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Preamp and phase inverter on the same tube?

Discussion in 'Shock Brother's DIY Amps' started by Che_Guitarra, Apr 14, 2019.

  1. Che_Guitarra

    Che_Guitarra Tele-Holic

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    I'm looking to rehouse my 6G2 build. I built it as a head but would now prefer it to be a grab-and-go combo.

    I only ever use the one input, thus i'm wasting one half of the V1 tube. And i'm not a tremolo user, so I built the amp without that segment of the circuit... thus i'm also wasting a half of the V2 tube.

    Seems logical to delete a tube during rehousing.

    Can I put the preamp and phase inversion triodes on the same tube, or is this likely to induce unforeseen problems?

    I haven't built that many amps that I know the answer to this.

    Thanks in advance :)
     
  2. FenderLover

    FenderLover Poster Extraordinaire

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    A 6G2 will have Gain > Vol/Tone > Gain > concertina. Looks like three stages without Tremolo. Does your 6G2 have two input stages?

    I don't know how common it is in guitar amps, but it is done in HiFi. The Dynaco MKIII is a popular example, so I don't see any electrical problems with it. Many guitar amps share the concertina with the trem, so there is not much difference sharing with a preamp stage.
     
    Last edited: Apr 14, 2019
  3. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    What you say doesn't add up, both inputs share the first gain stage, the signal path goes through 3 triode stages. Leaving the one out before the PI is not going to give you enough gain to get more than a weak drive to the power tubes. upload_2019-4-14_15-8-35.png
     
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  4. Wally

    Wally Telefied Ad Free Member

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  5. tubeswell

    tubeswell Friend of Leo's

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    The cathodyne inverter is (only) capable of unity gain. So if you only had a single inverting triode and a cathodyne stage, all the preamp gain* would only be from the inverting triode, which is not enough to drive a 6V6 grid.

    *You could try a cathodyne bootstrapped to V1 for more gain (as per the suggestion in Ch 7 of Merlin Blencowe's 1st pre-amp book) but then where would you put the volume control?
     

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  6. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    As Wally points out, a single small signal pentode such as the EF86 set up for high gain would deliver enough drive perhaps....but that would be something to do when shifting from a combo to a head, not the other way around....unless you like a good howl.
     
  7. Wally

    Wally Telefied Ad Free Member

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    I have a Gibson GA40 Les Paul amp that uses the 5879 in the input preamp stage.....no problem with microphonics.
     
  8. Che_Guitarra

    Che_Guitarra Tele-Holic

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    Oops! Right you are guys. I've tweaked the amp so many times in the past my memory is failing me!

    I only have a single unused triode... LTPI conversion it is then!
     
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