Tube amp: Bring it back alive

[Prologue]

Hello All,


I have an old handmade tube amp (EL84, 6CA4, 5814A) and due to a design flaw, the plus un minus cable of the speaker touched each other and kill one of the two output transformer... I don't want to ditch it so i corrected the design flaw but i need to find another (or compatible) output transformer, could you help me to source one or point me the right direction? Thanks!


Here are the specs i found:


Audax transformer
Type 50-60 (seems to be the size)
7000 ohms (@ 400hz?)

 

[carlthess40]

I have a few older consoles tube amps , maybe I could find one in the parts box
Not guaranteeing anything. Lol

 

[Alan4411]

@Prologue
I think you should say if the amp is single ended (1 valve per transformer) or push pull (2 valves per transformer).

 

[Ketje]

7000 Ohm yes, but the speaker side ?
You could probably use the Hammond 125B SE.
Mona

 

[Prologue]

I use 8 ohms old Kenwood KL-333 speakers with this amp, works great... on one channel

I have a few older consoles tube amps , maybe I could find one in the parts box
Not guaranteeing anything. Lol

It would be great ! Thanks

@Prologue
I think you should say if the amp is single ended (1 valve per transformer) or push pull (2 valves per transformer).

Indeed Alan, it is a SET i think, i add a picture:

 

[JMFahey]

Shorting speaker wires hardly kills a Tube amp OT; are you sure it´s dead?
What symptom or measurement makes you think so?
There are ways to measure it before chasing another.

Nice amp build, it even uses PCBs, looks like it was pulled from a Stereo console or something and remounted elsewhere.

 

[Ketje]

The Hammond could do.It is 5000 Ohm but that's accepable for a EL84.
Mona

 

[Prologue]

Shorting speaker wires hardly kills a Tube amp OT; are you sure it´s dead?
What symptom or measurement makes you think so?
There are ways to measure it before chasing another.

Nice amp build, it even uses PCBs, looks like it was pulled from a Stereo console or something and remounted elsewhere.

It was a kit sold during the 60's under the brand Arena (Danemark)
You can find another build with the same PCB here in french: Ampli single EL84 - Vintage Audio Heritage


I took it to a hifi repair shop and the tech told me the OT was dead, something about internal winding broken. The symptom is no sound at all except a few low harsh noise sometime. The tech didnt seems to want to do anything about it (or cannot do) so i try to source a new OT, it would be sad to trash this nice amp.

The Hammond could do.It is 5000 Ohm but that's accepable for a EL84.
Mona

Thanks for the tip, do you think that one wich seems to be closer would work?


OUTPUT TRANSFORMER 7K OHMS/4 OHMS

NOS OT AUDAX, primaire 7000 ohms primary, 4 ohms output for ECL82/ECL86, primary resistance 300 ohms self 11 H NOS
weight 950 gr, 80x50x70mm

 

[Ketje]

I took it to a hifi repair shop and the tech told me the OT was dead, something about internal winding broken.

It looks like (fotos on VintageAudioHeritage) the EL84 is in pentode mode.If the primary winding is broken the screen of the tube goes too, dead EL84 !
Seems to be a capacitor over the primary, could be short circuited.

OUTPUT TRANSFORMER 7K OHMS/4 OHMS
NOS OT AUDAX, primaire 7000 ohms primary, 4 ohms output for ECL82/ECL86, primary resistance 300 ohms self 11 H NOS
weight 950 gr, 80x50x70mm

If you use it with an 8Ω speaker the tube get loaded with 14k, worse than the 5k from Hammond.
Mona

 

[JMFahey]

I took it to a hifi repair shop and the tech told me the OT was dead, something about internal winding broken. The symptom is no sound at all except a few low harsh noise sometime. The tech didnt seems to want to do anything about it (or cannot do) so i try to source a new OT, it would be sad to trash this nice amp.

Not impossible but to confiorm your amp is not suffering a bad case of "lazy Tech syndrome" , please measure DC resistance across primary and secondary windings yourself.
I´d expect 100 or 200 ohm primary and less than 0.5 ohm secondary, but you tell me.

 

[Prologue]

Hello,


Thanks for the advice, i measured the OT ground and + wire to the speaker, it is 0.9 ohm both side. It is the secundary i guess


For the primary, i have one cable going from the OT to the pcb and another going from one OT to the other OT. should i desolder one on them and measure them with ground?


Thanks !

 

[Alan4411]

No need to unsolder anything. With your meter on resistance (2K range) go from the OT connection on the board to the connection to the other OT. You can check both transformers and see if they are the same. Good will be 100 to 200 ohms. Very low or very high is bad.
Then from ground to either of the OT primary connections just to make sure there is no internal short. The reading should be high resistance more than 5K ohms to 100Ks .

Tell us what you get.

 

[TG]

It's much more likely to be 300-500 ohms for the transformer of this size.

 

[Prologue]

No need to unsolder anything. With your meter on resistance (2K range) go from the OT connection on the board to the connection to the other OT. You can check both transformers and see if they are the same. Good will be 100 to 200 ohms. Very low or very high is bad.
Then from ground to either of the OT primary connections just to make sure there is no internal short. The reading should be high resistance more than 5K ohms to 100Ks .

Tell us what you get.

Left channel (bad one):










Right channel:








I must have done something wrong

 

[JMFahey]

Maybe

Both secondaries "show" 0.9 ohms, but that includes wire/connector/switch resistance which is NOT zero.

Touch one probe to the other and you´ll read a low number, not zero but, say, 0.3 ohm or something.
If you substract that "parasitic" number from indicated 0.9 ohm you´ll get the real secondary resistance value, about what we suggested.
So you are reading secondaries right

We do have a primary winding measurement error though, notice BOTH show OL, meaning "open" or simply "above measurement scale".

I strongly suspect you are NOT using the 2k scale, hard to see in your type of multimeter.

Not kidding and no offense intended, I prefer "noobs" to use an old/cheap multimeter with a rotary scale selector , think those yellow $10 multimeters available anywhere, simply because you set the rotary selector to some scale value, say 2k, and there is no doubt about that.

Autoscaling/"intelligent" thingies may self set to the wrong scale without warning and the screen "number" displayed makes no or little sense.

Again: even the "good" transformer shows OL.

Not sure how your multimeter sets scale but as a reference and on the same scale measure a resistor in the 1k range, anything between , say, 820 ohm and 1800 ohm , which should be displayed properly, then touching no setting measure transformer primary DC resistance.

Amp OFF and unplugged from mains for 1 hour or more, just in case.

"Good" transformer should read "something" from 100 ohm to 500 ohm as suggested by TG, DEFINITELY less than 2k

If both still measure OL triple check you are measuring across both ends of primary, not primary to ground.

 

[Alan4411]

Hi Prologue,
I believe you have missed a set of measurements.
The out side pair (yellow and black) I guess are the secondary as they measure less than an ohm. There is no internal short from your results.

But you need to check the resistance of the middle pair. There have to be 4 wires for the transformer to work. The red one and one which we cannot see on the pictures. I guess you need to measure the red to the 'free' tag. (2k range as before.) Alan

 

[Prologue]

Thanks guys,


Here are the measurement of the good OT:





and the bad one:





i guess it means my "bad" OT is dead

 

[TG]

Can you take and post the picture of the transformer side opposite to the "Audax Type 50-60 7.000" label? There should be some thin wires going from the transformer bobbin to its PCB - if one of them is broken outside the bobbin, there's a chance you can fix that.

 

[Prologue]

It must be broken inside


I ll order a hammond 125b as proposed previoulsy, will it be a problem to use it with the order Audax 7k OT or should i order 2 ? Thanks !

 

[TG]

I would definitely order two Hammonds (you'll need 125BSE, not 125B - please notice)