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Faulty tube, which one to replace, there are so many?

N

New2audio999

New Member
I'm currently using the Fisher 500c as a headphone amplifier & all of a sudden the bass got real distorted & noisy.
I think one of the power tubes might be dying died. I'm pretty sure of it, as one of them flashed blue before I started getting the noisy bass.

I plan to replace all 4 power tubes with Electro Harmonix 7591A to fix that (Iheard they are better than the stock tubes). By the way, are there there better options out there. Like the Tung Sol 7591A or the JJ Tesla 7591? Are any of them considered better than the other?

I also don't know which of the 16 additional ECC88/12AX7 tubes control the headphone input?
I don't want to buy replacements for all of them, especially if I won't be using the receiver for anything but headphones for now.
Which ones would I need to replace?

Do they impact the sound as much as the power tubes, do they all have to be matched as well? I'm thinking of just grabbing a few random 12AX7 tubes for them.
 
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New -- The EH 7591A would be a good choice for you as a good, respected output tube. The JJ has a significant history of premature failure, while the TS tube is the best of the bunch, but due to a particular characteristic it has, would require a slight modification to the unit for them to operate properly. In any event, you will want to get your tubes as a matched quad from a reputable vender like Jim McShane.

The headphone output is a reduced signal taken from the speaker outputs of the unit. Therefore, it is not a separate circuit that can fail on its own. And therefore, all tubes affect the sound in your headphones just as they would in your speakers.

The one tube flashing blue could be due to an issue in either the unit, or the tube itself. If the unit has not been restored, then that will make it hard to determine which it is without troubleshooting. If the unit has been restored, then the odds are more in favor of it being the tube.

Also, knowing if the problem is present in both channels or just one, whether it is volume dependent, present on all inputs, etc., is also helpful in narrowing the problem down.

If this is something you are inclined to take on yourself, then we can help you. Other wise, and particularly if it is not restored, it would be best to get the unit checked out by a competent tech to prevent doing any damage to the unit otherwise.

I hope this helps!

Dave
 
Is there anyone around you that might have a tube tester? With headphone use, those output tubes can probably be close to dead and still work correctly. Would be a shame to throw parts at it only to find the issue is somewhere else entirely.
 
The performance of the octal JJ tubes is excellent,when they are working.
Unfortunately,they're also very much like a grenade with a loose pin....
 
dcgillespie said:
The headphone output is a reduced signal taken from the speaker outputs of the unit. Therefore, it is not a separate circuit that can fail on its own. And therefore, all tubes affect the sound in your headphones just as they would in your speakers.

The one tube flashing blue could be due to an issue in either the unit, or the tube itself. If the unit has not been restored, then that will make it hard to determine which it is without troubleshooting. If the unit has been restored, then the odds are more in favor of it being the tube.

Also, knowing if the problem is present in both channels or just one, whether it is volume dependent, present on all inputs, etc., is also helpful in narrowing the problem down.
Click to expand...

The bass distortion is at all volumes, on both side of the headphones & I'm using the AUX inputs from the back with a few different RCA cables to connect to several different digital to audio converters.
The unit was fully restored. I don't have anything to test the tubes other than a different tube amp.
I tried taking some of the ECC88 tubes from the fisher to my other tube amp & when I turned the unit on, one of them glowed really bright yellow & then did not light up at all, while the other one lighted up normally.

Besides the 7591A tubes, what are the recommended tubes to replace the other smaller ECC88 tubes?
Should I just replace them with working ECC88 tubes or would 12AU7/12AX7 be a better option?
 
There are NO ECC88/6DJ8/6922 tubes in ANY VERSION of the 500C. The TWIN TRIODE tubes used in the 500-C along with 99% of the FISHER's out there is the 12AX7(ecc83). 12ax7, 12at7, 12AU7's are all twin triodes in the same family but they are NOT interchangeable.

Do not swap a 12au7, 12at7, 6aq8, 6dj8 into a socket for a 12AX7. It won't work right or not at all.

Looks like you put a 6V heated tube in a 12v socket, or one that had different socketing and burned up the filament.

I would seriously consider sending it out to a reputable shop, before any further damage to the unit, tubes, or yourself happens.
 
Oh o.k., I took a few of the tubes out & I could have sworn the lettering was ECC88 & not ECC83.

All of the print is faded & hard to read, especially the 8 number which can look like a 3 when the print is nearly gone.

That would explain why I burnt out one of the filament.
 
Not necessarily. First of all the fact that it is in both cannels means power supply. The fact that it is not volume dependent indicates that it is before the volume control. You probably have some 5K resistor that decided to become 15K. If the wrong tubes got put in either channel this is quite likely.
 
JURB said:
Not necessarily. First of all the fact that it is in both cannels means power supply. The fact that it is not volume dependent indicates that it is before the volume control. You probably have some 5K resistor that decided to become 15K. If the wrong tubes got put in either channel this is quite likely.
Click to expand...

Is there a diagram of the Fisher 500c board that shows which tubes slots control what part of the receiver?
I found a few, but they were all low res & I couldn't make out any of the details.
 
The closest you'll get to that is the tube layout guide in the service manual. Not only does it identify what tube type goes where, but also what it does. If you haven't already, you can get that from the digital docs section.

Dave
 
Thanks for the info.

My new Electro Harmonix 7591A tubes arrived today & I switched them up & that did not get rid of the bass distortion.
Voices are crystal clear, but bass is very distorted, the higher the volume goes, the more distortion it gets.

I'm just plugging the original two prong power cord directly to a wall socket. Can I do anything to feed it a cleaner power source? I tried plugging it to a sure protector, but that didn't change anything.

I did notice when i power the unit it on, there are two lights that light up the front panel, but the left one only stays lit up for about 10 or so seconds & then just turns off. The right tube stays lit though.
I circled the problem tube here:
http://i.imgur.com/p1WgZ3y.jpg
Is that normal?

I also noticed that there is a lot of noise when I plug in my headphones & turn the unit it on, but that noise only lasts for 5-8 seconds & after that it is dead quite!
 
The two lights are for your stereo beacon (one on the left), and your FM Signal strength (right one). The Left one will go on and off depending on whether the signal is MONO (OFF) or STEREO (ON). When starting up it will go on for a few seconds until the 6gk5 tube (V-7) starts to conduct then stut off until a Stereo signal is presented to the circuit by the MPX. It's NORMAL OPERATION. Suggest you get and read a copy of the 500-C OWNERS MANUAL to learn it's capabilities. http://www.fisherconsoles.com/non console manuals/fisher 500c om.pdf
 
Those look a lot like original power supply filter caps to me. If those are dying, its quite probably not going to perform properly. If those are original, its probably also got a bucketload of original caps under the hood that are due for replacement too.
 
New2audio, you say the unit was fully restored. As gadget said those look like the original power supply filter caps and could be the source of your problem. Can you provide a pic of the underside of the chassis? We can determine the extent of your restoration and help you better.
 
bhamham said:
New2audio, you say the unit was fully restored. As gadget said those look like the original power supply filter caps and could be the source of your problem. Can you provide a pic of the underside of the chassis? We can determine the extent of your restoration and help you better.
Click to expand...

Sorry, but I should have mentioned I just took a random picture of the board from the net to highlighted the light that goes off.

I'll take a picture of mine today & try to upload it later on tonight.

If this is something that can be fixed with just a few hundred, I would be fine with that.
I hope it won't cost as much as the actual receiver.
 
I see a couple of new caps, and what appear to be a fair number of originals. The cans up top don't look to have been replaced, and they are definitely in circuit. I suppose its possible they were cut open and re-stuffed but I can't tell. If you see any evidence of them being cut and re-attached on the topside, then that might answer things. The old EroFol caps underneath are probably original.

My guess is that it got new bias supply caps and a handful of others and sent out the door. It may have been working properly when it was sold, but if the bulk of the parts are original and pushing 50 years old, it was simply a matter of time before one quit. Different people have different notions of what "rebuilt" is.
 
Looks like the BIAS SUPPLY was at least partially rebuilt. There are a few lytics changed, films in the MPX section and 2 films in the Phono sections. Doesn't look to me that the Main Filter caps were stuffed/replaced, nor were any of the reliability upgrades made (Screen, cathode, grid return resistors, and output coupling caps.)

Basically he replaced a few lytics, a few film caps, diodes, added a varistor. Probably just enough to get it running and off his hands. But nothing that I would consider even 1/2way restored. If you do the work yourself, the parts can cost between $75.00 to $200.00 (depending on whether you stuff the can's or get new cans made). including all the standard upgrades for safety and reliability. That's the main filter caps, the rest of the film caps and the lytics that weren't replaced, some resistors, and the add on parts for the upgrades. NOT including tubes.

The Bass distortion could be a bad or off spec resistor or cap. You've pretty much ruled out the tubes which leaves caps and resistors, plus dirty switch contacts. Get a can of DeOxit D-5 and a can of FADERLUBE F-5. Read the sticky thread in the General Audio forum called ""The idiots Guide to DeOxit....Revisited". It will tell you how to clean the contacts on the controls. Do ALL of the switches, pots, and rotary switches. If it goes "In and out, up and down, or round and round" it gets DeOxit. Also the RCA jacks in the back can be dirty to the point where they will cause problems.
 
If it's acting up in both channels, pull and test V14 and V15 before digging under hood. Swapping in a spare X7 is the surest way and will make it quick work. I just had a '52 Sylvania X7 black plate with under 1k hours go hissy in one of these, and it still tests NOS. So it does happen.

That's a clean if mild rebuild as far as it goes. But I don't like to leave those old Ero-Fols around, as they all eventually get leaky and will cause a runaway output (and a blown cathode fuse here). But I don't see anything there that is going to burn up a winding or your house down. I don't think they are causing this, but replace those main couplers if you can. There are some couplers earlier on that I also replace that he left in as well, but I would be surprised if they are causing this. Again, this was a mild rebuild, and there is not anything wrong with that in many instances.

FWIW, I am STILL running original PSU filters in my own equipment (some of which is bumping 60), and they still work just fine. Yes, I'm a heretic here. But these are much hardier than people think. A lot of old bench techs will leave them in. If you're not getting significant ripple, yours may still be fine as well. Anyway, I don't think PSU filtering is causing your distorted bass in both channels . . . but I've been wrong once or twice before.

If a quick small signal tube check doesn't reveal anything, and you don't know what you're doing underneath, take it back to the tech who restored it. A reconditioning is not a life-time guarantee with these, and I wouldn't necessarily fault him.

Also, with new outputs, be sure to make sure the PIs are still strong, as they get beat up pretty good in these. Unless you're spinning vinyl, pull V16 and V17 - needless waste of two good tubes and unnecessary added load on the bias supply.

Good luck.
 
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