2204 circuit at lower output build

Discussion in 'Building the Classics' started by william vogel, Oct 16, 2021.

  1. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    I’m building a 2204 circuit expecting a lower output power than 50 watts. I’m thinking it’s probably going to be around 40 watts. Everything inside the chassis will be stock value. The transformers are the difference. I wound a power transformer from EI 125 size laminations M6. It’s 315-0-315 from 30 gauge wire with 84 ohms DCR on the secondary. I’m hoping it’ll hold 400 volts at idle. The 6.3 is 14 gauge wire and the 120 volt is 22 gauge. The output transformer is wound to 5.7k on EI112 M19 with 30 gauge primary and 22 gauge secondary. It’s the 784-139 interleaving pattern just with more primary turns. The choke is 10 Henry with 498 ohms to help pull down the screens a little to balance the higher impedance of the output transformer. I’ve never played a 2204 so I’m interested to see what it sounds like.
     
  2. South Park

    South Park Well-Known Member

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    The bias is the what you have to work out . You need to in crease the bias amps so the power tubes will work right you also will be dropping the preamp voltage .
     
  3. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Be careful with voltage and current drive levels to the output tubes. If the OT is underrated it'll blow up. If you demand too much of the PT it'll burn up. You start with building a reduced power amp section with appropriate power tube selection and then you refer to the tube manual that gives you the tube charts for your selected tube types. The charts will tell you what you want for plate voltage and bias current to stay within the power rating you want.

    Figure your power supply losses to the choke and rectification and spec the PT to deliver adequate voltage and current, and build your power supply voltage dropper network appropriately.

    There's a lot more to a reduced power version of a classic amp than just swapping out the transformers. You've got to design the power supply and drive the tubes correctly too.
     
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  4. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    This amp should be similar to the new model SC20 but with a little more power. Kinda middle of the road between the 50 watt and the 20 watt. I’ve been building transformers for a little while and having good success. The balance of current supply from the power transformer and current demand from the output transformer impedance is being followed. Also the wire gauge used has more than enough current capacity. I’m not worried about the transformers. The 315 volt secondary is unloaded voltage. It should pull down to about 300 or 295 which is going to be what I’m shooting for and it’s fixed adjustable bias. I’m going to try out different power tubes, EL34’s, 6L6GC, 5881 and KT66. All these tubes will work around the voltage and impedance.
    Something I’m seeing on different clone schematics is the omission of the 1000pF bright cap and the addition of a 100pF from pin 6 to pin 8 on V1b. My guess is that this is being done to reduce hiss and aggression. I’ll have to test it both ways and see how it performs. I can put the bright cap on a pull pot to give the option of with/without but the 100pF I don’t know. upload_2021-10-17_7-13-39.jpeg
     
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  5. neikeel

    neikeel Well-Known Member

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    That is how this started:
    [​IMG]
    [​IMG]
    I have not wound a PT (or any transformer for that matter) so the PT I chose was from a Quad II which has 300-0-300 HT. I used an old hi-fi OT with 4k (IIRC) primary. I think it worked well with EL37s.
    Look forward to hearing how your 2204 turns out.
     
  6. Gene Ballzz

    Gene Ballzz Well-Known Member

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    I really love how folks are endeavoring to embrace and advance the concept of getting "BIG SOUND" without the need for big watts and/or big sound pressure level. I believe this to be the new wave of the future of great fun and sounds in guitar amplification! You guys are fantastic!

    @neikeel Is that a rectifier tube I see tucked into the corner near the fuse holder?

    Great Innovators!
    Gene
     
  7. neikeel

    neikeel Well-Known Member

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    Yes it is I used a GZ34, I had contemplated a less efficient rectifier too but this one seemed just right so I left it.
     
  8. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    Just got the power transformer out of the varnish. Tested no load voltages. It’s got 314.2-0-314.3 on the HT and 6.58 on the heater winding. I’d say success. Going to dip it one more time for good measure and let it dry. Just got the board turrets installed and chassis drilled for everything except the transformers too.
     
  9. StingRay85

    StingRay85 Well-Known Member

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    Can't wait to find out how good this one will sound
     
  10. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    I’m curious how the power transformer is going to take to loading. I’ve never used M6 laminations. In my 1202-118 winds I’m using M19. I get a good sag of about 60 volts for a 350-0-350 and 70 volts for a 362-0-362. The 362 idles at 480 volts with EL34’s. I don’t want more than about 410 volts for this build but want it springy like the big transformers.
     
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  11. 2L man

    2L man Active Member

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    You can increase sag and decrease voltage installing current limiter resistors to rectifier tube anodes. Tube datasheet list them for several voltages ususlly next after max filter cap value is. I have used double resistor values and like the effect. They also increase tube life span and reliability. Current transformers seem to sag less than vintage PT s did. Also installing SS diodes to anodes is good practice.
     
  12. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    This amp will have solid state rectification. The two previous examples are also solid state rectifiers. The sag in those examples are purely from the transformers. Those transformers are EI 150 cores with 2 inch stack. This transformer is EI 125 core with 1.6 inch stack but it’s M6 steel vs M19 steel.
     
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  13. _Steve

    _Steve Well-Known Member

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    Hi William, is there a general rule with the size of the core/stack versus amount of voltage sag? Does the number of winding turns/guage play any role also?

    Loving this thread BTW!
     
  14. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    Everything plays into the sag. Having a large core increases the magnetic efficiency and allows a larger winding window for the copper mass. Everything in the copper mass is a balance. The total copper mass of the primary is ideally set to be equal to the total copper mass of the secondary. The wire gauge sets the current capability of the winding and luckily there’s enough space to get a significant amount of turns to fill the window. The heaters, to me set up the turns ratio because they have so few turns and the largest wire. I use 14 turns of 14 gauge wire for the 6.3. It makes a final layer on the outside of the core. There’s just enough room next to it for 11 turns of 16 gauge if a 5 volt heater is installed. I use a paper core tube to start. Every layer of copper wire is separated by a single .003 paper wrap that becomes an insulator once it’s impregnated with varnish. I have a vacuum chamber that I put the dry wound core in and vacuum it for 15 minutes to dry it out first. I then submerge it in the varnish and vacuum it again for 5-10 minutes. I release the vacuum to fill all the space with varnish. I re vacuum it 2 more times and then seal it up in the varnish for about 3 days. After the soak, I pull it out and stack the laminations. I dip each of the lamination plates to ensure full coverage of varnish. It’s hard to get the full stack in place because of the varnish layer but I pack them in. I hammer a hardwood shim in last to load the stack and core. Then I put it back in the varnish for 2 more days. They have to dry for about a week after they come out of varnish. I spoke to Chris Merren for a while about the varnish process and now I know why it takes so long to get transformers.
     
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  15. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    I would like to add the pros and cons of building transformers. The pros are short. I get the ability to set my voltages, current capacity and impedance. I don’t have to wait for them to get built and shipped. There’s a sense of pride being able to successfully build the transformers for my amps, it’s been one of my goals from the beginning.
    Cons: steel is expensive and very hard to get in small quantity. My time is worth x amount and it’s cheaper to buy transformers from other sources. Lots of math involved to predict output. You have to measure twice maybe three times before you cut when designing transformers. It sucks when a core is bigger than the lamination, 6-8 hours wasted.
    Like I said, I always wanted to make the transformers and the local people that critique my amps love them. We’ll see how this one turns out.
     
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  16. printer

    printer Active Member

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    Do you interleave the output transformer?
     
  17. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    Yes, this output transformer is wound with the same interleaving pattern as a Drake 784-139 that’s used in a 50 watt Marshall. It has more primary turns to achieve the 5.7k impedance but the secondary is identical.
     
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  18. william vogel

    william vogel Well-Known Member Gold Supporting Member

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  19. Gene Ballzz

    Gene Ballzz Well-Known Member

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    @william vogel
    > Whose chassis did you start with? Aluminum, yes?
    > On what side of the pond are you located?
    > You do excellent work!
    Just Askin'/Sayin'
    Gene
     
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  20. william vogel

    william vogel Well-Known Member Gold Supporting Member

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    I got the blank aluminum chassis from Tube Amp doctor hoping the corners were welded like their 100 watt blank chassis. The smaller 50 watt, jtm45 etc chassis I got wasn’t welded, oh well. I’m in Leesburg Georgia USA. I don’t like having to drill and punch everything but with the transformers I built, I had to do it because I’m not having a Swiss cheese chassis. You might have noticed I’m using all stainless steel hardware also, the brass stuff looks great but I like stainless steel because I can torque it down harder and not break it. On my vintage Plexi builds, I always use brass for the patina but this one is custom. I got a lot of stuff wired tonight. I really hope I like the 2204 circuit. I’ve built 3 Trainwreck Express amps and believe this should be similar gain wise and probably a bit fuller overall. Thanks for the compliment Gene!
     
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