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Old 17th December 2011, 02:59 AM   #1
Trileru is offline Trileru  Romania
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Join Date: Dec 2010
Default bad opt measured values

Hello all!
I decided to test my custom made opts (got them a year ago when I didn't knew this much about tube world, and ordered a cheap set from a local guy).
I am using a el84 push-pull schematic with 6n6p driver and concertina. Installed 5H choke in psu and voltage got up to 330-340V on center tap of opt (maybe I will regulate it to 320V - regulator suggestions are welcome) and using a lm317 CCS for el84 cathode bias, set it at 96ma with cathodes tied together.
Now back to my problem, as I couldn't remember what spec the opts are (when I got them the guy gave them to me with a poorly designed schematic for el84 push pull) I decided to somehow find out what's going on there. After doing a bit of research it became clear that I need a small AC signal so it was easy from there. I decided to use the 6.3V heater supply. With no load it was about 6.84V kind of stable as I did this at 5 in the morning (yeap, just another night spent with dmm in one hand and cigar in the other - kids, don't do it, it's bad!)
ok, so I connected the heater supply to the secondary of one opt and got a reading of 401V on the primary. doing some simple math the turn ratio came out about 58.6:1 (maybe more with losses?), and impedance ratio would be 3436:1. As I am using a 8 ohm speaker (8-ish as it varies across bw but still, let's call it 8) then I am actually using a 27.5k load for el84 push-pull? the other one came out with a 54.7:1 turn ratio meaning 2990:1 impedance ratio and into 8 ohm speaker that's a Ra of 23.9k. If my measurement tactic (and reference heater voltage) are ok then these must be really out of place in my setup. Now what's more that I'm interested, what is actually happening when using 24k load instead of 8-10k recommended? Do I lose power, do I stress the final tubes (actually I just installed 10 days ago a pair of 6p14p-ev's for break-in and final testing, matched pair that is, and made sure to match the current draw down to 0.1mA with a pot in cathodes and would hate to have done any harm to these babies). So this seems like I was doing all this fine tuning for nothing. Forgot to mention that I measured the individual primaries resistance and were off, one gave 280ohm the other 310. About the same for the other opt. If this is too bad for my setup (if and only if the measurement method is correct) then I will try to contact the seller and replace/improve, if this doesn't work out then maybe I could try to fix them myself. Don't have experience at all in this area but I don't mind getting my hands dirty. Maybe I could change the metal as well with something higher quality. Thinking that I need to unwind a bit from each side if it isn't too complicated. If not then last resort would be a pair of hammonds 1608A or somewhere like that. Maybe a 15W version. With 330-340V B+ and 48mA that's about 16W. Also I would like to keep an option for triode mode as I am likely to use either depending on situation. Haven't tried UL mode for obvious reasons, would do that as well if going for a Hammond opt. Now I must say that I enjoy the sound very much, but this wouldn't be an excuse for the opts as this is my first time listening to tubes, and don't know anything better than what I have at the moment. My background on music listening is shameful loaded with computer desk speakers, so anything above that I consider a wonder. That's why I am asking you, can it get better with another set of (decent priced) opts? Is it too much of a hassle to source C-cores and start learning? Making winding machines and stuff like that or a pair of Hammonds are going to be just fine here? Must mention that bass is deep, scope showed nice sine wave (tested to 1HZ and saw the freaky slow motion move on the speaker cone even). Lots of iron on these opts, heavy too!
Thank you for checking out my post
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Old 17th December 2011, 03:28 AM   #2
Trileru is offline Trileru  Romania
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Join Date: Dec 2010
I just realized that heater voltage is 50hz and maybe my whole measurement is compromised as I don't expect the opt will behave that good at this frequency. Is it possible to try with a 500hz sine from soundcard? Should put out 0.8-1v rms.
But that's for tomorow to do...
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Old 17th December 2011, 06:34 AM   #3
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Location: El Cajon, CA
The load is definitely too high for a pair that want to 'see' 8-10K plate to plate, but remember, transformers don't have pure impedance, they have ratios. If you were to put a lower load on the secondary output, you would get a lower reflected load to the tubes on the primary. If your speakers cant be changed to a lower load, they you are running the transformer too wide open (not loaded enough) and run the risk of voltage spikes damaging the transformer insulation in the primary. Don't worry about imbalances in resistance from the center tap- it usually is not equal because the wire is distributed unequally on the bobbin in the winding process to get equal turns ratio or voltages, and the resistance has to off a bit to get that right (smaller bobbin at the start gets more turns for an equivalent amount of wire than the larger, outer part of the bobbin that has to use more wire to get the same turns (so turns don't don't have equal resistance since the wire resistance is a constant and the length to get the right turns ratio is not).
It might be informative to play around building and winding your own, but it is science and an art to do it well (layer placement perfection, taping between layers, interleaving (the other winding) into the first to get the best frequency response and efficiency, metal and grain orientations and thicknesses, formulations, stamping insulation, bobbin material, assembly, coating and baking the coils, getting the ratios right with a proper window- allowing for taping between layers and different gauges of wire- it is a lot to do and still sound worse than a salvaged or more cheaply bought finished device from many sources. You could do better than winding your own output transformer using a power transformer in toroid form at low cost. A p-p pair of 6BQ5s to 8 ohms is 1250 to 1 impedance ratio (10,000:8). that gives you a turns ratio (or voltage ratio of 35:1) Using a 240 vac center tapped (2X120) as your primary, you need an output of about 6.8 volts. If that is 12.6 center tapped you just use one side (or use a 16 ohm speaker) 6vac center tapped can be used in parallel to get more current or use one half of a 6 volt winding to drive a 4 ohm speaker. It is close enough. Toroids work well since their magnetic field is so contained in the 'doughnut' and are by nature, well interleaved giving good performance even though they are designed for 50-60 Hz. Another advantage is they don't exhibit the 'unbalanced primary dc resistance syndrome' that typical E-I transformers do as you mention since the winding depth is rather shallow per watt (you tie two of the 120 volt windings together at the proper phase point and use that as your center tap). A 20 watt toroid is pretty small and can handle the 2 tubes fine.Some people even use 'normal; power transformers, but you need a center tap for push pull, so you need two 120 volt windings to get at that center tap you need. But you lose all the advantages of toroids but can get them cheap enough to build an amp with a 13 buck transformer on the output. If you need more highs or volume, turn it up and boost the treble!

Personally, I use vintage, good units designed for close to their intended use, or if I have the money, buy what I want ready made- nice and shiny with a return policy. Hammond, and Edcor have a large inventory at good prices.
Cheers!
Mark
To each His own!
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Old 17th December 2011, 03:01 PM   #4
Trileru is offline Trileru  Romania
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Join Date: Dec 2010
Hey thank you for the info. As it turns out it seems that I have about 60 turn ratio tested at 500 and 2000 hz. That's even worse I guess. I used signal from the grill of el84. 2 dmm, measured the input into the opt and another dmm measured the output. ratio is about 60 at more freqs. My question is, can you tell with an acceptable margin if the final impedance ratio is 3600:1 ? As I am using 8 ohm speakers that's 30k load.
As for the opts, I might just change the iron on them as I recall that was from scrap metal found in some UPS's
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