Hello:
I have a question about a NOS Heathkit output transformer I found...p/n 51-31, apparently from a T-4 Signal Tracer that used a 12CA5 tube.
https://frank.pocnet.net/sheets/049/1/12CA5.pdf shows typical plate resistance is 15-16 K and a load is 3500-4500 ohms.
Before finding this, I was mistakenly looking for 12CA6, and did some turns ratio experiments with the transformer.
Using a Variac, I applied 120.0 vac (60 Hz) across the full primary and measured 2.34 Vac output (unloaded) = 51.28:1 turns ratio. With an 8 ohm secondary load, the primary would look like about 21k.
I applied 60.0 vac across one half primary and got 2.33 vac output (unloaded) = 25.75:1 turns ratio. 8 ohms on secondary looks like 5.3 k on primary.
Does this make sense?
I thought to reduce the applied voltage on the half-primary because it might 'not like it' but I didn't consider exactly why. (Subconscious what the flux thoughts?).
As I type this, I wonder if 120 v at 60 Hz was too much for such a small core and if unloaded measurements are valid.
I did the variac test because the day before I tried to measure primary and secondary inductance with an inductance analyzer at 100 Hz, 1 V, then took turns ratio of inductance ratio because inductance is proportional to impedance. I usually only trust my iron-core inductance measurements if they agree with the turns ratio. 1 V is awfully far from a realistic OPT application. Full primary (blue-green) measured 24.2 H
The only thing I was satisfied with about the inductance measurements was that the two half primaries were very close to each other (but not exactly 1/4 the full winding): blue-red = 6.364 H, green-red = 6.367 H
Secondary (blk-blk) was 14.483 mH.
Lp/Ls gives 1670.9 for the full winding, 439.5 for the half windings. 8 ohms on secondary gives 13.3 k on full primary, 3.5 k for half primaries.
If all this is a useless educational exercise, any suggestions for other tubes usable with this OPT in a tiny SE guitar amp? (Unlike the T-4 owners, I only have the transformer).
Thanks for reading.
I have a question about a NOS Heathkit output transformer I found...p/n 51-31, apparently from a T-4 Signal Tracer that used a 12CA5 tube.
https://frank.pocnet.net/sheets/049/1/12CA5.pdf shows typical plate resistance is 15-16 K and a load is 3500-4500 ohms.
Before finding this, I was mistakenly looking for 12CA6, and did some turns ratio experiments with the transformer.
Using a Variac, I applied 120.0 vac (60 Hz) across the full primary and measured 2.34 Vac output (unloaded) = 51.28:1 turns ratio. With an 8 ohm secondary load, the primary would look like about 21k.
I applied 60.0 vac across one half primary and got 2.33 vac output (unloaded) = 25.75:1 turns ratio. 8 ohms on secondary looks like 5.3 k on primary.
Does this make sense?
I thought to reduce the applied voltage on the half-primary because it might 'not like it' but I didn't consider exactly why. (Subconscious what the flux thoughts?).
As I type this, I wonder if 120 v at 60 Hz was too much for such a small core and if unloaded measurements are valid.
I did the variac test because the day before I tried to measure primary and secondary inductance with an inductance analyzer at 100 Hz, 1 V, then took turns ratio of inductance ratio because inductance is proportional to impedance. I usually only trust my iron-core inductance measurements if they agree with the turns ratio. 1 V is awfully far from a realistic OPT application. Full primary (blue-green) measured 24.2 H
The only thing I was satisfied with about the inductance measurements was that the two half primaries were very close to each other (but not exactly 1/4 the full winding): blue-red = 6.364 H, green-red = 6.367 H
Secondary (blk-blk) was 14.483 mH.
Lp/Ls gives 1670.9 for the full winding, 439.5 for the half windings. 8 ohms on secondary gives 13.3 k on full primary, 3.5 k for half primaries.
If all this is a useless educational exercise, any suggestions for other tubes usable with this OPT in a tiny SE guitar amp? (Unlike the T-4 owners, I only have the transformer).
Thanks for reading.
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Half of the primary normally presents 1/4 of the impedance. Think about it -- the square of 0.5 is 0.25.
😉
Thank you.
Well, seeing your name, you might expect me to get the center tap = 1/4 inductance, or not get it...no in between😀...just joking...
I do understand it and was impressed the two half primaries seem well balanced...I think I have seen a transformer datasheet once with a spec for 1% tolerance on a center tap...I would think that would be voltage.
The part I don't understand, and have seen before when measuring inductance, is that the two halves are equal, but not exactly 25% of the full inductance. There is probably a reason...I'll guess it's a measurement phenomenon (test method). I have seen it with several inductive components, both iron- and ferrite-cored, on two different fancy pieces of equipment. I think with single-bobbin transformers there are things that happen due to concentric windings having the proper number of turns but different geometry...the inside winding has smaller average length per turn and the outer winding ends up requiring more wire length due to a larger average length per turn as the bobbin is progressively filling up with wire. Two windings with good voltage and inductance balance often have different resistance as a result.
Anyway, maybe I don't understand how plate resistance and primary load data on a data sheet relate to transformer selection, as my numbers don't seem to indicate this transformer agrees with the 12CA5 datasheet (but obviously is used with it in the Heath T-4.
I'm hoping someone can offer an opinion toward another compatible tube, to throw together a small instrument amp.
M
Thank you.
Well, seeing your name, you might expect me to get the center tap = 1/4 inductance, or not get it...no in between😀...just joking...
I do understand it and was impressed the two half primaries seem well balanced...I think I have seen a transformer datasheet once with a spec for 1% tolerance on a center tap...I would think that would be voltage.
The part I don't understand, and have seen before when measuring inductance, is that the two halves are equal, but not exactly 25% of the full inductance. There is probably a reason...I'll guess it's a measurement phenomenon (test method). I have seen it with several inductive components, both iron- and ferrite-cored, on two different fancy pieces of equipment. I think with single-bobbin transformers there are things that happen due to concentric windings having the proper number of turns but different geometry...the inside winding has smaller average length per turn and the outer winding ends up requiring more wire length due to a larger average length per turn as the bobbin is progressively filling up with wire. Two windings with good voltage and inductance balance often have different resistance as a result.
Anyway, maybe I don't understand how plate resistance and primary load data on a data sheet relate to transformer selection, as my numbers don't seem to indicate this transformer agrees with the 12CA5 datasheet (but obviously is used with it in the Heath T-4.
I'm hoping someone can offer an opinion toward another compatible tube, to throw together a small instrument amp.
M
Hmmm, found this...IT-12 used same OPT. other poster says 4k:4 with 4 ohm speaker.
Antique Radio Forums • View topic - OT replacement for Heathkit T-4 Signal Tracer
I still measured what I measured...
Antique Radio Forums • View topic - OT replacement for Heathkit T-4 Signal Tracer
I still measured what I measured...
So this OPT has a center-tapped primary but was used in a single-ended output stage? How exactly do you wish to use it; SE or PP?
Okay, I poked around a bit and discovered that your OPT was meant to be used as a quickie substitution in troubleshooting work, which explains a lot. I would consider running a pair of 12C5s in PP because ESRC has 'em for a buck each and you could get that big amp sound at a low power level. But I'm no guitar amp guy. You would probably get better advice by posting in the Instruments & Amps section. Try to look up the $100 Amp Challenge thread. Tubelab_com posted a complete amp circuit using some other $1 output tubes that looked extremely well executed to me.
Ok, thanks...I subscribed to $100 amp but sometimes find they are further off the track than where I'm looking.
Since finding SE OPT's in a box doesn't usually happen everyday, I was thinking single tube.
For a moment I was thinking it was a shame to not use it as SE, since PP design xfmr taunts us when wishing for SE...
I am just trying to make a minimalist amp as small as possible as kind of an end table novelty, but functional...kind of a follow through to a humorous challenge.
I've been hoarding some 5687W?'s...I might look at PP pair.
6BM8 triode/pentode I think also is usually shown with 5k primary...I have some of those.
I'll dig in the basement when I get home. Maybe some xC5's are there too. It's so often the lack of OPT than interferes with making decisions.
Since finding SE OPT's in a box doesn't usually happen everyday, I was thinking single tube.
For a moment I was thinking it was a shame to not use it as SE, since PP design xfmr taunts us when wishing for SE...
I am just trying to make a minimalist amp as small as possible as kind of an end table novelty, but functional...kind of a follow through to a humorous challenge.
I've been hoarding some 5687W?'s...I might look at PP pair.
6BM8 triode/pentode I think also is usually shown with 5k primary...I have some of those.
I'll dig in the basement when I get home. Maybe some xC5's are there too. It's so often the lack of OPT than interferes with making decisions.
I should probably post just the specs I measured once I think they are valid.
Maybe useful to someone else...
I think the resistance imbalance in center-tapped primary in interesting. I measured 438 ohms across full primary, but 198 and 240 for the two halves. AC balance as measured inductance and 60 Hz voltages was very good. About 20% difference in resistance so Si think that confirms inner/outer winding hypothesis.
Maybe useful to someone else...
I think the resistance imbalance in center-tapped primary in interesting. I measured 438 ohms across full primary, but 198 and 240 for the two halves. AC balance as measured inductance and 60 Hz voltages was very good. About 20% difference in resistance so Si think that confirms inner/outer winding hypothesis.
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