A friend of mine gave me a pair of very nice output transformers he has had for years, and I am trying to figure out the windings and impedance ratio and such, he says they were for an SE 300B or similar, and I could use some guidance to figure out the output windings.
I spent a few hours today driving the xfmr with a sig gen and measuring the output on my scope. It's not a normal secondary winding with the usual 4, 8, 16 ohm taps, it's 3 separate identical windings. I connected my sig gen to the primary winding, put in 40 VAC @ 1KHz and measured the following.
Each of the 3 output windings gave the same result, .75VAC out each. I tried connecting them all in series and made an LT Spice diagram so it's easier to see what's going on. The 300B SE could be anywhere from 2K to 5K plate load depending on plate volts and anode current. The measurements seem plausible but I could use some help to determine what I really have here. My ultimate goal is to scratch build an SE 300B with minimal driver stage, maybe xfmr coupled, not sure at this point, I'm working my way in from the back end. 😱 My speakers are Big Reds, 16 ohm. The turns ratios I came up with gave me what seem to be correct results for the 16 and 8 ohm windings, the 4 ohm seems way off. I won't say any more as I don't want to influence your thoughts....
Attached is the diagram I made of the transformer and AC measurements @ 1KHz. Ignore the Lx desinations, I couldn't figure out how to delete them in LTSpice.
Any help, much appreciated.
Cheers
I spent a few hours today driving the xfmr with a sig gen and measuring the output on my scope. It's not a normal secondary winding with the usual 4, 8, 16 ohm taps, it's 3 separate identical windings. I connected my sig gen to the primary winding, put in 40 VAC @ 1KHz and measured the following.
Each of the 3 output windings gave the same result, .75VAC out each. I tried connecting them all in series and made an LT Spice diagram so it's easier to see what's going on. The 300B SE could be anywhere from 2K to 5K plate load depending on plate volts and anode current. The measurements seem plausible but I could use some help to determine what I really have here. My ultimate goal is to scratch build an SE 300B with minimal driver stage, maybe xfmr coupled, not sure at this point, I'm working my way in from the back end. 😱 My speakers are Big Reds, 16 ohm. The turns ratios I came up with gave me what seem to be correct results for the 16 and 8 ohm windings, the 4 ohm seems way off. I won't say any more as I don't want to influence your thoughts....
Attached is the diagram I made of the transformer and AC measurements @ 1KHz. Ignore the Lx desinations, I couldn't figure out how to delete them in LTSpice.
Any help, much appreciated.
Cheers
40:2.25 = 17.78 turns ratio (so 316:1 impedance ratio)
40:1.50 = 26.67 turns ratio (711:1 impedance ratio)
40:0.75 = 53.3 turns ratio (2844:1 impedance ratio)
Recall that the impedances are scaled by N2 and voltages scaled by the turns ratio, N.
If you make the middle tap (711:1) an 8 Ω tap, you'll have 5.6 kΩ on the primary. That's about ideal for a low distortion SE 300B. The bottom tap (2844:1) will then be a 2 Ω tap.
It makes me wonder if the top tap (316:1) is supposed to be the 8 Ω tap. That would make for a 316*8 = 2.5 kΩ primary. 4 Ω on the middle tap would result in 711*4 = 2.8 kΩ on the primary. That leaves the bottom tap as a feedback winding.
Do you have any idea of what the primary inductance is? You need more than 10 H to get decent bass from a 300B.
Tom
40:1.50 = 26.67 turns ratio (711:1 impedance ratio)
40:0.75 = 53.3 turns ratio (2844:1 impedance ratio)
Recall that the impedances are scaled by N2 and voltages scaled by the turns ratio, N.
If you make the middle tap (711:1) an 8 Ω tap, you'll have 5.6 kΩ on the primary. That's about ideal for a low distortion SE 300B. The bottom tap (2844:1) will then be a 2 Ω tap.
It makes me wonder if the top tap (316:1) is supposed to be the 8 Ω tap. That would make for a 316*8 = 2.5 kΩ primary. 4 Ω on the middle tap would result in 711*4 = 2.8 kΩ on the primary. That leaves the bottom tap as a feedback winding.
Do you have any idea of what the primary inductance is? You need more than 10 H to get decent bass from a 300B.
Tom
Thanks Tom,
Those are exactly the numbers I got as well, and it was the low 2 ohm impedance that was the confusing point for me, but I hadn't thought it could be a feedback point, thank you.
I haven't figured out the inductance yet, I'll try tonight after work.
Looking at the WE data sheet, it seems 2K -3K is it's happy spot, @ 300-400 Vp most power and least distortion, but who knows if the current 300Bs conform to these data sheets.
I'll get back to it tonight, thank you for your help.
cheers
Those are exactly the numbers I got as well, and it was the low 2 ohm impedance that was the confusing point for me, but I hadn't thought it could be a feedback point, thank you.
I haven't figured out the inductance yet, I'll try tonight after work.
Looking at the WE data sheet, it seems 2K -3K is it's happy spot, @ 300-400 Vp most power and least distortion, but who knows if the current 300Bs conform to these data sheets.
I'll get back to it tonight, thank you for your help.
cheers
It seems the older designs were using a 3.6 kΩ anode load on the 300B for higher output power. 5 kΩ is a nice spot. It'll give you slightly less power but also lower distortion. I seem to recall that a classic operating point was around 360 V and 65 mA, which gives even lower power. I liked the 300B at 400 V, 85 mA but you'll probably want a newer 300B if you want to run them that hot. The JJ 300B is nice.
If you need inspiration for a 300B design, have a look at the DG300B I designed some ten years ago: https://neurochrome.com/pages/dg300b
The TubeLab SE and SSE are nice options as well: http://tubelab.com/designs/tubelab-sse/
Tom
If you need inspiration for a 300B design, have a look at the DG300B I designed some ten years ago: https://neurochrome.com/pages/dg300b
The TubeLab SE and SSE are nice options as well: http://tubelab.com/designs/tubelab-sse/
Tom
Okay using this method as pictured, I get 525H, seems a tad large. 🙄 I am using a dual channel scope and I get equal voltages all the way from 20Hz to about 200Hz, then a slight unbalance then there is a big dip around 4KHz then above that the voltages get closer but never equal again up to 10KHz. I'm using 40VAC P-P out of my generator
There must be something wacky?? If the signals are balanced from 20Hz up to 200Hz that gives a wildly different range of inductance's. Any other suggestions for measure L?
Cheers
PS going to try it on the secondary and use the formula Lp=N^2*Ls
There must be something wacky?? If the signals are balanced from 20Hz up to 200Hz that gives a wildly different range of inductance's. Any other suggestions for measure L?
Cheers
PS going to try it on the secondary and use the formula Lp=N^2*Ls
Got it! I used my Fluke 87III instead and found a very pronounced balance point at 1125Hz, which gives me 47H, much more believable.
I'll have to drag my VTVM out of storage to confirm, not sure the freq. response of the DVM, but I guess if it balances it shouldn't matter.
So now I need to figure out how best to wire these secondaries for 8/16, I don't have any 4 ohm speakers.
Cheers
I'll have to drag my VTVM out of storage to confirm, not sure the freq. response of the DVM, but I guess if it balances it shouldn't matter.
So now I need to figure out how best to wire these secondaries for 8/16, I don't have any 4 ohm speakers.
Cheers
You can get 4+8 ohms (or 8+16 ohms) by paralleling all three or paralleling two with the third in series. That seems the likeliest scenario, with 4+8 ohms giving 2500-3000 ohms primary impedance - which being more common seems more probable.
The trick is then to figure out which two windings to parallel for the higher impedance, for which you need to know how the windings are interleaved. DC resistances might help. If for example the secondaries are located one in the middle of the primary and one at each end, then paralleling the outer windings gives them each half the current, which is ideal to minimize leakage inductance.
The trick is then to figure out which two windings to parallel for the higher impedance, for which you need to know how the windings are interleaved. DC resistances might help. If for example the secondaries are located one in the middle of the primary and one at each end, then paralleling the outer windings gives them each half the current, which is ideal to minimize leakage inductance.
Awesome, thank you! I will dig out my Systron Donner 6 decimal place rack mount multimeter tomorrow so I can make the ultra accurate Ohm measurements. I could also try taking the cover off the xfmr to see if I can see the bobbin and where the windings come out.
I have so much to learn about transformer function, I had no idea you could achieve different impedances by paralleling different combos of windings..
Thanks again, we're getting closer to figuring this out.
cheers
I have so much to learn about transformer function, I had no idea you could achieve different impedances by paralleling different combos of windings..
Thanks again, we're getting closer to figuring this out.
cheers
Don't get fooled by that trap called "believable".
You need to understand that inductance varies due to amplitude and frequency, because flux density and losses change. Non-gapped cores are also less linear than gapped, so expect bigger variety in inductance values. If you measure at low flux densities, you'll potentially find yourself in the knee region of the core where permeability, hence inductance will be lower than high excitation regions.
To measure a tube output transformer's inductance more accurately, you need a high voltage AC value. One can use the mains voltage via an isolation or slight step down transformer, but mains voltage changes.
I believe Patrick Turner used mains voltage via isolation transformer and used a high pass 50Hz filter to reduce mains voltage lower frequencies fluctuations.
I'm using a simple LM1875 amplifier feeding an inversely connected power transformer and that gives me adjustable stabilized voltages from 60 to 150V with adjustable frequency from a generator.
Then you feed this voltage via a resistor and measure the voltage drop across the resistor. Then you can calculate the reactance across the DUT and finally the inductance.
Look what I found in my inbox when I got home! 
My friend found this hand drawn sheet for the transformers he gave me and it coincides with what Paul was saying about using parallel-series combo of windings, too cool. No other info but at least we are on the right track. This is drawn for 4/8 ohms with I'm guessing is the back of a DPDT switch.
So what would be your recommendation? I would really like to have them set for 8/16 rather than 4/8 ohms. I'm heading out to the shop to find my Systron Donner meter so I can start with some DC R measurements.
Thanks all!
Cheers
My friend found this hand drawn sheet for the transformers he gave me and it coincides with what Paul was saying about using parallel-series combo of windings, too cool. No other info but at least we are on the right track. This is drawn for 4/8 ohms with I'm guessing is the back of a DPDT switch.
So what would be your recommendation? I would really like to have them set for 8/16 rather than 4/8 ohms. I'm heading out to the shop to find my Systron Donner meter so I can start with some DC R measurements.
Thanks all!
Cheers