New Member, New ST-70

Recently picked up my first audio tube amp - an ST-70 with a PAS-3 preamp to go with it.  I intend to have it drive a pair of Yamaha NS-1000's.  Variac and inline current meter inbound (so I can have current on display on the variac without using my fluke).

This ST-70 seems older, with cloth wrapped transformer leads.  Bias pots appear to date to 1965.

Everything looks pretty original.  It has old black cat caps throughout, and the original selenium rectifier is there.  I don't see any leaking, waxy, or deformed caps.  Output transformer windings read 58 (with tube in place) to 68 ohms (no tube) on the output tube windings.  Most of the tubes are original Dynaco labeled, but it is missing one of the EL34 output tubes.  

I have searched for but not found solid answers regarding matched output tubes in this amp.  I need an additional output tube.  What's the point of a matched quad in this amp?  As two separate channels, it seems like a matched pair in one side would suffice (i.e., the side with the missing tube).  Should I just buy a single tube for now, a matched pair, or a matched quad?

Also, I've heard of installing a 10ohm 1/4W resistor on the cathode of each output tube to act as a fuse if the tube shorts.  Thoughts on this practice for these amps?

Was going to post some pics but I don't use any hosting service for pics.

Thanks!

Antarctica wrote:Recently picked up my first audio tube amp - an ST-70 with a PAS-3 preamp to go with it.  I intend to have it drive a pair of Yamaha NS-1000's.  Variac and inline current meter inbound (so I can have current on display on the variac without using my fluke).

This ST-70 seems older, with cloth wrapped transformer leads.  Bias pots appear to date to 1965.

Everything looks pretty original.  It has old black cat caps throughout, and the original selenium rectifier is there.  I don't see any leaking, waxy, or deformed caps.  Output transformer windings read 58 (with tube in place) to 68 ohms (no tube) on the output tube windings.  Most of the tubes are original Dynaco labeled, but it is missing one of the EL34 output tubes.  

I have searched for but not found solid answers regarding matched output tubes in this amp.  I need an additional output tube.  What's the point of a matched quad in this amp?  As two separate channels, it seems like a matched pair in one side would suffice (i.e., the side with the missing tube).  Should I just buy a single tube for now, a matched pair, or a matched quad?

Also, I've heard of installing a 10ohm 1/4W resistor on the cathode of each output tube to act as a fuse if the tube shorts.  Thoughts on this practice for these amps?

Was going to post some pics but I don't use any hosting service for pics.

Thanks!

Each channel needs a matched pair. And as the tubes are original dynaco you should benefit from fresh EL34 tubes.
If you buy a matched quad you may place them in any location, and if one tube should fail you still have a working pair plus a spare.

Don't bother with additional cathode resistors. Fresh tube ( matched !) will do what you need.
If you feel for some soldering, adding the "yellow sheet mod" ( adding series diod to the rectifier tube).

Congrats to a nice combination!

You can find the yellow sheet mod here:

Yellow Sheet Mod

The yellow sheet mod is for tube recitification. It can be improved to use only one diode.

BUT you'd be further ahead to dump the selenium rectifier, replacing it with a Silicon rectifier.

As far as a Quad matched set: It's a good idea as then both channels have similar response. Tubes wear out. If you lose one tube, it will be nearly impossible to find a match. You may be able to bias it the same, but the response will not be the same. Distortion and channel imbalance will likely increase. If the tubes have 1000 hours on them, buy a new quad.