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Thread: Vox AC15 (TBX) general Q's

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    Vox AC15 (TBX) general Q's

    Hi chaps-

    got myself a lovely AC15 TB (C'96) with a blue 'vox' speaker: I assume this was an addition as amp says only TB on, & therefore can now be called a TBX, & is this the usual monkey celestion G12 alnico just rebadged?

    Maxing the 'volume' (the preamp.. not overall 'master') I hear only very little OD.. weird I thought. Looking inside I find a jan philips 12AT7 in V1. So knowing all meant to be 12AX7, I swapped it for a JJ 12AX7.. expecting a bit more OD. Not so. Actually its a little -less- OD than with the 12AT7.

    I'm a bit confused.. not only the master doesn't seem to do much at all (& the reverb severely lacking too), but this tube swap not as expected. Its a fine amp, very well made, & looks gorgeous but I'm scratching my head a bit.

    Any thought? cheers, SC

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    Quote Originally Posted by Sea Chief View Post
    Maxing the 'volume' (the preamp.. not overall 'master') I hear only very little OD..
    I don't think it has a lot of gain in the pre-amp before the master so there won't be much OD. (Assuming I'm looking at the right schematic). It's like a Top Boost circuit but with lower gain. Cathode resistor R35 is a high value and not bypassed and the 3M3 reverb mixing resistor is also going to reduce gain.

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    Edit: Ignore what I said above. I've just noticed the input tube triodes are cascaded

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    Last edited by Dave H; 11-09-2019 at 10:37 PM.

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    V1 in the preamp is the phase splitter.

    My experience with these amps is that theyíre plagued with dry joints; itís best to reflow the pcbs.

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    Ah.. not sure what is meant by your edited line then DaveH. Thanks for schematic. I'm so used to fender ab763 etc types. pdf64 saying v1 is the phase splitter is totally weird.

    So what are the 'chain' of tubes from two inputs.. to power tubes then? is v5 the 1st preamp tube then? isa it maybe 'backwards' to fender v5,4,3,2,1 > power tubes?

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    Quote Originally Posted by Sea Chief View Post
    Ah.. not sure what is meant by your edited line then DaveH. Thanks for schematic. I'm so used to fender ab763 etc types. pdf64 saying v1 is the phase splitter is totally weird.

    So what are the 'chain' of tubes from two inputs.. to power tubes then? is v5 the 1st preamp tube then? isa it maybe 'backwards' to fender v5,4,3,2,1 > power tubes?
    I meant that it's not as low gain as I first thought because the two sections of the input trube (V5) are in series i.e. It has one more gain stage than the Top Boost pre-amp.

    I think the chain is V5 section A, V5 section B, V4A, V4B (cathode follower), V1A, V1B (phase splitter)
    V3 reverb driver, V2A reverb recovery, V2B trem oscillator.

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    Last edited by Dave H; 11-10-2019 at 07:35 PM.

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    Quote Originally Posted by pdf64 View Post
    V1 in the preamp is the phase splitter.

    My experience with these amps is that they’re plagued with dry joints; it’s best to reflow the pcbs.
    But unless I hear anything suggestive of a dry joint, there's not much point doing so surely:

    or,

    are you suggesting the things I find/ hear -are- suggestive of dry joints?

    (Its a bit of an ambiguous post/ sentence).

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    Quote Originally Posted by Dave H View Post
    I meant that it's not as low gain as I first thought because the two sections of the first triode (V5) are in series i.e. It has one more gain stage than the Top Boost pre-amp.

    I think the chain is V5 section A, V5 section B, V4A, V4B (cathode follower), V1A, V1B (phase splitter)
    V3 reverb driver, V2A reverb recovery, V2B trem oscillator.
    Ok roughly on board with 1st bit: but so I read & the manual says.. it -is- a 'top boost' type vox ac amp (but you say its got "one more" gain stage than this.. which would suggest to me, that I should definitely have some OD available using the MV knob).

    What exactly top boost is, even from reading some xyz about it is still totally lost on me. How on earth any' normal gtr player' (IE without having tinkered with the guts of a tube amp as I have) would have any idea I just do not understand. I mean its got a treb & a bass. Most gtr amps have a treb & a bass. If I want any gtr amp I almost expect a treb & a bass. pg1. What 'top boost' means.. god knows, but its something to do with the treb, & the bass is all I know.

    The chain I can get more sense from/ can see what you mean, looking at the schematic- thanks, I guess its just labelled 'opposite' to fender to be deliberately different.

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    Quote Originally Posted by Sea Chief View Post
    But unless I hear anything suggestive of a dry joint, there's not much point doing so surely:

    or,

    are you suggesting the things I find/ hear -are- suggestive of dry joints?

    (Its a bit of an ambiguous post/ sentence).
    And what do bad solder joints sound like? The answer is that it depends. It depends on the circuit in question and the nature of the joint failure (open, intermittent, low continuity).

    I don't think pdf64 would have mentioned it unless he thought it was worth doing. Are you looking for a reason to disagree? He's had more than a little experience at this.

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    Last edited by Chuck H; 11-10-2019 at 07:13 PM.
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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    Consider using freeze spray to identify cracked/bad solder joints.

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    Quote Originally Posted by Sea Chief View Post
    What exactly top boost is, even from reading some xyz about it is still totally lost on me. How on earth any' normal gtr player' (IE without having tinkered with the guts of a tube amp as I have) would have any idea I just do not understand. I mean its got a treb & a bass. Most gtr amps have a treb & a bass. If I want any gtr amp I almost expect a treb & a bass. pg1. What 'top boost' means.. god knows, but its something to do with the treb, & the bass is all I know.
    It's simpler than you're making it. On the early Vox amps the "top boost" channel had more HF gain than the "normal" channel***. Later incarnations that imitate the top boost channel were naturally called "top boost" as a name for the design. That's all there is to it.

    *** Actually what the top boost design does (among other things) is reduce LF through the circuit and boost overall channel gain. But that's beyond the subject of it's name.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Sea Chief View Post
    But unless I hear anything suggestive of a dry joint, there's not much point doing so surely:

    or,

    are you suggesting the things I find/ hear -are- suggestive of dry joints?

    (Its a bit of an ambiguous post/ sentence).
    Sorry, you mentioned a few issues (eg insufficient preamp gain for noticeable overdrive, master vol having little effect) that may indicate circuits not working as intended, which may well be due to poor connections; of course they may be due to something else, eg tired ecaps. A failing solder doesn't just cause crackles etc.
    One approach is to track down each perceived issue.
    Another is to blitz the boards and then reassess.

    Regarding the Blue speaker, I seem to remember reading that more recent Vox Blues (than this range, later than 2002ish) were made by Celestion in their Chinese site, whereas regular Celestion badged Blues have always been made in the UK

    Regarding the tube swap in V1, did you mean the tube noted as V1 on the schematic (ie phase splitter) or the tube that include the input stage (ie V5 of the schematic)?

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    Quote Originally Posted by Chuck H View Post
    And what do bad solder joints sound like? The answer is that it depends. It depends on the circuit in question and the nature of the joint failure (open, intermittent, low continuity).

    I don't think pdf64 would have mentioned it unless he thought it was worth doing. Are you looking for a reason to disagree? He's had more than a little experience at this.
    I'm certainly not looking for a reason to disagree, I'm sure he has a wealth of experience & I didn't for one moment suggest otherwise; I'm just looking for a reason he mentioned it.

    1) If it means I can dig into more OD.. great/ will do it.. but I don't think this is why it was mentioned.
    2) If it means I can ID my speaker.. great.. but don't think the reason it was mentioned either.
    3) Nor why I find an 12AT7 in V1.

    These are the 3 things I mentioned in the OP. Arbitrary opinions on other aspects non-related.. even if you mioght think tyhe amp total cr*p, may well have worth, but they don't tie with simply what I'm asking in th OP, & as a consequence just confuse me.

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    Quote Originally Posted by Sea Chief View Post
    1) If it means I can dig into more OD.. great/ will do it.. but I don't think this is why it was mentioned.
    2) If it means I can ID my speaker.. great.. but don't think the reason it was mentioned either.
    3) Nor why I find an 12AT7 in V1.
    You mentioned that the master volume and reverb didn't seem to have much affect. Making the amp work properly should certainly allow you to dig into more OD. Don't be obtuse about things.

    Perhaps what you meant was that the master volume doesn't seem to do anything to the tone of the amp? Volume not withstanding? Since the master volumes function is to set the master volume, when you say "it doesn't seem to do much at all" we can only interpret that to mean the control is not working.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by pdf64 View Post
    Sorry, you mentioned a few issues (eg insufficient preamp gain for noticeable overdrive, master vol having little effect) that may indicate circuits not working as intended, which may well be due to poor connections; of course they may be due to something else, eg tired ecaps. A failing solder doesn't just cause crackles etc.
    One approach is to track down each perceived issue.
    Another is to blitz the boards and then reassess.

    Regarding the Blue speaker, I seem to remember reading that more recent Vox Blues (than this range, later than 2002ish) were made by Celestion in their Chinese site, whereas regular Celestion badged Blues have always been made in the UK

    Regarding the tube swap in V1, did you mean the tube noted as V1 on the schematic (ie phase splitter) or the tube that include the input stage (ie V5 of the schematic)?
    Hi pdf64,

    I will consider this, but a bit drastic? I wasn't suggesting anything wrong with the amp. More I was puzzled by the design, which -afaict- & so far as I am using it correctly, & have the right tubes, seems to have such a small ammount of OD from a circuit I read alot about "top boost.. extra tubes in preamp.. more gain" & with an MV knob too.. that I'm left scratching my head. But every amp I have I find totally confusing, the OD infuriatingly impossible to achieve (without taking out half the village).. bar my small 10" Laney vc15, which is great but a bit like a toy version of this 90's UK made vox in terms of tone complexity.

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    Quote Originally Posted by Chuck H View Post
    You mentioned that the master volume and reverb didn't seem to have much affect. Making the amp work properly should certainly allow you to dig into more OD. Don't be obtuse about things.

    Perhaps what you meant was that the master volume doesn't seem to do anything to the tone of the amp? Volume not withstanding? Since the master volumes function is to set the master volume, when you say "it doesn't seem to do much at all" we can only interpret that to mean the control is not working.
    I'm not being obtuse. I'm trying to be as clear as possible, sorry if this is not coming across.

    The amp is working as it should, even with the small ammount of OD I achieve with volume maxed (the one near the gtr inputs). afaict there's nothing to suggest the amp is -not- working correctly.. but if anyone could second this/ know this amp at all/ or could decipher from the schematic if the small ammount of OD is to be expected it would be helpful: that's mostly what the thread is asking.

    From MV amps I've had or played, this function/ this very knob's use, is to achieve OD without turning the other 'main' volume knob up to destroy half the village.. with the caveat of having a 'lesser' OD tone. I think, this is what it is for (but the manual makes no mention of its use, nor mentions what on earth 'top boost' means) this is the secondary Q of my thread. These other amps, including a Twin Reverb you turn this knob to max, have the 'main' volume low.. & there's a decent ammount of OD (the quality dependent on the amp: for some reason the TR hopeless, & a diddy Laney vc15 excellent in this dept).

    If a very decent quality amp has this extra vol knob (one nr the gtr inputs) & its maxed, but only a very small ammount of OD achieved (& the amp is working as it should).. what is the design principle? why if the preamp circuit has "two extra tubes! top boost! extra gain!" do I seem to get the opposite of what I'd expect IE a very decent ammount of OD with this vol maxed-? this is the point of the thread.

    The 3rd minor Q is the speaker: its blue & it has 'vox' on it.. but what is it? its wattage, sensitivity?

    thanks, SC

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    Quote Originally Posted by Sea Chief View Post
    Ok roughly on board with 1st bit: but so I read & the manual says.. it -is- a 'top boost' type vox ac amp (but you say its got "one more" gain stage than this.. which would suggest to me, that I should definitely have some OD available using the MV knob).

    What exactly top boost is, even from reading some xyz about it is still totally lost on me.
    It has an extra gain stage but it only has a gain of about 10 owing to its large unbypassed cathode resistor (R35) then most of that gain is dumped by the 3M3, 470k divider before the following stage. You could try bypassing R35 with a 22u cap.

    Historically the AC30 was a double AC15 with an EF86 front end. The EF86 was found to be too microphonic and was replaced by an ECC83 which unfortunately reduce gain. To combat this Dick Denney came up with the “Top Boost” circuit which was an extra ECC83 to boost gain and drive a Treb, Bass tone stack to boost treble. Initially the “Top Boost” circuit was an add on bolted to the back panel but later on when it became popular it was incorporated into the amplifier.

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    The queries raised in post 1 might well indicate malfunction, hence my response. Unfortunately, after a couple of decades use, I find that some pcbs need reflowing in order for the integrity of their connections to be reliable, especially for lower spec, non through hole plated types.

    The master vol is after the phase splitter, so with it set low and the preamp controls set high, I would expect a reasonable degree of crunchy overdrive, even with lowish output pickups.

    Please clarify what is ineffective about the master volume control. eg with the preamp volume and tone controls set high, the master should be able to adjust the sound level from zero to deafening, does it do that?

    Please clarify which preamp socket the 12AT7 is in, eg V1, V5?

    I don’t think that the Vox Blue characteristics / rating have been published, but I’d expect them to be the same as a Celestion Blue of the same age, wherever it was made.

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    Last edited by pdf64; 11-11-2019 at 12:54 PM.

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    With respect pdf64, having just bought it on ebay & given vg feedback etc.. its working fine/ no indicators as to otherwise: its just the query of how relatively little OD this amp/ this circuits preamp just gives out, is just a bit strange. Similarly the reverb is very lacking too though, but this is a known-fact on these amps; the small preamp OD ammount is not written about anywhere hence my thread.

    But it still sounds vg. Its just the 'vol' knob is useless bar set at 10 & then only delivers a minimal OD ammount. I find this rather strange (& after my 30 yr attempt at amp OD.. pretty "I just cannot believe this!" infuriating).

    Most are probably lucky enough to dig into the real power-tube OD getting amp loud & treating the preamp 'volume' just as a gimmick they don't use.. but not me: its permenantly on 10 (& I just need it to go to 15 as it were).

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    If you're happy it's all working fine and there are no actual faults that require resolving, then that would seem to be that, and it's time to get on with enjoying your new amp

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    Quote Originally Posted by pdf64 View Post
    If you're happy it's all working fine and there are no actual faults that require resolving, then that would seem to be that, and it's time to get on with enjoying your new amp
    Yes fair point. And I do enjoy it, the OD I get is great you pick light/ its clean, you dig in/ it gets -a little- dirty. Its the 1st amp Ive ever had like so (we all know is the essence of a good player's amp). Its so frustrating is so little tho. And I wonder how from a design pov this might have been deemed "finished/ we're all happy/ all done" when it seems weirdly 'uncomplete'.

    IO guess the further Q is whether increacing the range of this volume knob by a mod, might be possible (but if so why wouldn't it have been done prior to 'signing it off' in the design dept-?)

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    I've tried to be clear in stating my view that your amp may not be working as intended, my apologies if you haven't understood that; as such, it would be bad practice to consider modification to it.
    Whatever, as Dave alluded to earlier, gain may be increased somewhat by bypassing R35.

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    Quote Originally Posted by pdf64 View Post
    I've tried to be clear in stating my view that your amp may not be working as intended, my apologies if you haven't understood that; as such, it would be bad practice to consider modification to it.
    Whatever, as Dave alluded to earlier, gain may be increased somewhat by bypassing R35.
    I have understood it pdf64, but unless you say why you think this I can only say that its not the case as there's nothing to suggest anything wrong with it. I may not have found the ideal amp in 30 yrs of trying, but I know & I buy an amp that is working as it should.. not agree to leave vg feedback having spent £500 if I have a smidgen of doubt its not. Its just obviously a curious design with a severe limitation deemed by the designers "fine- let's put it out". I just wanted to find out if the circuit/ schematic tallies with what you might expect (levels of OD on the preamp vol knob @ max)or if not.. then it must be just an odd amp design.

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    Quote Originally Posted by Sea Chief View Post
    I have understood it pdf64, but unless you say why you think this I can only say that its not the case as there's nothing to suggest anything wrong with it...
    Please consider that all I can know about your particular amp is what you have written, eg -
    Quote Originally Posted by Sea Chief View Post
    ...the 'vol' knob is useless bar set at 10 & then only delivers a minimal OD ammount. I find this rather strange (& after my 30 yr attempt at amp OD.. pretty "I just cannot believe this!" infuriating)...
    Quote Originally Posted by Sea Chief View Post
    ... if anyone could second this/ know this amp at all/ or could decipher from the schematic if the small ammount of OD is to be expected it would be helpful: that's mostly what the thread is asking...
    Quote Originally Posted by Sea Chief View Post
    ...I just wanted to find out if the circuit/ schematic tallies with what you might expect (levels of OD on the preamp vol knob @ max)or if not.. then it must be just an odd amp design.
    One last time, no, it is not what I would expect. The low preamp gain you describe suggests that the circuit may not working as intended (ie by its designer).
    Please don't carry on like this, every one of your threads always seems to follow this same pattern, rephrasing the same query over and over, and ignoring the response

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    Ok pdf64 point taken there might be a prob with it although I think its working ok. How would I establish if there's a problem tho without comparing to another amp I wonder.

    If anyone has this amp or knows it & can say if the small OD ammount with 'volume' maxed is normal on theirs, or not, Id be grateful.

    Its not the cc1 chinese one. That one's rubbish compared to this, I had one once.

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    Have we ruled out lifeless tubes?

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    Quote Originally Posted by eschertron View Post
    Have we ruled out lifeless tubes?
    If replacing v5 with a fairly new jj ecc83.. yes/ same. And swapping v4 for another.. yes/ same. Similarly v1.

    Not changed out v3, v2 but these are reverb & tremolo.. unlikely to add any chutzpa, I think.

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    What tools do you have available, eg multimeter, sig gen, scope?

    As there was a 12AT7 in there, it may be that other changes were made to further reduce gain.

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    I think with master volume set low and which ever channel volume full up, should give a reasonable amount of crunch. Tone settings will also affect this, so they should at least be up half way.

    I'm still not convinced we are all on the same page as far as physical tube layout (which is V1, etc.). A drawing of the layout would be quite helpful.
    I take it this is the C1 version of AC15.

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    Quote Originally Posted by pdf64 View Post
    What tools do you have available, eg multimeter, sig gen, scope?

    As there was a 12AT7 in there, it may be that other changes were made to further reduce gain.
    That's a good point pdf.. I have multimeter (rebuilt my twin rev '78 up from scratch) but not the other scope things.

    All looks pristine/ as stock on the pcb. Im pretty sure the AT was just put in to reduce gain yes.

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    Quote Originally Posted by g1 View Post
    I think with master volume set low and which ever channel volume full up, should give a reasonable amount of crunch. Tone settings will also affect this, so they should at least be up half way.

    I'm still not convinced we are all on the same page as far as physical tube layout (which is V1, etc.). A drawing of the layout would be quite helpful.
    I take it this is the C1 version of AC15.
    Hi g1 no this is not the C1.. its the "TB" basically you know those decent korg ac30 "TB" reissues circa mid 90's? this is the ac15 version. Made in UK (in the marshall factory in fact.. even more reason to think the preamp vol maxed should kick out some dirt, in my book!).

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    It should be feasible to identify if gain is being lost somewhere along the signal path without a scope, as long as the meter is reasonable. Need some sort of signal generator though, so it would very beneficial to get some tone generator software / app on a device with headphone etc output (eg pc / smartphone) which you can rig up a cable to plug into the amp's input jack.
    Looking generate a sinewave in the range 100Hz to 1kHz (lower is better for regular meters, which have a limited bandwidth) of 100mV to 500mVrms.

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    Quote Originally Posted by Sea Chief View Post
    Hi g1 no this is not the C1.. its the "TB" basically you know those decent korg ac30 "TB" reissues circa mid 90's? this is the ac15 version.
    Then this layout should be correct:


    Click image for larger version. 

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    Yes g1 that's like mine.. except.. mine has a 5Y3 rectifier.

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    Yes, the 2 that Iíve worked on had 5Y3.
    The chassis holes need opening out a couple mm in order to accept many modern tubes.

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    I think that gz34 is a typo.

    Thankfully my two el84 are old nos GE el84 (I think nos, they have "...phonic" written on a slant across them/ usa) so slim, & a jan philips usa made 5y3.

    So do you recall trying the input volume @ max on them?

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