Hello all,
at long last I am making a start on my Revox Model 40 stereo amp. I've had the amp about 30 years but many many years ago it fizzed and popped so I put it in the loft thinking one day I would have it repaired.
I think the fizz/pop probably came from the rectifiers as I believe they are prone to that and also they are bulged a little bit. So they are what I will do first followed by a re-cap.
I've uploaded a few pics so people know what I'm talking about.
Basically what I need to know is if the replacement rectifiers I have bought are suitable. I'm also puzzled by the smaller rectifier, it has five connections and I'm at a loss as to what to replace that with or should I just leave it alone? Two of the terminals are bridged if that helps.
The amp hasn't been turned on for about 27 years so I am hoping to do this work before plugging 'er in. I have the parts to build a dim bulb tester but sadly do not have access to a variac.
I have done a few re-cappiung jobs so i am reasonably okay with that side of it.
All help greatly appreciated!
Thanks in anticipation
Ian.
Lancaster, UK.
at long last I am making a start on my Revox Model 40 stereo amp. I've had the amp about 30 years but many many years ago it fizzed and popped so I put it in the loft thinking one day I would have it repaired.
I think the fizz/pop probably came from the rectifiers as I believe they are prone to that and also they are bulged a little bit. So they are what I will do first followed by a re-cap.
I've uploaded a few pics so people know what I'm talking about.
Basically what I need to know is if the replacement rectifiers I have bought are suitable. I'm also puzzled by the smaller rectifier, it has five connections and I'm at a loss as to what to replace that with or should I just leave it alone? Two of the terminals are bridged if that helps.
The amp hasn't been turned on for about 27 years so I am hoping to do this work before plugging 'er in. I have the parts to build a dim bulb tester but sadly do not have access to a variac.
I have done a few re-cappiung jobs so i am reasonably okay with that side of it.
All help greatly appreciated!
Thanks in anticipation
Ian.
Lancaster, UK.
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The two selenium rectifiers are wired in parallel and can be replaced with a single silicon bridge rectifier. Check the voltage rating on that rectifier - I tend to prefer higher voltage ratings. I'd recommend at least 800V or 1kV for the PIV rating even in relatively low voltage applications.
The filament rectifier can be replaced with a standard bridge with a 100piv rating or better at a couple of amps.
Replace the electrolytic caps as well before powering up!
The bulb will work fine for protection, start with low wattage bulb. (25 - 40W)
The filament rectifier can be replaced with a standard bridge with a 100piv rating or better at a couple of amps.
Replace the electrolytic caps as well before powering up!
The bulb will work fine for protection, start with low wattage bulb. (25 - 40W)
I have the same amp. It is very beautiful.
Had the same issue, of a noisy amp. The new bridge needs to be followed by a resistor to reduce the instreaming current and ensure the transformer keeps quiet.
I replaced the capacitor with a new version of the same (have one still laying around).
A slight capacitance multiplier does charms too.
The preamp section is a marvel.
The power section has a very low amplification.
I had to replace the pots - the balance specifically.
Had the same issue, of a noisy amp. The new bridge needs to be followed by a resistor to reduce the instreaming current and ensure the transformer keeps quiet.
I replaced the capacitor with a new version of the same (have one still laying around).
A slight capacitance multiplier does charms too.
The preamp section is a marvel.
The power section has a very low amplification.
I had to replace the pots - the balance specifically.
Thanks for the reply.
The new rectifiers I got are 4A 600V. I'd prefer to keep a pair of rectifiers if only to retain the original style/appearance.
Yes, the electrolytics are on the list to do before powering up and I've already got a set of Panasonics.
Cheers,
Ian.
Thanks for your reply. What resisitor do you recommend to put in after the rectifier? Or do I need to do measurements?
I am not particularly knowledgable about this kind of stuff. Hoping to mostly replace old with new! I have no idea what a capacitance multiplier is for example.
It is indeed a fine amplifier, when it works! It was my main amplifier for a number of years before the fizz/pop incident. I thought at the time it was something major so I just stashed it away for the future. The future is now here!
Ian.
Ian,
Some comments, if I may.
Unfortunately, that is not feasible.
Rectified voltages rate be higher using silicon, instead of selenium. Some additional resistances are likely needed.
The 6GW8/ECL86 is scarce. It will be difficult enough to source 4 specimens of the same brand, let alone a well matched quartet. The OEM schematic shows truly fixed bias. IMO/IME, that needs to change to a configuration with 4X bias trim pots., to deal with the reality of poorly matched O/P tubes.
Studer used the phono section heaters as resistors in the bias supply. I would bridge rectify the apparently unused 800 mA./12.6 V. winding with 4X 1 A./50 PIV Schottky diodes, cap. filter the resulting "raw" DC and regulate the rail with a 7812 3 terminal regulator, to obtain high quality DC for the phono section heaters. Isolate those heaters totally from the signal circuitry.
In the near future, the 6GW8/ECL86 will go the way of the dodo. Therefore, redesign of what is a fine piece is inevitable. The challenge is reasonably simple to deal with. Eliminate tone controls and use 6BQ5/EL84 O/P tubes. Sufficient 9 pin miniature sockets are already present to support that kind of thing.
Some comments, if I may.
Unfortunately, that is not feasible.
Rectified voltages rate be higher using silicon, instead of selenium. Some additional resistances are likely needed.
The 6GW8/ECL86 is scarce. It will be difficult enough to source 4 specimens of the same brand, let alone a well matched quartet. The OEM schematic shows truly fixed bias. IMO/IME, that needs to change to a configuration with 4X bias trim pots., to deal with the reality of poorly matched O/P tubes.
Studer used the phono section heaters as resistors in the bias supply. I would bridge rectify the apparently unused 800 mA./12.6 V. winding with 4X 1 A./50 PIV Schottky diodes, cap. filter the resulting "raw" DC and regulate the rail with a 7812 3 terminal regulator, to obtain high quality DC for the phono section heaters. Isolate those heaters totally from the signal circuitry.
In the near future, the 6GW8/ECL86 will go the way of the dodo. Therefore, redesign of what is a fine piece is inevitable. The challenge is reasonably simple to deal with. Eliminate tone controls and use 6BQ5/EL84 O/P tubes. Sufficient 9 pin miniature sockets are already present to support that kind of thing.
Hmm. I expect you're probably right about all that and I do know my limitations. I would consider taking this to someone for repair but think it would be uneconomical hence me wanting to do it myself. I have done some electronic work but my only experience with valves/tubes is modding a couple of SE Guitar amps (Valve Junior, Blackheart BH5) which is why I have come here.
All the valves are the original ones and were working last time it was used although I do appreciate that doesn't mean they will work now! Plenty of ECL86 on ebay at the moment but not cheap.
I'll ponder this for a while before I do anything.
Thanks for your input,
Ian.
I also have the ECL86 scarcity problem. I have 3 matched and 1 old.
The pentode section is same to EL84, so that is correct.
The triode section is a bit hard, but ECC83 should to do well (mu=100).
And yes, you can omit the RIAA section (I did) and reuse the sockets. But now my amp is not in original state anymore 😱
Taking a silicon bridge gives a higher Vb indeed.
My structure is as follows:
replace the GI 3 bridge too I think is a good idea too.
The pentode section is same to EL84, so that is correct.
The triode section is a bit hard, but ECC83 should to do well (mu=100).
And yes, you can omit the RIAA section (I did) and reuse the sockets. But now my amp is not in original state anymore 😱
Taking a silicon bridge gives a higher Vb indeed.
My structure is as follows:
5,5 uF MKP; an inductor (I have not documented what type or exact value, but it was at hand, about 100 ohms DC, somewhere 3-7 H), then a 200 uF cell; then with 3K to the next two cells of 50 uF. My new capacitor was 200+50+50 not 50+50+50.
One trick I did was configuring the pentode part as triode. I did not need more than 5 watts anyway. This works well sound wise, depending on speaker maybe (mine was a Fostex sigma 10).
replace the GI 3 bridge too I think is a good idea too.
Still plenty of PCL86 out there - it may be worth finding some space for an extra filament transformer. See ebay item 370793847380 as an example.
If I do go ahead what do you suggest I use to replace the G13 rectifier? It has five terminals although two are bridged.
Thanks,
Ian.
Selenium rectifiers are ticking, toxic, time bombs. It is a matter of routine to replace them with modern, silicon, parts.
As for the G13 bridge, 4X 1 A./100 PIV Schottky diodes will be a superior replacement. Unlike PN junction diodes, Schottky diodes don't exhibit a reverse recovery spike. Therefore, Schottky diodes are "noiseless".
As for the G13 bridge, 4X 1 A./100 PIV Schottky diodes will be a superior replacement. Unlike PN junction diodes, Schottky diodes don't exhibit a reverse recovery spike. Therefore, Schottky diodes are "noiseless".
Thanks for that. I understand how to connect the four diodes up but what about the fifth connection? That's the part that has me stumped.
Cheers,
Ian.
I found a document that indicates that there is more difference than just the filament between ECL86 and PCL86
Mihgt be the fifth is a shield connected to earth? I replaced it long ago, no memories left
Thanks for your reply.
The fifth terminal is unmarked but the wire bridges across to the - terminal so I wonder if they both make up the - side of the rectifier. There are only 4 wires attached so hopefully a modern four terminal rectifier will be the same. Detailed information (in a language a simple northern lad like me understands) about the B30C250 is hard to find but here is a picture I found where you can just make out the fifth terminal.
Cheers,
Ian.
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