B-15 5AR4 diode mod

Discussion in 'Amps [BG]' started by Rickenbackerman, Mar 23, 2014.


  1. Rickenbackerman

    Rickenbackerman

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    Curious if anybody's done this to their B-15. I actually have a Dynaco ST-70 and the rectifier arced on power up a couple of weeks ago and I haven't turned it on since. I finally got around to doing this last night and it seems to have done the trick!

    http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod

    Any benefits to doing this on your B-15? Jimmy, beans, love to get your thoughts on this one...
  2. okcrum

    okcrum in your chest Supporting Member

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    Do it with a diode plug in, so you can put the 5AR4 back if you want. You get a bit more power, and reduced B+ sag, the latter you might hear.
  3. Rickenbackerman

    Rickenbackerman

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    I know about the plug in SS rectifier. I like the tube rectifier. This is a modification that supposedly eases the amount of strain on the 5AR4 upon power up.
  4. okcrum

    okcrum in your chest Supporting Member

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    Sorry, misread the pictorial. Yes, this is worth it. It does not actually limit the surge current on power up, but it prevents negative voltage spikes on the 5AR4 plates. These are bad especially when the tube is not warmed up yet.
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  6. JimmyM

    JimmyM Supporting Member

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    I never tried it. Threatened to get one of those little plugs once, but eh, I'm fine with it as is.
  7. Rickenbackerman

    Rickenbackerman

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    I'm gonna do it since I've got exactly two extra 1N4007's... and the original rectifier tube which I'd like to last as long as possible...

    thanks for the clarificarion, okcrum!
  8. Jim C

    Jim C Supporting Member

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    Nov 29, 2008
    I don't have a B-15 or Bassman any longer but will use this on my Dynaco Stereo 70!
    While I do like the sag of a rectifier tube an a MI amp, I can't help but wonder if going to SS rectifier on a Dynaco might be a sonic improvement…
  9. beans-on-toast

    beans-on-toast Supporting Member

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    Yes I do that all the time in amps and have it in my B-15N. Take a look at the 57 Tweed Deluxe schematic on the Fender web site. They have a nice drawing. You use two unused connections on the tube socket to mount the diodes.

    When you use a cap that has a high turn on voltage surge, or if a large capacitance is used in the power supply, it can cause a flashover in the rectifier tube. Adding the diode increases the peak inverse voltage capacity of the 5AR4 and prevents the flashover. You can also use a current limiter to slow down the inrush.

    Details in the PF Wiki.
  10. beans-on-toast

    beans-on-toast Supporting Member

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    This isn't a SS rectifier, it is simply extending the specs of the rectifier tube. You can install a SS plugin module instead of the 5AR4 tube. They sell them here and a different type here. The WZ34 offers sag (not quite the same as a real tube) in a SS package. The WS1 is similar to the one at Antique Electronics. There are some additional details here.

    There is an interesting ST70 power supply upgrade board available from Triode Electronics. You can also buy just the board for around $20. Triode has a lot of interesting ST70 stuff. I use the Dynaco MKIII PS boards in prototypes. I have a couple in my SVT.
  11. JimmyM

    JimmyM Supporting Member

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    So this is just to keep the rec tube from arcing on startups and turn-offs and doesn't really affect the sound?
  12. beans-on-toast

    beans-on-toast Supporting Member

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    Yes. It's a safety measure. It doesn't affect the sag that the tube rectifier provides to the amp. It also doesn't make convert the amp to SS rectification, the tube is still there doing it's job.

    Turn offs aren't an issue, only turn ons. And only when the power supply caps are discharged. When the caps are discharged, there is an initial voltage surge till the caps get charged.
  13. AstroSonic

    AstroSonic Supporting Member

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    I suspect that some folks are having a problem with their 5AR4's because they are seeing more voltage on them due to the increase in line voltage (110vac - 122vac) since these amps were designed (1965-75). The tubes in these amps (especially the rectifier and output tubes) were often being used near their limits when they were designed. With todays higher line voltages they are being operated at near or above their safe limits. The addition of the diodes will of course add switching noise, but most will not notice this. Note that all the secondary voltages are affected, even the filaments. A better solution is to add a 12v transformer in series with the power transformer primary, with polarity resulting in subtraction of 12v from the primary voltage. It then sees about 110v, about what it was designed to see. The 12v transformer must be rated for about 1.5 time the current draw on the primary (or about twice the fuse current). This mod requires a bit more technical expertise and some chassis real estate, but solves all the over voltage issues and helps preserve the stock power transformer. Without a mod like this the power transformer delivers too much voltage on the secondaries, and it runs hotter, decreasing its useful life.
  14. Rickenbackerman

    Rickenbackerman

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    I'm glad I started this thread. There doesn't seem to be much info on here pertaining to this mod, but the hardcore Dynaco guys are ALL over it.
  15. Jim C

    Jim C Supporting Member

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    Yes, I am completely aware of the differences between solid state and tube rectification. I was just throwing out the idea of a SS conversion on a Dynaco because:
    1. It had never occurred to me and maybe it's a good idea to switch over
    2. Thought maybe someone here had already done it
    3. My memory from years ago is that the plug in SS unit did make bass amps more stout but I didn't care for it with the guitar amps; then again, I think the desert island amp is a Deluxe Reverb so I think the diode protection for tubes is a really good idea.
    Can hurt or affect the tone; right?

    Edit: Thanks for the links. I had no idea how inexpensive Dynaco upgraded boards are and thought that some of the DIY Fender and Marshall kits might be very nice assuming you have some spare time.
  16. coreyfyfe

    coreyfyfe Supporting Member

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    Interesting thread. Hadn't thought of something like this but I did have a rectifier flashover issue with my B15 recently. It worked fine before/after and didn't do it again that whole session so I forgot about it, but if this can prevent the issue altogether I might just try it out.

    That wiki link is a perfect diagram David. That wiki really turned into something quite amazing.
  17. Rickenbackerman

    Rickenbackerman

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    I went ahead and did it last night.

    [​IMG]

    However, as I was testing it out in my dim basement to make sure all was good, I noticed that my power tubes were redplating slightly! Time to tweak the bias...

    David, why do you have MKIII power supply boards in an SVT?
  18. BassmanPaul

    BassmanPaul Gold Supporting Member

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    This is a modification i intend to make to my B15N. I've had the rectifier flash over myself. To anyone contemplating replacing the tube rectifier with SS I caution you to check the value of the B+ line after the replacement. A tube will drop around 50-75V across it whilst SS it's less than a volt. The B+ will rise significantly and possibly be too much for the filter caps in the amp.
  19. beans-on-toast

    beans-on-toast Supporting Member

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    Thanks Corey. I look at it that we all help make it what it is. There is still a lot to cover with too little time to spend on it but it is evolving.
  20. beans-on-toast

    beans-on-toast Supporting Member

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    I wanted to use Panasonic industrial snap-in type, high current capacity caps. I boosted the SVT PS capacitance a bit but it is also the high current part that helped give my amp a deeper more responsive bass with more headroom. I feel that the first PS node in the amp is too small and can be helped by boosting it a bit. It's really an ongoing experiment to try tweaking the power supply and see how it affects the performance of the amp.

    I buy just the boards and use them on projects. They allow me to install up to four pairs of caps in series per board. I used two boards in the amp, one cut down. They are mounted as a unit on a base. I also wanted to attach the unit in a way that it is fully reversible. So the boards are mounted using only the original chassis holes. My intension, if I ever find the time, is to design a printed circuit board for recapping the amp.

    There are some pics here. The cap values that I used are here. As I recall one of the cap values isn't what I used in the end. I have the value that I used somewhere in my notes.
  21. Rickenbackerman

    Rickenbackerman

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    Ah, cool. My third (non-working) SVT has an overwound Heyboer power transformer and TONS more filtering than stock. Here's the power supply:

    [​IMG]

    It's basically done, but one of these days I need to figure out what's wrong with it. Bringing it up on the variac (out of standby) I think it was pulling 2 or 3 amps at like 50 volts. This was a long time ago, 8 or 10 years so I'm going from memory here. I shelved it out of fear of working on it and it's been sitting since. I think I may have gotten carried away with the amount of filtering I put in the thing. But Hiwatt 200's have ridiculous voltage and TONS of filtering, so.... I do remember it powered up ok with no power tubes. I should pull them and measure some voltages.

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