ST-70 amplifier advice.

cartop

cartop

AK Subscriber
Subscriber
Hi guy's, I have been looking for a tube amp to match my Frenzel tube preamp[in my avatar] and in my price range [$500-600] I can't seem to find anything new without a pre-amp built in. This amp is available but has been for a month and I wonder if anyone here can take a look and see if there is something obviously wrong with it. The seller say's he will demo it and the only thing missing is the cage. Also not being knowledgeable about tube amps is there anything I should know about this model besides all the good reviews I have read. The seller is asking $595 but I was thinking of offering $500. What do you think? Thanks.

00W0W_dtFka0L7Eblz_0x20oM_600x450.jpg
00000_aklJRfFVkRzz_0x20oM_600x450.jpg
 
Register to hide this ad
There are a few tube amps in your price range, several recapped/serviced, currently available in Barter Town.

Do you know if any work has been done on the ST-70? Condition of the tubes?
 
20220608_095906.jpg 20220901_111613.jpg It looks like the driver board has been replaced with and with better one with newer parts including Russian PIO caps. The electrolytic caps look like they have been replaced also.
It looks pretty good to me. The only thing as already mentioned, is the state of the tubes. You can get some decent power tubes for around $20 to $25 each.
The driver tubes are most likely fine, and still pretty cheap also. The rectifier tube isn't that expensive either, although you want to stay away from a couple brands like JJ. I use a TAD 5AR4 rectifier, the Tungsol's are supposed to be pretty stout also. I put EH 6CA7 fat boys in my ST-70 after trying various others and really like the EH's. I bought them for around $25 each for a matched quad. If you were to get this amp, I would stick with the driver tubes it has with the adaptors.
The supply for the original driver tubes used is getting thin, expensive, and can be michrophonic. The ST-70 has more support, documentation and mods than any other tube amplifier ever made. They never went out of production. When Dynaco went belly up, others just kept making kits. Like Dynakit, Tubes4Hifi, Triode USA, etc.
I would give it a good listen, and at higher volumes also before buying.
They're easy to work on and have loads of support here on AK and elsewhere.
Here is mine I built over 10 years ago using a VTA d4iver board I use in a second system.
 
I wouldn't worry too much about the power tubes as long as the amp sounds good, they are all the same brand and type and you don't see any of the internal plates glowing red. The heater glowing is normal though. You can check the bias also using a multimeter, the owner can show you what holes in the front sockets to use to test.
 
The good news is that cages can be found on Ebay and elsewhere; thousands of this unit were sold.
 
In working condition, $500 would be a pretty nice deal. Lots of mods and upgrades possible down the road, or keep it stock. I'm particularly pleased with the VTA boards/mods on mine (similar to what Lavane shows above). Very nice sounding amps! Good luck!
 
New reproduction cages are available at Dynakit and can be cheaper than used on eBay. They come in original brown with original logo, or black.
I built a ST-35 kit from Dynakit and bought the optional cage.
20220810_175713.jpg
 
Wow, You guys always come through. I checked BT and nothing really grabbed me. The seller of the ST-70 got back to me and said he would go to $550 and so I am going to have a listen. He said he went through the amp completely 2 years ago and it is working perfectly. I will post back when I can with an update and some pics. Thanks again everyone.....You are Marvelous.....:thumbsup::thumbsup:
 
cartop said:
Wow, You guys always come through. I checked BT and nothing really grabbed me. The seller of the ST-70 got back to me and said he would go to $550 and so I am going to have a listen. He said he went through the amp completely 2 years ago and it is working perfectly. I will post back when I can with an update and some pics. Thanks again everyone.....You are Marvelous.....:thumbsup::thumbsup:
Click to expand...
If you decide to get it, download the manual from hifiengine or here from the AK data base. Once transported, the bias should be checked. If you have to remove the tubes for transport, number them with a sharpie and put them back in the same sockets they come out of. Checking and setting bias is easy with a digital multi meter.
 
Lavane said:
If you decide to get it, download the manual from hifiengine or here from the AK data base. Once transported, the bias should be checked. If you have to remove the tubes for transport, number them with a sharpie and put them back in the same sockets they come out of. Checking and setting bias is easy with a digital multi meter.
Click to expand...

Great, Thanks so much. I am going to have my tech check it out if I get it. And will do on marking tubes.. :)
 
He did say this...........I restored it a few years ago. Complete disassembly and rebuild. Ive had no problems. The chassis is not ideal as the pics show. Hence the price.
 
OK, I talked in length with the seller of the amp, and he said that I should check my voltage in my house because if it varied, I guess more than just a few volts, It would mean the bias of the amp would have to be set at a deferent spec. He said he changed a resistor in the upgrade that changed what the bias is normally set at. OK, I can buy a meter and check my voltage but does anyone with the knowledge agree with this? I admit not knowing anything about tube amps, but would I have to go to a auto biasing amp to not worry about this kind of thing or is this not as important as it seems? He said if I run the tubes out of spec, which I would not want to do, it would run hotter than it should and shorten the life of the tubes. I am on the verge of just being happy with my Frenzel and my Onkyo m-504...........:confused:
 
I'm no expert, but my understanding is that the amps were built at a time when the standard outlet voltage was 117 volts vs 120 volts now, which increases the voltages throughout the amp from what they were originally designed for. There are two potentiometers for bias adjustment on the amp, so you should be able to fairly easily adjust the bias to keep the output tubes at a voltage range that they'll work well in. It's a fairly simple procedure that should only take a few minutes. The amps also use a rectifier tube, which should help bring up the voltage more slowly than solid state rectification, which should be easier on the amp. My best guess, and in my humble opinion, it's likely a non-issue, or a fairly mild one. I hate to see you give up a chance at a beautiful classic tube amp over it. If I'm wrong and it does prove to be a potential concern, it should be fairly easily remedied by you or by a tech.
 
Last edited:
Thanks Knotscott, I am going to buy a multi meter and try to set up a listen on Monday if possible. I will let you guy's know if I buy the amp and again thanks to all of you for all the help. Have a great day.......:bigok:
 
You probably know this already - but someone has removed all the markings on the front of the chassis (including the bias/test instructions) - FYI. I would point this out for negotiating.
 
Voltage at my house runs a pretty steady 123 volts. That may be the reason for the resistor change. I changed the resistor in mine because the bias pots were getting close to running out of adjustment. It wasn't a drastic change but it did make a difference. Couldn't tell you the specifics as it's been a while. 550 is good for a refurbished unit. Because of the tubes in mine I wouldn't sell it for less than 700
 
Have the seller show you the pin hole in the front sockets that you put your meter probe in. Positive goes to the socket, ground to the chassis.
A cheap digital meter will do. Since he replaced that resistor, ask what value to set it at now. I have several nice multi meters, but I also have 3 or 4 of those cheap $5 meters sold at harbor frt. A fresh battery is important.
Moving the amp from one house to another will probably change the bias some since your mains voltage is probably a few volts different from the owners.
Standard setting for a ST-70 is 1.56v per channel.
You could set it a bit lower at 1.40v for longer tube life also.
Once you have the amp setup and speakers connected, turn the amp on, let it warm up for 5 to 10 minutes then adjust without any signal input to the amp.
You will have to go back and forth between channels each time you adjust one if the pots to dial it in. After it's set, just check it every month or so. I would leave it on for an hour after setting the bias and give it a last check.
 
You must log in or register to reply here.
Back
Top Bottom