Mods to my Ampeg SVP-PRO!

[jazzbass_5]

The load imposed on the power transformer by the 8 volt rails probably didn't even amount to 80ma, almost nothing.

Not to take anything away from your abilities in electronics, but the reality is, tying two separate supplies to a common ground to create a split supply is 1st year electronics.

I've modified gear in the past, but before doing so, I ask myself these questions:
What specific problem(s) is this modification resolving; is there a clear benefit?
Is there any life safety issues to a user or technician that may arise from my proposed modification?
Is there any negative effects due to my modification; is it creating another problem?
What is the long term reliability of my modification?

From an educational standpoint, I'm not sure tinkering with things blindly offer much in the way of learning. The net is full of instances of people doing so, for no definite reason. For those wishing to learn, theory would be the first thing to tackle, and then start building and experimenting on your own circuits.

Agreed, theroy is always good.
I've built way to many of my own circuits and projects ranging from simple adjustable power supplies to 7 band EQs to 300 watt FET power amps and power amps built around STK packages.
I wasn't trying to correct a problem, I was upping the rails to see what would happen, experimenting if you will, see what would change, the comments about sagging voltage led me to the new power supplies. It worked.
I was tinkering.

 

[jazzbass_5]

The mute itself works, bu No difference if I push the mute knob or not.
Another idea was to took the grounding/earthing(ground loop) away (unsave).
Hiss/buzz remains.

There's also a mute circuit tied to the input, when you plug in your cord it disconnects it.

 

[ThisBass]

This not audible could also be


Again, there isn't a lot of difference. Maybe it's in my head.. I had no idea what changing the rails would do if anything, Bottom line is (still) I did it because I could.. wanted to see what would happen, Instead of asking first and getting all these answers and never even trying. It was fun, it's working and it's mine..
I'm confused about all the, what I can only interpret as, negatively persuasive comments leading me to believe maybe I shouldn't have done what I did, leave it alone, you're probably only going to make it worse, the engineers know what they're doing. Not one positive comment about how I did it, the Ingenuity behind using the (floating) 12 volt switch mode power supplies, fitting them in and making a +- 12VDC system. The fact that now the power transformer driving the HV and filiment voltage, (same transformer) has less work to do and less chance of overheating like my first SVP-PRO. (Btw, it was an open winding in the HV secondary, not the thermal fuse in my previous SVP-PRO transformer), no comments about the addition of a cooling fan.. all good. I hope someone sees this and maybe takes it a step further uses my silly ideas for something they're building. Saves some cash using thrift store wall warts as power supply or puts a fan in a project and uses the same idea behind what I did with my fan and a phone charger SMPS.
It's working folks, it's not noisey, I haven't opened any time space rifts yet powering it up and I have backup units that I use to play out with too.
If you're a proponent of leaving things stock and are upset that I violated this piece of equipment than I am truly sorry. I still have the components to return this to its stock original form if I don't end up destroying it,
Anyway.. back to work.

Learning by doing, that's not so bad.
I think its best to "eperience" theoretical considerations and practical implementation at just the same time and way.

Without any doubt the fan improves save operation for the mains transformer quite a lot.
Most amount of transformer power is reserved for the filament of the four tubes, while only very little power is neccessary for the SS schematic respectively the graphic EQ section. Extracting the supply for the SS section off from the mains transformer makes little to no dfference power wise for the mains transformer.

Of course I'm aware that any unhappy thermal environment conditions may ruin the mains transformer (at least over time).
So if the preamp was installed into rack case there should be enough space at bottom and top side of (metal) housing to provide possible air flow through the preamp.
Another "mod" that might be done to the preamp just to protect the mains transformer from overheating might be done with an additional thermal breaker that measures the temperature of the transformer iron and breaks off at about 100 degree F (rough estimation of course).

My favorite mod "solution" was to have a "clone" of the transformer build that was equipped with a reversible thermal breaker. I guess about $100 production costs to have a "custom" transformer build.
The most tricky thing was to specify transformer VA and secondary VA's just to meet electrical schematic requirements versus mechanical transformer dimension.

 

[jazzbass_5]

My cooling solution.
I used the old top for my gray face and opened up the vents above the tubes and also added a vent above the transformer. After that I added a fan so it would provide positive air pressure within the preamp and vents out the new (less restrictive) vents keeping the interior of the preamp cooler. Initially the fan was hooked up to one of the eight volt supplies. When I upped the supplies to 12 volts I found a noticeable noise coming from the preamp at the same frequency of the running fan. That's when I decided to use a small separate SMPS power supply to power the fan. That Supply is a 5.3 volt 650 milliamp ex-phone charger hooked into the switched AC line.

 

[agedhorse]

⠀
Mine hisses from firing up. No matter if an instrument/cable is plugged in or if the gain and master knobs are at "0" or "11".
It's a permanent hiss/buzz.

Hiss OR buzz? Completely different symptoms with mostly different causes.

Correct.
As you just now run 12V regulators versus 8V for the graphic EQ section the headroom is improved by about 3.5dB which (honestly speaking) isn't a ton of improvement. I don't think a tiny plus of +3.5dB could ever be sufficiant improvement just to fix "issues" if there really had been (complained) issues with "weak" headroom for the graphic EQ section.

So here you are what the schematic tells about headrom.

As I told already earlier tube stage V4B acts as buffer that drives both the graphic EQ input as well as the (active) midrange EQ section. If gain structure for the tube preamp signal level was set "correctly" with the gain pot for best SNR number all along the tube signal path then the V4b buffer outputs about 16V(peak) which is quite a lot. Then followed by a resistive voltage devider that does a signal reduction down to 0.4Vrms respectively 0.56Vpeak as (nominal) input level for the graphic EQ section.

For a "flat" setting of the graphic EQ the available headroom with 8V regulators predicts about +23dB versus about +27dB of headroom for 12V rail supply regulators.

Either way even the "smallish" 8V regulators provide pretty much headroom just to fully crank one or two sliders of the graphic EQ without clipping. Of course it was possible to crank the total graphic EQ section into clipping if all sliders was cranked and even the EQ level slider adjusted to "full" level. However even for 15V regulators as is for the svt II pro can't protect from clipping if everything was cranked all the way up.

Correct, this is what I was alluding to when discussing appropriate circuit design and scaling of gain structure. Of course most engineers that do this kind of stuff for a living and have a track record understand this better than most hobby-tinkerers. We usually do the math rather than guess, and apply that math to the proper theory to get to a workable end result.

I'm confused about all the, what I can only interpret as, negatively persuasive comments leading me to believe maybe I shouldn't have done what I did, leave it alone, you're probably only going to make it worse, the engineers know what they're doing. Not one positive comment about how I did it, the Ingenuity behind using the (floating) 12 volt switch mode power supplies, fitting them in and making a +- 12VDC system. The fact that now the power transformer driving the HV and filiment voltage, (same transformer) has less work to do and less chance of overheating like my first SVP-PRO. (Btw, it was an open winding in the HV secondary, not the thermal fuse in my previous SVP-PRO transformer), no comments about the addition of a cooling fan.. all good. I hope someone sees this and maybe takes it a step further uses my silly ideas for something they're building. Saves some cash using thrift store wall warts as power supply or puts a fan in a project and uses the same idea behind what I did with my fan and a phone charger SMPS.

What you are not understanding is that there are folks who are trying to learn and understand basic electronics here. What you have done makes no sense in that context, which is why people are questioning your changes. Why would somebody who is trying to improve their own gear using solid scientific information want to use your "silly ideas" (your words)? I simply do not see how it saves ANY cash compared with leaving an already solid working circuit alone?

My guess would be something wrong with the mute circuit.

So here's where we get into the need to understand theory and troubleshooting at the most basic level. This symptom just can't have anything to do with the muting circuit since the noise is present with the master volume at zero. The mute circuit(s) are all well before the master volume control. This is the sort of troubleshooting logic that (some) folks here are trying hard to learn about and understand why.

The load imposed on the power transformer by the 8 volt rails probably didn't even amount to 80ma, almost nothing.

Yup, I would guess less that 30mA, or less than 1/2 watt of electrical load, and with a transformer efficiency of say 75%, this would be about 1/8 watt of added heat to the transformer. Absolutely and completely insignificant and probably not even measurable.

I've modified gear in the past, but before doing so, I ask myself these questions:
What specific problem(s) is this modification resolving; is there a clear benefit?
Is there any life safety issues to a user or technician that may arise from my proposed modification?
Is there any negative effects due to my modification; is it creating another problem?
What is the long term reliability of my modification?

From an educational standpoint, I'm not sure tinkering with things blindly offer much in the way of learning. The net is full of instances of people doing so, for no definite reason. For those wishing to learn, theory would be the first thing to tackle, and then start building and experimenting on your own circuits.

I agree, and I fully support/encourage folks to get out there and learn more about the entire subject of electronics. The more you know, the more interesting it becomes, and the more successful projects and experimentation is.

The mute itself works, bu No difference if I push the mute knob or not.
Another idea was to took the grounding/earthing(ground loop) away (unsave).
Hiss/buzz remains.

Of course the mute is not going to make a difference, the mute is pre master volume, and when you turn the master volume control down it doesn't change. Is the noise a hiss or a buzz? This is a VERY important distinction.

There's also a mute circuit tied to the input, when you plug in your cord it disconnects it.

Correct, but this doesn't even come into play because the noise is entering post master volume, and he said the noise is present in every case, even with an instrument present (IIRC).

 

[GreyBeard]

I'm completely clueless about what you did, bro

Me too but, if it sounds good it is good, if it goes up in smoke well at least you tried. A lot of improvements are made just this way. Would I try this with any of my stuff? Not a chance.

 

[jazzbass_5]

Hiss OR buzz? Completely different symptoms with mostly different causes.


Correct, this is what I was alluding to when discussing appropriate circuit design and scaling of gain structure. Of course most engineers that do this kind of stuff for a living and have a track record understand this better than most hobby-tinkerers. We usually do the math rather than guess, and apply that math to the proper theory to get to a workable end result.


What you are not understanding is that there are folks who are trying to learn and understand basic electronics here. What you have done makes no sense in that context, which is why people are questioning your changes. Why would somebody who is trying to improve their own gear using solid scientific information want to use your "silly ideas" (your words)? I simply do not see how it saves ANY cash compared with leaving an already solid working circuit alone?



So here's where we get into the need to understand theory and troubleshooting at the most basic level. This symptom just can't have anything to do with the muting circuit since the noise is present with the master volume at zero. The mute circuit(s) are all well before the master volume control. This is the sort of troubleshooting logic that (some) folks here are trying hard to learn about and understand why.



Yup, I would guess less that 30mA, or less than 1/2 watt of electrical load, and with a transformer efficiency of say 75%, this would be about 1/8 watt of added heat to the transformer. Absolutely and completely insignificant and probably not even measurable.


I agree, and I fully support/encourage folks to get out there and learn more about the entire subject of electronics. The more you know, the more interesting it becomes, and the more successful projects and experimentation is.


Of course the mute is not going to make a difference, the mute is pre master volume, and when you turn the master volume control down it doesn't change. Is the noise a hiss or a buzz? This is a VERY important distinction.


Correct, but this doesn't even come into play because the noise is entering post master volume, and he said the noise is present in every case, even with an instrument present (IIRC).

Oh my @agedhorse
You're right about the mute circuit, and I respect all of your comments.
Still, I'm having fun doing it and it's working oh, and if you thought the 2 SMPS were bad and causing possible issues don't forget I've got a 3rd power the fan.
My cooling solution is easy, elegant and free heck, the transformer in my first SVP-PRO could have failed for any number of reasons that weren't heat related. Still, I installed a fan.
All good, you have valid points on all accounts, but I'm having fun, my unit works and I'm not good at leaving well enough alone.

 

[FenderOli]

Hiss OR buzz? Completely different symptoms with mostly different causes.

Of course the mute is not going to make a difference, the mute is pre master volume, and when you turn the master volume control down it doesn't change. Is the noise a hiss or a buzz? This is a VERY important distinction.

You decide. First click is preamp, second poweramp, then a "loud wave", after that the noise stays.

Svp pro noise

(hope it works...)

Noise comes from the speakers.

 

[agedhorse]

The first part is hiss combined with hum, the second part transitions into hum which can be caused by all sorts of things.

 

[ThisBass]

Actually either hum nor hiss sounds typical for the svp pro preamp. I'd guess the SOURCE for recorded hum and/or noise elsewhere but outside the svp pro.

 

[beans-on-toast]

A quick spectrograph check shows that the hiss is across the audio spectrum.

The EQ settings can increase the hiss. Bypass the graphic EQ, EQ switch out. Set the other EQ buttons out. Set the midrange switch to 3. Set the tone controls at noon. Set Gain, Master at 10, set Drive at 0. Insert an instrument cable between effect send and return.

The solid state EQ is now out of the signal path, tone is set flat. Check if the noise is the same with the graphic EQ in and out. This might tell you if the problem is in the graphic EQ stage.

Tubes can cause hiss and hum. So can a lot of other things in the amp. Here is a guide as to when tubes need to be changed: Tubes | TalkBass.com.

All amps need maintenance from time to time. It never hurts to have a tech clean the jacks, pots, internal connectors, and tube socket contacts. Then determine the source of any problems.


For reference:

[Invalid or Expired Link Removed]

http://music-electronics-forum.com/...961567-ampeg_svp-proa_-07s100-_schematics.pdf

 

[agedhorse]

A quick spectrograph check shows that the hiss is across the audio spectrum.

The EQ settings can increase the hiss. Bypass the graphic EQ, EQ switch out. Set other EQ buttons out. Set midrange switch in the middle to 3. Set the tone controls at noon. Set Gain, Master at 10, set Drive at 0. Insert an instrument cable between effect send and return.

The solid state EQ is now out of the signal path, tone is set flat. Check if the noise is the same with the graphic EQ in and out. This might tell you if the problem is in the graphic EQ stage.

Tubes can cause hiss and hum. So can a lot of other things in the amp. Here is a guide as to when tubes need to be changed: Tubes | TalkBass.com.

All amps need maintenance from time to time. It never hurts to have a tech clean the jacks, pots, and tube socket contacts. Then determine where the problem lies.


For reference:

[Invalid or Expired Link Removed]

http://music-electronics-forum.com/...961567-ampeg_svp-proa_-07s100-_schematics.pdf

If the noise is present with the master turned down as stated, then it can't be anything to do with V1-V4, eliminating all of that circuitry from the list of possible causes.

That leaves a makeup gain amp, cathode follower and a few passives.

 

[beans-on-toast]

If the noise is present with the master turned down as stated, then it can't be anything to do with V1-V4, eliminating all of that circuitry from the list of possible causes.

That leaves a makeup gain amp, cathode follower and a few passives.

Missed that the master was off, that does change things. Start by looking at V5. Tech time.

 

[byacey]

What kind of power amp are you driving with this preamp? Maybe you have too much gain?

 

[agedhorse]

My thoughts are a ground loop between the output and the power amp (a 1:1 audio isolation transformer is the proper and safe way to deal with this), or possibly noise getting into the V5 stages (via a defective tube, noisy heater supply, noisy plate supply or possibly an internal ground loop or poor ground current steering.

 

[Jim C]

I had borrowed a newish SVP-PRO back in the day and it hissed.
There have been many reports of this on TB over the years.
Conversely, this is not an issue shared by the CL.

Could it be that QC was loose and that brand new Pro's were shipped that had a high noise floor or hiss?
Perhaps wiring layout, shielding, or out of spec components.
Lastly, is it the overdrive potential or the graphic EQ that you folks like over the CL?

 

[byacey]

Has anyone tried a 12AU7 into the V5 socket? Perhaps the last stage doesn't require as much gain.

 

[FenderOli]

The first part is hiss combined with hum, the second part transitions into hum which can be caused by all sorts of things.

Actually either hum nor hiss sounds typical for the svp pro preamp. I'd guess the SOURCE for recorded hum and/or noise elsewhere but outside the svp pro.

A quick spectrograph check shows that the hiss is across the audio spectrum.

What kind of power amp are you driving with this preamp? Maybe you have too much gain?

I had borrowed a newish SVP-PRO back in the day and it hissed.
There have been many reports of this on TB over the years.
Conversely, this is not an issue shared by the CL.

Could it be that QC was loose and that brand new Pro's were shipped that had a high noise floor or hiss?
Perhaps wiring layout, shielding, or out of spec components.
Lastly, is it the overdrive potential or the graphic EQ that you folks like over the CL?

Thanks gentlemen,
I don't want to hijack that thread. I own that SVP-Pro since 1996, bought it used (nearly new) and it worked that way (audio above) since I got that.

No matter:
- Graphic EQ on/off
- each Knobs 0 or 10
- Cable (Instrument and patch-cable to poweramp)
- Power Amp (tried Peavey, Headphones, Crown or the poweramp-in from my '78 SVT)
- Earth loop (took the grounding away)
- 20 tubes tested all variations - until I got poor
- inserted an cable between effect send and return
- with or without the metal-lid/cover
- the "send out" is completly quiet - but you no master vol
- diffrent locations (home, rehearsal room, friend)

Once I took it to a tech, he said he never saw a tube schematic like that, more or less the tubes are in series/followers, he wanted to do the heating/filament via battery to see if that produce the hum. Too much effort.

Ampeg SVP PRO user's
Ampeg SVP PRO Club
SVP-Pro into a SVT-4 Pro through the XLR jack


What helped a bit:

In V1 - 60ties 7025 RCA (not cheap) it reduced a bit the hiss
In V5 - 12AU7 (in V5 you get less headroom but it reduces the hiss enormous)

Thanks again

 

[beans-on-toast]

Without test equipment, it helps to use a listening amp to trace through each stage of the amp from the first stage on through to the last stage. This tool allows you to hear what is going on at each point in the audio pathway. The input is a simple probe, a meter lead. The output goes into a small amp that allows you to listen for noise. I use a signal injector and monitor to do basically the same thing.

See the bottom of this page: Amp Tools. See attached pdf. A weber circuit is also included.

A similar circuit and a video showing how to use it is presented here: Billm Audio » Troubleshooting your amp with audio signal tracing

_____________________

Tube shields may help. A 9 pin medium height such as the military Cinch or IERC 6020-B fits a 12AX7: 4 X used USA IERC tube shield for 12AX7 12AU7 E88CC 6DJ8 6072 5687 ECC83 7025 | eBay.

Listed as available here: Leeds Radio - tube sockets and plate caps
Not as nice because it doesn't have the internal fingers but also works well: Invalid Link Removed

I'd want a shield on at least V1 and V5.
______________________

For the hum, a ground loop is a possibility. I would check out any ground connections to the chassis. This includes circuit board mounts that could be in contact with any ground traces. This ground loop problem occurred in the early Ampeg SVT reissues, detailed here: Ampeg SVT-VR&CL ground loop hum solution. With the volumes down, there was hum and noise. Not saying that this is the issue but something similar could be at play.

 

[jazzbass_5]

______________________

For those that want to modify their preamp.

A preamp should have a DC heater to keep line related hum down. This is a mod that pays off. I would rewire the heater circuit, convert it from AC to DC.

The schematic is confusing but the heater voltage is already rectified (full wave) to DC.
Seen here: