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Old 2nd July 2013, 12:51 PM   #1
osci is offline osci  Australia
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Default OPT impedances

Hi all, I'm hoping someone can offer advice on the following output transformer impedances?
I'm building an amp which has 2 x 6BM8/ECL82 in PP. My own research indicates that I need an OPT that has an approx 8kohm impedance.
I've been reading the reference material on the two following webpages which is specifically about using 100v line transformers normally reserved for PA systems as valve output transformers.
Here is the first link
http://www.ozvalveamps.org/optrans.htm and
The reference material and equations in particular in this second link http://home.alphalink.com.au/~cambie/6AN8amp/M1115.htm
I've been reading carefully, the speaker transformer that I may be able to use (and I say that lightly) has the following primary power taps, I've taken the table and added the values from my transformer and did the calculations and the table will show what I came up with.
Primary
Common or 0 tap
0.33W tap
0.66W tap
1.25W tap
2.5W tap
5W tap
Secondary 1 x 8ohm tap

I've copied some of the following text and the tbale below directly from the second weblink BUT have inserted the values from my transformer and did the calculations.

"The rating data is analysed by tabulating the spec for the various terminals as follows. In the left column are listed the original primary termination names. These all connect to a nominal 100V line, in the originally intended application, so this is shown in the next column, with respect to the primary "Common" terminal. The third column shows the rated power each of these terminations is used for into the nominal secondary load, shown in the fourth column. From secondary power and load comes secondary (RMS) voltage by V=(P.Z)0.5 shown at column five, and then comes the comparison (i.e. the ratio) of primary to secondary voltage, from the preceding figures. The square of this ratio gives the ratio of impedances between primary and secondary, and when multiplied by 8 Ohms, as shown in the last column, we see how an 8 Ohm load would appear, reflected back into the primary. All figures, at this point, continue to use the primary "Common" terminal as the point to which voltages are referenced."

these are my figures in the table




Based on my calculations I'm assuming the tranny has a 30kohm primary with respect to an 8ohm secondary?

Is it possible to use this transformer as an 8kohm primary impedance transformer based on the calculations in the equations particularly in the second weblink mentioned above?

If this transformer IS suitable, based on my figures in the table am I assuming correctly that in order to obtain the 8kohm primary impedance I would use the "common tap" & "1.25w tap" for the plates and the 2.5w tap for the CT?

I hope this isn't confusing and/or someone can offer advice?

thanks for taking the time in reading.

cheers Ron
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Old 4th July 2013, 10:19 PM   #2
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Hi Ron,

I have used this tranny for a number of projects (see 100 buck amp challenge).

For 8k anode to anode impedance you must use the common and 1.25 taps on the anodes and the 5w as your centre tap (not the 2.5w tap).

I currently have a breadboard amp running with EL91's into this trafo using the common and .66w taps with 2.5w as the centre tap.

(I bought a box of ten of these transformers for $1.25 on eBay)

Cheers
JimG
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Old 5th July 2013, 12:14 PM   #3
osci is offline osci  Australia
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Thanks heaps Jim for the reply, and thanks heaps for the correction on the CT,

I managed to come in contact with an old valve technician earlier in the week, he agrees that the tranny will most prpobaly do the job and that I've got the right impedance match but his concern is the small physical size of the tranny in that it may become easily saturated during operation?

However, from what your saying in your reply it sounds like that shouldn't be a problem?

anyway the only other valves in the circuit I'm using are one 12AX7 for the pre-amp and a 6V4 rectifier.


thanks heaps again

cheers Ron
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Old 5th July 2013, 10:36 PM   #4
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Hi Ron,

it would be possible to overwhelm the little 5 watter with a pair of 6BM8's, so either don't go for max power (even 1w output power in the bedroom has a very low wife acceptance factor), or use an Altronics 15w or 20w 100v line trafo - you can get 8k a-a impedance out of either.
(Or you can use 2x five watts in parallel).

What schematic?

Cheers
JimG
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Old 5th July 2013, 11:39 PM   #5
osci is offline osci  Australia
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Hi Jim,

the schematic I'm working from is the Gibson GA-8t, Lots of material I've read seems to indicate they originally had quite small OPT's from the factory but just how small i don't know.


I'm almost at the stage where I can power it up today, So i might use the ttranny and see how it goes, although I should get one of the Altronics 15watter


cheers Ron
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Old 6th July 2013, 02:59 AM   #6
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Yep. See how it goes with what you've got.
And let us know.
Cheers
JimG
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Old 6th July 2013, 03:28 AM   #7
osci is offline osci  Australia
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I just had a thought after doing some more reading,

I have a 40watt line transformer which has

5w, 10w, 20 40w primary's and then on the secondary's it has 0, 4ohm & 8ohm tap, I do havea 16ohm speaker which I could connect to the 4ohm tap to give me the 8k primary impedance?
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Old 6th July 2013, 03:58 AM   #8
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Yep. CT is 20w tap.
But try the little tranny too.
Speculating that a sub 8 watt amp with a 40 w OPT it might be like a mini with a Falcon gearbox - I don't know.
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Old 15th July 2013, 12:55 PM   #9
osci is offline osci  Australia
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Hi Jim,

I finally got the buggar fired, I had a lot of trouble since last thursday with cathode resistors in particular I had a 270kohm resistor on the cathode of the output tubes instead of a 270ohm resistor as soon as I changed this resistor, I have beautiful sound from the speaker using the 40watt pa output transformer.

The sound is almost like a chiming sort of sound with what appears to be a good breakup but not too heavy, I was going to try the smaller 5watt output trasnformer but its getting a bit late tonight so hopefully will do that tomorrow night or later in the week, but for now I have too much hum adn will on trying to reduce this somewhat, So i will post back later in the week

cheers Ron
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Old 16th July 2013, 06:09 AM   #10
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Cool. Excellent outcome.

JimG
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