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

[TubeNoob]

For sure. Just want to throw out there that it's all on a PCB with the PT in one corner and the OT's in another.

[TubeNoob]

Re-wiring the long un-shielded cables with short shielded ones... will report back after that!

[TubeNoob]

Replaced the long un-shielded wires (that run from the amp PCB to the control panel controls) with shorter shielded wires. Grounded them all together (only grounded on one side - at the control panel). No more ice pick problem!! I can crank all the dials now and it holds steady and sounds rippin'!!!
Lead dress - one of the most tedious things is one of the most important, for sure.

Chuck - Those 100pf bright caps on the volume controls that you suggested, I put them on toggle switches - and maybe i've lost my high frequency hearing but I'm not noticing any difference. I've already drilled the holes for these suckers, so i gotta put SOME kind of mod in there :-) heh hehhhh.
So, should I go for a smaller value cap maybe, or....?

Hope everyone had a great turkey day and a long weekend!

[TubeNoob]

@ChuckH: Hey, also I noticed that on your schemo for the mid section you changed C11 from .01uf to .022uf. This wasn't on the one full schemo you posted, and I didn't do it because it slipped past me.

What's the story on that mod? Boosting fewer lows in the mid-section controls?

[TubeNoob]

More questions on my mind:

I've just ordered a Cap tester - but, should I just go ahead and re-cap the PS? It's three 50UF caps.
If I luuuuuv the sound of the amp, should i try to match their current ratings or just go with the stock values?

What about the SS rectifier - is it something that should be replaced out of hand, or will it not jack stuff up too bad if and when it goes out on me?

The cathode bypass caps (16uf and 25uf) have a little orange foamy bulge out of one end. I'm assuming that's not a great sign. I have no problems replacing them, but am I tripping that its hard to find those two exact values at low voltage?!

The speaker jacks seem to ground when nothing's plugged in. They're very proprietary jacks so I'm replacing with regular jacks. Am I correct that by using shorting jacks (as they do) that the amp won't blow out the OT if no speaker is plugged in? I'm very careful about this but just curious about the logic.

[Chuck H]

I've just ordered a Cap tester - but, should I just go ahead and re-cap the PS? It's three 50UF caps.
If I luuuuuv the sound of the amp, should i try to match their current ratings or just go with the stock values?

Great question! You've been paying attention But there are other aspects than value to affect the tone of the amp WRT the filter caps. The impedance and ESR will affect how much NFB and potentially positive FB can occur in the power rail. But there's honestly not much to be done about that in practical application. So..

I'd say mach the value. Those old caps can be +/- by quite a lot from their spec. So to keep things as close to what you have now I would match the value.

What about the SS rectifier - is it something that should be replaced out of hand, or will it not jack stuff up too bad if and when it goes out on me?

Is it silicon or selenium? If it's selenium I'd replace it. If it's silicon I wouldn't worry about it.

The cathode bypass caps (16uf and 25uf) have a little orange foamy bulge out of one end. I'm assuming that's not a great sign. I have no problems replacing them, but am I tripping that its hard to find those two exact values at low voltage?!

15uf and 22uf would be close enough, provided that those caps measure close to their spec'd value. They probably don't. Further, if they are exhibiting any shorting it could be affecting the bias of those stages. That and the actual value will affect the tone of the amp more than differences in the power supply filters so you should test for these things. Measure cathode voltage at those stages and then lift one leg of the bypass caps. Then measure cathode voltage again. If it measures the same then you only need to replace the caps with something close to their measured value. If it measures different then you'll need to adjust the cathode resistor for those stages to get similar voltage that you had with the old caps. The caveat always being, if that voltage keeps the tubes operating in a safe zone.

The speaker jacks seem to ground when nothing's plugged in. They're very proprietary jacks so I'm replacing with regular jacks. Am I correct that by using shorting jacks (as they do) that the amp won't blow out the OT if no speaker is plugged in? I'm very careful about this but just curious about the logic.

Make sure there's no switching weirdness for multiple jack use going on. If the jacks are wired as just plain ol shorting jacks then do that. Does it mean the amp is safe with nothing plugged in? Not really. The tubes aren't intended to run into a dead short and will draw considerable current doing it. It's still preferable to an open load though. The shorting jacks would be the lesser of two evils. There's one old thread here where a guy was running his old Fender with the "ext" jack plugged into another amps input and had been doing it for years. That doesn't mean it's a good idea. I'll just say your amp is LESS likely to suffer catastrophic damage running into a short than an open.

[TubeNoob]

I'd say match the value. Those old caps can be +/- by quite a lot from their spec. So to keep things as close to what you have now I would match the value.

Is it silicon or selenium? If it's selenium I'd replace it. If it's silicon I wouldn't worry about it.

15uf and 22uf would be close enough.

Make sure there's no switching weirdness for multiple jack use going on... I'll just say your amp is LESS likely to suffer catastrophic damage running into a short than an open.

1) Will try to match those values! Yahoo, yeah I've been trying to pay attention; i refuse to get older without trying to get wiser!
2) They appear to be Selenium. Ugh. Ok, so what's the best option - 4 silicon diodes?

3) Ok, I have those values (15 and 22). I'm still gonna wait till my new toy arrives and I can measure them! heh!
4) No weirdness on the jacks that I can see. I'll def go with the switching jacks; but as I said I do take care with that sort of stuff (with the fear only a DIY'r can have).

Thanks so much, Chuck!!

[TubeNoob]

Oh and Chuck - still really hoping for your input for those previous two questions!! (100 pf bright cap and the C11 change to .022uf).

[Chuck H]

Oh and Chuck - still really hoping for your input for those previous two questions!! (100 pf bright cap and the C11 change to .022uf).

Ah, Right. Ok. Been a little backed up with other forum activities.

Yes, use plain ol 1n4007 diodes wired as shown in the schem.

If the 100p bright caps aren't making a difference then you can bump the value for more instant gratification on the switch. I promise that some others would hear it with the cap switched out. The 100p value is small, but helps preserve a little sweet chime that can get lost with no cap at all. It's a small affect, not really enough to merit a switch. Which is why I showed them hard wired. So, since you already have the holes, maybe hard wire the 100p caps on the pots and switch in another 100p or 150p cap with the switch. You really should be able to hear that.

The change from .01uf to .022uf was only indicated on the boost/cut switch part of the discussion. If you like what's happening with the .01uf value then absolutely do leave it that way. But the reason for the value difference as it pertains to the boost/cut switch is that the .022uf value gives the switch more effect by cutting deeper into the lower mid frequencies. It would voice the preamp more like a typical BF Fender with the switch in the "normal" mode. Which would be different from what you have now.

Did you include the boost/cut switch? And, if yes, does it give enough gratification between "normal" and "cut" settings? And, if yes, leave it the way it is.

[Helmholtz]

The effect of a bright cap reduces with increasing vol. settings. At max. volume it has no effect as it is shorted.

[Chuck H]

Good thing to point out.

Also, the actual knee frequency goes up. So there is less HF in the useful, audible treble as a result of that cap as the volume is increased. It's a double whammy.

The reduced affect of the bright cap with volume increase actually works out well most of the time because most amps start clipping well before the bright cap affect is disabled. And most amps sound better clipped if you DON'T push the very high treble frequencies. It does make for a switch on the front panel that doesn't operate at all amp settings though. I always hated that aspect for purely aesthetic reasons.

[TubeNoob]

Hey Chuck and the rest of you fellas... missed ya!

To put it mildly, life got in the way. But at long last, I am back in the USA and have all of my goodies out of storage and a 'real' work bench. Yay!

So, after fixing my 8-track recorders and soldering up some cables I needed for dumping all my horrible old demos into the computer, I have cracked this project back open.

(And btw, it sounds so good and has so much flexibility I've had 2 pro session guitarists play through it and ask me "how much to build me one?"). Yeah, not happnin'.

The last time I was playing this amp, it suddenly went out on me, so I shut it down and packed it before moving between countries, so I just waited till I had a full setup again.

Now as I'm doing a cursory visual inspection, I see that the thermal fuse has blown on the transformer. It's a 125 Celsius fuse, but it's literally this spring loaded hook. I dug around a bit but can't find any info on how to replace it in the original way, so my question is - can i just solder in a new-type 125c thermal fuse? Should it be thermal-glued to the side of the transformer or will the 'inner' hook (which connects to the big hook in the photos) be enough?

Obviously I will be troubleshooting the issue that caused this fuse to blow as soon as possible (and hopefully I can find my variac, over 300 boxes here, ooof).
I saw there was one of those wire tube holder things down in the chassis, so hopefully it was 'just' that! Darn it.

Cheers and hope you've all been doing really good.

[TubeNoob]

After a bit of research, this is called Rose Metal or Rose's metal. It's an alloy that melt's at 98C. I guess they applied it like solder (which it can also be used as) to stick the two connectors together.
I already ordered up some thermal fuse's, but now I'm wondering if I should get an ingot of this stuff - $13 bucks for a friggin solder joint, ooof!

[TubeNoob]

Gosh, I totally forgot to post this vid... Here's how this thing actually sounds. In the beginning of the vid is the 5f2a Princeton build I did, and then at 1:10 it goes to the turntable amp (the Secret Weapon!). Later, I do a blend of both amps.

I implemented basically all of Chuck H's suggestions, and put them all on switches (cathode bypass cap, NFB, presence, etc).
Just got my parts including the thermal fuse, a lot of work left on this but I am super excited to be back at it!

[TubeNoob]

The strange thing is Rose Metal has a melting point of 98c, yet on one of the schematics I have for this thing it says 125c.
Just got the caps and diodes I need to rebuild this PS and get rid of this selenium rectifier.
Anyone replaced a selenium with diodes before? - I understand I need to put a high-watt resistor in series since the silicon will put out more voltage.
I also have some 5 and 10 ohm thermistors, I was thinking to add one to the PS (after fuse and switch on the AC line) in an effort to take it easy on this old thing when its switched on.
Would love some feedback from anyone familiar with these 3 things! Cheers.