Just a quick and simple question, please.
How do you go about selecting an output transformer for a tube amp?
Output impedance for tube amp (say SE) with OPT is given by:
Rout=Rl*rp/Rl+rp right?
where: Rl=tranny primary impedance (assuming a secondary loading from speakers)
rp=tube internal resistance
For best power and results, best to match amp output impedance to prim tranny impedance, right? So, can Rout ever equal Rl?
What am I missing here?
Thanks,
Rick
How do you go about selecting an output transformer for a tube amp?
Output impedance for tube amp (say SE) with OPT is given by:
Rout=Rl*rp/Rl+rp right?
where: Rl=tranny primary impedance (assuming a secondary loading from speakers)
rp=tube internal resistance
For best power and results, best to match amp output impedance to prim tranny impedance, right? So, can Rout ever equal Rl?
What am I missing here?
Thanks,
Rick
Rick,
I was waiting for someone to explain it more eloquently than I can.
The way I see it, you are right about maximum power transfer, but that does not correspond with "best" distortion.
I believe the distortion goes down and down as the load impedance is raised. At the same time, the power also goes down.
The "rule of thumb" of Rl being 2 to 3 times Ra is just a compromise of power vs distortion.
Cheers,
I was waiting for someone to explain it more eloquently than I can.
The way I see it, you are right about maximum power transfer, but that does not correspond with "best" distortion.
I believe the distortion goes down and down as the load impedance is raised. At the same time, the power also goes down.
The "rule of thumb" of Rl being 2 to 3 times Ra is just a compromise of power vs distortion.
Cheers,
Thank you very much
John,
I appreciate your response...this stuff is difficult to comprehend without a background in electronics, I think.
The relationship you described power vs distortion and impedance seems to bear out since I use a Hammond 125ese which has different connections.
Your explanations are perfect for me!
Rick
John,
I appreciate your response...this stuff is difficult to comprehend without a background in electronics, I think.
The relationship you described power vs distortion and impedance seems to bear out since I use a Hammond 125ese which has different connections.
Your explanations are perfect for me!
Rick
OK, this is one where I only know things like ratios and the like. But... I thought distortion goes down as current goes up? Wouldn't a lower primary give less distortion due to higher current for signal?
I generally find this true for small signal design. But perhaps not for power output.
Higher impedance gives narrower bandwidth, no?
Curious,
Gabe
I generally find this true for small signal design. But perhaps not for power output.
Higher impedance gives narrower bandwidth, no?
Curious,
Gabe
CHOOSING YOUR OPT.
Hi,
Choosing optimum load for output tube /SE amp/:
For triodes choose - RL = (2-6) RP /optimum is around 4 Rp/
Higher RL for triodes means lower distortion and lower output power.
The output power is about 1/4 of PP - plate dissipation. Pout= 0.25 PP
For pentodes or beam power tubes - RL = (0.1) RP
For pentodes, tetrodes or beam power tubes the impedance is critical for best performance - higher power and lower distortion.
The output power is about 1/2 of PP - plate dissipation. Pout= 0.5 PP
WHERE: RL - load resistance /impedance of transformer primary/; RP - plate resistance of chosen valve.
Also determine loudspeaker impedance Z to match with the transformer - that is usually 4, 6, 8 or 16 Ohm. That will be reflected on the primary /transformed/ with the transformer.
Hope this helps.
Gabe,
The current flow through the outputstage may well affect distortion figures, it won't have a direct relationship to the OPT however.
Cheers,
Hi,
Choosing optimum load for output tube /SE amp/:
For triodes choose - RL = (2-6) RP /optimum is around 4 Rp/
Higher RL for triodes means lower distortion and lower output power.
The output power is about 1/4 of PP - plate dissipation. Pout= 0.25 PP
For pentodes or beam power tubes - RL = (0.1) RP
For pentodes, tetrodes or beam power tubes the impedance is critical for best performance - higher power and lower distortion.
The output power is about 1/2 of PP - plate dissipation. Pout= 0.5 PP
WHERE: RL - load resistance /impedance of transformer primary/; RP - plate resistance of chosen valve.
Also determine loudspeaker impedance Z to match with the transformer - that is usually 4, 6, 8 or 16 Ohm. That will be reflected on the primary /transformed/ with the transformer.
Hope this helps.
Gabe,
The current flow through the outputstage may well affect distortion figures, it won't have a direct relationship to the OPT however.
Cheers,
dhaen,
I know you didn't mention current... but it does come into play doesn't it?
fdegrove,
Why?
As an interesting side note, I measured the voltage across the primary of an SE amplifier with a signal at full output and it measured something like 30-40 volts higher than B+. Interesting, no?
Gabe
I know you didn't mention current... but it does come into play doesn't it?
fdegrove,
Why?
As an interesting side note, I measured the voltage across the primary of an SE amplifier with a signal at full output and it measured something like 30-40 volts higher than B+. Interesting, no?
Gabe
Thank you!
Thank you John, Gabe and Frank,
Very helpful!
Appreciate the info,
While I've got you, another simple question, please: How does the impedance from a vol pot affect the hum at the speaker in a two stage RC coupled, se triode?
That is, as the impedance of the vol pot goes up, the hum at the speaker increases. Using a 10k vol pot produces no hum but changing to a 100k or 250k ohm vol pot increases hum proportionately.
Source is CDP.
Any assistance is greatly appreciated,
Rick
Thank you John, Gabe and Frank,
Very helpful!
Appreciate the info,
While I've got you, another simple question, please: How does the impedance from a vol pot affect the hum at the speaker in a two stage RC coupled, se triode?
That is, as the impedance of the vol pot goes up, the hum at the speaker increases. Using a 10k vol pot produces no hum but changing to a 100k or 250k ohm vol pot increases hum proportionately.
Source is CDP.
Any assistance is greatly appreciated,
Rick
RE:IMPEDANCE MATCHING.
Hi,
Gabe,
The only reason I see for this becoming a problem is with badly specified OPTs, i.e. an SE OPT with a gap for say 100 mA and 120 mA run through it.
If an OPT is overloaded it will distort but that isn't actually related to the standing current of the outputstage.
Rick,
The 10K pot shunts alot more to ground than the 100 or 250K pot.
If this is an actual problem you have than there definetely is a hum problem with that circuit.
Cheers,
Hi,
Gabe,
The only reason I see for this becoming a problem is with badly specified OPTs, i.e. an SE OPT with a gap for say 100 mA and 120 mA run through it.
If an OPT is overloaded it will distort but that isn't actually related to the standing current of the outputstage.
Rick,
The 10K pot shunts alot more to ground than the 100 or 250K pot.
If this is an actual problem you have than there definetely is a hum problem with that circuit.
Cheers,
Hum Problem
Thanks Frank,
"The 10K pot shunts alot more to ground than the 100 or 250K pot.
If this is an actual problem you have than there definetely is a hum problem with that circuit."
Well, that is the case...so what is your best guess based on those symptoms? Circuit design or circuit layout/construction?
I would appreciate any guesses, thanks,
Rick
Thanks Frank,
"The 10K pot shunts alot more to ground than the 100 or 250K pot.
If this is an actual problem you have than there definetely is a hum problem with that circuit."
Well, that is the case...so what is your best guess based on those symptoms? Circuit design or circuit layout/construction?
I would appreciate any guesses, thanks,
Rick
No guesses necessary. The higher the grid resistor, the more sensitive the grid is to input. Tubes tend to be sensitive to almost anything (a generalization folks. No need to cite low mu tubes etc.). This is because of its very high inherent impedance. Make a single tube AM radio and listen to all the stations you pick up from hundreds to thousands of miles away, even just using the tube as a detector.
So... putting a low resistance at the grid lowers its apparent impedance, thereby its sensitivity to input.
In many cases topology... more like circuit layout... can help reduce hum to a minimum. Making sure the input is connected to something is another way. But, with tubes full volume setting will always allow stray EM, even from the house itself (remember you are immersed within a pool of EM from the power lines in your house) to be picked up. There is no avoiding it.
Gabe
So... putting a low resistance at the grid lowers its apparent impedance, thereby its sensitivity to input.
In many cases topology... more like circuit layout... can help reduce hum to a minimum. Making sure the input is connected to something is another way. But, with tubes full volume setting will always allow stray EM, even from the house itself (remember you are immersed within a pool of EM from the power lines in your house) to be picked up. There is no avoiding it.
Gabe
High resistance
Thanks Gabe,
Your assessment seems spot on compared with this experience!
There really appears a direct correlation between high resistance and hum.
I make a few changes and see what happens.
BTW, I really appreciate your website and your posts. I have learned a great deal from your designs.
thanks,
Rick
Thanks Gabe,
Your assessment seems spot on compared with this experience!
There really appears a direct correlation between high resistance and hum.
I make a few changes and see what happens.
BTW, I really appreciate your website and your posts. I have learned a great deal from your designs.
thanks,
Rick
Gymnastics, and swinging above the HT pole
In theory, because of the energy stored in the inductance of the output transformer, you should be able to swing linearly to twice the HT voltage. In practice, you never quite make it, but this is one of the better arguments for using inter-stage transformers. You can double the voltage swing (compared to a resistive load), then use a step-down transformer of 2:1 voltage ratio. This gives the same voltage as you could have achieved using a resistive load, but because impedances are changed by the square of the ratio, you now have a quarter of the output resistance of the circuit using a resistive load.
Gabevee said:
In theory, because of the energy stored in the inductance of the output transformer, you should be able to swing linearly to twice the HT voltage. In practice, you never quite make it, but this is one of the better arguments for using inter-stage transformers. You can double the voltage swing (compared to a resistive load), then use a step-down transformer of 2:1 voltage ratio. This gives the same voltage as you could have achieved using a resistive load, but because impedances are changed by the square of the ratio, you now have a quarter of the output resistance of the circuit using a resistive load.
fragman56
O gosh, I am blushing! Thanks.
EC8010,
I had once surmised that in an SE amp, the reason for its seeming dynamics was due to the fact that two inductances will actually have more AC (the key here) voltage across each inductance than the total input. That measurement proved it, inasmuch as when a signal is present the output tube acts as the other inductance.
Fascinating stuff!
Gabe
O gosh, I am blushing! Thanks.
EC8010,
I had once surmised that in an SE amp, the reason for its seeming dynamics was due to the fact that two inductances will actually have more AC (the key here) voltage across each inductance than the total input. That measurement proved it, inasmuch as when a signal is present the output tube acts as the other inductance.
Fascinating stuff!
Gabe
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