I went with the ST-120 as I wanted the extra power if I decided on less efficient speakers. Highly recommended.
I have 89db speakers and the el34 vta st70 is loud, plenty loud. Waiting on delivery of zu audio speakers.
"A long tail phase inverter rather than a concertina type like the Dynas."
'Long tail' layout- interesting- always learning from this site😎
Your opinion,
Is 'long tail' design better than single splitter? If so, then how so-
Jim
'Long tail' layout- interesting- always learning from this site😎
Your opinion,
Is 'long tail' design better than single splitter? If so, then how so-
Jim
Both can work well enough, but they have different advantages.
Long tail: large voltage swing, has voltage gain, nfb input node available.
Concertina: close output balance, uses only one tube section, only one input node.
Put another way, they have different disadvantages.
Long tail: inherent unbalance, needs large cathode resistor and negative supply (or current source) to work well,
needs extra coupling capacitor in some circuits.
Concertina: elevated cathode, unity gain at best, limited output voltage swing, outputs must be equally loaded.
Long tail: large voltage swing, has voltage gain, nfb input node available.
Concertina: close output balance, uses only one tube section, only one input node.
Put another way, they have different disadvantages.
Long tail: inherent unbalance, needs large cathode resistor and negative supply (or current source) to work well,
needs extra coupling capacitor in some circuits.
Concertina: elevated cathode, unity gain at best, limited output voltage swing, outputs must be equally loaded.
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I just finished Bob's 120 a few days ago. Love it. Running kt120 tubes, gz33 and Siemens chrome plate in middle.
I would suggest the ST120 over the ST70. KT88s are great tubes
There is also an argument for trying a pair of Bob's MarkIII kits.
I built a pair of Bob's M125's and to be honest I don't know how one is supposed to use that much power. The sound however is fantastic....
There is also an argument for trying a pair of Bob's MarkIII kits.
I built a pair of Bob's M125's and to be honest I don't know how one is supposed to use that much power. The sound however is fantastic....
A pair of big Magnepan speakers could easily "eat" that and yet more power. "Maggies" are low sensitivity, but exhibit a good for tubes "flat" 4 Ω impedance curve.
Thanks all! I picked Bob’s ST-70. I’ll just have to wait till my next deployment ends to build it.
A cathode coupled phase splitter with a true very high impedance current sink,
and perfectly matched plate loads (matched RL and Rg, versus RL and Rg of the push and pull stages) . . .
Is inherently balanced, etc.
Real world tubes, and it is inherently balanced.
It is resistive LTP that gives cathode coupled phase splitters the name un-balanced (not a high enough current sink impedance).
What balance do you need, 1%, 0.1%, then design it that way.
and perfectly matched plate loads (matched RL and Rg, versus RL and Rg of the push and pull stages) . . .
Is inherently balanced, etc.
Real world tubes, and it is inherently balanced.
It is resistive LTP that gives cathode coupled phase splitters the name un-balanced (not a high enough current sink impedance).
What balance do you need, 1%, 0.1%, then design it that way.