SZ13C zener replacement for 1964 JMI Vox T60? - UPDATE

[fazeka]

SZ13C zener replacement for 1964 JMI Vox T60?

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:

[sluckey]

Schematic shows 12.5V across the zener. A 13V zener would work just as well. However, it seems odd that a stud mounted zener would only be rated for 1 watt.

[fazeka]

However, it seems odd that a stud mounted zener would only be rated for 1 watt.

Thanks Steve. Maybe I should have picked something like 5W?

Or 2W?

FWIW, here's the original:

[PRR]

The gut-shots are not much use for the question about the Zener-- bless you for the schematic.

The job is 0.24 Watts. Using a stud-mount here must reflect a chaotic semiconductor market. I would just use a modern 1Watt 2-leg Zener.

> need to rebias

There's no simple way to re-bias this. The original depended on the driver windings but the tempco is wrong and for Si devices it matters. Unless you can re-design the stage?

This output stage WILL eat power devices. The 8 Ohm rating is over-ambitions for a stage amp. Even the lower rating for 16r load seems bold for chassis heatsink.

[PRR]

Join the diystompboxes.com forum and ask R.G. He knows a LOT about the Jennings/VOX amplifiers.
https://www.diystompboxes.com/smfforum/index.php?topic=124048.msg1177855#msg1177855
"I've spent pretty much my entire adult life fascinated by the Thomas Vox line, silly teenager that I was." -R.G.

[sluckey]

RG is also active at the amp garage.

[fazeka]

Thanks Paul and Steve.

I have asked also on the ampgarage forum.

Re: biasing this beast for Si, I got that from R.G. himself:

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/27218-vox-t60-ss-head-low-output?p=364306#post364306

[fazeka]

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? 

[PRR]

> 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!

Get a VOM, passive needle-meter (they are still in the electrical aisle at Home Store). Crank it till it does not rise any more. That's how Traynor did it in a similar time. 33W un-clipped on a peak or RMS will compute to about 60W totally gnarled.