Any ideas for a low gain triode (preferably IDHT). Ballpark gain around 6.
The lower the Rp, the better. Hopefully 6.3V heaters. No particular pinout. Dual or single triode is fine.
Used as differential input stage driving IT to differential 45/2A3/46 types.
Readily available please, no $25 tubes. Thanks !!
The lower the Rp, the better. Hopefully 6.3V heaters. No particular pinout. Dual or single triode is fine.
Used as differential input stage driving IT to differential 45/2A3/46 types.
Readily available please, no $25 tubes. Thanks !!
Use local feedback in the form of cathode resistors on your LTP to lower gain, use CCS (or long tail resistor) to maintain good cmrr in your LTP. Will raise rp of course, but assuming another stage following.
Very low gain tubes such as you are looking for are uncommon. The types I am familiar with like the 6C5 is in the ball park without additional degeneration in an LTP, others to consider might be 2C50 (dual triode) or similar..
Very low gain tubes such as you are looking for are uncommon. The types I am familiar with like the 6C5 is in the ball park without additional degeneration in an LTP, others to consider might be 2C50 (dual triode) or similar..
Cathode resistors increase plate resistance, quite opposite
the effect our friend was trying to achieve. But there is yet
another way to skin that cat (no live cats were harmed) ...
Local feedback from Plate to Grid will lower Plate resistance
and Mu. Read O.H.Schade (RCA 1938), search the forum for
a link to that chapter "Beam Power Tubes".
Mind you, Schade was making a fake triode out of a 6L6 by
external means, other than by obviously screen strapping.
For reason he wanted screen voltage as high as possible.
No reason his same trick can't be abused to make a high or
medium Mu triode into a lower one (with effectively lower
plate resistance too). No matter there is no screen.
Another 6n6p differential stage, but with plate feedback in
circuit, will behave quite differently.
the effect our friend was trying to achieve. But there is yet
another way to skin that cat (no live cats were harmed) ...
Local feedback from Plate to Grid will lower Plate resistance
and Mu. Read O.H.Schade (RCA 1938), search the forum for
a link to that chapter "Beam Power Tubes".
Mind you, Schade was making a fake triode out of a 6L6 by
external means, other than by obviously screen strapping.
For reason he wanted screen voltage as high as possible.
No reason his same trick can't be abused to make a high or
medium Mu triode into a lower one (with effectively lower
plate resistance too). No matter there is no screen.
Another 6n6p differential stage, but with plate feedback in
circuit, will behave quite differently.
hemgjord said:
Aha ! Now we're talking. Sad thing is, I know about that tube; should have thought of it myself.
While the other suggestions were good, I'm really not leaning toward making any design changes (other than tube choice and op point). The 6BX7 has its merits, but I haven't heard good things about finding matched triodes in the same bottle. Having individual triodes with the 12B4 is nice to hand select matches.
I understand these tubes are popular for linestages, so I'll do some searching to get perspective as to their linearity and sound quality.
Any other possibilities near the 12B4 ?
Remember in the case of the typical LTP your gain will be half that of a single triode in common cathode connection (the cathode of the driven section is seeing its internal resistance plus that of the other half of the LTP instead of a low impedance bypass if used, i.e 2X the internal cathode impedance alone) so your gain single ended would be no more than 3 (~10dB) and your differential gain 6 (~16dB). If this is ok the 12B4 may serve just fine..
Good point.
To make things easier, I've attached a tentative schematic. Thanks to Andy Evans for recommending the 46; going over the top for my abilities. Hopefully I can keep hum to a minimum. Still shying away from a DHT input tube, for a number of reasons, mostly an anal limitation of my design (wanting to avoid harmonics entirely in the heater circuits).
Anyway, with this layout I see a maximum possible output of 35.6W with ideal gains assumed. I want to maximize the utilization of my linestage, which is (for good reason) fixed at a maximum of 1.73V output. Hence my desire for a lower gain input stage. I formerly was planning on the 7119, mu of 24, but my input sensitivity was a little hot at 0.55V.
In fact, if there are any pin compatible tubes with the 12B4, I could play around with slightly higher mu's to get just the right playability.
To make things easier, I've attached a tentative schematic. Thanks to Andy Evans for recommending the 46; going over the top for my abilities. Hopefully I can keep hum to a minimum. Still shying away from a DHT input tube, for a number of reasons, mostly an anal limitation of my design (wanting to avoid harmonics entirely in the heater circuits).
Anyway, with this layout I see a maximum possible output of 35.6W with ideal gains assumed. I want to maximize the utilization of my linestage, which is (for good reason) fixed at a maximum of 1.73V output. Hence my desire for a lower gain input stage. I formerly was planning on the 7119, mu of 24, but my input sensitivity was a little hot at 0.55V.
In fact, if there are any pin compatible tubes with the 12B4, I could play around with slightly higher mu's to get just the right playability.
Then use JJ ECC99 same spec's linear frequentie spectrum and also super sound like 6n6p. I use also 6n6P( in shanling CD-T100se) and ECC99( in SE 813 amp) behind each other.zigzagflux said:
About the schematic just use one driver tube like 6n6p or ECC99.This works very good. My filisofie is " Less is more".
JoshK said:
It IS the Karna, with a few mods. I am hoping the negative rails w/ CCS is an improvement over resistor biasing. I originally was planning on the 7119, per his suggestion, but again, the gain is a little much for my source of 1.73 Vrms. So, I've pursued the lower mu devices.
Bandersnatch said:
That's my assumption; I basically calculate each stage's gain with the full mu of the tube, as all inputs and outputs are taken differentially. Net result is that I need a input stage with mu=6.5 to have 1.73 Vrms input produce 11.9 V output, equivalent to 35.6W (about the maximum). 12B4 is right there. Little more gain could be beneficial for lower level recordings.
For instance, Neil Young's Harvest seems to be a pretty 'quiet' recording, little more gain might help. But not as much as the 7044/7119/ECC99 types provide - those are just a little hot at a 0.55V input sensitivity.
mu anywhere from 6 to 10 should be really nice, aiming for low Rp.
Anyone NOT like the 12B4 sonics/linearity ? Most of the threads I've read rave about this tube, typically used in linestages.
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