Anyone familiar with my 1975 Ampeg SVT - diyAudio
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Old 2nd February 2011, 08:00 PM   #1
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Default Anyone familiar with my 1975 Ampeg SVT

I am recapping my original SVT and noticed the amount of filter cap is lower than I expected. Will increasing the amount of capacitance in the PS filter help to quiet my old beast? My recent amp projects have been SS and in that application more's better, so what if I double the amount of capacitance in the PS filter?? Will this provide a general reduction in hum as compared to the factory design? How about using snubber capacitors across the filter caps (as used on large SS power supplies) ?
I have obtained all new Sprauge Orange Drops for the coupling caps.
Any thoughts on (other) minor improvements on the 1975 design?
I have a technician friend that recommends I set the amp up with KT88's due to the poor quality of 6550 that is presently available... Any thoughts here?
My Ampeg SVT appears to be of the type that's schematic is readily available online on schematic heaven. (6550 power tubes)
I have owned this since bought new in '75 and I have no qualms about modifying it for MY use. It can be restored to original if need be for sale at a latter date (like after my DEATH).
It is time to tune up my beast and a little upgrading may be in order, I for one have NEVER seen or used the PERFECT amp...
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Old 2nd February 2011, 08:36 PM   #2
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I have also wanted to build an SVP preamp for ever but have not found much info on the midrange inductor. I searched this site again and found a reference to 0-200-400-800 henries can anyone provide a little more info on this or maybe someone has wound one of their own???
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Old 2nd February 2011, 09:17 PM   #3
cerrem is offline cerrem  United States
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I have owned quite a few old SVT heads and have worked a many of these...
No need to change out the coupling caps....you will loose some of the SVT charm if you do so... The original coupling caps are fine and do not dry up....
As for the HUM in these amps....this is mostly due to the poor grounding technique and ground loops that occur... The grounds need to be re-assigned... Bypassing the filter caps is great with high-end audio but the SVT has a certain sound just the way it is...
For modern tubes in the SVT I prefer the Svetlana KT88 next would be the JJ KT88 ....
Make sure the 6 tubes are not only matched, but they have strong gm.... Some tube vendors will send you matched tubes but the gm will be in the gutter and you will be lucky to make even 250 Watts with weak matched tubes..
ALso keep in mind that the screen resistors act as fuses..so don't put in some big power wirewound screen resistors....Instead just use the cheap white tub shaped ones at 5W.... and make sure they stand off the board by an inch....this way they won't burn the PCB up when they blow...
Over filtering will increase the peak currents in the transformer windings...this is due to the reduced conduction angle durring cap charge....you can go up a little by don't go over-board... The best thing I do for the power supply is remove all the EI-175 laminations from the power transformer and re-stack it with M6 and the vacuum varnish... The output normally has M6 laminations needed for the inductance so no need to do this on the output......
As for the inductor.... I have wound a few of these but it has been a while....I will check my notes on the values....

Chris

Last edited by cerrem; 2nd February 2011 at 09:20 PM.
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Old 2nd February 2011, 09:35 PM   #4
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When soldering those sand cast resistors to a PCB, make a small diameter loop (abt 5mm) of lead to provide a little thermal and mechanical isolation to the circuit trace.
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Old 2nd February 2011, 09:45 PM   #5
OneyedK is offline OneyedK  Belgium
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Not too much of experience with Ampeg, only one SVT done, but hum was ok.
Are you sure the "hum pot" is ok and well adjusted?
Same with the bias, if there's a current imbalance between upper and lower halves, you also get hum.
In that regard the 12BH7 is also very important, I used the
TAD 12BH7A-STR Premium Selected (balanced).

As for psu caps, stick to the values, there is not much room for bigger caps and they would put more stress on the diodes. You can't go wrong with snubbering.

Another option would be to go with Solen polypropylene:
Solen Electronique Inc.

As for the tubes, both 6550 and KT-88 current production are questionnable.
6550 winged C's are still available and the best sounding.
I hear good things about TAD 6550A-STR PREMIUM too, but only time will tell if they last.

Can't help you with the SVP.
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Old 2nd February 2011, 10:06 PM   #6
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Bassmeknik;

it has an error in ground wiring in preamp. I silenced one such amp for my friend John Pursel, he did not believe it was his amp anymore...

It causes as well HF oscillation in certain position of timbre pots and switches.
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Old 3rd February 2011, 09:12 PM   #7
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I have gone through a lot of these original SVT's to repair / rebuild them and if you want to be ambitious, then get new PCB's made for the amp made out of fiberglass PCB material.. What Ampeg used for PCB's in these amps is terrible considering the build quality of the rest of the amp.
If you are not willing to do this (understandable, as it is a huge undertaking) then go over all of the traces on the PCB's with a magnifying glass because they are prone to hairline cracks in the copper and other problems.
I always had wondered if there would be demand for a PCB retro-fit for these amps that used only 12AX7's and 12AU7's in order to get around having to use 12DW7's as is it hard to source quiet ones nowdays. It would involve a redesign of the PCB's.
Daniel
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