5K Tape Machine OTs Tested

paulbottlehead

Active Member
There has been a lot of interest in the small 5K output transformers that come out of tape recorders and how suitable they may be for HiFi amps, so Electrovice was kind enough to lend me an Akai and a Sony unit for testing. In order to test the iron, I will measure inductance with no DC current, then apply DC current to determine how much current can be run through these. After that I will install each transformer in some Paramour II amps setup to run 45s so I can test bandwidth, power, and distortion in this iron.

First up, the primary inductance and current tests!
Sony: 15H at 0mA DC, primary current should be no more than 50mA.
Akai M8: 24H at 0mA DC, primary current should be no more than 40mA.
One-Electron UBT-2: 25H at 0mA DC, primary current can exceed 100mA (my test setup doesn't go above 100mA).

Now for power, bandwidth, and distortion tests.
Sony:
Sony 1kHz.jpg
Here's the Sony at 1kHz and 2.8W at 10% THD.
Sony Frequency Response 1W.jpg
Here's the Sony's frequency response at 1W. This OT is -1dB at abt 39Hz and 12kHz.

At 35Hz, the Sony did 1.3W at 35Hz/10% THD.

Akai:
Akai 1kHz.jpg
This is the Akai doing 2.4W at 1kHz and 10% THD.
Akai Frequency Response 1W.jpg
Here's the Akai's frequency response at 1W. This OT is -1dB at about 32Hz and 8kHz. I flipped the primary connections around to see if this could be improved, but it is what it is.

At 35Hz, the Akai did 15W at 10% THD.

One-Electron:
One-Electron UBT-2 1kHz.jpg
This is the One-Electron doing 2.9W at 1kHz and 10% THD
One-Electron UBT-2 Frequency Response 1W.jpg
Here's the One-Electron's frequency response at 1W. This OT is -1dB at 22Hz and 16kHz.

At 35Hz, the UBT-2 did 2W at 10% THD.

As a little bonus data point, I put a Magnequest RH-60 in the mix. It did 3.4! watts at 1kHz and 10% THD and was -1dB at 28Hz and 14kHz.

I was quite sure that the Akai would beat the Sony, as the Sony has that 600 ohm winding working against the high frequency response, but that wasn't the case at all. Sure, the Akai goes lower, but the Akai has nothing going on up top. Naturally the two newer and more expensive output transformers are quite a bit better, but at the end of the day the plate impedance of the #45 starts to limit the performance of these transformers (just plugging in a 2A3 into the same circuit is going to flatten things way out!).

I also tried connecting the 600 ohm winding on the Sony as a feedback winding and that helped the bass tremendously (now -1dB at 22Hz), but the treble response started to look a lot like the Akai. There is probably happy medium where the feedback winding is used to get that nice bass response and a driver cathode resistor could be bypassed by a small capacitor to nudge the treble up a bit more.
 
That was informative.

In it's original application, the Sony tape machine used a 6AQ5 amplifier and a 4 inch "Holey Basket" speaker. The Akai machine used a 6BQ5 amplifier and an 8 inch Pioneer PIM speaker. Under the best of circumstances, a four inch driver will have limited bass response and an eight inch full range will have more bass but less high end response. My recollection was that the PIM drops off quite a bit above 8K or so. It would seem that the measured performance of these transformers match the requirements of their original applications.

Thanks Paul.
 

Audionut

Next Round Is On Me
Thanks for posting, now to do something with my two sets also....
Oh, if testing is done and those Sony fitted 45 amps are in your way......;)
 
I have never understood the fervor for the Hashimoto series feed OTs.
Never heard them. On paper, the 20W units look much better than the 7W. But really, for those prices there are some rather fine transformers available.

A bit off topic: One day I might try their the better interstage, apparently their answer to the NC-20 types... and I’d like to compare those types of refined-over-time transformers against quality, custom units.
 
The HATT Audiolab new HO 808 is very good 10Hz -80kHz -2dB
When Tango ceased production HATT Audiolab Japan reversed engineered most of their line as drop in replacements
The HO -808 is 2.5kg the old Tango U808 1.6kg . The new HO-808 is much better retaining 2.5K 3.5K 5K primary etc $120 USD each Japanese Orient Steel
 

John Frum

Secret Society Member
The HATT Audiolab new HO 808 is very good 10Hz -80kHz -2dB
When Tango ceased production HATT Audiolab Japan reversed engineered most of their line as drop in replacements
The HO -808 is 2.5kg the old Tango U808 1.6kg . The new HO-808 is much better retaining 2.5K 3.5K 5K primary etc $120 USD each Japanese Orient Steel

The price is right, but bringing up the U-808 will get you dragged around here. 😉
 
Yep the old Tango U808 was not very good , the HATT HO-808 is only similar in name only .
Its a very different piece of gear nearly a kilo heavier and was built to compete with Tango XE-20 , more extended on the bottom end slightly less on the top end, always thought the XE20 needed more bass support
 
I have also never understood the fervor for interstage transformers ;)
I’m only in the early stages of putting together an interstage coupled amplifier. No fervour here, just curiosity... wanting to learn through doing, when I have the chance. Some experienced builders prefer interstages so I’m willing to give it a good chance.

Do you still consider interstages might have something to offer or have you written them off? Can you share your interstage listening experiences - sonically, what is it you don’t like? Technically, what do you not like?

I think Bottlehead uses CCS and capacitor (or direct) coupling: I’m interested in trying that approach at some point.
 
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I’m only in the early stages of putting together an interstage coupled amplifier. No fervour here, just curiosity... wanting to learn through doing, when I have the chance. Some experienced builders prefer interstages so I’m willing to give it a good chance.

Do you still consider interstages might have something to offer or have you written them off? Can you share your interstage listening experiences - sonically, what is it you don’t like? Technically, what do you not like?

I think Bottlehead uses CCS and capacitor (or direct) coupling: I’m interested in trying that approach at some point.
I have Bottlehead....
6n1p CCS loaded cap coupled 300b
6n1p choke loaded direct coupled 2A3
3b7 cap coupled 45/2A3

I listen to the all DHT...I think you are headed in the right direction.
 

paulbottlehead

Active Member
Do you still consider interstages might have something to offer or have you written them off? Can you share your interstage listening experiences - sonically, what is it you don’t like? Technically, what do you not like?
Yes, they certainly have something to offer in terms of handling grid current a bit better when the amp is being pushed hard, but on the negative side they put some serious limitations on driver tube selection.
 
Yes, they certainly have something to offer in terms of handling grid current a bit better when the amp is being pushed hard, but on the negative side they put some serious limitations on driver tube selection.
Thanks Paul. Technical reasons then. Agreed regarding the driver (and a decent interstage); not insurmountable, but a perhaps costly challenge.

I listen to the all DHT...I think you are headed in the right direction.
Thanks mate. I hope that is the case. If not, well, I will try something different. 😊

I need to get it done.
 
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paulbottlehead

Active Member
I don't remember doing a whole lot of bench testing on it. The stock amp measured about -1dB at 20kHz and the one with the 3B7 shouldn't be any worse than that. Worn out tubes sure can sound like that though!
 
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