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Tube Amplifier Design and Rolling Tubes

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@Ruhled , I agree with much of what you wrote .We need to measure and look at the facts. There is no magic in tubes but they can still sound very nice even with the flaws.

This green plot is from my single ended class A Tube amp converted to headphone amp, triode strapped 6550/KT88..kathode bias. Sounds absolutely transparent and with no hum nor noise , The measurements tell a different story .
I just tested and measured 10 pairs of input tubes 6SN7 and 10 pairs of output tubes (6550/KT88/90/120): the channel balance varied between 0-1db , output levels 0-0.5db , and 2H distortion 0.05 to 0.35% ..all at 2.3V output. I do not need more as a headphone amp. I Optimized its output with a voltage divider for my headphones. At speaker levels things change of course..

The Blue plot in same amp at Ultralinear mode, quite horrible looking, but does not sound at all as it looks . But the engineer in me won’t allow me to listen to that.


Maybe time for a “Fun with tube measurements thread”? This will be my contribution, I sold my other tube amps to go for a class A transistor with perfect measurements, but not really cost effective…

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Ps. The set up is a horrible waste of money and energy of course…And any tube amp should have A balance control, or in the path somewhere, if it is not the tubes the rest of the construction especially the volume potmeter can cause imbalance
 
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First off, I hope everyone is having a great day.

I designed and posted this amplifier schematic over at AudioKarma and my first reply was it is a total waste of time and money, just set it up for the tube you want and be done with it. Also, the real knowledgeable people did not get involved. Below is what I wrote over there after I was told this. I am posting it here for a more scientific review

Thank you for the replies. A little background, and my case in theory.

This amp was designed using all Toroidal Transformers, thus the lower cost.

This all started as a training exercise for me and a challenge to myself to design an input stage that could actually roll the input tubes. As I have no one to sit down with and teach me and a friend who is willing to look things over by eyeballing it.

I was already looking at using new issue Tung-Sol 7581 output tubes and read a few places that they are real nice to 85% and 30 watts plate dissipation. I went with 84%. That puts the KT-88/6550 right at 70% plate dissipation using a 6.6K output transformer.

Then I thought, I could do the phase inverter stage like the input stage and ran scenarios for that. The results were just as good as the input stage. Russian tubes were used as an example for using shields and represent a higher amperage draw, 6.3V, 12AX7 input and 6CG7 phase inverter tubes. The 6N6P also has a higher 6.3V amperage draw and is supposed to be a superior sounding tube and, what if it's not or I want to compare tube sound, I can swap it out to a 6SN7. 6CG7 or 12AU7 with the same amplitude with minor adjustments. The same with the input stage and the 6N2P-EV could probably be changed to a 5751 tube with just a voltage change and maybe it would need a couple minor adjustments or any of the other tubes listed on the schematic for that matter.

It was never about the output circuit, that was just a quaint circumstance. It is about the rest of the amp and how one could find a great sounding, low distortion amplifier by rolling tubes. As for sockets wearing out, use socket savers. As for the output stage, set it up how you want.

All this stuff is a waste of money unless you are loaded so you must look at this as an investment in the future in some way. Adding some switches, trimpots and tube sockets to get something more superior is a minor expense in my opinion. Making it with long lasting parts and love is priceless.

I have changed the schematic to 5751 input tubes and 6550A output tubes with 5k 100 watt 40% UL tapped transformers to start with. I left room on in the chassis so Dynaco A431s transformers can be added at a latter date.

If anyone has the time to look this over, thank you. I will be looking forward to replies.

Regards, Tom
Listen to advice from SIY on circuit design.

Regarding the schematics posted - suggestions about "choices".
Power transformer secondaries - all center tapped secondaries. This allows 2 rectifiers for full-wave and proper voltage balancing if you must stack capacitors for sufficient voltage handling. For filaments the center tap allows for easy DC voltage offsets on filaments.
Plate resistor trim pots are a bad idea and should be replaced with fixed resistors. If you must adjust the operating points of the pre-amp tubes consider a ten turn wire wound pot for the cathode bias resistor or a precision adjustable negative bias supply for the pre-amp tubes. Another choice would be fixed plate and cathode resistors and an adjustable regulated B+ for the pre-amp sections to be changed when using different model tubes. Your conundrum is you may not be truly optimized for different model tubes unless you design for it with circuit topology and the right tube choice.
A fun exercise to be sure.
 
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