Mods to my Ampeg SVP-PRO!

[jazzbass_5]

This contains interesting modifications so read past the prelog.

Disclamer: never EVER try anything like this at home kids, get a 4 year (or more) degree before you even TRY to tinker with things. I'm just a simple tech messing with things I don't really know about and if I create a singularity or a rip in time space I apologize profusely.
You will see that I have questioning opinions hear about some of the things that I've done and that's okay too. My stuff, idc. Anyway, if you choose to modify anything of your own and use any of my ideas more power to you!

The last couple of days when I wasn't messing with my new Quilter Bass Block 800 I've been tube rolling my SVP-PRO and SVPCL for fun.. my SVP-PRO developed a buzzing at high volume and I suspected a bad connection due to the physical movement, so, I tore it apart and found the bad connection and touched up everything else (bad connection was pin 1 on J21)
Anyway, there's been talk about the preamp and how it's +- 8 VDC rails seemed a bit low and conjecture that this preamp may be better if the DC rails to the front were a bit higher. Giving the preamp and the EQ more headroom. So, yeah... I yanked out the LM79-8 and LM78-8 and replace them with LM79-12 and LM78-12 added heatsinks, just because, and upgraded the filter caps on both sides. The transformer seems has enough headroom to deliver the voltage needed, there's usually enough headroom engineered in and the solid state components don't draw a huge amount of current anyway. See pictures below.
So far, it's working, no problems!
It's early here and I can't crank it but it's sounding like a champ. I'll keep your posted. Right now I'm upgrading the fan power supply for the fan I had installed previously (I lost an SVP-PRO when a transformer failed due to internal heat, so I added a cooling fan to this one). The +12V rail was dipping with the fan on it so I added a separate power supply for the fan. I'll add those pictures when I finish.

 

[jazzbass_5]

Power supply added for fan. 5.3vdc, fan runs quiet no noise on the preamp.

 

[Jim C]

Thanks for this post.
Is the reason for this mod solely based on headroom?

Love my SVP-CL and have never even opened the case in 20 years as it sounds SOO good (afraid that the mojo might leak out)
I had great luck with original Telefunkens and Amperex is a Walkabout, but no or very little improvement in tone with a SM400 or Shuttle 12.2.
Curious as to how tube rolling went for you.

While I'm decent with a soldering station, I don't have enough experience to feel comfortable swapping out components to increase the output voltage on my early CL from .775V to greater.
Any experience with this?

 

[jazzbass_5]

I wasn't doing it to increase the output voltage and I'm not even sure if it effected that.
What I do know is the EQ section and the preamp op-amps were running at the low end of their designed DC voltage with +-8VDC and this in turn would effect the headroom of the entire circuit itself. Adding the extra voltage I presume will give the EQ section the added umph SVP-PRO users have complained about. So far with what I've heard it's working great and seems to have more adjustability in the EQ section . I would love to A/B with a stock unit.
As for my SVP-CL, mine also has the low output (.775v) and that (and upping the rails like I did today to my Pro) may be my next project. Although, it's a great preamp regardless.
As for tubes, I'm running in order in my Pro
V1 Raytheon 12AX7A
V2 Philips 12AX7WA
V3 Philips 12AX7WA
V4 JJ EC82
V5 Philips 12AX7WA
This seems to have a good sound for me.

 

[byacey]

You'll also want to up the voltage on any local power supply rail electrolytic bypass caps near any opamps, if they aren't rated for at least 16 volts; 25v would be better.
The heatsinks are a good idea. What's the input / output voltage differential on the regulators? You should have at least 4 or 5 volts. I've seen some manufacturers run the regulators so hot you can't touch them, mostly on poorly designed stuff like Behringer, etc.

87 is a great meter. I have a few of them; first one I bought in 1991, and it's still working great, although all the labels have worn off the function buttons years ago.

 

[jazzbass_5]

The input voltages are +14.01 and -14.38 with regulators like these you run into issues when the input voltage is excessively high compared to the regulated voltage.
I checked all the caps at the op amps and they are 35V or higher so there shouldn't be any issues there. I was worried about the indicator LEDS but the voltage increase hasn't seemed to make them much brighter so I'm sure their current draw is still within their specs.
I've had that 87 since the late 80s, note the buttons and lack of labeling. ..lol.

 

[byacey]

2 volts isn't very much differential. If your AC line voltage is low, the regulator will starve and cease regulating.

 

[jazzbass_5]

2 volts isn't very much differential. If your AC line voltage is low, the regulator will starve and cease regulating.

I guess if you look at the voltage readings that could be dispelled. Both rails are holding steady at 12v. There is no real excessive draw but if you'd like I could hook up the meter and try playing some to see of the rails drop below 12.

 

[jazzbass_5]

2 volts isn't very much differential. If your AC line voltage is low, the regulator will starve and cease regulating.

Also, I'm guessing in most situations ac line isn't going to dip a lot and even if it does, it's never going cause a drop at the regulators below the +-8VDC it was running at.

 

[BassmanPaul]

Did you apply heat sink grease under the new regulators?

 

[byacey]

The 8 volts is a moot point with 12v regulators. As soon as the input / output differential drops to about 1.2 volts, it will start passing power supply ripple as the regulator drops out below 12 volts.

If you have 14 volts @ 120V line, and the line voltage drops to 110 volts, you will only have 12.8 volts at the input to the regulators and the output will drop to about 11.6v; way past the edge for my comfort.

 

[jazzbass_5]

I'll have to get the oscope and my variac out and check that.. as for ripple, honestly, both sides are very over filtered, I replaced on the unregulated side 470uf caps with 2200 uf caps and on the regulated side I replaced 10uf with 100uf caps. Any ripple (I'm guessing) should be compensated for with that filtering, it's not drawing huge amounts of power. I'll do some testing.

 

[jazzbass_5]

Did you apply heat sink grease under the new regulators?

Very little, thin layer, they aren't even getting that warm really and had no sink prior.

 

[BassmanPaul]

Very little, thin layer, they aren't even getting that warm really and had no sink prior.

Thin is in tha knows.

 

[byacey]

I'll have to get the oscope and my variac out and check that.. as for ripple, honestly, both sides are very over filtered, I replaced on the unregulated side 470uf caps with 2200 uf caps and on the regulated side I replaced 10uf with 100uf caps. Any ripple (I'm guessing) should be compensated for with that filtering, it's not drawing huge amounts of power. I'll do some testing.

I would have left the 10uf on the outputs. These will generally have lower ESR for higher frequency noise.

 

[jazzbass_5]

I would have left the 10uf on the outputs. These will generally have lower ESR for higher frequency noise.

I thought that's what the .1uf caps were for.

 

[byacey]

The .1uf take over where the electrolytic impedance rises.

 

[jazzbass_5]

And if I leave it the way it is? What may happen?

 

[byacey]

I wouldn't worry too much about the output capacitance; I'd be more concerned about running out headroom with the power supply regulators.

 

[agedhorse]

I would worry about regulator stability driving 100uf on the output, especially under low line conditions.

2V is not enough differential, for 2V an LDO type is a better choice.

If the circuit is designed around +/-8V, you may not see any improvement anyway and if you drop out of regulation the noise floor may in fact suffer.