SVT clone power supply high B+

[g1]

Here are the reissue Power schematics.

[ATTACH]33851[/ATTACH]

[ATTACH]33852[/ATTACH]

One doesn't work and one is pretty much blank.

[Jazz P Bass]

Ooookay.

Ampeg SVT CL.zip

[MarkusBass]

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

[idlefaction]

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.

[idlefaction]

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.

[idlefaction]

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.

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!!

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. :-)

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.

[idlefaction]

I think you mean the screen voltages are 505V.

Right you are, I meant that the screens were at 505V.

[idlefaction]

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 =]

[guitician]

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!

[Leo_Gnardo]

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.

[g1]

Shouldn't the spec be adjusted taking this into account?

Mark

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

[Tom Phillips]

...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

[PauloP]

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...

[ATTACH=CONFIG]33836[/ATTACH]

[ATTACH=CONFIG]33837[/ATTACH]

[ATTACH=CONFIG]33838[/ATTACH]

[ATTACH=CONFIG]33839[/ATTACH]

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

[ATTACH=CONFIG]33840[/ATTACH]

[ATTACH=CONFIG]33841[/ATTACH]

[ATTACH=CONFIG]33842[/ATTACH]

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.

[catalin gramada]

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.

[Helmholtz]

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.)