Transformers and Impedance
One of the big questions that I had when I started building home brew amps was what transformers should I use. I generally don’t buy transformers new. I usually find junker phonographs, tape recorders, projectors and PA’s at garage sales, flea markets and piled on the side of the road on cleanup day. I am not interested in getting the perfect tone. I'll take what I get. I like making little amps and seeing how they turn out. I am usually pleased.
Power transformers are easy. Tube amps work well with voltages from 200v volts to 400 volts (measured from the center tap). Lower voltages give you lower power amps with a nice honey tone. Higher voltages give you nasty loud amps. I hook up a power cord with wire nuts, plug it into a variac set on 10 volts and measure the voltage across the output leads until I find the hot wires and the tap and then multiply by 12. If I want a more accurate reading, I’ll turn the variac up to 60 volts and then double the output volts. The variac, though, is there to let me play with the voltages with a degree of safety. Also the junker amps are usually junkers for a reason and I don’t want to fill my basement with smoke by plugging in a bad amp at full 117 volts.
Output transformers are more of a problem. If you are building Fender style amps you need to figure the output impedance seen by the power tubes.
First, output transformers are a bridge between the output tubes and the speaker. They reflect the impedance of the speaker. In this sense the output transformer has no impedance of its own. (A transformer, of course, has impedance when there is no speaker hooked up, but this is only of interest to those who wish to make silent amplifiers.)
For home brew nuts, you are interested in basically two configurations. That is the Champ and the Deluxe. The champs like to see an output impedance of about 5000 and the Deluxe likes to see an impedance of 6500.
The Champ is supposed to have a 4 ohm speaker so what is happening is that the 4 ohms is seen through the transformer as 5000 ohms. The transformer multiplies the 4 ohms by 1250 to get 5000 ohms. If you were to hook an 8 ohm speaker up to a champ the 6V6 would see 10000 ohms. It would work, but the high impedance would mess up the higher frequencies and the amp would sound muffled. Also, less power would flow through the speaker (because of the higher resistance) and you’d get less sound.
Fender Tweed type amps are supposed to have about 6500 ohms with an 8 ohm load. Fender black face amps have an impedance of 4300 at 8 ohms.
Older amps used the higher impedance to cut down on distortion at the cost of bandwidth and power. Newer Fender amps like the lower impedance for higher power but with some distortion.
How to figure the output multiplier for an unknown transformer.
I have an AC wall wart that does 13.7 volts. You have to measure the AC voltage because DC voltage won’t pass through the transformer. Hook the output from the wall wart to the tube side of the transformer. Measure the speaker side with a digital VOM set on AC volts. This will give you a number like 0.4 volts (actually somewhere between .4 and .5 as my VOM doesn't show more precision than 1 decimal point). Divide this number into 13.7 and you should get number like 34.25. This is the turn ratio. If you square this number you get the impedance multiplier which in this example is 34.25 * 34.25 = 1173. If you are using a 4 ohm speaker then the impedance will be 4 * 1173 = 4692. This is just right for a champ. (Remember champs like to see around 5000 ohms impedance.)
I worry that the wall wart might create enough current to heat up the output transformer. The transformer is used to seeing 300 or more volts across it, so this is probably unlikely. I worry anyway and I don’t leave this setup hooked up for more than I need to take a reading.
What happens if you mix and match?
Champs sound lousy with 8 ohm speakers instead of 4 ohm. A Bassman sounds lousy when you hook it up to an 8 ohm box instead of 2 ohm. This is because the higher impedance kills off the high frequencies and lowers the power output.
A Champ screams raw tone and distortion when you hook up two 4 ohm speakers in parallel (making a 2 ohm load which translates to 2500 ohms to the 6V6), but the higher power makes the transformer run warm and the 6V6 glow a nice cherry red. You might plug in a 6L6 and see if the transformers can keep up with the higher power usage. Supposedly the Champ’s transformers can take this, but I wouldn’t risk blowing up a nice little amp. Do this on your junker home brew box.
If you are building your own amp from junkers, you can get away with murder by using transformers that are too big for your amp. You can make a Champ with a deluxe (2 – 6V6) type output transformer by ignoring the center tap (put a wire nut on it and tape it up). Put two 4 ohm 8 inch speakers or four 8 ohm speakers in parallel and see what you get. Put a 6L6 in instead of a 6V6. This is essentially a Premier Twin 8. You get lots of power and a 6L6 tube that needs frequent replacement – but well worth it. Check below for rules about using center tap transformers on a single end amp
You can use a black-face type output transformer in a deluxe. You may want to hook up two 8 ohm speakers in series to give you 16 ohms so that your 6V6’s last longer.
Using the push pull transformers instead of a single end transformer is permissible in the cheap output transformers that are intended for guitar amps. The fancy ultra linear expensive transformers intended for high quality audio systems won’t work well if you ignore the center tap, but guitar amp push-pull transformers are just simple transformers with a center tap added. The rated impedance is usually for an 8 ohm speaker as seen by either half of the tube side – that is from the center tap to either end. So if you tape off the center tap the voltage ratio is doubled, but the impedance is 4 times the impedance of either end. A deluxe transformer now sees 26,000 ohms when hooked up to an 8 ohm speaker. This is way too high. You want to get it back to the 6500 ohm range for a tweed amp so you need to use two 4 ohm speakers or, three 8's parallel. If you used a single 4 ohm or two 8 ohm speakers it would still sound good. If the output transformer is tapped for multiple impedances, you want to hook the speakers up to the 16 ohm tap. (I am building a Gibson Gibsonette using a center tap transformer with the center tap taped up. I will be using the 16 ohm output connection to drive two 10 inch 8 ohm speakers in parallel.)
What are the dangers of hooking up the wrong speakers to an amp?
Too low impedance, for instance a 2 ohm speaker cabinet hooked up to an amp expecting 8 ohms, will overload the output tubes and the output transformer. The lower impedance will force lots of amperage through the tubes and out through the transformer. You would get a very loud amp for a short period of time. Something would blow. The tubes probably would go first, but you could also blow the output transformer or the speakers. It is possible that a small power transformer would not take the high amperage and blow up.
Too much impedance, for instance an 8 ohm speaker cabinet hooked up to a 2 ohm Bassman head can be damaging. Mostly you get crappy sound, but there is less danger to the amp. The tubes will operate at higher voltage because the load is not letting the voltage bleed off into output and you might get shorting in the tubes or the output transformer. This is another way to fry an output transformer and it is why you shouldn’t run your amp without a speaker load.
Generally you can mix and match output impedances up or down without too much of a problem. The biggest problem is that the amp’s tone will change with different speaker arrangements. Don’t be extreme. Don’t run a 16 ohm load on a 2 ohm amp. Don’t run a 2 ohm speaker box on an amp designed for 8 ohm speakers. Keep things approximate.
I've been talking about the speaker impedance. What happens if you get a nice Jensen speaker from a junker phonograph? How can you tell the impedance? Just measure the ohms of the speaker. A 4 ohm speaker shows a little more than 3 ohms with a DC ohm-meter. The 8 ohm speakers show about 7 and the 16 ohm spekars show about 14. I have several 24 ohm speakers that I don't know what to do with and a beautiful old 32 ohm speaker from a Bell and Howell projector that has a code of 220 (Jensen), but I don't know how to use it unless I find three more and make an 8 ohm cabinet out of them.
When you are building your own amp, it is best to over design. Put over-sized transformers in your box. Build your deluxe with a pro-reverb transformer. Build your Champ with Deluxe transformers. Build your Super with a Bassman transformer. If you can, always get an output transformer that has multiple taps for different speaker impedances. Then do the math and figure which one is best for you.