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Thread: SVT clone power supply high B+

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    SVT clone power supply high B+

    Hi team

    I've completed my SVT build, and it sounds amazing!!

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    I used the specs for the original 70's power transformer; it's resulted in massive B+ voltages and I was wondering if anyone could chime in on whether it's OK to run 6550's on 730VDC...? They have a 600V max anode voltage, but even the modern SVT amps seem to run them at 640-660V. Not sure what the story is here, hopefully someone can chime in?

    Cheers,
    --
    Darren

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    Old Timer Tom Phillips's Avatar
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    Beautiful build!

    The original 1970 SVT schematic lists 695V B+ for the unit that used 6550 power tubes. Looking at my personal notes I have confirmed B+ readings in that range on SVTs with 120VAC line input. Since it's common for line voltage to exceed 120V you could easily reach the 730V you measured. There are probably lots of opinions about how well modern tubes could handle that B+ but it is common for musical instrument amp designs to run tubs beyond their published ratings.

    One additional factor is that you are using only 4 of the standard 6 SVT power tubes. With a stock spec power transformer you will therefore have higher voltage due to the lower loading on the power supply.

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    Last edited by Tom Phillips; 05-05-2015 at 10:09 PM. Reason: Clarify stated voltages

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    Supporting Member Jazz P Bass's Avatar
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    I believe the key here, that is allowing that sky high B+ voltage on the plates, is the Screen Voltage being held low (345 some volts).

    Bias current being low will also let the B+ rise.

    The SVT CL indicates -45Vdc on the bias pins.

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    Quote Originally Posted by Tom Phillips View Post
    Beautiful build!
    Thanks Tom!! 4 years in the making. It sounds, in a word, amazing.

    Quote Originally Posted by Tom Phillips View Post
    Since it's common for line voltage to exceed 120V you could easily reach the 730V you measured. There are probably lots of opinions about how well modern tubes could handle that B+ but it is common for musical instrument amp designs to run tubs beyond their published ratings.
    Yeah, here in NZ we are on 230VAC but it usually measures 240VAC+ so that will be contributing. Are there any rules-of-thumb on how a high B+ might affect tube life? I haven't spotted anything online in my Google searching.

    I've only had it running for a couple of minutes at a time, so I'll give it twenty minutes or so tonight and see where things settle to. And will post some more photos of the amp with everything wired up, as I can see a few missing things in that guts shot above!

    Will also post the turret board layouts in case anyone else wants to build one.

    Just noticed I joined this board in 2011, and that was my first post. *heh*

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    Quote Originally Posted by Jazz P Bass View Post
    I believe the key here, that is allowing that sky high B+ voltage on the plates, is the Screen Voltage being held low (345 some volts).
    Bias current being low will also let the B+ rise.
    IIRC Screen voltage is also high at 495V, but I'll re-check that tonight. That is probably measured on the wrong side of the screen resistors, too. Did you mean that when you use a very high B+ you should use a lower screen voltage to compensate?

    I only have the output tubes biased at 50% plate dissipation at the moment, so I'll crank that up to 65% and see if it affects the B+ any.

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    Okay! The good news is that the B+ does come down from 730V to 705V, and the heater filament voltage also comes down from 7.35V to 7.10V. The wall was reading 238VAC, and I adjusted the bias up to 74mA/side, which is 62% max plate dissipation.

    The not so awesome news is that the plate voltage is 505V. As indicated above, really for optimum tube lifespan this should be a lot lower, but it's about what the original 70's SVTs had, so cest la vie!

    Some more photos of the finished build...

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    And as promised, here's my layout, in case anyone else is foolish enough to attempt to build one of these.

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    What a great job -- takes some berries to scratch build one of these!
    May I ask what you used for the mids inductor?

    What power tubes are you using?

    FWIW, I had a 70's SVT in for repair last month. It had the wrong driver tubes and was biased at around 25%, putting plate voltage upwards of 750V.
    It had been played hard this way for several years, the tubes were SED 6550's. They tested fine, and once the driver/bias was corrected they all idled happily within a few mA. Tough tubes.

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    Also FWIW, I'd be more concerned with your idle dissipation than plate volts. It's a super dynamic power amp, and Ampeg wisely advised cool bias.

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    Judging by the pictures it seems you missed out on 2 power tubes that would have helped soaking some juice out of the power transformer. This is probably keeping the B+ a tad high.

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    In this forum everyone is entitled to my opinion.

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    Wasn't Fender 400-PS amp using 6550s @700V? The schematic says "SPECIAL DESIGN AND SELECTED 6550 TUNGSOL TUBES".
    Why is the voltage too high if you used the spec for the original 70's power transformer?

    Mark

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    Who is the manufacturer of the blue perf board used for turrets? I like that idea a lot. The green stuff looks too home made.

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    Quote Originally Posted by MarkusBass View Post
    Why is the voltage too high if you used the spec for the original 70's power transformer?
    1) The line voltage is higher now than it was in the 70's
    2) He is only running 4 power tubes when the original PT was running 6 power tubes.

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    Quote Originally Posted by Jazz P Bass View Post
    I believe the key here, that is allowing that sky high B+ voltage on the plates, is the Screen Voltage being held low (345 some volts)...
    I agree. The Ampeg design used 350V on the 6550 screens. That’s from 1970 service information.


    Quote Originally Posted by idlefaction View Post
    IIRC Screen voltage is also high at 495V, but I'll re-check that tonight. That is probably measured on the wrong side of the screen resistors, too. Did you mean that when you use a very high B+ you should use a lower screen voltage to compensate?
    The Ampeg SVT power transformer had a separate winding for the screen poser supply. Does your amp use that approach? Anyway...that’s how they generated the 350V screen supply. The resistors we call “screen resistors” (one for each power tube) are there to limit the screen current under high power and overdrive conditions. If you were to insert a large voltage dropping in the screen supply to substantially lower the screen voltage at idle then I think the resulting saggy supply would adversely affect the amp. The original SVT design used 22Ω screen resistors. If you are using larger screen resistors, such as 1kΩ then you may be OK with the high screen voltage. Note, however, that the “Design Maximum Rating” for the 6550 screen voltage is 440V and the “Design Maximum Rating” screen dissipation is 6W. Of course we don’t really know what the ratings are for currently available 6550s since the few spec sheets that are published seem to be clones of the legacy spec sheets.


    Quote Originally Posted by idlefaction View Post
    I only have the output tubes biased at 50% plate dissipation at the moment, so I'll crank that up to 65% and see if it affects the B+ any.
    As hylaphone pointed out , Ampeg used a cooler bias. The original published “Calibration procedure” for the SVT specified 72ma per side of three 6550s. With 695V on the plates that gives 16.68W plate dissipation per tube which is 40%. I always thought that sounded fine and the amp produced 300 to 330W of clean power. I was never tempted to bias the SVT hotter. Give cooler bias a try and let your ears decide. I’m sure the tubes will like it.


    Quote Originally Posted by idlefaction View Post
    Okay! The good news is that the B+ does come down from 730V to 705V, and the heater filament voltage also comes down from 7.35V to 7.10V. The wall was reading 238VAC, and I adjusted the bias up to 74mA/side, which is 62% max plate dissipation.
    The not so awesome news is that the plate voltage is 505V. As indicated above, really for optimum tube lifespan this should be a lot lower, but it's about what the original 70's SVTs had, so cest la vie!.
    You lost me there regarding what caused the changes and why voltages seem to be all over the place. Anyway...the original SVT version (after they switched from 6146B power tubes to 6550s in 1970) used 695V B+, 350V on the screens, 6.3V for the heaters and was biased at ~17W per power tube.


    There are many tradeoffs that can be made but I’d recommend against simultaneously pushing the limits on multiple parameters.

    By the way, what version of the SVT schematic did you use as the basis for your build?

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    Quote Originally Posted by idlefaction View Post
    The not so awesome news is that the plate voltage is 505V. As indicated above, really for optimum tube lifespan this should be a lot lower, but it's about what the original 70's SVTs had, so cest la vie!
    I think you mean the screen voltages are 505V. You said your plates are in the 700V range. I don't know which "original SVT" voltage you are referring to here, but as was mentioned, screens in the original were around 350V.

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    Quote Originally Posted by Jazz P Bass View Post
    One doesn't work and one is pretty much blank.

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    Ooookay.

    Ampeg SVT CL.zip

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    Quote Originally Posted by g1 View Post
    1) The line voltage is higher now than it was in the 70's
    2) He is only running 4 power tubes when the original PT was running 6 power tubes.
    Shouldn't the spec be adjusted taking this into account?

    Mark

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    Quote Originally Posted by hylaphone View Post
    What a great job -- takes some berries to scratch build one of these!
    May I ask what you used for the mids inductor?

    What power tubes are you using?
    Its the Mercury Magnetics clone inductor. And those are JJ 6550s.

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    Quote Originally Posted by patlaw View Post
    Who is the manufacturer of the blue perf board used for turrets? I like that idea a lot. The green stuff looks too home made.
    Crap I can't remember where I got that perf board! I think it was tubesandmore.com but could be wrong.

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    Quote Originally Posted by Tom Phillips View Post
    I agree. The Ampeg design used 350V on the 6550 screens. That’s from 1970 service information.
    Great, I'll look at rewinding the screen transformer then, as all signs seem to point to the screen voltage being on the high side.

    Quote Originally Posted by Tom Phillips View Post
    The Ampeg SVT power transformer had a separate winding for the screen poser supply. Does your amp use that approach?
    Yep, I used the version that you can find all over the internets as 'Drawing by Joe Piazza'. The power transformer specified is supposedly the Ampeg P/N 8930072 which has the following specs:
    600v @ 0.75A plate
    330v @ 0.07A screen
    150v @ 0.025A bias

    Of course, bridge rectifying 600VAC gives you 840VDC plate, and full wave rectifying 330VAC gives you 460VDC screen, and full wave rectifying 150VAC gives you 210VDC bias, so I have no idea where any of these figures come from and really I should just have calculated the transformer specs to fit the voltages I was wanting to get!!


    Quote Originally Posted by Tom Phillips View Post
    As hylaphone pointed out , Ampeg used a cooler bias.
    It's with the guy I built it for now, but we biased it to 50% on everyone's recommendation. :-)

    Quote Originally Posted by Tom Phillips View Post
    By the way, what version of the SVT schematic did you use as the basis for your build?
    It's been a long time since I designed this originally and a few of the finer design decisions elude me now. I used the 1970's "Joe Piazza" schematic for the basic structure of the amp, but only built one preamp channel. I seem to recall using the SVT-CL schematic to decide how to replace out the non-ECC82/ECC83 preamp tubes, and after tracing both schematics when doing my pre-powerup checks I can say they're basically the same amp. The SVT-CL uses lower voltages and uses higher values through the bias section, eg. the bias pots are 50k instead of 15k, the rest is basically identical.

    I remember that I spent quite a lot of time doing stuff with photocopies of 6550 curves and rulers to figure out the output transformer specs, which were 2775 ohm primary, 2+4+8 ohm secondary.

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    Quote Originally Posted by g1 View Post
    I think you mean the screen voltages are 505V.
    Right you are, I meant that the screens were at 505V.

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    Quote Originally Posted by patlaw View Post
    Who is the manufacturer of the blue perf board used for turrets? I like that idea a lot. The green stuff looks too home made.
    https://www.tubesandmore.com/products/P-HTUR-D

    Found it =]

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    What is with web sites that give metric dim. You have to click "Product Measurements" to see inches. Are they trying to convert us in the USA to the metric system or what!

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    Quote Originally Posted by guitician View Post
    What is with web sites that give metric dim. You have to click "Product Measurements" to see inches. Are they trying to convert us in the USA to the metric system or what!
    USA - was SUPPOSED to have converted to metric by uh, 1975. Guess we got sidetracked a bit, had to bail out of Saigon, print up a couple WIN buttons, never did make the conversion. Maybe that's what's wrong? Better late than never.
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    Quote Originally Posted by MarkusBass View Post
    Shouldn't the spec be adjusted taking this into account?

    Mark
    Quote Originally Posted by idlefaction View Post
    Great, I'll look at rewinding the screen transformer then, as all signs seem to point to the screen voltage being on the high side.



    Yep, I used the version that you can find all over the internets as 'Drawing by Joe Piazza'. The power transformer specified is supposedly the Ampeg P/N 8930072 which has the following specs:
    600v @ 0.75A plate
    330v @ 0.07A screen
    150v @ 0.025A bias
    If you are going to get the screen winding rewound, then get the HV plate winding rewound also.
    I believe those voltages are the DC rectified voltages, not AC.
    For currents, because you are running only 4 of 6 power tubes, you should use 2/3 current ratings.
    So you want a HV winding that will provide 600VDC at .5A,
    screen winding that will provide 330VDC at .046A

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    Quote Originally Posted by idlefaction View Post
    ...The power transformer specified is supposedly the Ampeg P/N 8930072 which has the following specs:
    600v @ 0.75A plate
    330v @ 0.07A screen
    150v @ 0.025A bias
    Of course, bridge rectifying 600VAC gives you 840VDC plate, and full wave rectifying 330VAC gives you 460VDC screen, and full wave rectifying 150VAC gives you 210VDC bias, so I have no idea where any of these figures come from and really I should just have calculated the transformer specs to fit the voltages I was wanting to get!!
    Ampeg and then later SLM published power transformer information that listed the resulting rectified & filtered DC voltages rather than the secondary AC voltages. That practice is the source of confusion and the origin of the figures that you listed. Essentially the same figures have been listed in many newsgroup discussions adding to the confusion as readers mixed up their AC & DC voltages. Attached is an example from some SLM literature. Ampeg service information from 1970 confirms the figures. I choose this particular one because it listed both the quiescent and full load power supply voltages which I though was useful information. I hope this helps clarify the situation.
    Cheers,
    Tom
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    Quote Originally Posted by idlefaction View Post
    Okay! The good news is that the B+ does come down from 730V to 705V, and the heater filament voltage also comes down from 7.35V to 7.10V. The wall was reading 238VAC, and I adjusted the bias up to 74mA/side, which is 62% max plate dissipation.

    The not so awesome news is that the plate voltage is 505V. As indicated above, really for optimum tube lifespan this should be a lot lower, but it's about what the original 70's SVTs had, so cest la vie!

    Some more photos of the finished build...

    Click image for larger version. 

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    Click image for larger version. 

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    And as promised, here's my layout, in case anyone else is foolish enough to attempt to build one of these.

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    Good morning forist friends, I'm new to this forum and I'm starting to plan the mound of an SVT.
    I wonder if friends have this or another svt layout drawn not handwritten? I am having some difficulties with this layout.I also ask about the specifications of the transformers because I live far away and I will have to order the manufacture of the transformers.
    Thanks in advance.

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    Nice build, congrats! Just a notice at first pics sight: if those black piece on the board is a inductor I suggest you to tie it with some plastic stripes on the board, not with a screw bolt through. Avoid any fero-magnetic material through the core, and even as base support.Just a suggestion, if you don't mind.

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    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    I don't see a problem with a ferrous screw through the center of a toroidal inductor. As the magnetic flux is confined to the core and running at right angles to the axis of the screw, it won' be influenced (no magnetic coupling).

    (But any metal screw would be a problem if it was part of a conducting loop around the core.)

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    Quote Originally Posted by PauloP View Post
    Good morning forist friends, I'm new to this forum and I'm starting to plan the mound of an SVT.
    I wonder if friends have this or another svt layout drawn not handwritten? I am having some difficulties with this layout.I also ask about the specifications of the transformers because I live far away and I will have to order the manufacture of the transformers.
    Thanks in advance.
    PauloP - we may by comparing a lot of pictures and notes!

    I, too, am about to embark on the journey of building an SVT for a friend. He has been asking me since our college days over 20 years ago to build him a bass amp. Back then I built a tube hi-fi preamp and a couple 300B single ended tube power amps. Preamp was point to point, 300B was point to point with turrets. Since he saw me do that, he has been asking to build his dream amp. Then I put down the soldering iron for years (career, kids, 2 cross country moves, etc.) This year I have built and upgraded 9 First Watt / Pass Labs solid state hi-fi amps. Now I'm getting ready for his build.

    Discovering this thread is inspirational! The wiring is phenomenal, and the layout drawings are very nice. It makes a very intimidating project a little more approachable. Fantastic work!

    My buddy's dream bass amp:

    He runs his bass into Carvin X100B guitar amp using the Lead channel for processing (Gain, EQ, Distortion), out the effects loop, and into a 1970's black line SVT. I have his spare Carvin and his SVT to reference in my basement. He us using a 300W Sunn amp while the SVT is at my place.

    I ordered some cables to do some testing, and some dummy load resistors. I'm going to his band practice this week to get a better idea of how he's using the Carvin - exact settings, etc. I will mimic the settings on the spare Carvin, then run it through my oscilloscope to look at gain / eq and graph that.

    I also want to understand how he's using the SVT. It appears he's using channel 1 only. He has little paint dots on his default settings.

    Based on this, I want to make a sketch of the gain / processing stages to design his amp.

    Example:

    Carvin
    12AX7 input (? dB Gain)
    5 - band EQ: 75, 150, 500, 1500, 3000Hz (get the settings)
    12AX7 (20 dB Gain) - turned to "2" on the knob, very loud. 2.5 is deafening. Can this pot be changed to give better range?
    Out the effects loop send jack to the SVT

    SVT Channel 1 - Pre
    Input - Bright or Normal?
    V1 - 12DW7 Input
    Ultra Low switch set to +
    Bass knob set to about 4 o'clock
    Ultra-Hi switch set to off/zero
    Treble knob set to 0
    V3 - 12DW7
    Mid switch set at 220 hZ (left)
    Mid set at about 1:30 o'clock
    V4 - 12DW7
    V5 - 6C4

    SVT power amp section
    V1 - 12DW7
    V2 - 12BH7
    V3 - 12BH7
    V4-9: 6550 Power Tubes

    Ideally I'm not redesigning much.

    Things I'm thinking about at this early stage:

    Can I take SVT-CL design (one channel) and put a 5 band TH Custom EQ into the chain between input jack and first tube. Maybe jack, EQ, switch for normal/bright, and then to SVT-CL circuit? This will be simpler than the 2 channel design and use more modern tubes. Will it get the distortion the Carvin/SVT combo gets? Could adding one more gain stage do it?

    To test this theory I'm thinking of going with my buddy to a guitar store with a SVT-CL and use an EQ pedal (Behringer 5 band, MXR, or other) and see if he can get the sound he wants. If not, EQ pedal into a distortion or gain pedal...

    Turret boards vs. fabricating / buying PCBs. Does anyone make SVT PCBs? Does anyone have drawings / gerber files? I have had PCBs fabbed before using existing gerbers, but I have know knowledge or skills for the design/drawings.

    0 Not allowed! Not allowed!

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