I just built a Dynaco ST-70 tube amp. The kit was from Triode Electronics. I set the bias pots per the instructions. The tubes are all on and are glowing nicely but every once in a while I get a bright blue flash from the new GZ34 rectifier tube and it blows the 3 amp slow blow fuse. I hooked the amp up to a source and it operated fine for 15 minutes or so until it happened again. It popped the fuse and then the unit slowly shut down because the fuse was blown. The tube is a JJ Elctronic GZ34. Is it possible that I just have a bad tube that shorts out every once in a while? Does anyone have any other suggestions?
Either the rectifier is bad (now that it has arced over a few times its days are likely numbered) or you have a short somewhere in the B+ rail. What output tubes are you using? It could be an intermittent short in either a tube or a transformer. I'm inclined to think its a tube, possibly a screen wire that warps out of place once it is hot enough.
Beware of JJ KT77s. They seem to have issues with cathodes that like to slide out of the micas. The first quad I got arced the rectifier and blew the fuse as soon as B+ came up. Turned out one of the cathodes and slide far enough for its strap to contact a plate support wire. I was actually able to fix the tube by firmly smacking the end in my palm to slide the cathode back into place. I noticed at least one other that had slid.
AES swapped-out the quad and even replaced the rectifier. Kudos to them. The new set had centered cathodes and work great. Awesome tube for the ST-70 as long as you don't get bad ones.
Russ
Beware of JJ KT77s. They seem to have issues with cathodes that like to slide out of the micas. The first quad I got arced the rectifier and blew the fuse as soon as B+ came up. Turned out one of the cathodes and slide far enough for its strap to contact a plate support wire. I was actually able to fix the tube by firmly smacking the end in my palm to slide the cathode back into place. I noticed at least one other that had slid.
AES swapped-out the quad and even replaced the rectifier. Kudos to them. The new set had centered cathodes and work great. Awesome tube for the ST-70 as long as you don't get bad ones.
Russ
That sounds par for the course for the JJ rectifiers. I blew two of the damn things out within a week. They just don't cut it.
I modified all my Dynacos by installing a couple of 1N4007 diodes in the power supply. They rectify the AC voltage and the tube basically provides the slow turn on that we like so well. I haven't lost a single GZ34 after that. I will mention that the Sovtek GZ34 rectifiers seem to last a long time.
I modified all my Dynacos by installing a couple of 1N4007 diodes in the power supply. They rectify the AC voltage and the tube basically provides the slow turn on that we like so well. I haven't lost a single GZ34 after that. I will mention that the Sovtek GZ34 rectifiers seem to last a long time.
Yep, check the rectifier. I scratch built a 70 a few years ago, and it would flash, and blow a fuse. I thought I did something wrong, and tore the amp apart. I could not find a problem. I replaced the jj with a 5U4 as suggested here and it worked perfectly. A fter switching to another new JJ it still worked but will flash, and flow the fuse occationally. All my future builds will be based around a 5U4!!!!!!!!!!!!!!!!! I really like that tube. The other one sucks hard.
This is one case in where new production tubes really are garbage.
This is one case in where new production tubes really are garbage.
Thanks for the inputs!!! I'll go ahead and install a 1N4007 diode in series with each plate lead. I don't have any on hand so I ordered them. I also ordered a new Sovtek 5AR4. I hope that does the trick. If that doesn't work I'll go ahead and swap it out for a 5U4. The output tubes I'm using are EL34s. Thanks again for the replies. I'm really new to this and you guys are helping me out a ton.
Also I did have one other question.... The ST-70 build instructions told me to install the RCA shorting plugs to set the bias pots to 1V on each side. I wasn't sure about what an RCA shorting plug was so I went online and I saw a lot of talk about people building these with by shorting out the RCA jack with a 10 - 100 ohm resistor. I did this with a 47 ohm resistor (one in each RCA jack I had laying around) and didn't have any problems setting the 1V per channel. Being a complete newb on this... did I do the right thing? I didn't think it was a good idea to totally short out the RCA jack but maybe I'm missing something.
Also I did have one other question.... The ST-70 build instructions told me to install the RCA shorting plugs to set the bias pots to 1V on each side. I wasn't sure about what an RCA shorting plug was so I went online and I saw a lot of talk about people building these with by shorting out the RCA jack with a 10 - 100 ohm resistor. I did this with a 47 ohm resistor (one in each RCA jack I had laying around) and didn't have any problems setting the 1V per channel. Being a complete newb on this... did I do the right thing? I didn't think it was a good idea to totally short out the RCA jack but maybe I'm missing something.
That will work fine. There is already series resistance in the input to the circuit, so a completely shorted RCA plug works too. That's what I usually use.
1V of bias? What size resistor are they using for the output tube cathodes? I guess if that is what their instructions say...
BTW, the following forum is very Dynaco-oriented and you may find more specific help with certain kits there:
http://www.diytube.com/
It's not terribly active, but the archives have a lot of info for various kits.
1V of bias? What size resistor are they using for the output tube cathodes? I guess if that is what their instructions say...
BTW, the following forum is very Dynaco-oriented and you may find more specific help with certain kits there:
http://www.diytube.com/
It's not terribly active, but the archives have a lot of info for various kits.
The resistor on the cathode to ground is a 10 ohm resistor. This is the resistor I take pin 8 and pin 1 of the EL34s to ground with.
If I seem really green on this stuff it is because I am, I appreciate everyone's help and patience. I'm really trying to get a grasp on some of the theory of how these circuits work and react. That's why I'm starting out with a kit. After I get this ST-70 up and running I'd like to make a run at building a pre-amp using some 12AU7s. I've seen a few schematics I'd like to try out.
If I seem really green on this stuff it is because I am, I appreciate everyone's help and patience. I'm really trying to get a grasp on some of the theory of how these circuits work and react. That's why I'm starting out with a kit. After I get this ST-70 up and running I'd like to make a run at building a pre-amp using some 12AU7s. I've seen a few schematics I'd like to try out.
If both tubes share the same 10 ohm resistor, then 1V across this resistor gives you 50mA per tube (assuming a well balanced set). I believe this is where Dynaco also biased their tubes, though I tend to bias EL34s at more like 40mA (which would be 0.8V on Triode's test point).
The Triode chassis does not facilitate a twist-lock cap, so you have to get Triode's capacitor board. I don't remember the grand total capacitance, but it is a significant improvement over the "stock" Dynaco multi-cap.
The Triode chassis does not facilitate a twist-lock cap, so you have to get Triode's capacitor board. I don't remember the grand total capacitance, but it is a significant improvement over the "stock" Dynaco multi-cap.
Yep, both EL34 tubes share the same 10 ohm resistor (1 resistor per channel, matched set I believe).
You are right, it's not a twist lock cap, it's a cap board within the chassis. Triode Electronics ST-70 Capacitor Board (over 500uF's per their web site.)
Still waiting on the diodes and the new rectifier tube to come it. Hopefully they come in soon.
I'll have to give the preamp some thought. I'll really start gettin more series about the pre-amp after I get this ST-70 up and running solid and I read up some more about circuit theory. I ordered Valve Amplifiers by Morgan Jones and I'm hoping that'll help shed some more light on the subject. This all started when I built a K-16LS. It's really cheap and was VERY simple to put together. I swapped out the caps per some old threads on this board with some Ruby Gold ones and replaced the small bridge rectifier with a bigger one. It has a little bit of a hum when no source is pumped into it but it actually doesn't sound bad... it sounds better than the old Sony reciever I had driving my speakers. lol. I think I'm hooked.
You are right, it's not a twist lock cap, it's a cap board within the chassis. Triode Electronics ST-70 Capacitor Board (over 500uF's per their web site.)
Still waiting on the diodes and the new rectifier tube to come it. Hopefully they come in soon.
I'll have to give the preamp some thought. I'll really start gettin more series about the pre-amp after I get this ST-70 up and running solid and I read up some more about circuit theory. I ordered Valve Amplifiers by Morgan Jones and I'm hoping that'll help shed some more light on the subject. This all started when I built a K-16LS. It's really cheap and was VERY simple to put together. I swapped out the caps per some old threads on this board with some Ruby Gold ones and replaced the small bridge rectifier with a bigger one. It has a little bit of a hum when no source is pumped into it but it actually doesn't sound bad... it sounds better than the old Sony reciever I had driving my speakers. lol. I think I'm hooked.
The diodes are cheap insurance for sure. I have a JJ GZ34 in my ST70 now with JJ KT77s and it hasn't given me any trouble, though I have a pair of UF4007s on mine. I put an inrush limiter on the center tap after I smoked the first rectifier, so that would help too. I think it may be the bigger filter caps that are taking their toll on these, because folks arc over their expen$ive NOS 5AR4s in modded Dynos as well.
Russ
Russ
Quote:
I think it may be the bigger filter caps that are taking their toll on these, because folks arc over their expen$ive NOS 5AR4s in modded Dynos as well.
Well, Never lost anything but a JJ rectifier and a chinese one. After using the diodes haven't lost another rectifier. Bigger filter caps causing problems....humm ...well as long as the size is within the tube guidelines everthing should be ok.
I think it may be the bigger filter caps that are taking their toll on these, because folks arc over their expen$ive NOS 5AR4s in modded Dynos as well.
Well, Never lost anything but a JJ rectifier and a chinese one. After using the diodes haven't lost another rectifier. Bigger filter caps causing problems....humm ...well as long as the size is within the tube guidelines everthing should be ok.
While the 5U4 will work in the Dynaco, but is not ideal. The ST70 design depends on the output tubes being ready to draw current before the full B+ is available. The slow turn-on of the 5AR4 is well matched to the EL34's. Hang a scope or meter across the B+ and watch the turn on sequence of the ST70. The B+ will rise t or above the ratings of the filter cap with a 5U4 in place and even worse if only solid state rectifiers are used.
Dynaco "cheaped out" with a single 525v rated lytic cap. Much better would have been stacked caps (see the Heathkit Williamson series for an example of this)
As to increasing the capacitance - imho all electrolytics sound horrid and more is not better. My personal take is that if you go the electrolytic route use "just enough" capacitance to eliminate hum. Yeah, I've heard all the tighter bass claims. The ST70 is a tremendously good amp for the low cost - but if you want better bass you need a better design and better output iron - not more filter caps. JC spoke about this in Sound Practice many years ago and as usual he was on the money.
If you do increase the capacitance it should be after the choke, not before. If you increase the initial filter cap beyond 40uf you will go thru 5AR4's quickly.
I have minimal experience with modern 5AR4's. I purchased a handful of chinese 5AR4's a while back and probably should have realized that their quality control was poor when they were labeled "Made in Chian". They all arc'd over and ended up in the trash. If you're scratch building I'd consider some damper diodes which are inexpensive and very rugged.
- Gary
Dynaco "cheaped out" with a single 525v rated lytic cap. Much better would have been stacked caps (see the Heathkit Williamson series for an example of this)
As to increasing the capacitance - imho all electrolytics sound horrid and more is not better. My personal take is that if you go the electrolytic route use "just enough" capacitance to eliminate hum. Yeah, I've heard all the tighter bass claims. The ST70 is a tremendously good amp for the low cost - but if you want better bass you need a better design and better output iron - not more filter caps. JC spoke about this in Sound Practice many years ago and as usual he was on the money.
If you do increase the capacitance it should be after the choke, not before. If you increase the initial filter cap beyond 40uf you will go thru 5AR4's quickly.
I have minimal experience with modern 5AR4's. I purchased a handful of chinese 5AR4's a while back and probably should have realized that their quality control was poor when they were labeled "Made in Chian". They all arc'd over and ended up in the trash. If you're scratch building I'd consider some damper diodes which are inexpensive and very rugged.
- Gary
burnedfingers said:
The original Dynaco multicap had 20uF on the rectifier and 30uF after the choke. The cap that came in the kit I have has 40uF on the rectifier and 80uF after the choke. The Triode (SDS) board also puts 40uF on the rectifier and 195uF on the choke. That pushes the 5AR4 right to the limit. There are no series resistors or anything on the plates, either (not that I have ever seen a commercial amp that had them). So anything not up to snuff will likely not live very long.
Well... the diodes (1N4007) came in today and I soldered them in and swaped out the 5AR4 for another chinese (a cheap one but not a JJ) I ordered off ebay. It's been up and running for about 45 minutes now with no problems at all. It's sounding great!!! I've ordered a couple Sovtek's but I didn't want to install one if I did have something messed up with the build. Thanks again for all the help on this!!! Of course now if this is up and running well... now what am I gonna do? lol! Have start thinking about that pre-amp project maybe. Talk to you all later!
It's sounding better every minute! I'm gonna go and see how far and can throw that JJ GZ34/5AR4!
It's sounding better every minute! I'm gonna go and see how far and can throw that JJ GZ34/5AR4!Quote:
The ST70 is a tremendously good amp for the low cost - but if you want better bass you need a better design and better output iron - not more filter caps. JC spoke about this in Sound Practice many years ago and as usual he was on the money.
First of all don't believe everything you read. The dynaco responds very well to a proper power supply. I came to that realization the hard way. In my opinion the stock power supply needs to be upgraded and the driver board thrown in the dumpster. The dynaco wakes up from its long sleep after the driver board has been replaced and the power supply tweeked.
The iron is fine as it is. I have 4) of these amps that will totally turn your head and make you realize that the little amp has possibilities after all. In its stock state with the weak power supply I would have to agree the bass is nothing to write home about. A properly designed/sized power supply makes all the difference.
The ST70 is a tremendously good amp for the low cost - but if you want better bass you need a better design and better output iron - not more filter caps. JC spoke about this in Sound Practice many years ago and as usual he was on the money.
First of all don't believe everything you read. The dynaco responds very well to a proper power supply. I came to that realization the hard way. In my opinion the stock power supply needs to be upgraded and the driver board thrown in the dumpster. The dynaco wakes up from its long sleep after the driver board has been replaced and the power supply tweeked.
The iron is fine as it is. I have 4) of these amps that will totally turn your head and make you realize that the little amp has possibilities after all. In its stock state with the weak power supply I would have to agree the bass is nothing to write home about. A properly designed/sized power supply makes all the difference.
I've rebuilt many many ST70's in my life and still have 4-5 lying around here. Again - it's a great amp. I've heard some pretty elaborate audio systems that don't sound as musical as an ST70 driving something as boring as a pair of Large Advents.
I agree the ST70 responds to some improvements, but I quickly find that it's a case of diminishing returns. IMHO, the limiting factor to solid bass in the ST70 is the A-470 transformer, not the power supply. I've been down the path of increasing the capacitance of the ST70 power supply with electrolytics and don't find it sounds better. That's not to say that replacing the 40 year old electrolytic cap doesn't improve the sound, and a modest increase in capacitance is fine. But a system with 100uf+ of electrolytic filter caps isn't a route I'd choose.
There are many paths to audio nirvana.
I agree the ST70 responds to some improvements, but I quickly find that it's a case of diminishing returns. IMHO, the limiting factor to solid bass in the ST70 is the A-470 transformer, not the power supply. I've been down the path of increasing the capacitance of the ST70 power supply with electrolytics and don't find it sounds better. That's not to say that replacing the 40 year old electrolytic cap doesn't improve the sound, and a modest increase in capacitance is fine. But a system with 100uf+ of electrolytic filter caps isn't a route I'd choose.
There are many paths to audio nirvana.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.