Many years ago I was gifted with a pair of defunct and partially dismantled Kebschull 35/70 monoblock valve amps, since then collecting dust. Now I want to bring them back to life and for that I need your advice.
The power supply transformer is designed for 220V, but the mains voltage here is nominally 230V. Where I live it varies between 235V and 239V, so I will use a variac to bring it down to 220V.
Do you advise to rebuild it with just new capacitors, resistors, diodes and tubes or should I do something else?
https://www.diyaudio.com/forums/att...estauration-modification-project-advice-1-jpg
https://www.diyaudio.com/forums/att...estauration-modification-project-advice-2-jpg
https://www.diyaudio.com/forums/att...n-modification-project-advice-schaltplan1-gif
The power supply transformer is designed for 220V, but the mains voltage here is nominally 230V. Where I live it varies between 235V and 239V, so I will use a variac to bring it down to 220V.
Do you advise to rebuild it with just new capacitors, resistors, diodes and tubes or should I do something else?
https://www.diyaudio.com/forums/att...estauration-modification-project-advice-1-jpg
https://www.diyaudio.com/forums/att...estauration-modification-project-advice-2-jpg
https://www.diyaudio.com/forums/att...n-modification-project-advice-schaltplan1-gif
Attachments
Last edited:
Member
Joined 2009
Paid Member
If you want to go all out and don’t value the patina of age (it’s not furniture!) then I’d dismantle it, clean it (especially get rid of dust and grime around high voltage parts and get some flux remover on that pcb) and re-surface the chassis using your favourite method (e.g. hammertone paint), check integrity of the transformers (if Bad the project would possibly be junk), replace all electrolytic capacitors, and definitely make sure you have a modern 3-wire mains supply with safety earth to the chassis at a bare-metal point of connection. No reason tubes shouldn’t be good.
Last edited:
Member
Joined 2009
Paid Member
Power it up slowly on the variac, check grid voltages to make sure the coupling caps aren't leaky, check HT/B+ ripple etc and it shouldn't need too much replacing apart from any resistors that have drifted too much.
The schematic looks ok apart from the paralled caps everywhere, seems to be some daft audiophile idea,they're not needed.
Nice work on cleaning them up, Andy.
The schematic looks ok apart from the paralled caps everywhere, seems to be some daft audiophile idea,they're not needed.
Nice work on cleaning them up, Andy.
The 16V series electrolytics are more likely to have deteriorated, so check those and replace
with a single bipolar, if needed.
with a single bipolar, if needed.
For cleaning any metal plating the paste used for glass-ceramic cooktops works best in my experience. On filthy PCBs baking oven cleaning spray works wonders.
The elytics are all clearly below their nominal capacitance and have high ESR too. Foil capacitors are all fine. The resistors are all metal film, but I'll replace them as well.
I got a bunch of tubes for those amps, undoubtedly permanently driven with too high heater voltage as the previous owner used the amps on 230V mains. Without a tester, how can I check the tubes respectively what is the worst scenario using them anyway?
The elytics are all clearly below their nominal capacitance and have high ESR too. Foil capacitors are all fine. The resistors are all metal film, but I'll replace them as well.
I got a bunch of tubes for those amps, undoubtedly permanently driven with too high heater voltage as the previous owner used the amps on 230V mains. Without a tester, how can I check the tubes respectively what is the worst scenario using them anyway?
Last edited:
Don't worry about the heater voltage used in the past, that's still within the mfr stated tolerance.
The tubes should be ok.
The tubes should be ok.
I have an old Japanese tube amp that runs at 220 (2x 110) and the voltage here is 230. I checked the output and the heater voltages, when everything was loaded, were only up to about 6.7, instead of 6.3. So I left it alone.
You should definitely check your heater voltages and cross-reference with the tube data sheet. It’s probably best if you do that before making any modifications, if you find that they are too high for your liking. Measure first, cut later.
You should definitely check your heater voltages and cross-reference with the tube data sheet. It’s probably best if you do that before making any modifications, if you find that they are too high for your liking. Measure first, cut later.
Last edited:
I don’t know if it’s the picture or not, but the two el34 tube sockets seem to be discoloring the board. This might indicate that there is a problem with ventilation?
I won't do any modifications due to a lack of knowledge. Having up to 238V mains voltage the 220V mains transformer will give calclatory up to 6.8V on the heaters - too much for my taste, that's why I'll use a variac to be on the safe side.
There are quite a few threads on here discussing this problem, along with a few proposed solutions, ranging from the dead simple (a resistor in series with the heaters) to quite complex (a regulated heater power supply)
Yes, there is a problem with ventilation. The cathode resistors are getting really hot and touch the PCB. I'll refit them with longer leads giving some distance to the PCB and higher power rating.
Yes, I know ... The mains transformer is getting quite hot with higher than 220V mains voltage. So I think a variac is the best way to get around that.
Not enough space in the enclosures. As I have a nice and big variac I'll use it.
If it is made for Japanese domestic market it is designed for 100V not 110.
It is a common mistake to believe that mains voltage in Japan is the same as in the USA but it is not, it is 100V 50 or 60 Hz depending on region. Some equipment is OK to run at 10% higher mains, other things are more sensitive.
"Patina" 🙂🙂🙂 Looks beautiful now!
It looks like there may be space between the transformer coil and the windings to put in some counter-windings to bring the output voltage down if needed....
Last edited:
Would counter-windings make the mains transformer run cooler? The primary winding is on the inside, the 340-0-340V in the middle and the 6.3V on the outside. I bet counter-winding the primaries on the outside would introduce interesting but unwanted coupling effects ...
Last edited: