The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
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


5 posters

    Rectifier Mod

    avatar
    paulrmanning


    Posts : 6
    Join date : 2020-07-21

    Rectifier Mod Empty Rectifier Mod

    Post by paulrmanning Tue Jul 21, 2020 12:30 pm

    I was given this ST70 amp some time ago and am in the process of cleaning it up.  They have replaced the rectifier tube with a diode bridge rectifier and a couple caps.  I cannot find any data on the caps or this particular mod.  Anyone else seen this? the caps have TVLU 2925 and TVLU 1930 of them

    Rectifier Mod Img_2610
    Bob Latino
    Bob Latino
    Admin


    Posts : 3259
    Join date : 2008-11-26
    Location : Massachusetts

    Rectifier Mod Empty Re: Rectifier Mod

    Post by Bob Latino Wed Jul 22, 2020 4:58 pm

    Paul,

    See if you can post a larger photo of the entire inside wiring of the amp ?

    Bob
    peterh
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Rectifier Mod Empty Re: Rectifier Mod

    Post by peterh Wed Jul 22, 2020 5:32 pm

    paulrmanning wrote:I was given this ST70 amp some time ago and am in the process of cleaning it up.  They have replaced the rectifier tube with a diode bridge rectifier and a couple caps.  I cannot find any data on the caps or this particular mod.  Anyone else seen this? the caps have TVLU 2925 and TVLU 1930 of them

    Rectifier Mod Img_2610

    The best you can do for yourself is to restore the amp to factory state. A can cap
    and a 5AR4 is the main parts needed, schematic and pictorial is available
    at this site, look at the first threads.
    avatar
    paulrmanning


    Posts : 6
    Join date : 2020-07-21

    Rectifier Mod Empty Re: Rectifier Mod

    Post by paulrmanning Wed Jul 22, 2020 6:26 pm

    I am a bit of a novice but it appears that the added capacitors are adding additional capacitance to the quad and not really part of the rectifier mod.

    Rectifier Mod Img_2611
    peterh
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Rectifier Mod Empty Re: Rectifier Mod

    Post by peterh Wed Jul 22, 2020 6:29 pm

    You said that you should "clean up" this amp. That includes removing fluff and restoring
    damaged functions. The manual is your help here.

    ( there is no need for "extra capacitance" just remove ! )
    avatar
    Guest
    Guest


    Rectifier Mod Empty Re: Rectifier Mod

    Post by Guest Wed Jul 22, 2020 9:47 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 6:47 pm; edited 1 time in total
    avatar
    paulrmanning


    Posts : 6
    Join date : 2020-07-21

    Rectifier Mod Empty Re: Rectifier Mod

    Post by paulrmanning Thu Jul 23, 2020 10:26 am

    I was leaning towards pulling out the mod and reinstalling the socket and getting a solid state rectifier.
    avatar
    Guest
    Guest


    Rectifier Mod Empty Re: Rectifier Mod

    Post by Guest Thu Jul 23, 2020 1:37 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 6:48 pm; edited 1 time in total
    peterh
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Rectifier Mod Empty Re: Rectifier Mod

    Post by peterh Thu Jul 23, 2020 5:20 pm

    PeterCapo wrote:
    paulrmanning wrote:I was leaning towards pulling out the mod and reinstalling the socket and getting a solid state rectifier.

    You already appear to have solid-state diodes substituting for the tube rectifier.  You'd just have to figure out how to get appropriate dropping resistor(s) installed as well as a thermistor or time delay to ease startup.  If you pull the diodes out, reinstall the socket and then go with a plug-in, note that the Weber copper cap that emulates the soft start and voltage drop of a 5AR4/GZ34 is not recommended for hi-fi tube amps https://www.tedweber.com/wz34

    If you believe Weber's caution against using their WZ34 in a hi-fi amp, then the choice you're left with is a plug-in solid-state replacement that contains diodes only, like their WS1 or equivalents sold at other places.  In this case, you'd still want to find a way to add dropping resistors and also a properly rated thermistor or time delay to ease startup.

    Apart from all that, there's still the question of the resistors on the speaker terminals, the extra resistors on the power tube sockets and whatever else may have been changed for better or for worse.  Lacking complete documentation on it, I would not assume that the amp is otherwise functioning properly.

    Resistors at the speaker terminals might be an asset not a mystery ( and it won't harm
    the amp in any way). 100 at the 8 ohm outlet is an insurance against lethal flashower
    if speakers suddenly gets disconnected. 5w will do just fine and it will consume 3w of
    the 60w can supply.
    avatar
    Guest
    Guest


    Rectifier Mod Empty Re: Rectifier Mod

    Post by Guest Thu Jul 23, 2020 6:28 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:02 pm; edited 2 times in total
    avatar
    Guest
    Guest


    Rectifier Mod Empty Re: Rectifier Mod

    Post by Guest Thu Jul 23, 2020 6:43 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:01 pm; edited 1 time in total
    avatar
    Solder Slinger


    Posts : 36
    Join date : 2009-05-24

    Rectifier Mod Empty Re: Rectifier Mod

    Post by Solder Slinger Thu Jul 23, 2020 8:02 pm

    I think the terminal strip has 4 diodes, 2 each attached serially so maybe the original diode voltage rating wasn't high enough. At any rate, the TVLU-1930 is the middle cap, 30-50 mfd @ 525v, the TVLU-2925 is a 40/40 mfd at 500v where the 5AR4 tube was. That said, I don't see any reason not to use the solid state diodes, I would add a time delay relay ( I use one with a slow ramp up from K&K Audio, set it to at least 1 minute delay to allow the tubes to fully warm up ). I'd replace the carbon comp resistors in the power supply with wirewounds (3-5 watt), as well as metal films in the bias supply, carbon comps increase in resistance over time. The 25K Bias pots are a good choice, depending on the resistors used in the Bias power supply they would allow both 6550 / KT88 as well as EL34s to be used. I'm assuming the original Dynaco cap was replaced, if not replace it with a 40/80/30/20 mfd @ 525 v as well (the choke has wax that ran out indicating the cap was leaking). If it was mine, I'd add an NTC to the power cord (CL80) as well to help with the turn on surge as well as to knock the AC voltage down a bit. It looks like the previous owner added resistors to the output tube screens which is good, 100 ohm is the traditional value, some people advocate for up to 1000 ohm, using a 3 watt resistor. I'd add a Dynakit Parts Individual tube BIAS kit to be able to adjust the bias for each tube as well.

    I'd replace the carbon Comps on the input jacks with metal films as well.

    Can't see the top of the unit so I have no idea of what was done there, those recommendations will have to wait.

    Oh, and solder at least one of the middle cap feet to the chassis to insure a good connection.
    avatar
    paulrmanning


    Posts : 6
    Join date : 2020-07-21

    Rectifier Mod Empty Re: Rectifier Mod

    Post by paulrmanning Fri Jul 24, 2020 12:26 pm

    Wow so many options. I may be a bit over my head on some of this. Thanks for the values on the caps. I just replaced the quad cap. That got me back up and working. I was now just digging in a little deeper to see what I might replace next. I have a new power cord and input/output terminals on order. That said I am sure the next question will separate people into 2 camps. Are there any advantages/disadvantages to the 5AR4 tube vs the Solid state replacements?
    avatar
    Guest
    Guest


    Rectifier Mod Empty Re: Rectifier Mod

    Post by Guest Fri Jul 24, 2020 12:54 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:02 pm; edited 2 times in total
    avatar
    paulrmanning


    Posts : 6
    Join date : 2020-07-21

    Rectifier Mod Empty Re: Rectifier Mod

    Post by paulrmanning Fri Jul 24, 2020 1:23 pm

    My original thinking was that if I could not find replacements for the added caps to just start from scratch. Now that I have references for those I will probably just replace them and operate under the assumption that the previous owner (I do not know who that was. I acquired this some 40 yrs ago) knew what they were doing with the mods.
    avatar
    New2Tubez


    Posts : 184
    Join date : 2018-03-20
    Location : NY

    Rectifier Mod Empty Re: Rectifier Mod

    Post by New2Tubez Fri Jul 24, 2020 2:09 pm

    PeterCapo wrote:
    paulrmanning wrote:I was leaning towards pulling out the mod and reinstalling the socket and getting a solid state rectifier.

    You already appear to have solid-state diodes substituting for the tube rectifier.  You'd just have to figure out how to get appropriate dropping resistor(s) installed as well as a thermistor or time delay to ease startup.  If you pull the diodes out, reinstall the socket and then go with a plug-in, note that the Weber copper cap that emulates the soft start and voltage drop of a 5AR4/GZ34 is not recommended for hi-fi tube amps https://www.tedweber.com/wz34

    If you believe Weber's caution against using their WZ34 in a hi-fi amp, then the choice you're left with is a plug-in solid-state replacement that contains diodes only, like their WS1 or equivalents sold at other places.  In this case, you'd still want to find a way to add dropping resistors and also a properly rated thermistor or time delay to ease startup.

    Apart from all that, there's still the question of the resistors on the speaker terminals, the extra resistors on the power tube sockets and whatever else may have been changed for better or for worse.  Lacking complete documentation on it, I would not assume that the amp is otherwise functioning properly.

    For what it's worth, the Weber WS-1 is available with a thermistor option (WS-1t). I have this on my ST120.
    avatar
    Guest
    Guest


    Rectifier Mod Empty Re: Rectifier Mod

    Post by Guest Fri Jul 24, 2020 2:14 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:01 pm; edited 1 time in total
    avatar
    New2Tubez


    Posts : 184
    Join date : 2018-03-20
    Location : NY

    Rectifier Mod Empty Re: Rectifier Mod

    Post by New2Tubez Fri Jul 24, 2020 2:18 pm

    PeterCapo wrote:
    New2Tubez wrote:For what it's worth, the Weber WS-1 is available with a thermistor option (WS-1t). I have this on my ST120.

    That could be very handy.  But is there any information available as to exactly what kind of thermistor it uses?  Its ratings for cold resistance, maximum steady-state current, and hot resistance at different % of max current?  I suppose Weber might provide that info if someone were to inquire.  It would be important to know.

    I'd check with them.
    avatar
    paulrmanning


    Posts : 6
    Join date : 2020-07-21

    Rectifier Mod Empty Re: Rectifier Mod

    Post by paulrmanning Fri Jul 24, 2020 3:06 pm

    Someone asked for a top view.

    Rectifier Mod Img_2410
    avatar
    Guest
    Guest


    Rectifier Mod Empty Re: Rectifier Mod

    Post by Guest Fri Jul 24, 2020 3:31 pm

    PeterCapo wrote:The 5AR4/GZ34 has a voltage drop that puts the B+ voltages at the proper value.  The 5AR4/GZ34 also has a slow-start characteristic that prevents excessive stress on the amplifier's parts.  A simple solid-state diode replacement, whether diodes as you have right now or the kind that plugs into an octal socket, loses the slow-start, and the B+ values will rise.

    What is the typical voltage drop of this particular rectifier?
    Bob Latino
    Bob Latino
    Admin


    Posts : 3259
    Join date : 2008-11-26
    Location : Massachusetts

    Rectifier Mod Empty Re: Rectifier Mod

    Post by Bob Latino Fri Jul 24, 2020 4:52 pm

    Earl wrote:
    PeterCapo wrote:The 5AR4/GZ34 has a voltage drop that puts the B+ voltages at the proper value.  The 5AR4/GZ34 also has a slow-start characteristic that prevents excessive stress on the amplifier's parts.  A simple solid-state diode replacement, whether diodes as you have right now or the kind that plugs into an octal socket, loses the slow-start, and the B+ values will rise.

    What is the typical voltage drop of this particular rectifier?

    About 17 volts ... Check the link below ..

    Tube rectifier specifications

    Bob

    Sponsored content


    Rectifier Mod Empty Re: Rectifier Mod

    Post by Sponsored content


      Current date/time is Thu Mar 28, 2024 5:34 am