«New» vintage Hiwatt Custom 100 «Jimmy Page» build

Discussion in 'Building the Classics' started by shakti, Feb 9, 2020.

  1. shakti

    shakti Active Member

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    I'm quite pleased With how it turned out. Basically an amp with vintage sounds, looks and parts. I think this is possibly the very first time it has been done correctly down to the last detail since...1969!
     
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  2. BanditPanda

    BanditPanda Well-Known Member

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    Fascinating project and congratz on completion.
    Will you be providing some sound bytes for us?
    BP
    p.s. Sorry I've just seen that you've been asked a hundred times already!!
     
    Last edited: Feb 19, 2020
  3. shakti

    shakti Active Member

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    OK folks, you asked for it...so here it is, clams and all, very quickly thrown together. This is recorded on an iPhone with a cheap Fostex interface and I’m not even sure if it worked. Small room, it is *loud* even when attenuated -6dB with an Alex’ Attenuator. Cab is a ‘69 1982B and guitar is a 2002 R8 Historic Makeover with Throbak SLE101+ pickups.

    The amp needs some final tweaking, it’s lacking just a little bit of gain but I think tweaking the values in the phase inverter and adding the choke will take care of that. I heard some slight microphonics in a preamp tube but I’m not sure if that comes through in the recording.

     
  4. shakti

    shakti Active Member

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  5. shakti

    shakti Active Member

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  6. CROWEMAG

    CROWEMAG Active Member

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    OMG The sound is there, man. You nailed it. LOVE THIS AMP.
     
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  7. neikeel

    neikeel Well-Known Member

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    Sounds excellent, esp Moby Dick and Since I’ve been (you play it differently to me) but the tones are spot on.
    Maybe have to get a lower limb orthopaedic injury to secure a couple of months off work and finish this one and all my other projects.
     
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  8. shakti

    shakti Active Member

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    Thanks for the kind words! It’s been a long time since I really played any Zeppelin stuff so it’s rusty, and it’s always hard to get the feel and tempo right when playing all alone. All mistakes come through loud and clear with this amp! But I think it’s close. Some tube tweaks and the circuit tweaks should get it just right.
     
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  9. Gutch220

    Gutch220 Well-Known Member

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    That looks great. I didn't read through the whole thread, did you order everything (components) as a 'kit' or did you need to look at a schematic and buy everything yourself, individually.
    There were times when I'd be building things and, for example, I'd need a resistor(s) for 130k ohms, but you can't just order one, so I'd buy 10 for like $3 with free shipping, so they're basically free, but I'd get them, and measure all of them at home and get readings like 128k, 135k, 130k, 140k, etc.... and I'd choose one closest, or spot on. I'd do this for many of the components. I've always wondered if this made any difference or not albeit small or large. Then I would have a stash of exactly-measured components labeled with tape.

    I've always wanted a Hiwatt DR103 but they got real expensive real quick. They look pretty easy to work on based on Dave Reeves mil-spec layout
     
  10. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I personally wouldn’t add any more gain, amp sounds great the way it is. Page and Townsend actually have a pretty clean tone by today’s standards.
     
  11. BanditPanda

    BanditPanda Well-Known Member

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    Congratz Shakti.
    As stated above " You've nailed it".
    Brilliant.
    BP
     
  12. shakti

    shakti Active Member

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    Thanks again. It doesn’t necessarily need more gain. It’s probably more an EQ thing. It’s very close, but a bypass cap on V3 cathode and having a presence control (wired it up without) will be the ticket I think.
     
  13. shakti

    shakti Active Member

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    I haven’t had much time with this amp between building one more for a friend, and the Corona situation. But I just noticed a strange issue;

    - when I plug directly into a cab, there is a very noticeable hum, presumably a ground hum.

    - when I have an attenuator in line (Alex’ Attenuator in my case), the hum disappears completely even at the loudest setting (-3dB). I mean ZERO hum. The Attenuator has a bypass setting where the signal passes through directly, and the hum appears at that setting of course, but as soon as part of the signal is diverted to the reactive load of the attenuator, all the hum disappears.

    Any hints to what could cause this? I will admit that the two 220uF filter caps that are common for both the output tube screens and the phase inverter probably need replacement. I tried forming them with a 100k resistor in line before them when I first fired it up. I first formed the mains filter caps (4 x 100uF) and the voltage drop across the resistor came down to less than 2VDC so those are good to go, but I couldn’t get the two 220uF caps to less than a 19VDC drop, so one or both of them need to be replaced. But if those were the reason for this issue I would expect to hear the hum also at attenuated settings.
     
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  14. shakti

    shakti Active Member

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    I replaced the two old Plessey 220uF caps with a pair of NOS RS labeled ones (probably Erie or Daly, 1980 date codes). These caps formed beautifully to less than 2VDC within a few hours. But the problem remains - the amp has zero hum at any attenuator setting, but as soon as the attenuator is bypassed there is a hum.

    I thought at first it could be redplating as these tubes (a very well matched TAD quartet that I use for testing) are running very hot at about 22.5W dissipation each but again it would be odd that I only hear it with the attenuator bypassed. I can’t see any redplating. Although I *suppose* the attenuator could represent a slightly different load than the cab alone, and the change in load impedance when going directly to the cab is sufficient enough change that the tubes tip over into slight redplating. So I need to try a different set of tubes.

    Any other suggestions to what could be causing this behaviour?

    As an aside, I expected to like the sound even better with these NOS RS filter caps but I don’t...my ears might be turning tricks on me, but I preferred the sound of the Plessey caps in this amp. They seemed to accentuate the warm but nasally mids that this amp sound is all about. So I might give them another try and see if they form better on second try. They measure perfectly with my cheap cap meter for capacitance, ESR and voltage loss (albeit with just a 9V battery powered tester).
     
  15. neikeel

    neikeel Well-Known Member

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    Do you think you might have a ground loop? Did you use a choke in the end?
    I tend not to use filter caps if they do not reform to spec (I don’t even try if the ESR is sky high etc). The trick (apparently is to warm them up for a few hours (bowl of hot water or warming oven) to make sure electrolyte is liquid quickly install and reform. If there is oxidation on plates then ESR will still be sky high and reforming will not work.
    I will get those caps to you btw. (They will need reforming too!
     
  16. shakti

    shakti Active Member

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    I thought about a ground loop, but not sure. Since I have filter caps that are common for both the screens and the phase inverter, but are grounded by the output tubes, there might be a ground loop. All the preamp grounds are running along the preamp board, including the filter cap in the center of the preamp board, which is adding filtering for the «pre-phase inverter» circuitry on V3, but sits at the far end of a 100k resistor so there should be plenty of de-coupling (see schematic). I had it set up like this in my old Royal clone and I never had any issues with that amp (although I may have never tried it directly to the cab, always attenuating at least a little bit).

    In any case, I don’t understand how a ground loop could be very noticeable with no attenuation, but completely absent even with just -3dB attenuation.

    All the caps I have been using have measured fine on my cheap meter for ESR, capacitance and leakage. I’d be wary to use hot water to warm up old caps (what about water leaking into them?) but I suppose the oven at low temperature could be worth a try.
     
  17. shakti

    shakti Active Member

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    Update: I tried a different set of output tubes that bias up more conservatively. No difference.

    Then I tried removing the preamp tubes one by one until all were removed:
    V1: no change
    V2: no change
    V3: some increase in hiss and a faint crackling, but no noticeable hum
    V4 (phase inverter): increase in hum overall, and it is now audible also in attenuated modes.

    I still can’t understand it being a ground loop. Every Hiwatt I’ve owned or built has followed the same basic scheme; there’s a shared filter cap for the screens and the phase inverter. On a typical DR103 it is a 50+50 followed by a 220 in series, with a 1k decoupling resistor between the two sections of the 50+50. But the ground point is the same, at one of the output tubes. In this amp it has two 200uF in series and no 1k decoupling resistor, but that’s the whole difference.

    I don’t yet have any balancing/bleeding resistors across the filter caps. I will add those and see if it makes a difference. I also have to finish amp #2, so it will be interesting to see if it behaves the same.

    I am most interested in any educated suggestions as to what could be causing this.
     
  18. pleximaster

    pleximaster Well-Known Member

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    I had a very similar issue on a jtm45... with the attenuator... I redid a lot of things but especially the ground and I believe it was the grounding in some way...

    Sometimes one end up rebuilding many things so its hard to pin point the exact problem...

    plexi
     
  19. shakti

    shakti Active Member

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    Amp #2 is finished! But the hum is there on this amp as well, and even louder than #1. It is audible at all attenuator settings. So I need to look closely at the grounding again.
     
  20. shakti

    shakti Active Member

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    I think I am on the track to the source of this hum. First of all, I think V1 and V2 can be excluded. Adjusting either the input or master volume doesn’t affect the hum at all, it is there with everything on 0 and plugging in a guitar or adjusting the controls doesn’t make it better or worse. So I am focusing on the output stage and phase inverter.

    I cleaned all contacts and tube sockets, but no difference. Output tubes are balanced and biased in an acceptable range. I tried a different bias supply capacitor, but no difference there.

    Then I noticed that the negative feedback setup on this is slightly different from all of my other Hiwatts. These two amps were built on Slave amp chassis, which may have something to do with it. The NFB is taken from the 16 ohm tap as usual and run through a 10k resistor to the phase inverter. But it also has a 100pF cap across the 10k resistor which isn’t usual for a Hiwatt. Also, there’s no resistor and cap to ground at the «phase inverter» end of the 10k, like you find in my other Hiwatts. It’s usally a 100-470 ohm followed by a 10n cap to ground.

    I tried clipping in a 100R to ground, but that actually *increased* the hum so it was now audible also at attenuated settings. Same level of hum with the attenuator off. I didn’t remove the 100pF or add the 10n yet, but even though I don’t fully understand what’s going on here I think I am at least on the right track. Stay tuned. See schematic below to reference what I am looking at.

    [​IMG]
     

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