Seduction + C4S eating tubes. Compatibility?

[tiller]

Hi folks,

Been running my seduction for about 6 years now and really love it. Listening to it as I write this post, actually! Lately I've decided to try some new tubes as I've been running the EH 6922 that came with the kit since I built it. So, I picked up some NOS Tesla VRS 6DJ8 -- nothing fancy, just experimenting here.

Anyway, the seduction has killed 3 of these tubes in as many weeks believe it or not. One has begun with an unacceptable amount of hiss, the other is coming in and out sporadically, and the third is plain dead. What's odd is that the original EH tubes continue to run totally fine.

I was under the impression the Teslas should be a drop in replacement for the 6922 tubes. Is the Seduction more picky than I thought? Any suggestions for diagnostic checks?

Thanks!

Adam

[fullheadofnothing]

My vague recollection is that the operating point is more closely optimized for 6922 than 6DJ8. I also seem to recall that incoming voltage plays a role. Hopefully someone who knows more than me will chime in.

[Doc B.]

You might want to measure the voltages with the Teslas installed and see if there are any variations. In particular I would look at the heater voltage measured across pins 4 and 5 of either tube socket.

[tiller]

Thanks for the replies, gentlemen. I am just headed out the door to run some errands, but will check the heater voltages with Teslas installed when I return. Any other checks I should do?

[Doc B.]

Any other checks I should do?

Check to see if the seller will take the tubes back? 

[tiller]

Seller has been MORE then accommodating. He's sent me replacements. I just find it very odd that I've ruined 3 now in the span of a month after having gone 5-6 years on the same EH6922 with no issues.

So I just checked the heater voltages. Oor I hope at least I got the pinout correct -- looks like the heater pins, with a bunch of small ceramics on them and wire running from one socket to the other. With the EH tubes I get 4.3V on the tube closest to the PSU, and 4.7V on the other. With a brand new pair of Teslas I get the same. With a noisy and an intermittent Tesla (which was running fine until last night) I get 4.2V on the tube closest to the PSU, and 4.1V on the other. Also, with these bad tubes in there 2 of the 4 C4S LEDs are not lighting. If you are looking at the underside of the top plate with the IEC socket in the upper left hand corner, it would be the two rightmost LEDs that don't light.

Could it be one bad tube that is toasting the others?

Adam

[Grainger49]

Check in this thread:

http://bottlehead.com/smf/index.php?topic=1029.msg7557#msg7557

Those are the numbers to be looking for.  PJ has said the 6922 and 7308 are good if true NOS.  You can buy the 6N23P NOS all day cheaply.  I have used them in the Seduction and Eros.

[tiller]

Hi Grainger, these definitely should work, hence my post here for troubleshooting tips. They are NOS Tesla 6DJ8. I'm trying to determine if the Seduction is killing the tubes or if I've had a series of bad ones. My current theory is that 1 of the pair I first bought did not audible go bad, but was bad enough to ruin the other tube in the pair, twice now.

[Grainger49]

If your heater voltage is not out of range then the Teslas that were burned up were not new.  I went through a lot of tubes in my Seduction.  Most of them tube rolling.  None of them had such a short life.

[tiller]

I suspect something else is at play. My heater voltages are posted above and are a little on the low side. Where should I look to diagnose that the PSU is working properly? I don't really have the build instructions handy anymore.

[Paul Joppa]

The two most likely causes I can think of for low heater voltage are low power line voltage, and/or a bad solder or missing connection to the four Schottky rectifier diodes.

Two wires go from the transformer to those rectifiers (terminals 1 and 2). Check the AC voltage between those terminals, with and without tubes installed.

The four diodes are 1N5818s, while the two high-voltage rectifiers are UF4007s - it can be a challenge to make sure the right diodes are in the right places, but that's another possible cause of low heater voltage.

[tiller]

Hi Paul,

So, voltages between terminals 1 and 2 are: 6.5V with tubes, and 7.0V without tubes.

I verified that all diodes are where they should be.

[Paul Joppa]

OK, the AC input from the power transformer is good. Somewhere downstream the DC power supply is losing more voltage than it should.

Assuming all four rectifiers are properly soldered, and the capacitor is not failing (you'd know, it would be bulging or blow out, and it might be hot), the easiest fix would be to reduce the 1.2 ohm resistor. You can connect a cliplead or solder a wire to short-circuit that resistor, then check the voltage. From that I can calculate the right parallel resistance. It should ideally be in the range 5.7 to 6.0 volts.

By the way, this lower than spec heater voltage is only recommended by me, and only for this particular circuit.

p.s. it has happened that similar puzzling voltage variations were due to a run-down battery in the meter. I mention this just in case ...  :^)

[tiller]

Thanks for the clear instructions Paul. It's getting kind of late here and I'm in the midst of a spinning session (using the old EH tubes). I will see about finding a value for the resistor tomorrow. I may even have something to swap in there.

In general, what downstream could be causing this? Poor assembly with that pile of stuff on tube pins 4-5? I have to admit I do recall having a tough time with getting it all on there.

Thanks,
Adam
 
 

[Paul Joppa]

I am scraping the barrel for these....

It's possible one or more of the diodes is either not working as well, or has a high-resistance solder joint. In other amps, if it was outright failing, there would be pulsating DC in the power transformer and it would get both really hot and pretty noisy. I think you would have noticed, though I can't say I've heard of the same behavior with Seduction specifically.

It's possible the 1.2 ohm resistor is failing. You can check the resistance, but subtract the resistance you get with the probes shorted. I have heard of internal connections failing over time. Problem is, it's hard to get a good measurement at such low resistances.

If it were the filter capacitor, the small caps at the socket, or solder problems at the socket, all of these things would get hot - again, I think you would have noticed.

All the above can really only be tested by replacing the part that may or may not be at fault. That's the real reason I suggested changing the resistor - it will correct the problem whether it's the actual cause or not.

I think the 10000uF filter capacitor is an unlikely source of the problem, but it's far  easier to replace than the diodes, so that would be my second choice.

... by the way, not mentioned yet, the 6DJ8 does draw more heater current than the 6922; that pulls the heater voltage even lower, as your measurements showed. All these voltages are however well below the minimum 5.7v specification, indicating some other problem.