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

[TubeNoob]

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!

[TubeNoob]

Some pix ...

[Justin Thomas]

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

[TubeNoob]

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?

[Justin Thomas]

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

[TubeNoob]

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 :-)

[Justin Thomas]

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

Justin

[TubeNoob]

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.

[Jazz P Bass]

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.

[TubeNoob]

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.

[eschertron]

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]

+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

[Chuck H]

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.

[TubeNoob]

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!

[TubeNoob]

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.