Question as above. What is most important: high transconductance, low plate resistance, or something else? What about less important factors such as mu?
I may be wrong, but I read a post where Eli mentioned the 12AU7 as having the best characteristics for concertina duty. In some applications, maybe you could use other tubes, 12AT7's, 12AX7's etc.
My PP EL84 amp was designed with 12AX7's in the concertina, which I quickly removed in favour of 12AT7's that sound better to me.🙂
Jeff
My PP EL84 amp was designed with 12AX7's in the concertina, which I quickly removed in favour of 12AT7's that sound better to me.🙂
Jeff
kinda depends on whether it is just splitting or if it is also carrying out driver duties as well. Also what it is driving into...
The best concertinas have matched loads under ALL conditions. That means interposing something between them and any stage that will transition out of class A. The LTP that Wavebourn mentioned is a good one- also see the very clever Williamson circuit.
High gain, high transconductance, and low distortion, that's what you want. 12AU7 is not good in any of these categories. 12At7 is better but not ideal. Something like a triode-connected D3a or 7788 is near-ideal. A 6DJ8 is a good compromise practical choice.
High gain, high transconductance, and low distortion, that's what you want. 12AU7 is not good in any of these categories. 12At7 is better but not ideal. Something like a triode-connected D3a or 7788 is near-ideal. A 6DJ8 is a good compromise practical choice.
Jeff,
You misconstrue my remarks. I stated that cathode followers and "concertina" phase splitters were the only jobs the 12AU7 is suited to. As SY stated, there are any number of types better than the 'U7 in those roles. I abominate 'U7 section voltage amplifiers.
A "concertina" phase splitter is a tricked out cathode follower. As such, high gm rates (IMO) 1st. In spite of its μ of 22, the ECC99 is good for the job, thanks to high gm and low RP.
Right; in both cases common mode errors are minimal due to Concertina usage. Common mode errors cause syn-phase currents in output tubes that waste power. But in Williamson case 2 coupling caps are needed; in mine only one. It gives a possibility to stabilize working point using output from the long tail to the screen grid of the 1'st pentode; also my pentode LTP is convenient to drive linearized by parallel feedback by voltage output tubes that working as pentodes with low Cmiller have lower output resistance than triodes may have. Such a way wider frequency band may be obtained from output transformers, also phase intermodulations are minimal.
However, in my case Concertina + LTP form an APF, so big time constant is needed for stability on low frequency end.
An advantage of Williamson is possibility of better balance of amplification factors between 1'st tube and a driver, so choice of the 1'st tube and it's regime is less critical. Common triode may be used, while in my amp a very linear pentode stage is required.
Sorry Eli, I am the misconstruer. After two days of struggling to rid this computer of malware & trojans, my head is spinning, therefore I shouldn't be quoting anyone.
Is there any way of implementing CCS in the concertina of my HK A224, or is this just a waste of time?
Thanks,
Jeff
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Uhm.... Jeff, who made such a thingy with feedback from OPT to Concertina? 😱
It needs to be thoroughly discussed...
It needs to be thoroughly discussed...
CCS? You mean fold the top end of the concertina back to earth?
Who? I dunno, he said HK? Looks suspiciously similar to my Fisher x100...
Perhaps trying to minimize cap shifts in the loop?
Who? I dunno, he said HK? Looks suspiciously similar to my Fisher x100...
Perhaps trying to minimize cap shifts in the loop?
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Yes, even a complete idiot noob like me thought that was a bit odd.😀
The guilty party was someone at Harman Kardon.
Jeff
You lost me.😕
There's actually another resistor in series with the 47mmf feedback cap not shown in the schematic. The amp oscillates wildly without it.😀
Jeff
Pretty good considering. That's why I'm wondering if anyone has any ideas on ways to improve it.
The tone circuits are bypassed, coupling caps have been replaced, using GE Command 6679 for the concertina and RCA 5814A for the input gain.
Jeff
I have a followup question: what exactly happens on overload? Can a strategically placed buffer help?
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