5F2a: this cant be right...

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Jun 2, 2020.

  1. Jewellworks

    Jewellworks TDPRI Member

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    ive posted about this lil amp before, but once again, i ask you pardon my ignorance...

    after some experimenting, ive reverted it back to the basic 5F2a circuit, with the exception of 2 things:
    ive up'd the filter caps to 22/10/8/8 uf, and in place of the 220k ohm grid leak resistor in front of the 6V6, i added a 250k volume pot.

    heres what im running into... if i have my 1 knob tone control in the front (between V1a and V1b) then i get the treble peak i expect. but if i move it to the rear, in front of the 6V6, then the treble peak doesnt work. ive gone so far as to disconnect the .005 cap just to make sure im not hearing things, and it doesnt work in that position. it doesnt make sence to me. its wired exactly the same.

    the simple answer is just leave it in the front. but i have other plans for this amp, and i really want to move it to the rear. so why cant i get it to work in that position.

    and frankly, im really surprised by how much gain and overdrive this lil amp has. did Leo really intend for an overdive monster like this? as it is, im getting either a lot of blocking distortion with the gain full up (and in the bridge position only), or getting a ton of amp sag. i really dont know the difference. as an experiment, i (temporarily) put in a .01uf coupling cap after V1a, thinking there just too much bottom end coming through as well as a 33k Grid Stop before V1b. ive read that helps with blocking distortion, but i still get a nasty volume drop and "bloom". it didnt make much difference with those (temp) tweaks anyway...

    any help or direction with these 2 thimgs would be MOST helpful.

    thanx again
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    It's not clear to me what the circuit looks like when your tone control does not work as expected. I can imagine that you are running into impedance problems. The tone control has two 1Meg pots in it. The grid leaks are 220k. Assuming ("Assumption is the mother of all ****-ups") your circuit is as I think is it, you might have a high impedance output driving a 'low' impedance input. That is asking for problems.
     
  3. Jewellworks

    Jewellworks TDPRI Member

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    20200603_081142_1591186429548.jpg

    This is what's in here right now.
    With the tone control in the rear, I get no treble control. (high pass, treble peak) Not at any volume or any gain. If I move this to the front, the treble works. I can use it as a distortion control.
    As for blocking distortion or amp sag, if I turn up everything all the way, and switch to my bridge p'up, I get a volume drop, and as it sustains, it gets louder. If I turn the Volume down (Gain still up) then I dont get the drop. Which tells me the problem is at the rear, not the front.

    My "plans" for this amp are to add another tube. After the 12ax7 in this circuit, to a cold clipper then cathode follower feeding the volume and tone controls to the 6V6. Basically a JCM800 preamp. And have a switch to flip between the 2 preamp circuits. This is why I really want to get this tone control to work in the rear of the amp. I have a very small chassis and don't have enough room for any more than the 3 pots I already have.

    It seems to me this should work as-is. It's frustrating
     
  4. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Seems wrong to me in more than 1 way.

    Coupling cap seems missing. Now dc is present at the grid.

    High is dumped to ground with the tone knob turned one way. OK. But normally with the knob turned the other way, the highs bypass a part of the other pot. Here they bypass a wire. Not much bypass needed...

    The volume pot takes the role as grid leak. I can imagine that is also not what you want.
     
    Last edited: Jun 3, 2020
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  5. tubegeek

    tubegeek Tele-Afflicted

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    Yeah, your power tube grid is DC coupled to the driver tube in front of it. That ain't right.

    What is written next to the pot between the first two tubes?

    I think this is more likely to work but I'd expect wicked interaction between the two controls. At least you would have 1) a DC blocking cap between the driver plate and the output tube's grid (the top cap in the tone control) and 2) a reference from output grid to ground (the resistance of the volume control).

    But you have an arrangement where either of two pot wipers failing open-circuit would lose your ground reference!

    5f2a mistake copy.jpg
     
    Last edited: Jun 3, 2020
  6. Jewellworks

    Jewellworks TDPRI Member

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    damn. i forgot to draw the coupling cap in. trust me, its there .02uf

    the first pot between the tubes is GAIN
     
  7. Jewellworks

    Jewellworks TDPRI Member

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    i looked at that too, but if you were to put the same circuit at the front, before the Gain pot, and turn the gain up all the way, it still acts like a "wire bypass". cept i actually get a treble peak effect. im failing to see the difference

    i dont? even with the Volume up all the way, i get 220K to ground. if i turn it down, im sending more to ground and lowering the volume. is that a problem? isnt that how its supposed to work? its basically exactly what the Gain pot is doing...
     
  8. Jewellworks

    Jewellworks TDPRI Member

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    View attachment 727777

    more accurate drawing with the .02 Coupling Cap after V1b. its there, just drew it too quickly this morning before work...

    Tweed Tone.jpg

    if you turn the 1M VOLUME up all the way, it in effect, bypasses the pot, and all your left with is a wire. turn the tone up all the way, and it bypasses the 1M TONE, and passes through the 500pf cap. right? yet i still get a treble peak when this circuit is in the front. if i move this to the back, i dont get the treble peak. i have the Volume wired slightly different, and ive tried wiring it exactly like this, but it doesnt seem to matter. i dont get the treble peak when its at the rear.
     

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    Last edited: Jun 3, 2020
  9. tubegeek

    tubegeek Tele-Afflicted

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    No - the gain pot is working as a voltage divider, choosing between full signal (at the "up" end of the pot) or zero signal (at the ground end of the pot.) The control you have doing the volume now is working as a variable resistor ("rheostat" connection) which is essentially just loading down the tone network with something between 250K and 0 ohms.

    The reason @Tom Kamphuys is saying you wouldn't want the volume rheostat as the grid leak - I'm guessing - is that failing-wiper-open condition I mentioned above. That could be avoided by adding a safety resistor of 500K grid-to-ground. 500K is the max grid R for 6V6 in cathode bias from the data sheet. Since it'd be more-or-less out of circuit (bypassed by the 250K volume pot) unless there's a fault, you could probably get away with 2.2Meg there and that would load down the tone network a bit less and reduce the interaction factor. But I think you'll still get a lot of interaction.
     
    Last edited: Jun 3, 2020
  10. tubegeek

    tubegeek Tele-Afflicted

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    I'm thinking the difference has something to do with a) the rheostat connection instead of divider, and 2) 250K vs. 1M. Either or both.

    That drawing shows that, with the volume up all the way, with the tone towards the .0047 cap, you'd have .0047 bypassing the volume control to ground. Highs rolled off. With the tone at the 500pF end, you have no effect until you turn the volume down, and then you'd get a treble bleed, maintaining highs as the volume is reduced. In between, in between.

    You probably switched to 250K to avoid exceeding the data sheet value? See what happens with the divider connection as shown in the drawing, and 500K instead of 250K, that's an allowable value.
     
  11. Jewellworks

    Jewellworks TDPRI Member

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    @tubegeek , @Tom Kamphuys ,
    ok, i understand the safety Grid Leak resistor. i can add a 500k tonight. but im still baffled by the tone control.
    mind you, im not trying to argue, but to understand... that said...
    if i wire the Volume Pot EXACTLY like i have the Gain, (not the Variable Resistor/rheostat) i STILL dont get the treble peak to work at the rear. im also failing to see how the Volume Pot ISNT a voltage divider, (full signal (at the "up" end of the pot) or zero signal (at the ground end of the pot.) taking 250K to ground, no matter what. -clearly im missing something.
    i can try it again tonight, but i know ive been down this road before...
     
  12. tubegeek

    tubegeek Tele-Afflicted

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    I think the problem is the 250K is too small a resistance for the tone network to work into, and then when you lower the volume, it gets even smaller. If you take the signal off the volume wiper, you are now better able to control the load on the tone network and hopefully raise it high enough to allow it to work properly. You're basically adding a too-small resistance to ground in the middle of the tone network (off the tone wiper) and throwing off its behavior. I'll scan a sketch in a bit when I get a chance.
     
  13. Jewellworks

    Jewellworks TDPRI Member

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    i have a 250k in there because thats what was on the original 5F2a schematic. (it actually shows a 220k, and if i measure my Volume pot, it comes out to EXACTLY 220k. -no other reason
     
  14. Mongo Park

    Mongo Park Tele-Holic

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    It may be easier to build something closer to what you want than try and cut and paste different parts of amps together. Which people do but as you can see it takes some work to get the parts to match together properly
    From what you described as the final amp maybe the swamp thing might come close to what you want. Maybe have a look and see if it is something that might give you the drive you want.
    http://www.cs.cmu.edu/~ehn/ax84/schematics/Visio-Broiler2.pdf
    from what you said this amp has a cathode follower, drive AKA master volume, tone stack just before the power tube a extra gain tube, and a Marshall like preamp tube. Single end as the 5f2a is.
    it would be way easier to build since you are hitting the wall with the 5f2a and you got a ways to go. Just a idea to consider, or cary on and you will learn a lot along the way.
     
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  15. Jewellworks

    Jewellworks TDPRI Member

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    @Mongo Park
    thanx. its always great to look at other circuits to see how "they" do it, but as ive previously stated, i have a very small chassis and i dont have enough room for any more pots, so the TMB tone stack is not happening. all i really want is a single knob tone control.
    and if i REALLY had my way, id get the treble side to work as an actual hi-pass/Low CUT like its supposed too. ive tried and failed to get an actual low end roll off and thin the sound, especially at higher volumes. i know i can get the treble peak to work, but only in the front.
    and this brings be to 2 points of confusion:
    the cathode follower is a LOW impedance feed to the tone stack, correct? (im still baffled by what is high and what is low, and how to spot the difference, and how to measure the 2, and why its "better that way")
    if im not feeding a full TMB tone stack, and have a single pot for the tone, do i still need the Cathode follower?
    i dont want to get too side-tracked here, but ive got so many questions...

    and for a little background, im a guitarist and former recording engineer with a strong background in Audio. ive made a living as a pro AV installation tech, for permanent installs of TV and Radio and pro AV systems. i know "equipment" but havent gotten into circuits and components very much. so much of the reading i do on amp circuits and theory, is another language. im thrilled my lil amp works at all and sounds as good as it does, and that i havent set fire to anything or electrocuted myself in the process. that said, i love the midrange "honk" i get, but i also know it can sound better. i want to use this amp as a platform for learning, as reading is just reading.
     
    Last edited: Jun 3, 2020
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  16. tubegeek

    tubegeek Tele-Afflicted

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    You may have to experiment with the .0047 cap value (and in conjunction, the 500 pF) to get it to do more of what you want. Try 1/10 and 10x, that is, .000470 pF (with that, I'd swap the 500pF for 50 pF) and .047. See if either works more how you want it and then tweak from there.

    The different resistances in your modded version change the frequency at which the control works. If you're turning up & down the treble above 10K, for example, you'll never know it.

    Same with increasing/decreasing the bass below 100 Hz. Nothing there to increase/decrease, nothing to hear.

    That tone control is a little tricky to figure out to begin with - it'd be great to run a simulation on it and then you could see the EQ curves change.


    GOOD NEWS: this circuit is built into the Tone Stack Calculator program!

    Downloadable software or Java app in the browser:

    http://www.duncanamps.com/tsc/

    https://www.guitarscience.net/tsc/info.htm

    This is a variant of the Princeton, I started there and changed values to match what you've got. Then I messed with the .0047 (C2 in the sim.)

    I just f'ed around with the browser version. It looks like .01 or .022 instead of .0047 is what you want, but check it out!
     
    Last edited: Jun 3, 2020
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  17. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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  18. Jewellworks

    Jewellworks TDPRI Member

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    i think youve recomended it to me 3 times now. i owe you $3 ;)

    my birthday is coming up. its on my list
     
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  19. Jewellworks

    Jewellworks TDPRI Member

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    i didnt see the Princeton Tone control in the Duncan TSC, but its on the web browser version. this is SO MUCH EASIER! i can SEE what its doing! thanx @tubegeek ! youve been very helpful! over the top-out of your way helpful!

    that said... im confused by some of the values im seeing. i get the RV and RT and Cap values, but what is RIN? it comes up as 38k. where do they get that? and on the Big Muff circuit (something else i tried and failed to get working) their RIN value is 1K. where are they getting these values?
     
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  20. tubegeek

    tubegeek Tele-Afflicted

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    38K for Rin is derived from the source impedance of the circuit driving the "input" point. It's the rp value of the tube (there's a correction for the cathode resistor when it's unbypassed) in parallel with its 100K plate load resistor.

    The values in the online simulator are straight out of the schematics, I guess that's what a big muff has - it's probably a transistor in a follower configuration. Impedances in transistor circuits are often 10x to 100x smaller than tube circuits.
     
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