Members ChrisAlbertson Posted June 1, 2009 Members Share Posted June 1, 2009 I've been looking at schematics trying to figure this out and don't see the light. Why does running a tube amp with no speaker connected damage the amp? Seems to me an infinite impedance on the OT's secondary would mean the primary would also present an infinite impedance to the power tubes. No current would flow from the power supply and no damage. OK, I'm missing something. Link to comment Share on other sites More sharing options...
Members WRGKMC Posted June 1, 2009 Members Share Posted June 1, 2009 You wont find the answer in a schmatic, you need to study basic electronics and read up on why you need proper impediance matching and loading of output transformers. If you want the scientific answer its here Transformer Operation Under No-Load http://www.tpub.com/content/doe/h1011v4/css/h1011v4_57.htm It has to do with how the current and voltage in the two coils are balanced and how its done using the magnetic flux in a process called hysterisis. Coils create a magnetic field when its charged, it also supplies it back to a magnetic field when they collapse. What occurs is the primary and seconsary, Magnetic fields, The voltage and current have different rise and fall times allowing the two coils to push and pull on eachother. This balance limits what the tubes caps ps etc need to supply. Without an output the voltage on the primary ahds nothing to work against and will push the primary current above its maximum rating and cook the coils in the output transformer primary till the enamel wire cooks off and shorts out, wiping out the output transformer. The short answer, running no load cause the primary to increase above safe levels for the winding till it cooks. With no load on the secondary to absorb what the primary is generating, the voltage in the primary continues to be supplied by the tubes and caps. This generates heat and cooks the primary. Transistor amps are the oposite of a tube amp. You generally wont blow the amp with no load. With a tube amp running it shorted is better than no load. In both cases, its best to ONLy run an amp at its recomended impediance or use a Hot Plate to simulate a load. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted June 2, 2009 Moderators Share Posted June 2, 2009 You wont find the answer in a schmatic, you need to study basic electronics and read up on why you need proper impediance matching and loading of output transformers. If you want the scientific answer its here Transformer Operation Under No-Loadhttp://www.tpub.com/content/doe/h1011v4/css/h1011v4_57.htmIt has to do with how the current and voltage in the two coils are balanced and how its done using the magnetic flux in a process called hysterisis. Coils create a magnetic field when its charged, it also supplies it back to a magnetic field when they collapse. What occurs is the primary and seconsary, Magnetic fields, The voltage and current have different rise and fall times allowing the two coils to push and pull on eachother. This balance limits what the tubes caps ps etc need to supply. Without an output the voltage on the primary ahds nothing to work against and will push the primary current above its maximum rating and cook the coils in the output transformer primary till the enamel wire cooks off and shorts out, wiping out the output transformer. The short answer, running no load cause the primary to increase above safe levels for the winding till it cooks. With no load on the secondary to absorb what the primary is generating, the voltage in the primary continues to be supplied by the tubes and caps. This generates heat and cooks the primary. Transistor amps are the oposite of a tube amp. You generally wont blow the amp with no load. With a tube amp running it shorted is better than no load. In both cases, its best to ONLy run an amp at its recomended impediance or use a Hot Plate to simulate a load. I don't see the answer there. I see a question assuming that the open circuit power is 70W for a 10kVA transformer with a 90% power factor. 70 watts out of 9000 watts is less than 1%, and isn't likely to damage anything. Below that is an explanation of how a transformer responds with no load:"If the secondary of a transformer is left open-circuited (Figure 6), primary current is very low..."Again, nothing about damage to the transformer resulting from no load. Link to comment Share on other sites More sharing options...
Members WRGKMC Posted June 2, 2009 Members Share Posted June 2, 2009 In my case with a Fender Blackface Bassman head, The amp uses resistors on the 6L6 tubes to keep the screen grid at a lower potential than the plates. Without a load the voltage goes up, takes out the resistors, then the tubes and possibly the transformer. I was luckey in my case, I was young and running a high impediance of 12 or 32 ohms or something crazy like that. It only took out the tubes and the resistors. The resistors acted as fuses saving the transformer. Guess when they blew it screwed with supply voltages, and blew the mains fuse which shuts down the amp. If I was running open it just as easily taken out the transformer but Fenders are a beefy amp. There are many different tube amp designs and the failures can vary alot. Its extremely difficult to give any one explanation for failures. By buddy had his 60s bassman 4X10" tweed output transformer blow out when his speaker went. Didnt take very long. He was always pushing the crap out of that amp using a Stramp with it and cranking the input gains too hard. The main thing is though the secondary when connected to a load generates a emf field in the iron core of the transformer as the field generated by the primary collapses. This feedback is balanced with the primary. Without it it throws the bias potential of the tubes out of wack and eventually damage is done. It may flake or short tubes, tubes may loose power because of this. I was always having to replace my tubes running a high impediance. Dont think the tubes lasted more than a few months. I though it was just because they were Radio shack tubes, but they were all that was available to me at that time. Depending on the build and quality of the parts theres plenty of other issues. Tube sockets can arc from the voltage imbalance, if they are the bakalite type sockets the arc carbonizes the fiberous material making it a conductor. They mar snap crackel and pop or they can smoke completely and take out both transformers. The damage can be caused by an impediance imbalance. depending on the bias it can take a tube out fast or slow, It can turn the tubes orange like a toaster with the grid resistors gone, then you get some idiot who sticks a car fuse into the back and completes the damage to both transformers as he pumps 20 amps through both. I'm sure there are exceptions. There are hybrids out there that can be left unplugged for recording and other class designs I'm not as familure with like class A, C G etc that may have different results. Just believe me though, They do blow in most cases. Most players realise a cabinet isnt plugged in and quickly correct it and arent playing through the amp with it cranked. Left Idol, I may be wrong but I dont think theres going to be alot of damage done. With all those recording out there, unplugging the speaker and cranking the head so it can be recorded out of a like out or effects loop can be a death nell waiting to happen. Even a purely resistive load may save the amp. Not the best thing cause the resistance doesnt vary like a speaker does with frequency. After repairing enough beasts and asking how they were being run the results arent always the same. The item that was the same though, is the idiots using them werent running them correctly. Usually bad impediances or lack of maintainence. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted June 3, 2009 Moderators Share Posted June 3, 2009 Interesting. I'm looking at the schematic for one of my Sunns. It's an ultralinear design, not particularly popular among guitarists, but I like them. Maybe because I'm a bass player. Anyway, I notice that the screen grid is gets its voltage from the output transformer primary, at a higher voltage than the plate. See the schematic here:[deleted, possible malware site] It's possible I'm missing something. I have some background in electronics, but not much in tube amplification. But the function of the screen grid is to reduce interelectrode capacitance between the control grid and the plate. Seems to me that it will do that at a voltage a bit lower, the same as, or higher than that of the plate. I don't see how it would cause the effects you describe. Link to comment Share on other sites More sharing options...
Members A150PoundGuy Posted June 3, 2009 Members Share Posted June 3, 2009 Amp with no speaker, why is it bad? Because you cant hear what you play!!!! does that answer your question? Link to comment Share on other sites More sharing options...
Members WRGKMC Posted June 3, 2009 Members Share Posted June 3, 2009 Issac, Thats a sunn all right. Its a different setup all right than what I was explaining. I have a 200w Sunn concert lead thats all transistor and uses the output transformer in a simular way. Makes for a very plush sound. The additional taps in the transformer make up for the differences in potential. In my case resistors are used as you can see here, the output transformer is less complex on bothe the primary and secondary. Very durable though. In my case I musta blown those grid resistors and had the power tubes light up like a toaster bright orange 4 or 5 times with the improper load. Mine is a 4 ohm head, I'm running at 8 which is fairly acceptable for its design, so long as I dont go higher. Yours has speaker taps that should definately be used. I'd hate to see the damage to one of those. That transformer may be a bitch to find. http://www.schematicheaven.com/fenderamps/fender_bassman50.pdf Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted June 3, 2009 Moderators Share Posted June 3, 2009 Issac, Thats a sunn all right. Its a different setup all right than what I was explaining. I have a 200w Sunn concert lead thats all transistor and uses the output transformer in a simular way. Makes for a very plush sound. The additional taps in the transformer make up for the differences in potential. In my case resistors are used as you can see here, the output transformer is less complex on bothe the primary and secondary. Very durable though. In my case I musta blown those grid resistors and had the power tubes light up like a toaster bright orange 4 or 5 times with the improper load. Mine is a 4 ohm head, I'm running at 8 which is fairly acceptable for its design, so long as I dont go higher. Yours has speaker taps that should definately be used. I'd hate to see the damage to one of those. That transformer may be a bitch to find. http://www.schematicheaven.com/fenderamps/fender_bassman50.pdf Not easy to find, nor cheap. In general, it's almost as cheap to buy another head as another transformer. Link to comment Share on other sites More sharing options...
Members WRGKMC Posted June 3, 2009 Members Share Posted June 3, 2009 Not always, with a Sunn maybe so unless fender has copied that design. I have an old Sound City in the closet. I blew the PT when I ras replacing the Power cap. I found the exact same replacement for it and figured I'd throw some new tubes in there at the time. I did nothing wrong as far as replacing the components and all, but instantly blew the secondaries. After looking at the diagrams carefully I saw the original had had EL34s in the diagrams. The amp had 6L6s. I just replaced them with another set of 6L6s. The old cap was leakey and probibly didnt provide the same juice so that extra clean boost was enough to do it. If I had checked the diagrams I would have seen the amp wasnt set up right which would account for the half cooked grid resistors and all. I did check out there and saw the original Partridge transformers arent made anymore. I believe a Hiwatt which bought out Sound City may have the same transformer. I might even get a Marshall transformer to work. The high voltages needed for the tubes in my case are the same for most amps. I have 4 other guitar heads so its not a priority but I do want to get it working again. it was a loud SOB. It had a boost switch in the back that would kick it up to 60w and it does have a midrange presence etc which my fender doesnt. It's very simular to a marshall point to point setup but may have to rework the power section to get it all working right. I may just have it done by this local guy. He'd got more modding experience than myself, and he said he could get her running for around $200 so it may work out better fo me. If I had exact replacements I dont have a problem. Biading the thing with transformers I'm not familure with and getting it wired up if all the taps arent available would require rewiring the power section. In your case if the Output wasnt available you would either have to find something close or get what you can and redesign the power stage to something else the transformer could handel. I'm sure it would depend on the voltages of the PT but again I believe its very doable. You would just need to find a guy with the right kind of experience. Fitting the transformer in there physically is one item. The other would be picking a design. Any standard push pull may work. its just comparing diagrams and figuring what and were voltages may need to be routed. A marshall output transformer is $88. with some rewiring by someone who knows you may have a great setup for $200 as well. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted June 4, 2009 Moderators Share Posted June 4, 2009 No, any old output transformer won't work. You need the extra taps on the primary for the ultralinear circuitry. OTOH, I suppose one could reconfigure the output circuitry, make it a different kind of amp, more like a Fender or Marshall. Fortunately for me, all of the transformers on my Sunns are still working. For the time being, anyway... Link to comment Share on other sites More sharing options...
Members WRGKMC Posted June 4, 2009 Members Share Posted June 4, 2009 Thats what I said, you may have missed it. You would have to modify the power section to another design to match a different transformer. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted June 4, 2009 Moderators Share Posted June 4, 2009 So you did. Link to comment Share on other sites More sharing options...
Members WRGKMC Posted June 4, 2009 Members Share Posted June 4, 2009 I wouldnt mind having one of those old tube Sunns. they were butt kicking amps. My bass player had the Model T bass amp with a single 2X15 folded cab. It could drown out my 100W plaex marshall if he cranked it. My 200W concert lead head is an interresting design. http://www.richbriere.com/images/1972cat3.gifMine came with the 6X12 flat faced cab. I found a 6X12 v shaped bass cab for it which will throw the sound in all directions. I've used it for bass on several occasions. Right now I have the head on a 4x12 peavey cab for my other guitar player. I need to get some new speakers for the sunn cab. The originals suffered a flood here in TX before I bought the cab for $20 and the cones, though still functional are pretty old. I patched them but they do need replacement. My buddy bought the head in the 70s and gave it to me a few years ago. I did a cap job and in studying the diagrams I saw it uses a transformer with the power transistors kind of like your diagram to give it a tube like sound. Probibly the most realistic sounding tube amp sound from a transistor head I've ever heard. With 200w its a real air pusher on stage. Link to comment Share on other sites More sharing options...
Members gretchy Posted June 8, 2009 Members Share Posted June 8, 2009 so is this similar in concept to revving to redline in neutral rather than cruising in 4th @ 3000rpm? Link to comment Share on other sites More sharing options...
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