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Post subject: Adding SS diodes to new stock GZ34/5AR4
Posted: Thu Jun 07, 2012 6:43 pm
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Thought this maybe an interesting mod. To quote from another amp board:

" Avoid the JJs!!

My experience says the Sovtek handles higher currents better than the Shuguang/Chinese. Conversely the Shuguang seems to handle voltages above 450 or so better, but isn't happy with higher currents.

So one of those two will be the best match for your needs. "



Here's a mod that a lot of Dynaco ST-70 owner use for their older dual EL34 amps, so that new Sovteks can be safely used. The mod maintains the slow-startup of the GZ34. But, allow much greater voltage handling-- avoiding arc-over, which is a blight of Sovteks. And keeps the tube rectifier "sag" that we've all come to like.

The only good added suggestion to this mod (or any SS diode sub out) is using "UF" fast recovery diodes ]in place of the 1N4007, use a UF4007.


Though the diagram is for the Dynaco app you can use the same wiring for any AB763 amp (or any one using a GZ34) with high voltage output (B+ = greater than 400VDC).



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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Thu Jun 07, 2012 8:05 pm
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Interesting, rectifying the voltage to.....a rectifier. Does that substantially reduce the voltage out of the tube rectifier?

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Thu Jun 07, 2012 8:36 pm
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Shimmy, My impression is the voltage and current stress on the tube rectifier during transients is reduced a lot with the ss-diodes. Which is needed with many of these modern day made rectifier, which are not built to the same standards as the NOS ones.


Last edited by BMW2002Ti on Fri Jun 08, 2012 10:21 am, edited 1 time in total.

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Fri Jun 08, 2012 9:02 am
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No complaints regarding the JJ GZ34/5AR4 I use in my 5F6A clone. Why the warning to "Avoid the JJs!!"?

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Fri Jun 08, 2012 10:20 am
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Bluesky, in the recent two years, there have been a number of early failures with JJ rectifiers and output tubes. Related to lot and date of manufacture. Due, in part, to uneven QA/QC with certain JJ related plants.

With rectifiers, the failures are related to the stress placed on them by the units. With higher demanding hi-fi units displaying these failures quicker. Such as the Dynaco ST-70, which has one GZ34 powering four EL34's.

This is why it is a good idea to buy rectifiers and output tubes from someone who test them in amp-like conditions, and not just on a typical tube tester.


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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Fri Jun 08, 2012 1:24 pm
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BMW2002Ti wrote:
Bluesky, in the recent two years, there have been a number of early failures with JJ rectifiers and output tubes. Related to lot and date of manufacture. Due, in part, to uneven QA/QC with certain JJ related plants.

With rectifiers, the failures are related to the stress placed on them by the units. With higher demanding hi-fi units displaying these failures quicker. Such as the Dynaco ST-70, which has one GZ34 powering four EL34's.

This is why it is a good idea to buy rectifiers and output tubes from someone who test them in amp-like conditions, and not just on a typical tube tester.


Hmmm. Interesting. Thanks. I buy my JJs from Eurotubes. No problems with anything from them to date. :D

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Fri Jun 08, 2012 2:11 pm
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bluesky636 wrote:
BMW2002Ti wrote:
Bluesky, in the recent two years, there have been a number of early failures with JJ rectifiers and output tubes. Related to lot and date of manufacture. Due, in part, to uneven QA/QC with certain JJ related plants.

With rectifiers, the failures are related to the stress placed on them by the units. With higher demanding hi-fi units displaying these failures quicker. Such as the Dynaco ST-70, which has one GZ34 powering four EL34's.

This is why it is a good idea to buy rectifiers and output tubes from someone who test them in amp-like conditions, and not just on a typical tube tester.


Hmmm. Interesting. Thanks. I buy my JJs from Eurotubes. No problems with anything from them to date. :D


JJ rectifier here, too in my AB763 no issues...


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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Fri Jun 08, 2012 5:17 pm
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Really, the AB763 circuit is not as hard on rectifiers as, say a Dynaco 70 watt amp (35 watts per channel). But, this is a good way to maintain long-term effective use. Even in a dual 6L6GC Fender amp. Just an alternative, maybe to keep in mind for the future.


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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Fri Jun 08, 2012 6:08 pm
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BMW2002Ti wrote:
Really, the AB763 circuit is not as hard on rectifiers as, say a Dynaco 70 watt amp (35 watts per channel). But, this is a good way to maintain long-term effective use. Even in a dual 6L6GC Fender amp. Just an alternative, maybe to keep in mind for the future.


What makes a Dynaco hi-fi amp that is probably running at fairly low average power levels with only occasional peaks (depending on the source music, of course), harder on a rectifier tube than a guitar amp running cranked up and highly compressed?

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Fri Jun 08, 2012 7:40 pm
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Bluesky, prolly the total amperage demand of running 4 X EL34, versus 2 X 6L6GC. It's a pretty well known fact that these Dynaco amps are hard on rectifiers. Most ppl need to run NOS or good testing pulled Mullard or Amperex GZ34. As most new production makes are avoided, due to early failure.

Anyhow, if the Sovtek or JJ is working fine, don't worry about it. However, if you have issues with it later... or with future rectifiers, there is a solution that seems to work.


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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Fri Jun 08, 2012 9:02 pm
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BMW2002Ti wrote:
Bluesky, prolly the total amperage demand of running 4 X EL34, versus 2 X 6L6GC. It's a pretty well known fact that these Dynaco amps are hard on rectifiers. Most ppl need to run NOS or good testing pulled Mullard or Amperex GZ34. As most new production makes are avoided, due to early failure.


Do you have a link to the problems with the Dynacos? As an engineer I am curious about the problems they are having.

EL34s are 25 watt tubes vs 30 for 6L6GCs, right? I would think that for hi-fi applications the 34s would be biased somewhat cold to keep them in a more linear operating range than 6L6GCs in a guitar amp. I know that the average power output of a hi-fi amp is pretty low compared to the peak power while the guitar amp is operating closer to its limits. I wouldn't think the current draw of a 70 watt Dynaco would be that much greater than the 50 watts or so of a Fender amp. Could it be that the guitar amp is overbuilt enough to handle the continuous high current compared to a hi-fi amp? Or is the high peak to average power draw of a hi-fi amp the real culprit due to the sudden surge in current required to support a high dynamic range signal?

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 4:03 am
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Bluesky, Bob Latino's Dynaco board has been through this issue several times over the years. I can't find the specific reference, but this is a link to some of the topic:

http://dynacotubeaudio.forumotion.com/t ... y-in-st120


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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 6:03 am
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BMW2002Ti wrote:
Bluesky, Bob Latino's Dynaco board has been through this issue several times over the years. I can't find the specific reference, but this is a link to some of the topic:

http://dynacotubeaudio.forumotion.com/t ... y-in-st120


Thanks! I'll check it out. :D

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 10:14 am
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This actually makes a lot of sense, especially to protect the amp as well as the tube. Although not mentioned in the article (below), I can see how adding these diodes would also eliminate the possibility of arcing in the tube. The diodes would be a wall against reverse current.

Much cooler than I originally thought, thanks BMW2002Ti :D

http://www.premierguitar.com/Magazine/Issue/2008/Jul/The_Immortal_Amplifier_Mod.aspx

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Post subject: Re: Adding SS diodes to new stock GZ34/5AR4
Posted: Sat Jun 09, 2012 10:24 am
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shimmilou wrote:
The diodes would be a wall against reverse current.


"Flyback" is an issue of concern, especially if the rectifier is a pricey NOS bottle such as an Amperex or a Mullard.

Arjay

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