Small Size Output Transformers

[el156]

Hi !
I found a small size transformer that can be interesting for low power amps. It is from RS, and code is 210-6475 , the primary impedance is between 4K - 15 K , the secundary 3 - 15 Ohms ,and allows 40mA DC, with 3.5 Watts max power. I tested it ,and the sound is OK for the size!
Perfect for a mini tube amplifier! The cost is about 10 Euros!
Hope is usefull
Silvino

 

[Brouilly]

I just bought a pair of those for my 6DT5 SET, havnt fired it up yet, so im glad to hear that they work well. Plus they are really cheap!

Thanks for the info

Charlie

 

[bigwill]

Hmm, I think it's guitar amp time!

 

[artosalo]

Note the primary inductance, only 12,4 H.
This means that the actual primary impedance at 50 Hz is only 3,9 kohms and at 100 Hz 7,8 kohms respectively. With proper amount of NFB this could be used without too poor low frequency response.

 

[Hi_Q]

As this has a centre tapped primary, could this transformer be used in push-pull mode with suitable Triodes having a low Ra? I was thinking of something like the 5687. Might make a good headphone amplifier.

 

[el156]

Hi !
I used this transformer in SE with ECC804 with the triodes in parallel for headphone amplifier,but i think it can be used in PP as well ! I will make a test in PP and post the results! I have some 6N7 ,maybe it worth a test!
Just for Fun
Silvino

 

[Brouilly]

Hi Silvino,

I hooked these OTs to my SE 6DT5, and a few seconds after turn on there is an increasingly loud hum.

I thought it might be my wiring, so ive just rewired the whole amp, and am sill noticing the same hum. Im concerned it might be the transformers, as the old transformers were fine.

Have you noticed a problem like that?

Can you, or anyone offer some suggestions?

Thanks

Charlie

 

[el156]

Hi Silvino,

I hooked these OTs to my SE 6DT5, and a few seconds after turn on there is an increasingly loud hum.

I thought it might be my wiring, so ive just rewired the whole amp, and am sill noticing the same hum. Im concerned it might be the transformers, as the old transformers were fine.

Have you noticed a problem like that?

Can you, or anyone offer some suggestions?

Thanks

Charlie

Hi Charlie!
No ! No hum at all! and as far as i know a OT doesnt make any hum ! If the wiring is ok,the hum has to come from somewhere else,i think ! Are the OTs mounted very close to the Power Transformer?

Silvino

 

[Brouilly]

Hi Silvino,

Ive fitted the components into an old radio, so there isnt much space, but ive made sure that ive given the OTs enough room from the PT.

When testing the B+ on the primary the 'buzzing' intermitently gets worse and cuts out. Ive checked as much as I can, but my knowledge isnt good enough to diagnose the problem.

It seems there is some serious interference from somewhere.

Do you have any ideas?

Thanks

Charlie

 

[Brouilly]

Ive checked for faulty diodes, caps, and ive moved the transformers around.

Ive also checked my wiring again, and to me it seems satisfactory.

Im a bit lost

 

[DF96]

Likely sources of hum: heater wiring, unscreened input cables, hum loops, poor HT smoothing, poor grounding.

Is it 50Hz or 100Hz? Hum or buzz?

 

[Brouilly]

Hi DF96,

Thanks for your help, think this is currently being discussed on 2 threads now, so sorry for that.

The problem was 'buzz', this was solved by a ground lead from the volume pot not being connected to star ground.

Now its just a hum, but the hum isnt present when the pot is fully turned left or right.

If you can help, maybe reply on the 'power supply buzz' thread to save confusion.

All I can suggest is that the pot is faulty, but maybe there is a more detailed reason for it...?

Thanks again

Charlie

 

[M Gregg]

Charlie,

Is the shaft of the pot earthed. Is the pot screw thread on metal or wood?

Regards
M. Gregg

 

[Brouilly]

Hi M. Gregg,

This is currently being discussed on the 'power supply buzz' thread.

The pot thread is on metal, but the metal is on glass, so not sure if that helps your question.

Charlie

 

[M Gregg]

Charlie,

Wrap some wire around the pot thread and earth it to star Gnd.

 

[M Gregg]

O.K.I will look at Buzz thread.

 

[45]

As this has a centre tapped primary, could this transformer be used in push-pull mode with suitable Triodes having a low Ra? I was thinking of something like the 5687. Might make a good headphone amplifier.

Yes you can and you don't have to worry about DC balance.

The low primary inductance is not as bad as it looks because it has been measured at 100Hz with only 0.274V at 40 mA DC. With higher voltage it will increase a lot and at lower frequency it will be also higher because of the lower losses.
This specification is more conservative than most hi-end transformers which are measured in the most convenient conditions....
In a PP situation with lower o no DC it will also be a little bit higher and will have more headroom.
I would be more worried about high frequency response and loop feedback could not be straightforward....
It appears to have 4000 turns on the primary that are a lot. I would use it as 7K (connection 1-4 both in SE and PP plate-to-plate) 8 ohms (connection A-D) to avoid additional loss due to unused turns. It is already high (more than 1K primary DC resistance and 1.5 ohms secondary DC resistance). Power loss could be lower using 16 ohms load (for 14K primary using the previous connection) but then inductance could be a limiting factor at low frequency.

45

 

[45]

Yes you can and you don't have to worry about DC balance.

The low primary inductance is not as bad as it looks because it has been measured at 100Hz with only 0.274V at 40 mA DC. With higher voltage it will increase a lot and at lower frequency it will be also higher because of the lower losses.
This specification is more conservative than most hi-end transformers which are measured in the most convenient conditions....
In a PP situation with lower o no DC it will also be a little bit higher and will have more headroom.
I would be more worried about high frequency response and loop feedback could not be straightforward....
It appears to have 4000 turns on the primary that are a lot. I would use it as 7K (connection 1-4 both in SE and PP plate-to-plate) 8 ohms (connection A-D) to avoid additional loss due to unused turns. It is already high (more than 1K primary DC resistance and 1.5 ohms secondary DC resistance). Power loss could be lower using 16 ohms load (for 14K primary using the previous connection) but then inductance could be a limiting factor at low frequency.

45

P.S
I forgot about the phases......
You have to reverse the phase of one half-primary. Under the white tape take connection 2 and disconnect the two halves one is the termination from 1 and the other is the start that ends at 4. Then take 4 and connect to the end of 1 (or take 1 and connect to the start of 4) and the two remaining terminations will be your plate connections.