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pi/phase inverter?? v1?? help!......

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mcdes, Jan 29, 2008.

  1. mcdes

    mcdes Member of no importance

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    pretty much as the title suggest, i have a vox ac30 heritage head and am replacing the tubes and hear all about these two terms.

    i want to make sure i have the right/best valve in the right position, i know a lot of it is move around till it sounds good, but, still, it will be handy to know.:confused:

    is the phase inverter and v1 the one and the same?

    also, how do i know which valve socket in amp is v1 or pi for that matter?


    thanks in advance
     
  2. hamfist

    hamfist Member

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    When you take the chassis out you will find an ef86 at one end, then three 12ax7's. The power tubes and rectifier are on completely the other side of the chassis.
    V1 refers to the first tube that the input signal reaches once it reaches the amp and is standardly the nearest tube to the guitar input jack. The Heritage Ac30 almost has two V1's, as the ef86 is V1 for the EF86 channel, but the 12ax7 next to it is the V1 for the top boost channel.
    My understanding is that the phase inverter basically amplifies and distributes the signal to the power tubes. I have certainly seen the PI being actually considered as part of the power amp. The PI will therefore be the last 12ax7 before the power tubes (the one nearest to the power tubes in most amps).
    Interestingly Vox chose to use chinese 12ax7's in V1 and V2 of the top boost channel, but a Tung Sol for the PI. Usually putting a quality tube in V1 has the biggest sonic effect, followed by the PI.
    As I've stated in other threads, I found that a NOS EF86 in my AC30HH made a noticeable tonal improvement in that channel. I really liked it before, but I love it now !!
    However, I stuck a known good Mullard 12ax7 in V1 of the TB channel and I can't really hear much difference. I've got some more playing around to do with 12ax7's though. I haven't yet tried playing with the PI yet, but will at some point.
    Here's a shot of the preamp side of the AC30HH, tubes from left to right are EF86, 12ax7 (V1 of TB channel), 12ax7 (V2 of TB channel), 12ax7 (PI).
    [​IMG]
     
  3. hamfist

    hamfist Member

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    woah, sorry about the enormous picture !!
     
  4. mcdes

    mcdes Member of no importance

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    well hi again mr hamfist!

    pic is perfect, the answer given is exactly what i needed, cheers bud.

    i tried the nos tesla ef86 which was real nice, but it is kinda mid range so have got a nos (60'S?) amperex ef86, ill give that a wirl and see if it can sound even better than it already sounds.

    i have mullard re-issue el84's and the 12ax7's i have used all tungsol reissues to match the one in pi position.

    still cant get over the ef86 channel, but so far, as far as top boost is concerned, maybe more clarity but, not much difference, as you say.
     
  5. mcdes

    mcdes Member of no importance

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    oh, hamfist:

    have you changed the rectifier tube? and to what? also did it make a difference?
     
  6. hamfist

    hamfist Member

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    In fact I just got a NOS Japanese GZ34 in the mail today. I have stuck it in the amp, but only just had time to check that the amp still works with the new rectifier (which it does !). I've got a band practice this evening, so I'll give it a whirl then.
    To be honest, I don't expect to notice any difference in the amp at all. It's just that all the currently produced GZ34's seem to be renown for only lasting months (if you're lucky) in an AC30. When they go, they take out a fuse or two, and (if you're unlucky) possibly one of your transformers (I forget which).
    So I've gone NOS on the rectifier purely for longevity and reliability purposes.
    It's my understanding that the Japanese GZ34's are still a very good quality tube, but nowhere near the silly money you will pay for a Mullard.
     
  7. hamfist

    hamfist Member

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    Just got back from band practice. The new NOS Jap rectifier worked fine. As I suspected, I didn't notice a difference from the Sovtek, but I hope it'll last way longer !
     
  8. mcdes

    mcdes Member of no importance

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    awesome, thats handy to know! would hate to blow transformer!

    i dont suppose you'd know? but i'll ask anyway...... can i swap a 12ax7 tube for a 12au7 tube? wont damage anything? will it work at all?
     
  9. hamfist

    hamfist Member

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    Should be completely safe. the 12au7 has a much lower gain than a 12ax7, so if it's in the pre-amp, you will have an overall quieter amp, with possibly greater clean headroom (depending on the circuit).
    I have read of people putting a 12au7 in V2 of the TB channel of AC30's with some success for increasing clean headroom. Is that your plan ?
     
  10. mcdes

    mcdes Member of no importance

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    yea, been thinking about it, i got a spare nos 60's tungsol 12au7 ive been thinking of trying.
     
  11. hamfist

    hamfist Member

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    Hey man, you need to change the title of this thread to "the Mcdes and Hamfist thread" !!
    I'm not sure anyone else is listening.
     
  12. mcdes

    mcdes Member of no importance

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    ha, must be quite low on the love for the vox heritage! dont know why! ha

    anyway, i tried 12au7. didnt like it, was real dull. even tried dampers on the tubes after that and it went kinda sterile too. so, no dampers, all 12ax7's.

    changed the nos 70's ef86 for a nos 60's amperex ef86, huge difference! just bigger sound with more clarity, and warmer. bit like changing from stock speakers!

    oh well. ill keep experimenting, wouldnt mind trying some genalex gold lion tubes, maybe one.... they cost alot, ha. gotta sneak it past the wife first, ha!
     
  13. hamfist

    hamfist Member

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    Hmmm, I thought about tube dampers, espec on the EF86, but decided that they would push on the wiring and circuit board, as it is all very close to the tubes. I felt the risk of disturbing a contact was greater than the perceived benefits of the dampers.
    Did you find yours had enough space to fit the dampers in there ?
     
  14. mcdes

    mcdes Member of no importance

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    mine had enough space but was getting close on the ef86 to the wires, it was touching, but i thought it might be better as it would keep the wires away from a hot tube and not melt through. they dont touch without the dampers.

    im kinda keen to rip off the heatshrink on the stock ef86 and see what tube they used, have you had a peek yet? or keep original! ha
     
  15. Groovey Records

    Groovey Records Member

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    As far as EF86 goes I've used the current Electro Harmonix twice, a current Red Wing and Just Yesterday a NOS Dario Mini Watt in my TopHat Supreme 16. The S16 is based on the early Vox AC15 Circuit.

    To my ear all have been Microphonic to some degree. A good friend has recomended the RFT EF 86 and I will try those next. He also said he didn't use a dampener but if he did he would use a Halo. Thinks the effect of the tube could be diminished by dampening. Said to use your own ears and decide.

    I'd also like to find some NOS Mullard EF86's since I have NOS Mullard EL 84's for power and 12 ax7's in the pre

    To my ear all the EF86's l have tried so far have been unstable and microphonic with the ring a ding ping when driven hard at 2 or 3 pm on the AC 15 circuit. Thats where the tonr lives and breathes Is this something I have to live with ?

    So the PI in this circuit whats it for? If there's no reverb or tremlo what's goin on.

    Hamfist since I can't find stable EF 86's why would I want to put a second one into the circuit? What is it doing for you ?


    Oh the Mystery of it all

    Regards
    Groovey Records
    PS what is under that shrink wrap ?

    Listening to Johnny Winter-Second Winter
    one and a half record set on Vinyl-very rare
     
  16. hamfist

    hamfist Member

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    McDes, I'm not going to cut the heatshrink off the EF86, as when I sell it as a NOS tube in 30 years time, I'll get loads more money for it !! I have read somewhere that it is a Sovtek, BTW, but am not really sure.

    The PI is involved in splitting the signal and feeding the power tubes. This really is stretching the limit of my understanding here. I know that a PI never has anything specific to do with a tremolo or a reverb.

    And I didn't realise that I was suggesting putting a second EF86 in the circuit. I certainly didn't mean to. I think one is probably enough.
     
  17. mcdes

    mcdes Member of no importance

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    ha ha ha, i cant bring myself to take heat shrink off either!, ha

    WELCOME, to the discussion mr groovy record!

    alot of good info with your input. im pretty much of the mind at the moment that, if im only going to use the ef86 channel, any other tube relating to top boost doesnt get alot of thought and attention. so if the p.i. isn't doing much, im not worried! ha

    groovy record, bummer your getting microphonics with the ef86! mine seems fine. the new amperex ef86 is really doin it for me, but id still try others, the search continues.......

    hamfist. do you have a slight low hum with your amp? when i listen for it, it seems to come from the transformers.? wonder if i should replace them?
    thinking of replacing power switch to a on/on/on switch or something, just so i can install a standby, thought?

    i wanna put on standby between sets to save valve life, and to give an option to cut out speakers so if i have to change pentode/triode switch settings, i wont get (hopefully) the pop that goes with it!
     
  18. hamfist

    hamfist Member

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    the only hum I get with mine is for a few seconds as the tubes warm up. Literally just for about 5 seconds after you get sound, and then the hum fades away and it's as quiet as a mouse.
    to be honest I wouldn't bother with putting a standby switch in. there's no real evidence that they increase tube life at all, only theory ! The rectifier tube warming up gives the other tubes a slow warm up anyway - that's why you can't get any sound for 10-15 seconds after switching on. I just switch the whole thing off between sets. If I wanted to avoid a pop when switching from triode to pentode, I would also just switch the amp off for a few seconds. The tubes stay hot for a long time anyway, so when you switch it back on, they are "good to go" straight away.
    My opinion is that EL84's are pretty cheap, and I don't have to pay anyone to bias them, so I'm not too bothered if they don't last too long. if I get more than 6 months out of them I'll be fairly pleased.
     
  19. mcdes

    mcdes Member of no importance

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    yea that makes sense, it does run nice and hot. and it does go quiet once warmed up after bout a minute.

    i read on another thread that someguy may have pulled sleeve of ef86 and it was a eh ef86 that came stock with heritage?

    you dont think turning amp on and off will destroy the tubes with no warm down tho? i blew a valve in my custom classic turning it off after 20 seconds of playing.
     
  20. Groovey Records

    Groovey Records Member

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    I Tried this today no opinion yet

    "For jumpering, plug your guitar into the high input of the EF86 channel and take a guitar lead from the low input EF86 into the high input Topboost channel. Just play around with blending the levels of each channel until you come across a sound you like. You can try all ways of jumpering from high to low or low to high inputs from and to each channel - let your ears decide which is best.. "

    I still can't get past the EF86 rattle unless that channel is at 3pm and the guitar at 3 or 4 or 7 even !!!

    I guess we have a bit of a time difference

    Best
    Groovey Records
    [​IMG]
     

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