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Thread: New build 5F1 & AA764 champ, 2 in 1

  1. #36
    Bent Member Chuck H's Avatar
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    Not sure if this will be anything like Robinette's description (I didn't look ). But here is a (very) simplified description of the standard TMB tone stack we all know. And it may not seem like a simple description either. But read it twice and most will sink in. Also download Duncan TSC. It's a great tool and playing with it will give you great insight into how the tone stack works.

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

    EDIT: Opening the attachment at the bottom will make it easier to read the text.

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    Last edited by Chuck H; 01-13-2020 at 04:28 PM.
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  2. #37
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    Nevermind my last post, I'm still figuring it out.

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  3. #38
    Bent Member Chuck H's Avatar
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    Ah! I think I see now where the confusion might be. Are you interpreting the physically lowest cap in the diagram as the bass cap? In order from top to bottom on the diagram it is treble, bass, middle. If you look at the operation description I posted it makes this clear.

    I'm working something up for you now to show both incarnations (stock and modified) in operation.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Chuck H View Post
    Ah! I think I see now where the confusion might be. Are you interpreting the physically lowest cap in the diagram as the bass cap? In order from top to bottom on the diagram it is treble, bass, middle. If you look at the operation description I posted it makes this clear.

    I'm working something up for you now to show both incarnations (stock and modified) in operation.
    Exactly, sorry, I deleted my post to avoid confusion among other members who might be reading this thread because I figured that out and calculating the filters again from scratch.

    EDIT: I give up, because I've done again the calculation but forgot that the treble pot, bass variable resistor and the mid fixed resistors are all in series and the 100k slope resistor is in parallel with the treble pot and in series with the bass VR and mid resistor.

    Need to take a break.
    Hi-Fi amps tone stacks (when present) are so much more straightforward, sigh.

    Here all the controls are interactive with one another and thank god almighty at least the mid pot is a fixed resistor in the Champ schematics. How the hell does it interact with the others? Doesn't it work just like a general "boost"?
    I might be talking out of my ass at this moment, I don't know why I'm having so much issues understanding this circuit.

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    Last edited by donluca; 01-13-2020 at 06:44 PM.

  5. #40
    Bent Member Chuck H's Avatar
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    It's because of all the variables and interactions, with all three frequency bands in the same passive circuit, that it's much easier to model this sort of thing with Spice.

    This may not be your tone stack values, but it demonstrates that it works and there isn't any significant difference in performance.

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    Last edited by Chuck H; 01-13-2020 at 08:29 PM.
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  6. #41
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    Nice!

    I mean, I'd love to understand why the results are similar, but I guess that for now that'll do.
    Ironically, I've been using LTSpice for years (albeit it's been a while since I last used it), don't know why sometimes I still stubbornly decide to do things with pen and paper, the old fashioned way.

    I'll definitely import the tonestack on LTSpice, put the correct values in and then play a bit with them to make your solution behave as close as possible to "the real thing" and then post results here.
    I've found other threads discussing the topic of taking a blackface AA764 and make a mod to switch back and forth from the AA764 circuit to a 5F1 with the tonestack bypassed.

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  7. #42
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by donluca View Post
    I'll definitely import the tonestack on LTSpice, put the correct values in and then play a bit with them to make your solution behave as close as possible to "the real thing"...
    That's something I've done for myself on another project. The trick is finding a way to get exactly equivalent performance with standard values! And then there's the reality that the amp is going to be modified anyway and the existing values may no longer be ideal. And that they aren't going to be the same as any vintage amps anyway. Not to mention pot tapers. I'd probably just roll with the modification for now without refinement and then tweak values by ear and tapers for aesthetics (bass on 5 rather than 7 for example). JM2C from a guy that's done a lot of this.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Today I was wondering... how accurate are Fender's schematics regarding voltages and component values?

    I've adjusted the B+ voltage and kept plate and cathode resistors as per AA764 schematic and on the V1A cathode I get 1.385V (instead of 1.8V) and 1.42V on V2A (instead of 1.7V).

    All the components in the circuit are the same (1.5k on cathodes bypassed by 22uf on first half of the 12AX7 and 2.2uF on the second half), same B+ voltage.

    In order to get in the 1.7V-1.8V ballpark as per schematic I should raise the cathode resistors to ~2k each.

    For reference:



    Does anyone here have an original Fender Blackface/Silverface Champ AA764?
    I know for sure that the voltage reported are lower than they should be now due to higher wall voltages, but still...

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  9. #44
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    Some original Fender schematics show a note stating voltages within +/-20%. So your cathodes voltages should roughly be within spec.
    Cathode voltage depends on tube variation (Gm), plate voltage and actual resistor value.

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    Last edited by Helmholtz; 01-19-2020 at 06:55 PM.
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  10. #45
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    They are at the lower extremes:

    1.8V can go from 1.44V up to 2.16V (I'm at 1.385V)
    1.7V can go from 1.36V up to 2.04V (I'm at 1.42V)

    Maybe I should bump up a bit the B+ in order to get those guys closer to the intended values.

    Not even sure if it would be worth it, honestly, if the ampbook's 12AX7 calculator is correct ( https://www.ampbooks.com/mobile/vacuum-tubes/12AX7/ ) the difference in headroom should be minimal.

    EDIT: and of course, then there's the Gm variable which I've not considered in my calculation. My modded VHT has its original chinese tubes which I've read around they're not particularly good.

    I'll probably scoop some nice NOS Soviet tubes like the 6n2P-EV and do a very minor rewire to have them working instead of spending big bucks on other 12AX7s.

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  11. #46
    Lifetime Member Enzo's Avatar
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    Hi don. About my earlier remark, I think you might have looked too deeply at it. I was only chiding you for saying you got "no resistance" between two points, and I said I think you mean infinite resistance. meaning "no resistance" means zero ohms - a dead short. Just a terminology nitpick.

    As to voltages, it says right on MANY Fender schematics, including the AA764 Champ: voltages read +/-20%

    Something I harp on a lot: these are just guitar amps, not precision equipment. If your 360v B+ winds up 345 or 370, the amp will still work just fine.

    Spice is a wonderful tool, but you can also to to Duncanamps and download the free "tone stack calculator", which is already loaded with a variety of tone stacks, and in real time you can adjust the tone controls and watch the effect on the freq response. It also allows you to change the component values and see what that does. it is a delightful little thing.

    Big can caps full of 20uf sections are common. 20uf (or 22uf) individual caps are common. The only place to find high voltage 8uf and 16uf caps is in the nostalgia guitar amp suppliers catalogs, and you pay extra for that. They used those values back when because caps were not cheap then. They saved money with 8uf over larger caps.

    CAthode voltages? 1.4v instead of 1.8v, and 1.4v instead of 1.7v? Oh geez, I wouldn't even think about it. Close enough. If you have 200v at the plate per the drawing, and the plate resistor measures exactly 100k, and the B+ node for that was 330v also on the drawing, then there is 1.3ma flowing through the resistor. That means through the tube too, and of course thus the 1.5k cathode resistance. I get 1.95v across the cathode resistor. SO right there the schematic disagrees with itself.

    In the second stage, I can pretty much ignore the 40-some extra ohms in the cathode (it represents maybe 3% of the 1500 ohms), and see with the same 100k load from the same B+ they show different numbers. And of course the 19v the 6V6 generates across its cathode 470 ohms is totally dependent upon the individual tube. Bottom line is that the EXACT voltages on the schematic don't matter, and changing 1.5k resistors to 2k resistors to chase after it is wasted energy. Fender voltages are ball park figures only. When you see 1.7v on a cathode it is there to tell you not to expect zero volts and not to expect 8 volts. A volt or two is fine.

    Look at the 10k B+ resistor. It has 20v dropped across it on the drawing. That means 2ma through it. But adding the two sides of the 12AX7 circuit we get more than 2ma. SO again, the schematic disagrees with itself - if we take it literally.

    Schematic also notes 10% tolerance resistors. Older models were made with 20% resistors. That means a 100k resistor can be anywhere from 80k to 120k and be on spec. 10%? that still means 90k to 110k.

    Just my opinion, of course.

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    Thanks Enzo, that's EXACTLY what I was thinking and I was going nuts because Ohm's law it's not an opinion, so I thought that either the voltages were just there as a guide or there was something up with the resistor values (or the current draw from the 12AX7).

    Also, thanks for confirming that the difference is not significant. For what is worth, when I changed a resistor to bring the B+ voltage down from 330V to 300V I couldn't really detect much difference in sound by ear (very un-scientific, I know) so I debated about mucking around the B+ and cathode voltages.

    The point is that I'm still waiting for several parts to come here from all over the world and I'm kinda bored, so I wanted to do some (small) tests and see their effects.

    Right now I have 300V B+ voltage, 200V on the plates and 1.385V and 1.42V on the cathodes, with 100k plate resistors, 1.5k cathode resistors bypassed by caps and 1M AC load.

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  13. #48
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    Did you measure the cathode resistors? Did you try different tubes?
    Some chinese ECC83s have very high Gm, which could be a reason.

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    Cathode resistors where changed to film metal oxide 1% and measured 1.5K dead-on (those are 1W or 2W, can't remember).

    Unfortunately I don't have other 12AX7/ECC83 to test and there aren't any signs on the tube to find a datasheet. It's one of those no-name chinese tubes, just like the 6v6 power tube.

    But, again, I don't want to make it too much of a big deal as the amp sounds great as it is so I might just leave it alone and put better tubes in.

    About power supply and plate voltages, I'll have to update the schematic with the new readings, so I can give you all the information needed.

    Going from memory, 300V on B+ and ~200V on plates.
    Bear in mind we're speaking about a modded VHT 6 Special which has a SS rectifier.

    I'll post the updated schematic later with the various voltage readings.

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    Right now I have 300V B+ voltage, 200V on the plates and 1.385V and 1.42V on the cathodes, with 100k plate resistors, 1.5k cathode resistors bypassed by caps and 1M AC load.
    That in itself isn't completely consistent (assuming resistor values are exact): 1.42V across 1.5k means a plate plate current of 0.95mA and a voltage drop across the plate resistor of 95V. So with a B+ of 300V plate voltage should be 205V.

    But I am with Enzo, it really doesn't matter. Plate voltages between 190V and 210V mean close to center biasing and allow for a plate voltage swing of at least +/-90V.

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  16. #51
    Supporting Member loudthud's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Some original Fender schematics show a note stating voltages within +/-20%. So your cathodes voltages should roughly be within spec.
    Cathode voltage depends on tube variation (Gm), plate voltage and actual resistor value.
    Back in the day, Tektronix made a special scope called the 570. Using it you could display the plate current curves of many tubes. Connections to the tube socket were made by short jumper wires. When you plugged a tube in, the very first thing you did was adjust the heater Voltage (with a small internal variac) because all the curves depended on it. So that 125V mains puts the heater Voltage in your old Fender to something near 6.8 to 7.0 VAC. It makes a big difference.

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    Sorry for suddenly disappearing, life/work stuff took over.

    Here's where I stand now (still waiting for components from China to arrive ):



    As you can see, I've stripped it down to a very simple 5F1-like circuit.
    When the other components will arrive I'll change it back to the version with the AA764 tonestack and a switch to bypass it.

    For reference, the components underlined in red are those which have been changed/added from stock. The original amp is a VHT Special 6.

    In my last post I was wondering if I should change the cathode resistors because, when compared to the AA764 circuit which has 200V on the 12AX7 plates, I have only 1.38V and 1.46V instead of 1.8V and 1.7V.

    Still, the amp sounds great and so far I'm loving it so I'm not sure if this is a non-issue.

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