Buying Tips: ST70

Buying Tips: ST70

Postby papa-audio » Sat Jan 28, 2006 8:04 pm

What things to be considered or to be checked when buying secondhand ST70 for it to be considered good buy...

just a newbie in tube hobby! :D
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Postby mozilla » Tue Jan 31, 2006 5:26 pm

If you are a collector, a good buy would mean a STOCK, mint condition unit with clean chassis, original power transformer and original OPT's (output transformers), original resistors, caps, driver board, speaker terminals and sockets. Not to mention quality tubes. The ST-70's main strength is in it's simplicity of design and the sought after iron. The best OPT are those marked A470 with CLOTH covered leads. Busted ST-70's are being snapped up all over the place so DIYers could cannibalize them for the OPT's which they use in their own tube amp projects. Note that the original driver board was made w/ compressed pulp. Heat and age may have taken its toll on most units, thus causing the circuit traces to peel.

Bear in mind that there are 2 camps in vintage amp use. Those who RESTORE and those who MODify.

When one restores, he tries to match the original parts and sound as much as possible.

Once you start putting modern or boutique parts like auricaps, metal film resistors, cerafines with larger capacitance you are modifying. This normally makes the sound more " modern" which means that it sounds more extended, tighter bass, cleaner, more detailed sound. This at the expense of the sweet, smooth, natural tonality of most vintage stuff.

Now back to the ST-70. The St-70's churns out 35Wpch of class AB power. It runs at class A up to about 15wpch. It has 4 EL34's, 2 7199's and 1 GZ34 rectifier tube. The 7199 is practically no longer in production. There is I beive a JAN/Philips but it does not sound very good.

Its appeal is that it's cheap and sounds great and is easy to tweak. It can hold its own vs many modern amps out there. Some change caps and resistors and some change the entire driver board. Joe Curcio (curcioaudio.com) for example claims that his ST70 mods will sweep the floor off $6,000 amps.

The stock ST-70's sounds smooth and mellow. Its main flaw is its wooly bass. The stock caps can sound a little nasal in comparison to something like say, auricaps.

On the other hand, a modded ST-70 with a good combination of parts can sound dynamic with good, extended & defined bass. The highs can sound more airy and clean, with liquid mids. Some change the power supply for added headroom.

If you know how to operate a DMM, check the voltages if they are the same and w/in spec.

In summary, look for:

STOCK ITEMS:
1. Stock A470 OPT with Cloth leads
2. Check the condition of the driver board
3. Condition of chassis & lettering

MODS:
1. Type of parts used in the driver board (caps, resistors)
2. Have the PS caps been upgraded?
3. New RCA Jacks to take modern IC's
4. New speaker binding posts

OTHERS:
1. Tube complement
2. Upgraded power cord
3. Upgraded internal wiring
4. Upgraded sockets (usually ceramic w/ GP pins)
5. Upgraded driver board

As always, try before you buy. System matching is important.

Hope this helps
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Postby mozilla » Tue Jan 31, 2006 6:00 pm

BTW, check if it still has the original selenium rectifier. This should be replaced asap as they are bound to fail anytime from age and may just decide to take with them some other parts when they do.
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Postby papa-audio » Thu Mar 30, 2006 8:49 am

very nice comments/advise mozilla... thanks! :wink:
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Postby JackD201 » Thu Mar 30, 2006 9:40 am

This should be a Sticky. Great advice as usual Grant.
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