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Old 10th January 2010, 06:48 PM   #1
Reidy- is offline Reidy-  United Kingdom
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Default 8ohm resistor in series with 8ohm speakers for a 16ohm OT?

Probably a bad idea but as I am on a tight budget I thought I'd get some opinions before throwing it out the window.

There are some cheap 40watt PP OT's that I am considering buying, but they have a 16ohm secondary (3.2k primary), well my speakers are rated at 8ohms.

what will be the outcome soundwise? I presume that there will be a gain in output power but will it be at detriment to the sound quality?

I havnt really decided upon a circuit yet, but I have bought 2 KT88 JJ-electronics tubes for use and am currently weighing up PP or SE.

if I where to wire a 8ohm resistor inbetween the speakers and the amp would it enable the use of my speakers and again what kinda power/sound differences should I expect
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Old 10th January 2010, 08:56 PM   #2
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Using the wring speaker tap will affect performance & sound. The 8-ohm resistor is electrically a good solution - it corrects the loading - the drawback is that it drops the efficiency by 50%. You'll have an output of exactly half the power the amp can deliver.

It also greatly increases the damping factor - which is never huge with tube amps, so that's not a major problem. The bass might be a bit more soggy than 'normal'.

The resistor will turn that 50% lost power into heat, so it better be a big one.

If the transformer is designed for PP, you can't make an SE amp from it (not without a lot of added headaches trying to generate a balance current for the transformer to stop the core saturating)

Since you have only 2 KT88s, you could go for mono PP, and series connect your two speakers for 16 ohm.
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Old 10th January 2010, 09:11 PM   #3
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Im afraid that it doesnt increase the damping factor, but totally destroys it.

Maybe there is a better solution. If the OPT can handle the power, you could double the output tubes, and connect the 8 ohm speakers directly to the 16 ohm taps.
If you do that the transformer behaves like a 1.5k OPT. I dont know the ideal primary for KT88's PP, but if it was 3.2K, than for 4 tubes, it should be about 1.5K.

woops I read now that its only 40 watts, thats way to low for 4 kt88's. Or you need a very low V+.

Is it for HIFI? because you bought only 2 KT88's... If its for guitar, you could put 2 speakers in series.

If for HIFI, I think you'll have to buy a different, better OPT. Sorry...
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Old 10th January 2010, 09:19 PM   #4
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Reidy- View Post
Probably a bad idea but as I am on a tight budget I thought I'd get some opinions before throwing it out the window.

There are some cheap 40watt PP OT's that I am considering buying, but they have a 16ohm secondary (3.2k primary), well my speakers are rated at 8ohms.

what will be the outcome soundwise? I presume that there will be a gain in output power but will it be at detriment to the sound quality?

I havnt really decided upon a circuit yet, but I have bought 2 KT88 JJ-electronics tubes for use and am currently weighing up PP or SE.

if I where to wire a 8ohm resistor inbetween the speakers and the amp would it enable the use of my speakers and again what kinda power/sound differences should I expect
This is a very bad idea. While a resistor will not degrade the sound, 50% of your amp's power will now be dissipated as heat!

Additionally, 3.2KΩ primaries are a little low for a single pair of KT88s. Typically, 4.2KΩ*is the right number for PP. Using these transformers, even if driving a 16Ω speaker will not generate a satisfactory frequency response and I would imagine your distortion ratings will not do much better.
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Old 10th January 2010, 09:20 PM   #5
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Quote:
Originally Posted by pauldune View Post
Im afraid that it doesnt increase the damping factor, but totally destroys it.(
DANG! I wrote increases, it of course decreases it, when you actually want it quite high.
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Old 10th January 2010, 09:30 PM   #6
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A pp transformer can not be used for a single ended amp. Single ended and PP transformers are built differntly. A single ended transformer is built with a gap in the
magnetic circuite to prevent saturation. The pp transformer does not need this because the curent through both sides of the primary sort of cancle out the magnetic field and it does not saturate.
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Old 10th January 2010, 09:43 PM   #7
Reidy- is offline Reidy-  United Kingdom
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Yeh the idea was to build a Hifi amp.

I have a simple SE PCB waiting for me at the post office so it looks like I'll carry on with that, just didnt look forward to spending £100 on transformers then keeping my fingers crossed to escape customs

I understand the fact that SE & PP circuitory/transformer don't mix

Just I wanted something with a bit more guts than the simple SE what kinda wattage am I looking at with 2*kt88's a (375-0-375) power transformer. hopefully running it in ultra linear mode.
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Old 10th January 2010, 10:24 PM   #8
Sendy is offline Sendy  Czech Republic
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Too many questions in one place.

Output transformer for PP can not be used for SE.

When using resistors in series with the speaker change 8/16R load will be a problem with a reduction damping factor. Power loss is less significant problem. Significant problem may affect the course of frequency response - the speaker impedance is dependent on frequency. This applies to single speaker, and also for multi-way speaker with passive crossover box etc.

I think the best of bad options is to change the winding of the transformer secondary side. 16R winding can have several series and parallel sections and the reconnection can be achieved by proper impedance. Another option might be to eliminate part of the upper section of the secondary winding, if available. A third option might be adding a further short section of winding, which connects in series with secondary winding in the opposite polarity.
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Old 10th January 2010, 11:25 PM   #9
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Reidy- View Post
Yeh the idea was to build a Hifi amp.

I have a simple SE PCB waiting for me at the post office so it looks like I'll carry on with that, just didnt look forward to spending £100 on transformers then keeping my fingers crossed to escape customs

I understand the fact that SE & PP circuitory/transformer don't mix

Just I wanted something with a bit more guts than the simple SE what kinda wattage am I looking at with 2*kt88's a (375-0-375) power transformer. hopefully running it in ultra linear mode.
If the transformer's output is rectified with a 5U4GB you might get 450 to 480 VDC for the KT88s' plates, but I don't know what the transformer's current rating is.

At those voltages you might expect between 50 to 60 Watts. The closest thing that you might use as a measuring stick would be the Dynaco Mark III, which runs 480 VDC to the center tap of the output transformer into two KT88s. Dynaco rated that mono block at 60 Watts ultralinear.
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