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Old 7th April 2009, 10:21 AM   #31
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But what about the self noise of such a huge resistor? Won't be amplified by first stage, strongly worsening SNR?
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Good for ppm, size and sonic fingerprint, but nV/Hz^0.5 isn't still related to nominal resistance and Johnson?

This is true, but for all the passband frequencies between the 20M resistor and the 47K, the Johnson noise-resistance is the parallel value of the two resistors. i.e.: it is 47K. You might get some large very-low frequency noise, but that could be lost by the output coupling capacitor.

I think this sounds worthy of some building... I have always avoided valve RIAA first-stage, not for the noise, but the avoidance of dc current (+ turn-ON transients) in the cartridge.

The Euro equivalents PCF806 and 30Fxx are very available too.

I'd be tempted to use transistor-cascoding to get the gain high in these..
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Old 10th April 2009, 05:49 PM   #32
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Assuming grid leak bias with 20 MOhm parts at both 6X8 section control grids, I make a 159 pF. cap. coupling the 47 KOhm cart. load resistor to the pentode as taking care of the 50 Hz. (3180 S.) pole of the RIAA curve.

I garbled things pretty thoroughly. RIAA playback EQ is a trio of LOW PASS poles. A 159 pF. coupling cap. between 47 KOhm cart. load and 20 MOhm grid leak resistor is a 50 Hz. high pass pole. NOT GOOD!

Lets start over. A 2122 Hz. (75 S.) low pass can be achieved by paralleling the 20 MOhm part with 3.75 pF. of capacitance. If the pentode 1st gain block is DC coupled to a ZVN0545A source follower, the same parallel cap. "trick" across the anode load resistor can be used to implement the 500 Hz. (318 S.) pole. I'm thinking that the high pass "corner" at the pentode's I/P should be about 14 Hz., which seems a reasonable compromise between infrasonic noise rolloff and in band phase shift.

Sufficient net gain from the preamp should not be problematic, even if the triode in the 2nd gain block has a modest mu. Just make the pentode's load resistor "tall" enough.
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Old 14th April 2009, 12:22 AM   #33
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I wonder what the gain is of the pentode section of the tube?
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Old 14th April 2009, 01:56 AM   #34
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Originally posted by michaelsamra
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I wonder what the gain is of the pentode section of the tube?

Mike,

The gm of the pentode is spec'd at 4.6 mA./V. Working into a 27 KOhm load resistor and ZVN0545A source follower, stage gain should easily be 40 dB.
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Old 14th April 2009, 09:49 PM   #35
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As for noise in the pentode section, we could always try triode connexion and then cascoding with a bipolar transistor. Triode connexion would lose "partition noise" found the pentode mode. In the tests I made on cascodes for my 300B driver circuit, even high-capacitance transistors (BU208 TV sweep part) sounded better than 6SN7s in the upper cascode position.

perhaps a bigger problem with cascodes or pentodes in RIAA modules is the increase in sensitivity to power supply noise.

The GE data sheet for the 6X8 does show characterisation for operating both sections at 50V, and maybe that could be a good solution - minimize noise by operating at high current, low voltage and run the whole thing off lead-acid batteries. A quick look at my favourite disti shows 10x 12V/2,2Ah are only 54 now (USD80, Euro60) - so why not? 120V at low impedance, no hum, no chokes, no induced fields, no reason for power supply noise. With the series-string 300mA versions of the common-cathode valves, you could run the heaters and 4 x 10mA anodes currents for 6+ hours betweeen charges.

http://www.rapidonline.com/Electrica...atteries/73355
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Old 15th April 2009, 02:10 AM   #36
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perhaps a bigger problem with cascodes or pentodes in RIAA modules is the increase in sensitivity to power supply noise.

Rod,

It's a given that the PSU will be well filtered and regulated. If you look over on AA, you'll find remarks about batteries being noisy.

Grid leak bias rates to yield less than 2 V. of potential differential between grid and cathode. I'm considering a 75 V. 0C2 for g2 B+, given that bias limitation.
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Old 31st January 2010, 03:03 AM   #37
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Default GE specs for 6X8

The GE specs describe the 6X8 as the equivalent of one 6j6 triode and a 6ag5 pentode. The 6ag5 specs show triode curves.

I am only a beginner, but the curves for both the 6j6 and 6ag5 triode strapped look pretty respectable. They both have a mu in the low 40s.

I made a preamp using the two triode sections of the 6X8s (and ignoring the pentode). It sounds quite good. Next I will try to make a preamp out of the two 6ag5s triode strapped. Given the common cathode, the solution seems to be use just one of the two sections of the tube.

I can't take it any further because I don't know how to use an oscilloscope, but I think that the tube is definitely not a plinker.

Al link to the GE specs is on the Duncanamps.com site here

TDSL Tube data [6X8]

The oldradio link is the one you want.
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Old 31st January 2010, 03:36 AM   #38
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Default errata

It's the Frank Phillipse site that has the GE spec sheet for the 6X. Sorry for the error.
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