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#1
Posted to rec.audio.tubes
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Line transformers as output
I recall seeing info once that PA line matching transformers could be
used for speaker matching in guitar amps. Can anyone elaborate on the pro's and con's of this relating to both single ended and parallel configurations? Given a certain level of compromise, are there other or better design alternatives that do not require the use of expensive OT's? Thank you, Mark Robbins |
#2
Posted to rec.audio.tubes
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Line transformers as output
Mark Robins wrote: I recall seeing info once that PA line matching transformers could be used for speaker matching in guitar amps. Can anyone elaborate on the pro's and con's of this relating to both single ended and parallel configurations? Given a certain level of compromise, are there other or better design alternatives that do not require the use of expensive OT's? Thank you, Mark Robbins The line matching trannies are all ungapped trannies which normally have far too few turns per volt to allow hook up to any tubes except perhaps to low Ra trioders such as 6AS7. They are all mainly made to connect multiple speakers to a "100V line" from a solid state amp so many speakers around a factory, shop or school can be wired without using thick copper wires on long runs which would have far too much inductance to maintain the HF response. At each speaker there is a 100V to say 10V step down tranny, so that the tranny is actually a 100 : 1 Z ratio, so about 800 ohms to 8 ohms. Usually such trannies are crude and not well interleaved and have high winding resistances unless they are toroidal, in which case then they can be wide bandwidth. Its no good to try to hook up a pait of KT88 to such a tranny. But were you to have say 10 x KT88 in parallel SET with a choke feed for the DC supply, then you could cap couple to a line tranny with 600 ohm input impedance and say 100 watt rating. The load seen by each KT88 will be 6,000 ohms, about right, and the primary is coupled via a suitable cap of about 100uF mimimum, with one end of the primary grounded. Because no gap is in a line tranny, and there is usually no primary CT, then their use in normal configurations in tube amps is not usually feasable. Patrick Turner. |
#3
Posted to rec.audio.tubes
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Line transformers as output
"Mark Robins I recall seeing info once that PA line matching transformers could be used for speaker matching in guitar amps. Can anyone elaborate on the pro's and con's of this relating to both single ended and parallel configurations? ** Do you know what a Google Groups Advanced Search is ?? http://groups.google.com.au/group/re...ad98cc e8dbc2 Given a certain level of compromise, are there other or better design alternatives that do not require the use of expensive OT's? ** Since 70 / 100 volt audio line and AC power transformers are NOT designed for push pull operation and also cannot accept the DC current of a class A stage - you have a problem. ......... Phil |
#4
Posted to rec.audio.tubes
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Line transformers as output
flipper wrote: On Sat, 14 Oct 2006 05:38:55 +0930, Mark Robins wrote: I recall seeing info once that PA line matching transformers could be used for speaker matching in guitar amps. Can anyone elaborate on the pro's and con's of this relating to both single ended and parallel configurations? Given a certain level of compromise, are there other or better design alternatives that do not require the use of expensive OT's? Thank you, Mark Robbins Well, it's been done for a modest 10 watt 'stereo' amp, and he seems pleased with it, but I don't know if it could handle sustained power like a guitar amp. http://home.alphalink.com.au/~cambie...mp/6an8amp.htm I breadboarded up one using PP 6EM7s and a Speco 70 volt line transformer and it worked 'decent' enough that I'm building a whole one, triode mode 6GK6s this time, just for the fun of it but I wouldn't suggest it as anything more than junk box amusement. I've also breadboarded a 20 Watt guitar amp using a power tranny I had lying around as OPT but it's not really much, if any, cheaper than a cheap 20 Watt guitar OPT if you have to buy it, and especially not when you consider the total cost of the whole amp. Like, say, this one http://cgi.ebay.com/Deluxe-Output-Tr...QQcmdZViewItem (I couldn't get http://www.triodeelectronics.com/ to come up) You can forget SE into either a line or power transformer because they're not gapped and PP requires precise bias balancing (I'm using a one sided servo). You missed the point I made about line trannies and SE amps. It is quite possible to use a line tranny to connect to any SE amp if the SE amp's DC flow is brought to the tubes via a choke which allows the primary of the line tranny to be coupled via a cap to the anode/cathode circuit of the tubes. Its the parafeed connection. DC gapping is thus not needed. PP operation is also possible where you have two tubes in series with DC flow from a B+ supply through both to 0V, and with top tube cathode connected to bottom tube anode. Output is via a cap to the OPT which again does not need a gap or a CT. The arrangement is very similar to the topology of OTL amps based on Futterman circuits. The Circlotron circuit can be employed to allow PP use of tubes in an OPT where the DC flow is through one winding in both directions, and no gapping or CT is needed. Since the line trannies have such low primary load values in hundreds of ohms rather than thousands of ohms, tubes such as the 6AS7/6080 can be used but far better are tubes such as the 6C33C which is used for OTL and which will power into loads of say 500 ohms to give excellent class A performance which is better than the nearly class B performance used in OTL amps which requires large amounts of NFB to make them anywhere near as linear as a normal transfromer coupled amp with proper load matching. Altronics supply 150 watt toroidal line trannies which have wide BW and which would be fine for a pair of 6C33C. For guitar amps the idea is to have conservative load matching to allow the tubes to be grossly overdriven without their Pda rating being exceeded. OTL amps are thus quite hopeless as musician's amps and are never used, but using a line trannie with an OTL amp makes the tubes loading benign and Pda much lower so such OTL amp topologies could be tried by a musician. Whether it sounds as good a s apair of 6L6 and conventional deign is a moot point. The trend now is towards SE class A guitar amps with low power, and they are simply miked up and the sound amped up with thousands of solid state amp watts. There is a trend away from musicians having to lug huge heavy amps into venues where a "house sound system" is a permanent facility. Patrick Turner. |
#5
Posted to rec.audio.tubes
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Line transformers as output
On Sat, 14 Oct 2006 05:38:55 +0930, Mark Robins wrote:
I recall seeing info once that PA line matching transformers could be used for speaker matching in guitar amps. I've tried it in a 1 watt amp... Can anyone elaborate on the pro's and con's of this relating to both single ended and parallel configurations? They can't take much DC so forget parallel... or high power... or wide frequency response. (They sound like ****!) Given a certain level of compromise, are there other or better design alternatives that do not require the use of expensive OT's? Sure, buy cheaper transformers... or salvage used equipment... or wind your own... or use enough tubes to direct couple... or use a resistive load or a choke and a capacitor... Thank you, Mark Robbins |
#6
Posted to rec.audio.tubes
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Line transformers as output
On Sat, 14 Oct 2006 05:38:55 +0930, Mark Robins
wrote: I recall seeing info once that PA line matching transformers could be used for speaker matching in guitar amps. Can anyone elaborate on the pro's and con's of this relating to both single ended and parallel configurations? Given a certain level of compromise, are there other or better design alternatives that do not require the use of expensive OT's? Thank you, Mark Robbins Yeah - you can also use filament trsformers - like 6V and 12V ones as output transformers. It may not sound as good or run as efficiently, but in some cases they will have a close enough match they can sound just as good as a tranny intended for Loudspeaker Output service. |
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