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Sansui 1000a trouble

I plan on replacing with the new Tung Sol when I am finished with the recapping. The power tubes look rough and my tube tester does not have the settings for 7591 to test.
I do have a question one channel is weaker than the other. I have switched the tubes with no difference. I have not replaced all the caps yet but have done all the coupling caps and most of the tone section.
Could this be the issue or should I be looking elsewhere or just be patient until I complete the recap? Mine is the 4 pot version
Thanks
 
jcon109 said:
I plan on replacing with the new Tung Sol when I am finished with the recapping. The power tubes look rough and my tube tester does not have the settings for 7591 to test.
I do have a question one channel is weaker than the other. I have switched the tubes with no difference. I have not replaced all the caps yet but have done all the coupling caps and most of the tone section.
Could this be the issue or should I be looking elsewhere or just be patient until I complete the recap? Mine is the 4 pot version
Thanks
Click to expand...

Could be biasing, could be a weak cap, weak tube, dirty pots, lots of possibilities..
 
Ok replaced two 200uf caps on the 4 pot board seems like the channels are even now but now I have a tube red platting. I checked pin 5 to ground and my voltages were very high

.134
.071
.071
.135 then while adjusted it down heard a pop and started red platting and now measures .204
Help!
 
Found a cold solder joint on the 200uf cap no more red platting but the bias is still high not sure i can adjust it down that low or am I measuring it wrong?

The pots seem to be a little flaky to me.

I have the input set to aux, volume down then ground the DVM on the case and the measure on pin 5 with the 10 ohm resistor.

thanks

JC
 
Had a similar red plating issue , it was one of the biasing resistors soldered on the back of the Tube socket feeding the grid coming apart. The metal moulded case attached to the lead had become weak over time and came apart from the resistor when touched. Just worth checking.
 
mano

You are correct it was the bias resistor not the cold solder joint I thought it was. Just touched it and the end fell off.

Still having trouble with the bias adjustment and one channel weaker than the other. The strong channel I can only adjust down to .060 on both tubes. On the weak channel I can get it to the .030 to 032 like Larry suggested. I tried setting all 4 tubes set the same @ .050 and the right channel still much weaker. Swapped 6an8, 7591 and the 12ax7 with no change
 
Finally got back to work on this. All capacitors replaced and both channels are equal now, no red platting but still have an issue with trying to adjust bias on one channel.

I have about 490 volts so I am looking for .029 to .032 but the pots on the left channel will only go to .048 On the right channel I can adjust to .032

My question is this a problem with the pots or is something else causing the issue and will this cause any problems running it that high?

Oh by the way, this beast sounds incredible with my JBL C40's
Thanks

JC
 
Combination of the flaky pots, redplated tube (replace), and probably some more of those metal capped resistors. Check them all and replace as necessary. For the pots I'd use multi turn bourn's sealed pots. They aren't so touchy, and you can dial them in without overshooting.

Start at the power supply and check the voltages at each component (L&R SIDES). You might have too much of a bias voltage (closer to ZERO) on one side than the other. It should be ideally about -20 to -21v then adjusted at the pots to get you .32 across the cathode resistors.

Are you still using the old output tubes or the new ones. If the old ones, they aren't pulling down enough to bring the plate voltage down to about 475-480. Mine when it warms up settles at 478 and stays there unless I'm really pushing it, then it'll sag a bit at the plate.
 
jcon109 said:
My question is this a problem with the pots or is something else causing the issue and will this cause any problems running it that high?


JC
Click to expand...
JC
I know you have swopped the Output tubes, Just try swopping again now that you have sorted the bias to these. I had similar issue with the aged output tube characteristics when I resurfaced into the Tube technology after my self exile of 30years. I was almost ripping my head out until I realised the tube itself was so...so unmatched and causing the correct bias current to be unattainable.
Mano
 
Mano
I will swap them again and recheck the voltage. I guess running them until my new tubes come in is really not a big deal.

Larry
I will check all voltages again and let you know. The pots need to be 10K correct?

Thanks for the help!

JC
 
Yeah 10K. Try moving the two front tubes and resetting the bias. With the non matched tubes you may now have closer matching pairs which mano alluded to. Sometimes with really worn tubes you may not be able to get down to the spec'ed numbers. If you can't get them below 35ma, shut it down and wait for the new tubes.

I've got a quad of Old Toshiba 7591 I use just for testing circuits. They won't give me any more than 20-22 ma. Which is enough to prove out the circuits. Then I throw in the good tubes and bring them rightup. Too high is also possible, epecially with a high BIAS B- (less than -14V and closer to zero). You risk tube and transformer damage running them too high. Make sure you have the 10 ohm cathode resistors to ground on pin 5.

Larry
 
I have the 10 ohm resistor installed. Looks like I need to order some new tubes! I am thinking about the new Tung Sol, I just want to make sure everything is running ok before I install new tubes. I will swap some tube around and see if it makes any difference.

Thanks

JC
 
Worn out tubes will not load down like new or good tubes, giving you spurious readings. The Tung-Sol 7591 Re-Issues are really nice in the 1000A compared to the EH's. They both sound good, but the T-S's with the smaller bottle size, radiates a little less heat, and the bottles aren't right up against each other like the EH's. They do tend to bias hot, so you'll need to watch where you bias them. I'd recommend no more than 32ma which is a good setting with the 470-480V plates on the 1000A. When I mean hot I mean for a given bias voltage they tend to output more than comparable new tubes. For instance on my 1000A @ -21v my EH's bias about 27 to 28ma. While the T-S's bias 32ma-33ma for the same voltage.
 
Ok

I received my second set of Tung-Sol's today, installed and set bias to:

-19.22 vdc on pin 6
32 ma
32 ma
32 ma
32 ma
Steady for 1 hour no deviation on voltage.
I cannot thank everyone enough for the help. I now have a much better understanding of tube technology... still have a lot to learn though.

I think I put this one in the rotation!!!

JC
 
GOOD JOB JC! You've got it on the Run.


What's your plate voltage (pin 3). Take that and multiply by ma (in your case .032 for all tubes) to get watts per tube. It's approx and will vary as the plate voltage changes.

70% to 80% of the tube's maximum dissipation of 19.5w is a recommended good safe operating area. This will give you between 13.65 and 15.6 watts per tube.

Check bias once daily for the 1st week or so. After that when the mood strikes. I find that the new T-S's and the EH 7591A are very stable as far as setting the bias and keeping it.

Larry
 
Thanks One of the best looking airplanes ever built!

After a few days of running I have an issue, I have static and popping in the left channel. I comes and goes. Looks like when I touch the front panel in different locations or move the control pots up and down a little I can make start or stop. I originally thought it was in the volume control but seems to be several places along the front panel. Any thoughts?

JC
 
jcon109 said:
Thanks One of the best looking airplanes ever built!

After a few days of running I have an issue, I have static and popping in the left channel. I comes and goes. Looks like when I touch the front panel in different locations or move the control pots up and down a little I can make start or stop. I originally thought it was in the volume control but seems to be several places along the front panel. Any thoughts?

JC
Click to expand...

Could be a tube or a dirty switch - those are the two likely culprits. Try swapping tubes in the pre amp and phase inverter stages, and then if it remains, clean the switches and pots. Sometimes it takes a couple cleanings to get it all sorted.
 
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