What bias point are you using for modern production tubes? I know 1.56v is stated on the manual and is printed on the amp, but I have read somewhere that modern tubes requires a lower bias point as there not as sturdy as tubes made in the 60's
That can be true, especially for tubes sold as "guitar amplifier" tubes.
But the better quality audio tubes should be in the right range for the original Stereo 70.
But the better quality audio tubes should be in the right range for the original Stereo 70.
Where do you measure these 1.56 V? Without schematics there's no possibility to give a valuable statement!
Best regards!
Best regards!
We are discussing a Dynaco ST-70. Here the bias is measures at the "biaset" outlet of the octal plug in the front.Just
as described in the manual.
as described in the manual.
I don't know this amplifier, nor do I have the service manual. Isn't it somewhat annoying if someone asks questions without providing such basic informations like schematics?
Best regards!
Best regards!
The manual including schematics is freely available at nearest file storage. Google on "dynaco st-70 manual".
They're everywhere in America, about half a million (!) made, including kits, but probably not well known everywhere. Older folk have probably had at least a half dozen pass through their hands, 50 years ago for about US$100, and know them by heart. They're the '57 Chevys of audio (probably another American idiom - meaning hot-rod body donors for guys of a certain generation).
Bias was set for 1.56 VDC because it could be calibrated against a fresh single cell battery, across a 15R6 precision resistor (a luxury in the early 1960s) for 100mA for both output valves of each channel.
The kits originally sold for US$99.95, which was a lot back when grown men worked for $2 an hour, but a fair price and well received.
All good fortune,
Chris
Bias was set for 1.56 VDC because it could be calibrated against a fresh single cell battery, across a 15R6 precision resistor (a luxury in the early 1960s) for 100mA for both output valves of each channel.
The kits originally sold for US$99.95, which was a lot back when grown men worked for $2 an hour, but a fair price and well received.
All good fortune,
Chris
Note that the bias resistor was custom made for Dynaco. The 1.56 volts was chosen for ease of accuracy. Back in those days, the standard dry cell battery was carbon-zinc. When fresh, the voltage was always 1.56 volts. This made it easy for anyone with a half-decent DC meter to calibrate the reading with a fresh carbon-zinc dry cell and then adjust the bias on the amplifier.
Also note that the bias resistor was for the output pair, i.e., both tubes on a channel. With the 15.6 ohm resistor, that was 100ma of current for the pair, or 50ma per tube. Needless to say, matched pairs were required.
For the 60-watt MK-III mono amplifiers, these used 6550 or KT88 outputs and the bias resistor was 11.2 ohms. This resulted in about 140ma of current, or 70ma per output tube.
Back in those days, most manufacturers ran the output tubes at or near their maximum published ratings. That gave them better power and lower distortion for the most part, while sacrificing tube live. Then again, back in those days tubes were relatively inexpensive and easily obtainable.
Also note that the bias resistor was for the output pair, i.e., both tubes on a channel. With the 15.6 ohm resistor, that was 100ma of current for the pair, or 50ma per tube. Needless to say, matched pairs were required.
For the 60-watt MK-III mono amplifiers, these used 6550 or KT88 outputs and the bias resistor was 11.2 ohms. This resulted in about 140ma of current, or 70ma per output tube.
Back in those days, most manufacturers ran the output tubes at or near their maximum published ratings. That gave them better power and lower distortion for the most part, while sacrificing tube live. Then again, back in those days tubes were relatively inexpensive and easily obtainable.
Yes, maybe. But why should I google for someone else asking for advice when this someone could/should have googled already or even scanned the instructions/schematics? Honestly, I'm too lazy for that.
Here's an example for good practice. I've been very pleased to answer his question.
Best regards!
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The author asked for info about a Dynaco st-70. If one does not know anything about it one better keep quiet.
I have fond memories of riding around in my friends '55 "Shoebox" 283 4-speed. Good times. 🙂
jeff
Sounds like more internet BS. Just buy your tubes from a trusted dealer, not some mystery seller on ebay. That's just asking for trouble.
jeff
I was making $2.10/hour in 1975.
And, I set my tube bias voltage on my ST-70 at ~1.5V. The KT66 tubes have liked it for the past 10 years or so...
Note: amplifier is powered off, so voltage meter displaying zero.
It's possible that some modern-production tubes will require a change in the actual bias circuit to provide the correct bias voltage, but you should still set the bias for 1.56 volts. The EL34's in the ST-70 are not running terribly hot, and any decently-made new tube should not have a problem handling the current.
This could quickly devolve into the Monty Python skit about "We had to live in a pothole in the road..." etc. Great stuff, but I really did have a totally rusted out (already!) '55 two door, a '56 convertable, no top of course, and a '57 four door, only one motor between them and couldn't afford a battery good enough to avoid parking on a hill, in high school. It was a real chick magnet to need to bring a sister or two along to push start the thing.
4-speeds would have been James Bond level tech for lots of folk. Air conditioning? Are you kidding?
All good fortune,
Chris
