Hi this is my first post here. I'm hoping someone can help.
I recently got hold of a Michaelson Audio Odysseus valve amp. As far as I know this is a class A ultra linear amp. It has 8 EL34 power tubes, 2 ECC83 and 2 ECC85 pre-amp tubes. It outputs around 45 watts.
This is a photo of one:
http://www.hifiengine.com/phpBB2/album_page.php?pic_id=210&sid=407972df91bcd82abbd16f31f8491d5f
The original owner had not experienced problems.
When I received the amp it worked and very nicely so I am determined to get it working again.
I have a little experience of fixing my Fender Twin guitar amp which I completely rebuilt (it was also an ultra linear - not so common for Fender Twins). However I have forgotten some know how since then and this Odysseus amp is obviously a little more complicated.
This is what went wrong and what I have done so far:
The right channel of the amp stopped working and fused. There are fuses connected to the screen grid circuits. I looked inside the amp and immediately saw some problems. The main problem being burning around the high watt wirewound resistors at the filter caps. Bear with me because I have forgotten some knowledge about what everything does. You will understand what resistors I am talking about here - whatever they are called. These resistors read 23.5 ohms on my meter (so bad a state that reading the colour code was impossible). They are not the same type as appear in the original Odysseus. They look too small and low wattage and were almost touching the PCB. The right channel had such a bad connection here, due to heat, it was loose. I sorted these by replacing with suitable value high wattage wirewounds (I actually stuck two 47 ohm in parallel here - which I presume should be OK) and I checked all connections, PCB tracks etc. No other obvious problems in this respect. I also noticed that the wirewounds on the power tube cathodes were in terrible physical state. These were 470 ohm wirewound. They were only marginally off but again not the same resistor types as was obviously on the original amp. Possibly too low wattage again. Originally, it had the standard oblong shaped ceramics here. I replaced these resistors. I also discovered a completely blown screen grid resistor. Once again these resistors don't look the same as the originals. Possibly too low wattage yet again but I have left them, only replacing the dodgy one for the time being.
As you will now deduce, work has been previously done to this amp. What I know of is that the Filter caps were replaced along with 100uf caps on each power tube cathode and two 220uf caps in the preamp stage. This is documented as being done two years ago. I have not yet tested all the caps because this amp is hell to work on. The replaced caps are all correct spec as far as I know. Some resistors were also replaced but not the ones I replaced. All resistors seem OK. I've tested them all.
Next stage was turning the amp on again. I make the assumption an EL34 valve had gone because it's screen grid resistor immediately went up in smoke and the amp fused again. I replaced this tube (I was provided with new spares of the same Harma valves when I bought the amp) and the amp then turned on OK except I get serious motorboating. I even returned the original resistors (the ones that were working but not entirely satisfactory as far as I can tell) to return the amp to how it was when it worked but to no avail.
My questions start with what causes motorboating and where should I go from here? Testing all the caps may seem like an obvious start but unfortunately that is going to be physically hell on this amp due to it's design. What caps, if any, might be causing my problem? It it the filter caps? Did the blown valve or loose connected resistor damage anything else? My knowledge is not that good but it's more a question of brushing up what I once knew so I can provide reasonably detailed info.
If anybody can help me get this amp working it wold be greatly appreciated. I can supply photos if that is of any use.
Thanks a lot.
I recently got hold of a Michaelson Audio Odysseus valve amp. As far as I know this is a class A ultra linear amp. It has 8 EL34 power tubes, 2 ECC83 and 2 ECC85 pre-amp tubes. It outputs around 45 watts.
This is a photo of one:
http://www.hifiengine.com/phpBB2/album_page.php?pic_id=210&sid=407972df91bcd82abbd16f31f8491d5f
The original owner had not experienced problems.
When I received the amp it worked and very nicely so I am determined to get it working again.
I have a little experience of fixing my Fender Twin guitar amp which I completely rebuilt (it was also an ultra linear - not so common for Fender Twins). However I have forgotten some know how since then and this Odysseus amp is obviously a little more complicated.
This is what went wrong and what I have done so far:
The right channel of the amp stopped working and fused. There are fuses connected to the screen grid circuits. I looked inside the amp and immediately saw some problems. The main problem being burning around the high watt wirewound resistors at the filter caps. Bear with me because I have forgotten some knowledge about what everything does. You will understand what resistors I am talking about here - whatever they are called. These resistors read 23.5 ohms on my meter (so bad a state that reading the colour code was impossible). They are not the same type as appear in the original Odysseus. They look too small and low wattage and were almost touching the PCB. The right channel had such a bad connection here, due to heat, it was loose. I sorted these by replacing with suitable value high wattage wirewounds (I actually stuck two 47 ohm in parallel here - which I presume should be OK) and I checked all connections, PCB tracks etc. No other obvious problems in this respect. I also noticed that the wirewounds on the power tube cathodes were in terrible physical state. These were 470 ohm wirewound. They were only marginally off but again not the same resistor types as was obviously on the original amp. Possibly too low wattage again. Originally, it had the standard oblong shaped ceramics here. I replaced these resistors. I also discovered a completely blown screen grid resistor. Once again these resistors don't look the same as the originals. Possibly too low wattage yet again but I have left them, only replacing the dodgy one for the time being.
As you will now deduce, work has been previously done to this amp. What I know of is that the Filter caps were replaced along with 100uf caps on each power tube cathode and two 220uf caps in the preamp stage. This is documented as being done two years ago. I have not yet tested all the caps because this amp is hell to work on. The replaced caps are all correct spec as far as I know. Some resistors were also replaced but not the ones I replaced. All resistors seem OK. I've tested them all.
Next stage was turning the amp on again. I make the assumption an EL34 valve had gone because it's screen grid resistor immediately went up in smoke and the amp fused again. I replaced this tube (I was provided with new spares of the same Harma valves when I bought the amp) and the amp then turned on OK except I get serious motorboating. I even returned the original resistors (the ones that were working but not entirely satisfactory as far as I can tell) to return the amp to how it was when it worked but to no avail.
My questions start with what causes motorboating and where should I go from here? Testing all the caps may seem like an obvious start but unfortunately that is going to be physically hell on this amp due to it's design. What caps, if any, might be causing my problem? It it the filter caps? Did the blown valve or loose connected resistor damage anything else? My knowledge is not that good but it's more a question of brushing up what I once knew so I can provide reasonably detailed info.
If anybody can help me get this amp working it wold be greatly appreciated. I can supply photos if that is of any use.
Thanks a lot.
Welcome to the forum - shame about the amplifier. I once had a Michaelson & Austin TVA10. If you so much as looked at it in a funny way (or removed its input or loudspeaker load) it would oscillate violently at RF and the output valve anodes would immediately glow red. I suspect you may have the same problem here. RF oscillation causes heavy power supply currents and that can often provoke motorboating. The motorboating may be because a different value was fitted when those HT capacitors were replaced. The amplifier clearly has a serious fault that has been there for some time and is going to need an oscilloscope and a fair degree of intelligence to fix.
I looked after a few of these amps in the early 90s when I was working for an MF dealership. Don't remember any instability problems with this particular design - we had one pretty much permanently powered up in one of the listening rooms which was used with a wide range of different equipment.
They all ran really hot though, so 15 years on I would be looking for dried up filter caps and probably more cooked resistors. Iffy soldered joints on the pcbs are likely at this age as well.
It would be worth going through the whole amp and replacing the components that are past their sell by date - caps, power resistors etc. Check the tube sockets for the EL34s and the soldered joints between these and the pcb below.
Good luck
They all ran really hot though, so 15 years on I would be looking for dried up filter caps and probably more cooked resistors. Iffy soldered joints on the pcbs are likely at this age as well.
It would be worth going through the whole amp and replacing the components that are past their sell by date - caps, power resistors etc. Check the tube sockets for the EL34s and the soldered joints between these and the pcb below.
Good luck
OK, thanks for the replies.
I tested as many of the caps as possible but my multimeter can't go high enough uf to test them all.
The latest is that when I run each channel and test two valves in it. The amp runs with no motorboating. If I hook up 4 valves in any channel then I'm back to motorboating. Maybe this can give a clue for you to help me track down what is wrong.
I tested as many of the caps as possible but my multimeter can't go high enough uf to test them all.
The latest is that when I run each channel and test two valves in it. The amp runs with no motorboating. If I hook up 4 valves in any channel then I'm back to motorboating. Maybe this can give a clue for you to help me track down what is wrong.
Another update here.
As I've said, when I run 4 valves I get the motorboating. There are two separate PCB boards with the valves attached and related circuitry such as cathode resistors and caps, grid resistors etc. These are connected to metal sections which also house the output transformers so when open these parts of the amp can be physically moved away from the main PCB. This movement cures the motorboating but with 4 valves in a channel I get a lower level hum (doesn't occur with 2). The amp has some kind of insulating spacers which are not there while I work and a lot of connections had shrink wrapped insulation which is obviously removed as well. It is possible some noise would be eliminated when rebuilding the amp but the motorboating does not disappear unless the valve PCBs with transformers are moved well away from the main PCB.
Apart from the electrolytics which I can not test with a simple uf on my multimeter, all the rest of the amps caps and resistors appear to be OK and I can not find any dodgy connections. A possible problem is burnt PCB around where the power resistors were but it is not serious. The PCBs seem a bit weak. some of the copper tracks around the power resistors were damaged and I had to bend the new resistor wires back across the PCB to make good contact but I can't see anything that now looks like a dodgy connection.
As I've said, when I run 4 valves I get the motorboating. There are two separate PCB boards with the valves attached and related circuitry such as cathode resistors and caps, grid resistors etc. These are connected to metal sections which also house the output transformers so when open these parts of the amp can be physically moved away from the main PCB. This movement cures the motorboating but with 4 valves in a channel I get a lower level hum (doesn't occur with 2). The amp has some kind of insulating spacers which are not there while I work and a lot of connections had shrink wrapped insulation which is obviously removed as well. It is possible some noise would be eliminated when rebuilding the amp but the motorboating does not disappear unless the valve PCBs with transformers are moved well away from the main PCB.
Apart from the electrolytics which I can not test with a simple uf on my multimeter, all the rest of the amps caps and resistors appear to be OK and I can not find any dodgy connections. A possible problem is burnt PCB around where the power resistors were but it is not serious. The PCBs seem a bit weak. some of the copper tracks around the power resistors were damaged and I had to bend the new resistor wires back across the PCB to make good contact but I can't see anything that now looks like a dodgy connection.
EC8010.
You mention about the HT caps. I have discovered that I think they were replaced with a different value. The only thing I have to go on is that I have an invoice for the previous repair. The invoice says that 6 470uf 220v caps were replaced but in the amp these caps are actually 560uf 250v. I know sometimes you have to go higher because of lack of availability of the correct value. I make the assumption that 470uf caps were what was originally in there but they used higher value even although the invoice says different. Could the problems have stemed from this? I also have no idea about the make of these caps. All the other Electrolytics are Jamicon. Should I simply upgrade all the electrolytics? What are good quality or at least worth the cost? The 6 big ones may well be poor quality and I don't think Jamicon is that good either from what I remember.
Also. Oscilloscopes. I have considered getting one of these before. Any ideas about what is a good oscilloscope to buy? Not too expensive but a good make. Something that I might find on ebay for example.
This is my first valve hifi amp and I've had a taste of what a great sound it is. I'm prepared to go to a fair degree of expense and time just to get the sound back and better if possible which I'm sure it is. This amp had a gloriously powerful and detailed sound.
You mention about the HT caps. I have discovered that I think they were replaced with a different value. The only thing I have to go on is that I have an invoice for the previous repair. The invoice says that 6 470uf 220v caps were replaced but in the amp these caps are actually 560uf 250v. I know sometimes you have to go higher because of lack of availability of the correct value. I make the assumption that 470uf caps were what was originally in there but they used higher value even although the invoice says different. Could the problems have stemed from this? I also have no idea about the make of these caps. All the other Electrolytics are Jamicon. Should I simply upgrade all the electrolytics? What are good quality or at least worth the cost? The 6 big ones may well be poor quality and I don't think Jamicon is that good either from what I remember.
Also. Oscilloscopes. I have considered getting one of these before. Any ideas about what is a good oscilloscope to buy? Not too expensive but a good make. Something that I might find on ebay for example.
This is my first valve hifi amp and I've had a taste of what a great sound it is. I'm prepared to go to a fair degree of expense and time just to get the sound back and better if possible which I'm sure it is. This amp had a gloriously powerful and detailed sound.
Hi TJ,
I telephoned Musical Fidelity who serviced the amp a couple of years ago. This is Michaelson's company. They wouldn't give me info. Basically they wanted me to send the amp for servicing. Judging by an invoice I have here from the service two years ago I will definitely not be sending it. Huge expense to replace a few caps, volume control and selector switch. I want to learn anyway.
One thing I can't understand at the moment is some resistors on the screen grids. They have colour code for 10 ohm yet all read around .9 ohm. I'm sure they are not the original resistors either but I have no schematic so I have no idea what was here originally. Two of these have gone. One was gone first time I looked in the amp (it had literally cracked apart). and another went up with the dodgy valve. They are tiny little wirewounds. At least that's what they look like to me judging by the one that has blown apart. I've not seen resistors like this before.
When I've said valve I mean tube of course. I presume non UK people know that we call them valves? It might sound confusing.
I've ordered Valve Amplifiers by Morgan Jones (I guess everyone will know we call them valves then). I need to learn more before I start doing anything else with the Odysseus. My guess at the moment is Filter Cap problems but I need to make sure all other components are in order.
I was able to completely rebuild my Fender Twin amp and do some repairs - change the balance control to bias etc, but I found that much easier to understand. Easier to see the wiring rather than looking at a circuit board and this Michaelson is a nightmare to work on unless you simply dismantle the whole thing and take it all out the chassis.
I telephoned Musical Fidelity who serviced the amp a couple of years ago. This is Michaelson's company. They wouldn't give me info. Basically they wanted me to send the amp for servicing. Judging by an invoice I have here from the service two years ago I will definitely not be sending it. Huge expense to replace a few caps, volume control and selector switch. I want to learn anyway.
One thing I can't understand at the moment is some resistors on the screen grids. They have colour code for 10 ohm yet all read around .9 ohm. I'm sure they are not the original resistors either but I have no schematic so I have no idea what was here originally. Two of these have gone. One was gone first time I looked in the amp (it had literally cracked apart). and another went up with the dodgy valve. They are tiny little wirewounds. At least that's what they look like to me judging by the one that has blown apart. I've not seen resistors like this before.
When I've said valve I mean tube of course. I presume non UK people know that we call them valves? It might sound confusing.
I've ordered Valve Amplifiers by Morgan Jones (I guess everyone will know we call them valves then). I need to learn more before I start doing anything else with the Odysseus. My guess at the moment is Filter Cap problems but I need to make sure all other components are in order.
I was able to completely rebuild my Fender Twin amp and do some repairs - change the balance control to bias etc, but I found that much easier to understand. Easier to see the wiring rather than looking at a circuit board and this Michaelson is a nightmare to work on unless you simply dismantle the whole thing and take it all out the chassis.
Initially one side of the amp went. The fuse on that channel went. There are fuses on each channel as well as a main fuse. A valve had obviously gone as I found out when I turned the amp on again and a screen grid resistor blew on the dodgy valve and that channel fused again. A new valve I put in did not cause any fusing. I'm gradually working through what does what but there is more circuitry in here than on my Twin. The Odysseus auto biases itself but what is the difference between auto biasing and self biasing? My Fender Twin is fixed biasing meaning I adjust the bias myself.
There are no signs of glowing red valve plates or anything like that. The valves appear to be OK now. There was never any glowing or flashes. New and old valves are all bought from Watford valves. Exactly the same Harma STR E34L type. The problem now is motorboating. It seems something got damaged or something that was an accident waiting to happen has been pushed over the edge by whatever changes have occurred. I have made a few changes myself but not changed any values of anything or type except resistor wattage. I even put back the original resistors but no changes. The problem is that from simply looking at a few photos of Odysseus I know some resistors have been changed. Maybe not changed in type or even value but definitely different wattage handling. Also, the cathode capacitors were all changed. These are 100uf 100v electrolytic. These were changed in the repair job done by Musical Fidelity. However, the resistors that Musical Fidelity changed are not the ones that look completely different from photos. I should probably take some photos of what looks different from the photo I linked to in my first message.
Once I get this Valve Amplifier book I'll get better understanding so be able to be more specific. I will also have to dismantle the amp more to be able to see routing properly on the main PCB. I may also get a better capacitor tester so I can test the big caps.
Screen grid seems a dodgy area at the moment. I can section out an area where every adjoining component has had a problem including a valve but this may not necessarily be related to just one valve socket or valve. The new valve has not had a problem yet. The screen grid on my Fender Twin is a different arrangement. I need to look into exactly how the screen grid works. I know what it does in the valve but not exactly how the current flows for this. All I can do is give clues until I know more. I also need to work out routing around the filter caps and adjoining power resistors. I can't see this without looking at the back of the PCB again.
By the way, just to add here. I was told it is very dodgy to have no speakers connected to this amp. My Fender Twin is the same. It must never be turned on without speakers so I have been very careful that the Odysseus has never been turned on with no speakers connected. The speaker connections have been totally solid.
There are no signs of glowing red valve plates or anything like that. The valves appear to be OK now. There was never any glowing or flashes. New and old valves are all bought from Watford valves. Exactly the same Harma STR E34L type. The problem now is motorboating. It seems something got damaged or something that was an accident waiting to happen has been pushed over the edge by whatever changes have occurred. I have made a few changes myself but not changed any values of anything or type except resistor wattage. I even put back the original resistors but no changes. The problem is that from simply looking at a few photos of Odysseus I know some resistors have been changed. Maybe not changed in type or even value but definitely different wattage handling. Also, the cathode capacitors were all changed. These are 100uf 100v electrolytic. These were changed in the repair job done by Musical Fidelity. However, the resistors that Musical Fidelity changed are not the ones that look completely different from photos. I should probably take some photos of what looks different from the photo I linked to in my first message.
Once I get this Valve Amplifier book I'll get better understanding so be able to be more specific. I will also have to dismantle the amp more to be able to see routing properly on the main PCB. I may also get a better capacitor tester so I can test the big caps.
Screen grid seems a dodgy area at the moment. I can section out an area where every adjoining component has had a problem including a valve but this may not necessarily be related to just one valve socket or valve. The new valve has not had a problem yet. The screen grid on my Fender Twin is a different arrangement. I need to look into exactly how the screen grid works. I know what it does in the valve but not exactly how the current flows for this. All I can do is give clues until I know more. I also need to work out routing around the filter caps and adjoining power resistors. I can't see this without looking at the back of the PCB again.
By the way, just to add here. I was told it is very dodgy to have no speakers connected to this amp. My Fender Twin is the same. It must never be turned on without speakers so I have been very careful that the Odysseus has never been turned on with no speakers connected. The speaker connections have been totally solid.
Hi
Maybe this link might be of help to you
http://www.ampslab.com/SCHEMATICS/Michaelson&AustinTVA10.gif
Thanks
Maybe this link might be of help to you
http://www.ampslab.com/SCHEMATICS/Michaelson&AustinTVA10.gif
Thanks
Thanks Ben Goh but unfortunately that amp bears no similarity to mine.
My amp is ultra linear. Is this common place on hifi amps? My guitar amp is ultra linear but that is very rare with guitar amps. On the Odysseus the ultra linear taps from the output transformer are supplying DC to the screen grids from what my understanding is but there are other connections to the screen grid which are not present on my UL guitar amp.
My amp is ultra linear. Is this common place on hifi amps? My guitar amp is ultra linear but that is very rare with guitar amps. On the Odysseus the ultra linear taps from the output transformer are supplying DC to the screen grids from what my understanding is but there are other connections to the screen grid which are not present on my UL guitar amp.
This is wht i guess , hope this help
this is a a common problem on the EL34 valve . when this tube near end of it life mostly will cos internal short circuit . destroy the cathode resistor and some power supply components . it depend how the designer to fussing the amp to protect the main and audio transformers . you cannot repair the amp without drawing the circuit diagram . please check
1, PSU heater for EL34 , how they connect ,components
2, check the cathode , components
3,Main supply
4, coupling cap
this is a a common problem on the EL34 valve . when this tube near end of it life mostly will cos internal short circuit . destroy the cathode resistor and some power supply components . it depend how the designer to fussing the amp to protect the main and audio transformers . you cannot repair the amp without drawing the circuit diagram . please check
1, PSU heater for EL34 , how they connect ,components
2, check the cathode , components
3,Main supply
4, coupling cap
OK, Thanks TJ.
I plan on making a circuit diagram. I am still waiting for the Valve Amplifiers book which should help me understand more.
Cathode resistors are OK but there are some other components there that I will get back here about once I have some diagrams. Heaters seem simple. They just come from the power transformer to the EL34s and then to the preamp tubes.
It certainly looks like the tube shorted at the screen grid. I am also sure that the fuses on each channel were not there on the early Odysseus amps. I can't see them in any pictures. However my amp has had these since built. I can tell they have definitely not been added.
I plan on making a circuit diagram. I am still waiting for the Valve Amplifiers book which should help me understand more.
Cathode resistors are OK but there are some other components there that I will get back here about once I have some diagrams. Heaters seem simple. They just come from the power transformer to the EL34s and then to the preamp tubes.
It certainly looks like the tube shorted at the screen grid. I am also sure that the fuses on each channel were not there on the early Odysseus amps. I can't see them in any pictures. However my amp has had these since built. I can tell they have definitely not been added.
keep in mind , if valve failure/short circuit , the current will looking a path to ground !! something will have to burn !! you have to follow the route where the current found it way to ground !! remember in a valve amp half the main current is from the heater circuit and the other half is from the main . hopefully your output transformer still ok .
btw you wont get much help from the book you order . this amp too complicated and not reliable . My advice is send it to MF , get the repair done and play it for couple year then sell it .
btw you wont get much help from the book you order . this amp too complicated and not reliable . My advice is send it to MF , get the repair done and play it for couple year then sell it .
OK, thanks a lot for the replies.
The amp is complicated all right but this kind of makes it intriguing.
It looks to me like the problem is that the amp is powerful with a hell of a lot of heat but lacks reliability due to underrated wattage on a lot of resistors. I don't even need to get my multimeter to see that there are problem resistors. Resistors that have clearly taken some severe heat. Clearly way under wattage from what I see in pictures. It's hard to tell if they have been in there from square one or not. They are not documented as having been replaced in the repair. The cathode resistors look like power surge resistors. Possibly 2 watts. They're coating is all cracking off. Someone had painted them with something as some kind of makeshift repair which seems odd to me. Why not just replace them with ones that are up to the job? My Fender Twin had been totally abused by previous users for over 30 years, played into the ground, blown output transformer, wrong speakers etc etc but I hardly needed to replace any resistors, only those near the valves. The cathode resistors on the Odysseus were reading OK but I did try replacing them. In pictures I see oblong ceramics that must be at least 7 watts. Same story with a couple of the resistors at the filter caps. The screen grid resistors look like around 1/2 watt. This doesn't seem right to me at all. Again in the pictures these are much bigger.
I don't trust Musical Fidelity. They were the last ones to handle this amp as far as I know and it doesn't look good. Even the invoice does not match what they did. One phone call to them was enough. The guy I spoke to said he had serviced these amps for five years at one stage but was not forthcoming when I asked him about specific details. He gave me the expected blurb about dangerous voltages etc but I know about this. If an owner electrocutes themselves it's not Musical Fidelities responsibility but they sure want to repair it all right to earn a few bucks. This is why I got into learning about these things. I wouldn't let anybody touch my Fender Twin now. Cowboys had already been at that.
I'm certainly no expert on valve amps but that's why I've ordered the Valve Amp book. It may not relate to this amp but it will help me build up a better picture on how amps work. It should maker it easier for me to sort out a schematic.
How would I know if the output transformers were damaged? Would they simply not work at all? All I can say is that everything runs OK when I have two valves in each channel and not four. No motorboating or anything. Are there any tests I can run on them, bearing in mind I have no specs for them other than what I can work out from the rest of the amp?
As long as the transformers are OK then the amp is good in my opinion. I won't mess further with it yet though. I'm going to do some research first then I may be back.
The amp is complicated all right but this kind of makes it intriguing.
It looks to me like the problem is that the amp is powerful with a hell of a lot of heat but lacks reliability due to underrated wattage on a lot of resistors. I don't even need to get my multimeter to see that there are problem resistors. Resistors that have clearly taken some severe heat. Clearly way under wattage from what I see in pictures. It's hard to tell if they have been in there from square one or not. They are not documented as having been replaced in the repair. The cathode resistors look like power surge resistors. Possibly 2 watts. They're coating is all cracking off. Someone had painted them with something as some kind of makeshift repair which seems odd to me. Why not just replace them with ones that are up to the job? My Fender Twin had been totally abused by previous users for over 30 years, played into the ground, blown output transformer, wrong speakers etc etc but I hardly needed to replace any resistors, only those near the valves. The cathode resistors on the Odysseus were reading OK but I did try replacing them. In pictures I see oblong ceramics that must be at least 7 watts. Same story with a couple of the resistors at the filter caps. The screen grid resistors look like around 1/2 watt. This doesn't seem right to me at all. Again in the pictures these are much bigger.
I don't trust Musical Fidelity. They were the last ones to handle this amp as far as I know and it doesn't look good. Even the invoice does not match what they did. One phone call to them was enough. The guy I spoke to said he had serviced these amps for five years at one stage but was not forthcoming when I asked him about specific details. He gave me the expected blurb about dangerous voltages etc but I know about this. If an owner electrocutes themselves it's not Musical Fidelities responsibility but they sure want to repair it all right to earn a few bucks. This is why I got into learning about these things. I wouldn't let anybody touch my Fender Twin now. Cowboys had already been at that.
I'm certainly no expert on valve amps but that's why I've ordered the Valve Amp book. It may not relate to this amp but it will help me build up a better picture on how amps work. It should maker it easier for me to sort out a schematic.
How would I know if the output transformers were damaged? Would they simply not work at all? All I can say is that everything runs OK when I have two valves in each channel and not four. No motorboating or anything. Are there any tests I can run on them, bearing in mind I have no specs for them other than what I can work out from the rest of the amp?
As long as the transformers are OK then the amp is good in my opinion. I won't mess further with it yet though. I'm going to do some research first then I may be back.
These are some of the dodgy resistors. The top is a picture of one at the filter caps. Doesn't need much further explaining to see problems here. I was able to pull this out easily. The PCB burning is not quite as serious as it seems to look in this photo. The board was almost touching the resistor. The bottom is some of the Cathode resistors. Looks bad again but surely these resistors are not correct. Photos of other Odysseus amps show standard type oblong high watt ceramics. Bear in mind that the amp did actually work at first. This burning damage seems to be long term heat exposure. I don't think it just happened suddenly or at least not while I had the amp.
An externally hosted image should be here but it was not working when we last tested it.
Efflux: My advice is give up on this amp!!
I owned an Odysseus for about 3yrs and although it sounded absolutely wonderful (powerful, valvey, musical and sweet to the max), I had nothing but trouble with it. It went backwards and forward to my technician ( a highly competent valve amp builder) about ten times over that period of time.
The main problem with the amp is that basically 'heat-and-pcbs don't mix'. It is a lot of amplifier housed in a small box, meaning HOT HOT HOT! The two vertically placed pcbs housing the EL34's virtually disintegrate and fall apart over time. Michaelson should not have placed the EL34 sockets directly on the circuit board, the heat transfer situation is not good!
If you are keen enough though, you could view it as a long term hobby project and completely rebuild and hardwire the whole amp - the beautiful sound of the amp may certainly be worth it!
Regards,
Steve M.
I owned an Odysseus for about 3yrs and although it sounded absolutely wonderful (powerful, valvey, musical and sweet to the max), I had nothing but trouble with it. It went backwards and forward to my technician ( a highly competent valve amp builder) about ten times over that period of time.
The main problem with the amp is that basically 'heat-and-pcbs don't mix'. It is a lot of amplifier housed in a small box, meaning HOT HOT HOT! The two vertically placed pcbs housing the EL34's virtually disintegrate and fall apart over time. Michaelson should not have placed the EL34 sockets directly on the circuit board, the heat transfer situation is not good!
If you are keen enough though, you could view it as a long term hobby project and completely rebuild and hardwire the whole amp - the beautiful sound of the amp may certainly be worth it!
Regards,
Steve M.
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