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Thread: Mod'ing a tube circuit (from early 60's stereo turntable)

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    Mod'ing a tube circuit (from early 60's stereo turntable)

    Hello all,

    I saved an early 60's turntable from the trash bin. The turntable is missing parts and doesn't spin, but the stereo amp does work.

    I alligator'd my guitar cable into where the needle's leads come in to the circuit and it works great (for a 'clean' sound).

    A couple of questions:

    1) I would like to add a switch to make the amp break-up sooner (it barely does now at full volume). How can i do this in the most simple fashion, change the V1 cathode resistor or do i have to mess with the B+?

    2) Does lifting the tone-stack only increase 'clean' gain (headroom) or would it help overdrive the power tube?

    3) How best to lift the tone-stack, de-couple the ground or should i truly bypass it (and how?)

    I've attached the PDF including the schematic.

    Thanky much for any input!
    Attached Files Attached Files

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    Some pix ...

    dscn3569.jpg

    dscn3568.jpg

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    To start,I'd just bypass the "Volume" controls and then add a cathode bypass cap to each first stage... easy & quick. The "Balance" controlsnseem to just be Volume controls, anyway...


    Justin
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
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    Hi Justin, thanks for the info!

    So, are you saying bypass the 'balance' controls, or those and also the main volume pot?

    What value cathode bypass cap should i start with you think?

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    I meant bypass the two Volume controls - the ones BEFORE the first stages. The Balance controls just look to be regular Volume controls, anyway. I just think make the circuit topology more like a Champ - not necessarily a clone, but lay the different circuit parts out in the same orderr... in a Champ, the Volume goes after the tone stack, right before the driver. There is no Volume before the first stage. I think you can get a nice little grinder with a lot less effort than you think!

    A new speaker or two wouldn't hurt, either! I did the bypass caps & speaker thing to an old Lectrolab and it went from clean on 9 to reging Neil Young sound (with musical feedback) on 10 and a whole lot in between. And nothing else sounded like it.

    Justin
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Ok cool. So could i just bridge from patch point 1 to patch point 3 (the black boxes near the input) or do i need to include C6 and R9 in there?

    Where is Chill-Ville, VA? I'm from Alexandria :-)

  7. #7
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    Yup, disconnect the Volume & stick a wire in its place. Chill-Ville is Charlottesville.

    Justin
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Gotcha, i'll give that a go. Thank you, Justin.

    Any suggestions on value for the cap on the cathode? 25uf / 25v like on the Princeton?

    Been to Charlottesville several times, fun town.

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    Supporting Member Jazz P Bass's Avatar
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    Not too sure how much music you listen to, but that stereo amplifier would probably sound pretty good through a decent pair of speakers.

    Those EL95's are rated at 6.5 watts, Class A.
    Richard and TubeNoob like this.

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    Hey Justin, tried tapping in to both sides of R10 but didn't really hear any difference. I guess the vol pot maxed out is about the same.

    Hi Jazz P Bass: I listen to a lot of music, though i have some KRK Rokit 8s for that. I want to have the option on this tube amp to revert back to its original 'clean' tone which i would use for a few things - minijack input for MP3 players, running stereo synths into it and then mic'ing the speakers (to warm things up a bit), re-amp'ing, etc. But then i'd like to throw some switches and make whatever source material i'm running thru it get dirty. I don't have any other speakers/cabs on hand, but that's a great idea and this unit does have the outputs for external speakers - will definitely give that a shot.

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by TubeNoob View Post
    Hello all,

    I saved an early 60's turntable from the trash bin. The turntable is missing parts and doesn't spin, but the stereo amp does work.

    I alligator'd my guitar cable into where the needle's leads come in to the circuit and it works great (for a 'clean' sound).

    A couple of questions:

    1) I would like to add a switch to make the amp break-up sooner (it barely does now at full volume). How can i do this in the most simple fashion, change the V1 cathode resistor or do i have to mess with the B+?

    2) Does lifting the tone-stack only increase 'clean' gain (headroom) or would it help overdrive the power tube?

    3) How best to lift the tone-stack, de-couple the ground or should i truly bypass it (and how?)

    I've attached the PDF including the schematic.

    Thanky much for any input!
    Nice find! Anything you do with it is better than letting it rot

    You can experiment and get an idea of what lifting the tone stack may do by using an alligator-clip jumper (or equiv) from point 4 to point 9 on ch1, same process on ch2. This should boost the gain considerably. Any gain here will help overdrive the power tubes. Ensure that the volume control is all the way up Don't remove the volume controls until you have resistors to use for grid leak on those stages. The grids need to see 1M or so to ground for reference voltage.

    If you still don't get the grind you want, put a boost/overdrive pedal in front. Better yet, use a stereo chorus and route the signals right and left for pure ear candy!
    Justin Thomas and TubeNoob like this.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  12. #12
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    +1,000 on the stereo chorus - I recommend an Arion SCH-1! Or anything old and not-Boss... I love the sound of a chorus with SE amps... cuts out any fart but leaves all else. It's a great and lovely sound for sure!

    Justin
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  13. #13
    Supporting Member Chuck H's Avatar
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    Clean boost pedal I use to use a BOSS GE7 EQ so that I could tune the boost frequencies. Noisy as hell, but very effective.

    Other than that you'd need to add a tube. Doing so would allow you to add another stage to either channel OR to cascade existing stages for a mono preamp and then wire an effects loop for stereo to feed the separate power amps. But that starts to get ambitious.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    Thanks for all the suggestions, everyone. Yes, i am stoked it's stereo so that i can not only run stereo effects, synths and DAW busses, but also split my mono signals into different fx chains and mic up the different speakers.

    @eschertron: Ok, i will try out bypassing the tone stack and will report back! Thanks a lot for the input!

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    by the way, i plan to put a metal plate in place of the whole turntable assembly (saving a good 15 lbs!) and install some home-built fx there, like a booster, an overdrive, a tape-saturation sim using diodes, a multiplier and a mixer with an FX send. hoping the motor power lines can be used to power those FX... need to bust out the DVM and see what they're throwing out.
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    @eschertron: Wired up the tone-stack bypass - really added a nice warm distortion to things (and bumped the volume up). Not at all over the top, in fact it's pretty laid-back, so i'd definitely need to find a way to crunch it up further. But a really, really nice sound option for sure. Thanks for guiding me on that one.

    Aside from pedals, is there anything further that can be done to the circuit (without major changes) to get some more dirt?

    A V1 cathode bypass cap would give it more headroom (and less crunch), would it not?

  17. #17
    Supporting Member eschertron's Avatar
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    A pair of cathode bypass caps on V1 (a and b) will increase gain and reduce input HR. By about a factor of two. Definitely noticeable, if it's already getting 'warm'. The other thing to do is investigate the NFB loop on the output stage. Hi Fi usually has more NFB than guitar amps, so you could relax that a bit to get more crunch. I didn't look at the values, so can't tell you what to expect at this point. But from your description, it appears you will be happy with modding this thing. Won't be a metal monster, but could get nice and bluesy!
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    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Fantastic, thank you Eschertron - so adding those cathode caps will overdrive the power tubes more? Going for it!

    Just ordered up the parts from Tayda Electronics, a good cheap place to go!

    Will have the following: Multiplier, Muff Boost (clean and dirty on a switch), Tape saturation simulator, FX Send section, 4 channel mixer with pan and level controls, mini-jack and two mono 1/4" inputs direct to amp.

    Should be a cool little color box when its done.
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  19. #19
    Supporting Member Chuck H's Avatar
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    Sorry I'm late to the game. I sketched up the changes "I" would make, short of anything too crazy. Some have already been covered but I included them to consolidate everything on one schematic. These would make it pretty much a pair of 2W Champs Probably with a little more crunch though.

    Changes in red:
    Attached Thumbnails Attached Thumbnails propturn2.jpg  
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    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    Hi Chuck! You've helped me before, really glad to have your input here and thank you so much for mocking up that schematic!!!

    Is there any way i can trouble you to walk me through what these changes do in particular? It's a lot to throw on a switch (so i can revert back to the 'clean' sound) and i do have a home-built Princeton 5f2a, but i'd really love to understand what these changes do and implement any and all options i can.

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    ...by the way, Chuck - i just ordered up all the parts you suggested just in case!!

    dang i love forums like this. makes an agoraphobic tinkerer like me feel like a part of the world! heheh! and the best part is communicating with some really brilliant people who share a common interest...

  22. #22
    Supporting Member Chuck H's Avatar
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    There's no need for a switch to get clean. The amount of grind you'll get from the above changes isn't dramatic and the clean tone at lower volumes (around 4 or 5 probably) will be as loud as the amp will get anyway. So a switch buys nothing in headroom. The amount of distortion will be a little greater than a BF type Champ. Well into a rock grind, but not metal by any stretch. The actual number of gain stages is the limitation here. Something you COULD put on a switch would be the classic tone stack bypass. The normal circuit for this "mod" just lifts the tone stack ground resistor and disables it. That would buy a lot of gain. Maybe too much, but that'll be up to you. I'll draw it up and re post, along with some explanation of what the changes I proposed above do.

    P.S. Since you've already ordered parts it may be too late, but you DO want log pots (or a log dual ganged pot) for the new volume controls.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  23. #23
    Supporting Member Chuck H's Avatar
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    As promised:
    Attached Thumbnails Attached Thumbnails propturn2.1.jpg  
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    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    Chuck you're a monster!

    1) I did order log pots and not linear.
    2) So you're saying these mods wouldn't need to be on a switch since i could basically back the volume off enough so that the 'clean' sound is still an option? I want to use this for re-amping stuff and warming things up sometimes (not just for guitar crunch - i'm trying to have the crunch be optional).
    3) How much of a difference would the replacement of balance controls with volume controls make? That's the only mod i'm a little hesitant on.
    4) I did try bypassing the tone stack and it added a really lovely (yet subtle) tube dirt flavor. Really, really nice.

    Thank you so much for taking your time to do this.

    You should do a book with common schematics and then have 'overlays' for your suggested changes!!!
    Justin Thomas likes this.

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    Chuck - can you explain what's happening on that tone-stack bypass? The original value was 10k, but it looks like you've got a 2.2M there 'normally' and then switch in a 15k?

    The way i tested the tone-stack lift was completely bypassing the whole thing - is there a problem in doing that way?

    I'm curious how your version sounds compared to the way i did it (and i do love the way it sounds).

    Thanks man!!

  26. #26
    Supporting Member Chuck H's Avatar
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    The 2.2M resistor is there to keep the caps in the tone stack connected to ground when the REAL load (the 15k) is lifted. This keeps the switch from going POP! when you flip it.

    The actual difference in gain between actually bypassing the TS and just lifting that resistor is really small and simply lifting the resistor only requires a SPST switch. Along with the other changes it'll be PLENTY of dirt. I promise.

    Changing the balance controls to volume controls is an imperative part of this mod. It allows you to bypass the (very non guitar amp like) existing volume control. This is where a lot of your added gain will come from. An extra few dB at the input triode is reamplified by the next making it exponential. Relocating the volumes to replace the balance control also puts the volume controls in the circuit where every other instrument amp does. Making their use more intuitive.

    The way the balance control will be wired has one channel like a typical volume control and the other wired opposite, as if rotating the knob counterclockwise turns the amp up. So this one gang on the pot is wired "backwards". This is the only thing you'll need to wrap your head around. If you still want "balance" control you'll need to use two 1M pots (and drill a hole I suppose). If you're alright with the stereo channels going up and down in gain simultaneously you can use a dual ganged pot (and skip drilling a hole).

    The amp should be well behaved used as a typical "vintage" style amp. It'll be clean with most guitars up to around 4 or 5 on the volume control and should be real crunchy at 10. The actual location on the knob where the amp will start to break up will, of course, depend somewhat on pot taper. If you ordered the typical Alpha pots the taper on their 1M is 15% I think. That should be fine.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    Ok, excellent explanation, Chuck. Thank you!

    No probs on doing the balance controls as two volume pots.

    Just as a note, i don't want to use it all for guitars, i want to have the option of it being 'clean' enough to just warm up some mixer buses and let me re-amp anything from keys to vocals (and have these grittier options available!).

    So now i guess i need to look into how much the power lines to the turntable's motors have got for me so i can hopefully power these solid state circuits from that.

    I should probably have the tube amp's power on a separate switch if possible so that i can just use the SS effects without running the tube circuits. Thoughts on this, anyone?

    Otherwise i would probably need to go with an adapter.

  28. #28
    Supporting Member Chuck H's Avatar
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    since you'll be using it for things like vocals and keys as well you'll probably want more mid control. I'm thinking a SPDT on/off/on make before break toggle on the mid control for normal/tone stack bypass/mid cut selection. It WILL be a guitar amp and it IS a little amp. That means it's going to sound a bit boxy or lack headroom in the midrange for vocals or other instruments without a mid cut. I'll draw it up
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    ooooh i can't wait to see it! thank you for your suggestions and expertise, Chuck.

    if i run vox or keys through it, it's going to be for some saturation, so i think i do understand the sonic trade-offs.

    i guess i'm just going for overall flexibility wherever possible.

  30. #30
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by TubeNoob View Post
    i guess i'm just going for overall flexibility wherever possible.
    Not asking too much amplifier duty from a sixty year old turn table, are you?
    Attached Thumbnails Attached Thumbnails midstuff1.0.png  
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    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    Quote Originally Posted by Jazz P Bass View Post
    Not too sure how much music you listen to, but that stereo amplifier would probably sound pretty good through a decent pair of speakers.

    Those EL95's are rated at 6.5 watts, Class A.
    That thing's clean, too.
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  32. #32
    Supporting Member eschertron's Avatar
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    If we're putting a switch on the mid control, why not have a 25k mid pot and a SPST lift control? That aught to give 'flexibility' to the tone stack
    Maybe even a pot with a pull-switch?
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    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Not asking too much amplifier duty from a sixty year old turn table, are you?
    ohhhh no! I would NEVER do that!!!

    If we're putting a switch on the mid control, why not have a 25k mid pot and a SPST lift control? That aught to give 'flexibility' to the tone stack
    right on!

    @ Richard - yeah i need a nice pair of cabs for sure! And it being 'clean' is something i like, that's why i'm driving Chuck crazy with my 'flexibility' issues :-p

    @Chuck - thanks for the schemo on that!!!

    Ugh...10 days to go until my parts are due.

    If you live near an actual electronics store, i just want you to know, i hate you.

  34. #34
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by TubeNoob View Post
    If you live near an actual electronics store, i just want you to know, i hate you.
    Here on Whidbey "Aint no 'lectronic parts." Well, we have a Radio Shack...(in case I need one of the twelve components they carry)

    Anything I order has to get to me via a ferry or over a two lane bridge that also happens to be a tourist attraction in a county park (so it isn't used by commercial vehicles much). We also have very "small town" post office. That is, not fast or accurate. Neighbors get each others mail sometimes. Sometimes when a parcel is dropped at the post office they fail to notify me so I have to go in and ask if there's any mail for me.?. When I "track" orders on line they get across the country in a couple of days and into Seattle. From there it takes longer than it should to get onto the island and then longer than it should to get to my door or mailbox. In other words, Over night shipping is two or three days, three day shipping is a week, etc. And that's when everything goes right.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    man i remember back in the 80's when the guys at radio shack were EE's!! (at least where i was!) and they had a ton of DIY parts. i've thought for a long time they should go back to that DIY ethos (to, you know, avoid bankruptcy!)

    yeah when i lived in the mountains in CA it was the same thing; but there was a radio shack for those 12 parts!

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