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Which amp to trade for Dynaco ST70 or VTL ST80?

There's been no discussion yet on details of the VTL. What frontend tubes does it use, what topology does it use? How does it measure? Is there a schematic? Seems like that should be part of the discussion and decision. I assume the 80 watts rating is total power summed from both channels.

One issue I see with the dynaco is the scarcity of the 7199. A mod is very likely necessary these days to convert that tube slot to something more obtainable. Looks like you can still find this tube on eBay and elsewhere but what quality are they? I expect they have been picked over for years and it’s getting harder and harder to find ones that measure well.
 
ST70 was a budget tube amp, from the early '60s, most of us know it and love it. Great sound, especially at the price.

VTL was High End in the late '90s. Their flagship was the Wotan, 1,250W monoblocks, for $25K/pair. I went to a "Stereophile Expo" in 2001; several rooms used the Wotans to drive their uber-expensive speakers. One room used 10 Wotans per channel to drive Martin Logan's ultimate "Statement System."

There's a review in Stereophile (google VTL Wotan). The conclusion on page 3 is an absolute rave: "A masterpiece of Tube art."

We're talking about a lower model of course, but has the same genome. I wouldn't even think about it, I'd grab the VTL fast. 80W of tube power is ample for me (certainly compared to 35W); it's 30 years newer; and it was a supreme component in the High End, not just "great for the price." There are also many reviews for your VTL online — and keep in mind they're comparing it to the very best competition.

It's a choice between every starving student's "dream" amp for his dorm room, and every rich audiophile's dream amp for his 'High End' system. And you say the price is identical?!!

I'm not overly concerned about product support: it's a basic tube topology.
 
I only said I would choose it, over a Dynaco — one man's preference. Many feel differently, some don't seem to know the VTL brand.

You are as free to have your preference, as I am to have mine.
 
The St 80 has a pair of EL34's and a pair of 12AT7's per channel. I believe that it was rated as 40W x2.
 
So I was curious how hard you can push a pair of EL34s. Are there conditions under which you could obtain 80 watts or more from a single pair of EL34s running in AB1 with fixed bias?

The short answer is yes...but you are pushing the tube right to the edge. The conditions would be:
  • 6.8K primary (plate to plate)
  • 700V plate voltage
  • Fixed bias with -32V on grids
  • Pentode mode with 360V on screens
  • Idle current 35 mA.
  • The power supply would need to stoutly deliver 370mA peak current, or 260 mA RMS per pair of tubes, or 520 mA RMS for two channels @ B+ of ~710V DC, or so! That's a mighty beefy power transformer.

Plate power is 116 watts per pair of tubes, or about 90 watts at the speaker terminals (given 20% loss in the output transformer). So it does appear plausible the ST80 could run at 80 watts per channel, 160 watts total..assuming the tube has time to cool during the off cycles.:dunno:

But I wouldn't really want to run any new production reissue EL34 tubes under those conditions--nos maybe, but not new production. I think you'd be burning through tube sets every year or so. For example look at point P on the graph. Instantaneously, when one of the tubes in the PP pair is conducting, it is dissipating 300V * 240 mA = 72 watts (instantaneous). But every half cycle it gets to rest. Still, that's pushing them pretty hard.

EL34 Eval.png
 
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Well,when it comes to EL34s you can push them pretty darned hard! I have a pair of these Hiwatt 200 rackmount units leftover from my touring days,and subsequently used in my stereo for a period of time. Unfortunately,a dozen power tubes generate more heat than I am comfortable with,Canadian winters notwithstanding. Each unit has 6x EL34,1x 12AX7,1X 12AT7.They run very close to 700V on the plates,and those Partridge output transformers can level buildings. The JJ E34L has proven to be perfectly reliable in these amps.:)

(internet pics)


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Excellent info Peter. Thanks for posting that. Much easier to get a pair of EL34s to do 55 watts than 85 watts.
 
Todays update - I just picked up a Yamaha C-85 preamplifier. I am thinking this will be better for testing than using the preouts from the Sansui 9090db or the home brew SS pre.

I really like the C-85s parametric eq, variable loudness, inverted pre amp outputs and the phono stage. The inverted outputs will let me run the stereo amps channels out of phase to ease the load on their power supplies. I will just have to remember to switch the polarity on one of the speakers so that they are still in phase. The phono stage has various capacitance loads for bot MM and MC cartridges!

I did speak with Vee today at VTL. She seemed really on the phone. She said that the VTL ST-80 predated her arrival at VTL and that the ST-80 was built before they went digital with all of their manuals. Nothing is available from them. She did say that the website has biasing info though that is generic to the amps.

The Dynaco and VTL should be here this evening so I can begin my listening tests.
 
petercapo said:
Are you able to change out the 5R4 for a 5AR4/GZ34? I believe the Stereo 70 will run at reduced power output with a 5R4 in it.
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I think that I have a 5AR4 in my Pioneer tube amp.

So far I have the VTL ST-80 hooked up and running. I really like it with the NS-200M. Next I will try the Tannoys.

We did have a hickup at first. There was a loud buzz from one channel. We tried various interconnects and the buzz moved from one channel to the other. Nothing seemed to be consistent. Then I remembered an issue I had with my Ampeg 6446 amps. I shut off the ceiling fan and the buzz went away!!! I think that it is the light dimmer in the fan that is the culprit. Why does the dimmer only affect some tube amps and none of my SS amps???
 
Potato_Head said:
I think that I have a 5AR4 in my Pioneer tube amp.

So far I have the VTL ST-80 hooked up and running. I really like it with the NS-200M. Next I will try the Tannoys.

We did have a hickup at first. There was a loud buzz from one channel. We tried various interconnects and the buzz moved from one channel to the other. Nothing seemed to be consistent. Then I remembered an issue I had with my Ampeg 6446 amps. I shut off the ceiling fan and the buzz went away!!! I think that it is the light dimmer in the fan that is the culprit. Why does the dimmer only affect some tube amps and none of my SS amps???
Click to expand...
Maybe higher input impedance making them more sensitive to rf interference? Maybe the cable has a bad shield connection that the low Z sand sloughs off.

Correction, that should be EMI (obviously not radio frequency).
 
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petercapo said:
If possible... it would be helpful to see images of the Stereo 70 top side and/or inside to further ascertain its condition....
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Pictures coming tomorrow. Been too busy listening to the VTL. I really like it except for a constant 60 cycle hum. It is very low level but I can still hear it with the Yamaha NS200Ms. The Sansui 9090DB didn't drive the Yamahas this well.
 
There has been some work done to the Dynaco. Looks like the Choke(?) has gotten very hot and melted the wax out of it. I think I recognize the hot melt glue. It is not holding the cap in place so the cap can bounce around. Maybe Rick Caldwell(?) worked on this amp before. He like to use the hot melt a lot. I have a 5AR4 to swap in when I hook the amp up tomorrow. Didn't the factory use two colors of wires to denote the channels? Maybe that was just the PAS 2 I had. Are the traced on the PCB supposed to look like that or did someone flow solder on top of etched traces? The driver board looks to be original except for one capacitor.
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Not quite sure if I will open up the VTL or not. There are lots of small screws holding the cage on. The hum does bother me. I might not end up getting either amp At least the bias text points and adjustment pots are accessible from the top.20190606_230351_HDR.jpg 20190606_230345_HDR.jpg 20190606_230224_HDR.jpg
 
The chokes are often a bit drippy on the ST-70, its not usually an indicator of any real issue. They get slightly warm and the wax doesn't take a whole lot of heat to run.

The traces look typical for the stock driver board. Lots of old PCBs were done that way. I suppose it was to keep the copper foil from oxidizing. Most of them also seem to use all one color wiring inside too.

Not a big fan of the hot-snot mounted multicap.
 
So, the VTL is newer and in better condition, but has a hum issue and no service info, schematic or support is available. I've got a boutique 80s tube preamp like that, and it's pretty frustrating.

The Dynaco is older, and has some changes that need undoing, but has all the service information and upgrade options you'll ever need.

It's your decision, based on what you need and whether you could get help from a skilled local tech if the VTL needs work. It would be hard to find such help online. If the resistances of the transformers in the Dynaco check out, it would be quite easy to get it working again yourself and forum support here and elsewhere would be available.

Do they both come from the same owner? If so, I think you definitely need to look under the hood (ie the underside) of the VTL before making your decision. Hopefully, there will be no 'hot melt mods' there.

P.s. After rereading this thread, I would *temporarily* lift the ground from the VTL power cable, just to see if that loses the hum. Like others here, I want to love the VTL. It's just the support issue that remains.
 
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I think there is far too much nervous nelly going around regarding servicing the VTL.It is a couple of 12AT7's driving a pair of EL34's.
Does any skilled tech (or even a talented amateur) really require a schematic of such a simple unit? I don't think so ;)

For future reference,take very clear,high resolution photos of the chassis interior,and both sides of the pcb.This will allow you to have a record of all component values in the event that you ever ''smoke'' a component and it's value is no longer legible.What more do you need?
 
If guys here are having issues researching the VTL80, imagine what others will run into trying to figure it out.
Get the ST70. It's a very well known item, with a ton of modern options and will be much more marketable should you want to sell it in the future.
 
arts said:
I think there is far too mto atic of such a simple unit? I don't think so ;)

For future reference,take very clear,high resolution photos of the chassis interior,and both sides of the pcb.This will allow you to have a record of all component values in the event that you ever ''smoke'' a component and it's value is no longer legible.What more do you need?
Click to expand...
:idea: I regularly see posts from AK members struggling to find anyone who can help them with a straightforward repair or servicing query, and that's with well-known brands and schematics. Maybe I'm being over cautious because I have a number of amps without schematics, and it does make life hard for amateurs like me.

It would be nice to see photos of the chassis interior and both sides of the PCB, but Potato Head is reluctant to remove the cage and nobody else on this thread has ever seen one of these. We'd like to be in PH's shoes, but aren't. :)
 
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