Dynaco VTA-ST70 Vacuum Tube Power Amp Troubleshoot and Repair

An old Dynaco ST70 power amp that went pop goes up on the bench.

[avahifi]

Gap between bottom cover and chassis is to allow space for cover sides to fit in between (not installed)

[lyntonprescott]

What a brilliant video! I was also confused about the HT (B+) coming from the cathode of the rectifier valve (tube) but could not find any explanation in any book or on the net. You have solved a mystery.
I’m going to watch your explanation over several times to make sure I fully understand this.
Ok on the lamp limiter and the fact that a speaker has to be connected, I could appreciate these from comments from other U tube vids but no one ever explained the rectifier.
Many thanks
Lynton

[coyote_den]

Big color CRTs are notorious for building up a high static charge long after the 2nd anode has been discharged and disconnected from the chassis. I mean, it's a glass jug with conductive surfaces so it's basically a Leyden jar.

It might not kill you, but it can jolt you bad enough you drop it, which doesn't end well for the tube.

[olipito]

Never had such a good explanation of the function of the rectifier tube, now I get it! Thanks!!

[pliedtka]

This one has improved bias board. Ah plenty of articles in Audio Amateur for ST-70. Yes they are easier to work on, but not always. I didn't know that tube amp shouldn't be powered up w/o load. Thanks for the tip.
I have Radio Craftsman mono from 50s, possibly designed by Sid Smith before he left Chicago to work for Saul Marantz - the Model 9. In 1994 I heard Earl Zausmer BMW: B&W tweets and mids powered by Millbert tube amps, one of the best car audio system I heard - non of the boom-boom you hear most of the time. Fast, very articulate bass, smooth mids and highs with great resolution and transparency - it was my sort of a base on which I would judge other cars and home audio back then.

[alexispieltin]

This amp has apparently suffered from a Cascade of failures, and you found one of the original culprit very soon. But I'm also surprised and delighted to see you find the other tube failure with that simple current limiter, many lab people have missed this simple bulb trick! Maybe European higher voltages have rendered this system difficult to apply, or simply we used other forms of current limiter, sometimes far more complicated, like some ampmeter with a variable needle limit assorted with a relay (Metrix stuff from the 1930'…), a 1950 much more modern current limiter using a bridge and a Thyratron as a fast variable current limiter, and last versions using a Hall detection cell, permitting fast and indirect current measurement, including DC ones, useful for total safety or DC operated (battery) systems, like many early radio sets.
However, I'm surprised you didn't tried the tubes on a specific tester, a good and safe way to get shorts and remaining transconductance datas. The full wave rectifier you suspect initially should have rendered different values from one side to the other if the initial forgotten solder was to suspect, but when you found one of the power tube was shot, I suppose you also clearly saw something different in the top getter deposit. I'd say it's not whiteish as a gassed tube, but you can immediately suspect a overheated tube or some form of shorts: if a tube is bad, that's the one. Another thing is important you partially notified, these tubes are supposed to be matched pairs, so this will cost much more to repair than a simple tube. Anyway, you should also check for the output transformer integrity, as a catastrophic failure is always possible when it comes to this point. As it seems these tubes are not the original types, it's always a good thing to measure the anode currents, bias voltages and output transformer values to get an idea of the tube operating point and check with the data sheets: this amp has been modified, and the polarization point should have been checked, but when it comes to simple switches to change from pentode to triode mode, you have to verify if these are OK (it's not a good practice to operate these on a hot amp!). You also have to check the coupling caps to finally ensure the initial culprit is not there. With the initial tube choice that was different, that make many suspects in what looks like a "hot" tube failure, particularly when client change tubes when they don't know if it's matching the transformer specs, polarization point and so on. People read in magazines "this tube is better than this one, you should change it, it's as easy as to change bulbs" and they finally pay for a catastrophic system.
Last, I guess you will also replace the rectifier tube, as the budget is nothing compared to the matched power tubes.
Another good advice is to double check what you supply if you use matched pairs. They generally pair them using transconductance or Mu values at a certain B+ voltages and polarization points, that could be very different in your circuit! The response curves may differ significantly, specially in pentode mode where non linearities could be suspected if you push the limits.

[waynemckinnon]

Thanks Dave. My brother should thank you too. We'll see. I'll be using this amp for some time more! As I now own a piece, (although I doubt he'll see it that way) I think it's fair.
As always, I learned a lot but still don't really understand too much. What I do know is that this thing makes nice music with that modified Pat 3x preamp that you fixed up for me last winter (?). Very nice.

[robinsonsoto]

With all due respect. You know a lot. Therefore. you are KNOWLEDGEABLE ELECTRONICS ENGINEER.

[robinsonsoto]

WITH ALL DUE RESPECT. YOU ARE A VERY KNOWLEDGEABLE ELECTRONICS ENGINEER.

[jakeb]

You hit a little about lineman work. Most of the time we work it hot and use rubber gloves, sleeves, and sticks. In order to work it dead we will ground it before where we need to work and after the work area. so that for some reason it will blow the breaker/fuse. This is the only way we can work on the line, substation, or transformer without safety equipment. Also just a line in the air can pickup electric that can kill you. We ground everything if we work without our PPE

[FindLiberty]

I use that same current limiter trick (switchable between shorted for startup, and then through a 25W bulb) while setting the Idle Current (external pot mod) on each bandpass-modified 200W subwoofer plate amp that I use as main L/R channels for keyboard. This let me find the exact best compromise between annoying output stage cross-over distortion and running too warm. You could also use a two speed fan switch or three step light switch to select multiple wattage lamps (if needed for a wider range of protection) with a separate bypass switch for full juice. It's a handy bench item that save lots of fuses.

[moodyga]

the GZ34 is the best rectifier to use in valve amps slow warm up stops the EHT from being dumped on cold tubes

[thermionic]

You woulda thought the person who modded the amp would have come across the roblems you corrected!

[RegiPavan]

It's ridiculous how expensive tubes amps have become lately. They had their time, got rejected by it bulk and electric bills and now they returned... Just waiting for the future CD's comearoundagain. Got a bunch of then getting dust.

[sfx]

I noticed another thing. The ST-70 originally used EL34, not 6L6G. EL34 has a higher plate dissipation and draws more heater current. It's possible the lower current draw of the heater current caused a higher than usual heater voltage or the amp could have been misbiased and exceeded the 6L6G's plate dissipation. Assuming the heater voltage is fine, 6L6GC would be the tube to use, assuming the bias and screen current are correct.

[Giblet]

I considered modding mine for a shoulder tube like 6L6G or KT88. Shoulder tubes just look better than EL34s, and if it looks better, then it has to sound better. It's the law.

[johnrobinson]

Have a pair of dynaco 90's from the way back days. They worked last time i had them out for a spin around the vinyl. But they are original and just not into the resto thing anymore. They hold down that side of the shelf very well. I got the kits for them 20 or 30 years ago to hot rod them but never did. I like my late 70's technics low power receivers, re cap them and they just keep on going. Great tuners too. Way better than the junk of today.
Got more than enough tube stuff, with too much of it to look after it becomes a job of sorts. And the real estate the spares take up.
You get old enough and you just want to simplify things.

[TheMilford]

Looks like a current version of the kit... also looks like an upgraded driver board. The chassis has that gap because there’s a cage that slots into the gap.

[_Hertz]

I had one of those for a while. I played around with different tube types for the long-tailed amplifier section. I found that the frequency response was a bit excessive in the bass region and played with a current controlled dual-triode long-tail amplifier. The result was very much like listening to a well-controlled MOSFET amplifier. I enjoyed the big iron of the design, but I didn't like that if the negative bias diode blew, it'd take out the finals in nothing flat. Fun to play with, lots of heat but a nice sound for about 1000 hours of use and then the finals would start going soft.


Sovtek 5AR4's never gave me much happiness. I probably got a bad batch from the factory as I burned out 2 of them.



It was far cheaper to have tube amplifiers before the boutique-inspired audio fools came into the picture. Now, it's too expensive.

[JacGoudsmit]

Great video as usual, but some inaccuracies. Not that I'm a tube expert (not even close) but I think most home equipment with tubes works on anything between 100V and 300V. Lower and higher are possible but not common. Also, I think 6.3V is most common as a filament voltage and it can vary between 1V-24V or so, but maybe I'm mistaken because my limited tube knowledge is from Europe (mostly Philips/Mullard tubes).
Beginners may also want to know that "B+" was originally short for "Battery Plus", the positive side of a high-voltage battery.
And the reason why an incandescent light bulb works so well as a current limiter is because Tungsten has a positive temperature coefficient (PTC): When it's cold, it's almost a dead short but when it warms up, the resistance increases. So if the Device Under Test doesn't draw a lot of current, it gets the full voltage from the mains because the light bulb filament stays relatively cold. But if the DUT has a short, the light bulb gets the full 120V (240V) from the mains, and limits the current to less than 1A (0.5A) (I = W / V) and conveniently lights up to indicate that there's a problem. It's about the most useful thing for an incandescent light bulb, besides lighting up an oven that gets too hot for an LED :-).