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#1
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Tube amp 16 ohms impedance speaker has 4 ohms
Hi,
can the OPTīs from a tube amp be damaged if: I use the 16 ohms tap and connect 4 ohm speakers? and I use the 4 ohms tap and connect 16 ohm speakers? Thanks, Heinz |
#2
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#3
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In article ,
Sander deWaal wrote: (Heinz) said: Hi, can the OPTīs from a tube amp be damaged if: I use the 16 ohms tap and connect 4 ohm speakers? The tubes could be damaged, or at least one can expect a limited life span. What's the mechanism for this? I started audio in the tube era, ca 1963, and no one I know worried about this. However, tubes were cheap. Mike Squires -- Mike Squires (mikes at cs.indiana.edu) 317 233 9456 (w) 812 333 6564 (h) mikes at siralan.org 546 N Park Ridge Rd., Bloomington, IN 47408 |
#4
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In message , Michael Squires
writes In article , Sander deWaal wrote: (Heinz) said: Hi, can the OPTīs from a tube amp be damaged if: I use the 16 ohms tap and connect 4 ohm speakers? The tubes could be damaged, or at least one can expect a limited life span. What's the mechanism for this? I started audio in the tube era, ca 1963, and no one I know worried about this. However, tubes were cheap. Mike Squires I would say that you are more likely to damage the amp by using a 16R speaker on the 4R tap, as this is going to cause larger than expected voltage swings at the anodes. -- Chris Morriss |
#5
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In article ,
Chris Morriss wrote: I would say that you are more likely to damage the amp by using a 16R speaker on the 4R tap, as this is going to cause larger than expected voltage swings at the anodes. -- However, the voltage swing can't be more than the B+ rail; the total dissipation might increase. Mike Squires -- Mike Squires (mikes at cs.indiana.edu) 317 233 9456 (w) 812 333 6564 (h) mikes at siralan.org 546 N Park Ridge Rd., Bloomington, IN 47408 |
#6
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Heinz wrote: Hi, can the OPTīs from a tube amp be damaged if: I use the 16 ohms tap and connect 4 ohm speakers? The OPT is unlikely to be damaged, but if high volume levels are used constantly with 4 ohm speakers on the 16 ohm tap, the tubes might start glowing red hot and fail. But at low level listening, you should be OK, except that distortion will be 6 times more. and I use the 4 ohms tap and connect 16 ohm speakers? This way distortion will be 3 times less, but there is *no* risk of damaging the tubes or the OPT. There will be less maximum power. Patrick Turner. Thanks, Heinz |
#7
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In message , Michael Squires
writes In article , Chris Morriss wrote: I would say that you are more likely to damage the amp by using a 16R speaker on the 4R tap, as this is going to cause larger than expected voltage swings at the anodes. -- However, the voltage swing can't be more than the B+ rail; the total dissipation might increase. Mike Squires Don't try testing it under these conditions with square waves though, or you will soon see voltages higher than the B+ rail at the anodes. Don't forget that a transformer coupled valve amp is perfectly safe under s/c conditions. It's an open-circuit load that can cause flashover. -- Chris Morriss |
#8
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Chris Morriss said:
can the OPTīs from a tube amp be damaged if: I use the 16 ohms tap and connect 4 ohm speakers? The tubes could be damaged, or at least one can expect a limited life span. What's the mechanism for this? I started audio in the tube era, ca 1963, and no one I know worried about this. However, tubes were cheap. I would say that you are more likely to damage the amp by using a 16R speaker on the 4R tap, as this is going to cause larger than expected voltage swings at the anodes. But that would still be more or less controlled. A lower load would result in too much dissipation, especially in g2. -- Sander deWaal Vacuum Audio Consultancy |
#9
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Patrick Turner said:
and I use the 4 ohms tap and connect 16 ohm speakers? This way distortion will be 3 times less, but there is *no* risk of damaging the tubes or the OPT. There will be less maximum power. With triodes the distortion would be less, with pentodes it would be more. -- Sander deWaal Vacuum Audio Consultancy |
#10
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Sander deWaal wrote: Patrick Turner said: and I use the 4 ohms tap and connect 16 ohm speakers? This way distortion will be 3 times less, but there is *no* risk of damaging the tubes or the OPT. There will be less maximum power. With triodes the distortion would be less, with pentodes it would be more. -- Sander deWaal Vacuum Audio Consultancy The amp in question probaly has a lot of nfb applied, arranged so there is say 20 dB when 4 ohms is connected to the 4 ohm tap. If the amp works in pentode mode, the Dn will be probably 0.4% at 1 dB below clipping with the above 4ohms to 4 ohm tap. When 16 ohms is connected, the gain of the pentode output stage rises by around 3 times, or 10 dB, so the applied nfb leaps to 30 dB, and any increase in pentode distortion is mainly prevented, depending on the amount of class A used. With no load at all, there is a danger of excessive anode swings, and pumping a large input signal into the output grids will excite electro magnetic phenomena resulting in perhaps 2,000 vrms deveoped across the OPT primary. This can cause an arc between the primary and anything earthy nearby. But a 16 ohm load connected to the 4 ohm tap is an ample sized load to absorb such excessive voltages caused by back emfs in the OPT. In practice, speakers have a very varied impedance, and its not uncommon for] a speaker to have an impedance peak of say 40 ohms at say 50 Hz. Thus when 50 ohms is connected to the 4 ohms tap, the load seen by the tubes is way higher than designed for, and there isn't quite enough R to prevent the spurious over voltages caused by back emfs. It can still happen if the 16 ohm tap is used with a speaker a peak impedance of 50 ohms. To prevent such phenomena, and the resulting strain on OPT insulation, diodes should be connected from the anodes of each output tube to ground to prevent the anodes from going negative, or to a potential below 0V. If one end of a an OPT primary goes so negative that current flows from 0V into the winding via the very low on resistance of the diodes, then the other end which goes +ve cannot rise beyond the same volatge value in the side being shunted. See the schematic with such diodes at http://www.turneraudio.com.au/htmlwe...00ulabmono.htm There are 3 x IN4007 in series, each with 1M resistors strapped across them, to form a clamping network which prevents the peak -ve voltage swing at each primary to never exceed the value of the B+ voltage. Normally, the diodes sit there doing absolutely SFA, and have an utterly negligible effect on the Dn. But if some dude turns the volume way up but with no speaker connected, the amp clips without straining the hell out of the insulation between P&S windings. Most tube amps don't have such clamping diodes, and so you get the occasional OPT failure because of excessive voltages. Its true that when triode operation is used, 16 ohms connected to the 4 ohm tap would result in definately less Dn. But the increase in gain in the trioded output stage is not great, and there is no huge increase in the effectively applied nfb when the RL the tubes see is a high ohmic value, so the amp tends to be more stable, but only if the quality of the OPT permits this. There are plenty of triode amps with FB that oscillate at HF just as easily as a pentode amp especially if the load is capacitive. Before fiddling around with the mode of operation, one must be sure about stability issues. Most ppl prefer the sound of triodes. The proviso is that there must still be an adequate ceiling for power. Say you have an EL34 pentode AB1 output stage which makes 35 watts with 4 ohms load connected to the 4 ohm outlet. The anode to anode load might be 4k. Suppose one only requires 1 average watt per channel. Then one doesn't need 35 watts max power ceiling, and one could settle for 15 watts which is all you'll get using AB1 triode connection with 2 x EL34. If 16 ohms load is connected to the 4 ohm outlet, then the load value seen by the output tube leaps from 4k to 16k, and the power becomes totally class A power, but you then will only get about 10 watts of power. But its a very nice 10 watts, and its probably enough if 1 watt average is all that's used. To find out if this is the case, a DVM with a peak and hold function should be used whilst playing the loudest music one wishes to ever hear with the chosen speakers. From this we can calculate the maximum instananeous power level being used, so if we read 12v peak on our meter, this is equal to 8.48 vrms, and if the load was 16 ohms, then the instaneous maximum power is 8.48 x 8.48 / 16 = 4.49 watts, and even if there was a dip in the 16 ohm speaker impedance down to 8 ohms, the power out max would be 8.98 watts, so triode operation would be permissable. With the OPT configured for 4k to 16 ohms, the voltage ratio is 15.8:1, and if the B+ max swing was 400v peak at each end of the OPT pri, then you have 565 vrms across the OPT pri, so the max output voltage would be 565 / 15.8 = 35.7vrms = 50v peak. With an impedance peak of 50 ohms being common in 16 ohm speakers, one could expect to see perhaps 45 peak volts of swing at the output of a triode amp. All this may seem confusing, tedious, and boring for those without a clue or experience, but if one assumed that the load was indeed 16 ohms only, and that triode power into 16 ohms was a max of 10 watts, then the output voltage is 12.6 vrms, then the max peak voltage permissable should be 17.7 peak volts. So if you have a 16 ohm nomimal Z speaker, and you measure no more than 12 peak v at the output, triode operation should be fine, and give pretty decent sound, and possibly better sound than pentode operation. I say possibly, because ppl have been arguing this one way or other for at least as long as the beginning of the use of multi grig power tubes in the early 1930s, and the use of NFB before that. I find most well designed ultralinear amps to be better than plain pentode amps or triode amps. I also find the use of well implemented cathode feedback windings to give about 13% of NFB to the output tube cathodes is superior to any other configuration of multigrid output tubes. The feedback from my customers tells me all this. Patrick Turner. |
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