Output Tubes Biasing method - Luxman SQ-38FD

[brud]

Hello all

I am having difficulty in determining biasing method for the output tubes on the Luxman SQ-38FD. Is it Cathode or Grid biased. I feel it is Grid biased . If it is cathode biased how do I calculate/measure the Cathode Resistance ??

I would appreciate a hand here - thanks in advance.

I am wanting to check the bias on these output tubes.

 

[mrk229]

That schematic is pretty low res hard to make out some of the notations. It certainly appears to be grid bias.

To the right of the switch chart in the center of the page are the bias level pots, marked b? & b?. Looks like B4 for the top amp.

The pot in the center of the output tube grid circuits is the bias balance pot. Wiper to the bias supply in center.

Cathodes go direct to ground, so no easy to measure bias that I see.

Interesting circuit. Some of the tube heaters are in series and fed from that same DC bridge.
Quite hard to tell I can't make out the "V" numbers to see which ones are in series. Maybe the phono pre tubes?

 

[brud]

That schematic is pretty low res hard to make out some of the notations. It certainly appears to be grid bias.

To the right of the switch chart in the center of the page are the bias level pots, marked b? & b?. Looks like B4 for the top amp.

The pot in the center of the output tube grid circuits is the bias balance pot. Wiper to the bias supply in center.

Cathodes go direct to ground, so no easy to measure bias that I see.

Interesting circuit. Some of the tube heaters are in series and fed from that same DC bridge.
Quite hard to tell I can't make out the "V" numbers to see which ones are in series. Maybe the phono pre tubes?

Here is the schematic again - hopefully cleaner and easier to read . Thankyou so much for you input .

 

[brud]

Here is the schematic again - hopefully cleaner and easier to read . Thankyou so much for you input .
View attachment 2581466

Im sorry tge image is still poor - i found a better copy on Radio museum though

The tubes are a 50CA 10 with a 50v heater . I think i might use the output transformer resistances and measure centre tap to plate voltage to calculate cathode current and power dissipation to see if they ate biased correctly ? Your thoughts

 

[gadget73]

sure looks fixed bias to me. I see grounded cathodes and adjustment pots on the grid feed for what appears to be bias and balance. Doesn't appear to have resistors in the cathode circuit to monitor current. What does the factory say about it? Seems odd to me that they wouldn't have instructions. The Lux MB-3045's I worked on had a resistor between cathode and ground and it was just a matter of measuring across them to set bias and balance on the tubes.


if you have to measure across the output transformer you might be better setting the meter to current mode and reading current directly. The DC resistance in the transformer will skew that slightly but not significant enough to matter. Never been a big fan of that method either way though, don't really like poking into high voltage circuits unless there isn't a choice. I might modify this to have small value cathode resistors to make setting this easier and safer.

 

[mrk229]

Im sorry tge image is still poor - i found a better copy on Radio museum though

The tubes are a 50CA 10 with a 50v heater . I think i might use the output transformer resistances and measure centre tap to plate voltage to calculate cathode current and power dissipation to see if they ate biased correctly ? Your thoughts

Last time I saw a 50 volt heater was a 50C5 in an all American five AM radio. I found a little better schematic.

See the attached PDF.

I see the output filaments connected right across the ac line. I also see that this appears to be a Japanese 100vac amp. Are you using an appropriate device to drop the line voltage to 100vac?



Orange box shows the output tube filaments and the 100vac primary voltage rating.
Blue box shows what looks like V1 through V4 series string filaments.

I never thought of using the transformer resistance to measure the voltage drop, it certainly seems to fit the ohms law criteria and I suppose under no signal conditions that it would work,

Maybe some others will chime in.

 

[gadget73]

has resistors on each end of the 100v heater string, guessing thats how they make it work on 120v. I'm assuming different transformer or at least different taps for 120v operation too.

 

[brud]

Last time I saw a 50 volt heater was a 50C5 in an all American five AM radio. I found a little better schematic.

See the attached PDF.

I see the output filaments connected right across the ac line. I also see that this appears to be a Japanese 100vac amp. Are you using an appropriate device to drop the line voltage to 100vac?

View attachment 2581470

Orange box shows the output tube filaments and the 100vac primary voltage rating.
Blue box shows what looks like V1 through V4 series string filaments.

I never thought of using the transformer resistance to measure the voltage drop, it certainly seems to fit the ohms law criteria and I suppose under no signal conditions that it would work,

Maybe some others will chime in.

This unit has a 240Vac transformer for us here in Oz - all good here .

 

[brud]

Unfortunately there does not appear to be any service info on this model except the schematic . Unless anyone has found more service and calibration information ??

 

[gadget73]

possibly all 4 tubes in series plus resistors for 240v operation. 200v out of the heaters, drop another 40 in resistors. Makes more sense than dumping 140 in resistors at least.

I don't suppose you know what the plate voltage is on this? The datasheet gives us a few operating conditions for various voltages in push-pull so at least thats something to use as a target. Its rated to 30 watts of plate dissipation, thats a pretty stout tube. Looks like the schematic says 350 volts going into a bridge for the HV supply, so thats in the ballpark of 500vdc on the output if that is correct.



TDSL Tube data [50C-A10] (duncanamps.com)

I'm going to assume some things but it looks like for anywhere between 300 and 400 volts in AB1 push-pull you want 100 ma of idle current (says plate volts, but the unit is in mA). I'm also assuming thats for two tubes because 100ma at 400 volts is 40 watts plate dissipation on a tube rated for 30 and that just doesn't make sense. I don't speak Japanese so I'm just relying on the English notes on the sheet. Using that same current level at 500 volts would give 25 watts plate dissipation, which is acceptable but not exactly nice to the tubes. If you keep it under 45ma per tube it would make them live longer.

 

[brud]

possibly all 4 tubes in series plus resistors for 240v operation. 200v out of the heaters, drop another 40 in resistors. Makes more sense than dumping 140 in resistors at least.

I don't suppose you know what the plate voltage is on this? The datasheet gives us a few operating conditions for various voltages in push-pull so at least thats something to use as a target. Its rated to 30 watts of plate dissipation, thats a pretty stout tube. Looks like the schematic says 350 volts going into a bridge for the HV supply, so thats in the ballpark of 500vdc on the output if that is correct.



TDSL Tube data [50C-A10] (duncanamps.com)

I'm going to assume some things but it looks like for anywhere between 300 and 400 volts in AB1 push-pull you want 100 ma of idle current (says plate volts, but the unit is in mA). I'm also assuming thats for two tubes because 100ma at 400 volts is 40 watts plate dissipation on a tube rated for 30 and that just doesn't make sense. I don't speak Japanese so I'm just relying on the English notes on the sheet. Using that same current level at 500 volts would give 25 watts plate dissipation, which is acceptable but not exactly nice to the tubes. If you keep it under 45ma per tube it would make them live longer.

Thankyou ! I plan to measure the plate volts today . I was also going to measure the overall resistance and the centre tap to plate . Also the voltage from centre tap of output transformer to plate . Then divide this by the centre tap resistance to give me the plate current .
There is a pot on what i believe is the grid bias of the output tube .
Thoughts on this ?

 

[gadget73]

looks like one pot per channel to set the grid voltage, so that will change your output tube current. I can't quite read what the control numbers are but they are both there on the lower left corner of the power supply section of the schematic.

also looks like a balance control for each side too. That should be set for equal current, but you should also be able to measure hum at the output and null it with that. Minimum hum = equal current.

 

[brud]

looks like one pot per channel to set the grid voltage, so that will change your output tube current. I can't quite read what the control numbers are but they are both there on the lower left corner of the power supply section of the schematic.

also looks like a balance control for each side too. That should be set for equal current, but you should also be able to measure hum at the output and null it with that. Minimum hum = equal current.

Further investigation today:

I have measured the Plate voltage it is around 460V - for an LIne Voltage of about 248Vac.

I have also measured the Grid voltages into the Output tubes it varies between -48 to -54 vdc .

It appears that the pots on the power supply supplying b4 and b5 ( output tube bias pot) supply the input of the Channel grid bias adjustment pot.

I used the input transformer method to determine plate current:

1. Measured from diode bridge output to plate resistance .
2. Measured voltage from centre tap of output transformer to plate of each tube .
3. Calculated plate current by dividing 3/1 .

However I have silly numbers 8-14mA !

?? Should I place my DVM in series with the plate supply and set to current mode and measure directly .

The othe interesting issue I found was one of the 50CA10 output tubes appears to have a faulty grid. When measuring the grid voltages 3/4 measured as above however the 4th one measured silly number 47mV. I checked the voltage into the. 100k resistor it was -47v. I then moved the valve to 2 different spots and this issue moved with me ??? Open grid on this tube

If I do place my DVM in series with plate voltage and measure current , then as you suggest set grid voltage to obtain a plate current of -45mA ??

The other question - where should I set the voltage on the pots delivering b4 and b5 supplies ?

Really appreciate your help here - learning heaps

 

[gadget73]

wouldn't be an open grid if the voltage is zero at the grid end of the resistor but normal at the supply end. Thats a grid to ground (cathode) short. Fair guess you can stick an ohm meter on the tube directly and confirm that.

personal opinion, I'd break the connection from cathode to ground and install small value resistors to measure the current across. No need to fool around with breaking a high voltage circuit or any of that. If you use precision 1 ohm resistors, mv = ma, so 50mv = 50ma. No muss, no fuss. Setting the grid voltage doesn't really get you a known current unless its a known good, strong tube that lines up well with datasheet conditions. If its old, all bets are off.

unfortunately you may have a whole set of junk tubes here.

 

[brud]

wouldn't be an open grid if the voltage is zero at the grid end of the resistor but normal at the supply end. Thats a grid to ground (cathode) short. Fair guess you can stick an ohm meter on the tube directly and confirm that.

personal opinion, I'd break the connection from cathode to ground and install small value resistors to measure the current across. No need to fool around with breaking a high voltage circuit or any of that. If you use precision 1 ohm resistors, mv = ma, so 50mv = 50ma. No muss, no fuss. Setting the grid voltage doesn't really get you a known current unless its a known good, strong tube that lines up well with datasheet conditions. If its old, all bets are off.

unfortunately you may have a whole set of junk tubes here.

Thankyou for advice

would a 2watt resistor be ok for the job ?

what do you mean “Setting the grid voltage doesnt …..

thanks again

 

[gadget73]

1/4 watt would be sufficient, these will dissipate no significant amount of current.

What I mean by the grid voltage is that just because its at a particular voltage doesn't mean the tube is flowing a specific current. There are tube curves and all that will tell you given plate voltage X and grid voltage Y, it will flow current Z but once tubes get well used that doesn't necessarily apply anymore. I tend to dwell more in the land of half used-up stuff and at that point taking a measurement is a much more reliable way of figuring out whats going on vs using the graphs to speculate at what it should be doing.

you can however compare your measurements to the graphs and figure out how far it is away from what it should be to make some determinations about whether the tube is any good or not.

 

[brud]

1/4 watt would be sufficient, these will dissipate no significant amount of current.

What I mean by the grid voltage is that just because its at a particular voltage doesn't mean the tube is flowing a specific current. There are tube curves and all that will tell you given plate voltage X and grid voltage Y, it will flow current Z but once tubes get well used that doesn't necessarily apply anymore. I tend to dwell more in the land of half used-up stuff and at that point taking a measurement is a much more reliable way of figuring out whats going on vs using the graphs to speculate at what it should be doing.

you can however compare your measurements to the graphs and figure out how far it is away from what it should be to make some determinations about whether the tube is any good or not.

I understand , its really easy to do the resistor . I just checked the tube that had now grid voltage - it did show 1-2 ohms between grid and cathode , all the others showed open cct. Looks like I will need to buy a nos set of 50CA tubes , probably a good idea as they are becoming rarer.

If the output tubes have had a lot of use would it be a reasonable finding to find low Plate current value for a specified negative grid voltage at idle - no signal in ?

I will let you know how I go , I really appreciate your input.
Cheers
Bruce
Oz

 

[gadget73]

yeah, typically weak tubes will flow less current than the chart says they should. Power output test would confirm it, weak ones just won't make rated output. I have a set of 6CA7 tubes that bias OK but won't make more than 7 watts. Good tubes in the same amp make 40 watts, so just relying on the idle current as a measure of tube strength isn't the final word.

should be open from grid to cathode, 1-2 ohms is a dead short. That would have flowed a whole lot of current. The tube would basically be a rectifier at that point, no control at all.

 

[brud]

yeah, typically weak tubes will flow less current than the chart says they should. Power output test would confirm it, weak ones just won't make rated output. I have a set of 6CA7 tubes that bias OK but won't make more than 7 watts. Good tubes in the same amp make 40 watts, so just relying on the idle current as a measure of tube strength isn't the final word.

should be open from grid to cathode, 1-2 ohms is a dead short. That would have flowed a whole lot of current. The tube would basically be a rectifier at that point, no control at all.

thankyou . While i have your ear

i have always struggled as to what input voltage i should run my signal generator when running my 1KHz test sine wave into the amp to check to output voltage/watts. What value do you use and does it vary with tube vs solid state gear ?

 

[brud]

thankyou . While i have your ear

i have always struggled as to what input voltage i should run my signal generator when running my 1KHz test sine wave into the amp to check to output voltage/watts. What value do you use and does it vary with tube vs solid state gear ?

And us that value RMS or pk-pk