New to the Dynaco ST70 Community

[Quadaptor!]

So far I have tried the Fosi P4 active preamp and a simple passive preamp (volume control). So far I am liking the P4 mainly because of the multiple inputs and the tone controls (gain is not needed however and is set to minimum). As long as I use my Marantz receiver as the input controller I will never need a full preamp as in a PAS. Sounding great though I will recap the input board at some point (I have the parts waiting) and I also installed a matched quad (it had 3 German tubes and some no name 4th tube). I might also install a 3-prong power cord that I have.

Sounds like fun, keep it going. Also, keep this post going too.

 

[albusark]

View attachment 3820639I’m trying to replace the can cap but I don’t want to undo all of the wires at once. The guide am going by said that he clipped the lugs on the cap but what can I use to do that? The only big diagonal cutters that I have are too dull and the newer small cutters can’t do it either. Can you just twist them off of the can? What do people here do?

I always draw a detailed diagram for a can cap replacement. Dynaco already supplies overhead drawings of wiring so you can just use that in your case, no matter what it is a time consuming effort but when you're finished it's very rewarding.

 

[pac1085]

Also the new one doesn’t have terminals with holes but only thin posts and no twist tab perforations View attachment 3820641

That’s a PCB mount can capacitor. You should send it back and buy the correct capacitor for the ST-70

Edit: didn’t see the post where you already installed it. Ah well. For anyone else reading in the future, you can still get the correct twist lock caps for the st70 from a number of manufacturers

 

[mcgjohn]

The dyna 70 is tough to beat at that price point. Over the years have owned 4 or 5 copies of the ST-70. Very listenable amp. Never wears on your ears.
There are already tons of comments. The only adds I have is make sure you get a robust rectifier tube. The diode mod trick on the rectifier socket is definitely worth doing.
If you replace the main filter cap, you may want to look for one from Authenticap since they offer a cap with a bit higher voltage rating.

 

[RDW]

That’s a PCB mount can capacitor. You should send it back and buy the correct capacitor for the ST-70

Edit: didn’t see the post where you already installed it. Ah well. For anyone else reading in the future, you can still get the correct twist lock caps for the st70 from a number of manufacturers

I ordered the cap linked to in the article I mentioned and never noticed it was a PCB mount until I went to use it. I should have been more careful what I ordered.

 

[RDW]

The dyna 70 is tough to beat at that price point. Over the years have owned 4 or 5 copies of the ST-70. Very listenable amp. Never wears on your ears.
There are already tons of comments. The only adds I have is make sure you get a robust rectifier tube. The diode mod trick on the rectifier socket is definitely worth doing.
If you replace the main filter cap, you may want to look for one from Authenticap since they offer a cap with a bit higher voltage rating.

What is the diode mod trick on the rectifier socket that you mention? The rectifier tube is the only one that I haven’t replaced and I also didn’t change the selenium rectifier as several mentioned that it is a low current design and is not a problem. What rectifier tube would you recommend?

 

[Lavane]

What is the diode mod trick on the rectifier socket that you mention? The rectifier tube is the only one that I haven’t replaced and I also didn’t change the selenium rectifier as several mentioned that it is a low current design and is not a problem. What rectifier tube would you recommend?

The yellow sheet mod.

 

[Lavane]

Instead of a rectifier tube, I'm now using a WZ-34 Coppertop solid state rectifier in my ST-70. It has the same voltage drop as a 5AR4/GZ34 rectifier tube and a thermistor to add a bit of a slower start up compared to just two diodes.
The 5AR4 gets stressed in these amps at higher volumes. The softer start is really the only thing the 5AR4 has going for it when used in these amps.
I've been using the WZ-68's, (same as two 5AR4's) in my KT-88 mono blocks for several years without issue, so I got the WZ-34 for my ST-70.

 

[mcgjohn]

What is the diode mod trick on the rectifier socket that you mention? The rectifier tube is the only one that I haven’t replaced and I also didn’t change the selenium rectifier as several mentioned that it is a low current design and is not a problem. What rectifier tube would you recommend?

See @Lavane comment. He found it faster than me. This trick lowers the PIV a bit and makes life easier on the rectifier tube.

 

[RDW]

Instead of a rectifier tube, I'm now using a WZ-34 Coppertop solid state rectifier in my ST-70. It has the same voltage drop as a 5AR4/GZ34 rectifier tube and a thermistor to add a bit of a slower start up compared to just two diodes.
The 5AR4 gets stressed in these amps at higher volumes. The softer start is really the only thing the 5AR4 has going for it when used in these amps.
I've been using the WZ-68's, (same as two 5AR4's) in my KT-88 mono blocks for several years without issue, so I got the WZ-34 for my ST-70.

This might be the best bet instead of a tube. I noticed as with the output tubes prices can range drastically for these tubes so a WZ34 may be the best and simplest solution. I’m trying to keep prices reasonable as I am retired and don’t have the best hearing anyway. I already replaced the output tubes with a matched quad of JJ EL34s since the condition of what was in there was unknown. Or I could get a JJ 5AR4 for somewhat less.

 

[Lavane]

This might be the best bet instead of a tube. I noticed as with the output tubes prices can range drastically for these tubes so a WZ34 may be the best and simplest solution. I’m trying to keep prices reasonable as I am retired and don’t have the best hearing anyway. I already replaced the output tubes with a matched quad of JJ EL34s since the condition of what was in there was unknown. Or I could get a JJ 5AR4 for somewhat less.

DON NOT buy a JJ 5AR4! They have a high failure rate. I had two fail with just a few hours on them. One in my ST-70, and a second one in my 300B SET amp.
Others have had similar stories about JJ's 5AR4 failures as well. If your going to use a 5AR4, maybe get a Sovtek. You can also use a NOS 5U4GB. It has 17 volts more voltage drop, and isn't a soft start rectifier, but it won't be stressed in the ST-70 like the 5AR4 is. I think Tubes4hifi sells 5U4GB's now for there amp kits instead of 5AR4's. They also sell a dual rectifier board for the ST-70. You could use two 5AR4's with it then I guess. I thought about getting one of those board kits but its just so ugly looking sitting on top of the amp. The best tube rectifier for the ST-70 would be a GZ37, but they don't make them anymore and a NOS GZ37 will set you back $150 each or more.

 

[RDW]

DON NOT buy a JJ 5AR4! They have a high failure rate. I had two fail with just a few hours on them. One in my ST-70, and a second one in my 300B SET amp.
Others have had similar stories about JJ's 5AR4 failures as well. If your going to use a 5AR4, maybe get a Sovtek. You can also use a NOS 5U4GB. It has 17 volts more voltage drop, and isn't a soft start rectifier, but it won't be stressed in the ST-70 like the 5AR4 is. I think Tubes4hifi sells 5U4GB's now for there amp kits instead of 5AR4's. They also sell a dual rectifier board for the ST-70. You could use two 5AR4's with it then I guess. I thought about getting one of those board kits but its just so ugly looking sitting on top of the amp. The best tube rectifier for the ST-70 would be a GZ37, but they don't make them anymore and a NOS GZ37 will set you back $150 each or more.

Thanks for the warning! What do you think of the WZ-34 Coppertop?

 

[Lavane]

Thanks for the warning! What do you think of the WZ-34 Coppertop?

I like it. I don't have to worry about thrashing a 5AR4 when listening at higher volumes. I've had it for some time now without issue.
I've been using the WZ-68's in my mono blocks since I built them in Feb. 2020. Webber states on there web sight that these copper tops shouldn't be used in hifi gear. I dunno why though. If they can survive being used in guitar amps why not hifi equipment?
The GZ68 is whats recommended for my Tubes4hifi mono blocks.
These copper tops don't have anything exotic in them. Just a couple diodes, a couple reaistors, and a thermistor for a bit of a soft start. Tubes44hifi sells a small board that delays the B+ voltage until the tube heaters warm up. Some folks add one of those. I haven't added one to my ST-70, or the mono blocks though. If using one of those plug in SS rectifiers that are just a tube base with a couple diodes inside it might be a good idea to get a delay board.

 

[RDW]

I’ve got about10 hours on the new output tubes so far and it sounds great to me. I see no reason at the present time to replace the rectifier tube. I may take it back to the shop and recap the input board but only because I already have the parts. So far I’m loving this new (to me) amp!

 

[gadget73]

Good deal. besides outrite fireworks, the usual failure mode for the rectifiers is low output voltage. You'd see that as lower output power but more easily noticed is it won't bias up properly on either channel.

 

[Quadaptor!]

Are those power tubes JJ? I see you have the adaptor sockets for the 7199s. I used those on a little (about 14-watts per side) Heathkit AA-30 stereo amp and have nothing but praises to say about it.

 

[RDW]

Are those power tubes JJ? I see you have the adaptor sockets for the 7199s. I used those on a little (about 14-watts per side) Heathkit AA-30 stereo amp and have nothing but praises to say about it.

Yes it is a matched set of JJ tubes. I bought these adapters and preamp tubes when I bought the caps in case I needed to replace the 7199s which seemed to be working ok but since I had them I went ahead and installed them too. The rectifier is the only tube that hasn’t been replaced.

 

[Quadaptor!]

Yes it is a matched set of JJ tubes. I bought these adapters and preamp tubes when I bought the caps in case I needed to replace the 7199s which seemed to be working ok but since I had them I went ahead and installed them too. The rectifier is the only tube that hasn’t been replaced.

I have a couple ST 70s (mind you I fell into the Dynaco hole back in the mid 80s) and I always thought changing out the rectifier tube for a SS Webber was a good idea. I haven't had a ST 70 up and running for a while and never tried a Webber myself so I do encourage you to do your research. But here's an observation. My house dates back to 1946 and as an electrician once frankly put it, electrical service back then was not designed to power the amount of appliances we take for granted today. Having said that, I had issues with the input transformer humming and short lives with rectifier tubes on my ST 70. But a couple years ago I tried a DC Block from Audiolabs and, outside of observation with nothing to support it, I do believe the DC Block made a difference. At least it did when I put it between the the wall outlet and my Heathkit AA 30 that too uses a GZ34/5AR4. And I had problems with the rectifier tube on that one too. Just something to keep in mind as you are navigating the universe of tube amps and ST 70s.

 

[RDW]

I have a couple ST 70s (mind you I fell into the Dynaco hole back in the mid 80s) and I always thought changing out the rectifier tube for a SS Webber was a good idea. I haven't had a ST 70 up and running for a while and never tried a Webber myself so I do encourage you to do your research. But here's an observation. My house dates back to 1946 and as an electrician once frankly put it, electrical service back then was not designed to power the amount of appliances we take for granted today. Having said that, I had issues with the input transformer humming and short lives with rectifier tubes. on my ST 70 But a couple years ago I tried a DC Blocker from Audiolab and, outside of observation, I have nothing to support this with, I do believe it made a difference. At least it did when I put it between the the wall outlet and my Heathkit AA 30 that too uses a GZ34/5AR4. And I had problems with the rectifier tube on that one too. Just something to keep in mind as you are navigating the universe of tube amps and ST 70s.

I’ll keep that in mind, thanks!

 

[peterh]

Nice to see, enjoy the music !

As for the rectifier tube ; they use to keep alive very long. Two things that kills them ( independent of brand) is overcurrent and hotswitching. Both these kills by creating arc that shorts the elements and physically damages them.

The "yellow sheet" diode mod does some help here as the arc won't be continous as the diodes block during the negative half period. This is the only thing i would do if this was my amp.