I know that putting an 4ohm speaker on a 8ohm secondary tap will halve the input impedance as seen by the output tube(s). But the diagrams below have me confused. Why does the input impedance on the 18037 vary with each tap, when the 18081 states 6Kohms (regardless of tap used?).... Is this just an average, and the 18037 states it more precisely? I'm looking for an OPT for a Marshall 18W project. Thanks!
Wow....I would hope they would tell me that, when I want to have as close a match to what tube data sheets recommend. I know the amp will still work, but when I'm changing things to optimize the sound, I would like to know what I'm working with. Do all mnfrs average this primary impedance the same way? Can I use the more precise information as a usable guideline to what the tubes are seeing when I change the speaker tap? I had thought using a 4ohm spkr on the 4ohm tap was THE SAME as an 8ohm spkr on the 8ohm tap.....
You might find life a lot easier of you would lose the word "precise" from discussions of guitar amps. They are not remotely precise.
Imagine tabletop height in a dining room was 33 inches. Now imagine frustration over concern whether your table was 33" exactly or 31.5" or 34". or maybe tire pressure on your car is supposed to be 32 pounds. Is 34 pounds less than optimal? Is it even wrong?
Your tubes have curves, and the impedance points are points on these curves. Your signal then expands along those curves either way from rest point. Now imagine the band of signal moving up and down along that curve as impedances vary. Note that little will change if you move the signal along either way somewhat.
And consider the load, which is almost always a speaker. Pick one. Look at its response curve. It is only 8 ohms on an "8 ohm" speaker at one point on the response. Example:
With that in mind, the transformer has no impedance of its own, all it does is transform the secondary to primary impedances in a ratio. So a 6k primary impedance ONLY exists when the 8 ohm exact load is on the secondary. Put 4 ohms across that same secondary and that same primary no longer shows 6k to the tubes.
Look at a tube curve:
The blue line is at 300v, so imagine signal going 100v either way from there. See how little anything would change if you shoved that 200v band either way left or right some amount? Also note if you went to a higher or lower current range, the curves are roughly parallel, so again, little will change if you move things vertically.
CLose enough works in a guitar amp. precise works in laboratory equipment.
Imagine tabletop height in a dining room was 33 inches. Now imagine frustration over concern whether your table was 33" exactly or 31.5" or 34". or maybe tire pressure on your car is supposed to be 32 pounds. Is 34 pounds less than optimal? Is it even wrong?
Your tubes have curves, and the impedance points are points on these curves. Your signal then expands along those curves either way from rest point. Now imagine the band of signal moving up and down along that curve as impedances vary. Note that little will change if you move the signal along either way somewhat.
And consider the load, which is almost always a speaker. Pick one. Look at its response curve. It is only 8 ohms on an "8 ohm" speaker at one point on the response. Example:
An externally hosted image should be here but it was not working when we last tested it.
With that in mind, the transformer has no impedance of its own, all it does is transform the secondary to primary impedances in a ratio. So a 6k primary impedance ONLY exists when the 8 ohm exact load is on the secondary. Put 4 ohms across that same secondary and that same primary no longer shows 6k to the tubes.
Look at a tube curve:
An externally hosted image should be here but it was not working when we last tested it.
The blue line is at 300v, so imagine signal going 100v either way from there. See how little anything would change if you shoved that 200v band either way left or right some amount? Also note if you went to a higher or lower current range, the curves are roughly parallel, so again, little will change if you move things vertically.
CLose enough works in a guitar amp. precise works in laboratory equipment.
Enzo-thanks for putting my concerns in the proper perspective....now I can sleep tonight..... ��. I will keep it all in mind, and use my ear to tell me when it is right.
Another possibility would be an OT19PP transformer (for Marshall 18W amp) from Matt at
Musical Power Supplies, Inc. - Home
I've been happy with the transformers I've bought from him, and his communication and helpful attitude are great. His transformers are mostly made in USA, and I like dealing with smaller businesses.
Thanks VG, took a look at their offerings. I have a Weber kicking around, will start with that, I want to hear how EL34s compare to 6V6s, but the OPT does have a lot to do with the sound as well. The amp power section is cathode biased rather than grid, so will play with that a bit also.
EL34 or 84?
I think that 6BQ5/EL84 tube is part of the '18-watt sound', but that may be part of the reason I never got really attached to my '18-Watt Lite (non tremolo)' build. I guess I'm a 6V6 (and less loud) amp guy at heart!
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