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Any amp techs have any advice on this circuit?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by guitarcapo, Jan 19, 2013.

  1. guitarcapo

    guitarcapo Senior Member

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    I bought an AO35 and was thinking of converting it into an 18 watt of some kind, but before I gutted the whole thing I thought I'd try simply modifying the circuit. here's what I started with:

    [​IMG]


    ...and here was some online advice:

    [​IMG]


    Well I did the mods for ***** and giggles and fired it up. Sounded like crap.
    Here's the sound as I can best describe it: Very little sound unless you slam on the strings HARD and then this nasty distorted sound sort of breaks through the silence like a loud fart. Interestingly, if I switch off power, before everything falls silent, it starts sounding normal.

    I'm thinking there's something wanky in that first gain stage that needs to still be addressed. Only 60 volts on the plate? I'm also wondering if R3 and C2 are helping being there. And maybe I should lower the value of r4 and r5 to get a higher plate voltage? Any ideas before I gut this thing and build a whole new amp?
     
  2. teemuk

    teemuk Member

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    Damn... I wrote a wrong reply but apparently the forum keeps failing today and it never got through.

    In summary:
    - 60VDC at plate is lowish but still provides ample headroom for first gain stage.
    - The whole preamp in general has plenty of headroom, power amp overdrives first
    - The preamp has way too much gain to get subtle overdrive. Couple that to - not very nice - overdrive characteristics of a power amp with poor damping open loop and plenty of negative feedback applied
    - Still, with your mods the circuit should work decently with lowish input signals (e.g. guitar volume rolled somewhat down)
    - Check your mods for errors!
     
  3. Davidg1974

    Davidg1974 Member

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    I am sorry to disagree with 1st reply but there is no way that circuit will ever sound good without a pretty extensive overhaul and he is way of about 1st gain stage but that is only one of many problems. A 390R cathode res with 60V on plate is basucally going to get u fart noises at best especially with evrything else going on there. And u are going to need to change some of your power supply dropping res too-thst is why u only have 60V on plate.I just glanced at it but do yourself a favor and save a lot of headaches and find a 18 watt circuit u like and copy it and dont forget about power supply.It probably wont sound exact but will get u in the ballpark so u can start tweaking from there.And if u dont really know what u are doing please be very careful.You dont have to know everything about whats going on in there but u absolutely must know how to work on it safely! Mainly draining caps and not wiring caps backwards and such.Ceriatone has some good layouts as well as triode electronics and Hoffman Amps.Good Luck and let us know how it goes
     
  4. gldtp99

    gldtp99 Member

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    Agree with this---- also consider this circuit: http://www.el34world.com/charts/Schematics/files/fender/DELUXE_6G3.pdf

    Build a single channel version and keep the EL84 outputs
     
  5. guitarcapo

    guitarcapo Senior Member

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    I'm going to play around with the first gain stage some more before gutting the whole thing. All that crap at the cathode doesn't look right for an electric guitar amp. Other people have gotten their amps to sound good though. Maybe I'll take voltages and post them here later today.

    Update: I clipped out everything except the 390 resistor. No change. Looks like I'm gutting this.
     
    Last edited: Jan 20, 2013
  6. guitarcapo

    guitarcapo Senior Member

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  7. Keyser Soze

    Keyser Soze Member

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    What are the quiescent voltages at the grid and cathode of V1a?
     
  8. smolder

    smolder Gold Supporting Member

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    that's a lot to fit into that tiny chassis. remove the standby and the imp selector... and it's still tight. not a lot of room inside around that PT. might end up pretty noisy.
     
  9. teemuk

    teemuk Member

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    Why not convert it to Carmen Ghia? You have already done half the mods.
     
  10. guitarcapo

    guitarcapo Senior Member

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  11. guitarcapo

    guitarcapo Senior Member

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    Here's one I found that CLAIMS to be a Carmen Ghia...but your assesment that I'm half way there seems wrong:

    [​IMG]
     
  12. Prattacaster

    Prattacaster Member

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    That is a carmen Ghia, relabled
     
  13. smolder

    smolder Gold Supporting Member

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    did you read through the AO-35 thread just below this one?
     
  14. Tone_Terrific

    Tone_Terrific Supporting Member

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    I'd try reconfiguring the 1st stage in a normal Fenderish way before giving up on the whole thing.
     
  15. guitarcapo

    guitarcapo Senior Member

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    I was the one who brought it back from the dead
     
  16. teemuk

    teemuk Member

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    I'd concentrate on fixing the thing before worrying about the first gain stage... or any other stages on that matter.

    I ask you this: Did Hammond build an Organ amp in which "distorted sound sort of breaks through the silence like a loud fart!"...?

    No.

    Definitely they didn't. The amp should work well as is, despite minor mods.

    The first gain stage with low plate voltage and mixed bias is "unconventional" for those who expect to see a Fenderish circuit in a guitar amp, yes, but it still has plenty of headroom for first gain stage and it will amplify and operate normally despite minor mods like higher input impedance or removal of the HF shunt cap in input. These mods or an "unconventional" first gain stage should not turn the amp into mess you describe in the first post.

    By the way, the first gain stage isn't all too different from what you could find from the highly praised Magnatone amps.

    So, the amp's misbehaving has to do with something else. Perhaps you made an error while you built those mods. Have you verified your wiring? Have you scoped the amp or done anything concrete to locate what is -really- causing the amp to misbehave? It's an old chassis.... parts may have gone bad due to aging... Fix the problems first, then worry about if the first gain stage - or any other stage - even needs mods.

    Start from the fact that the Hammond's circuit works pretty allright as is. If it doesn't now then it's due to something YOU made. Your mods were pretty minor (other than killing a lot of upper mid and high frequencies with the swap of 1K to 33K in a RC circuit with high value of C, and a substantial increase to power amp gain from altering the negative feedback loop) so I would suspect you just made some kind of an wiring error or alternatively some part just decided to fail in the particular time period.

    With your mods the amp should still work. It might sound somewhat dark because you kill a lot of treble right in the input, and it may have way too much gain so it overdrives the power amp pretty much instantly unless you have guitar volume rolled very low, but despite that - with lowish input signal - the amp shouldn't fluctuate in volume, fart excessively - or suddenly behave normally when the power supply voltage drops during power off. (Yeah, doing that will really drop that plate voltage in the first gain stage). All those symptoms on the other hand speak of some major fault in the circuit.

    Fix the issue(s), then worry about mods.
     
    Last edited: Jan 21, 2013
  17. Tone_Terrific

    Tone_Terrific Supporting Member

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    ^
    By all means fix it , or at least identify the bad parts, before reusing them.
    If you haven't been through it with a meter and injected a signal into the power stage and worked back... you should do the basic troubleshooting.
     
  18. ekp

    ekp Member

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    HI,

    I am an electrical engineer from the days when they did still teach tubes in school. There are a few issues that I would like to take issue with:

    The first issue is the .0022 microfarad capacitor at the input. With the original 1K ohm resistor, it would create a larger and lower resonance with your guitar pickups. With the 33K resistor, it will create a low pass filter with a time constant of about 70 microsecond time constant or about 2.5 KHz, just doing the math in my head. Please also bear in mind that the 33K resistor will be the largest source of input thermal noise.

    The cathode bias is strange. I would never wish to inject power supply ripple into the first stage. It is much better to not use the 68K resistor, use a larger resistor in lieu of the 390 ohm resistor and bypass it with a capacitor. The size of this capacitor is like 22 microfarads if you want a flat response and less if you want to have a treble accentuated response.

    The 1 meg plate resistor certainly limits the current through the first stage and does not help the gain much. Nominally, the gain is a voltage divider between the plate resistance and the plate resistor (the 1 meg). At low currents the plate resistance is high and reduces the expected wonders of the high plate resistor. Also, the high value of the plate resistor produces more thermal noise than a lower value. Typical values are 1.5K for the cathode resistor and 100K for the plate resistor.

    But then the schematic is a bit blurry. I am not sure whether that is 1 meg or .1 meg...

    The second stage looks OK, but again, if you want to have treble emphasis, use a small cathode bypass and for more gain in general use a large cathode bypass.

    The output stage is configured like a negative input amplifier akin to an inverting op amp circuit. That is not a real issue. I do that in my amps too. The gain is determined by the 3.3 meg feedback resistor divided by the net resistance looking back into the second stage. Substituting the 220K for the 2.2 meg will change the output stage gain from about 1.5 to about 15 or bit less.

    However, the phase inverter circuit is a bit strange as well. Again, why inject power supply ripple into the signal? The 1 meg from 42 volts only increases the voltage across the 22K resistor by about a volt. Most of the current in that resistor is from the phase splitter triodes.

    So now, to your complaint. Your description sounds like there is a bias problem somewhere. The lack of sound suggests that there is a tube cut off. So I would check the grid to cathode voltages, keeping in mind the polarity of your measurement. The grids should be somewhat negative when compared with the cathode, but not a lot negative. To put that in the right frame of reference, the grid should be about 1.5 volts below the cathode or so. However, if it is five volts or more below the cathode, the tube is cut off.

    The there is the opposite problem, saturated. However after finding ones at
    http://www.google.com/imgres?imgurl...7-UOe7CorpqgGI4oHoBw&ved=0CDYQ9QEwAQ&dur=3254

    It does not seem to be OK if the resistors are 100K.

    This leaves you with bad wiring or mistaken part values. And those are not hard do to. Some resistor manufacturers manage to produce background colors that alter the appearance of some of the resistor colors.

    Good Luck, Eric
     

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