Hello everybody,
I'd like to recap an old VTL 75/75 amplifier and I'd like tho have some advice to choose the best value for many devices to use in the power supply (later, I'll ask for other issues).
As starting point I've drawn the schematics using the VTL 50/50's I've found on the web because it is very similar.
Though the VTL stereo 75/75 was a 75+75 watt amplifier it has only 90 uF (per each channel) to filter AC 😱... I'd like to increase this value to 340 uF (2x 400V-680 uF caps in series) as the "cousin" Manley Lab Stereo 75+75 watts shows 4 X 330 uF (series/parallel that is 330uF per channel) in its schematic and used the same power transformer (as far as reported in its original 1995 schematic as later it used a similar power transformer produced by another manufacturer).
I would not exceed 340 uF (per each channel) because I don't want to stress the power transformer too much with an excessive inrush current and because some people says that increasing to much the filter capacitance can negatively affect the sound.
Obviously I'd like to change the integrated bridge (with discrete diodes) even if the Manley Lab Stereo 75+75 watts schematic exactly reports the same Fagor bridge actually used in the VTL.
At the moment I'd use these caps: LGW2G681MELC50 Nichicon | Capacitors | DigiKey
and some diodes such as these (ultrafast): HERAF808G C0G Taiwan Semiconductor Corporation | Discrete Semiconductor Products | DigiKey
Any advive and opinion will be greatly appreciated.
Thank you
I'd like to recap an old VTL 75/75 amplifier and I'd like tho have some advice to choose the best value for many devices to use in the power supply (later, I'll ask for other issues).
As starting point I've drawn the schematics using the VTL 50/50's I've found on the web because it is very similar.
Though the VTL stereo 75/75 was a 75+75 watt amplifier it has only 90 uF (per each channel) to filter AC 😱... I'd like to increase this value to 340 uF (2x 400V-680 uF caps in series) as the "cousin" Manley Lab Stereo 75+75 watts shows 4 X 330 uF (series/parallel that is 330uF per channel) in its schematic and used the same power transformer (as far as reported in its original 1995 schematic as later it used a similar power transformer produced by another manufacturer).
I would not exceed 340 uF (per each channel) because I don't want to stress the power transformer too much with an excessive inrush current and because some people says that increasing to much the filter capacitance can negatively affect the sound.
Obviously I'd like to change the integrated bridge (with discrete diodes) even if the Manley Lab Stereo 75+75 watts schematic exactly reports the same Fagor bridge actually used in the VTL.
At the moment I'd use these caps: LGW2G681MELC50 Nichicon | Capacitors | DigiKey
and some diodes such as these (ultrafast): HERAF808G C0G Taiwan Semiconductor Corporation | Discrete Semiconductor Products | DigiKey
Any advive and opinion will be greatly appreciated.
Thank you
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That is a 555V rail! The energy in 90uF charged to 555V is 13.86J. In a solid state amp with 40V rails (More than a 75W SS amp) u'd need 17330uF to get the same amount of energy.
So 90uF aint half bad. U have a hum problem with the amp as is?
I do understand ur need to modify...I can never leave well enuff alone either.
So 90uF aint half bad. U have a hum problem with the amp as is?
I do understand ur need to modify...I can never leave well enuff alone either.
As far as diodes I find UF5408 to be more than good enough in my KT120 amp. I use them in series (for a bridge you'd need 8 diodes for 2kV PIV) It has 1200uf capacity at 540V (175J!) and these diodes handle the inrush perfectly.
Thank you,
Why did you use the diodes in series?
What is the maximum current (not peak current) running through these diodes in your amplifier?
This thread was moved from "power supplies" to "tubes / valves" section.
I do hope to be more lucky here and get some opinions and suggestions from people that have already gone through a direct experience with this kind of modifications.
Thank you
I do hope to be more lucky here and get some opinions and suggestions from people that have already gone through a direct experience with this kind of modifications.
Thank you
Sorry for the delay in responding.
I use them in series out of habit from when I used to use a large choke input. I found that sometimes when I turned off the amp the back EMF would blow the diode, so putting 2 in series increased the PIV to 2kV.
The first caps in the filter are NOS Nichicon, followed by Kemet PEH534, and the current of the B+ load is about 500ma. All of the caps after the chokes are Chinese but work perfectly fine. The stage after the phase splitter has 300R grid stoppers, but I forgot to put them on the schematic.
I use them in series out of habit from when I used to use a large choke input. I found that sometimes when I turned off the amp the back EMF would blow the diode, so putting 2 in series increased the PIV to 2kV.
The first caps in the filter are NOS Nichicon, followed by Kemet PEH534, and the current of the B+ load is about 500ma. All of the caps after the chokes are Chinese but work perfectly fine. The stage after the phase splitter has 300R grid stoppers, but I forgot to put them on the schematic.
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I will say my design is overkill, most people use a 1uF to 1ma ratio. In this case 470uF would work fine.
SS parts of all types are electrically fragile. The inductive kick back spikes of choke I/P filters can easily destroy SS diodes. Diodes of a PIV rating sufficient to deal with the RMS voltage are satisfactory, provided a 0.01 μF. ceramic cap. whose WVDC is in the multi-KV. range is placed in what would be the 1st position of a CLC filter. A reasonable alternative to the ceramic cap. is a carefully selected MOV.
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Take care also on the inrush current on the primary of the power trafo.
I suggest you, if you want to go up with value, the use of a Ntc in series with a primary or a delay circuit.
In my mono amps I use 8 x 470 uF 550 Vdc Nichicon each channel and I use a delay circuit that put in series a Ntc of 15 ohm-12 A for a 5 seconds delay, it works fine.
Of course you can left the Ntc in series with primary without any delay because after just 1 minutes it reach less than 1 ohm
Walter
Walter
I suggest you, if you want to go up with value, the use of a Ntc in series with a primary or a delay circuit.
In my mono amps I use 8 x 470 uF 550 Vdc Nichicon each channel and I use a delay circuit that put in series a Ntc of 15 ohm-12 A for a 5 seconds delay, it works fine.
Of course you can left the Ntc in series with primary without any delay because after just 1 minutes it reach less than 1 ohm
Walter
Walter
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Thank you Walter,
The problem of inrush currents is a bit controversial and it's very difficult to find information about the inrush current a power transformer can withstands without any problem.
The opinion of some transormer manufacturers I asked for is that the big resistance in the high voltage secondaries (in the VTL power transformer the DCR of each high voltage secondary is 65 and 70 Ohm respectively) would strongly limit the inrush current even if big caps are used.
Moreover some people say the biggest capacitance and low ESR caps in a power supply fklter are the goal to get a very good sound, but on the other side some people say that increasing too much the capacitance of a power supply filter could negatively affect the sound quality!
I'm not an electronic engineer, so I'm a little confused about all these so different thinkings.
Since I usually repair electronics without modifying them I don't know if the "upgradings" I'm going to do will really improve the amplifier performance. I hope they will!
I will apreciate your opinion and each experienced member's of this forum about the sonic result that can be reached with the modifications I'm going to do.
I do think that the reability of an electronic item is of paramount importance and so I'm very interested in the delay circuit you used to protect the power transformer from the inrush current in your amplifier.
Could you please post a schematics of that delay circuit?
Thank you
Hi
The power trafo from VTL are strong.
To check the impulse on power on if you put a 0,1 ohm/10watt in series with wire to the fiter cap ( after the diodes) and you use a analogic tester (as voltmeter ) you can see the value ( is not so fine but you can have an idea) of the inrush current.
But if you want to have a bigger caps ( and this is a very good idea for sound mainly on bass) you must have some precaution; David Manley on his VTL book explained why is always better the use of a bigger filter caps.
In all my mono I have 568 joule.
A NTC, as told you before, is the simplest way.
I paid a 12 A -15 ohm NTC about 5 euros; in series with primary works fine.
About the delay I will check on my paper the schematic that is very simple.
Walter
The power trafo from VTL are strong.
To check the impulse on power on if you put a 0,1 ohm/10watt in series with wire to the fiter cap ( after the diodes) and you use a analogic tester (as voltmeter ) you can see the value ( is not so fine but you can have an idea) of the inrush current.
But if you want to have a bigger caps ( and this is a very good idea for sound mainly on bass) you must have some precaution; David Manley on his VTL book explained why is always better the use of a bigger filter caps.
In all my mono I have 568 joule.
A NTC, as told you before, is the simplest way.
I paid a 12 A -15 ohm NTC about 5 euros; in series with primary works fine.
About the delay I will check on my paper the schematic that is very simple.
Walter
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Here is my supply. I put in a big 55uF 630V film cap. Don't know if you can hear a difference. I can say that more uF's do improve the base for sure.
I bypass my NTC after 1 second which seems to work well. The Hammond 378CX is a big transformer ( 20 ohms in the whole secondary) and it starts fine with only a 3 amp fuse @ 120V primary.
I bypass my NTC after 1 second which seems to work well. The Hammond 378CX is a big transformer ( 20 ohms in the whole secondary) and it starts fine with only a 3 amp fuse @ 120V primary.
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Thank you Walter,
I'm looking forward to your schematic!
I'd like to use a delay circuit to bypass the NTC by a relay a few seconds after switching the amp on.
If I limit inrush current by using a NTC, what would be a reasonable value of energy storage for the VTL amp? I remind you that It has a pair of KT90 and about 80 watts continous per channel.
At the moment the amp has filter caps connected in series. The B+ is about 555V.
Moreover I'd like to know if I could safely use 600 V caps (NOT in series) like these Nichicons: https://www.digikey.com/product-detail/en/nichicon/LGN2X221MELC50/493-14693-ND/6049397 .
I could put them in parallel to reach the desired value avoiding series cap connection and the consequent increase in series resistance. Unfortunatey, I couldn't find 105°C caps withstanding more than 600V. Using 600V caps my concern is both the voltage peak at switching on, (that is before current starts flowing through the tubes) and the consequence of an increase in mains AC voltage. The latter would reflect on the tranformer secondaries and a 10% increase in the primary would cause 610 V in B+.
On the other hand, I've examined some Audio Research power amps and they use normal 105°C, 450 V Nichicon caps with a B+ of 420 V. A 10% increase in AC voltage mains would cause more than 460 V in B+!
Please let me know your opinions.
Thank you
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Thank you MelB .
I could not understand how you exactly bypass the NTC after 1 second. Could you please post a circuit?
Thank you
It's cheaper to just use one of these: NE555 DC 12V Delay Relay shield Timer Switch Adjustable Module 0 To 10 Second | eBay
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