Fisher X-100 Help Needed

[goatropercu]

Hello,

I have a Fisher X-100 that has no filament voltage at tubes V1, V2, V6, and V7. There are very likely other issues, but this is the first thing that stands out. Fella that gave it to me said it was blowing smoke and I believe this may have been coming from the 2.2K resistor in the power supply.

Voltages appear to be good at the rectifier V11, C27A and C27B, and the heater supply to the other tubes.

As far as I can tell I have 0v at C15A. Also, I'm not sure where the heck C16 and R52 are physically, nor can I put my finger on R61, but like I said C27 seems to be kosher.

I looked at the guts of another X-100 on another AK thread and it seems like my cans are wired correctly, as long as what I was looking at was wired correctly. I installed new hayseed cans thinking/hoping that C15 was malfunctioning and all my woes would be relieved, but I am encountering the same readings with the new cans. After looking at the cans more closely it almost appears as if someone may have already replaced C15. This new-ish looking can along with a piece of paper w/ the #3 on it inside the chassis are the only indication of previous "work" so far. Can't find any obvious re-work.

Below are some voltage readings taken with the amp on dbt.

C27-B: 345vdc (384vdc)
C27-A: 300vdc (320vdc)
**DO NOT SEE R61 across C27-A and C27-B?

C15-A: 0v
C15-B: 297vdc (265vdc or 285?)
C15-C: 300vdc (285vdc)
**pretty sure I had meter on correct terminals
**don’t see R52 or C16

Rectifier Tube V11 GZ34/5AR4 (ON DBT)
2: 5vac (5vac)
4: 540vac and climbing (across 4 and 6 730vac)
6: 540vac and climbing (across 4 and 6 730vac)
8: 345vdc (approx 384vdc)

I’ll do some more studying and snooping, but any help identifying why my four 12ax7's are not glowing is greatly appreciated.

Thank you for your time.

 

[dcgillespie]

Check the plate (pin 7) and screen (pin9) voltage at each output tube socket. They must be correct, and good output tubes installed for the heaters in those particular 12AX7 tubes to operate, because it is the current flowing through the output tubes that powers those heaters.

Dave

 

[goatropercu]

Check the plate (pin 7) and screen (pin9) voltage at each output tube socket. They must be correct, and good output tubes installed for the heaters in those particular 12AX7 tubes to operate, because it is the current flowing through the output tubes that powers those heaters.

Dave

Full transparency I did not realize the interdependency and had the output tubes pulled for fear of frying one. Below are the voltages with them installed and with the amp on dbt.

V4:
Pin 3 (cathode): 0 at power up, gradually climbs to around 10vdc (spec 44v)
Pin 7 (plate): 222vdc, climbs to >300vdc at power up then drops (spec 380v)
Pin 9 (screen): increased to 200vdc at power up then dropped to 180vdc (spec 320vdc)

V5:
Pin 3: same as above
Pin 7: 222vdc
Pin 9: 180vdc

V9:
Pin 3: same as above
Pin 7: 223vdc
Pin 9: 181vdc

V10:
Pin 3: same as above
Pin 7: 222vdc
Pin 9: 181vdc

Juncture of R82 & R83: 7.5vdc (34vdc)
Juncture of R84 & R85: 7.5vdc (34vdc)

With output tubes installed I no longer have filament glow at V3 and V8 and they're only getting around 4vac . Similar to the other voltages this has been observed starting off strong at initial power up, then fizzling down. The output tube cathodes behave in an opposite manner.

Output tube V5's Pin 3 (cathode) connects directly to tube V1 pin 5 (heater). As stated above this voltage is slow to rise, but eventually gets up to around 8-10vdc. I'm clearly no tube gear expert, but shouldn't that be sufficient supply for some glow action? Per the 12ax7 data sheet heater voltage "12.6v Series / 6.3v Parallel". V1 heater goes to V6, then V7, then V2, and then to chassis ground (just as illustrated in the schematic)

As stated I am using a dbt (40w bulb) in an effort to not smoke something critical, unobtainable, or expensive. I have had different experiences using a dbt on tube gear and in this case I'm not sure if it is helping or hurting. Surely it is causing the voltages above to sag, but I'm reluctant to rip off the band aid based on the fact that it was smoking previously. I have a variac, but rarely use it as I am under the impression that it will just tell me at what line voltage whatever smoked, smoked at. If I should give the variac a whirl please help me understand which voltage points are most critical to monitor.

Lastly, caps have been giving me more issues than normal lately and I can't help notice this big 20uf/350v ancient looking electrolytic. Any idea what this cap's purpose in life is and if it may have anything to do with anything? I'm going to replace it, but don't have anything suitable on tap. I see its positive terminal connects directly to pin 1 (plate) of V1. Speaking of which the plate voltages at V1, V2, V6, and V7 peaks around 200vdc at power up then settles in around 155vdc.

Thank you for your time.

 

[goatropercu]

Well I got bold and fired up on variac. I’m at 110vac, plate voltage at outputs is 326, and I am now getting some glow action on all 12ax7’s. No smoke yet either. More to come.

 

[goatropercu]

I would really appreciate it if someone could help me validate the value of this capacitor. It’s labeled “033K”. I do not see a strikingly obvious decimal point like those labeled “.039K”.

Is this a .033uf/250v?

 

[jvstratman]

Here's a shot of the x-100 schematic. C13 & C14 are .033uf @ 250 volt. Are yours connected to the tube socket pins as per the schematic?

 

[goatropercu]

Is that an excerpt from an X-100 derivative (A, B, C) schematic? The schematic I am looking at for the plain jane X-100 does not have values listed.

To your question they are connected to tubes V2 and V7 as follows:

Caps clearly labeled .039K/250v to pin 6
Caps clearly labeled .047K/250v to pin 3
Caps that I believe are likely labeled .033K/250v, but question if the spec i see is a decimal point to pin 1

Thank you.

 

[larryderouin]

It's a decimal point. Look at the upper of the two. Rotate it slightly by hand (it won't hurt it), and you can clearly see the decimal.

 

[goatropercu]

Thank you Larry. Just wanted to make sure. So .033uf/250v it is.

I was able to adjust the bias per the sm with ease. Controls cleaned. Moved to scope and dummy loads. Left ch seemed weaker than right and instead of a clean sine wave it was putting out what I would describe as a mountain range of peaks and valleys at about 1/4 throttle. I fiddled around with flipping switches and turning controls when suddenly the left ch got really noisy on the scope. I cleaned the controls again, but not banking on that to resolve the issue. Going to go ahead and order the remaining caps and see if I can get her motor firing on all cylinders.

Thank you all.

 

[jvstratman]

Is that an excerpt from an X-100 derivative (A, B, C) schematic? The schematic I am looking at for the plain jane X-100 does not have values listed.

To your question they are connected to tubes V2 and V7 as follows:

Caps clearly labeled .039K/250v to pin 6
Caps clearly labeled .047K/250v to pin 3
Caps that I believe are likely labeled .033K/250v, but question if the spec i see is a decimal point to pin 1

Thank you.

Whoops, wrong schematic.

I think this might be the plain x-100

https://www.google.com/search?q=fis...d=chrome-mobile&ie=UTF-8#imgrc=GWmlFx5QhSkmBM

C19 & C20

 

[goatropercu]

The good news...
After replacing the remaining caps the amp agreed to pass a clean signal on both channels. Bias was again adjusted with ease. I moved to test speakers and was immediately reminded that a fisher tube amp is tough to beat. It sounds pretty fantastic in my opinion, and is dead silent at no signal.

The questionable news...
Output tubes' plate (pin 7) voltages are around 408vdc (380vdc per spec) and screen (pin 9) is hanging around 352vdc (320vdc per spec). These seem a little too high for comfort. Per the 7189 data sheet looks like plate voltage of 400v is max. Any ideas? As far as I can tell all four output tubes are old fisher branded tubes from various countries of origin. I noted previously R61 that connects the positive terminals of C27A and C27B was not present, but the fact that this amp appears totally stock and another file photo of a seemingly stock X-100 is also without this component makes me believe it never existed in this amplifier; not even sure if that would make a lick of difference, but it would touch these two voltage supplies if installed.

Thank you.

 

[jvstratman]

It looks like (R61) is the green resistor tied to C27B. Is it 3 watts? Is it 2.2 kohm? If so, the other end ties to a terminal strip. Is there an orange wire on the other side of that terminal strip? If so, does it make a loop to the output tube pin 9's and then back to C27A?

Lots of ifs for sure but I'm trying to decipher from your attached photo in post #1.

If it does end up looped, maybe they did it because of the resistors physical size hampering the installation across the two capacitor tie points.

This is all a guess so...

 

[jvstratman]

Looked at your photo again.

Maybe there's 2 orange wires on C27A.
One to the output tube pin 9 and the other all the way back to the green resistors terminal strip. If so, they did it for lead dress to keep all the wires running as a bundle.

 

[larryderouin]

Rarely(Hen's teeth are more prevalent) does the physical layout of an amp match the schematic. So you DO have to go digging some to make sense of it.

Confirming jvstratman's R61 theory. My 101-B is set up the same way.

 

[goatropercu]

Thanks Gentlemen.

Should I be overly concerned w/ the high output tube plate and screen voltages?

I listened to it for several hours yesterday. Had a hard time unplugging it.

Thank you both for your help.

 

[larryderouin]

Check the heater voltage on the 12ax7's. V1/2/6/&7 should be right around 11V ea. Also check the voltages on pin 3 of the output's (44V). If those are good, everything else will fall into line. It's possible that your output tubes are past their prime too. Weak output tubes won't come up to load spec (44V). Less load there = higher plate and screen voltage.

10% variance from schematic voltages can be expected per FISHER due to component tolerances and variance.

 

[jvstratman]

Both my assumptions were feasible but shortcoming. After thinking about it once more and looking yet again at your photo, the main reason for those three power resistor's locations were to align them directly under all those strategically drilled chassis holes to allow heat to escape. We're looking at it upside down.