Ampeg SVT-5 Pro Misc Questions

[Grant_More]

Hi Guys,

Two unrelated questions on the SVT-5 Pro. I'm using bridged power amp mode here.

1. I've got a background mains hum which is there no matter what knob I twiddle. What kind of ripple should I be seeing on the high-voltage power amp rails? With no load and no input I'm seeing 300mV pk/pk of ripple on each rail. Seems a lot to me. Is that normal or are my jumbo smoothing caps knackered? I've checked the bias of the easy-to-reach power amp stage per Ampeg specificaiton and it looks good. Does the power amp just pull lots of quiescent current or should I be worried about those caps at that ripple level? Do they just hum because they are mega high-power amps and consume lots of quiescent with the whopping great output stages?

2. Perhaps a generic toob-noobie question, what are the characteristics of a knackered/aged pre-amp tube? I have some odd low-level high-frequency distortion/buzzing, the frequency of which seems to be unrelated to the input signal frequency and which seems to be related to the gain at the very first stage of the amplifier - possibly the tube. Turn the first stage gain right up and it goes away/is masked by the noise floor, same with right down. Seems to come straight through to the speaker no matter what the gain level of the master gain is. Not a problem at high volume levels, but is a pain at practice levels.

Any guidance very much appreciated!

Cheers,
Grant

[Enzo]

Hum comes from many sources. One quick check is the freq. Power supply ripple is twice the mains freq, so if the hum is 120Hz, or 100Hz for you, then we have ripple currents somewhere. If the hum is at mains freq 60Hz or 50Hz for you, then it is grounding, or radiated or unshielded, etc.

I doubt your hum is from "weak" filter caps. Even if it is power supply ripple, it is more likey a return path sharing copper with a signal ground.


And tubes? I find that having a spare tube is the fastest answer. I could spend a lot of time pondering and measuring, but a quick tube swap tells me instantly if the tube is involved or not.

[Grant_More]

Thanks Enzo!

I'll go analyse the hum frequency - I just assumed it was PA supply ripple, but you're right it could be induced directly from the unrectified supply. I'll check it out.

I'll go order a couple of spare tubes, easy solution to try!

Great tips, thanks!

Cheers,
Grant

[Steve Conner]

Enzo's right (as usual ) - the ripple on the big power supply caps shouldn't matter. 300mV is quite normal, and it'll get a lot bigger under load. 10V p-p wouldn't be surprising. The power amp is designed to reject it, using the wizardry of negative feedback.

Unless it's some weird design that doesn't use much feedback. Warwick had some hybrid tube/MOSFET amps that might fall into this category, but I think the solid-state SVTs are more conventional.

Does the amp have an effects loop, pre-out/power-in jacks, or something similar where you could break the signal path, and see if the unwanted noises disappear along with the signal?

[Grant_More]

Hi Guys,

Just wanted to post back with a quick update on this and the fix in case anyone else has a similar issue. After a season of light wedding-style gigging with this amp, the noise issue progressively got worse. Strange and random noises high-frequency and occasionally a rising noise floor culminating in the DC offset protection operating (which is what prompted me to open it up!) and the associated pop. Low-frequency hum sometimes apparent.

Threw the amp on the bench at work and hooked up to an AP2700 (luckily my work have 20 of these bad boys!). I managed to trace the issue down to the power amp stages. Couldn't recreate the pops, but by lifting the corner of the amp with the lid open it was possible to change the THD by around 15dB on one channel (I use the stereo power amp in bridge/BTL mode).

Main board out, got it under a magnifier and carefully analysed. Found broken or stressed solder joints at the MJE15032/15033 power amp pre-driver FETs Q102, Q105, Q202 and Q205. These FETs have clip-on TO-220-style heatsinks and I guess must be subject to thermal stress.

My general comment would be that the standard of the original soldering in the amp is poor. You'll not see this unless you get the PCB out of the can and flipped over. Not at all what I would expect of a £1500 bit of kit! So, I spent an afternoon, carefully re-soldered all the joints on the PCB, cleaned it off and put the amp back in can - working perfectly. THD rock solid, all noise issues eliminated, amazing audio restored!

I have a full set of schematics, anyone wanting talked through this mammoth task is welcome to email me for a guide! Also have a set of AP performance plots for the power amp if anyone is interested. Can PDF.

Thanks to Glen at Mars Amp Repair for the tip to pull the main board out of the can and look specifically at these transistors. To make sure it all gets labelled and bagged to go back in as it came out takes around 3 hours out and 2 hours in so I wouldn't have done it if Glen didn't recommend it in another post!

Cheers,
Grant