Amimaster,
Thanks for mentioning in your Post # 1, that this is about a Guitar tube amplifier!
This is the Tubes/Valves thread area. Now both guitar amps and speakers are being discussed in this thread.
Since solid state guitar amps also are mentioned in this thread, I am giving my $0.02 here (two cents):
How to make a solid state guitar amplifier that will have a very low damping factor . . .
Use a PNP and NPN totem pole collector output (PNP collector connected to NPN collector; emitters to the + and - power supplies, respectively).
Be sure to design it Open Loop, No negative feedback . . . that gives high impedance output. Damping factor << 1 (<< Unity).
I gave my $0.02 worth, because tube guitar amplifiers are allowed to be posted on the Instruments and Amps threads.
. . . Tradeoffs.
Thanks for mentioning in your Post # 1, that this is about a Guitar tube amplifier!
This is the Tubes/Valves thread area. Now both guitar amps and speakers are being discussed in this thread.
Since solid state guitar amps also are mentioned in this thread, I am giving my $0.02 here (two cents):
How to make a solid state guitar amplifier that will have a very low damping factor . . .
Use a PNP and NPN totem pole collector output (PNP collector connected to NPN collector; emitters to the + and - power supplies, respectively).
Be sure to design it Open Loop, No negative feedback . . . that gives high impedance output. Damping factor << 1 (<< Unity).
I gave my $0.02 worth, because tube guitar amplifiers are allowed to be posted on the Instruments and Amps threads.
. . . Tradeoffs.
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Just for the records, I'm inspiring my amp to this schematics, I'm going to keep the power amp pretty much as you can find there, except for some "improvements" like individual bias trimmers and a more sophisticated power supply, while I'd like to experiment much more with the pre-amp since I already own pretty much the same (original) amplifier (Mesa Quad Preamp) but with a 500W power amp, which is completely overkill for every situation except playing in a stadium.
Thanks everyone! I'll update as soon as I have some time to investigate more on it!
The B+ should indeed be around 483V, according to schematics and my simulations... However the loadline I'm getting should cut right through the max power zone for such a low load, so something doesn't add up.
One of the two meters was connected to the power supply (it's an handheld oscilloscope with a dc meter function..). The other I suspect it was a little low on battery since it doesn't give much stable measures.. I'll try replacing the battery or the meter.
The "non-simul-class" version of this transformer is commonly used in 100W amps with parallel tubes and only supports 6L6s (so they just have two leads and a B+), typically four of them. They state it's around 2K Raa.
I noticed it too. I double-checked the wires before plugging the power. I didn't test the B+ lead, but maybe I should. I actually don't know who wires these transformers (they are branded by TAD but I don't know whether they are sourced from a third party, wires come from Italy...). I think it's unlikely they mixed up the wire colours, but who knows...
Yes, it should use two 6L6 and two EL34 at the same time, as in the schematics. If the findings are correct then I'm not sure I understand how they could be. For what I remember, parallel tubes only "see" half of the load each, which should lead to even lower values. In this case they are even taken from different taps, so I struggle visualizing the situation..
Thanks everyone! I'll update as soon as I have some time to investigate more on it!
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I ran a more thorough measurement using a sine wave with 500mV amplitude at 1KHz on the secondary, then I read the peak-to-peak values for each wire combination (as suggested by this video). These are the results. They are even lower than those measured with the variac. I really don't understand what's going on here...
Were both primary and secondary voltages measured using the same instrument?
If not, make sure that both meters give the same reading when connected to the same voltage.
Remember that DMMs read RMS and Vrms = Vp/1.41 = Vpp/2.82.
If not, make sure that both meters give the same reading when connected to the same voltage.
Remember that DMMs read RMS and Vrms = Vp/1.41 = Vpp/2.82.
For windings ratio , measurements must be made at 50/60 Hz directly at mains voltage , like a power transformer ... no errors from multimeters at 1KHz , higher losses in transformer and so on . Using a variac should be ok but is another variable ...
But it is not clear what transformer is that , if it is for that weird schematic using EL34 and 6L6 in the same time or not ...
But it is not clear what transformer is that , if it is for that weird schematic using EL34 and 6L6 in the same time or not ...
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Yes, In this last measurement I was using an Hantek 2D42 oscilloscope/function generator, both for generating signal and measuring the secondary (in channel 1) and primary (in channels 2). I both inspected the waveform by counting the divisions and summed the min/max values of the waveform as shown by the oscilloscope meter function (i.e., I'm measuring the peak-to-peak values, not RMS as i'm not using the DMM funciton).
I see many people using these methods for estimating primary impedance, they seem to have no issues in doing that. However, the transformer is this, and yes, it is specifically for that weird schematics using EL34 and 6L6. That circuit is being used since the 80's and it's one of the most famous and sought after guitar amplifiers used in blues/rock/metal music. The original transformer was a Schumacher SC-152019B. They are quite rare, so I ordered a replica from the site above. I could never find exact specs for the SC-152019B, and the only specs I have are those you can read in the link itself. For this reason I was trying to measure them...
Rikaro,
You are correct, always use the same measurement equipment to measure on both sides of the transformer.
Or as you said, connect both to the same signal, and compare the readings.
Setting up an Oscilloscope and a pair of probes, to do 2 Channel Differential measurement, requires checking that the readings of both channels and probes are equal when they connect to the same point, and measure the same signal. Then, do not switch the probes from channel to channel.
It is more complex than your formula, it only works for an Un-distorted Sine Wave.
Many meters are calibrated to properly read out the RMS voltage of an undistorted Sine Wave.
But, they do not all respond the same to other waveform shapes.
Meters generally actually Respond to one of the following characteristics:
Peak Voltage (many VTVM and TVM)
Average Voltage VOM and inexpensive DMMs (old Simpson and Triplett)
True RMS Voltage (more expensive DMMs)
But most of the readouts 'read' RMS Voltage, But for any waveform other than an Un-distorted Sine Wave, the reading is Not correct, unless the meter is actually True RMS Responding.
If I remember correctly, some old VTVMs have scales for Both Peak and RMS; even though they only respond properly to Peak.
"All generalizations have exceptions"
"All meters are equal, but some meters are more equal than others"
Measurements are an Art and a Science.
Your Mileage May Vary.
Have Fun Measuring!
You are correct, always use the same measurement equipment to measure on both sides of the transformer.
Or as you said, connect both to the same signal, and compare the readings.
Setting up an Oscilloscope and a pair of probes, to do 2 Channel Differential measurement, requires checking that the readings of both channels and probes are equal when they connect to the same point, and measure the same signal. Then, do not switch the probes from channel to channel.
It is more complex than your formula, it only works for an Un-distorted Sine Wave.
Many meters are calibrated to properly read out the RMS voltage of an undistorted Sine Wave.
But, they do not all respond the same to other waveform shapes.
Meters generally actually Respond to one of the following characteristics:
Peak Voltage (many VTVM and TVM)
Average Voltage VOM and inexpensive DMMs (old Simpson and Triplett)
True RMS Voltage (more expensive DMMs)
But most of the readouts 'read' RMS Voltage, But for any waveform other than an Un-distorted Sine Wave, the reading is Not correct, unless the meter is actually True RMS Responding.
If I remember correctly, some old VTVMs have scales for Both Peak and RMS; even though they only respond properly to Peak.
"All generalizations have exceptions"
"All meters are equal, but some meters are more equal than others"
Measurements are an Art and a Science.
Your Mileage May Vary.
Have Fun Measuring!
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I think you realise that transformers using EL34 and 6L6 in the same time are "special" , no surprise if Raa for tubes is not standard ...
If you measured in two ways and the results are pretty close than it is what it is .
If you want to check your methods of measurement you should use a normal output transformer that should be close to Raa stated in datasheets .
Yeah, we experimental physicists love measuring.
30+ years of professional experience in electronics helps.
You're correct, the simple RMS conversions I mentioned only work with a sine wave.
So they apply to the 1kHz signal the OP used.
Testing with a 1kHz signal is fine as long as the meter has a sufficiently large bandwidth (meaning considerably larger than 1kHz).
Unfortunately many cheap DMMs can only be used up to around 400Hz. Moreover accuracy often changes with signal level.
To determine the effective Raa of an OT I use a different method:
With my LCR meter I measure the primary AC resistance @1kHz with a load resistor of designated value connected to the secondary.
This gives me the actual Raa including the effective copper resistance of the primary and secondary windings.
With a small OT the result can be up to 20% larger than the reflected load resistance found from the voltage ratio.
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If you drive a transformer winding with a low impedance, and leave the other winding Open,
The Turns Ratio = The Voltage Ratio
The Voltage Ratio and Turns Ratio with an unloaded winding does not take into account the DCR of the two windings.
If you drive a transformer winding with a low impedance, and load the other winding,
The Impedance Ratio = The Square of the Voltage Ratio.
The Impedance Ratio does include both the primary DCR and the secondary DCR;
You get the actual impedance ratio, because the loaded winding takes into account the DCRs.
Any other losses are also accounted for because you drove one winding with a low impedance, and you loaded the other winding.
You have determined the actual impedance ratio.
Don't forget: Impedance Ratio = (Loaded Voltage Ratio) squared
The Turns Ratio = The Voltage Ratio
The Voltage Ratio and Turns Ratio with an unloaded winding does not take into account the DCR of the two windings.
If you drive a transformer winding with a low impedance, and load the other winding,
The Impedance Ratio = The Square of the Voltage Ratio.
The Impedance Ratio does include both the primary DCR and the secondary DCR;
You get the actual impedance ratio, because the loaded winding takes into account the DCRs.
Any other losses are also accounted for because you drove one winding with a low impedance, and you loaded the other winding.
You have determined the actual impedance ratio.
Don't forget: Impedance Ratio = (Loaded Voltage Ratio) squared
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