Qualified orders over $99 will ship to you without incurring a shipping . Nope. Finally, in order to drive normal loudspeakers, I decided that I needed a power rating of at least 20W. Hi there. This varying load impedance is reflected back to the primary, so the tube, and output transformer must work into a varying impedance range. Solid State amps are much more forgiving. Tube amps with no output transformers (like the Bottlehead Crack), for example, . Popular audiophile lore has it that a mismatch in impedance between the preamp and amplifier will result in bass loss. In regard to the construction projects in TUT 3 & 5 the most common specified output transformer for 50 watt power amps is a Hammond 1650KA. If we put 100 ohms on the secondary, the tube would see 400 ohms (100 . 81) Posted by John EH on 2007-12-16, 16:36:20 (138. This is similar to the Trainwreck where you can use EL34, 6L6, 5881, or 6V6 without any bias adjustments. The output transformer reflects the speaker impedance back to the tubes. If you must mismatch, better to have a somewhat lower impedance than too high. I have been doing this since 1967 and never had a problem with my vintage Fender amps, that's all I own. Again, 8 is generally worse than 2 on a tube amp rated for 4 ohms. To function properly, their primary winding must match the impedance of the . Speaker mismatch and how it effects the output transformer ... Figure 1: The 25W OTL amplifier circuit. Into a purely resistive load, the flyback voltages aren't bad at 2x mismatch, but at 4x mismatch they can get rather high, primarily dependent upon the leakage inductance of the transformer used. A guitar speaker has no fixed impedance over the frequency range. Impedance Matching :: Rick Onslow :: Tube Amp Mods Unlike solid-state amps, valve amps are basically self limiting . The K-701 is a 62 Ohm headphone and also very inefficient. For tube amps a low mismatch is typically safer than a high mismatch. Modern Tube Amp Technology In a princeton-type amp that's able (good OT w/4 and 8 ohm taps, but no impedance switch) to run 6v6 or 6l6 tubes (after a rebias), I understand that the 6v6s output to 8 ohm, and that the 6l6 output to 4 ohm. Transistor/solid-state amp. If we connect a 4 ohm load to the transformer then the load on the tubes is 4x400 = 1600 ohms. The speaker impedance is always much larger than the amp output impedance. I don't, but looking at my trusty RCA Receiving tube manual, the 12BH7 plate resistance of the output tube on the AO28 preamp is about 5.5k ohms. If the 2-k headphones you mention are classical magnetic headphones, concerns of "fidelity" -- as in "frequency response" -- are pretty much out the window. 8 percent for the 12 months ended in July. Can a 4 ohm amp be connect with 8ohm cabinet? - Ultimate ... Question about dissipation vs output and OT impedance mismatch « Reply #1 on: January 22, 2021, 02:24:38 am ». Output Transformer Impedance A fine amp that had a choice of transformer impedance taps. Could cause the Output Transformer to short. When you increase the load to 8 ohms and connect that to OPT's 4 ohm secondary tap, the primary impedance increases to 10k ohms. a tube amp uses impedance matching via a transformer to load the circuit, that is the mathematical principle: to try and match the output of the amplifier with the cabinet rating. The obvious choice of output tube was the Russian designed 6C33C triode, because a single pair can deliver 2.5A into an 8Ω load from a moderate 150V rail. Unlike an 8 ohm system there is no attempt to match the load impedance to the amplifier, in fact the mismatch is used to limit the load power. Need help on impedance match for 115-65 | Music Man Amps Addressing a couple of questions concerning performance of vacuum tube amplifiers when an eight ohm load is placed on the 4 ohm and 16 ohm output taps with T. Reflected Impedance Impedance Mismatch Due to Tube Change | The Gear Page The consequence of using a lower-impedance speaker is that the amp will start passing too much current. Whenever you mismatch a tube amp with a much higher load (speaker) impedance, a higher signal voltage is produced across the primary of the output transformer. We know that class B amps have a maximum efficiency of 78%, class C and digital even higher. Massive 5 pound (4 lb 15 oz) push pull tube output transformer came from working 7591 amp seems to have apprx 27. Output transformers (OPT's) serve a critical purpose in tube amplifier circuits. You may hear distortion, and, if the impedance mismatch is extreme, it can also destroy the power tubes or output transformer. 16 ohm amp into 8 ohm speaker | The Canadian Guitar Forum For example the blackface Fender Twin Reverb has a 4 ohm transformer, and the internal 212 cab provides a 4 ohm load. I built an amp a while back with an output transformer primary impedance of 6.6K. In the case of a mismatch of 16-ohm loudspeakers with an 8-ohm output impedance, the output power for a tube amplifier decreases only to about 89% and not to 50%, as in the above calculation. It usually causes a dip in output power and usually you'll get some extra distortion because of the mismatch but unless its at full power its likely nothing you'd ever notice. I am considering constructing a multichannel amplifier using thermionic valves for amplification. If the load is 4 ohms, then you choose an OPT of 5k:4 ohms impedance (35.4:1 turns ratio). The reason is called reflected impedance and it's bad news. The smaller the mismatch, the safer it is. But the tubes are still generating power so where does it go? If the amp's input impedance is too low weaker bass will be the result. If you are going to mismatch VALVE amps it is actually less bad, (for the amp or more specifically, the Output Transformer), to mismatch to a lower impedance, (16 ohm into 8 ohm cab), rather then higher. The bottom line is the greater the low impedance mismatch the harder your amp works, and the greater the high impedance mismatch the more likely you are to burn out your output transformer and/or power tubes. At some point the impedance becomes too high, but a 2 to 1 mismatch either way will be fine. To function properly, their primary winding must match the impedance of the . So in our example the primary impedance would be 8x400 = 3200 ohms. The forty ohms in this example is the impedance that a tube would see at the primary with a 2:1 turns ratio output transformer and 10 ohms on the secondary. Usually lower than that of a typical loudspeaker. I have a 100 watt tube head that runs at either 8 or 16 ohms. The only time a mismatch is dangerous is when using a tube (valve) amp and setting the OHM Selector higher than the speaker's impedance. The problems occur when the output stage is heavily overdriven into a square wave. Using too high an impedance is fine for solid-state (although it does reduce the potential output), but not for tube amps. Operating a tube with a higher load impedance than it wants generally results in lower power output and lower distortion. If we put 100 ohms on the secondary, the tube would see 400 ohms (100 . Although the primary impedance changes with a speaker mismatch, the turns ratio to the NFB tap doesn't. The change in primary impedance has a potent effect on the power tube gain while the gain of the OT for NFB remains constant. That can cause the tube sockets or worse the output transformer widings to arc, and your amp tech bill just got expensive. That's on the low side for impedance and most inexpensive OTL tube amps will struggle, due to their high output impedance. The first amp like this that I built I used a 5K ohm transformer because I had one, The transformer was made for a 45 but it was a mismatch for the other tubes. In spite of the theoretical electrical "advantage" of using the 4 Ohm or 6 Ohm taps, the Maggies sounded much better on its 8 Ohm taps. I've used a 4 ohm speaker to a switched impedance ss amp (with the switch at 8 ohm) and I blew a fuse. On solid-state amps, which behave linearly to the load, it is perfectly acceptable to increase speaker impedance to decrease output. Unfortunately, to match the high output impedance of these devices to the low input impedance of the ordinary speaker, the use of a transformer usually indicated -- and as tube amps have grown rare, transformers with the required characteristics [broad frequency response, &c.] have also become . The voltage ratio (like turns ratio in transformers) is equal to the square root of the impedance ratio, so √20≈8.7. Some of the silverface amps had two output taps and switching jacks. This is the same as the domestic power mains where, if you matched the point-oh nothing output impedance of the power station you would get its entire power output. Output transformers I figured failed from stuff like being played without a load. I did a search for "impedance mismatch" and looked for my situation, but didn't see it. WW1WW, May 24, 2017. If we put 5 ohms on the secondary, the tube would only see 20 ohms (5 * 2^2). The speaker impedance is always much larger than the amp output impedance. Julius Futterman's article "An Output Transformerless Power Amplifier" appeared in the October 1954 issue of the Journal of the Audio Engineering . This changes the impedance plane to match the impedance of the speaker, so the audio output transformer is no longer required to match the tube to the speaker. An Impedance Converter accomplishes this match up. An 8 ohm amp can easily and safely run a 4 or 16 ohm speaker. It increases at higher frequencies. Four is the the turns ratio (2) squared. Impedance mismatch per se usually does not directly affect frequency response; impedance mismatch (thinking of AF power tubes) has a greater effect on power output v distortion. 16 layer output transformers. A mismatch is not a good thing either way, but higher Z is actually worse (harder on the output transformer) than lower. All Tube Output Transformer-Coupled Headphone Amplifier. The output transformer is a push-pull, so I can't use it for this amp but its primary is 5700 ohms impedance @ 400Hz and the voice coil is 2.28 ohms impedance (not resistance) which makes it exactly a 50 : 1 WR output transformer. And this again applies only to fixed ohmic impedances. This means that the output voltage will be 8.7 times bigger, so it will be equal to 8.7 mV. The bias is within range regardless of which tube I use. The bottom line is the greater the low impedance mismatch, the harder your amp works, and the greater the high impedance mismatch, the more likely you are to burn out your output transformer and/or power tubes. The forty ohms in this example is the impedance that a tube would see at the primary with a 2:1 turns ratio output transformer and 10 ohms on the secondary. This can destroy the power tubes or output transformer. Re: Question about dissipation vs output and OT impedance mismatch. Four is the the turns ratio (2) squared. What is meant is that a tube amplifier with a specified output impedance of say, 8 Ohms, will safely work with a load of between 4 Ohms and 16 Ohms, and for many tube amplifiers this is true. IF the amp is designed to run the tubes at 3200 ohms all is well. On tube amps, just buy an attenuator. Hence a matching transformer is not needed (Some very early SS amps did use a matching transformer). It has a beautiful breakup when overdriven. Tube amps (which are coupled to the speakers via an output transformer) are different than most solid-state amps (which are generally not). But as @beans-on-toast said, a 1:2 or 2:1 mismatch is usually not tragic with old Fenders. If we put 5 ohms on the secondary, the tube would only see 20 ohms (5 * 2^2). In a lot of cases this is not good for Output Transformers. The exact opposite. They don't want to see a higher impedance than the rating. Like if you put a 6L6 tube in a 6V6 amp and the extra current draw made the power transformer so hot that you can't even touch it etc. A tube amp is happiest when the speaker load impedance matches the setting of the impedance switch on the rear of the amp. A guitar speaker has no fixed impedance over the frequency range. On solid-state amps, which behave linearly to the load, it is perfectly acceptable to increase speaker impedance to decrease output. With the speaker unplugged you have an extreme impedance mismatch. out, the amp would see 2:1 Amp/Cab impedance mismatch. To calculate the output voltage after the matching circuit, we need to know the ratio of impedance, in our case, 1500 Ω/75 Ω=20. . Figure 1: The 25W OTL amplifier circuit. A lot of how much/well mismatch is tolerated depends on the primary impedance of the amp in question's OT given the power tube compliment (as well as other factors like plate voltage and the OT's power bandwidth rating), which is why you will hear some manufacturers state mismatch is OK one way or another (higher/lower) - so it can and does . transformer, a balun transformer, an output matching transformer, a reversing transformer, a 4 As connected in push-pull output circuits, the device functions as a 1 to 4 impedance transformer. It increases at higher frequencies. Tube amplifiers (again, not those amps with a tube - like a 12AX7 - in preamp section, we're talking a heavy monster with a bank of big momma tubes in the power output section) have specific outputs that are designed for specific impedance loads.Tube-powered amplifiers CANNOT safely operate into an impedance load that is not properly matched to the amplifier's rated load. For tube amps a low mismatch is safer than a high mismatch, that's why most speaker jacks have a shorting switch--a short is better . How hot is too hot for a transformer?. On the old forum Speedracer (Joe from Obsolete Electronics) posted this as "Common Output Tranny Primary Z ratings" Amplifier Primary Impedance Marshall, 50W 2xEL34 3,400 Ohms Marshall 100W 4xEL34 1,750 Ohms Marshall JTM45 2xKT66 8,000 Ohms Vox 30W 4xEL84 4,000 Ohms Vox 15W 2xEL84 8,000 Ohms Fender 50W 2x6L6 4,100 Ohms Yes, on the 4 Ohm taps there was a VERY SUBTLE sense of SLIGHTLY more powerful and better controlled bass, but the overall sound was cloudy and . It's also why running a tube amp without a load (the highest impedance possible - open load - infinite Z) will cook your output pretty quick. Chosing A Replacement Output Transformer Now lets assume we have a set that has an output transformer with an open winding that must be replaced, and the output tube is a single 6F6. For example, do not set a tube amp to 8 OHMS when using 4 OHM speakers. In that case, a 5k ohm primary impedance will get you the max power out from that triode. Example, using the amp with no speaker or no shorting jack attached to the output can damage the OT. On the secondary side, if you want to drive a balanced line, 600 ohms is ideal. The impedance match/mismatch is rooted in the amp's output impedance facing the speaker's input impedance. Tube amps with no output transformers (like the Bottlehead Crack), for example, . Output transformers (OPT's) serve a critical purpose in tube amplifier circuits. or shorted out (infinite or zero impedence) But I'm an eye doctor and not an electrical engineer/amp tech. No this is 2.28ohms AC (impedance). It doesn't, unless it has an aftermarket output transformer. This will result in an increase in current and runs the tubes harder. Finally, in order to drive normal loudspeakers, I decided that I needed a power rating of at least 20W. When the transformer is impedance matched to the load maximum power is transferred, but when there is a mismatch of impedance less power is transferred. Which is better for bass and actual power output? Whether a 4-ohm load is safe with a transistor amp depends on the particular transistor amp's design and especially the size of its power supply (output transformer plus filter capacitors), but generally speaking they're going to be more low-impedance friendly . And this again applies only to fixed ohmic impedances. Actually a greater load is bad for a tube amp. The secondary windings often have taps to adjust the output Impedance for 8Ω or 4&Omega speakers. When the primary tap is changed from 8K to 5K, the turns ratio to the NFB tap changes in addition to the power tube . So, for example 100 ohm output impedance is a genralized measure across all frequencies. This tube amp seems like a much more durable design since it takes both 4 and 8 ohm impedances easily. Link Output. FWIW, vendors that design amps to match well with most any pre-amp, including most higher output impedance tube pre-amps, may go even higher with their amps input impedance as an insurance policy, in that actual impedance varies with frequency. So you would be looking for an impedance of around 5K ohms on the primary side of a single ended transformer. Impedance matching between the output impedance and the headphone impedance is critical for power going from the amp to the headphones. You mentioned a lot of impedance mismatch when the source goes more than 2.5-8 times the Headphones, But what happens if the source is say <1ohm and the impedance of headphone is 18, in this the source is quite a lot lesser than 8 times. Mercury offers Transformer Restoration Services for rare and vintage transformers that are failing or underperforming. . These are specs I got for this speaker from the Philco Phorum website. 6W in SE would result in 3W ideally (50% of power delivered to load) typically that is around 40-45%, so <2W output with 6W applied in this case is not unreasonable with such a gross mis-matched load. A typical vacuum tube amplifier's output impedance is around 3,000-ohms while recent post World War II era hi-fi dynamic loudspeakers had an impedance rating that hovers between 8 to 16-ohms. The flyback voltage is a factor of the overall AC output voltage and the level of impedance mismatch. But who would do that with a guitar amp, anyway??? The transformer provides isolation, facilitates multiple outputs, and allows for voltage adjustment by 10. the output tube is loafing . pabloxyz Senior tube assistant Posts: 156 (8/21/03 6:34 pm) 206.231.12.101 Reply | Edit | Del Re:output impedance----- January 21, 2018. by Paul McGowan. for a 8 ohm amp, a 4ohm speaker is ok, but a 16 ohm speaker is a bad idea. But fortunately Fender took this into consideration when these transformers were designed to handle this type of impedance mismatch. If you need to make a mismatch it's better to have the speaker impedance lower than the amps impedance (as described above). The obvious choice of output tube was the Russian designed 6C33C triode, because a single pair can deliver 2.5A into an 8Ω load from a moderate 150V rail. If speaker impedance is higher than amp impedance, the amp won't produce more power than it can give away. A better way to express acceptable mismatch would be to say: a tube amplifier will work safely with an impedance mismatched by a factor of no more than 2. To avoid an impedance mismatch when selecting speaker cabinets, you will need to know what the output impedance of your amplifier is (this should be stated on the back of the amp). 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