Tode First Power Up - Sparklers!

[TubeTim]

Well, my luck ran out...  Was able to build a Mainline that worked at first power up.  For the Tode, it didn't go so well.  The EL84 tube did a nice Sparks show internal to the tube.  I figure that tube is toast - and any further testing with that tube is futile -  Correct?

A missed solder joint on a ground connection was likely the cause. 


BTW what does Tode mean?  Google search didn't help.  Not a musician, so the meaning escapes me.

Thanks

Tim

[Paul Birkeland]

"Tode" was derived from "pentode", as both the tubes in the Tode are pentodes.

If you ended up discovering a funky joint, you can try powering on the Tode again with the same EL84.  It can be shocking and somewhat magnificent to observe how much abuse certain tubes can take! 

[TubeTim]

Thanks for the explanation of Tode. 

I don't have a guitar (building this amp was for fun) therefore I don't have a proper input device.  So, with a 2 wire jack plugged in and the 2 wires shorted to each other (input grounded), the sparking EL84 settled down.   It is however mighty hot, to hot to touch.  The feedback is set to 'creamy'.

I realize tubes get hot but this bad boy seems a bit to much.  Maybe I'm to used to parts that run on 1.1 V-DC.

Removing the input short, one can touch the input lead and hear a nice 60 cycle hum, as expected.  The brake works as expected with hum getting louder / softer based on brake setting.  The input is mighty sensitive and I wonder if  anyone can suggest an input device to use to test?  Maybe a line-out dropped thru a 10:1 voltage divider?

The voltage readings on the back terminal strip are:

1 - 0
2 - 0
3 - did not measure
4 - did not measure
5 - 414
6 - 0
7 - 0
8 - 0
9 - 207
10- 414
11 - 300

The AC power shows 119.3 V-AC on meter.   Terminals 3 & 4 I couldn't not probe comfortably.

Do these voltages look in the ball park?

A 1.6k resistor is between terminals 5 and 11, we have power in said resistor =
   V² / R =  114² / 1600  = 8.122 watts. 
That is within the power limits but that puppy sure gets hot also.  Also the 560 ohm 5 watt resistor gets to hot to touch.  Maybe I'm to used to micro-power stuff....

I'll order up a new EL84 just to be on the safe side.

Many Thanks for your help!!

Tim

[Paul Birkeland]

Yeah, you're in the ballpark.

Yes, the resistors get hot.  They are inside the chassis so that you won't touch them

[Doc B.]

  The input is mighty sensitive and I wonder if  anyone can suggest an input device to use to test?

How about a guitar player?

Electric guitar pickups put out about 250-500 mV. You could also try a dynamic microphone or an electronic keyboard.

[TubeTim]

Hi Doc B,

I didn't think I knew any guitar players but a buddy of mine just happened to mention he played and agreed to give it a go.  Will also try the dynamic mic.  For some strange reason I'd like to build a Crystal radio and give it a go thru this amp.

Thanks for the info about a guitar pickup. This is very good to know.

Attached is a (rather poor) image of the front portion of the Tode.  It was a fun kit to build and not particular difficult.

Will I hate myself for routing the A-C filament wires in parallel with the signal wires from A to B tube?  The filament wire is shielded so it should be okay, but???

Best Regards,
Tim

[TubeTim]

Took The Tode to a friend who has several guitars and agreed to test the new amp.  He plugged one in and nadda.  Absolutely no sound.  I knew that couldn't be right as I could touch the tip of the input on the test bench and hear AC hum.  Luckily he had a second sound cord, plugged it in and beautiful sounds!   They commented how clean the amp sounded.  It's also loud enough to use in a small bar gig they thought.  We only had it cranked up 2/3 of the way too.

Tim

[TubeTim]

...
It can be shocking and somewhat magnificent to observe how much abuse certain tubes can take!

I have done some reading about tubes.  If one searches for 3cx800A7 you'll eventually land on a Ham Radio ops page who knows a lot of the care and feeding of these tubes and their big brothers.  At $1k a pop they take good care of them.  Some of the tubes, (3-500Z) they say to run slightly cherry red to act at the 'getter' function. 

After reading about these, your are absolutely correct, the tubes can take a lot of abuse.  Being hot to the touch is not even close, of course.

Now have to figure out how to drive a 3cx800A7 with the Tode and get 800 watts out. 

TiM