secondary impedence

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geoff 1965
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secondary impedence

Post by geoff 1965 »

s-l1600 (7).jpg
just picked this little beauty up on ebay,it has a valve line up of 1 EZ81,4 ECL82's ,1 ECC83. my intention is to turn it into a push/pull ECL86 guitar amp using one of the output transformers if i can,there is just a + and - terminal on the secondary winding so doe's anyone know how i can check the impedence?
the PT has 270-0-270 HT unloaded so that should put me in the 10 watt ballpark and there is two output transformers so if i can use one of them it would be good.
note; i don't have a variac or signal generator to determine the turns ratio like the instructual vids on youtube show but i might be able to improvise and use the 6.3V filament supply from my other amp.
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fenderbass39uk
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Re: secondary impedence

Post by fenderbass39uk »

Good find Geoff- looks like those trannies were dipped in wax! It was a stereo hi- fi amp I presume...any makers name on it?
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geoff 1965
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Re: secondary impedence

Post by geoff 1965 »

no name on it Colin but all the components are top qualty,yeah the OT's have been dipped and are quite beefy, it will have been hi-fi because the primary's have linear taps for screen and there is a capacitor between the centre tap and each plate lug which i'll remove if they are suitable for guitar.
ecl82 plate load for 2 in push pull is 10K ohms compared to el84's 8K and ecl86's is 9K1 so hopefully i might be able to use them.need to work out the turns ratio for the speaker load.
i bought it mainly for the power transformer and paid £30 for it!
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fenderbass39uk
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Re: secondary impedence

Post by fenderbass39uk »

I'll have to check out ecl82's as I'm not familiar with them. Yeah the PTx looks useful anyway. The o/p tx impedance testing is an interesting point as I've posted a thread about testing my strange o/p tx but had no response as yet "every day is a school day "
Last edited by fenderbass39uk on Mon 02/24/20 2:32 pm, edited 1 time in total.
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geoff 1965
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Re: secondary impedence

Post by geoff 1965 »

yeah i was hoping someone knew a simple way with a multimeter but it's not as easy as that!
there's less plate voltage and current draw with the ecl82's but this PT handled four of them so it will easily cope with 2 ecl86's.
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geoff 1965
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Re: secondary impedence

Post by geoff 1965 »

just put 6.3 volts into the secondary of both OT's and measure 363 and 362 volts on the primary plates so they are very closely matched. that gives me a turns ratio of 57.5 to 1,now onto the hard part of working out the speaker load, uncle doug vids here i come!
as these are push/pull do i need to measure the voltage from the centre tap to each plate on the primary?
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geoff 1965
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Re: secondary impedence

Post by geoff 1965 »

okay, the winding ratio is 57.5 so i square that to get my impedance ratio = 3317 now the effective load resistance of 2 ecl86's in push/pull with 300 plate voltage is 9100 ohms so if i divide the plate load by the impedance ratio that gives me a speaker load of 2.74 ohms.
the plate load of the ecl82's that were in the amp at 250 plate voltage is 10K which gives a 3 ohms speaker load,look's like they are suitable for a hifi speaker not guitar amp,shame,i would have liked to use one of these!
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Daviedawg
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Re: secondary impedence

Post by Daviedawg »

For me, (without checking your maths) that is close enough to use a 4 ohm speaker load with those valves. But you maybe need to satisfy yourself on that matter.

Dd
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geoff 1965
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Re: secondary impedence

Post by geoff 1965 »

looking for 8 or 16 ohms ideally Dd, the plate load calculation for the ecl86's was at full 13watts but i'm going to have a lower plate voltage and bias them to around 10W and look out for a nice used 20W greenback.
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Re: secondary impedence

Post by zaphod_phil »

geoff 1965 wrote:
Sun 02/23/20 5:52 pm
ecl82 plate load for 2 in push pull is 10K ohms compared to el84's 8K and ecl86's is 9K1 so hopefully i might be able to use them.need to work out the turns ratio for the speaker load.
ECL86s run happily with an 8k primary impedance and 300V on the anodes. I know this from one of my favourite builds, a "13W Lite II", which uses an 18W OT (an RS clone built by Ingo). It sounds fabulous, as well as loud. :D
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geoff 1965
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Re: secondary impedence

Post by geoff 1965 »

sound's cool ZP, i had a half power "9W" tap on the PT i've just replaced on my lite2 and i was expecting a considerable drop in volume but actually it was just as loud as full 18W! the tone was pretty much the same as well just a little earlier breakup.
something is confusing me here regarding primary impedence,you have 8K with 300V on the plates but my mullard valve data book show's 9K1 for ecl86's push/pull with 300V plate?
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Re: secondary impedence

Post by zaphod_phil »

geoff 1965 wrote:
Fri 02/28/20 6:39 pm
sound's cool ZP, i had a half power "9W" tap on the PT i've just replaced on my lite2 and i was expecting a considerable drop in volume but actually it was just as loud as full 18W! the tone was pretty much the same as well just a little earlier breakup.
something is confusing me here regarding primary impedance,you have 8K with 300V on the plates but my Mullard valve data book show's 9K1 for ecl86's push/pull with 300V plate?
Impedance matching doesn't have to be very exact for valve guitar amps. Keep in mind the Mullard books were intended for audio/hi-fi. Plus I couldn't let this lovely 8k 18W OT, made for me by Ingo, go to waste. The result turned out really good, with a sweet, creamy 18W-style distortion, and a big, loud sound! :D
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geoff 1965
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Re: secondary impedence

Post by geoff 1965 »

that's good to know it does'nt have to match perfectly,apart from the 18W's there's quite e few 10 & 15W's to consider for the ecl86's. i need to add another string to my bow and learn all i can about OT's,time and time again i see what look like good used transformers on ebay like ferranti's etc that don't have specs only a resistance reading which puts me off buying.
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