of the power transformer. If you have another meter, clamp one lead to the ground, and with one arm tucked safely away, use the other one to probe for some test voltages, and log them onto a sheet, then turn off and ponder the data. Usually just checking each valve pin on all sockets gets a good baseline of data. If you aim for 50VAC first time up, then you are looking for 20% voltage figures, although the voltages go down a lot when the valves are in and are warmed up and conducting.
Be extra vigilant of power supply capacitors, because with valves and possible blown fuses, they sometimes can be isolated with a full charge. Probably a good mod at the outset is to fit a bleeder resistor on the big electrolytic in the power supply.
The good habit to get into is to always suspect that the caps are charged, and routinely short them with a suitable resistor. I made one that plugs into my meter leads so I can test the capacitor, and discharge it at the same time. I did once pick up an amp that had no baseplate, and the tip of my finger was against the end of an empty inline fuse, and it was not a nice experience!
Be extra vigilant of power supply capacitors, because with valves and possible blown fuses, they sometimes can be isolated with a full charge. Probably a good mod at the outset is to fit a bleeder resistor on the big electrolytic in the power supply.
The good habit to get into is to always suspect that the caps are charged, and routinely short them with a suitable resistor. I made one that plugs into my meter leads so I can test the capacitor, and discharge it at the same time. I did once pick up an amp that had no baseplate, and the tip of my finger was against the end of an empty inline fuse, and it was not a nice experience!
Exactly my thoughts also. First of all, and as yet mentioned by trobbins in #43, I'd replace both the EL 34 plate resistors and the common screen grid resistor before ever powering the unit again. Hopefully the EL 34's aren't damaged already by the missing anode load, and the burnt screen resistor is just a follow-up.
Then take a closer look at that yellow WIMA capacitor from #49. Gently turn it's body, or better lift one leg, and read what's written after that WIMA acronym: If it is Durolit, replace this capacitor. Durolits are paper capacitors that by experience get soaked with moisture over the decades. If there's written anything else, the capacitor is not that suspicious - as is the blue prismatic WIMA.
Best regards!
Waiting for my replacement capacitors to arrive. These include a pair of 16uf 500v electrolytic's for the power supply to replace the big old original combined one that's in there now.
So I've drawn out the power supply again, hoping it's right this time... Done it with 2 separate caps this time as it makes more sense to me that way. Hoping it's right this time!. Would be real grateful if someone could clarify whether it looks correct.
I've missed a connection which connects one of the caps to the power valves screens via a resistor.
So I've drawn out the power supply again, hoping it's right this time... Done it with 2 separate caps this time as it makes more sense to me that way. Hoping it's right this time!. Would be real grateful if someone could clarify whether it looks correct.
I've missed a connection which connects one of the caps to the power valves screens via a resistor.
I don't know what exactly TFF means, perhaps Therephtalate Film Foil, but I guess this isn't a suspicious capacitor.
Best regards!
Best regards!
It’s a polyester derivative. Falls in the same class. Will almost never give any trouble. More dissipation factor than MKP, but whose counting here?
Thank God it isn’t all Z5U ceramics in the signal path. Technically, nothing will ever be wrong with them, but the audiophile in you says NOOOOOO!
Thank God it isn’t all Z5U ceramics in the signal path. Technically, nothing will ever be wrong with them, but the audiophile in you says NOOOOOO!
One quick method to get the resistor values back to 'normal', is to parallel another resistor so that the final value is sort of what you want, The advantage is you don't have to unsolder the old resistor and even if it has drifted high over 4-5 decades, it is not going to change markedly over the next few years. The disadvantage is you need to have a calculator or webpage for parallel resistor calculation, and access to resistors. If aesthetics or morals nag you then you can replace with a single new resistor once all operation is settled. Or you can solder a new resistor across the old resistor and snip out the old resistor - to avoid having to unsolder tags with multiple wires wrapped around the tag. For old vintage aesthetics a small modern resistor can even be 'hidden' underneath an old dogbone style resistor and you wouldn't know it was there.
The schematic change for the two filter caps in the same can is correct.
Do you have a meter that suits valve circuits? Eg. a meter with high input resistance so that it doesn't load down a circuit, and one that can measure Vac when there is Vdc, and vice versa, and an extra benefit is one that can measure low ac and dc voltages. It's worth checking out a meter's detailed specs, as even some Flukes can be a bit limiting.
Your amp has global feedback, which can cause unexpected results/operation, and can be quite technical to confirm correct operation with. Most people have an oscilloscope and a signal generator for bench testing, but there are other ways to make measurements that give confidence that all is ok. And given the amp has some grunt power, it may be wise to use a cheap high-power speaker for initial testing, rather than risk damaging something with hi-fi credentials.
The schematic change for the two filter caps in the same can is correct.
Do you have a meter that suits valve circuits? Eg. a meter with high input resistance so that it doesn't load down a circuit, and one that can measure Vac when there is Vdc, and vice versa, and an extra benefit is one that can measure low ac and dc voltages. It's worth checking out a meter's detailed specs, as even some Flukes can be a bit limiting.
Your amp has global feedback, which can cause unexpected results/operation, and can be quite technical to confirm correct operation with. Most people have an oscilloscope and a signal generator for bench testing, but there are other ways to make measurements that give confidence that all is ok. And given the amp has some grunt power, it may be wise to use a cheap high-power speaker for initial testing, rather than risk damaging something with hi-fi credentials.
To your power stage schematics: I guess the upper 0.15 µF capacitor is connected to the other side of the 22k plate resistor, i. e. to the tube's pin 1 instead of the plate supply voltage. Only this connection would make sense, as the last triode works as a Concertina phase splitter.
Best regards!
Best regards!
That's a good shout, yeah.
Not too bothered about aesthetics for this.
Great stuff. I think once I looked at it again the penny dropped pretty quickly. Happy it's right now.
I'm not sure, it's just an Amazon one. It's got a lot of features, I had a Screwfix (think Bunnings...) own brand one before this which was dreadful. I'm not entirely sure of the input resistance.
Might be time for an upgrade?
I don't own a scope but it's something I've thought about and will likely look for one soon.
Another good shout. I'm looking on FB marketplace and eBay at the moment for spares/repair combo Amps or cabs that I could nick a speaker out of. I'm planning to make a cab from scratch eventually, refinishing the amp in a similar quilted leatherette fabric (like the original stuff), the speaker cab will be finished in the same to match. If I manage to get a working speaker for next to nothing from a broken combo amp (I've seen them going for next to nothing), I can use it for testing. Might be some other salvageable bits for other projects as well. Then later I'll look for a decent speaker for the new purpose built cab.
I had another idea, where the redundant 10 pin input socket is at the top, a magic eye VU meter. 😀
Meter should suit - but only accurate from 10Hz to 1kHz, so keep that in mind for some tests and use other methods to confirm the amp isn't accidentally oscillating.
My caps arrived. Annoyingly I'm a 50uf Axial electrolytic short 😞. I had 47uf or 100uf caps to hand, I fitted a 47uf polerised electrolytic where a 50uf is supposed to be wired in parallel with the 6BR7 cathode/G3 bias resistor. Not sure what effect , if any, this will likely have. The 6BR7's are the mic stages. I thought for initial testing I'd probably use the Gram inputs anyway. I can always order another 50uf. I'll likely be making a few changes to the input stages as others have suggested if all goes to plan.
Bought another combination can 16uf+16uf for the power stage. I've taken a wire over to a connection point for the neutral which is connected coming back to the star point right below the capacitor! Which will work obviously but a bit daft. This connection originally was for the shared ground at the opposite end of the old cap. It serves nothing else. I will amend this tomorrow. Time to stop when you're tired.... 😴.
Getting closer to a power up I hope.
Bought another combination can 16uf+16uf for the power stage. I've taken a wire over to a connection point for the neutral which is connected coming back to the star point right below the capacitor! Which will work obviously but a bit daft. This connection originally was for the shared ground at the opposite end of the old cap. It serves nothing else. I will amend this tomorrow. Time to stop when you're tired.... 😴.
Getting closer to a power up I hope.
Aim to take the CT of the HT winding direct to the neg terminal of the 16+16uF combo can, as that constrains the charging pulses to just the CT loop and the first 16uF of that combo. That combo neg terminal can then go to the 0V of the output stage cathodes, as that is the main load on the second 16uF cap. I couldn't easily see where the wires are going from that second photo.
The cathodes of the 6BR7s and ECC81s are close to 0V, so likely than any bypass e-cap from 16V up would be fine - they function as a short circuit to signal voltages which then achieves maximum signal gain from each of those stages.
The cathodes of the 6BR7s and ECC81s are close to 0V, so likely than any bypass e-cap from 16V up would be fine - they function as a short circuit to signal voltages which then achieves maximum signal gain from each of those stages.
That makes sense, thanks.
I'll do a bit more tomorrow. I have a Variac which I've never used, so I need to get that wired up and make a little enclosure for it. I have a lamp limiter which I have used but not for ages, so I'll need to check that over as it's been moved around a bit while stored. Really looking forward to getting it powered up now.
I'll do a bit more tomorrow. I have a Variac which I've never used, so I need to get that wired up and make a little enclosure for it. I have a lamp limiter which I have used but not for ages, so I'll need to check that over as it's been moved around a bit while stored. Really looking forward to getting it powered up now.
Checked the screen resistor again tonight, it actually measures 416 ohms. Don't know where I got 44k from!
I've replaced the anode resistors as well with 100ohm resistors