Heathkit output transformer
- Thread starter kyle
- Start date
Register to hide this ad
The primary impedance is 10K, with the screen taps located at 50% of the winding. It is a 20 watt transformer, so max current will be in the 150 ma range. Max voltage should not be a concern unless you're considering using transmitting tube voltages!
Dave
Dave
kyle
Super Member
Dave, one other thing...
I got a link to some testing done by John Atwood in the 1990's (see link) on a lot of different transformers. The tests were all done with 6550's at 475volts and 75mA. Would this affect the outcome for transformers that were not rated for this operating point?
Specifically, I'm looking at the Acro TO-300 and the previously mentioned Chicago 52-17.
I have both and the Acro specs show a 6600 ohm primary while Mr Atwood measures 7800 ohms. He also shows 10K for the 52-17. That seems high for a pair of 5881's at the same operating points as the Acro in the heathkits amps. These are the outputs for the Heathkit w3m (acro) and w4m (chicago).
http://www.one-electron.com/Trans_Tests.html
I got a link to some testing done by John Atwood in the 1990's (see link) on a lot of different transformers. The tests were all done with 6550's at 475volts and 75mA. Would this affect the outcome for transformers that were not rated for this operating point?
Specifically, I'm looking at the Acro TO-300 and the previously mentioned Chicago 52-17.
I have both and the Acro specs show a 6600 ohm primary while Mr Atwood measures 7800 ohms. He also shows 10K for the 52-17. That seems high for a pair of 5881's at the same operating points as the Acro in the heathkits amps. These are the outputs for the Heathkit w3m (acro) and w4m (chicago).
http://www.one-electron.com/Trans_Tests.html
Vaguely related, but how close is the Chicago 52-17 to a Chicago BO-13 ? I have a BO-13 and have had no luck finding a twin to it. If the W4M's stock iron is the same, then that would give me a better chance of finding a mate so I can finally do something with that thing.
FRom the transformer catalog, a BO-13 is a 10k primary, rated to 20w output, with 4/8/16 ohm output and the screen grid taps. I don't know what percentage the taps are though.
FRom the transformer catalog, a BO-13 is a 10k primary, rated to 20w output, with 4/8/16 ohm output and the screen grid taps. I don't know what percentage the taps are though.
Last edited:
I'm not sure how John developed his data for impedance, but using a low impedance source (variac), an Acro TO-300 is in fact a 6600 ohm transformer, as measured on two that I have. Using a low impedance source minimizes any error in measurement.
The 10K impedance of the W-4 transformer is higher than normal. On the other hand, it produces somewhat less distortion as a result, and also allows for extending the class A portion of the class AB operating conditions employed.
The Acro transformer is the original UL transformer, but as is so often the case, it was introduced into designs with compromises of economy even by Acrosound themselves. In this case, the TO-300 is ideal with 6L6 class tubes when fixed bias is used -- not cathode bias, as is so often seen. As such, this transformer can produce the most power output, and very low distortion when fixed bias is used. On the other hand, transformers with 7600 ohms (such as those used in the Eico HF-20/22/35 series) work very well with these tubes when cathode bias is used (and frankly better than the Acro transformer does with cathode bias). So does the Heath transformer as well.
The biggest problem with the Heath transformer, is that its winding capacitance is such that the transformer actually limits HF power output before the core saturates on the low end. As a result, the transformer is really only good for about 16 watts RMS -- which is just under or right at about what a stock W4M can produce, depending on AC line levels.
The Eico transformers are very good up to about 25 watts RMS (making poor devices to use in the HF-35), as are the Acros -- all as related to producing a flat 20 Hz to 20 kHz power curve.
I hope this helps!
Dave
The 10K impedance of the W-4 transformer is higher than normal. On the other hand, it produces somewhat less distortion as a result, and also allows for extending the class A portion of the class AB operating conditions employed.
The Acro transformer is the original UL transformer, but as is so often the case, it was introduced into designs with compromises of economy even by Acrosound themselves. In this case, the TO-300 is ideal with 6L6 class tubes when fixed bias is used -- not cathode bias, as is so often seen. As such, this transformer can produce the most power output, and very low distortion when fixed bias is used. On the other hand, transformers with 7600 ohms (such as those used in the Eico HF-20/22/35 series) work very well with these tubes when cathode bias is used (and frankly better than the Acro transformer does with cathode bias). So does the Heath transformer as well.
The biggest problem with the Heath transformer, is that its winding capacitance is such that the transformer actually limits HF power output before the core saturates on the low end. As a result, the transformer is really only good for about 16 watts RMS -- which is just under or right at about what a stock W4M can produce, depending on AC line levels.
The Eico transformers are very good up to about 25 watts RMS (making poor devices to use in the HF-35), as are the Acros -- all as related to producing a flat 20 Hz to 20 kHz power curve.
I hope this helps!
Dave
Thanks for the kind words, and good luck with your project!
Dave
Dave
kyle
Super Member
You're mistaken. 5K is perfectly acceptable for a single tube operating point of 350V at 50mA and -76 on the grid. That's straight class A. In stereo it's about twice in practice, so about 10 to 14 watts, depending on the power supply. It's in the Westinghouse specs.
http://www.tubebooks.org/tubedata/we300a_b.pdf
It came to me on a mono DIY amp project. And yeah, potted can, with bottom terminals. No leads. I still wonder if its basically the same thing other than the leads vs terminals.
Tom, thanks for the further data on the BO-13. I guess that could be considered nominally a 10k primary with a 50% tap?
battradio
Electron trainer
Yes but you will be using it in PP not single ended , and 5K PP is where you want to be , 10k will be about 6 to 8 watt at clipping for PP
http://tubedata.tubes.se/sheets/136/3/300A.pdf
Last edited:
kyle
Super Member
I see you're using the older 1936 300A spec sheet. You should update your files with the link I provided. It has the more up to date version from 1939. Just kidding.
I would use a 5K trans for 2A3's but for 300B amps I like a higher impedance. In my SE 300b's I use at least 3.5K and more often 4 to 5 K primaries. Also, I have extra parts sitting around which is the main motivation for using this transformer.
Can 'splain why you think 5K would be better?
I would use a 5K trans for 2A3's but for 300B amps I like a higher impedance. In my SE 300b's I use at least 3.5K and more often 4 to 5 K primaries. Also, I have extra parts sitting around which is the main motivation for using this transformer.
Can 'splain why you think 5K would be better?
