How to bias a Dynaco ST-70

[mediumjim]

My Dynaco ST-70 arrived today and the first impressions are real good.  I want to bias in a quad of Mullards and have a BK 2408 Multimeter.   Here's a link to it:

http://www.bkprecision.com/products/model/2408/mini-pro-digital-multimeter-w-non-contact-voltage-tester.html

I'm inept as they come, so please forgive what seems to be a simple question!  My guess is to set it at 2V and the positive probe to the socket and ground the negative probe to the chassis?  Is this correct? 

Back to the Dynaco, I'm running it mated with a McIntosh MX-110 and it on first impressions is a great mating, dead quiet.  I opted to get a Subwoofer (Polk Audio 10")  to go with Meadowlark Swifts.  The sub is still breaking in, it nice and taught and fast, but does need to open up a bit.  The ST-70 has no problem driving this set-up well over 100db effortlessly.   The highs are real sweet and not at all fatiguing, the mids are a little forward, but that may be the sub setup right now.  The bass is there but still needs to be dialed in to achieve smooth and seemless response between the Swifts and Sub.  

The soundstage is big/wide/deep and with great instrument placement.  It will only get better when the tubes get some burn in.

Anyway, it would be real nice if someone would help me figure out how to bias them with the meter I have.

Thanks,
Jim

 

[Grainger49]

The manual tells how to bias on page 6 & 7 (Initial Adjustment, ignore getting a new D Cell, just measure the voltage):

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

This assumes the pairs in each channel are matched.  Without a match one or both of them will not have the current they specify for them.  

The procedure also assumes that the 2 white resistors under the chassis, attached to the output tube pair sockets and ground, are still 15.6 ohms.  They were 1% resistors 50 years ago and still might be.  But first measure them using your meter on the lowest resistance scale (200 ohms).  If they are 15.6 ohms you should get 1.56 volts.  If they are 1X.Y ohms you will get 1.XY volts where the X and Y are what you read in ohms on each resistor.  Recheck after 15 minutes and 30 minutes running.  Be sure to measure DC volts not AC volts (V~).

 

[mediumjim]

Grainger:

Good to go on the resistors...my question was the setting on my multimeter to be 2V. 

Jim

 

[JC]

Neg. to ground, Pos. to bias pin on front socket.  You may wish to set the meter to the 20V range to start, in case the bias is off enough to have your reading exceed the 2V scale; then, after an initial tweak to get the reading under 2V, reset the meter to the 2V scale for fine adjustment to 1.56V.  Most digital meters react to an over-scale reading pretty benignly, but you may as well play it safe until you know for sure.

 

[2wo]

Yes, the 2V is were you end up.  I start with a higher scale and work my way down. The reasoning is say there is 3V at your test point when you start. You'll get some sort over range on your meter that can confuse you for a second, then you have to switch up anyway. On a 20V or 200V scale you can still read 1 or 2V, just not as precisely...John   

 

[mediumjim]

Yes, the 2V is were you end up.  I start with a higher scale and work my way down. The reasoning is say there is 3V at your test point when you start. You'll get some sort over range on your meter that can confuse you for a second, then you have to switch up anyway. On a 20V or 200V scale you can still read 1 or 2V, just not as precisely...John   

You are so right...I end up at 20v and I was able to dial both channels in at 1.48...It will hold 1.56 no problem, but its a bit more relaxed running a little cooler.  The soundstage is totally 3D and the inner detail really showed up. 

It is all coming together as the subs got some life after playing some pink noise through them and I was able to properly integrate them with the Meadowlarks and it is now seemless and the mids are now really good. 

My Marantz 9's have some competition...David Hafler had it right back in 1959. 

I may leave the JJ's/Tesla mixture in her as they work real well.  I put two of my 1980's Tesla's in her as one of the JJ's arrived gassed and save the Mullards for down the road.  I haven't even put in the NOS 7199's as the original Dynaco 7199's
are still strong.

I did replace the 5U4GB it arrived with with a 1957 Metal Base Amperex GZ34 that I was hoarding!

Thanks to all,
Jim

 

[corndog71]

Valve Art makes some really nice EL34's too!

 

[mediumjim]

Valve Art makes some really nice EL34's too!

Not sure if Valve Arts makes them or is like Groove Tubes who sources them and puts them through rigorous testing and them brands them VA.  I do have to agree that there has been a positive shift in the modern EL34's.  They will not rival the older vintage EL34's in overall quality and will not burn for as many hours. 

Jim

 

[mediumjim]

Now that this technologically declined individual knows how to bias his ST-70's with a a meter, he will now look for the sweet spot.  I heard some actually bias theirs as low as 1.40.  I'm sure that each ST-70 will be slightly different as is the nature of assembled resistors, capacitors and other electonics. 

Too cool and the bass definition gets floppy and slow, go to hot and the highs sizzle.  My Marantz's are real happy about two needle lines low, seems to give them more headroom, clarity and detail.  Tubes will last a whole bunch longer if not pushed too hard. 

Thanks again,

Jim

 

[JC]

The thing to keep in mind is crossover distortion.  If your bias is set too low, one or both tubes in the push-pull pair will stop conducting before/after the zero crossing of the waveform.

Like all things tube, this is not a precise one-size-fits-all setting; it falls within a range.  Dyna, for instance, picked a setting where they felt comfortable that the vast majority of EL-34s manufactured then would be "on" during the zero crossing, and stop conducting shortly thereafter.

The "real" way to set the bias, then, would involve a signal generator and a scope to observe the waveform at the zero crossing; amp manufacturers obviously could not rely on many of their customers being so equipped, especially in the '50s and '60s, so they developed methods to allow customers to get the bias into a range they felt would accommodate most tubes most often. 

Interestingly, most guitar amps did not provide any user-accessible facilities to set the bias on their push-pull amps.  Many did not even include a cathode resistor to measure in order to facilitate setting the bias on the output tubes.  Some had no way to adjust the bias, even if you could measure it.  They evidently relied on the consistency of the tubes being manufactured to fall within a usable range of bias, and didn't worry about the occasional outlier.  Or, the resulting distortion they might cause.

 

[mediumjim]

JC:

True, most if not all early guitar amps were based upon a simple Western Electric Circuit with fixed bias.  One needs to make sure that the output tubes they're using in this situation are balanced and with spec. 

Later amps were cathode biased and have bias pot(s).

If I really wanted to do it right, I would put in my tube socket bias tool and check each tube for ma output and balance them that way...right now it sounds most excellent, so I will just enjoy the ride. 

Jim

 

[JC]

Remember the qualifier: "user accessible."  Most often, one had to pull the guitar amp chassis out of the box to do anything with the bias.

Leo Fender's goal was to build amps that generally worked no matter what tubes or adjustments, or lack thereof.

Marshall, in their hey-day, required removing the chassis.  Ampeg was iffy; in much later reissues, they sometimes used a comparator circuit and a few LEDs to "set" the bias.

HiWatts were inside, too, as a rule.  Same with most Gibsons.

Now, I know there were many, many more brands than just those I mentioned, but it's been my experience that guitar amp manufacturers either expected a tech to set the bias, or for it not to get set.  Of course, when tubes were being turned out in great quantities, I'm sure the consistency from tube to tube was much improved.

 

[mediumjim]

JC:

Vintage tubes in general were better in all area's and the major brands tested and rejected those tubes out of spec.  Agreed, the design of Guitar Amps were not for the benefit of the general public and biasing was done by a Fender, Marshall, Gibson, Ampeg, etc., technician. 

The key tube in most amps is V1 (preamp) and the phase inverter which should be balanced on both triodes for optimal performance.

Fender in general used high quality transformers, Triad, Schumacher Transformers that were very forgiving. 

Jim