New ST-70 blowing 3 amp slow blow fuse - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 18th April 2009, 05:12 AM   #1
diyAudio Member
 
Join Date: Apr 2009
Default New ST-70 blowing 3 amp slow blow fuse

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?
  Reply With Quote
Old 18th April 2009, 06:46 AM   #2
rknize is offline rknize  United States
diyAudio Member
 
rknize's Avatar
 
Join Date: Feb 2009
Location: Chicagoland
Send a message via AIM to rknize Send a message via Yahoo to rknize
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
  Reply With Quote
Old 18th April 2009, 12:33 PM   #3
diyAudio Member
 
Join Date: Nov 2002
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.
  Reply With Quote
Old 18th April 2009, 01:28 PM   #4
kmtang is offline kmtang  Canada
diyAudio Member
 
kmtang's Avatar
 
Join Date: Nov 2004
Location: Vancouver
Try use a 5U4 rectifier which can stand higher voltage and current. The NOS 5U4's are cheap.

JOhnny
  Reply With Quote
Old 18th April 2009, 01:38 PM   #5
diyAudio Member
 
TubeMack's Avatar
 
Join Date: Apr 2006
Location: Florida
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.
  Reply With Quote
Old 18th April 2009, 02:32 PM   #6
rknize is offline rknize  United States
diyAudio Member
 
rknize's Avatar
 
Join Date: Feb 2009
Location: Chicagoland
Send a message via AIM to rknize Send a message via Yahoo to rknize
The Sovtek 5AR4 has been pretty reliable for me...
  Reply With Quote
Old 18th April 2009, 04:18 PM   #7
diyAudio Member
 
Join Date: Apr 2009
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.
  Reply With Quote
Old 18th April 2009, 07:05 PM   #8
rknize is offline rknize  United States
diyAudio Member
 
rknize's Avatar
 
Join Date: Feb 2009
Location: Chicagoland
Send a message via AIM to rknize Send a message via Yahoo to rknize
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.
  Reply With Quote
Old 19th April 2009, 12:38 AM   #9
diyAudio Member
 
Join Date: Apr 2009
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.
  Reply With Quote
Old 19th April 2009, 03:07 AM   #10
PB2 is offline PB2  United States
diyAudio Member
 
PB2's Avatar
 
Join Date: Sep 2004
Location: North East
Blog Entries: 1
Wondering how much capacitance you have in the main plate supply? Did you or Triode up the value from the original design?

Triode uses an improved PS transformer I believe with more iron, is that the one you have? How many ohms are the HV windings out of the PS transformer?

Pete B.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
NAD fuses, slow blow or fast blow? AllCaster Solid State 3 25th October 2009 06:11 PM
Fuse housing melting but fuse not blowing, ppi amp rawadia Car Audio 21 2nd August 2008 10:39 AM
Sa-5.1 blow fuse jpweaner2 Tubes / Valves 0 3rd May 2007 01:36 PM
fast / slow blow fuses millwood Everything Else 3 5th January 2004 05:49 PM
Fried a slow blow 4 A fuse chris ma Everything Else 2 10th October 2003 06:59 AM


New To Site? Need Help?

All times are GMT. The time now is 06:10 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2