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    paulrmanning

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    Post by paulrmanning on 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

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    Bob Latino
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    Post by Bob Latino on 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
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    Post by peterh on 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.
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    paulrmanning

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    Post by paulrmanning on 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.

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    peterh
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    Post by peterh on 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 ! )
    PeterCapo
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    Post by PeterCapo on Wed Jul 22, 2020 9:47 pm

    They're probably home-grown mods. What appear to be extra capacitors seem to be cracked along their bottom sides. Without dropping resistors to accompany the diodes, the amp will run at higher than normal voltages, which could damage it - maybe that's why the extra capacitors are cracked along their bottoms, I don't know.

    It also has resistors on the speaker terminal strips and extra resistors on the power tube sockets that were not there in the original.

    Two paths to take here: 1) reverse-engineer a schematic and then try to figure it all out, or 2) tear it down and rebuild it according to the original design. Fortunately, the assembly manual and pictorial wiring diagrams are readily available: https://www.dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf Even if you wanted to try one of the currently available mods for it, you'd need to start with the amplifier in the original Dynaco configuration.

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    paulrmanning

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    Post by paulrmanning on 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.
    PeterCapo
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    Post by PeterCapo on Thu Jul 23, 2020 1:37 pm

    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.
    peterh
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    Post by peterh on 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.
    PeterCapo
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    Post by PeterCapo on Thu Jul 23, 2020 6:28 pm

    peterh wrote: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.

    It "looks like" it's the original 15.6Ω Biaset resistors that were moved to the speaker terminals, one each.  It seems like they're in parallel with the brown lead for the 4Ω output taps.  Are you saying the 15.6Ω resistors provide a load so the amplifier can be powered on without a speaker connected, is that it?  Wouldn't it also lower the impedance the OPT sees on those taps and thereby increase distortion?  And, those resistors are rated at 1-watt.  What would happen if the speaker wire became disconnected while the amp is playing loudly?  Wouldn't the 1-watt 15.6Ω resistor get fried?  I am not familiar with this arrangement and would like to understand.

    Also, the two pots appear to be marked as 25KΩ instead of the original 10KΩ.  Wondering how this amp is being biased with the Biaset resistors moved to the speaker terminals and with different value pots.

    I think you might also be referring to screen stability resistors connected to the power tube sockets?  Looks like they are on pin 4 for G2 of each EL34 socket?  Can't tell from the photos what value resistors are in use here.  The only other resistors I see on the power tube sockets are the grid stoppers.


    Last edited by PeterCapo on Thu Jul 23, 2020 6:46 pm; edited 1 time in total
    PeterCapo
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    Post by PeterCapo on Thu Jul 23, 2020 6:43 pm

    Furthermore... that terminal strip next to the 5AR4/GZ34 socket seems to have at least three silicon diodes on it. Replacing the tube rectifier needs only two. What's the third diode for? Can't quite tell where their leads go. Given the repurposing of the Biaset resistors and other differences noted, I'd not be inclined to just remove the terminal strip with the [at least] three diodes without thoroughly understanding what they're all for. Again, reverse-engineering a schematic and then interpreting it.
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    Solder Slinger

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    Post by Solder Slinger on 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.
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    paulrmanning

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    Post by paulrmanning on 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?
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    Post by PeterCapo on Fri Jul 24, 2020 12:54 pm

    paulrmanning wrote: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?

    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.

    With solid-state diodes in place of the rectifier tube, the higher-than-spec B+ could damage parts in the amplifier, and this might be why the two capacitors appear cracked.  If you want to keep the solid-state diodes, then it would be wise to install appropriate voltage-dropping resistor(s) as well as a properly rated thermistor or time-delay to ease startup.

    At the same time, without understanding how your amplifier is now set up, removing the solid-state diodes in favor of reinstalling a tube socket and a 5AR4/GZ34 runs the risk of having problems due to the amplifier not otherwise being in its original configuration.  New power cords, I/O jacks, speaker terminals, etc., won't matter much if the amplifier breaks.

    I suppose the easiest thing to do is to just keep running it as it now is, and if it breaks figure out what to do then.


    Last edited by PeterCapo on Fri Jul 24, 2020 1:23 pm; edited 1 time in total
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    paulrmanning

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    Post by paulrmanning on 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.
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    New2Tubez

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    Post by New2Tubez on 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.
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    Post by PeterCapo on Fri Jul 24, 2020 2:14 pm

    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.
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    Post by New2Tubez on 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.
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    paulrmanning

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

    Someone asked for a top view.

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    Earl
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    Post by Earl on 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?
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    Post by Bob Latino on 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

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