500C Weak Bias Current - Is it tubes?

D

davefr

Active Member
Guys,
I'm giving my 500C a quick once over prior to a full rebuild. (I want to start from a known good state!) I'm setting my variac to get 430 VDC as measured by pin 3 on V8,9,10,11 (7591's). Variac set at 117 VAC to get 430VDC. (Pins 8 really close to 375 VDC)

I have 10 ohm resistors on pins 5 to Gnd to measure cathode current.

Right channel pair is closely matched at .160 mv (16 mA). Left channel pair is pretty well matched at .190 mv (19 mA). Isn't that rather weak bias current?

Tubes are old Realistic and RCA but they pass with plenty of margin on my TV-7U (36 is pass/fail threshold and they're all around 50)

I plan to do IBAM/IBBA as part of the rebuild but will it allow me to get bias up to the sweet spot of around 30 ma? Are my old tubes simply weak? I doubled checked the entire output circuit and all components check out just fine.

Receiver works and sounds just fine. Anything else I should be looking at?

Thank you!!
 
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16ma @ 430V = 6.88W Total dissipation
19ma @ 430V = 8.17W total dissipation

Subtract 3ma for screen dissipation
13ma @ 430V = 5.59W
16ma @ 430V = 6.88W

If your pin 6 voltage is about -17v These tubes are literally "Rode Hard and put away Wet" , or 6 pins in the grave and 2 pins on a Wet banana Peel". Already have a lot of distortion and you can't push them hard before they break up (sound wise).

Minimum ma idling before increasing distortion is about 28-29ma. You want wattage (with all the upgrades in the unit) to be between 70% and 80% of Max Wattage of 19W. 13.3W to 15.2W (Shoot for 14.3-14.6) 37ma Total per tube gets you closest. 37ma-3ma =34ma 430x0.034= 14.2 W per tube.

IBBA/IBAM will help these tubes some as you can drop the Pin 6 from -17V down toward Zero some, (I don't like going below -12V) but don't go below -10V. I suspect you'll probably get down to about -12 to -13v to get them somewhere in the low 30ma range. Better off getting a quad of Tung-Sol Re Issues(if you must have the original envelope size for aesthetics) or EH's (if you don't mind the slightly larger envelope size), along with the IBBA/IBAM. Note that the Tung-Sols need a more negative voltage than the originals or EH's (for example my EH's Range from -16.5V to -18V to get 37ma, while a set of T-S need -21 to -23v on average.
 
Thanks Larry,
I tried a new set of EH7591's I had on hand and they all biased at around 29-30 ma each @120 VAC. Looks like my old tubes are nearly worn out. I hope these new tubes fit the Fisher cabinet.
 
EH tubes run at a higher current for a given grid voltage. If the grid supply is overly negative it may just be causing the tubes to run too low without them actually being bad or weak. Need to know what the voltage at the grids is.

Also would be good to know the voltage at the screen, since that has major influence on current flow as well.
 
The schematic says Plate, Grid and Screen should be 430/375/-17

At 117 VAC I'm seeing 422/390/-18.5 with the old set of tubes back in. There's negligible variance from tube to tube except for bias. The RCA's are a lot weaker then the Realistics.

I guess IBAM/IBBA could get me some more bias current out of the old tubes but I'll probably just use the EH tubes and use IBAM/IBBA to get them to around 37ma. (if they fit my cabinet)
 
its that extra volt and a half of negative bias thats causing them to do this. If the selenium rectifier was changed, I expect thats where the extra voltage is coming from.
 
gadget73 said:
its that extra volt and a half of negative bias thats causing them to do this. If the selenium rectifier was changed, I expect thats where the extra voltage is coming from.
Click to expand...
Yes, a silicon rectifier replaced the original one a few years ago. At 117 VAC Variac setting the output at the bridge is -25V vs -22V (schematic). At line voltage it would be even lower.
 
usually there is a resistor added in there to drop the voltage back down. Also keep in mind this feeds the phono tube heaters, not good for those to get excessive voltage. Stock they were starved a bit, probably for noise reasons.
 
Thanks. Adding the IBAM/IBBA mod will take care of 7591 Bias but I haven't read anything about downstream tweaks needed for the V14, V15, V16, V17 heaters. If the silicon bridge is outputting 25VDC that means each 12AX7 phono tube is getting 12.5 V vs 11VDC. Is that too much or non optimal for noise? Should I add a resistor in series with the 12AX7 heater feed to get them back down to Fisher's spec. of 11V?
 
IIRC a 27ohm 1/4W resistor with a 47uf 50V cap. Someone check me on this please.

Bias to tube heaters .jpg

As for Heater voltage, FISHER Starved the tubes (less noise and greater longevity were main results of this). Really you can run the tubes on the 12V circuit @ 12.6V + or - 5% and they'll be fine. I prefer a little less than prescribed 12.6v nominal voltage, right around 12.0v to 12.2v. Your range (5%- to 5%+ is roughly 11.97 to 13.23V) But you CAN EXPERIMENT with LOWER Voltages. On some of the slightly older receivers the phono amp tubes can see about 10V. Takes forever and a day to warm them up (but it works well.) Rumor has it that Human gestation of 9 months co-incides with the 10v Warm up of these tubes.
 
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larryderouin said:
IIRC a 27ohm 1/4W resistor with a 47uf 50V cap. Someone check me on this please....

As for Heater voltage, FISHER Starved the tubes (less noise and greater longevity were main results of this). Really you can run the tubes on the 12V circuit @ 12.6V + or - 5% and they'll be fine. I prefer a little less than prescribed 12.6v nominal voltage, right around 12.0v to 12.2v. Your range (5%- to 5%+ is roughly 11.97 to 13.23V) But you CAN EXPERIMENT with LOWER Voltages. On some of the slightly older receivers the phono amp tubes can see about 10V. Takes forever and a day to warm them up (but it works well.) Rumor has it that Human gestation of 9 months co-incides with the 10v Warm up of these tubes.
Click to expand...

Some time ago I had a brief exchange with Dave on this subject. Like Larry, I settled for slightly cooler operation. You really have to play with the resistor until you're happy:

Fisher 12AX7 Heater Voltage
 
BTDT with my cell phone. Never call it. Then when some chair warmer asks me for my # I give them the house #. "Do you have a cell phone #??" while he's looking at it on my hip..............!!! Spend 5 minutes looking thru junk trying to find # end up asking them to just use the house #. Uh, what was the question again.............:naughty::naughty:
 
I implemented IBAM and now all my tubes are right at 32 mA at 120 VAC. I also added the 10 ohm resistor to the downstream 12AX7 heater feed and it reduced the voltage from about 26V to 23.5V for each pair. I also did the screen resistor mod. Now just waiting on Hayseed caps for the rest of the rebuild.

P1100278 (1).JPG
 
I would also ask -- now that the quiescent current is correct for each tube, what is the value of the negative bias voltage that is being provided to each tube from the IBAM circuit to achieve that current flow?

Dave
 
At 120 VAC line voltage, I'm seeing 423 V on the plate and -15.7 V on the Pin 6 grid. That's producing bias that build up to around 32 mA as it warms up for a few minutes. I would like to choose a fairly conservative Bias to preserve the tubes and try and minimize heat. I doubt I'll be going for all out performance with this unit since my speakers are high efficiency Klipsch Fortes. Is there an optimal conservative Bias setting you'd recommend.

Thanks!!
 
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423 x .032 = 13.53W per tube. about 71.24% of max dissipation. Not accounting for the screens. -3ma.

Accounting for the screens 423 x .029 = 12.26W per tube. or 64.5% of max dissipation. Right now you're flirting with increasing distiortion as the dissipation drops @ about 28ma.

Bring your plate dissipation up to 35ma, which will give you 32ma total dissipation (accounting for screens). You'll be at 32ma total, or 13.53W per tube. You'll be in the 70% to 80% ideal range. Which is good for both OEM and Russian tubes.

Don't rely on the pin 6 value except as a warning as you get closer to ZERO VOLTS. With the IBBA, Balance and bias the tubes evenly ( equal load distribution). Fisher originally used highly matched tubes that met their specification on output's and could get away with just setting the pin 6 voltage. Some tubes draw more and some less current. With tubes becoming less matched from dealers, and / or using random tubes , the IBAM and IBBA were born, which allows tubes that aren't closely matched at a set point to become matched by current load. So the pin 6 voltage, as long as it's a NEGATIVE VOLTAGE, is not a primary concern. I make the cutoff for my 7591's and 7868 running 75% of max dissipation -10v to -11v on pin 6. Then I use them for plinking or as test mules for proving circuit when I do rebuild one.
 
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