Fisher 500 mono output tubs running hot?

[MrVelvito]

Happy Holidays everyone!

I'm a newbie to this hobby and am enjoying it a lot, but I could really use some guidance on how to proceed.

BACKGROUND:
I rescued a 500 mono chassis out of an old console that was in terrible shape and quite moldy. To this chassis, I:
1) recapped it
2) put in a 3-wire outlet cord
3) put in a thermistor to limit inrush current

When I powered up the unit (slowly on a variac) before the recap, one of the 5881 output tubes got much hotter to the touch than the other. To my pleasant surprise, the unit worked and I got sound out of FM. After the recap, the same output tube ran hot.

I swapped the two output tubes in the chassis and the one that ran hotter in the old location ran hotter in the new. Soon after the swap the sound faded away and now both 5881s test bad for emissions.

I purchased new Sovtek 5881s but am hesitant to install them for fear of burning them up, too.

The schematic I have shows that output tubes (KT66s/EL34s) running with a plate voltage of 400V and a grid voltage of -37V. Before they died, my 5881s were running with:

Plate on V11: 521V
Plate on V12: 515V

Grid on V11: -24.4V
Grid on V12: -23.5V

Which seem too high for both the plate and grid according to the 5881 spec sheet (if I'm reading it correctly).

ADVICE NEEDED:
So... does someone know what the plate and grid voltage should be for the 5881s in a Fisher 500 mono? If I'm running too hot, what can I do to fix the problem? Other recommendations?

Any help would be greatly appreciated!

TIA
-MrV

 

[electrode101]

For the 6L6 tube family in that circuit, you need that -37volts of grid bias or the tubes will draw way too much current, and as you observed, self destruct.

In checking the schematic, the bias voltage is developed between the junction of the main filter cap, and the bias cap. This junction is biased with a 400ohm resistor, and feeds the filament string of tubes v8-v10. The output of this voltage source is -37 volts.

This voltage is sent to the output tubes via a 33K resistor, to the junction of the two 470K grid resistors of the output tube to provide the bias. If any one of these resistors is out of spec, including the one in the power supply, you'll end up with insufficient bias.

I've checked the circuit schematic with 5881's vs El34's, and the values are the same.

Good luck!


jD

 

[MrVelvito]

Thanks electrode101!

I'll check out the schematic and try to find those resisters in the actual chassis. I'm very green with this stuff so it may take me a while...

Really appreciate the answer.
-MrV

 

[whoaru99]

The plate voltages listed at over 500V were with all the tubes installed?

 

[MrVelvito]

Greetings whoaru99,

Yes, all tubes were installed.

Happy New Year!,
-MrV

 

[MrVelvito]

For the 6L6 tube family in that circuit, you need that -37volts of grid bias or the tubes will draw way too much current, and as you observed, self destruct.

In checking the schematic, the bias voltage is developed between the junction of the main filter cap, and the bias cap. This junction is biased with a 400ohm resistor, and feeds the filament string of tubes v8-v10. The output of this voltage source is -37 volts.

This voltage is sent to the output tubes via a 33K resistor, to the junction of the two 470K grid resistors of the output tube to provide the bias. If any one of these resistors is out of spec, including the one in the power supply, you'll end up with insufficient bias.

I've checked the circuit schematic with 5881's vs El34's, and the values are the same.

Good luck!


jD

electrode101,

In the schematic that you attached, I read the 400 ohm resistor to be R85, the 33K ohm resistor to be R76, and the two 470K ohm resistors to be R75 and R78. In the attached bottom view of the chassis from the Photofact Folder, the resistors and their general location are identified. Going to those locations, I didn't find the resistor values that I expected.

Instead of: I found:
======== ======
400 ohm 920 ohm (R85)
33K ohm 68K ohm (R76)
470k ohm 330K ohm (R75, R78)

The resistors look original. Could the manufacturing plant vary the components this much?

I measured the resistance of R76, R75, and R78 with the output tubes removed, They measured close to their stated values; R76 - 68K ohm, R75 and R78 - 343K ohm and 345K ohm respectively. R85 I tested in place (without desoldering one lead) and it read 43 ohms. I assume that I have desolder one leg to get a more accurate reading, right?

TIA and have a Happy New Year,
-MrV

 

[electrode101]

They are all off. the 33k resistor @68K is double the original value. The 470k's are at least 20% off. While I don't think these values alone are responsible for you insufficient bias voltage, I would still change them.

You said you replaced the capacitors. Were these just the power supply caps, or the audio caps as well?

The relevant caps here would be the output stage coupling caps, C69, C70. If these are the original caps, or even not very good ones, they are probably "leaky" which means they'll pass DC from the previous section, which would be a positive voltage, reducing the bias on the grid of the output tubes.

If you want to test them, remove the output tubes, turn on the unit, and check the bias voltage at the power supply side of R76, and the junctions of C69, R75, and C70, R78. Then power down the unit, remove tube V10 and try again. note if there are any changes in the bias voltage.

Report back.

 

[MrVelvito]

HYN, electrode101.

Hope you had a good one!

They are all off. the 33k resistor @68K is double the original value. The 470k's are at least 20% off. While I don't think these values alone are responsible for you insufficient bias voltage, I would still change them.

I'm a little concerned that these values are close to their stated resistance, but different from the schematic (if I'm reading it correctly, which I may not be as I have a hard time correlating the physical layout to the schematic).

You said you replaced the capacitors. Were these just the power supply caps, or the audio caps as well?

I replaced most of the capacitors, both audio (orange drops, brown drops) and power supply (MIEC). All caps were purchased from justradios.com. I did not stuff the metal electrolytic can but put the caps on the inside of the chassis. This took up too much room and I did not have the space to replace the paper tube canister. I measured the paper tube canister caps with and ESR meter. They're supposed to be 40 mF but turned out to be about 72 mF each. The ESR value for both caps were within tolerance according to the ESR meter manual, 1.39 ohm and 1.35 ohm.

The relevant caps here would be the output stage coupling caps, C69, C70. If these are the original caps, or even not very good ones, they are probably "leaky" which means they'll pass DC from the previous section, which would be a positive voltage, reducing the bias on the grid of the output tubes.

I replaced the old C69 and C70 caps with new orange drops. The ones that I removed were .1 mF. The schematic shows that they should be .047 mF. What's going on? Are we working off the wrong schematic? The resistors that I looked at (see my first response) and now these caps do not match up with what the schematic says. Do you have any ideas?

-MrV

 

[Tom Bavis]

EL34 and KT66 were never used in the 500 - only EL37 or 5881 - mistake on Sams part. Fisher had a service bulletin, I believe, to increase the bias voltage when 5881 were used, adding a 50 Ohm 2W resistor in series with the cathodes to ground. There's a K-100 manual (console version of 500, with an eye tube) on my website: http://www.audiophool.com/Schem_A/Fisher_K100_SM.pdf. This shows -31V bias, probably right for EL37, but Sams shows -36V for 5881. The three preamp tube heaters are the negative B+ lead, giving around -36V for bias - if it's higher, then add a cathode resistor to reduce idle current. It shows 395V B+, which would go higher with low-emission output tubes (and preamp tubes would be barely lit!), The original 5881s were rated for 23W, and probably were run close to that (55 mA @ 400V)

 

[electrode101]

The bias voltage shown for the 5881 is the same for a 6L6GC, however the 6L6 has a higher dissipation rating, and will provide more output power.

Still, the bias reading the OP measured of -24 shows a fault, and that bias will not work with either the 5881 or EL37.

The difference between the .047 and .1uf caps is minor, however it's never a good idea to change these inter stage caps far from their designed values as you may run into instability issues. You're probably ok here.

Have you checked the 135voltage source that's at the end of the string from the 6x4 rectifier?

Also, try removing V10, V6, and V* and measure the bias voltage again. This needs to be fixed before you can move on.

jD

 

[MrVelvito]

Thanks Tom and jD!

Just got back in town and will dig into your recommendations.

 

[MrVelvito]

Okay, here are the #s that I came up with after testing the unit:

Note, the 500 was plugged into an isolation transformer when these measurements were taken. Also, the old 5881 tubes that are bad were in the unit for all tests.

6X4 rectifier (V14):
-plate pin #1: 203VAC -- schemaitc says 165VAC
-plate pin #6: 203VAC -- (ditto)
-cathode pin #7: 186VDC -- schematic says 180VDC. The voltage rose to 197 volts and then slowly stabilized around 186 VDC
-135V source -- 135VDC

R80 (270 ohm) -- 312 ohm
R81 (270 ohm) -- 287 ohm, 135V source measurement taken after this resistor

5881 output tubes:
-base bias V11, pin #5 -- -12.5V, the bias voltage took a very, very, VERY long time to reach these levels. The bias made it as low as -15V and then started rising to the levels indicated.
-base bias V12, pin #5 -- -12.5V, (ditto)

-with V10 (Mullard 12AX7) removed:
-V11, pin #5 bias -- -45V
-V12, pin #5 bias -- -45V

-with V6 (noname 6AU6) removed and V10 re-installed:
-V11, pin #5 bias -- -10V
-V12, pin #5 bias -- -10V

Please let me know if I tested the unit improperly. Do these numbers point to a source of the problem?

Again, thanks for your help,
-MrV

 

[ivan100000]

Good luck with your 500; I think it's the best vintage Fisher value. I've owned many 400/500/800 stereo receivers from the 60s over the years, and it's this TA-500 with the quintessential tube sound I held onto. The tuner is exceptional, and I miss living in Los Angeles where I would listen to K-Jazz 88.1 before going to bed. The sound through my KLH Six speaker was magic; it was as if a small jazz combo was set up in my apartment. Too bad that I'm living on a small island now with poor radio reception. So it goes.

 

[MrVelvito]

Good luck with your 500; I think it's the best vintage Fisher value. I've owned many 400/500/800 stereo receivers from the 60s over the years, and it's this TA-500 with the quintessential tube sound I held onto. The tuner is exceptional, and I miss living in Los Angeles where I would listen to K-Jazz 88.1 before going to bed. The sound through my KLH Six speaker was magic; it was as if a small jazz combo was set up in my apartment. Too bad that I'm living on a small island now with poor radio reception. So it goes.

Greetings and felicitations Ivan100K!

I really like this amp, too! It's my first endeavor into this hobby and, as can be expected, I'm in a 'learning curve,' which is the PC way of saying that I don't know what the h*ll I'm doing...

Funny that you should mention the jazz combo, when I first slowly turned on this amp, it was set on FM and to the local station that plays a lot of music from the 30s, 40s and 50s. The TA-500 worked at that point and played that radio station. It was magical, listening to music out of the 40s on this old amp. Unfortunately, the output tubes didn't like the amount of voltage they were getting and decided to quit. I'm trying to solve that problem now with the amp.

Hope all is well with you and yours,
MrV

PS. Here's a thought -- if you have a halfway decent internet connection, you can pick up KKJZ 88.1 on your computer (they broadcast over the internet) and then feed that to your TA-500.

 

[danno2000]

They are all off. the 33k resistor @68K is double the original value. The 470k's are at least 20% off. While I don't think these values alone are responsible for you insufficient bias voltage, I would still change them.

You said you replaced the capacitors. Were these just the power supply caps, or the audio caps as well?

The relevant caps here would be the output stage coupling caps, C69, C70. If these are the original caps, or even not very good ones, they are probably "leaky" which means they'll pass DC from the previous section, which would be a positive voltage, reducing the bias on the grid of the output tubes.

If you want to test them, remove the output tubes, turn on the unit, and check the bias voltage at the power supply side of R76, and the junctions of C69, R75, and C70, R78. Then power down the unit, remove tube V10 and try again. note if there are any changes in the bias voltage.

I also am working on a 500 mono , with the first 12ax7 pulled I have -36 volts , once the 12ax7 are back in the chain it drops to -10..weird though when the power tubes are in I get about -27v...so I had to install 100 ohm resistor on each EL37 cathode, this get the bias down in the 55-58ma range...why wouldn't Fisher have a dedicated tap off the transformer and create the neg bias...I ohmed out the heater wiring and all seem oj, I am going to replace the wiring and I even tried swapping out different 12ax7's think it might be one bad heater causing to load down the -bias circuit.