ECL 86 output transformer

[saint gillis]

My Davoli guitar amplifier's output transformer died.. Primary winding broken wire, nothing to do, it is deep inside the coil.

Here's the schematic : http://sub-continental.com/diy/davoli/davoli.jpg

Here's the ECL 86 datasheet : https://frank.pocnet.net/sheets/010/e/ECL86.pdf

I've never done this, where should I start to try findinding a replacement output transformer?

 

[saint gillis]

I've read that the EL 86 is an EL 84 and ECC83 combo, and I've also read that EL84s and 6V6s are compatible.. I guess that opens the doors for several options..

 

[RuudNL]

Is it me, or is the feedback from the secundary (center tap) of the output transformer to the cathode of the lower ECL86 a bit strange? (Because there is no ground reference.) Or is the ground at the output simply not drawn?

 

[rock soderstrom]

I've read that the EL 86 is an EL 84 and ECC83 combo, and I've also read that EL84s and 6V6s are compatible.. I guess that opens the doors for several options..

The pentode section of the ECL86 is weaker than EL84/6V6, so they are not identical. Nevertheless, you can check for suitable transformers among that iron designed for EL84/6V6. Hammond offers some transformers that might work because of their flexible connection possibilities.

 

[saint gillis]

Yes, the speaker - is grounded

 

[rock soderstrom]

Something like this, maybe even a smaller type C could work. Check the attached datasheet.

https://www.tube-town.net/ttstore/hammond-125d-universal.html?language=en

 

[abbey road d enfer]

I've never done this, where should I start to try findinding a replacement output transformer?

You need a xfmr with a primary impedance of about 10k and a secondary impedance in accordance with the speaker.
It's a push-pull amp. The maximum linear power is about 10W, but when overdriven, it goes up to about 20W.
The Hammond 125D should work. I wouldn't opt for smaller.

 

[saint gillis]

Thank you for your help here are the windings' DCR measured with an ohmeter, and the ratios I've found injecting 1VAC at the output :

 

[abbey road d enfer]

Thank you for your help here are the windings' DCR measured with an ohmeter, and the ratios I've found injecting 1VAC at the output :

DCR ratio is not a good indicator of the actual transformation ratio. But the 20.68V:1V seems adequate.
What Z is the speaker? If it's not writtent on it, measure the DCR and multiply by 1.3 to get a ballpark figure.
You said the primary is open, how did you measure the DCR?

 

[rock soderstrom]

What's the impedance of the loudspeaker?

 

[rock soderstrom]

You said the primary is open, how did you measure the DCR?

Good question!

 

[saint gillis]

One winding is open .. I mesured the side that is still working, and assume the center is correctly centered.

I don't know what speaker was used at the origin, but it worked something like 6 years with an 8ohm speaker.

 

[rock soderstrom]

Good answer!

Your primary target impedance is with a 8R loudspeaker atm:

8×20,68*20,68=3421 ohm. Thats looks pretty low. Maybe a lighter load would be better, something like 5 to 8k?

The Hammond 125 is very flexible, you can checkout what fits best.

There is still the feedback winding we should take care of.

 

[saint gillis]

The ratio for the feedback winding is just a bit higher, 21,76:1 instead of 20,68:1 ..

 

[rock soderstrom]

I do not see that much problem with that. Maybe you have to adjust the feedback resistor.

Lets see what Abbey thinks...

Did you like the sound of the amp before?

 

[abbey road d enfer]

Let's assume the use of an 8 ohm speaker. The original xfmr probably had a tap at 4 ohms.
So the connection would be:
Speaker negative and ground to pin 2
Speaker positive to Pin 5
NFB to pin 4
If there is a tendency to oscillate, connect NFB to pin 3.

 

[abbey road d enfer]

There's an interesting point in the schemo. The heaters seem to be "elevated"; actually I should say "lowered" because the DC voltage superposed seems to be negative if I follow the capacitors, but positive if I follow the diode rectifier.
Can you check this? I have quite often seen elevated heaters but never lowered.

 

[saint gillis]

Let's assume the use of an 8 ohm speaker. The original xfmr probably had a tap at 4 ohms.
So the connection would be:
Speaker negative and ground to pin 2
Speaker positive to Pin 5
NFB to pin 4
If there is a tendency to oscillate, connect NFB to pin 3.

Awesome, I'll try that, thanks all.
Yes this amp sounded real good, something reminding good old Creedence Clearwater Revival guitar sound. I hope it will live again.
Maybe I'll go with a 125E instead of a 125D, the dimensions seem closer to the original one.

 

[abbey road d enfer]

Maybe I'll go with a 125E instead of a 125D, the dimensions seem closer to the original one.

+1

 

[saint gillis]

There's an interesting point in the schemo. The heaters seem to be "elevated"; actually I should say "lowered" because the DC voltage superposed seems to be negative if I follow the capacitors, but positive if I follow the diode rectifier.
Can you check this? I have quite often seen elevated heaters but never lowered.

Well done, the diode is reversed on the schematic, I'm going to correct it, thanks.