The 6C4, 7A4, and 6J5 are approximately identical electrically (They are both about 1/2 a 6SN7), but they each need their own different socket of course.
Soft start kit alternate tubes
- Thread starter johnsonad
- Start date
Paul Joppa
Moderator
6C4 is half a 12AU7. There are many other more-or-less suitable tubes. I recommend browsing the GE "Essential Characteristics" handbook looking for a suitable mu and plate resistance. It's as amusing, and as fruitful, as Facebook! :^P
Paul Joppa
Moderator
As described in my Dec. 25 post.johnsonad said:
What's the Kreg voltage coming off the adjustable side?
It may be that PJ had the bigger pot in mind to allow more signal voltage into the amplifier.
It may be that PJ had the bigger pot in mind to allow more signal voltage into the amplifier.
Paul Joppa
Moderator
According to the published curves, a 10K pot will be very close to the extreme end of its adjustment. I think it makes better sense to have the pot in the middle of its range, to accommodate variations among tubes, hence the 20K suggestion.
I have been cobbling adaptors together to try different driver tubes in the 300b paramount with soft start. A CV181 runs nicely at the preset mA current values with IMO unbeatable midrange but they are pricey. The 2c51has better soundstage and dynamics. Makes sense as it was optimized for the circuit values. I did bolt on a bigger heatsink and changed R1 to get about 12 mA's on the first C4S(increased by 2 mA) and changed R1 on the driver tube C4S to add the 2 mA's thru the driver( about 5.4 mA vs 3.4). I tried the CV181's again with same great midrange but now drive and soundstage like the 2C51. I tried a 6J5 as driver using one triode of the 2c51 as VR tube. Nice sound but I think I need more current thru the 6J5 maybe 6-7.5 Ma. My questions are since .6 Ma's are being shunted thru the 300k driver plate to ground resistor, can I remove this in the cap coupled 300B to get more current thru the driver and is the 300k resistor there for the soft start operation to shunt current before the driver conducts? I would also like to try PB's suggestion of running both triodes of a 6J6 as a VR tube using 4 mA's of current rather than 4.7 of the 2C51. I am thinking I would hook both plates up to 300v B+, grids to ground with 220ohm cc resistors and since there is one common cathode, hook that up as before. Is this reasonable or should I further my education in tube electronics before trying this at home? Thanks, Paul S
The 300K resistor isn't super helpful in the context of the 300B circuit. If you're going to remove that, you could also remove the 33K and 10,000uF capacitor to roll back to a more V1.0 style Paramount driver setup.
As far as currents go, your rationale seems about right. You may or may not need both halves of the 6J6.
As far as currents go, your rationale seems about right. You may or may not need both halves of the 6J6.
Paul Joppa
Moderator
The 33K/10000uF is the soft start, and eliminates an audible thump even with cap coupling.
The soft start almost completely eliminates the arcing that occurs with a (very few) 2A3s, but there remains a very small "tic" that is audible from the speaker. The 300K eliminates that. To the best of my recollection, there is no such tic when using the cap-coupled 300B circuit.
The soft start almost completely eliminates the arcing that occurs with a (very few) 2A3s, but there remains a very small "tic" that is audible from the speaker. The 300K eliminates that. To the best of my recollection, there is no such tic when using the cap-coupled 300B circuit.
I haven't heard the tic in my V1.0 Paramounts, just what I presume is the TL431 turning on. (Kind of a funny chirp)
Paul Joppa
Moderator
The tic - and the arcing - only happen with a few specific tubes.
johnsonad
New member
Would it be possible/ideal to use the soft start kit with a 45 as a driver tube in a (my oddball) Paramount circuit? Filaments and shunt reg side aside, could it work?
I'm going to change the OP of this amp to 350 P-k, -74v, 60mA, 3k from the current 350 p-k, -76v, 50mA, 5k. This may be enough alone to improve the sound (they are not driving the 755a's as well as I would like) but a full DHT amp would be pretty cool!
Thoughts??
I'm going to change the OP of this amp to 350 P-k, -74v, 60mA, 3k from the current 350 p-k, -76v, 50mA, 5k. This may be enough alone to improve the sound (they are not driving the 755a's as well as I would like) but a full DHT amp would be pretty cool!
Thoughts??
The actual question is whether it would be possible to use the 45 as a driver tube in the Paramount, which it would, but you would need a very unusual (unusual means hard to find and very expensive) interstage transformer to make this possible.johnsonad said:
This is a bit of a separate question from the driver tube question, are you running 2A3's or 300B's?johnsonad said:
johnsonad
New member
Thanks for the reply PB. As to the OP, that is for a 300B. As to the 45 as a driver, as far as an IT goes, something like a Tango NC-20/F? I'm just throwing ideas around. I had considered something like the 801A too after looking at a few of Thomas Mayer's builds but Dan had mentioned the 45 as something to consider.
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