SZ13C zener replacement for 1964 JMI Vox T60? - UPDATE

[seveneves]

Hi all,

Recently acquired a 1964 Vox T60 chassis that I feared was headed for the scrap bin. Get the feeling the chassis was unloved for the last 30+ years as it sat on a shelf and was very grimey and dirty.

I get it; not a very reliable amp and thus probably not very desirable. Also not very powerful. It had a lot going against it... But if I can get this repaired, would probably make a cool little recording amp for bass.

Anyway, the original Arrow power switch plastic bat handle was broken and so was the voltage selector. The front panel was bashed in which may have been the reason for the aforementioned.

But the goal is to save this from the trash.

Did not know if the amp was funcitoning upon arrival as various components in the output section (including the output transistors) were replaced rather amateurishly.

The driver transformer was measured having DC resistances of 32 ohms on the primary with 4.5 ohms on each of the secondaries so I'm presuming that is fine. Power transformer confirmed at 55 VAC on the secondary (27.5VAC CT).

Disassembled down to the chassis. Worked the front panel dent back out (carefully; aluminum chassis!). Straightened the faceplate out, hardwired the PT to mains voltage, new power switch. Replaced all the eletrolytics and a few of the resistors in the output (R23, R27, R30, R31). Will be using silicon PNP MJ15025G for the outputs (of course need to rebias).

I attempted to reuse the original G.E.C. SZ13C zener (stud mount) but the cathode lead broke off inside the body...

I tried to find a datasheet for it, no such luck!

Is it safe to assume that this is essentially a basic 13V 1W zener? If so, would there be a problem using something like a BZX85C13 or 1N4743A?

Some pics before and during the refurb:













And the schematic if anyone's curious:

[Stevem]

Thanks for the schematic.

Why not use a modern 220 type negative regulator for that 3 transistor section of the preamp.

I put silicon type T03 outputs in my 66 reverb twin and found that the higher idle current needed only served to slightly reduce the amps RMS and peak output with really no improvement in tone.

Nice job on the rebuild by the way, I love when early gear is saved, just because and even if it had issues and short comings.

[seveneves]

Why not use a modern 220 type negative regulator for that 3 transistor section of the preamp.

Ah, never thought about that. You thinking like a 7912 or 7915?

I put silicon type T03 outputs in my 66 reverb twin and found that the higher idle current needed only served to slightly reduce the amps RMS and peak output with really no improvement in tone.

I only thought about using silicon instead of germanium because they are more readily available... never thought about silicon reducing output power due to the higher idle current requirements of the silicon. I'm still learning here...

[R.G.]

The note "stud" for mounting bothered me, but the math says that the preamp uses about 30ma. A 500mW or 1W zener would be fine. The 7912 would work and would be a superior supply for the preamp, much more noise rejection. You could even put a diode in its ground lead and get it up to -12.6 or so for more accurate voltages.

@Stevem: the point of silicons is not to improve tone. It's to keep the amp alive if you can't source germaniums that will work and bias.I would not expect much if any change in tone. A loss of a watt or two, even a few watts, doesn't much cause an issue because of the human auditory logarithmic response.

[seveneves]

Thanks R.G. as usual.

Found this post of yours from 13 years ago on another forum. Am planning to use the MJ15025s and insert an 18 ohm resistor coming off the driver transformer secondaries and into the 1k/base junction. Am I understanding that right?

FWIW, here is the stud mount zener with its broken cathode lead:

[Stevem]

Hi RG!

Oh yes I know, but with mine only kicking out the low 18 rms that it does I did not thank that the original where stressed all that much where a silicon would be a big advantage.

Now in the bigger amps I know it’s a life giving up grade.

And by the way, thanks for all the knowledge sharing you do with these Vox amps!

[seveneves]

Update: just finished the refurb of the T60.

Appears to be an October or November '64 as it has the link added (late Sept. '64) but still has R14 = 330k (changed to 82k in mid Dec. '64).

Good amount of work put in and it's not the most desirable of JMIs but if it means keeping it out of the dumpster then it's worth it.

Was in a rather sad state upon arrival but gave it some TLC and it turned out pretty good, best it's probably been in decades I reckon.

Everything disassembled and cleaned. Front of the chassis was bashed in but was carefully "body worked" nearly straight. Also took care of the various front panel dents.

Replaced all the electrolytics, a few of the input resistors, rectifiers, the zener and the output transistors. Used MJ15025s and rebiased according to R.G. here.

Plastic power switch bat was broken off so replaced that with a Carling. Also, the voltage selector was cracked and non-functional so it's hardwired to 115 VAC (US).

Still needs a few minor things to finish 'er up (hardwire the speaker cable and mains cable, etc.). Sounds pretty good, actually!

Output before clip is 33W @ 9.5 ohms (that's what the big wirewound resistor I have measures at), 26W @ 16 ohms. "T60" (as in 60 watts), huh? Hah!

Anyone got a spare empty T60 head cab lying about?

[seveneves]

Everytime I try to edit my post, the site is posting a new reply quoting the post?!?

[Stevem]

With the ability you have displayed in the great reserection of the chassis you should be able to make a cab for the head easy!

You might check out North coast music, as they have,or had repro cabinets.

No Zobel network added to the output stage for safety?

It seems everything back then was rated for peal EIA music watts.