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1964 DR: Bigger filter caps or not? A trade-off of sorts...

Discussion in 'Amp Tech Center' started by Sea Devil, Mar 23, 2021.

  1. Sea Devil

    Sea Devil Friend of Leo's

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    I'm really just thinking aloud here... I currently have the standard 16 uf filter caps, and I'm running a 5V4 as a rectifier (I have a bunch of old US-made 5V4s), so the rectifier's already seeing 2uf if over nominal spec, which seems fine.

    I have new 22uf caps (for the first two positions) and I can't decide whether to install them or not. I know I'll have to stop using the 5V4s if I do.

    I've reduced operating noise to the level of a mouse fart, but it's a slightly louder mouse fart with these caps and the standard 5AR4/GZ34 rectifier than it is with any of the 5V4s. (Plate and bias voltages seem to "travel with" each other, and I can swap between the two rectifier types without rebiasing.)

    Am I likely to experience the same level of noise (again, totally within acceptable limits) with the GZ34/2 X 22uf combo as with the 5V4/2 X 16uf set-up?

    I'm aware that attack, articulation, and bass response will likely undergo subtle changes too, but I doubt I'll care one way or the other. Quiet operation, particularly at lower volumes, is a higher priority (at stage volume, the proportional level of noise is basically non-existent).

    I have a more-than-a-lifetime supply of nice old 5V4s with nowhere else to call home, and only modern JJ 5AR4/GZ34s. That's a factor too...

    EDITED!! I originally wrote 5R4 by accident. Fixed!
     
    Last edited: Mar 23, 2021
  2. schmee

    schmee Doctor of Teleocity

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    The 22uf are often used in those type amps. I've had both. But I never use a 5R4. I use 5V4's a lot though. If you like the sound of the amp with that weak rectifier, you may want to just use it as is.
     
  3. Sea Devil

    Sea Devil Friend of Leo's

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    B6D20F24-9B3C-4BDD-8D51-CD35D9F05858.jpeg

    Oops... inexcusable lapse!

    They are 5V4s. That's about a 35V difference! Sorry 'bout that!

    This one's a 1955, IIRC.
     
    Last edited: Mar 23, 2021
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  4. JSMac

    JSMac Tele-Holic

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    I've been wondering about this. When I worked on old audio amps we typically increased the filter cap values as a matter of course. Might be different in a guitar amp though. I went from 16uf to 33/22/22uf in a 5E3 and wonder if that was a mistake.
     
  5. Jon Snell

    Jon Snell Tele-Holic

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    A GZ34/5AR4 is happy up to 60uF as the first tank capacitor, so 32uF is ideal, 47uF is also ideal.
    The 5R4 however is unhappy at more than 4uF. Be careful if that is what you are using.
    The 5V4 is a little better but may get damaged with a tank capacitor exceeding 8uF.
     
  6. Sea Devil

    Sea Devil Friend of Leo's

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    Fixed my error... 5V4, so not much more voltage drop than standard, and spec'd to 30 uf in the first filter stage.
     
  7. Sea Devil

    Sea Devil Friend of Leo's

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    Also thinking of going to a 100uf/100V cap (common but misguided, I know; only the voltage increase is helpful, and over 68uf is pointless) in the bias section, again good only with the 5AR4 due to its slow warm-up.
    I guess I could just replace all three caps and try out the 5AR4, and put everything back if I don't like it. It wouldn't take long either way.
     
  8. schmee

    schmee Doctor of Teleocity

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    I tried 20uf to 44 uf in hot rod Princeton 6L6 experimental builds. It seemed to make very little difference. 20/20/20/20 then 33/33/33/20, then 44/44/44/20
     
  9. Sea Devil

    Sea Devil Friend of Leo's

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    I just chucked that JJ GZ34 in there and turned it up a little more than usual practice level.

    It sounded ridiculously perfect. I have no reason to open the damn thing. Everything is spec for the circuit, albeit with wall voltage 3V higher, and all is right with the world.

    I hadn't tried the GZ34/5AR4 since the final round of out-of-spec resistor replacements (only four or five swapped at that stage, IIRC). I guess I owed it to Leo to bring everything as close to spec as possible and listen to the circuit the way it's supposed to be. Sounds pretty %$#@ good to me!
     
    Last edited: Mar 23, 2021
  10. timfred

    timfred Tele-Meister

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    Has anybody actually fried a 5R4 or 5V4 in a DR? I’ve been using a 5R4GYB (20uF rated) for years without issue. My B+ is a tad under schematic, sounds great, plus they are dirt cheap.
     
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  11. Nickfl

    Nickfl Friend of Leo's

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    People often fail to understand that the capacitance spec of rectifier datasheets is not a max value, but a typical value. This isn't to say that an excessive reservoir cap value won't damage the tube, it will, but its not as simple as that variable alone being a strict limit.

    Also, older datasheets list lower typical values because larger capacitance caps were not commonly used then, later datasheets sometimes show higher typical vales for identical tubes because as the technology improved larger caps became more cost effective and therefore more common, and these are "typical" values.
     
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  12. Sea Devil

    Sea Devil Friend of Leo's

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    In spite of my total satisfaction with the sound of the amp with the stock filter cap values, I tried out the two 22uf caps. My first impression was that there was actually less bass than before, but a closer listen revealed that this was not the case.

    It's never been clear to me just what "tighter bass" meant in the many descriptions of the effect of increased capacitance in the first stage of the filter circuit. We all know what that means when it comes to speakers: no farting out, greater clarity, etc. I hear something very different here, though.

    There isn't an overall reduction in bass. In fact, it sounds as though the range of the bass pot has shifted slightly downward and is more widely dispersed across the frequency spectrum -- a softer-shouldered peak, with a lower Q value, if you will. A little more of a hi-fi, EV RE-27 mic classical-radio-announcer velvety lushness rather than the boisterous deep honk produced by some low, single-note Duane Eddy runs with the lower filter values.


    What about sag? I didn't notice any change at low volume. The sound feels slightly more "detailed," but with no noticeable alteration in dynamic range or envelope characteristics, other than the oddly frequency-dependent salience of certain passages with the smaller caps; frequencies from about 96 Hz to maybe 120-160 Hz seemed more pronounced with the 16ufs.

    Noise was definitely, quantifiably lower. The very, very low 120V hum that seemed to be emanating from the PT or OT itself/themselves rather than the speaker seems to be totally gone. (At one point that had me worried, but I measured tranny voltages and the amp's fine, especially after some key resistor swaps. Several devices on the one and only circuit in my apartment make some variation on this noise.)

    I like the lower noise better, although it was already reduced far below what I once thought possible. I think I like the other minor changes too, but I've liked everything this amp has ever done, regardless of what tubes and speakers it's had in it, even when it was far off spec, other than making unwanted noise. From "perfect" to "more perfect"? Perhaps.
     
    Last edited: Mar 25, 2021
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