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pabloxyz
Senior tube assistant
Posts: 155
(8/21/03 12:27 pm)
206.231.12.101
Reply | Edit | Del All output impedance
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related to the 1/2 power switch question, how do you calculate the output impedance of a tube amplifier? what's the equation and what are the variables?
and - how does an impedance mismatch affect the sound? does an impedance mismatch affect reliability of tubes, tranny or speakers? Would you ever intentionally create a mismatch in output impedance to create a certain sound?
can anybody recommend some reading on the subject?
thanks, pab
coap01
Junior tube assistant
Posts: 29
(8/21/03 12:35 pm)
207.241.167.7
Reply | Edit | Del try this
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i saw this on the other message board. math is pretty intense, but rearanging some variables, you can use the same equations
http://pub58.ezboard.com/fampworkshopfrm5.showMessage?topicID=690.topic nonreglA
Junior tube assistant
Posts: 3
(8/21/03 3:28 pm)
128.155.29.14
Reply | Edit | Del Re: try this
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impedance formula
Tiny Daddy
Senior tube assistant
Posts: 112
(8/21/03 5:14 pm)
67.30.35.146
Reply | Edit | Del Re: try this
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But the important thing about tube amps is not the amp's output impedance, but rather, how much load are you placing in the plate circuit of your output tubes when you connect the speakers. The output transformer reflects the speaker impedance back to the tubes. The speaker impedance is always much larger than the amp output impedance.
pabloxyz
Senior tube assistant
Posts: 156
(8/21/03 6:34 pm)
206.231.12.101
Reply | Edit | Del Re:output impedance
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thanks guys - these are all helpful.
Tiny D. it's just these kinds of things I am stupid about. When you say the speaker impedance is reflected back through the transformer to the output tubes plate circuit, that means the plates like to have something to push against, right? So, say for a given plate circuit and OT, a lower speaker load would make the amp sound . . . louder? Would that make the tubes run hotter? How would you figure a "nominal" speaker load for a given plate circuit/output tranny? And, how far can you vary from that nominal load and not destroy your equipment?
<-- still in 2nd grade. . .
pab
Casey4s
Junior tube assistant
Posts: 55
(8/21/03 7:53 pm)
205.188.209.108
Reply | Edit | Del Re: Re:output impedance
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This is how I figure OT/ tube impedance:
Finding Unknown Values for Output Transformers
(A little Math is required)
If you have an Output Transformer with unknown values and want to find the intended speaker impedance for a given tube type there are several things to consider. An OT reflects the impedance load (the speaker) on the secondary back onto the primary winding.
There are a few abbreviations I am going to use for the purpose of this explanation.
VR = Voltage Ratio
IR = Impedance Ratio
TR = Turns Ratio
IL = Impedance Load (The speaker)
ELR = Effective Load Resistance (from tube manual)
ELR Examples for two tubes in PP AB1 or AB2
6L6 ELR = 3,800 ohms
6V6 ELR = 8,000 ohms
EL84 ELR = 8,000 ohms
EL34 ELR = 6,500 ohms
The Turns Ratio (TR) can be found by applying a small AC voltage to the secondary and measuring the result on the primary. If you put 1v AC on the secondary and measure say, 31.6v on the primary you have a Voltage Ratio (VR) of 31.6 which means that the turns ratio is 31.6:1.
Finding a 1v AC source is difficult unless you have access to a Variac, small transformer or similar device. You can use a 5v rectifier winding from an existing PT or the 6.3v heater winding. You must then divide the answer by the number of volts you applied to the secondary. For example the 31.6:1 TR would measure 158v with 5v AC applied to the secondary.
The Impedance Ratio (IR) is the square of the Turns Ratio (TR), in this case:
31.6 (sq) is 998.6
To determine the expected Impedance Load (IL) or speaker impedance you will divide the known Effective Load Resistance (ELR) by the Impedance Ratio (IR)
So: ELR / IR = IL
In this example we want to use a pair of 6V6 tubes that have an ELR of 8,000 ohms with this transformer and want to determine the proper speaker impedance (or IL).
8,000 / 998.6 = 8.0 ohms
So you would use an 8-ohm speaker with this transformer and a pair of 6V6 or EL84 tubes in AB1 or AB2.
What about a pair of 6L6 tubes? RCA says the 6L6s have an ELR of 3,800 so:
3,800 / 998.6 = 3.80 ohms
So you would use a 4-ohm speaker with this transformer and a pair of 6L6 tubes in AB1 or AB2.
Casey4s
02/04/01
REV. 07/08/01
Edited by: Casey4s at: 8/21/03 7:56 pm
Tiny Daddy
Senior tube assistant
Posts: 113
(8/21/03 10:21 pm)
67.30.35.83
Reply | Edit | Del Re: Re:output impedance
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Yes lower speaker impedance makes your tubes work harder, assuming you crank the volume up. Add that second cabinet for a full stack, and you halve the impedance your amp sees, meaning half the plate load impedance for the output tubes. With the volume cranked, AC current flows through the tubes and transformer to your speakers. Check the RCA tube manual or other data sheet for nominal plate loads at various plate voltages. These "nominal" values are often exceeded in guitar amps.
As for how far you can vary and not destroy your equipment, we don't know because RCA and others only guarantee their product for the conditions in the data sheet. If you exceed those, all bets are off.
One more thought, the max power out from your amp will be limited somewhat by the size of the output transformer.
Edited by: Tiny Daddy at: 8/21/03 10:29 pm
pabloxyz
Senior tube assistant
Posts: 158
(8/22/03 12:37 pm)
206.231.12.101
Reply | Edit | Del re: cathode vs. fixed bias
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many thanks! I have very little background in electronic circuits so these things dawn on me slowly, but I am having lots of fun learning. Do you guys have copies of these tube manuals at home? where can you get 'em?
The way I understand it an OT has taps at different points along the secondary winding for matching to different speaker impedances. Casey I guess if you wanted to measure the turns ratio you'd apply the 1VAC to the full secondary winding. Does that sound right? Is there another formula for finding the ELR for class A?
coap's link above led me to a reference to a book called "The Electric Guitar Amplifier Handbook" by Jack Darr, now out of print. I found some chapters from it in PDF format at:
Jack Darr chapters
these look pretty good - I will print them out do some studying as soon as I get a new printer cartridge.
pab
Tiny Daddy
Senior tube assistant
Posts: 116
(8/22/03 1:24 pm)
67.30.31.61
Reply | Edit | Del Re: re: cathode vs. fixed bias
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Jack Darr... Service Editor for Radio Electronics, helped me with television questions (black-&-white!) when I was a kid. This was done by snail mail in the 60's.
Antique Electronics Supply is selling reprints of RCA tube manuals. I have RC-21 and RC-30 and the Transmitting Tube manual.
Also useful:
www.mif.pg.gda.pl/homepages/frank/ Jonas Gemini
Senior tube assistant
Posts: 183
(8/22/03 8:48 pm)
162.40.96.167
Reply | Edit | Del Re: re: cathode vs. fixed bias
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So how does the WATTAGE RATTING of the transformer need to be addressed? Using a 40 watt 4 - 8 - 16 multitap secondary transformer with a 3800 ohm primary to run a pair of 6V6's, using the appropriate tap to reflect the impedence to 7600 ohms?
Jonas
pabloxyz
Senior tube assistant
Posts: 160
(8/23/03 12:16 am)
206.231.12.101
Reply | Edit | Del Re: estimating power amp output in watts
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ok Jonas you asked for it
estimating an amplifier's output
I hope this is what you were looking for - I had to take an alka seltzer after reading it.
Jonas Gemini
Senior tube assistant
Posts: 186
(8/24/03 11:20 am)
162.40.96.167
Reply | Edit | Del Re: estimating power amp output in watts
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Two more question................
Using a 50 watt 3800 ohm primary 4 - 8 - 16 ohm secondary output transformer as an example, could I use this transformer to run a pair of 6v6's, under the assumption that I set the secondary impedence to reflect 7600 ohms (3800 x 2)?
Taking into account the wattage rating of another transformer, what would happen if a 25 watt 8000 ohm primary 4 - 8 - 16 ohm secondary output transformer was used with a pair of 6L6's, under the assumption that the secondary was set to reflect 3800 ohms (7600 / 2)?
Thanks
Jonas
Tiny Daddy
Senior tube assistant
Posts: 122
(8/24/03 12:10 pm)
67.30.33.123
Reply | Edit | Del Re: estimating power amp output in watts
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Well here's my thinking:
Both situations would be fine if the correct speaker load was always connected. But i
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