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Old 11th September 2006, 06:27 AM   #21
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Hello efflux,

I'm new around here and this is my first reply. Although I'm not new to electronics. I'm 63 with over 40 years experience as a designer, builder & repair tech. I learned electronics on tubes when there were no transistors.

Motorboating: To combat motorboating it helps to understand that it is a form of low frequency oscillation caused by feedback. The source of which is not always apparent, but usually originates in the power supply. Specifically from a supply that has too high of an output impedance. This condition will not supply proper current to the circuit when loaded past a certain point. Typical causes are deteriorated filter capacitors, changed resistor values and even bad or improper DC rectification.

Another source of feedback is a lack of isolation between stages. This can happen through the power supply when a signal is seeing a high resistance to ground. (as in a bad filter cap) or is more closely coupled through a changed/lowered divider resistance. I've even seen a shorted filter choke cause motorboating.

So you will need to check the filter capacitors either by direct substitution or by looking at the ripple with an oscilloscope. Temporally bridging them with a new one of the proper rating will also help find a defective unit. The larger value caps installed are not the problem and are OK so long as they were not smaller. BTW, Jamicon's are not so good to begin with. They're cheap Asian imports.

Screen resistors: EL34/6CA7s have fairly low screen dissipation levels that are componded when running ultra linear. This is because they can actually run at higher then plate voltage do to being connected from taps on the output transformer that are closer to the B+ source then the plates themselves. A screen limiting resistor of 1000 ohms is typical and highly recommended. This goes double for most of the Chinese imports of today which IMHI are much inferior to the NOS American stuff.

An overheating screen will kill an otherwise good tube in short order. Especially when the tube is mounted horizontally as yours are which hurts proper convection cooling. A design layout that I do not approve of. You can visually check to see if your screen are running hot. In a darkened room, look very closely at the tube while it's running and peer inside the structure. If you see the screen glowing orange, it's running too hot. This may be difficult to see at first so keep looking. Increase the value ot the resistor in series with it. Doing so will not hurt the sound or reduce the power, but your tube will be much happier and live a lot longer.

Cathode resistors: Looking at the picture you posted, I see what appear to be either metal or carbon film types. If so, these are a poor choice since they will not stand any sort of overload. Not even for a few seconds unless they were being used to act as a fuse. If one resistor should open. the other tube in parallel with it must take up the load. Carbon composition resistors are very forgiving and will withstand an overload well, just like a tube.

I also tend to agree with Steve about the overall mechanical design of the amp. It's too cramped with too much heat from those horizontal fire bottles.

I hope this will help you.
Victor
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Old 11th September 2006, 10:35 AM   #22
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One of the difficulties in trying to repair an amp like yours is working out where to start. Current problems are likely to be the result of faults, general wear and ageing as well as poor previous repair work.

If there are lots of poor soldered joints around then further fault finding will be near impossible as the circuit will change every time you move something. Spend a couple of hours going over the entire amp and remaking soldered joints which either look suspicious or are likely to have been stressed (sockets, power resistors etc).
Replace anything which is obviously damaged or in poor condition e.g. screen and cathode resistors. Unless you are 100% sure of an electrolytic capacitor then replace it. Check wiring and grounding arrangements.

Start checking the voltages for power supply rails, plate, screen, grid and cathode of output tubes and drivers. Make sure that these are within safe limits for the tube and compare working and faulty channels. It can be helpful to sketch out a rough schematic and write in the voltages at each point. It that doesn't identify the problem then at least it will give other diyaudio forum members something to work with.
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Old 13th September 2006, 12:10 AM   #23
efflux is offline efflux  United Kingdom
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OK, thanks very much for the opinions.

Thanks for the long message HollowState. Much appreciated. I have one question to ask if you don't mind. What about wirewound resistors on the cathodes? The original Odysseus definitely had large oblong shaped ceramic wirewound resistors on here, presumably chosen due to easy and cheap availability of high wattage. I can see these in some photos I have of an Odysseus dismantled. Possibly Michaelson Audio decided this was not a good idea and changed it on later models or somebody else changed it to the resistors you see in the pictures. Maybe whoever did it wanted to make sure the amp would be coming back for repair. Who knows. Also, if you look at the photos of the cracked up cathode resistors, they do have windings inside. I thought this meant they must be wirewound but I've rarely seen inside resistors anyway. However, I have seen resistors just like this that are sold as wirewound surge resistors rated at 2 watts I think. Am I right in presuming that, with all the heat issues in this amp, where a resistor is best higher wattage, then wirewound would be more robust due to availability of much higher watt.

After looking carefully at various photos (not many) of the Odysseus, it seems my version is a revised one, specifically the main PCB (ISSUE 02). The PCB is different. In mine you can see certain PCB tracks that are left overs from the first version.

If anybody else out there knows of anyone who has one of these amps, that could be very helpful but I'm guessing it's quite rare.

I got the Valve Amplfier book which is excellent. I guess this is going to be my tube amp bible.
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Old 13th September 2006, 01:41 PM   #24
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Hi, I'm not sure what version of the amp I have (this is a piccy of mine on the kitchen table) and I know nothing about valve amps, but if you want some close up images of the inside I should be able to oblige? As long as it doesn't involve stripping down any further than the top covers and heat shields as mine is working fine and I don't want to upset the boat
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Old 14th September 2006, 12:23 AM   #25
efflux is offline efflux  United Kingdom
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Hi vinylengine.

I'm glad to hear that someone else has one of these and without problems.

There is a picture of my Odysseus at that same site. Posted by the previous owner. The one with the paint stripped from the knobs to reveal the brass. The brass was all dirty. I've polished them to a high finish and will lacquer them. That was before the problems started. More serious issues to deal with now.

You could be of HUGE help to me because I am not entirely sure about a few things in the amp - what they were originally. Has your amp ever had any repairs done? Is it all original?

Taking off the parts that you mention is all that is needed. However the cathode resistors are not easy to see. Here is a picture to show you some of the parts I would like to know about. Is this your amp because it's at that same site as the photo you linked to and it looks like it's sitting on the same wooden surface?

Click the image to open in full size.

The red circles are the thing I need to know most. They are resistors mounted vertically on the valve PCB. If you can tell me anything about these, any colour codes or markings it would be much appreciated. Photos would be cool. The green circles are 6 big electrolytic capacitors. Be careful if you open the amp. Probably you know the dangers anyway but these caps can hold big voltages even when the amp is off. You won't need to touch anything anyway. I think I know what value these caps are but any markings with uf or v would be a help. The blue circles are more electrolytic capacitors. There are 4 the same. Again, uf and v markings. There are another two small electrolytic capacitors that look just the same but very small. It's not absolutely necessary to know all these others but I want to make sure. Some of mine have been replaced.

The next thing would be helpful but these are not so easy to see. Here is a photo I found of an Odysseus valve PCB. It is upside down so the components I have marked are on the bottom when viewing an open Odysseus amp. The yellow circle is a resistor. Any info about these would be helpful. The purple circle is another electrolytic capacitor. You should be able read markings on this from the top of the amp without problems. There are 8 or these resistors and 8 capacitors. All the same. One on each valve.

Click the image to open in full size.

The final parts are resistors connected at these squares marked with yellow circles from a photo of my amp, taken before fixing these parts. Any info about these would be further added help but I'm sure I have this right now anyway. In this picture your amp will most likely have different resistors here. Except for this photo I have used photos of other amps because they will probably look exactly like yours.

Click the image to open in full size.

Also at the back of the main PCB you will see ODYSSEUS D.I.P ISSUE and a number. I don't think this is really that important. Just minor design changes.

If you can give me some of this info it would be very helpful. Thanks for offering to help.
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Old 14th September 2006, 09:48 AM   #26
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Hi, Yeah that's my amp! I don't know if it's ever had any repairs done though? I bought it S/H and only went inside to clear out some of the dust before firing it up as it had been stood some time. I ended up wiping off some of the markings off the valves with a damp cloth (doh!) but they all used to say Michaelson Audio on them so even they may be original. I'll try to get some close-ups of the inside by the weekend - it might even help me if anything ever does go wrong; it always useful if you can see what component values are before they get burnt off (!)
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Old 14th September 2006, 11:29 AM   #27
efflux is offline efflux  United Kingdom
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OK. Thanks.

The valves will probably all be original then. Also, from looking at the photo of your amp I can tell it's all original. Your amp will have the parts mine should have which probably explains why yours is still running.

The resistors encircled with red are the ones I really want to know about before anything else because on my amp these don't make sense and my problems point to a possible issue caused by these. The ohm value was reading so off from from the colour code as to not make sense and the colour code was not even difficult to read. The size of these doesn't make sense either, the smallest resistors in the amp when it looks like they should be of reasonable wattage handling.
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Old 14th September 2006, 09:15 PM   #28
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Hi efflux,
Regarding cathode resistors and the use of wirewound units. This usually depends on what they are being asked to do by the designer. Please note, I'm unfamilier with the circuit of this amp as to whether it uses fixed or cathode bias. So here's what I mean.

If the output tubes are using cathode bias, the resistor needed will be on the order of several hundred ohms to produce the required negative voltage at the grid. (an educated guess assuming 50ma.) This will be a power level of a few watts and will require a wirewound part. And the original parts you described certainly sound like wirewounds.

If the tubes use fixed bias, and the resistors are there only to monitor bias current with a metering circuit, then they will be a much lower value (with lower voltage drop) and need only dissipate fractional wattage. So a non-wirewound type would suffice here.

Since the originals seemed wirewound, I would replace with the same type. They will not easily burn out if a tube shorts or runs hot from loss of controlling bias. And this reminds me of one of my pet audio peeves.

Some will say that wirewound resistors are bad for audio because they are inductive and should not be used. Ignore most of that. This is audio, not RF. If we were dealing with RF it would be different. But at audio frequencies any inductive reactance in a resistor of this small physical size in moot.

I looked again at the picture of the resistors you removed and they sure look like film types. Although I could, perhaps, be wrong. Most of my experience is with American made equipment. The exception being with some fine Bruel & Kjaer and Marconi stuff. European parts could be, or look different.

Film resistors (carbon & metal) are made on a ceramic core with a thin film deposited on the surface. A laser is used to trim the value by making a spiral cut along the length. To the unfamiliar, this may appear as a wound wire when it really isn't. If you have actually peeled off some wire then, of course, it is.

Victor
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Old 14th September 2006, 10:13 PM   #29
efflux is offline efflux  United Kingdom
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OK. Thanks HollowState.

From what you say, the cathode resistors on my amp are not wirewound then.

Michaelson Audio definitely used wirewound resistors on the Original version of the Odysseus. You can see this clearly on one of the pictures but these are totally different from the cathode resistors on mine. They may have changed the design later.

My amp is cathode biased which ties in with what you are saying about the resistors.

You can actually get non inductive wirewounds but they are very expensive.

So I will replace the cathode resistors with larger wirewounds of the same value as already in there. Exactly the same resistors as in pictures of the original amp which will probably be 5 or 7 watts or thereabouts.

The most important thing is getting the screen grid resistors right. Vinylengine might be able to shed light on that because my resistors here don't make a lot of sense. Tiny wirewounds (these are definitely wirewound - I can see actual wire on a blown one) with ohm value at around a tenth of what the colour code says.

Any thoughts about where metal oxide is good to use? I did use these to replace a few resistors on my Fender Twin.

If you think the Odysseus is bad what about my Fender Twin guitar amp. Upside down tubes where all the heat flows up into the chassis!
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Old 15th September 2006, 02:29 AM   #30
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When installing new screen resistors it isn't totally necessary to stay exactly and precisely with the value that may have been originally used. (Reread my earlier post on them.) They are there to limit screen dissipation. Typically 1000 ohms are used. This is not an arbitrary value on my part, but comes from published application data for EL34s in ultra-linear service.

If your tubes were 6L6s or 6550s or another beam pentode type this would, perhaps, be less important. But with EL34s it is. And especially with the Asian imports that are produced in different factories having varying degrees of tolerance and quality control. To use a low value, say around 50 or 100 ohms, would be of little value in reducing screen dissipation. They might serve as a parasitic suppressor or to couple to the plate in triode mode, but that is not the issue here.

Metal oxide resistors. Personally I'm not a big fan of them. Their application is better relegated to RF circuits and low power audio unless you want them to act like a fuse and not fire burn. Metal films will not take an overload and they open very quickly if stressed or pulsed. They're good for high frequency because of their deposited surface resistance. (skin effect)

Carbon composition resistors are just the opposite. You can stress them for short periods and they'll still work. Even if they smoke a bit, but not crack, they'll work. Can't do that with film resistors. I think the reason you see them used so often nowadays is because that's what is available. Comps are not made anymore and haven't been for quite a while. That's why NOS A-B, Ohmite and etc. command their high prices. Fortunately for us DIYers, there's enough still around, but not for manufactures.

Be carefull how many metal oxides you use in one amplifier. Many years ago, I replaced all the carbon resistors in one of my Ampex recorders with metal films thinking I would get better sound. I also replaced the paper caps with polyesters. What I did get was a gawd awful exaggerated high frequency response. That was a real ear/brain opener for sure. Never again.

I'm very familier with the older guitar amps from Fender and Ampeg. Many of them had their tubes upside down. But they were mounted on steel chassis that both sank and deflected the heat away from the wiring inside. Not circuit boards. As I remember, they were reliable with minimal heat related problems.

Victor
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