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Which amp to trade for Dynaco ST70 or VTL ST80?

I've ended up with a number of mystery things as well. At some point the only solution is to reverse-engineer the thing and draw up your own schematic. Its not that difficult with point to point stuff, but PCBs can make it considerably more difficult to trace out.
 
gadget73 said:
Not a big fan of the hot-snot mounted multicap.
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Learned a new phrase today. Looking for ways to use it now. :)

Dandy said:
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.
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A good idea for starters. And it can be done easy enough if in the US with a ground lift adapter plug available at your local hardware store for just a few bucks.

If hum still persists, I’d scope it to learn if it’s 60 or 120 hertz. If 120 hertz, I’d next look at replacing the reservoir/filter caps in the power supply. If 60 Hertz I’d look at the input cabling between input jack and the first gain stage.
 
arts said:
Does any skilled tech (or even a talented amateur) really require a schematic of such a simple unit? I don't think so
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EXACTLY! And VTL amps sound fantastic.. there's very little inside one of these amps, and it should be obvious if it has been worked on or not (repairs leave tell tale signs like flux on the board, different looking solder, etc)

I'd be all over that VTL - the hum issue can likely be solved without too much effort. The thing about the Dynaco is at the end of the day, it's still an ST70... the definition of an entry level tube amp.
 
petercapo said:
gets you much or most of the way to the sound quality of much more expensive amps
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Isn't that kind of the definition of entry level? I don't mean to group it with the $500 chifi creations, or console pulls - I mean it is an entry level high fidelity amplifier. The ST70 has been many people's entry into tube amps, and it serves well for this - both as a way to learn about tube amps, and as a reliable component. I certainly wouldn't call it bad - but it isn't really on the same level as say a VTL, or an Audio Research - those kind of amps are for people who already owned a Dynaco and want to level up.

For those photos - the top one looks all original, the middle one looks original on the board but there's something going on with the disconnected filter capacitors. The bottom one is too small to see anything.
 
if the guy who designed it is still living, the best option would be to contact him and see about an actual schematic. Absent that you draw up the unit in front of you and see if it makes sense. I'm more used to doing this with stuff designed by people who are long dead, so contacting someone for a sanity check simply is not an option. It may not be ideal but sometimes you have to work with the options available.
 
Maxhifi nailed it. A company like VTL is sure to have nice build quality on the boards. Any mods will be easily detectable with a careful look. I do it all the time on remanufactured boards on industrial gear.
VTL is high end gear. A ST70 can be found 7 days a week. I know what I would pick.
Heck, if you get the VTL and regret it, I have a clean unrestored ST70 with cage that I would trade for it.

BillWojo
 
I'm not a tube genius either and if I had that amp in my hands I would reconstruct the schematic and post it here. There is more than a few smart guys on here that could help out.

BillWojo
 
It was a fun process to decide which of the two amps to keep. The ST-70 was very quickly ruled out by the condition of previous repairs. Supposedly a well known shop had performed the work on it and charged close to $500.00 for the results you have seen in the pictures.

The hum issue has been resolved - along with the issue of the light dimmer in the ceiling fan - Simple grounding issue. The power strip I was using had a broken ground pin on the plug. Once I connected the preamp and amp to the same wall socket the low level hum went away and the ceiling fan light did not cause noise.

The cage wasn't that hard to take off. Just had to remove an end cap.

Then I checked the biasing. Not so good
I checked the resistance between the test point and negative speaker leads. They were all 9.9 to 10 ohms. Spec is 10 Ohms
The biasing was all over the place. the lowest was .201 and the highest was .399. The specs said that it should be between .250 and .300
I was able to get all of the tubes biased to .267 thinking that it was a good mid point. I will run the amp again to check to see if it is holding the bias.

The VTL works well with the NS200Ms.

I will take the bottom cover off of the amp for some pictures to share.
 
If you want to play the dating game,see if you can read the numbers on all of the electrolytics,especially the big cans. Most will have the date of manufacture printed on them.

The board area chipped away for cap clearance is kinda cheesy,although it could be an early production dimension oopsie that needed some fudging to make things fit.Also,the soldering on one of the 9-pin sockets looks a bit ratty. Judging by the board and components, I think this is an early one.Those transformers and that chassis are a great foundation for any amp,and due to the simplicity and low overall parts count, should be economical to completely restore,if you are so inclined.

Overall,it's not bad at all. Believe me,I have seen far bigger clusterf**ks.

Congrats:)
 
petercapo said:
Make sure the insulation layers over the large electrolytics' contacts stays in place. You wouldn't want one of those to contact the chassis unexpectedly... :angel: Keep the safety ground in use.
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Hello Peter, YES! I was surprised to see that in earlier posts there were no insulators. I did look under them. The caps in my VTL have screw terminals instead of spade connectors see in the other amps photos.
 
petercapo said:
That should be interesting. Giving up on tubes? Yamaha has made some good-sounding gear in their time. Please let us know how it sounds.
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Hello Peter,

Nope - not giving up on tubes. I really like the VTL ST-80.

The P2200 does provide more headroom when I really want to crank the volume to 11. It needs a very complete cleaning and checking of all of the components before I put it into regular service.

I need to sort out some grounding issues in my entire system though but I will start another thread for that.
 
petercapo said:
Here is an image I found online when I searched on VTL ST-80. Can someone tell me how to know if it is original or if it has been changed?
View attachment 1529994

Here's an image of another VTL amp, possibly an ST-75, that I found with a Google search:
View attachment 1529995

This one was labeled as the current production ST-85. This newer PCB is apparently now designed to have parts mounted underneath in a more orderly manner than in the older ones:
View attachment 1529999

With regard to the Dynaco, I must beg to differ about it being simply an entry-level amplifier. It is a fine-sounding unit that, when properly refreshed, does what it always does - it gets you much or most of the way to the sound quality of much more expensive amps. I think it is even better than some amps that are only moderately more expensive that it is.
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My stereo 75/75 looked just like that
 
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