5F1 Modification: Questions and Advice

Discussion in 'Shock Brother's DIY Amps' started by bad_rudy, Nov 4, 2019.

  1. bad_rudy

    bad_rudy TDPRI Member

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    Hello All,
    Several years ago I completed a 5f1 build in part with the advice / help of some folks here. I really quite enjoyed it, and have been using it exclusively (other than a pignose I keep in my office at work) since it was completed. However, I have also been itching since that time for another project amp, but life has not generally been particularly friendly to that goal (live in an apartment, have a 2-year old, money tight, etc). In addition, I have also found myself occasionally wishing that the amp had a few more options for tone given that it is my only rig and I would rather not have a cumbersome pedal board. Had various mods in mind, but as is commonly noted the form factor of the 5f1 isn't very forgiving on space.

    So, I have decided to gut and rebuild the amp to address these problems and also clean up a lot of beginner sloppiness that has been bothering me from the beginning. My plan is to basically modify it to more of a 5f2a, but with an added tremolo circuit, attenuator (I`m exclusively a bedroom player, so getting some crunch without one is a no go), and switchable NFB. I am making a chassis that is copied from the ga-5 skylark, in part because I like the look of it but more so because it is accessible to me with the tools/resources I have available. I`ll also make the cab as I did with the original build. I know the 5f2a had a 10" speaker, but I am planning to stick with the 8" weber sig8 from my 5f1. The goal here is three-hold: 1) a new project to work on; 2) expand the tonal options of the amp, as I am a bit of a minimalist gear wise and this is my only rig; and 3) provide a platform that is amenable to later tinkering and modification. However, I wanted to ask a few specific questions before starting, and also solicit input from the community about what other changes might be good ideas.

    Circuit: My understanding is that the principal circuitry differences between the 5f1 and 5f2a are the tone pot and the addition of a second 16uf filter cap. My plan is to simply copy those verbatim, but someone please tell me if I have overlooked some detail.

    Tremolo: I am planning to copy the tremolo circuit from the aa764 vibrochamp and splice it into my 5f1/5f2a circuit. The aa764 overall schematic appears very similar, and I can see where the tremolo enters it on the cathode of v1b. However, the aa764 has an extra 47 ohm resistor after the "normal" 1500 ohm resistor heading into the ground. In addition, there appears to also be a cathode bypass cap on v1b, which is not present in the schematic of either the 5f1 or 5f2a. So, I`m curious what the origin of these differences is, and are they required to make the tremolo work with the 5f1/5f2a circuit?

    Also, if I use a pot with on/off functionality similar to the volume control of the 5f1 for the intensity control of the tremolo, will that effectively cut it out of the signal path when in the "off" position, or is some other mechanism required to isolate the circuit?

    Suggestions: I would happily take any other suggestions regarding what other modifications might provide the most "flexibility" in the amp. The chassis will have plenty of space for other things, although I of course want to strike a balance between flexibility and complexity.
     
  2. King Fan

    King Fan Friend of Leo's

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    I haven't thought about the diffs for a while, but IIRC in addition to the tone pot, bright cap, tone cap, and extra 16uF filter, and partly depending on how you built the 5F1, the 5f2a adds the 25uF bypass cap on V1a and uses a bigger 15W OT. People also often add a one-switch or two-switch tone-circuit defeat to get back to the classic Champ sound.

    The right amount of NFB cut on these guys needs some research -- and some personal listening tests. A pot instead of a resistor would be a smart start. If you found a resistance setting you really liked, you could replace it with a resistor.

    I'll leave the trem part for the smart kids who understand trem. :)

    Finally, I know you're looking for low volume, but I'd really reconsider and look at a low-efficiency 10. Weber 10a100T is ideal and very vintage -- or a Weber Sig 10A if $ is really tight. Why? Well, if you want a more diverse sound palette, adding those 2" plus the tone circuit will give you a *lot* more sonic options.
     
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  3. muchxs

    muchxs Doctor of Teleocity

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    It's like trying to hot rod the family car. What are you gonna drive while you're building it up to a street and strip monster?

    I'd suggest a junk box build on a Zackman Vibro Champ chassis.

    http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=223351604508

    The trem is footswitchable.

    A Vibro Champ can be modified to do just about anything aside from giving you a foot massage. It may be the wrong tool to bring to a shouting match with a Marshall stack.
     
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  4. muchxs

    muchxs Doctor of Teleocity

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    Cue Mad Scientist insane laugh sound effect. :cool:

    Zero NFB is a good starting point.

    I always leave the NFB disconnected on my SE builds. That way I don't get a herd of hungry piglets squealing urgently on start- up.

    Grandma's old electronic organ is the start of any well stocked junk box. I view antique organs as well stocked junk boxes in general. Get the organ for free, you get a couple free musical instrument speakers with it. Just don't be cuttin' up tonewheel Hammonds although I understand some people do.

    Free is cheaper than inexpensive every time.

    :) :) :)
     
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  5. King Fan

    King Fan Friend of Leo's

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    For sure, and I shoulda said (and built) a 3-way switch including a full cut. OTOH like the OP I wanted a housebroken beast, and the small cut I ended up with (I forget) kicks it up just a single notch to get that great edge-of-breakup sound at 6 on the dial instead of 7.

    Perfect for my little music room -- but that's in my big ol' almost-soundproof house. YMMV. :D
     
  6. bad_rudy

    bad_rudy TDPRI Member

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    Thanks for replies so far. I can certainly consider a 10". Yes, volume is a concern but as I said I am planning on an attenuator anyways so presumably I can tame it down to a level that is more friendly.

    My plan for the NFB switch was to basically be an off vs on type of deal. My circuit is built from the tube depot plans so it actually already has a variable NFB adjust on it. I would just like an option to disable it altogether readily accessible on the control panel.

    Re; hot-rodding the family car. Too late! I've already gutted it, so the family has to wait until it gets put back together ;). I do like the idea of a junkbox build, and it seems like that could be my route toward a new project down the road. For now, its time to get the mainstay back online. One thing you have me thinking about though is maybe I would be better off just saving the current circuit board and actually just building the vibrochamp circuit from scatch. That would certainly solve the problem of wondering how easily the vibrochamp tremolo would splice into the 5f2a. It seems like I could use all of my current hardware, and its not like the various board components or pots are terribly expensive.

    re: junkbox builds in general. Where do you target for the old gear? My grandmother does not own an organ that I am aware of...
     
  7. muchxs

    muchxs Doctor of Teleocity

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    I looked at the Madison Craigslist. It's a wasteland for old organs. Lotsa solid state junk. You don't want to bring those back to an apartment.

    You might need to drive to Milwaukee for good junk.

    https://milwaukee.craigslist.org/atq/d/milwaukee-vintage-dynavox-phonograph/6997475023.html

    Don't know what's in there. If you get to Milwaukee and see a power transformer, an output transformer and a couple tubes you pretty much have your amp.

    https://milwaukee.craigslist.org/atq/d/east-troy-record-player/7011965567.html

    I don't claim to be familiar with every old record player console, just a few of the good ones. The usual deal is look for tubes.

    These old Magnavox consoles often contain enough parts for several amp builds.

    Ask the owner if they have sentimental attachment to the console. If not, strip out the speakers and the amp. Smash the cabinet flat.

    There's that nice rest area on the way back into Madison, the one with the old fire tower. You could start a cheerful campfire to get rid of the wood.
     
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  8. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    Junkbox build sources.
    • Old Hi-Fi/Radiograms can provide valves and transformers.
    • Old PA and school tannoys.
    • Ham radio meets. Generally nice, approachable, knowledgable folk too.
    • Recycling centres. Get to know the staff, be prepared to take some sow's ears before the silk purse.
    • Old CRT televisions. The ones based around an 'entertainment console' type are a goldmine!
    Small ads, house clearances etc can yield some gems. Local charity shops ( thrift stores ) sometimes get old stuff they cannot handle/resell. Talk to them, get known. Think outside the default 12AX7 for the preamp. How about a 6f1p Pentode/Triode, or an EF86, or a pair of 6AU6, or a Soviet 6n2p-EV instead?. Why the endless EL84/6V6 for low volume amps?, why not an 832 in push/pull or parallel single ended?, or the 6p1p-EV, a small bottle variant of the 6V6. There are even single Triode/Tetrode ( triode first gain stage feeding a tetrode power stage ) that have been mantled into guitar amplifiers. You may need to buy an output transformer or two, some sockets, caps, resistors and pots, but with a little imagination, you can build some extraordinarily fun little noise makers.
    Baking tins, biscuit tins, ammo boxes etc have all been pressed into service at one time or another. Old radios look spectacular when converted to guitar use. My next project, when I finish the three I've currently got on the go...
    ps. Tone controls are for wusses :)
     
  9. old wrench

    old wrench Tele-Holic

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    Concerning those old CRT televisions - :)

    Educate yourself thoroughly regarding their components before poking around in them ;).

    I'll never forget the time (about 40 years or so ago) when I removed the protective cover from that big capacitor and put myself in the ground path when it discharged.

    Holy **** !!!

    g
     
  10. Phrygian77

    Phrygian77 Tele-Afflicted

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    I just finished up my second Vibro Champ, this one with an Eminence 1028k which is 97dB. 4-5 watts with a 97dB speaker is still very loud for a bedroom.

    I highly recommend an Eminence Maverick over an attenuator if you can afford it. I did some testing with the new Vibro Champ build and the Maverick that I have in my blackface Bassman combo. It's a much more reasonable bedroom level with the Maverick turned all the way down to 92dB.
     
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  11. Mongo Park

    Mongo Park Tele-Holic

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    Here is one in Broadhead which is not that far, it has a killer transformer and a op Transformer. I can't see what the tubes are you could ask for a better photo. With luck a 6V6 and a 5Y3 and it is worth the trip.
    Muchxs is right with a little searching you can save a fair bit. Some mighty nice iron can be had for little money and it is kinda fun picking these things up. About once a week I search adds for old radios and have quite a collection of transformers for that next build. An old radio transformer is perfect for your type of build.

    https://madison.craigslist.org/atq/d/brodhead-antique-radio-turntables-with/7006318524.html

    Cheers Ron.
     
  12. Uncle Daddy

    Uncle Daddy Tele-Holic

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    I built a 5f1 with an L-pad attenuator between the output and the speaker. Worked really well.
     
  13. bad_rudy

    bad_rudy TDPRI Member

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    This was eye opening. Lots of emails out to craigslist ads. Suspect my wife will grow to dislike all of you strongly soon...
     
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  14. bad_rudy

    bad_rudy TDPRI Member

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    Picked up an interesting old magnavox console pretty cheap. Its a "berkeley" model with a radio chassis marked "cr-198b". Fully functional. Some poor research had me under the impression it ran on a pair of 6V6's in push pull, but it turned out to be SL6's in push pull with a 5U4G rectifier. Also many other tubes associated with the radio that I do not recognize and assume are of little utility to me (6J5, 6SN7GT, 6U5, 6SK7, 6SA7). My understanding is it is rated at 20 watts of output. The PT is massive (at least to my little 5 watt'er attuned eyes). Having trouble finding specs on the PT and OT, so I`m not sure what they are rated and as a result its entirely unclear to me if I can use them for anything I might want. Has two 12" field-coil speakers (I know nothing about these, assuming not great for guitar amps as I never see them in use).

    So, I have no idea if this will turn out to have been useful, but as I said it was quite cheap and I got to have a fun conversation with an old enthusiast. Fun experience either way.
     
  15. Nickfl

    Nickfl Tele-Afflicted

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    My first step with something like that is to pull out the transformers and test them. The PT voltages may be pretty high if it was driving field coil speakers. Start by identifying and labeling all the transformer windings and then you can apply power to the primary and see what your unloaded secondary voltages are, which will give you a starting point.

    Here is a good article on testing unknown transformers:

    http://www.angelfire.com/electronic/funwithtubes/Transformer_Lead_Colors.HTML

    And another one on output transformers:

    https://www.radioremembered.org/outimp.htm

    Once you have some your transformer specs, you can start to figure out what you can potentially build with them.
     
  16. Mongo Park

    Mongo Park Tele-Holic

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    Label the wires before you cut them it makes it easier. Look where they go to the tubes and you can figure out the heaters and the high voltage and any center taps. I searched Google but nothing comes up for SL6 tubes.
    Here is the pin out for a 5U4G rectifier tube so you know what transformer wires are going to it. This tube and a 5Y3 have the same filament heater Voltage so that is a good thing.

    https://www.radiomuseum.org/tubes/tube_5u4gb.html
    6SN7 is a amp worthy tube.
    Here is a starter video from Uncle Doug as a starter on transformers


    Cheers Ron
     
    Last edited: Nov 5, 2019
  17. bad_rudy

    bad_rudy TDPRI Member

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    Sorry that was a typo. The tubes are 6L6GT's, so pretty well known but I am most familiar with seeing them in amps much larger than what are on my radar for the next build.
     
  18. Nickfl

    Nickfl Tele-Afflicted

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    That's a good find, should be a nice beefy set of transformers! Definitely not appropriate for a 5f2 style build though. I'd save these parts for a bassman/ marshall or something like that and keep looking for small amp parts.

    In my experience most old radio consoles were single ended 6v6 units and you are likely to run across something like that sooner rather than later.

    Also are those 6l6 gc, gb, or metal 6l6 tubes? Big difference between gc and the others. I wouldn't expect 6l6gc in a unit old enough to have field coil speakers.
     
    Last edited: Nov 5, 2019
  19. Mongo Park

    Mongo Park Tele-Holic

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    Well go figure your score is so good it is not worth wasting on a Champ, go figure. Yes the op transformer is not for single end Champ. You could re sell the transformers and tubes as guitar worthy if you are not going to build something bigger, trading up so to speak.
    Still worth figuring out the transformer as practice and helps for resale. Do you have a walwort AKA poor mans Variac:
    [​IMG]
    If not they are easy to get at the second hand store. This will power your Transformer at low Voltages and you can measure the wires on the other side to see what they are putting out multiply the voltages to get the 120 Voltages. So if your wall wort is 12V into the transformer and you get 37 volts out on one of the red wires. 12 to 120 is 10 times, so multiply 37X10= 370Volts, perfect for a tube amp.
    The miliamps is another thing but any radio with 5 or 6 tubes will run a champ no problem.

    Cheers Ron
     
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  20. bad_rudy

    bad_rudy TDPRI Member

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    No, but I do have about 14 real variacs ;) I recently inherited an experimental apparatus at work that had a bank of them. Never the less, great idea for determining PT levels. Had not occurred yet.
     
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