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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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Bob Latino
New2Tubez
rjpjnk
vtshopdog
deepee99
Drummerboy2
tubes4hifi
nmchiefsfan
Mr C
HarryY
LeGrace
Dale Stevens
Peter W.
17 posters

    A bright flash, a low dull thunk and diminished volume

    deepee99
    deepee99


    Posts : 2252
    Join date : 2012-05-23
    Location : Wallace, Idaho

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    Post by deepee99 Mon Aug 26, 2019 5:16 pm

    LeGrace wrote:Recently I had someone advise me, in very strident language, that I needed to immediately replace the rectifier leg of my quad cap with a standalone cap 1/10 the value when running a GZ37, for safety reasons. I now always pursue a 2nd opinion, so posted a checking question here. Turns out this advice was same quality as your rectifier only advice, ie not good. Soon after someone else posted an actual literature reference confirming that the quad cap as currently configured is appropriate for use with a GZ37. Sadly many posers out there.
    I don't think that's wrong, LeGrace. That's a 40 uF cap, which is 10x that recommended for a GZ-37. With a -33, no problem. My tube guy says you don't usually get fireworks with the -37, but it's a definite lifetime shortener.
    LeGrace
    LeGrace


    Posts : 358
    Join date : 2016-08-07
    Location : Ontario, Canada

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    Post by LeGrace Mon Aug 26, 2019 5:27 pm

    deepee99 wrote:
    LeGrace wrote:Recently I had someone advise me, in very strident language, that I needed to immediately replace the rectifier leg of my quad cap with a standalone cap 1/10 the value when running a GZ37, for safety reasons. I now always pursue a 2nd opinion, so posted a checking question here. Turns out this advice was same quality as your rectifier only advice, ie not good. Soon after someone else posted an actual literature reference confirming that the quad cap as currently configured is appropriate for use with a GZ37. Sadly many posers out there.
    I don't think that's wrong, LeGrace. That's a 40 uF cap, which is 10x that recommended for a GZ-37. With a -33, no problem. My tube guy says you don't usually get fireworks with the -37, but it's a definite lifetime shortener.

    Yeah I was told to immediately reduce it to 4 uF based on a value referenced in the GZ37 data sheet. But as with a lot of tube stuff the devil is in the details. Turns out up to 60 uF is fine with this tube.
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    nmchiefsfan


    Posts : 66
    Join date : 2012-03-21

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    Post by nmchiefsfan Mon Aug 26, 2019 10:32 pm

    I was under the impression that the VTA board would draw enough current to keep the voltage under the 550V rating of the caps. If it doesn't that is a learning for me. The fact the the fuse blows right away...before the rectifier has a chance to warm up and conduct...points to a dead short somewhere prior to the quad cap (IMHO). It seems like the only thing there that would take out the rectifier is the rectifier. It has been a long time since I had to do the initial setup of my VTA120 and my paperwork is packed away in the shed. Maybe when the new rectifier shows up the initial setup can be done after checking for shorts to ground on the rectifier socket and the legs of the quad cap. If memory serves, it is done with 2 power tubes (left or right channel) in place.
    Mr C
    Mr C


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    Age : 66
    Location : Visalia CA.

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    Post by Mr C Tue Aug 27, 2019 12:14 am

    Here's what I've done so far.  Reflowed the solder on all the octal tubes sockets.  resoldered the quad cap, after cleaning and fine sandpapering the terminals.  Reflowed the solder on the ESL connections and SCM.  Touched up a few other solders.  Took an acid brushed and brushed around everything.  Probed around the quad cap terminals with a little stick and made sure they where not touching the chassis and nothing was lodged.  Then vacuumed with the little vacuum.  Looked for solder bridges.  I didn't find anything.  But hopefully if there was a short I cleared it.  Hopefully if there was a dodgy solder I got it.   I'm not sure what the next step is.  Do I put in the new tube and turn it on?  Any other things to check first?  Now with the original tung-sol tube in I get 425 v on pin 4 & 6 and 0 on pin 2 and 8.  All the filaments glow. Didn't blow the fuse but doesen't seem to be passing DC through.
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    StevieRay


    Posts : 63
    Join date : 2009-01-09
    Age : 59
    Location : Central VA

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    Post by StevieRay Tue Aug 27, 2019 9:43 am

    With the recitifier tube pulled out, check to see if you have filament voltage -- ~6V AC across pins 2 & 8 of the rectifier socket.

    Is that correct?  You say filament glowing on that tbe, but just making sure.  2 & 8 should have white wires coming from the transformer.  Then pin 8 should ALSO have a red wire going to the first section of the quad cap.
    Mr C
    Mr C


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    Location : Visalia CA.

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    Post by Mr C Tue Aug 27, 2019 1:49 pm

    Stevie. I have 5.6 VAC across the white wires on pins 2 & 8. Wire from pin 8 to the 40Mfd terminal on the quad cap (square symbol).
    markeby
    markeby


    Posts : 24
    Join date : 2019-02-13
    Age : 63
    Location : Near Austin, Texas

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    Post by markeby Wed Aug 28, 2019 11:26 pm

    I have been using telefunken GZ34-TK.  The only one I ever fried was when I had a short after putting in the auto biasing board.  I found it on sale somewhere a while back and bought a few.
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    tuberadios69


    Posts : 5
    Join date : 2012-09-02
    Age : 67

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    Post by tuberadios69 Wed Aug 28, 2019 11:32 pm

    I think you have a bad quad cap. I had the same issue with two NOS 5AR4's a couple years ago. My amp was the original Sound Values Stereo 70 introduced in 1988. Sound Values had re-introduced the Dynakit Stereo 70 kit after they bought the Dynaco parts inventory from ESS in the late 1980's, who had bought a truckload of parts from Hafler earlier in the 1980's. One of my 5AR4's that suddenly arced and blew was an original Mullard and the other a Philips (Sylvania) big bottle 5AR4. Both flashed and went kaput. Turns out the quad cap from the 1988 kit (rated at an incredibly low 450 volts) had finally given up the ghost. i replaced it with a Mallory 40/20/20/20 cap rated at 500 Volts that I had bought NOS back in the late 1990's. I'd bought a bunch of both NOS 5AR4's and the Mallory quad caps over the years. The quad cap replacement solved my issue and the amp runs fine now.
    Mr C
    Mr C


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    Post by Mr C Thu Aug 29, 2019 2:23 am

    To test capacitance you need to remove the capacitor from the circuit, correct?
    LeGrace
    LeGrace


    Posts : 358
    Join date : 2016-08-07
    Location : Ontario, Canada

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    Post by LeGrace Thu Aug 29, 2019 5:37 am

    Mr C wrote:To test capacitance you need to remove the capacitor from the circuit, correct?

    You don't need to remove a cap from circuit when using a cap meter. In fact I just checked the rectifier leg on my quad cap a few days ago while still in place, came up bang on it's 40 uF spec. Also checked the coupling caps and they were fine as well.
    Bob Latino
    Bob Latino
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    Post by Bob Latino Thu Aug 29, 2019 7:37 am

    Mr. C has Emailed me on the side and mentioned to me that his line voltage varies between 119 and 124 VAC. If your line voltage gets up to 122 VAC (or higher), you should run the amp off a variac set to about 118 VAC when your voltage is at its highest point. For the VTA ST-70 or ST-120, either of the 5 amp models at the link below will work fine.

    Variacs at Circuit Specialists in AZ

    Bob

    Mr C
    Mr C


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    Post by Mr C Thu Aug 29, 2019 5:57 pm

    With a good 5AR4 I brought it up with a DBT and I was able to get bias current.  I ordered a variac and it should be here in a couple of days.  I'm not going to do any more till the variac gets here.
    Mr C
    Mr C


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    Location : Visalia CA.

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    Post by Mr C Thu Sep 05, 2019 9:15 pm

    A happy ending! I got one of those made in china variacs. It feels a little cheesy but works. Replaced the 5AR4. Set the variac to 116 and it fired right up. But after a couple of minutes it made a funny noise and one of the 6550s started glowing red. Yiikes! Shut it off and let it cool a bit and moved it over to the other side. Again after a couple of moments the same tube red plated. Moved it to the back socket, and it red plated again. That told me it was the tube. Replaced it and no more problems. It sounds absolutely wonderful. I have maybe 10 or 12 hours on it now. I went easy on it at first but this afternoon I cranked up a bit. Wow! It'll drive the Polk Monitors with authority. It'll drive the wife out of the room! I'm still listening to it but it seems to have a nice wide and deep sound stage and very detailed. It digs pretty deep and the highs are smooth. I put on Kind of Blue and I could pick out every musician and where they where. I but on Exile on Main Street and it rocked. From Bill Wyman's bass to Kieth Richards playing in the upper register. It's an impressive amp.
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    Andy Jersey


    Posts : 71
    Join date : 2012-09-29

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    Post by Andy Jersey Thu Nov 18, 2021 7:29 pm

    I ran an ST120 for years with a coppertop. I did experiment with a gz34. I never blew a rectifier. I wasn't using NOS Mullards or the like. I think there's an underlying issue causing your problem. Perhaps as Bob said, a cold joint on the quad cap. If you run amp hard you will overload a tube rectifier eventually. Unless you have very inefficient speakers or listen very loud, i don't see this being an issue. At idle you're not taxing a chinese GZ34. Unless i misunderstood, you're blowing fuses without source applied.

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