RESOLVED - Rectifier sparked, fuse blew. Both replaced. But Now, one channel is 'out'

Running a ST-70 can be like having a tempermental spouse.  All seems good until, BANG.

Since my build in 2014, I've habitually watched the tubes as I power on, just in case something doesn't look right.  Last week I saw the rectifier spark internally, then the fuse blew.
While ordering a replacement GZ34 and some extra fuses, I also purchased a new set of output tubes.  With everything new in place and the bias turned down slightly, upon startup the rectifier and one channel illuminated (both tubes), but not the other channel (neither of the two tubes).  The new fuse did not blow.

Referring to Bob Latino's troubleshooting guide, item 4 (below), this would seem to cover it.  

 

The quad capacitor was original to the kit build and is now 3 and 1/2 years old.  Should I start my investigation there with the multi-meter?

Thanks in advance for your help and suggestions.

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@PeterCapo : I did find a broken connection in the socket, and have repaired.

First, without the rectifier and without speaker load, inserting the two output tubes to the side that previously failed to light/power on, the tubes now light up.  Powering down and inserting tubes for the second channel, all four output tubes have power.

Inserting the rectifier tube and connecting the speakers to both channels, then restarting (with fingers on the power switch), the rectifier arcs inside.  Immediate shut down before the 3A fuse blows.

I have one brand new rectifier tube left...

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Just a few suggestions, going right back to basic troubleshooting and what to do first!

1. With the amp off, unplugged and cold for at least 24 hours:
a) Mark and remove all tubes. Tighten and clean the sockets carefully. Use a wooden toothpick, alcohol and a dental pick. If you have non-metallic cleaning brushes, those too. Be very careful with the miniature-tube sockets.
b) Check *every* connection under the chassis. Use that same dental pick to pull on each wire - sometimes the connection will look good and be held in place by the insulation alone.

This will get past bad connections and/or loose sockets, a key start for basic troubleshooting.

PeterCapo wrote:Wait - was it a new rectifier tube that arced or was it the old rectifier tube?

Peter asks a very important question, did you reuse the rectifier that originally arced?  Once the GZ34 arcs, it will continue to arc when power is reapplied. In other words, once it arcs it is bad and should be discarded.

Tubes4ever wrote:

PeterCapo wrote:Wait - was it a new rectifier tube that arced or was it the old rectifier tube?

Peter asks a very important question, did you reuse the rectifier that originally arced?  Once the GZ34 arcs, it will continue to arc when power is reapplied. In other words, once it arcs it is bad and should be discarded.

All true. However and at this moment, I would like to see the OP establish that ongoing problems are not mechanical in nature. Also agree that short-cycling is NOT good for the amp.

@Peter Capo - The second time restarting, I mistakingly used the old sparky rectifier. A new one is now in place.

V3, tab 1 needed to be resoldered. This was done, and all other wiring appears to be solid.

Reconnected the speakers, preamp and CD player. Before turning everything on, I turned the bias setting slightly counter-clockwise, lowering the voltage. Powered everything on. All four output tubes illuminated/glowed, as did the new rectifier. Left this running for five minutes. Nothing looked too 'hot'; nothing sparked internally - everything appeared normal.

However, now there is no sound, not even a hiss to my speakers. When I attempted to set the bias to 400, I got no signal from my multi-meter (set on the 2 volt dcv selection). (The multi-meter correctly measures a 1.5volt AAA battery).

The assembly manuals included on the "Tubes4hifi" site - are those manuals for the original Dynacos and not the contemporary models? I ask due to the references to the 1.56V bias rather than the contemporary, recommended 400 bias setting, and the $7 service fee if the unit needs to be returned for service. I want the contemporary (new) manual so that I can measure voltage at each tab as was done when I finished the build, before the initial start.

Thanks for all your time/responses to date.

I am near Buffalo, NY. Are there any forum members within a six hour drive that would have the patience, time and knowledge to help me diagnose this issue? I hate to ship the unit due to the possibility of damage.

I'll supply the chicken wings, Weber's mustard and Shalen's hot dogs if you're willing...

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SnugDynaco wrote:I am near Buffalo, NY.  Are there any forum members within a six hour drive that would have the patience, time and knowledge to help me diagnose this issue?  I hate to ship the unit due to the possibility of damage.  

I'll supply the chicken wings, Weber's mustard and Shalen's hot dogs if you're willing...

The Dynaco Clinic *Will* still happen. Zip 19027 may be at the hairy edge of a 6-hour drive. I will let you know.

Sounds to me like no B+. Bad rectifier?

I'd like to attend the workshop. Just over 6 hours according to Google maps, and have friends in nearby Lititz, PA.

Was this a genuine GZ34 or one of the modern ersatz?

Over 40 years of owning tube amps... I never had these problems .....any one that owns a tube amp... need a Variac!.....I guess before thay get any.... tube amp or preamp.....for the cost of tubes an better sound.
Even the china tube amps are still come in with 110v ac input....all the Ac here in the US is high...an not comeing down ...ever...good luck....hohoho

Are you able to safely probe the amp while it is powered on? If so, for starters measure the dc voltage on pin#8 of your 5AR4. Then measure pin #5 on each EL-34. Do this with all tubes installed.

nmchiefsfan wrote:Sounds to me like no B+.  Bad rectifier?  

After obtaining a copy of the assembly manual, I ran all of the test numbers for initial startup and found everything was in acceptable ranges.

Then went back an re-read Bob Latino's quick troubleshooting guide. No. 2 - even if a rectifier lights does not mean it is good. No sound and no bias would seem to confirm this.

Having nothing to lose, I removed the first "new" rectifier and installed a second new recitifier. Lo and behold, the music has returned, and the unit now will bias.

Both new rectifiers were supplied recently by a well known source. I will be contacting them for a replacement. The rectifiers are Sovtek GZ34.

I still am running the older set of EL34 Mullards, and in the morning will install the new set. The older set has about 18 months of use approx. 2000 hours. Bias was beginning to drift a bit.

Thanks for all of your advice on helping to track down the problem. I did have a loose connection at one of the pins and was able to make that repair, and learned a fair amount from this incident.

Dynalover wrote:Was this a genuine GZ34 or one of the modern ersatz?

Both genuine Sovtek GZ34's

SnugDynaco wrote:

Dynalover wrote:Was this a genuine GZ34 or one of the modern ersatz?

Both genuine Sovtek GZ34's

Those are not "real" GZ34s. Only Mullard made 'em.

It's not going into a Leak Stereo 20 so a fake GZ34 should be fine.

Jim McShane's got some pretty sturdy new-issue GZ-34 types. I'd give them a whack before searching for NOS Mullards.

I had the same issue two years ago with a new rectifier, no music, no bias voltage. I had dropped the new rectifier, a 5AR4, but it certainly wasn't the seller's fault the tube didn't work. My rectifier of choice is a 5V3A. Though it has more voltage drop than a 5AR4, it handles a lot more current, over 400ma. The same rectifier's been in use now for more than two years.

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Should've mentioned I did the yellow sheet diode modification. Not sure if it contributes to the life of the 5V3a. Time will tell. (I built a tube guitar amp from a kit and did the yellow sheet mod to it also.)