Problem with Tung Sol 5998 (solved)

[buddha2015]

Hi there,

I  finished building my Crack amp with Speedball upgrade. It measured and worked flawlessly right from the start with stock tubes.
To lower output impedance I purchased a Tung Sol 5998 from a seller on Head-Fi. Right from the start it just got hot but it was not glowing and there is no output at all. I tried with different ECC82s with the 5998 in place but no difference.

I measured voltages with the Tung Sol installed:
B1: 82V B2: 210 V B3: 1,4 V B4: 80V B5: 210 V B6: 1,4 V B7: 0V B8: 0V

How to interpret this ? Could anything be wrong with my Crack ?

The seller says the tube was working perfectly when he sent it.
Is there any way of knowing if the tube could have been damaged in shipping ?

thanks for your help
Guenther, Austria

[Doc B.]

Pull the 5998 out of the socket. Set your meter to read resistance in a low range. Measure resistance across the 5998 pins 7 and 8. You should read some low number. If you read OL or infinity the heater is open and the tube is dead. If you read a good, low resistance from 7 to 8 plug the tube back in. Set your meter to read AC volts in a low range, like 20VAC. Take a voltage reading from B7 to B8. It should be around 6.3 volts. If the voltage is correct turn your room lights down and look for any kind of glow from the 5998 that would indicate the voltage is getting to the heater. The glow may be faint. If it is not glowing at all, try cleaning the pins and rocking the tube in the socket a tiny bit.

[buddha2015]

Thanks for your help, Doc.

The resistance between pin 7 and 8 is somwhere around 1 Ohms or below.
The meter reads 5,5 V AC voltage between B7 and B8.
The tube does not glow at all but it gets hot !

I have plugged it in and out numerous times with same results.

Strangely I found the 4 red LEDs on the larger circuit board of the Speedball Kit are not glowing with the 5998 plugged in. All the LEDs are glowing with the stock tube in place.

[Nathan]

I run a 5998 with great results. I'm inclined to think you have a bad tube

[Doc B.]

5.5V is a little low, and that may have some bearing on whether the tube will conduct. What is your AC mains voltage? Also, what AC voltage do you measure from B7 to B8 with the stock 6080 installed?

[buddha2015]

My AC voltage measures 233 V.
The AC voltage from B7 to B8 is 5,6V with the stock tube.

The 4 LEDs not glowing - does that indicate any problem ?

[Doc B.]

The LEDs not glowing are indicative of the same problem as the low B3 and B6 voltage, the tube is not conducting. My hunch is the 5.5VAC is just a bit low for the 5998 (I think it wants more like 5.7VAC minimum) and it's getting warm but not hot enough to emit properly. The 6080 you have seems to have a heater that emits better at the slightly low voltage.

You might try removing the 5998, leaving the ECC82 in its socket and measuring AC voltage across terminals B7 and B8. That will give us an idea of whether the 5998 is pulling the heater supply voltage down excessively. 

[buddha2015]

The voltage is 6.15 V  from B7 to B8 with the tube removed.

[Doc B.]

OK, that's about right for the line voltage you are getting. The 5998 is just pulling that voltage down quite a bit, probably just below the threshold of sufficient emission. My hunch is that to get the tube to work you would need to increase the AC mains voltage to the amp by 3%, about 7 volts. The easiest way to do this is with a variac.

You might also reflow all of the solder joints from pins B7 and B8 back to the power transformer. Just a little bit of added resistance from a cold solder joint could affect the heater voltage. For example, an added .1ohm of resistance from a cold joint with the  2.4A current draw of the 5998 heater could drop the heater voltage .24V. It may or may not help, but it's something simple to try. Also clean the tube pins really well.

[buddha2015]

Ok I will resolder the joints tomorrow, it is getting close to midnight where I am.

At my workplace we should have variacs and different high voltage power supplies. Both are not tools that I want to use at home but they should do for testing.

Is there any way of stabilizing the voltage at B7 B8 or cranking it up somehow by changing components on the circuit boards ?

[Doc B.]

The heater supply is very simple and direct, going from the transformer right to the tube socket. There aren't any components in between that could be changed to get more voltage. To see higher heater voltage you either have to reduce any unnecessary resistance in the wiring connections, use a tube that draws less current like the original tube (and thus pulls the voltage down less), or put more voltage into the primary side of the power transformer.

The Variac will be a great way to see if a little more voltage will get the 5998 hot enough to play.

[buddha2015]

OK so I used a variable transformer to crank up AC mains voltage up to 250V which brought up AC voltage from B7 to B8 to 6 V.

No change whatsoever, the 5998 gets hot but no glowing and the LEDs on the large Speedball circuit board do not glow either.

Bad tube ? Any other tests I can run ?

[Paul Birkeland]

Sure sounds like a bad tube!  Just double check that the amp is still happy with your other 6080/6AS7 tubes.

-PB

[buddha2015]

The amp happily accepts the stock tube.

Anything else I can try/measure with the 5998 tube ?

Can a tube get damaged like that from shaking/in transit ? I paid more than $100 and I need to argue with the seller...
 

[Paul Birkeland]

It would be best if you could get the tube tested locally, if at all possible.

-PB