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Old 16th Jan 2020, 3:42 pm   #41
ronbryan
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

The DAC90A has the same output transformer as the DAC90, which uses a CL33 output valve. That valve (CL33) also has a Ra of 4,300 ohms, which may have inflenced the Bush designers to use a standard part for the DAC90A.

* Correction: The Mullard data sheet for the CL33 on the Radio Museum lists the Ra as 4500 ohms, so that implies a turns ratio of 39:1

The UL41 Ra is also listed as 3000 ohms for HT of 100V and 170V.

Ron

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Old 16th Jan 2020, 3:44 pm   #42
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

PS: on the motor speed controller - I ordered a couple of speed controllers from ebay to see if they have better control at low speeds. I am wondering if running the speed controller at a higher voltage will help the low speed starting.

Thanks
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Old 16th Jan 2020, 3:45 pm   #43
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Thanks Ron

I bet your correct. Making best use of a common parts bin and the valve won't care about the exact matching.

Chris
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Old 16th Jan 2020, 4:06 pm   #44
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Just an observation from me, ri increases as the g2 voltage is reduced while the anode voltage remains the same, if you look at the valve data curves you can get some idea of what I mean:

http://www.r-type.org/pdfs/ul41.pdf

The data for Class A operation gives ri as 20k for Va = Vg2 = 170 volts and Vg1 = -10.4 volts

If you look at the Ia Va curves for Vg2 = 170 volts and do a quick ri calculation centered on a Va of 170 volts with Vg1 at -10 volts you will see that it approximates the 20k that was quoted.

Now look at the Ia Va curves for Vg2 = 100 volts (approx. Vg2 voltage in the receiver) and do the same calculation centered on a Va of 190 volts (approx. Va voltage in the receiver) with Vg1 at -5 volts (approx. -Vg1 voltage in the receiver) you will see that ri has increased somewhat, I make it approx. 32k (ish) which is considerably higher than the 20k first quoted.

Lawrence.
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Old 16th Jan 2020, 4:45 pm   #45
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Hello Lawrence and Thanks

Thanks for the data sheet, that's the one I originally looked at when I got my 3000 figure. I don't really follow the maths on this but I will have a look through the graphs etc. But from the datasheet it says the optimum load is always roughly 3000. I think your saying the impedance value changes with anode current so will be constantsly changing anyway etc

Unfortinately I do have a bad habit of mixing up my RA and Ra or is it ra. To be clear I am interrested in the load impedance not the anode impedance. (Lots for me to learn here)

Thanks
Chris
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Old 16th Jan 2020, 4:49 pm   #46
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

RL is the one of interest when calculating transformer ratios.
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Old 16th Jan 2020, 6:01 pm   #47
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Doing a quick measurement of the impedance it’s about double what I was expecting. I wonder if anyone has tried these measurement in the past The last couple of transformers I have wound have both measured a similar factor out using a variety of impedance bridges and loads . I’m not going to loose sleep over it but it Is concerning when the theory doesn’t match the reality. This transformer is a perfect match into 1 Ohm if I had one speaker that is LOL

I’m pretty sure that I have wound the transformer with the same turns count and wire gauges etc. The results disappoint me!

Chris
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Old 16th Jan 2020, 6:02 pm   #48
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

The lack of a cathode bypass capacitor, giving current negative feedback, together with the very low screen grid voltage (about 90V in practice) modifies Ra - the load impedance.

Some time ago I tested some of Ed Dinning's transformers for this set, which were a very close match to the original. Perhaps he could confirm the turns ratio he used.

Without extensive distortion testing, perhaps unwarranted on a budget radio, it's hard to get a difinitive answer. My feeling is that the Bush turns ratio is correct. A larger core would have been an enhancement, which would have improved reliability with thicker primary wire.

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Old 16th Jan 2020, 6:23 pm   #49
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

The optimum load for a pentode, or beam tetrode, is approximately 0.9 x Vak/Ia. It is largely unaffected by whether the cathode resistor is bypassed; if there's overall negative feedback; etc.

The DAC90a actually runs the UL41 fairly gently, at less than its rated maximum figures. So the standing Ia is less than maximum. This means that the optimum load in the DAC90a will be greater than the data-sheet figure.

It also means that the maximum output power will be less, of course.

Presence of feedback would affect the 'looking backwards' impedance, and the damping that the valve imposes on the speaker (which as the valve's ra is high, is precious little for a pentode by itself anyway). But that is a minor concern.
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Old 16th Jan 2020, 6:56 pm   #50
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Clearly I have a lot to learn LOL. All I can be sure of is the turns ratio that I used, was the same as the original transformer that I unwound and wire gauges were same as the original. Like most subject the rabbit hole goes deeper than expected when looking in from the outside etc.


Thanks so much for all the helpful comments and advice.
My Best Regards Chris
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Old 16th Jan 2020, 7:16 pm   #51
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

You might find the first bit of chapter 2 in this classic book is worth a read, chapter 2 section 2 pages 15 and 16 shows the basics of extracting a valves anode resistance, mutual conductance and amplification factor from its anode characteristics:

https://archive.org/details/RadioDes...dbook/page/n55

Lawrence.
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Old 16th Jan 2020, 7:51 pm   #52
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Thanks Laurence that will be useful as I am thinking about trying to design a simple TRF from scratch as a project.

Chris
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Old 16th Jan 2020, 7:51 pm   #53
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Quote:
Originally Posted by ms660 View Post
You might find the first bit of chapter 2 in this classic book is worth a read, chapter 2 section 2 pages 15 and 16 shows the basics of extracting a valves anode resistance, mutual conductance and amplification factor from its anode characteristics
You can indeed - though the optimum load is not the same as the valve's anode resistance ra, and indeed for a pentode, has very little association with it.

One method of being sure you have loaded the valve optimally, is 'scope the output, feed in 400Hz, wind up the level slowly, and if both peaks start being clipped symmetrically the load is about right. But as most pentodes go into clipping in a soggy manner, and when they do, the speaker cone's inertia tends to fill in the blanks anyway, it's not as useful as ideal theory would predict.
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Old 16th Jan 2020, 8:26 pm   #54
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by ms660 View Post
You might find the first bit of chapter 2 in this classic book is worth a read, chapter 2 section 2 pages 15 and 16 shows the basics of extracting a valves anode resistance, mutual conductance and amplification factor from its anode characteristics
You can indeed - though the optimum load is not the same as the valve's anode resistance ra
I've known that for the last 60 years or so, but for anyone that's missed it.....it's in UL41 valve data link I posted earlier.

Lawrence.
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Old 16th Jan 2020, 11:45 pm   #55
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Hi Folks, I'm away from home at present so no access to my data. However I can say that several of these transformers were reverse engineered and the turns were pretty consistent. I'll let you know the numbers later.
I did find that the core was very similar to a modern EI 48 and I now use these bobbins instead of gluing up the old ones
Kalee, can you confirm the bobbin size please?

Ed
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Old 17th Jan 2020, 8:36 am   #56
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Quote:
Originally Posted by Ed_Dinning View Post
Kalee, can you confirm the bobbin size please?

Ed
This thread has the relevant information and diagrams:

https://www.vintage-radio.net/forum/...ad.php?t=87213

Hope that’s useful.
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Old 17th Jan 2020, 11:21 am   #57
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Default Re: Rewinding Bush DAC90 Audio Output Transformer

Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by kalee20 View Post
You can indeed - though the optimum load is not the same as the valve's anode resistance ra
I've known that for the last 60 years or so, but for anyone that's missed it.....it's in UL41 valve data link I posted earlier.
I wasn't implying you didn't know, Lawrence! Just a caution for anyone reading up about gm, ra, μ... that optimum load is not ra. (For a Class A triode, load and ra are related, but not for a pentode).

Quote:
Originally Posted by Ed_Dinning View Post
Kalee, can you confirm the bobbin size please?

Ed
I can't ATM! However, if Chris can measure the width of the central limb of his transformer laminations, we can work from that.

One problem with transformers is, as a 'black art' not many people tackle them, so suppliers hardly exist who supply the bits in hobby quantities rather than industrial quantities.
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