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Old 18th June 2008, 05:50 PM   #1
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Default Low gain triode recommendation plz

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 !!
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Old 18th June 2008, 06:11 PM   #2
kevinkr is offline kevinkr  United States
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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..
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Old 18th June 2008, 07:08 PM   #3
Koifarm is offline Koifarm  Netherlands
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Use 6n6p in srpp config. Gain = 11 x
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Old 18th June 2008, 07:28 PM   #4
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6n6p is used in my linestage. Lovely tube. Good suggestion.

However, I would prefer to avoid duplicate tubes in successive stages. Just personal preference; I'm sure it wouldn't hurt.

Thanks, keep it coming !
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Old 18th June 2008, 08:23 PM   #5
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The 6BX7 pretty much matches your criteria (a little high on the gain: u= 10). It's also quite linear as well. And they're under $25.00.
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Old 18th June 2008, 08:38 PM   #6
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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.
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Old 18th June 2008, 08:49 PM   #7
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12B4 - single idh triode:
rp~1k, mu~6
Heater with ct to accomodate both 6,3 and 12,6V heater

/Olof
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Old 18th June 2008, 10:06 PM   #8
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Quote:
Originally posted by hemgjord
12B4
rp~1k, mu~6
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 ?
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Old 18th June 2008, 10:33 PM   #9
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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..
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Old 19th June 2008, 01:13 AM   #10
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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.
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