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Choosing the best EL34 pp opt
Choosing the best EL34 pp opt
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Old 14th September 2005, 11:00 PM   #11
Johan Potgieter is offline Johan Potgieter  South Africa
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Hi Nigel,

Not to underestimate your level of knowledge; do not want to carry coals to Newcastle (pun intended), but so far nobody warned that one also has to take feedback stability into consideration (if you have NFB) when using an output transformer. One cannot just slap an "unknown" output transformer into a circuit and compare, without checking that there are no peaks at either extremity of the audio band. This is not a problem that cannot be sorted out by proper design (simple aligning of time constants), but it is possible that a better transformer can give poorer results simply because it has been slapped into a circuit designed and compensated for something else. A signal generator/square wave generator and 'scope are all that is needed, but just keep it in mind.
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Old 15th September 2005, 03:30 AM   #12
ray_moth is offline ray_moth  Indonesia
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Avoid guitar amp OPTs, they are not designed for high-fidelity purposes. As another poster has said, Lundahl C-core OPTs might be worth a try. They are reasonably priced and have a gtreat reputation. One thing that attracts me is the air-gap that is present, even in the push-pull models. This gives you a bit more leeway in the OP tube balance, which can be a critical issue with some other models, especially the toroids of Plitron (Canada) and is sister company Amplimo (Netherlands). The Lundahl web site is here.
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Old 15th September 2005, 03:58 AM   #13
Geek is offline Geek
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No one has mentioned Hammond yet

Great bang for the buck!
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Old 15th September 2005, 07:05 AM   #14
cerrem is offline cerrem  United States
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Before choosing the OPT...
What is the intended plate voltage and screen voltage... Since this will dictate the optimum plate load to use... "Optimum" meaning best power output for lowest harmonics.....
Since this is Hi-Fi application....I hope you regulate the screen voltage for mucho better linearity...

Chris
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Old 15th September 2005, 09:57 AM   #15
amperex is offline amperex  United States
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Default Edcor new Line of transformers

Edcor has a new line of audio transformers that is about 30% less expensive than Hammond & I suspect of equal or higher quality. They been in business a long time.
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Old 15th September 2005, 09:19 PM   #16
andyjevans is offline andyjevans  United Kingdom
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O-netics make good OPTs and are excellent to deal with - Bud Purvine is a nice guy and interesting to email with. Prices are also good - or were when I bought. I got three pairs of 6.6K - sound nice and clean and clear. I had mine sent to UK and he sent them as samples so no tax. Useful. The OPTs are mentioned here: http://www.nutshellhifi.com/triode2.html

Email is HPurvine@aol.com
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Old 15th September 2005, 11:52 PM   #17
Johan Potgieter is offline Johan Potgieter  South Africa
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At the risk of sounding like an old goat (the former is unfortunately unavoidable, the latter hopefully not earned yet), I am surprised that even quality transformer companies do not list leakage reactance and equivalent winding capacitance. Primary inductance (defining the low frequency end) is faithfully quoted, but what about the high frequency end? Arguably more problems can lie there than at the bull fiddle end. It can usually be taken care of, but one would at least like to know what to expect.
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Old 16th September 2005, 01:11 AM   #18
gingertube is offline gingertube  Australia
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Nigel,
I'm in Adelaide and have a couple of prototype PPP Amp Monoblocks running with EL34s driving Plitron VDV2100 Toroidal Output Trannies. I run the EL34s in Triode Mode to give about 35 Watts total, zero global feedback.
The Toroids DON'T give that "traditional" 2nd harmonic rich, artificially strong bass, tube amp sound, instead they are VERY articulate, detailed and balanced, with lower lows (but SUBJECTIVELY weaker) and higher highs.

I've also run the same Monoblocks with Hammond 1650Ts. They DO give that traditional tube amp sound BUT lack some of the high frequency detail of the Toroids. They also sounded good with the tubes in Ultralinear Mode (80 to 90 Watts Out) and about 6dB of global feedback.

My theory (and it only a theory) of why the bass is different is that as the impedance of the Primary Inductance drops with frequency and approaches the effective rp of the tubes you get a fair ammount of 2nd harmonic ditortion generated by current distortion in the tranny. As frequency goes up so does the impedance of the primary inductance and this effect disappears. That is you get extra 2nd harmonic distortion at low frequencies ONLY. This is the "traditional" strong bass tube sound. Its great for guitar amps BUT can sound artificial in HiFi Amps. The fact that the Toroids have 10 times the primary inductance of the EI trannies shifts this effect off the bottom of the audio band.

The Toroids NEED for each tube to have individual bias adjustment (or a bias servo) to match push and pull idle currents to better than 1 mA. They don't like idle current imbalance.

The Toroids did not sound good in Ultralinear Mode - ONLY I suspect because the Zout was too high and I had a damping factor of no more than about 1.5. I did'nt get round to trying global feedback on them to fix the damping problem, just converted to triode mode instead. The bandwidth of the toroids would however make applying global feedback fairly easy compared with the Hammond (can get away with a higher dominant pole due to less phase shift in the output tranny at higher frequencies).

I bought both from Art at Evatco in Melbourne. I have since also got some VDV2100-CFB/H to try which are similar but with 30% Ultralinear Taps and 10% Cathode feedback taps. Have'nt had a chance to drop these in yet.

CAUTION: I ordered these direct from Plitron, by the time I paid transport costs and GST to the import agent they turned out to be very expensive.
I also bought the matching power transformers. Plitron have a significant price break at a quantity of 5 because they manufacture "on demand". It was actually cheaper for me to buy 6 power trannies than it was to buy 4.

Anyone wanting Plitron trannies, might I suggest you try to organise a group buy.

Cheers,
Ian
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