Help me bias my dynaco st-70

[Fortune]

I've just ordered some new quads of EL34's for my highly modded dynaco st-70.

This is the first tube amp that I've ever owned that did not have a bias meter built in to the amp.

How do I go about biasing each tube? I have a meter... is there any other equipment I need?

Do I have to open it up? There are 2 bias adjustment knobs on the top of amp but there are 4 EL34's. I'm confused. Please help.

 

[Dougigs]

No, you don't need to open it up, and if you have any kind of meter that measures voltage you should be fine. The bias controls are for left and right -- you bias each pair of tubes. That's why you need to buy matched pairs/quads for this amp (or rebuild it to have individual-tube bias like I did). But the pair-bias approach works just fine.

Here's what you should do:

1) Install the new quad of el34s

2) Turn the volume of your preamp-source down to zero

3) With the speakers hooked up etc, turn on the amp

4) Leave it on for half an hour, without any signal, so the tubes can warm up

5) Now you need to set your meter to measure low voltages -- you're aiming for a voltage of 1.56V here.

6) You want to connect the ground tip of the meter to any ground point (the sleeve of the bias-test jack or anywhere on the case.) Then attach the tip of the meter to the bias-test point (I usually start with L). You should use a small slotted screwdriver to adjust the bias-adjust pot on that channel very slightly in one direction or the other until you get roughly 1.56V. Don't worry about being too precise on this round...

7) Now do the same for the other channel (R)

8) And then go back to the L channel -- you'll often find that the bias-adjust on the R has thrown it off. Again, don't aim for super-precision at this point.

9) You can turn up the volume and listen now. Let the amp run for several hours, or even a day. Then repeat the bias adjustments.

10) You might want to readjust the bias a couple more times over the next 2 weeks or so, as the tubes break in -- their characteristic changes most in the early weeks of use. After a couple weeks, they tend to be quite stable, though it's wise to check them every couple months.

I find the ST70 can be a magnificent amp if a) it has new coupling caps, of larger value (up to 1uf) and better quality (polypropylene film) than the original, and b) new capacitors placed in the power supply (the SDS labs boards are great), and c) It has reasonably fresh tubes, both for the front end and the power tubes. Other mods are nice, but these three will turn it into a really grand amp.

Also it does a very nice job into headphones -- use the 16 ohm tap.

 

[Fortune]

Thanks so much for the detailed instructions. It seems much easier than I thought it would be.

When you are talking about testing the voltage with ground and the left and right signals, are the signals the speaker binding posts? I assume that I use the positive.

Thanks again.

I really like the st-70. It is my favorite tube amp so far. I was lucky enough to get my hands on one that was rebuilt from the ground up. It has all new and improved parts (insert technical jargon) and, from what I've heard when comparing it to a friends "stock" st-70, gives more impact, speed and instrument separation than the stock amp.

I'll be holding onto this one for a while.

I ordered some of the sed winged c el34's after reading some really positive things about them on audio asylum. I'm also considering trying the groove tubes as well. Have you had any experience with any of these?

How about the 5AR4 or 7199. My current 5AR4 is a sylvania brown bakelite base and the 7199's are stamped dynaco but I believe they are philips.

I love tubes

 

[rickcr42]

You really should download the original dynaco ST-70 manual From the curcio audio site so you will have not only the step by step build manual but the set up and operating manual as well.
You will also find some invaluable ST-70 info at the Audio By Van Alstine site in the "Audio Basics Magazine" area

 

[Dougigs]

The manual is indeed very handy:

http://www.curcioaudio.com/st7_mnl.pdf

The 7199 tubes are the biggest problem with that amp (and with a number of other amps from that era including HH Scotts). Modern makes of EL34s are great, comparable to the old ones and well worth using, but the 7199 is no longer made (if you see Sovtek-branded "7199," don't use them; they're not). It sounds like you have a good stock of the old ones. Those Dynaco-stamped ones are most often RCAs, which are very nice, but Sylvania are also great. Keep in mind that they don't last forever, though (if you see a lot of stuff that looks like scorch marks inside, it's probably time to change them) and fresh ones will improve your amp's sound considerably. Then you face a dillemma: The NOS ones cost upwards of $25 each. For that price, you can get the driver board replaced with a new one that uses better, cheaper new triode tubes like the ECC99. (Triode Electronics in Chicago sells a great one).

The rectifier tube you've got should last forever, or at least a very, very long time. You don't need to get too excited about 5AR4s -- they're rectifiers, and they either convert AC to DC or they don't. For a long time, the modern ones (incl. Sovtek) had a tendency to fail after short periods (like a year). There are more brands now, and maybe some are better. Still, this is a case where buying an old one, even used, was probably wise. The old rectifier tubes just keep on ticking.

It really is a great sounding amp with new components, though... I like its sound better than the bigger MKIII -- it has a comfortably low operating point, with plenty of class A operation, and very modest feedback, and most importantly it has really fantastic output transformers (which saturate just a bit on the low bass, giving it that plump-bottomed sound). As Sheldon Stokes said, it has a loose-tailored sound, like a comfortable old shirt, rather than the starchy sound of other amps.