walloffound
Active Member
I have a Fisher X-202. Keeps blowing the rectifier tube. Im thinking of converting the unit to ss by replacing the tube with a couple of silicone diodes as are the later fisher models. Has anyone else done this?
Tube to ss rectifier conversion
- Thread starter walloffound
- Start date
UncleBingo
Super Member
Have you done the diode mod on the tube socket and added an MOV? Between that and a decent rectifier tube (avoid some of the newer production models discussed at length on many forums) you should be good.
It will bump the voltage considerably unless you add a resistor to drop it.
Better question is why does it eat rectifiers? I would investigate that first otherwise you may start frying less replaceable items like the power transformer. Schematic says 200ma load on the rectifier, should be able to measure 25v drop across the pair of 250 ohm resistors if thats the case. If its dropping more than that, figure out where the current draw is. If all is normal, steady state 200ma is pushing the limits on a 5AR4 and listening with any sort of volume is going to exceed the abilities of the tube to keep up. If you make it SS, bump the values of those two 250 ohm resistors, or add another resistor between those and the rectifier to put voltage back where it belongs.
Better question is why does it eat rectifiers? I would investigate that first otherwise you may start frying less replaceable items like the power transformer. Schematic says 200ma load on the rectifier, should be able to measure 25v drop across the pair of 250 ohm resistors if thats the case. If its dropping more than that, figure out where the current draw is. If all is normal, steady state 200ma is pushing the limits on a 5AR4 and listening with any sort of volume is going to exceed the abilities of the tube to keep up. If you make it SS, bump the values of those two 250 ohm resistors, or add another resistor between those and the rectifier to put voltage back where it belongs.
walloffound
Active Member
Thanks guys, what value resitor would i need to add between the two 250 ohm and the rectifier?
walloffound
Active Member
I already had tha amp checke out by a tech and he said everything checked out. Im just tired of buying new rectifier tubes
UncleBingo
Super Member
"Everything checked out" is incredibly vague. Not knocking the tech. There's no way to check "everything" without generating a massive bill.
Are the tubes failing at power-up or in some recognizable fashion?
Are the tubes failing at power-up or in some recognizable fashion?
What brand Rectumfrier tube are you buying? I've also got a X-202 and use a Mullard or a SOVTEK. Haven't hurt either one. But I did do the DIODE install on the socket, which I do feel helps.
UncleBingo
Super Member
Iirc this is the diode mod. Hopefully larryderouin can confirm. https://www.audioasylum.com/cgi/vt.mpl?f=tubediy&m=225965
A solid-state rectifier mod is a step backward in my opinion if you value the sound most. A really good rectifier tube can have quite an impact on sonics. And they should last a looooong time if everything is running kosher.
A rectifier tube eater usually has a bigger problem. Often in the filters. If the supply is eating up rectifier tubes, take a good hard look at the first filter coming off of it. If it is strongly leaking or shorting, it is going to take out a rectifier tube in short order.
But better a rectifier tube than a transformer secondary. That is the other advantage of tube rectifiers. If properly spec'ed, they can save your power transformer by sacrificing themselves first.
Simply dropping in diodes, or plopping in a more robust rectifier tube substitution, if there are filters shorting or leaking heavily, is asking for much bigger trouble.
And as others have noted, a SS mod is going to abruptly raise your WV on the downstream supply, with all that implies. Make sure the filters are up to it in any event.
A rectifier tube eater usually has a bigger problem. Often in the filters. If the supply is eating up rectifier tubes, take a good hard look at the first filter coming off of it. If it is strongly leaking or shorting, it is going to take out a rectifier tube in short order.
But better a rectifier tube than a transformer secondary. That is the other advantage of tube rectifiers. If properly spec'ed, they can save your power transformer by sacrificing themselves first.
Simply dropping in diodes, or plopping in a more robust rectifier tube substitution, if there are filters shorting or leaking heavily, is asking for much bigger trouble.
And as others have noted, a SS mod is going to abruptly raise your WV on the downstream supply, with all that implies. Make sure the filters are up to it in any event.
Man, that's where I would go!
UncleBingo
Super Member
Looks like one of the filter caps has been bypassed with newer parts.
By “bypassed”, are you saying the original filter is now out of circuit? Piggybacking a new cap on the old one will not help with a leaky one, but can lift the total capacitance past the maximum design current of the rectifier. The first filter past a tube rectifier should not be much higher in capacitance than originally specified. Otherwise the inrush current is going to eat up the rectifier.
In other words, you can not throw a mountain of extra filtering at a tube rectified supply (i.e. no Dr. Stereo mods). You can always bump up the WV values a little, but keep the capacitance values reasonably close to design here.
I’m not going to tell you to recap it just yet, and I don’t have the schematic for that one in front of me right now. But if still original, those filters are now pushing six decades. The only good thing about that is if they are not leaking, they have certainly reduced in capacitance, as far as your rectifier is concerned. Otherwise, if they have been replaced, double check the replacement case values to make sure they are not excessive.
Of course, if this problem has only recently surfaced, then it is more likely a failing part than a mis-specified one. On older units, it sometimes is a combination of failings over time that finally drags down a supply, and you have merely reached the final straw on a long overloaded back. Drifting resistors, slowly leaking filters, outputs with a drifting hot bias . . . they all ask a little more from the supply. So it may not be just one thing to fix if you want it back to original specs.
But start by measuring the values on any resistors coming off the supply rails. If any have drifted low, it can sag the supply. It will also sometimes drag down the voltage on that leg.
In the most basic terms, you have an excessive load on the supply somewhere. That’s either leakage or an excessive working path to ground somewhere along the way. Don’t ‘fix’ it by beefing up the rectifier. That’s pushing the problem back on the power transformer. Find the leakage point(s) and fix that (them).
If you have access to a scope, you can often nail down a leaking filter simply by seeing the heightened ripple that accompanies the compromised capacitance. Otherwise, you will need to isolate each leg and chase the supply circuit values down the line. It sounds harder than it is.
Make sure your outputs are not running hot or shorting. That will quickly load down a supply, too.
The 202 in good running order is a sweet sounding unit. Get it straightened out and stick with that tube rectifier.
Not sure, but I'd probably start with about a 50 ohm 10w and adjust from there.
but yeah, be absolutely sure you aren't putting a band-aid on a different problem first.
walloffound
Active Member
I am one to keep things original so its not really my first choice to make any changes but i really like this amp and want to keep it, the tech is a local guy who did the original restoration and he does a lot of work on expensive and rare old tube gear and has a good reputation from what ive heard, the local vintage stereo shop use him for much of their more expensive vintage equipment. I thought of changing to ss because it looks like Fisher got rid of the tube rectifier after this model so maybe it was a known problem. i figured it looked like an easy enough change that i could do and it is not irreversible.
If you know the tech who worked on this previously and he is still available, and this is more than you want to tackle, take it to him and let him look at it. If it’s remained untouched since he last restored it, then it is still his baby, so to speak.
To my knowledge, a 202 devouring rectifiers on a regular basis was never a “known problem” with them. If I were driving horns on the cheap, a 202 is always on my short list.
And all the remaining for-hire techs left whose work I think is competent I can now count on one hand. Almost all the really good alignment techs are gone. Perhaps I am just an old stickler now. So if you have a good tech available to you, keep him fed.
To my knowledge, a 202 devouring rectifiers on a regular basis was never a “known problem” with them. If I were driving horns on the cheap, a 202 is always on my short list.
And all the remaining for-hire techs left whose work I think is competent I can now count on one hand. Almost all the really good alignment techs are gone. Perhaps I am just an old stickler now. So if you have a good tech available to you, keep him fed.
UncleBingo
Super Member
Looks like those new caps near the can cap are on terminal strips, bypassing the original can cap. Not piggybacked onto the old cap (which I agree would be bad practice).
fred soop
Super Member
Speaking as an engineer, filtered DC voltage at the output terminal of a capacitor is ..... filtered DC voltage at the output terminal of a capacitor. There is no audio signal present at this point, or at least there shouldn't be. Doesn't matter if the voltage, if correct, got there through a tube, solid state rectifier, battery, or motor-generator. Please explain in terms that would pass technical scrutiny why a tube rectifier would lead to better sound.
That's the one I used in my TA 600, X-101B, and X-202. As I understand it, the diodes are in series they take the load more than the tube does, but the tube still does a slow warm up. Also as I understand it, the diodes keep reverse current out of the transformer in case the tube shorts. I'm using Sovteks in the 101 and the 202 for over a year now and they seem happy. The Mullard in the TA-600 hasn't hiccupped either.
I would echo my own experience here. A properly operating X-202 as built by Fisher will not eat rectifier tubes. In fact, mine still has the original Mullard tube that Fisher installed in it, that tests and operates just fine -- and my unit showed up with a serious amount of mileage on it. I will say this however, at nearly 200 mA of quiescent current draw, this is not a candidate for a JJ rectifier tube.
The JJ GZ34 gets a lot of bad press -- and deservedly so in many applications when compared to the original device it is attempting to replicate. Used in less demanding applications however, it seems to work just fine. I've had a number of them powering 350 volt 125 mA applications, where they perform quite well. But in applications of higher voltage and current levels -- where the original device would still perform with distinction, the JJ product will typically fail, and do so rather early in life. The X-202 is one of those applications. The X-202 is asking basically as much of its rectifier tube as the Dynaco ST-70 does, and the failure rate of this manufacturer's tube in that application is near legendary. Therefore, if this is the tube you've been using, stay away from it in this application. On the other hand, I have had very good results from the (relatively) new Genalex tube in high demand applications (stock SA-300 at full sustained power output in both channels), so if you must use modern manufactured tubes, I would offer this example up as a good choice for the X-202.
Dave
The JJ GZ34 gets a lot of bad press -- and deservedly so in many applications when compared to the original device it is attempting to replicate. Used in less demanding applications however, it seems to work just fine. I've had a number of them powering 350 volt 125 mA applications, where they perform quite well. But in applications of higher voltage and current levels -- where the original device would still perform with distinction, the JJ product will typically fail, and do so rather early in life. The X-202 is one of those applications. The X-202 is asking basically as much of its rectifier tube as the Dynaco ST-70 does, and the failure rate of this manufacturer's tube in that application is near legendary. Therefore, if this is the tube you've been using, stay away from it in this application. On the other hand, I have had very good results from the (relatively) new Genalex tube in high demand applications (stock SA-300 at full sustained power output in both channels), so if you must use modern manufactured tubes, I would offer this example up as a good choice for the X-202.
Dave
