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Old 9th September 2006, 01:47 AM   #11
efflux is offline efflux  United Kingdom
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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.
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Old 9th September 2006, 02:47 AM   #12
ben goh is offline ben goh  Singapore
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Hi

Maybe this link might be of help to you

http://www.ampslab.com/SCHEMATICS/Mi...ustinTVA10.gif

Thanks
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Old 9th September 2006, 03:35 AM   #13
efflux is offline efflux  United Kingdom
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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.
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Old 10th September 2006, 12:22 AM   #14
TJ is offline TJ
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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
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Old 10th September 2006, 10:59 PM   #15
efflux is offline efflux  United Kingdom
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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.
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Old 10th September 2006, 11:40 PM   #16
TJ is offline TJ
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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 .
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Old 11th September 2006, 12:21 AM   #17
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Quote:
Originally posted by TJ
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
Or gut it and start over (assumming the iron is OK.

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Old 11th September 2006, 01:40 AM   #18
efflux is offline efflux  United Kingdom
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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.
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Old 11th September 2006, 02:28 AM   #19
efflux is offline efflux  United Kingdom
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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.

Click the image to open in full size.
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Old 11th September 2006, 03:49 AM   #20
Steve M is offline Steve M  Australia
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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.
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