[Mjork]

Today I have soldered 6 new resistors on a loose print and connected those in instead of the ones on the onboard ones. Connected between C8/C9 and the ECC99. Checked all values again throughout the amp.
All end up the same, output signal also. So the onboard resistors are fine.

All has been checked now and is correct all the way up to the ECC99 inputs and also the ECC99 is OK. What
remains is the output transformer it seems to me.
When one side is disconnected I get amplification from 10 to 40 VAC, when I connect the other side also the output signal collapses from 40 to less then 10 VAC.

As can be seen on attached pictures, the output signals are in counterphase (as they should be), but this is only seen when I close the volume pot to around 5%.
Then I go up to 20% and you seen the dent in both wave kicking in on the pos side.
Then when going up further to 50% the signals look like two signals that are in phase.

So why could cause this to happen?

Anyone ideas? Thanks, Marc

[günther]

to see if the transformer is the problem, you can use two resistors.
Anything from 1k to 10k will be fine for testing.
it will only change the plate voltage and maybe some clipping will show up.
grid signal should be less than 12V for nice curve. Above the signal shows some distortion but will stay out of phase.

[Dirk]

Anyone ideas?

You have a sineweave generator and a scope so you can check the OT. To get some ideas please see: http://www.marstran.com/Measuring%20Primary%20Impedance.htm

How did you connect the OT ? All pins not connected are "open" and not connected to ground or something else ?

Best regards, Dirk

[Mjork]

@ Gunter, I tested the system with two 10k resistors instead of the OT. Good suggestion!
Signal from the plates is now a nice sine and they are exactly in counter phase, like a mirror.
You are correct, when going up in volume over 10VAC on the grid the outsput signal starts to flatten on the top.
But no dent is coming down and the signal phase stays mirrored.
Does this invalidate the OT now? Then I will  order a new one straight away.
N.B. The resistance I measure in the OT are 160 and 130 Ohm per  coil, way lower then the 10kOhm installed now.

@Dirkj: I did not get to  do the OT tests yet, but indeed all 4 other pins are just left open.

[günther]

a coil has two resistors. once the dc resistor, in this case 130/160 ohms. and then there is a second resistor in a row. this
is called impedance.
this changes with the frequency. the 125c has an impedance with an 8 ohm speaker of about 13kohm at 1000 Hz.

Looks like that there is somthing wrong with the output trafo.

[Mjork]

@Dirk: I do not have a function generator, but I used a 1kHz signal I got from youtube and got in the aux input of my looper.
This works well and the looper volume knob increases the output voltage.

 If I follow correctly the example in http://www.marstran.com/Measuring%20Primary%20Impedance.htm
these are the results
- when connecting the looper to the OT input wires blue and brown I cannot get more than 580mVAC (RMS, measured with DMM). Red wire is left open.
- input 580mVAC, output between pins 4 and 6: 3 mVAC
- this gives a VTR of 193
- Imp ratio then is 37249
- multiplied by 6 (pin number) gives 223494. According to the example this then is the Primary Impedance of the OT?

What do you conclude from this?

[Dirk]

Hi,

if you have an other output transformer then give it a try. I have never seen such a kind of failure but it looks like that the source of the problem is the OT.

Best regards, Dirk

[Mjork]

No I did not have one so I ordered one from some obscure company called Tube Town?

New OT arrived and performing the test from http://www.marstran.com/Measuring%20Primary%20Impedance.htm
gave really different results:
- when connecting the looper to the OT input wires blue and brown the output did not drop like with the old one, so I could keep  the 1 VAC.
- input 1000mVAC, output between pins 4 and 6: 24 mVAC
- this gives a VTR of 42 (old one 193)
- Imp ratio then is 1764 (old one 37249)
- multiplied by 6 (pin number) gives 10584 as a primary Impedance (old one 233k)

So I had good hopes on going to use this new OT, and indeed now the Emma works as to be expected!
17VAC on the input of the ECC99 gives 95VAC out.
Both sides perfect sine in counterphase and signal to speaker is 4,8VAC.

Seems like I can pick up the gitar again instead of scope and soldering iron and finally get to experience that so called tube sound!

Pfffffft.... It has been a long ride. But with a steep learning curve which you do not get when first time right!
Thank you to the supporting members!

[günther]

 

[Dirk]

That's good but still a little bit strange. I guess there is an internal connection between the two windings or something like this. Never seen before and I would really like to know what it is. Anyway, please contact us via the contact form for a  refund.

Best regards, Dirk

[günther]

Meine Vermutung ist, das das Eisenpaket durch irgend einen Umstand magnetisiert worden ist. Dadurch geht  der Trafo schon Bei kleinen Signalen in eine Art Sättigung.

[Mjork]

@Dirk, do you want me to send the OT back, so you can check it out?

[Dirk]

@Dirk, do you want me to send the OT back, so you can check it out?

no, this will be a waste of money.

Best regards, Dirk