Douk EL34 Amp kit

[M Gregg]

Something looks strange on the photo Pin 4 EL34?

One side looks like G2 to cathode? The other side looks different.

Check G3 and G2 on each tube..connections look wrong!

Compare here:
https://www.google.co.uk/search?q=e...gCVal959M:&usg=__JHbzNtDJ6OIqH5q94pMr_bZL3BY=

Grid 3 should be to cathode..
https://www.google.co.uk/search?q=e...r3NaEWHyM:&usg=__JHbzNtDJ6OIqH5q94pMr_bZL3BY=

Pin 4 should go to the 10K in the PSU on both EL34's..Pin 1 to Cathode...on both EL34..

Perhaps its the photo..but it looks strange..


Regards
M. Gregg

 

[Avo111]

Checked it and all correct. Pin 4 both sides on PZ metering at 330v not what it says on schematic. Pins 1and 8 strapped together to connect cathode to grid 3. Wiring on pin 4 right heads off in wrong direction and runs under tag strip to end of 10k

Thanks for all the help it is really annoying that I can't find the problem. I tried disconnecting feedback and these resistors are both reading right. What next?

Regards AVO 111


Caber

 

[M Gregg]

OK,

we can use the meter as an input probe. use the meter with one probe connected. Set to DC volts.

Connect some old speakers to the amp and touch the meter probe on pin5 of each el34.

Let me know if you get a hum or buzz.

If the output section is OK and the EL34's are working we should get a buzz out of both sides.

Regards
M. Gregg

 

[M Gregg]

If this doesn't work,

Connect your meter back up as DC volts what voltage do you have from the cathode to Ground across the 300 ohm resistor?
On each tube..EL34.

If the heaters are on and G2 is connected have you got the capacitors the right way round in the cathodes? + to the tube.

You have voltage on the anode via the output transformer the input grid G1 is grounded via the 220K.

It doesn't make any sense..What voltage do you have on the EL34 heaters on the tube AC volts?

__________________________________________________________________________________________

The next step is resistance checks.

Power OFF set your meter to Ohms.

Check the ground of the cathode resistors on the EL34's to supply ground.

Is the centre tap on the power transformer connected to Ground? (must be because you have power across the PSU caps)

Regards
M. Gregg

 

[Avo111]

OK back on this. Left channel there is hum and the input valve is microphonic. Right channel there is very slight crackle when I touch pin 5.

I confirm all caps + towards tube at least according to markings, the identified connectio being marked on the sleeve

heaters on EL34s 6.15 vac left 6.17 vac right

heaters on 6n9p 6.23 vac paralleled between both

volts cathode to ground left 0Vdc right 43.8 Vdc

 

[M Gregg]

volts cathode to ground left 0Vdc right 43.8 Vdc

Is this measured on EL34's cathode to ground?
Ie with meter set DC volts meter between pin1 and ground .

Regards
M. Gregg

 

[Avo111]

Yes, just tried it again this time Left 30 vdc right 0vdc i swapped input valves over!

 

[Avo111]

I am thinking we may be chasing a couple of simultaneous faults making it hard to diagnose

 

[M Gregg]

Yes, just tried it again this time Left 30 vdc right 0vdc i swapped input valves over!

Forget the input valves...if the output tubes are not biasing correctly you can forget the rest! You need voltage across the cathode resistor on both EL34's.

So if you have an EL34 that gives voltage across the cathode resistor use that one in the other channel and see if you get voltage across the cathode resistor.
IE remove the EL34 that gives 0 Volts and put it to one side. then do all other tests with the working EL34.

The coupling capacitor isolates the input tubes from the output tubes..so in this circuit you can treat the input and output as separate things. (from a DC perspective).

Regards
M. Gregg

 

[M Gregg]

Here is something to test to give you an idea,

Remove both input tubes and test the EL34's on their own.
See what you get across the cathode resistors and post the voltage.

Regards
M. Gregg

 

[Avo111]

now getting 4.9vdc on left and 11vdc on right hum now coming from right speaker

 

[M Gregg]

now getting 4.9vdc on left and 11vdc on right hum now coming from right speaker

read post 69 and 70 again..

Does this reading still stand on the cathode's of the el34's?

if it does mark the EL34 with 4.9v and put it to one side then try the EL34 with the 11V in the other channel. see if it gives 11v.

ie the EL34 side that you hear the hum is working the other isn't.
So for the moment we will assume the other EL34 is faulty..it might not be but we need a known situation.

Regards
M. Gregg

 

[Avo111]

left 10vdc right 29 vdc with input valves out i note the schematic says I should have 15 V there

 

[M Gregg]

left 10vdc right 29 vdc with input valves out i note the schematic says I should have 15 V there

OK this is on the cathodes of the EL34's?
Plug in one of the input tubes and try it again.

Regards
M. Gregg

 

[Avo111]

This is on pins 1&8measured to ground and measured across the 300 ohm resistor same thing should be?

 

[M Gregg]

This is on pins 1&8measured to ground and measured across the 300 ohm resistor same thing should be?

Plug in ONE input tube..

We don't want to lose the cathode voltage..check the voltage across each EL34 cathode we should still have voltage.

Regards
M. Gregg

 

[Avo111]

swapped el 34s again, left 18.7vdc right 19.9 vdc

 

[M Gregg]

swapped el 34s again, left 18.7vdc right 19.9 vdc

That's correct! now try one input tube..Just one!
Don't touch the EL34's leave them alone..See if it effects the cathode voltage on the EL34's

Regards
M. Gregg

 

[Avo111]

put in left input tube left el34 00.3vdc right el34 32.5vdc

 

[M Gregg]

put in left input tube left el34 00.3vdc right el34 32.5vdc

Unplug that input tube put it to one side..try the others one at a time each time test the cathode voltage on the EL34's..let me know if you get any input tubes where the cathodes on the EL34's remains good.

Regards
M. Gregg