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

[TubeNoob]

HA!!! oh y'all are funny....needed them laughs.

yeah i totally get it. but i know of a store in Glendale, CA that just does 3D printing stuff and they pay the rent there.

i mean if RS (which has the distribution resources, the advertising, etc) focused on DIY (3d printing, arduino, low and high-voltage components, had workshops, put together DIY kits, etc) they could maybe re-invent themselves where they originally started off? not talking about NOS tubes or anything, just the stuff that the mass majority of DIY'rs need on a regular basis.

yeah, i'm a dreamer. but hell if i wouldn't spend the extra to have it same day and be able to return it if needed (and save on shipping).

that's how guitar center et al survive with higher prices than online stores - people want "same day" and they want "a knowledgeable salesguy" and they want to "return stuff". that was my experience working there in 98, anyway.

[Enzo]

One of my career goals was to teach electronics. I found out many years ago that electronics was disappearing. The local large community college used to have an electronics two year program, but they dropped it 30 years ago. Now ther is only the very rudimentary element in like a computer course. No one fixes computers at a component level, they teach you enough to find out if the power supply works. Electronics as a hobby has disappeared. Used to be we used short wave radio to communicate and just listen to the rest of the world. When I was a kid for example, I subscribed in the mail to Radio Budapest, and got schedules etc. Nowdays, people go on the internet for that.

What little electronics there is finds people less into it. Today if someone wants to build a project, they need a small power supply for it. If there is no "kit" for a power supply, they don;t build the project. What could be simpler than a power supply? But even that is beyond most hobbyists. But the hobby isn't there. Used to be several electronics hobby magazines on the news stand, now ther are almost none. And a lot of that leans heavily on the computer end of things.

The ham radio crowd wonders where the new members will come from. You may hear about teaching "STEM" in your schools - science, technology, engineering, math. Kids are not particularly interested. But we need to get them interested because we will need people in those disciplines in the future. The kids are not building things, they are playing with APPs on their smart phones.

Greater Lansing is about 400,000 souls, a medium size city. We used to have two real electronics stores - the places TV repair shops go for parts. Local TV stations bought connectors there, etc. One of them folded years ago. The other I was in yesterday. It used to fill the building. Now it has shrunk to just the showroom up front. Their component line is down to NTE. They sell to professionals still, lots of TV exterior antennas, and lots and lots of security systems. Components? Not so much. We used to have a bunch of RS in the area, most are gone. The remaining ones are basically Sprint stores. They sell a lot more phones than they ever sold resistors.

What do home guys buy? Speaker wire, cable TV cables, RCA cables for "home theater". Oh and fuses. Just not a sustainable model selling small parts.

[TubeNoob]

Yeah it's sad. I do think DIY is doing well, though - at least viewing it through the scope of the internet it seems so popular (hack a day, the DIY synth and effect pedal sites, all the forums, etc which make it easier to jump into the hobby) but with the Net you lose the local shops selling the stuff we need/want same-day.

Anyway, my parts are due by the end of the week - Yahoo! Only two days to go from Thailand to Paris... we'll see how the French Postal Service does with it now that its in their hands.

Anybody got any ideas on what kind of power i can expect from the supply going to the two turntable motors? From what i can see on the schemo they look high-voltage but maybe i'm not reading it right.

I want to hopefully use that power for the FX stuff (and if so, put them on separate switches so i can run either Amp AND FX, or just FX) but if they're high voltage i guess i'm looking at burning off a lot of power with some high-watt resistors?

And Noooo Chuck, not expecting too much from a 60 year old turntable!

[g1]

The turntable motors are normally run off the 120VAC.

[Chuck H]

The turntable motors are normally run off the 120VAC.

That's how it looks in the schem. Though there does seem to be some weirdness with how the AC is routed through the motors through the PT primaries .?. Any guesses about why they did this?

[TubeNoob]

Hi Guys,

Though there does seem to be some weirdness with how the AC is routed through the motors through the PT primaries .?

Do you mean where it says 'no connections' around caps 24 and 25? Yeah, that part of the schemo is weird to my low-level mind.

What would be the best way to use this for the effects (two of them use 9 volts, the other 2 effects use NJM2068's at 4-18 volts)? There's room to put a wallwart inside the chassis in case that's a better route than high-watt resistor dividers.

[eschertron]

Hi Guys,
What would be the best way to use this for the effects (two of them use 9 volts, the other 2 effects use NJM2068's at 4-18 volts)? There's room to put a wallwart inside the chassis in case that's a better route than high-watt resistor dividers.

My guess from looking at the schem is that while there is line voltage across both TT coils is series, you do not have line voltage on the switch. If you absolutely must build your own LV power supply, test the voltages between the jumper pins. The 2/3 jumper and the 9/7 jumper will have line voltage between them, but the 4/5 jumper will have some intermediate voltage. And the diff between 2/3 and 4/5 will be the voltage that appears across the two poles of the switch. Useful? Measure and find out. Otherwise, stuff a wall wart inside the box. No one will be the wiser

[TubeNoob]

@ eschertron: thanks a lot. you said 'if you absolutely must build your own LV PS' - would there be an issue doing so? Sounded like you were advising against it?

still gotta test those jumper points you mentioned, but if i were to put an adapter in there, couldn't i tap straight off the motor supply line?

one last question - i want to upgrade the electrolytic caps while i'm in there - there are 3 50MFD 300v caps in the supply (in one can), shouldn't i increase the voltage or capacitance on those? Like 57MFD+ / 400v+?

[eschertron]

@ eschertron: thanks a lot. you said 'if you absolutely must build your own LV PS' - would there be an issue doing so? Sounded like you were advising against it?

still gotta test those jumper points you mentioned, but if i were to put an adapter in there, couldn't i tap straight off the motor supply line?

one last question - i want to upgrade the electrolytic caps while i'm in there - there are 3 50MFD 300v caps in the supply (in one can), shouldn't i increase the voltage or capacitance on those? Like 57MFD+ / 400v+?

OK> I took another look at the schem and noticed the "shorting pin" voltage selector doodad blurb. so you do have basically 120vac across the TT windings, essentially 110/127 * line voltage if the jumpers are set as per the schem (I think). The 42R and 6.5R windings at the bottom of the PT primary function as a kind of auto transformer, dropping a slightly higher line voltage to a slightly lower one for the TT and the filaments.

If I were doing the FX level power supply, I'd weigh time and materials for my options. And yes, maybe I'd run into trouble with noise if I put a SMPS inside my chassis. I might have some old linear wall wart supplies laying around, but that's me. Your scenario is unique to what resources you have. My comment was really pointed at "how would I feel to put a modern space-age component into a vintage de/re-construction?" So, how do you feel about it?

[TubeNoob]

It's a European box, so it's a 230v line.

I have no issues aesthetically with putting new stuff in there obviously, but i think you mean in terms of other components' tolerances?

I was just thinking that e-caps are not things that fall into the 'if it ain't broke don't fix it' category on an amp this old.

No issue using a wallwart though i would have to find/order one.

[TubeNoob]

(Found it near a trash bin in Italy but currently powering it here in France)

[eschertron]

(Found it near a trash bin in Italy but currently powering it here in France)

No tariffs/customs charges?

So whatever line level is where the unit was abandoned... you may want to verify the jumper device to know absolutely what it's doing. At EU voltages you'll have control over 220/240-ish supply.

Yes, I was thinking about the low voltage supply purely from an aesthetic POV. If I wanted a (Frankenamp) showpiece, I'd definitely build my own LVPS. But if I'm being expedient, I'd do whatever is quicker. That's all.

And Yes, recapping the electrolytics is a warranted option. If you're not depending on the amp for your livelihood, however, you may try re-forming the caps first and see if they act/sound OK. You'll KNOW if they are bad.

[TubeNoob]

No tariffs/customs charges?

Ha! That's one benefit on a short list of good things about the EU, free movement of goods!

Sounds good, will check around on the adapter vs. lvps aspect.

To reform the caps, wouldn't I need a variable power supply? The stuff i just read about it seems to indicate that's the case.

[eschertron]

how does the reservoir cap look? have you checked it for dead short? too bad you're not in a 110V zone so you could set the PT for half voltage.
you want to protect the PT when you power up the first time. There's no glass recto so that's not a problem. You'll find plenty of help on bringing the amp up; if you want to replace the caps first, that's your call. Each tech/designer has a level of caution that they don't want to cross. Repair techs will absolutely not risk having to replace a PT with out-of-pocket money. Or at least the good ones don't! Others may just bill the customer anyway

So need a variable power supply? Hmmm. ummm. You'll have to use what you have. Or find another strategy. If the caps look OK, and the PT doesn't get REAL HOT when power is applied, you may still find that the caps 'sound' bad. Then they get replaced. Or play it safe.

[TubeNoob]

All the caps 'look' fine. And i'm not hearing anything bad sounding. The amp is actually pretty quiet, too. PT does not get super hot.
The only thing i've noticed is the heaters do flare up for a second or two before settling back down.