• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

What type of audio transformers to use for tubes / valves

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sco1t

sco1t

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Joined 2010
mng.dyndns-home.com/
#1
Hiya all, i've made a amplifier useing a pcl84 valve useing this plan 😎http://www.diyaudio.com/forums/attac...4&d=1152603590
and it goes pretty loud 😱 and sounds really good 🙂 but i don't have a proper audio transformer so i'm useing a mains transformer insted that only works well with 16 ohm speakers 🙁

I was wondering if one of these transfomers would work well. what do you think???
--|P037T|100V TRANSFORMER 8W | CPC

600-OHM INPUT AUDIO TRANSFORMER FOR R390 R390A RECEIVER on eBay (end time 03-May-10 14:18:26 BST)

Thanks for looking :trapper:
 
#2
First one is for commercial sound speakers tapping off 100V lines. Each roof or wall speaker in large buildings has one inside. Bad quality, mainly for the vocal range. Primary voltage I don't think they can take safely over 100V.

The second one is milspec. Very low primary impedance. I don't see the circuit for your link.
 
#7
Hi thanks Salsa and Rotaspec for the tips and help.
I'm new to valve amplifiers 🙂. You know on the pri side of the audio transformers they seem to have about 2000-5000 ohms. does that mean if i mesuerd it with a ohm meter the ohm meter would read the same????? E.g A output transformer saying 5k would read 5k on a ohm meter????
 
#8
Hi sco1t,
No, it wouldn't.
Transformers and things like speakers are measured for an AC impedance, not a DC resistance. Normally the impedance is rated in mid-band. If you were to measure the resistance of a speaker, it would only be about 75% of the rated Impedance of the speaker.

For instance, a power transformer may only have a resistance of a few ohms. Yet if you calculate how much current it would draw connected to the AC mains, it's clear that the working impedance must be a lot higher (it is).

Instruments that measure inductance, capacitance and impedance (an LCR meter or impedance bridge) use an AC voltage or current to develop a voltage drop across the component. They calculate the impedance from that. Often the test frequency has several values to cover a larger range.

-Chris
 
#13
sco1t said:
Hi thanks Salsa and Rotaspec for the tips and help.
I'm new to valve amplifiers 🙂. You know on the pri side of the audio transformers they seem to have about 2000-5000 ohms. does that mean if i mesuerd it with a ohm meter the ohm meter would read the same????? E.g A output transformer saying 5k would read 5k on a ohm meter????
Click to expand...

No. Not at all. 2000-5000 ohms is what the tubes "see" when the speaker is connected. A transformer, any transformer will reflect the inpedence of the load times the squesrt of the turns ratio onto the primary. the 2000-5000 ohms figure is that.

When you look for an OPT look for one that matches a (say) 5000 source to a 8 ohm load. and then has the correct power handling and also look at bandwidth specs. As said Edcor is a good place to buy OPTs And they will answer question via email if you make it easy for them by being specific.
 
#14
sco1t said:
Aww thanks every one. I think i have the hang how transformers work now 🙂.
but one more thing. how could I work out what impedance would work best with my valve (pcl84) in this circuit http://www.diyaudio.com/forums/atta...03590-suitability-ecl84-tubes-audio-ecl84.jpg. I guess 5k to 10k????

All i need to do now is think about how much to spend on the transformer 🙂
Click to expand...

Rule of thumb: If you aim to keep this amp and listen to it often, or if you want to re-use the transformer in a future amp (single-ended like this, but may have more power than the 0.5 to 1W this will be producing), then buy a GOOD transformer that is rated much higher in power (10W minimum). It will pay off in the long term. The Edcor CXSE series seem to be good quality at reasonable prices. Shipping from the US is a killer though.
BTW, you can bump more power out of this circuit quite easily, as the pentode section of the ECL84 is connected as a triode as is (highest quality, lowest power). Reconnecting it as a pentode will probably double the output power with very little sonic degradation. The other way to increase power out, is to find a ECL86 and use that. Little or no component changes needed for that valve vs the 84, but the pins are connected differently.
A ECL86 in pentode mode will produce about 3W in that circuit with a 5k transformer. I know, I have one here that is very similar (I just rewired it to use separate pre and power tubes, as one of my ECL86s died). It did (and does again) sterling service as my computer amp.
 
#15
Here is a manufacturer of transformers for your country! SP wound.co.uk...
They have been very helpful to me when I have had some questions. Their customer service is superb!

_______________________________________________________________Rick....
 
#17
Thanks guys 🙂 and Ladies is there was any lol.
I have found a nice transformer that is 10K for a 4 ohm speaker 🙂 😀
i'm waiting to find out it's frequency response. i can't hear past 18KHz or lower then 35Hz so as long as it goes up to that i'm fine lol. oh and i'll have to put up with Lily Allen's music the poor thing.
 
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