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Theoretical question about interstage transformers
Theoretical question about interstage transformers
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Old 12th June 2018, 07:22 PM   #11
kodabmx is offline kodabmx  Canada
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I used to use Hammond 124B for driver/phase splitter and while I liked the sound of it, the scope revealed the HF transient response was terrible. 10kHz square looked like shark fins! I switched to cathodyne and the results are orders of magnitude better, but since music doesn't contain square wave (well, not often anyway) it's generally good enough. It's just all of the points that DF96 made and the added cost for a quality part that really drive a nail in the coffin for me. Still, the idea of making an amplifier that only used coils and tubes is pretty cool IMHO. Only for the "wow" factor though.

Always love reading those posts, GoatGuy. Very informative as usual.
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Old 12th June 2018, 07:32 PM   #12
Lingwendil is offline Lingwendil  United States
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Theoretical question about interstage transformers
+1

GoatGuy is my spirit animal. Him and PRR are my favorite posters, always well put and enjoyable reads, with great information and explanation.
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Old 12th June 2018, 07:56 PM   #13
waxx is offline waxx  Belgium
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I think it's a matter of taste, but anyway...

All thanks for the replies, i learned something extra, wich is always good
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Old 12th June 2018, 08:38 PM   #14
analog_sa is offline analog_sa  Europe
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Theoretical question about interstage transformers
Interstage transformers have lots of advantages when driving output tubes. It has been discussed in numerous threads. As for phase-splitters...i have failed to find an equally good sounding solution using active splitters.

As for voltage gain - i think it is a generally bad idea to use an interstage for this.
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Old 12th June 2018, 08:41 PM   #15
45 is offline 45  Italy
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One technical reason why IT's are mostly found with DHT's is that these are generally the most linear devices and work fine without feedback. Applying global feedback with two transformers in the loop can be quite challenging and sometimes impossible. Of course less linear devices could be used with local feedback and it always seems to be a waste for most people....not for me. I am just building such an amp!

There are indirect types (both triodes and triode-strapped pentodes/tetrodes) that show very good linearity but are only found in the low power range. The best low power types are very good drivers for sure (several are Russian types and are also cheap!) and they are often found in amplifiers. I am not aware of indirect type power triodes (more than 10-12 W plate dissipation) that are very linear except for the 6HS5 and its equivalents but this needs to work at least around 900-1000V anode voltage to get some useful power and the output transformer for it is a challenge. There might have been some medium-high power ones in the past but to me they are extinct. Maybe there are some made for non audio applications but don't seem to be common...

P.S.
I do not recommend interstage transformers with step-up, except for few cases of splitters like 1:1+1 designed for very specific applications. They usually bring more troubles than advantages. For me (moderate) step-up transformers have only place at the input of the amplifier (and driven by a suitable pre-amp or source).

Last edited by 45; 12th June 2018 at 08:53 PM.
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Old 12th June 2018, 09:00 PM   #16
45 is offline 45  Italy
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Quote:
Originally Posted by kodabmx View Post
I used to use Hammond 124B for driver/phase splitter and while I liked the sound of it, the scope revealed the HF transient response was terrible. 10kHz square looked like shark fins! I switched to cathodyne and the results are orders of magnitude better, but since music doesn't contain square wave (well, not often anyway) it's generally good enough. It's just all of the points that DF96 made and the added cost for a quality part that really drive a nail in the coffin for me.

Still, the idea of making an amplifier that only used coils and tubes is pretty cool IMHO. Only for the "wow" factor though.

Always love reading those posts, GoatGuy. Very informative as usual.
Do you really think that the Hammond is a reference to draw general conclusions? They are honest transformers, some type are also quite good, but not really a reference. Cost is subjective.
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Old 12th June 2018, 09:03 PM   #17
6J7 is offline 6J7
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I have LL1660 waiting to be used, is it possible to apply plate-to-grid (Schade) feedback between anode of output tube and anode of the driver tube?

Only interstage transformer in the loop, so it should be stable?
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Old 12th June 2018, 09:13 PM   #18
45 is offline 45  Italy
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I haven't tried that. You might need some compensation. I am using cathode feedback only with a special winding in the transformer.

Another solution for you might be: for the power stage you could just use the Schade fb between plate and grid of the power tube exactly as was presented in the original work. At the end of that article there is the 6L6 power stage driven by a SE driver + splitting transformer and could use the plate-to-grid fb also at the driver. The input stage might not be the usual one with tiny tubes and will have to work a bit harder but don't think is a problem if it has to swing moderate signal.

P.S.
Send me a pm with your specific application. We might see if you could use your IT in a more "creative" way.

Last edited by 45; 12th June 2018 at 09:21 PM.
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Old 12th June 2018, 10:19 PM   #19
Zoran is offline Zoran  Serbia
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Try to understand two types of interstage. one with magnetization current and other without. (Small amount of current with leakage curent of the C and grid curents of the tube are exist but it is very low...) Interstage transformer is not a driver, the part transforming source from prim to sec with square of transfer ratio, or square of ratio of turns (without losses...) In that transformation not the voltage or impedance going into transformation but capacitance too...

So with step up of say 1:4, the capacitance transforms from primaru 14 times. And that is causing potential damage at high end of BW. For good response at low end the inductance of primary should be high or matched with driver source. the internal resistance of tube. As more inductance we need (for the higher Ri tube driving TX, as much number of turns we have resulting in increase of capacitance at the primary and squared ratio multiplying to the secondary.

When we have magnetization currents trough the primary (without cancelation from the secondary or other extra layer for instance), the things are getting more complicated. So when You use interstage transformer, try to use 1:1 or slight step down, higher L(Hy) of primary, lower capacitance, lower leakage inductance, potent drive tube with lower Ri, and with as low of reactive load on secondary... So when ordering or considering interstsge transformers please note that is from vital importance to have more datas, like Lp(Hy), Ls, Rdc-primary, Rdc-secondary, Cp, Cs, Coupling factor, magnetic gap value and so. To determine the best driver tube for interstage. Unfortunately most of manufacturers are not giving that datas. Only transfer ratio is not enough. and My opp is that with proper interstage sound is much better And for Push pull spliting phase, interstage almost is a must, and good and proper interstage will perform better against any phase splitter.
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Old 12th June 2018, 11:33 PM   #20
kodabmx is offline kodabmx  Canada
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Quote:
Originally Posted by 45 View Post
Do you really think that the Hammond is a reference to draw general conclusions? They are honest transformers, some type are also quite good, but not really a reference. Cost is subjective.
I think it's a good general representation. Only a fool would pay 400+ dollars for an IT when a great tube based circuit would cost about 50 IMHO.

Also I should mention that I was running open loop.
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