impedance matching

[tonequester]

tonequester here.

I am not nearly "up to speed" with the vast majority of what I read and try to study here.Please excuse my ignorance. Can anyone explain to me just
what the consequences are of running the output impedance or a tube amp(example : 50 Ohms) into a s.s. amp with an input impedance of 150kOhms ? I realize that
the conservation of the "sound" of the end result won't be the same. I also realize that there will be power consequences. Are these the only considerations to be
concerned with. I am grateful to anyone who has the time to clue me in. tonequester.

All men by nature desire knowledge. Aristotle.

[Austin]

tonequester here.

I am not nearly "up to speed" with the vast majority of what I read and try to study here.Please excuse my ignorance. Can anyone explain to me just
what the consequences are of running the output impedance or a tube amp(example : 50 Ohms) into a s.s. amp with an input impedance of 150kOhms ? I realize that
the conservation of the "sound" of the end result won't be the same. I also realize that there will be power consequences. Are these the only considerations to be
concerned with. I am grateful to anyone who has the time to clue me in. tonequester.

All men by nature desire knowledge. Aristotle.

Theres a 1000 analogies for it, usually involving water. Like filling a bucket with water in a certain amount of time with a small piece of tubing the size of a straw (high impedance) would require high pressure (high voltage), but if you had a garden hose (lower impedance) even a small slow trickle (low voltage) could transfer the same amount of water in the same amount of time.

Electrical power is measured in watts, a 100 watts at 100 ohms would take 100 volts. 100 watts at 10 ohms would only take about 32 volts. The amperage is of course much higher with the 10 ohm load! So to transfer the same amount of power with high impedance takes high voltage, to transfer the same power with low impedance requires high amperage.

Transformers are used as impedance matching devices, 10 turns on the primary and 100 turns on the secondary would step up the voltage by 10 times and lower the amperage by 10 times. Transformers work both forward and backwards too.


A transformer is two inductors coupled by magnetic flux, an inductor is like a long garden hose (wire) with water (electricity) flowing inside.. Put your finger in the end (resistance) and the pressure builds (voltage), let your finger off and it squirts out faster (up to twice as fast) than it was before for a brief time. If water is flowing, it has since it has mass it also has inertia- if you stop it or start the flow suddenly it sends a shock wave thru the hose.. This is reflected back to the primary of the transformer!

Check out wiki for a better explanation of the water anology: Hydraulic analogy - Wikipedia, the free encyclopedia

Basically, if you hook the output of a transformer coupled tube amp into a higher resistance than what it was designed for it can arc inside the power tube and damage it because the inductor effect of the transformer can cause up to double the voltage on the plate.

[tonequester]

saving me smoke and fire.

Theres a 1000 analogies for it, usually involving water. Like filling a bucket with water in a certain amount of time with a small piece of tubing the size of a straw (high impedance) would require high pressure (high voltage), but if you had a garden hose (lower impedance) even a small slow trickle (low voltage) could transfer the same amount of water in the same amount of time.

Electrical power is measured in watts, a 100 watts at 100 ohms would take 100 volts. 100 watts at 10 ohms would only take about 32 volts. The amperage is of course much higher with the 10 ohm load! So to transfer the same amount of power with high impedance takes high voltage, to transfer the same power with low impedance requires high amperage.

Transformers are used as impedance matching devices, 10 turns on the primary and 100 turns on the secondary would step up the voltage by 10 times and lower the amperage by 10 times. Transformers work both forward and backwards too.


A transformer is two inductors coupled by magnetic flux, an inductor is like a long garden hose (wire) with water (electricity) flowing inside.. Put your finger in the end (resistance) and the pressure builds (voltage), let your finger off and it squirts out faster (up to twice as fast) than it was before for a brief time. If water is flowing, it has since it has mass it also has inertia- if you stop it or start the flow suddenly it sends a shock wave thru the hose.. This is reflected back to the primary of the transformer!

Check out wiki for a better explanation of the water anology: Hydraulic analogy - Wikipedia, the free encyclopedia

Basically, if you hook the output of a transformer coupled tube amp into a higher resistance than what it was designed for it can arc inside the power tube and damage it because the inductor effect of the transformer can cause up to double the voltage on the plate.

tonequester here.


Thanks much for the bottom line on my query Austin. I,ve many things reguarding tonal loss, and many suggestions on "how to". I hade not found the simple answer that I had the feeling was laying wait for me. DESTRUCTION ! Don't feel that YOU may be a little out of your league. I must be the lowest of the low on this forum.
However, my intent was to learn, and I am succeding at that because of good folks like you. i will check up on the "leads" that you suggested. Thanks again.
tonequester. Quote :" Be less curious about people and more curious about ideas".........Marie Curie, discover of Radium.

[JoeM]

This link may help understand using a mismatched load on a tube OT:

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#XformImped

(written by forum member R.G. Keen)

[Austin]

I was jamming out on my home made stereo se el84 amp one day a while back, I kept noticing a flashing coming from one of the tubes. I had two 4x12 cabs hooked up to it, and it seemed to sound ok still... After looking at it I realized I was playing with a bad speaker cord and was getting an intermittant connection failure with the vibration. It was arcing inside the little el84! Still works tho and I am still using them. Vacuum tubes must be pretty tough if they can endure all the abuse I have dished out over the years!

[tonequester]

tonequester here.

I don't know if you got my reply on your last post, as it doesn't show up. I probably screwed up something. I wanted to tell you that I appriciate the true to life story. I'm
going to keep an I on my speakers wires from now on, regularly. I've always soldered the connectors on, but never given much thought about a broken wire. It sounds like you almost had an early 4th of July with an EL-84 display. By the way, that's the tube that I've decided to do my first build based on. It's going to be a Class A, cathode-biased, single ended, with two EL-84's in parallel. I'm just stashing away the cash for the essentials. That alone will give me the better part of 6 months to "bone-up". I appreciate all of your replies to my questions, which I realize are at times pretty well ground level stuff. Thanks for the patience. Keep an eye on the "fireworks", and GOOD ROCKIN' ! tonequester.

[Austin]

tonequester here.

I don't know if you got my reply on your last post, as it doesn't show up. I probably screwed up something. I wanted to tell you that I appriciate the true to life story. I'm
going to keep an I on my speakers wires from now on, regularly. I've always soldered the connectors on, but never given much thought about a broken wire. It sounds like you almost had an early 4th of July with an EL-84 display. By the way, that's the tube that I've decided to do my first build based on. It's going to be a Class A, cathode-biased, single ended, with two EL-84's in parallel. I'm just stashing away the cash for the essentials. That alone will give me the better part of 6 months to "bone-up". I appreciate all of your replies to my questions, which I realize are at times pretty well ground level stuff. Thanks for the patience. Keep an eye on the "fireworks", and GOOD ROCKIN' ! tonequester.

You know, one el84 can be very loud with a 4x12 cab, plenty loud enough to play in a band without miking the cab even outdoors. Mine is two channel but I played outdoors for the first time with it at a derby party where they had alot of musicians in a big jam not long ago and accordingto my friends in the audience, I was the loudest guitar there. This is with two 4x12 cabs full stack style one on top of each other, one for each channel. I wasn't miked and they all had 50 and 100 watt amps Mesa Boogie etc miked thru the pa. They looked at my home made amp and I could tell they were chuckling to themselves, until they heard my tone..Since I wasn't sure if I would be loud enough, I just let it rip... They couldn't beileve it was only 5 watts! That's per channel tho, I guess it really counts as ten watts then... Cranked up sounds lots better than a big amp or something barely turned up to 2 in my opinion, more genuine and tangible feel to it rather than the guitar notes buried in a buncha pedal effects to attempt to get the same tone. Whatever the case may be, I was grinning from ear to ear listening to my ugly home made amp tone compared to the 1000$ dollar amps I was playing along side of. So, you might consider building two single ended (not parallel) el84 amps and then you can use it as your home stereo too or play mp3s through between sets or something if you wanted and get to use all those cool stereo effects pedals. The cost wouldn't be that much more than making one big parallel el84 amp, especially if you used the same chassis for both channels. Two output transformers and one shared power supply for both channels is going to be your biggest cost. Sounds great with home stereo speakers hooked up and plugged into computer for mp3s or whatever too. My 2cents worth.

[tonequester]

tonequester here.

Hey Austin. I gotta say that i liked your 2 cents worth. This is the first time that I've heard 5-10 Watts(done even properly) can keep up with a Mesa Boogie. The thought of having 2 single ended EL-84 powered amps, as you explained, is well worth some consideration. Even the two channel idea hadn't really crossed my mind. I plan to make my own speaker cabs as well, so I may be copying your set-up in the future. One of the reasons for me to build an amp is to learn by experience anyway, and this would be twice the experience. I hope to get to the point that I can supplement my Social Security Disability by doing some part-time guitar and amp repair and modifications around where I live. I know for a fact that there isn't
a "free-lance tech" within about 50 miles from where I live. If you don't mind telling, I'm curious about what speakers you used in your 4x12's. I have to say that your comment about your
sound preference is cranked-up, is very true for me also. With all of the effects and hybrid amps out there making claims about getting THAT tone, you'd think one of them would come real close. I haven't found that one yet ! Thanks for sharing your open air experience. I won't worry nearly as much about being under-powered, and that alone will save bucks.
Have a good one.....down by the river ! tonequester.

[Austin]

tonequester here.

Hey Austin. I gotta say that i liked your 2 cents worth. This is the first time that I've heard 5-10 Watts(done even properly) can keep up with a Mesa Boogie. The thought of having 2 single ended EL-84 powered amps, as you explained, is well worth some consideration. Even the two channel idea hadn't really crossed my mind. I plan to make my own speaker cabs as well, so I may be copying your set-up in the future. One of the reasons for me to build an amp is to learn by experience anyway, and this would be twice the experience. I hope to get to the point that I can supplement my Social Security Disability by doing some part-time guitar and amp repair and modifications around where I live. I know for a fact that there isn't
a "free-lance tech" within about 50 miles from where I live. If you don't mind telling, I'm curious about what speakers you used in your 4x12's. I have to say that your comment about your
sound preference is cranked-up, is very true for me also. With all of the effects and hybrid amps out there making claims about getting THAT tone, you'd think one of them would come real close. I haven't found that one yet ! Thanks for sharing your open air experience. I won't worry nearly as much about being under-powered, and that alone will save bucks.
Have a good one.....down by the river ! tonequester.

I think a source of cheap low watt sensitive speakers is easily gotten from those old magnavox console stereos. I found several old consoles in the trash over the years, I have managed to save the 12" speakers of some of them. I have two cabs, one is a cheapie cab I got on ebay for 100$ and came loaded with 4 12 inch eminence legends but no more... The other cab I have is a Traynor 4x12 pa cab with 4 12 watt Marsland 12" speakers in it. I ended up putting two of these speakers in the bottom cab and filling the two holes with one columbia console speaker and one magnavox console speaker. I also put the same setup in the bottom speaker. So I have 4 unused Eminence Legend 50w 102 db speakers left over. They just dont sound as sweet as the older console speakers... The paper in the cones are very light and bring out alot of detail in the playing I think, although the eminence have good (better?) bass response. The cabs are wired in series parallel, so in my mind adding one of these huge magnet type high watt speakers might upset the similarity of the speakers and the impedance reactance because the Marslands are very similar to the Maggy speakers.

I have two horn tweeters from one of those old stereos and you might be surprised how good an mp3 player sounds playing through your guitar cabs with them adding the missing top end!

[tonequester]

tonequester here.

Thanks for sharing your cabinet recipes and that tip about playing an Mp3 through amp and cabs.I haven't heard really good home stereo since I was playing vinyl on a 200
Watt Sansui set-up, through 4 big 3-way speakers. Heck, I even have an Mp3 jack on my little Blackstar Ht-1R practice amp that I've never thought to try out. No doubt about it, I gotta get caught up on the new tech stuff. Like my music, my brain seems stuck in vintage mode. Thanks again for your helpful replies Austin. "May the river never rise above the floor-board of your van". tonequester