wiring tubes in series

[M Gregg]

if the two 6v tubes are the same type eg KT88 then they can be wired in series.
I wired 2 (6.3v) 6922 and a 12AX7 to run from a 12v supply easily.

I think you need to read the thread these are discharge tubes.

Regards
M. Gregg

 

[Aran Everitt]

ok connected as told - still the same readings unfortunately.

 

[M Gregg]

ok connected as told - still the same readings unfortunately.

I have to sign off for a while,

I will try and get back to you.
If you have tried the G1 to 12V and G2 is signal then in theory it should conduct. The only other thing is the cathode resistor is to high.

Regards
M. Gregg

 

[Aran Everitt]

ok i will try to lower the ohms.

Aran

 

[M Gregg]

ok i will try to lower the ohms.

Aran

Just a quick drop back this circuit uses 12DU7

NB notice the cathode resistors or should I say lack of resistors.
You could try this and then touch a Meter probe on the signal input to the 12DU7 and see if you get any sound out of the speaker. (hum or crackle)
If it works then you can check the preamp.
The grid resistor is 2.2 Meg I cut it off sorry.

The only thing to watch is the tube current I don't know what resistance is the primary of the output Transformer in your circuit.

Try 100 Ohm and measure to see if you have voltage across the cathode resistor...if that doesn't work try 50 Ohm etc.
Its probably a good idea to put some resistance even 20 ohms so you can calculate the tube current. Its a bit suck it and see I'm afraid.

Its easy to calculate the tube current its the voltage across the cathode resistor divided by the resistance value and that gives you the current.
(Basic Ohms Law)


Regards
M. Gregg

 

[Aran Everitt]

i have attempted this and it looks much better now however i am still stuck on why the 12du7 doesn't glow anymore i hope i haven't blown it

 

[M Gregg]

i have attempted this and it looks much better now however i am still stuck on why the 12du7 doesn't glow anymore i hope i haven't blown it

Did you have any resistance in the cathode?
Pull the tube and check the anode voltage...
The number One thing is the output Transformer<<do you have voltage on the anode connection?

Regards
M. Gregg

 

[Aran Everitt]

measuring the cathode to ground? or with the resistor between it?

 

[M Gregg]

measuring the cathode to ground? or with the resistor between it?

Remove the tube and check the anode voltage from anode to ground.
On the Tube socket to make sure the output transformer is OK
You had 12 V on that connection before..

Regards
M. Gregg

 

[Aran Everitt]

ok without the resistor i am getting around 5-3 ohms

 

[Aran Everitt]

and after removing the tube i measured the voltage to ground and got 12v

 

[M Gregg]

ok without the resistor i am getting around 5-3 ohms

Remove the tube

And check the voltage from the transformer and anode connection to Ground.
Ie is 12 volts still on the anode connection with the tube removed from the amp.

Regards
M. Gregg

 

[Aran Everitt]

yes it is.

 

[M Gregg]

I was thinking the primary of the output transformer had blown open.

so you still have 12 V on the anode connection with the tube removed?
Put the tube back in..

Ok so now try the values of resistance in the cathode..
And measure the voltage across the cathode resistor..phew,,,

Regards
M. Gregg

 

[Aran Everitt]

yepp 12v on the anode to ground. just the tube isn't lighting up. checked all the connections.

 

[M Gregg]

yepp 12v on the anode to ground. just the tube isn't lighting up. checked all the connections.

I don't suppose you know the Primary impedance of the output transformer.
Its obvious that 330 ohms is to high..

Did you say you have used these in PP before?

Regards
M. Gregg

 

[Aran Everitt]

ok i think i have a sign of life, when you said remove the res cap bias and directly ground it, i did it and the tube lights up when i probe it...

 

[Aran Everitt]

the primary impedance is 5k, the tube outputs about 2.7k. the difference is not too big...

 

[M Gregg]

ok i think i have a sign of life, when you said remove the res cap bias and directly ground it, i did it and the tube lights up when i probe it...

Still put a resistance in the cathode.

You may draw to much current so get the largest value you can in the cathode with the tube still working.
When you get to this point see if you get anything out of the speaker..

Then you can work on the preamp section.
You can calculate the current and set the resistor..

Regards
M. Gregg

 

[Aran Everitt]

how do i work it out?