Balanced Triodes for Phase Inverter.

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tekbowtekbow Frets: 1687

Does anyone here buy the concept of balanced triodes in an guitar amps phase inverter (class AB amp i guess)?

You see it a lot these days, with a fair upcharge in many cases. when i was having my amp serviced a while back i bought one for the PI just cuz. Tech didn't even use it, all he said mattered was plate length for heat dissipation because the position runs hotter, something called a Long Tail Pair was mentioned, and also that the position doesn't contribute anything tonally, it doesn't even matter if its particularly noisy.

So therefore I'm in a position of not accepting the claim of balanced triodes, unless anyone has other information that i should be aware of.

The tube he didn't use was a JJ ECC83s, which to be fair had a short plate by even short plate standards.

Apparently my amp eats Sovtek LPS, so I'm gona give a JJ ECC803 a try, with a 17mm plate.

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Comments

  • RiftAmpsRiftAmps Frets: 3113
    tFB Trader
    Only really beneficial if the PI is perfectly balanced (which in a gtr amp is most likely not). HiFi maybe...

    Merlin says on LTPs...

    "a slightly unbalanced phase inverter is often quite beneficial to guitar tone, due to the additional 2nd harmonic it introduces"

    Full article on AC coupled LTP design here - http://valvewizard.co.uk/acltp.html


    My own personal stance is that I don't bother with balanced triodes for a PI, just simply roll a few 'normals' in that position and use my ears to find the best sounding one.
    *I no longer offer replacement speaker baffles*
    Rift Amplification
    Handwired Guitar Amplifiers
    Brackley, Northamptonshire
    www.riftamps.co.uk

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  • jpfampsjpfamps Frets: 2723

    It's mainly marketing flapdoodle.

    Even if a perfect balance in the PI is desirable (and it's unlikely it is) then having a "balanced" valve in the PI almost certainly won't help, and this myth seems to be perpetrated by a) people who don't understand how the PI works and b) valve vendours who want to up charge you.

    I've not heard the larger plates work better in the PI, and have no evidence to support this based on failure rates of valves in the PI; they seem to be no more or less prone to failure than any other "pre-amp" valve.

    The idea that the PI valve is driven harder than the other valve is nonsense too.
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  • tekbowtekbow Frets: 1687
    fair enough, works for me. what about selecting for quiet operation? Does the PI contribute anything tonally to the amp?
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  • tekbowtekbow Frets: 1687

    @JPFamps

    Did you used to be on the D*A*M or Bareknuckle Forums? Name is familiar..

    When i say "used to be" you may well still be, i haven't posted on either of them in a few years.

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  • jpfampsjpfamps Frets: 2723
    tekbow said:

    @JPFamps

    Did you used to be on the D*A*M or Bareknuckle Forums? Name is familiar..

    When i say "used to be" you may well still be, i haven't posted on either of them in a few years.

    I was on the BK forum for a while. Can't remember the D*A*M forum (maybe the solder fumes are lacking effect?).

    I've always used the same name, which is the name of my business.
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  • ICBMICBM Frets: 71960
    tekbow said:

    all he said mattered was plate length for heat dissipation because the position runs hotter, something called a Long Tail Pair was mentioned, and also that the position doesn't contribute anything tonally, it doesn't even matter if its particularly noisy.
    Personally, I think that's all nonsense apart from the technical description of the circuit being a long-tailed pair.

    A longer plate doesn't really help with heat dissipation, phase inverter valves don't usually run particularly hot anyway, the position does contribute to the tone (particularly as in some amps it can be where the onset of power stage overdrive occurs), and noise can still be a problem - although less than in other positions, that is true.

    But the long-tailed pair is inherently mismatched, so using a matched or balanced valve does nothing useful.

    tekbow said:

    Apparently my amp eats Sovtek LPS
    That could be because the phase inverter has a significantly higher cathode voltage than a standard preamp valve, and most Russian 12AX7s have a problem with cathode-to-filament insulation - which is also why they shouldn't be used in cathode-followers, eg a vintage Marshall V2 position.

    I would just put the JJ you have in - although I have seen a few JJ failures in cathode-followers too, they don't seem to be affected as much in phase inverters, which are intermediate in terms of cathode voltage.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • tekbowtekbow Frets: 1687

    @jpfamps

    Yeah, it'll probably have been BKP.

    @ICBM

    Nice one, cheers for the info WRT to the LPS, How do chinese 12ax7's get along in those positions? i gave the 83S to a mate, but I've got a couple of spare chinese tubes sitting around.

    @Riftamps

    Cheers for the link, that explains the long tail pair well.

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  • ICBMICBM Frets: 71960
    The standard cheap Chinese 12AX7s are fine in cathode-followers and phase inverters, ironically!

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • tekbowtekbow Frets: 1687
    edited September 2018
    ICBM said:
    The standard cheap Chinese 12AX7s are fine in cathode-followers and phase inverters, ironically!
    Great, I'll ram one in there! It's one of those Watford Harma 250's at the moment(I'm pretty sure EHX), therefore russian, therefore sovtek.
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  • hywelghywelg Frets: 4302
    Dumble always fitted a trim pot to the ODS amps and in those it is important, but its function is to set the unbalance if the PI to correctly balance the output at the speaker with the aim of maximising harmonics, bloom and feedback control. 
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