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Looking for design idea

Discussion in 'Shock Brother's DIY Amps' started by Hachiko, Mar 1, 2021.

  1. Hachiko

    Hachiko TDPRI Member

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    Good day all,

    I am currently planning on building an amp with a Trainwreck-like preamp.

    I would like to use the transformers that I have on hands to build the amp:

    OT (PP): 8K-8ohms, 20W
    PT: 650V CT/300mA; 6.3V/4A; 4V/4A

    Would anyone have a suggestion for the power section using these two transformers? Since the PT is fairly powerful, I would prefer to avoid tubes like EL84/6V6 that wouldn't use much of the PT's heaters capacity. I looked into EL34, but don't think I could get it to work with a B+ that high and also because I prefer fixed bias.
     
    Last edited: Mar 1, 2021
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  2. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    You will be selecting your output tubes based on the output transformer specifications. 8k primary 20w. Sounds like class A/B 6V6GT to me. The power transformer voltages are not out of line and it has lots of current which gives you options like multiple channels, reverb, gain stages, etc... you could make a very cool amp with this set. One channel train wreck and one channel something completely different.
     
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  3. 2L man

    2L man Tele-Meister

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    That voltage and OPT impedance is quite little for EL34 pair!

    If you use PT as Center Tabbed it changes to half 325V and mathematical max DC comes multiplying using 1,41. If you use much less than 300mA voltage might stay close to 350VAC.

    Use this easy to use loadline calculator. When you select the tube it comes average single ended values. Then tick PP box and it changes to average push pull. Then change voltage, current=bias, impedance and screen voltage and when you hit "enter" you see immediate change.

    If is interesting to test how other builders have set their amp tubes entering their values. If you design a cathode biased power amp you need to add g1 and anode voltage together to get B1. If you test tube datasheet values note their low screen voltages which instrument amp designers rarely follow. If you design parallel tube circuits use double the impedance because tubes share it so one tube gets its load from half the impedance which technically doubles it on loadline.

    https://www.vtadiy.com/loadline-calculators/loadline-calculator/#calculator

    I like Tung-Sol 5881 sound and earlier breakup and its 23W is bit lower power than EL34 25W.
     
    Last edited: Mar 3, 2021
  4. Nickfl

    Nickfl Friend of Leo's

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    You seem to be starting from a point of worrying about using the heater supply to its fullest potential which is in my opinion the last thing you should be worrying about.

    Your output transformer is going to be your limiting factor, it's impedance matches a pair of 6v6 or el84. You could mismatch the speaker and cut that in half, which would be about right for 6L6 but with it's power handling at only 20 watts, it is not going to be sized for those tubes. That's going to give low output power and lead to early saturation and lots of compression, which is not probably going to be ideal for a TW style circuit, since I understand that Ken Fisher liked big iron for his amps.
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Sure, build the Trainwreck with 2 x EL84 or 6V6. The OT works best with those tubes.

    B+ voltage for the different amps vary:
    The *Liverpool* B+ could be obtained using a rectifier tube. The four EL84 would have to be reduced to only a pair of EL84. It is only a reduction of 3dB and still loud.

    The *Express* B+ would use SS diodes.

    The Trainwrecks were built with big iron so your PT is right in line ime.
     
    Last edited: Mar 3, 2021
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  6. Hachiko

    Hachiko TDPRI Member

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    I did like the idea of multiple channel and started looking into that; I now have 2 projects in mind. Thank you.

    That's great, I didn't know there was something like that. THank you for all the information!

    Totally agree! I only wanted to avoid buying any parts if possible and also didn't want to "underuse" that big transformer since those irons are pretty expensive. I listed 4A for the heaters but I'm fairly sure it could handle even more, 4A was the heater consumption of the amp I pulled it out of and it also has a 2nd 6.3 secondary that was unused.

    Thank you for the info! Since I'll be buying a pt for the Trainwreck, I might even go for dual rectifier, ss/tube switchable.



    So, with all that info and a few days to think I've decided to go for a Trainwreck like amp with a el84 pp AND a custom/mod SLO amp build with a KT66 pp.

    I will be buying a power transformer for the Trainwreck and an output transformer for the SLO. Both Toroidals from Antek.

    I'll be making a thread for whichever amp I decide to build first. Cheers!
     
  7. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    I can’t wait to see how these go!
     
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