Power Transformer PT009 - CTM 33302 info

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Lani
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Post by Lani »

So I picked up 7 of these on the Bay for a great price thinking they would be good in tube amp projects. I measured all the resistances for them (see below image) and I'm pretty confident I know what wires are the primary and what are the secondary. The primary is a little strange and worries me because you would think the resistances would be the same on both halves which they aren't. Is there a conventional color coding scheme with the wires? I know black & white for the primary is Line & Neutral but I'm unsure on brown (maybe a 100v tap option). I'm planning on feeding it with another transformer, say 6VAC (I know, I need a Variac) and take measurements and do the math, (don't worry, I've been an electrician for 25 years now) but first wanted to post here to see if anyone knows where to get a datasheet or if I'm way off base here. Am I correct that the manufacturer is CTM? Searched Google all morning for info on it and found notta. It does look like it has a high voltage CT winding as well as a low voltage CT winding. Any help would be appreciated! Time to go snow blow the driveway..... :x
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lolbou
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Post by lolbou »

Lani wrote: 15 Mar 2023, 11:35 worries me because you would think the resistances would be the same on both halves which they aren't
Actually the extra coils (BK to BR) may be to switch between 115 or 120 input and or 230 / 240 when you use the primaries in series or parallel..

You can also use a signal generator at 60 (or 50) Hz and measure the secondaries, and if you don't have a variac, use another transformer to get some 12V AC input (you'll be isolated from the grid regarding current).

The outputs looks center tapped as you mention, but the 300V you're after will then have to be rectified from the opposite ends of the secondary with a full wave rectifier.

TBH I wouldn't expect very high voltages...
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Post by george giblet »

Technically the manufacturer's spec for the primaries could be 0V (WH), 110V (BR), 115V (BK) or 0V (WH), 115V (BR), 120V (BK). It's hard to split that difference.

To get the full transformer rating you normally need to parallel the primaries for nominal 115V etc. supplies. So if you use the White + Black primary both White's connect together and both Black connect together. Similarly if you use the White + Brown primary both White's connect together and both Browns connect together.

My guess is the transformer rating is around 80VA to 100VA. Perhaps a better estimate could be given if we knew the core outer dimensions (width and height) and the total thickness of the laminations (the stack).

What you don't know is if both secondaries. are rated at 50VA. You can estimate that once you measure the actaual secondary voltages.

Regulation of a transformer that size is likely to be around 6.5% to 7%. That means with no load the output voltage are 6.5% to 7% higher than the manufacturers secondary voltage rating.

From the winding resistances, and assuming 240V in (both WH + BK = 120V, in series), it looks like the secondaries are 500Vct (250-0-250V) and 40Vct (20V-0V-20V). This is a very unreliable estimate with a lot of assumptions. You have to measure the output voltages directly.

When using a full-wave rectifier and a filter you need to derate the DC current.

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Lani
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Post by Lani »

Thanks for getting back to me guys. I will test with a small AC voltage in the morning and report back what I find.

Core dimensions are roughly 55mm x 55mm Laminations are W=85mm L=72mm H=28mm.
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Post by george giblet »

Core dimensions are roughly 55mm x 55mm Laminations are W=85mm L=72mm H=28mm.
Those dimensions look fairly consistent for that size transformer.

The core size: EI86x29

Based on standard temperature winding, I estimate rating is in
the order of 80VA to 85VA. Higher temp windings will increase
the rating but there's no simple way to know this is the case
and it's a high risk to make this assumption.

The primary resistance of 18.6 ohm is indicating a 60Hz transformer.
For 50Hz, even if the primaries limited to 230V max (115V + 115V)
it still looks like the core is pushed a little too hard (ie. high Bpk).
It will take some detailed measurements and testing to resolve
the finer points for 50Hz.

Transformer regulation is in the order of 5% to 6%.
(Easy to measure actual with a dummy load.)

Keep in mind these are estimates. It's very difficult to know categorically
what the manufacturer's specs are. It's a good idea to use the low
VA estimate.

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Post by Lani »

Took a bit but I finally got around to testing this out and it ended up being 325VCT (162v-0v-162v) for the HT winding and 38VCT (19.2v-0v-19.2v) for the LV winding.... Thanks for all the help everyone!!
Check out this contraption I just whipped up for testing it :lol:
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Post by lolbou »

Good catch then, thx for the feedback! Seems like you have a B+ and negative bias transformer!!

Very neat setup indeed! :D [smilie=a_goodjob.gif]
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Post by george giblet »

Took a bit but I finally got around to testing this out and it ended up being 325VCT (162v-0v-162v) for the HT winding and 38VCT (19.2v-0v-19.2v) for the LV winding.... Thanks for all the help everyone!!
Check out this contraption I just whipped up for testing i
Interesting the secondary is only 325V ct. The 325V ct + 146.7 ohm is a little inconsistent with the resistances of the primary and the 38V ct. It's certainly possible to make transformers like that - that's the thing about a custom transformer. What it means is the 38V ct winding is rated at a higher VA than the 325V ct, for example 50VA and 30VA. You might not even get the full 80VA.

Do you know what your mains voltage was when you measured 325V?

As a side note you might be wondering why the *smaller* Signal Co DP-241-8-12 on your test jig is 100VA but the larger transformer is only 80VA! For one, the Signal Co. has a higher temperature B-class winding. I also suspect the 100VA rating is at an ambient of 25 degC whereas it's quite common to rate transformers at an ambient of 40 degC (that includes the my 80VA estimate). Without those two points a transformer the size of the Signal Co DP-241-8-12 might only be rated at 65VA! I couldn't confirm the ambient for the Signal Co. was 25 degC but I found this almost identical Triad F8-12 and you can see it is 100VA with a 25 degC ambient.

https://catalog.triadmagnetics.com/Asset/F8-12.pdf

The 25 degC issue is a common problem with ratings on modern transformers. FWIW, the signal DP-241-8-12 transformers we made as far back as 2001.

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Post by Lani »

As it was around 6pm (EST) when I tested it and yeah the line voltage was a little low at only 114v so I'll measure it again here a little later tonight when demand is lower and I have closer to 120v and see what I get then.
Here are the voltages I got while stepping up the voltage little by little
6.5v in > 19.2vct out (9.4v - 0v - 9.4v)
13.4v in > 38.9vct out (19.2v - 0v - 19.2v)
17.5v in > 49.8vct out (24.5v - 0v - 24.5v)
35.4v in > 100vct out (49.8v - 0v - 49.8v)
114v in > 325vct out (162v - 0v - 162v)
118v in > 337vct out (168v - 0v - 168v)
120v in > TBD
Been looking at Rob Robinettes site specifically the deluxe micro ( https://robrobinette.com/Deluxe_Micro.htm ) for a little practice amp. Would this power transformer be ok for an amp like that? I think it's only 4 watts or so max. Planned on using that Signal DP-241-8-12 for the heaters which is overkill but leaves room for upgrades / more tubes if I want down the road.
So I take it you work for Signal Co?
Appreciate the help bud!! Have a great weekend!!

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