Ampeg SVT-II no pro no go

[nevetslab]

Svt-ii photos-orientation time

Saturday, and I finally got the covers off the working SVT-II from our rental Dept @ CenterStaging, LLC Burbankc,CA. I first thought I'd capture and present a series of photos of the amp, just to put things into perspective when what is easy to see on the schematic, and a nightmare to try and get to in the real world. My apologies for not having a current-higher-resolution camera than this old Sony Mavica that I'm still beating up...low res, stores on 3.5" floppy disc!





There's plenty more images but this will get us started. First, I'm curious to see what the heater voltage is. Here in California, I'm running on 120VAC. 5.84VAC. At 110VAC Mains, its' 5.37VAC, and at 127VAC (hi line), it's 6.24VAC. Interesting. This has the same Heater voltage problem (is it?) as you've found. No matter...that's where they set the Heater xfmr windings.

Just looking to see what the bias voltage is set at...it's probably been 3 years since I've had this in the shop. TP 1 is 62mV DC, TP2 is 59mV, so it's a little low from the nominal spec of 72mV. Voltage at the cathodes of the two 12AU7 drivers are -47.9VDC, right in the ball park.

I'll post this for now, and resume either tomorrow after church or Monday morning. My eyes are in terrible shape, and driving at nighttime is now very scary...scheduling for Cataract surgery on both eyeballs.

[Jazz P Bass]

"scheduling for Cataract surgery on both eyeballs"
They 'do' one eye at a time.
So you have to wait a week or 2 until you get the second one done.

I am here to tell you that you will not want to wait!

I took the patch off of the first one on the way home.
I could not believe how vibrant colors where.
Bluer than Blue.
Greener than Green.
Redder than Red.


Good luck.

[nevetslab]

SVT II Output Waveforms, Data

I had hoped to have the whole ay with this amp, but didnt' get started until nearly 2PM. But, got some waveform meassurements & scope photos.

First, the amp won't put out 300W into 4 ohms It will do 200W with slipping, and 225W into 4 ohms with severe clipping, as you'll see. Here's some saveforms, with details below going left to right



All of the output waveforms are from the monitor output of the Amber 3501a, and aren't to any specific scale. The other waveforms ARE to scale, and will be noted.

The first scope photo is @ 20VAC Out into 4 ohm resistive load (100W). The positive-going half-wave is the cathode voltage across the 1 ohm cathode resistor with it's diode across it The image next to it is the Amber 3501a Audio Analyzer, monitoring the output voltage, showing 20VAC on the meter, and is also sourcing the 400Hz sine wave input to teh power amp stage.

The next waverforms are @ 28VAC into 4 ohms, with the output now showing some clipping, along with the voltage across the 1 hm cathode voltage (now @ 500mV/Div), definitely showing saveform clipping, while we don't see much of it on the output. The next two waveforms are showing teh same 28VAC output, along with the positive and negative driver tube outputs to the grids of the power tubes. 20V/Div via X10 probe. We're starting so see some assymetrical clipping on them at this drive level



These two waveforms are at 30VAC output ito 4 ohms (225W), and definitely clipping. The driver stage waveform is starting to show a bit more distortion, but still only moderate non-linearity

The next two waveforms are driving higher resistance loads on the 4 ohm output tap, with the cathode voltage showns with the output waveform.



These last two waveforms are at 40VAC output, 1st one into 16 ohms, and 2nd one into 24 ohms.. The half-wave is the waveform across the cathode resistor on the positive half of trhe power tubes. I'll have to add the voltage measurements on the amp @ idle when I get home

At 120VAC Mains, I find the following:

Power Tube Plate voltage: 647VDC
Screen supply voltage 383VDC
Driver tube cathode (bias) -47.8VDC
Driver tube grid voltage -70VDC @ V3B, -72VDC @ V2B
V3A Plate Voltage 181VDC
V3A Cathode Voltage 7.4VDC
V2A Plate Voltage 182VDC
V2A Cathode Voltage 7.4VDC
V1A Plate Voltage 217VDC
V1A Cathode Voltage 70.5VDC
V1B Plate Voltage 186VDC
V1B Cathode Voltage 1.2VDC

I forgot to get grid voltages on V1A, V2A & V3A. The Cathode Voltages @ TP1 & TP2 were recorded in the prvious post, as was the heater voltage.

I haven't yet made measurements under 2 ohm load, and should get that tomorrow.

[shzmm]

I had hoped to have the whole ay with this amp, but didnt' get started until nearly 2PM. But, got some waveform meassurements & scope photos.

First, the amp won't put out 300W into 4 ohms It will do 200W with slipping, and 225W into 4 ohms with severe clipping, as you'll see. Here's some saveforms, with details below going left to right

[ATTACH=CONFIG]47224[/ATTACH] [ATTACH=CONFIG]47225[/ATTACH] [ATTACH=CONFIG]47226[/ATTACH] [ATTACH=CONFIG]47227[/ATTACH] [ATTACH=CONFIG]47228[/ATTACH]

All of the output waveforms are from the monitor output of the Amber 3501a, and aren't to any specific scale. The other waveforms ARE to scale, and will be noted.

The first scope photo is @ 20VAC Out into 4 ohm resistive load (100W). The positive-going half-wave is the cathode voltage across the 1 ohm cathode resistor with it's diode across it The image next to it is the Amber 3501a Audio Analyzer, monitoring the output voltage, showing 20VAC on the meter, and is also sourcing the 400Hz sine wave input to teh power amp stage.

The next waverforms are @ 28VAC into 4 ohms, with the output now showing some clipping, along with the voltage across the 1 hm cathode voltage (now @ 500mV/Div), definitely showing saveform clipping, while we don't see much of it on the output. The next two waveforms are showing teh same 28VAC output, along with the positive and negative driver tube outputs to the grids of the power tubes. 20V/Div via X10 probe. We're starting so see some assymetrical clipping on them at this drive level

[ATTACH=CONFIG]47229[/ATTACH] [ATTACH=CONFIG]47230[/ATTACH]

These two waveforms are at 30VAC output ito 4 ohms (225W), and definitely clipping. The driver stage waveform is starting to show a bit more distortion, but still only moderate non-linearity

The next two waveforms are driving higher resistance loads on the 4 ohm output tap, with the[ATTACH=CONFIG]47231[/ATTACH] cathode voltage showns with the output waveform.

[ATTACH=CONFIG]47235[/ATTACH] [ATTACH=CONFIG]47236[/ATTACH]

These last two waveforms are at 40VAC output, 1st one into 16 ohms, and 2nd one into 24 ohms.. The half-wave is the waveform across the cathode resistor on the positive half of trhe power tubes. I'll have to add the voltage measurements on the amp @ idle when I get home

At 120VAC Mains, I find the following:

Power Tube Plate voltage: 647VDC
Screen supply voltage 383VDC
Driver tube cathode (bias) -47.8VDC
Driver tube grid voltage -70VDC @ V3B, -72VDC @ V2B
V3A Plate Voltage 181VDC
V3A Cathode Voltage 7.4VDC
V2A Plate Voltage 182VDC
V2A Cathode Voltage 7.4VDC
V1A Plate Voltage 217VDC
V1A Cathode Voltage 70.5VDC
V1B Plate Voltage 186VDC
V1B Cathode Voltage 1.2VDC

I forgot to get grid voltages on V1A, V2A & V3A. The Cathode Voltages @ TP1 & TP2 were recorded in the prvious post, as was the heater voltage.

I haven't yet made measurements under 2 ohm load, and should get that tomorrow.

nice work! I will post my shots tomorrow, but your amp is much cleaner than what I have here!

Looks like the voltages are pretty much in line with what I had already posted however...

[nevetslab]

Some added notes to what i posted last night.

V1A Grid voltage 25.2VDC
Test Point 1 68mV
Test Point 2 63mV

AC Mains @ idle 120VAC @ 2.01A/204W

Power @ 20VAC Output/4 ohms (100W) 4.36A/442W @ 120VAC mains
Power @ 28VAC out/4 ohms (196W) 5.8A/563W @ 120VAC
Power @ 30VAC out/4 phms (225W) 6.4A/647W @ 120CAC

I haven't opened up the insides thus far. And, I don't have component layout drawing for the PCB's. Thus far, I haven't identified the bias supply. Nor have I checked the min/max range of the bias control and the corresponding voltages at the Test points TP1 & TP2.

Some client work came in so I need to address that for now.

[shzmm]

ok here goes,

at 240VAC:

Power Tube Plate voltage: 657VDC
Screen supply voltage 308VDC
Driver tube cathode (bias) -45VDC
Driver tube grid voltage -66VDC @ V3B, -66VDC @ V2B
V3A Plate Voltage 191VDC
V3A Cathode Voltage 7.1VDC
V2A Plate Voltage 186VDC
V2A Cathode Voltage 7.4VDC
V1A Plate Voltage 237VDC
V1A Cathode Voltage 86VDC
V1B Plate Voltage 188VDC
V1B Cathode Voltage 1.3VDC
V1A Grid voltage 25.5VDC

Test Point 1 19mV
Test Point 2 20mV

The waveform doesn't get any prettier after around 4 VAC at 4 OHM. 2OHM doesn't look any better either.

As you can see the boards have had a lot of work done on them not by me however. As well there is quite a lot of heat damage as well as the caps all look tired.

I'm still pretty convinced this amp is a lost cause but let's see...

[Chuck H]

Is it just me, or do all those electrolytics look crowned?

[shzmm]

Is it just me, or do all those electrolytics look crowned?

no it isn't just you. They look bad but there is no hum per se (all ampegs seem to have a base hum.) They have plastic covers that are what is bulging not the caps themselves.

Either way, you can see the heat buildup in these amps is just silly. Even with a fan, having 6 6550s in close proximity in a rack mounted amp was probably not the best idea of the engineers of Ampeg.

[Chuck H]

Well I can't imagine those covers are bulging on their own.?. Get those caps outta there! They may be shorting while the amp is conducting and ruining performance. It's not uncommon for a bad cap to seem ok when the amp is just idling.

[Justin Thomas]

Perhaps they're bulging on top because the "explosion end" is blocked with goop. Sucks that the goop used to secure the caps is the exact same color as electrolytic cap spooge...

And, not all hum is bad caps, but not all bad caps hum, either...

Justin

[Chuck H]

That was my thinking too. They're bulging = bad. (<period)

Might be the whole problem. Don't forget the bias caps, etc. I'd strip every single aluminum cap out of that amp and replace it.

[shzmm]

I just checked the caps. The covers are there to protect against shorts I suppose. There is no bulging on the top or bottom. The goop is ancient silicon or whatever they used back in the 90's...

I have seen lots of amps with bad caps but never had an issue like this one. I can replace them all of course but it will take some time-- my usual shop doesn't have these sizes.

[Enzo]

Just heat around them often causes end covers on caps to bulge, it is not a sign of a bad cap. Slit the plastic sleeve to free the end cap and look underneath. I bet the caps are perfectly fine under there.

[nevetslab]

I'm wondering about the screen supply. The Power Tubes plate voltage is in the same ballpark as I see on CenterStaging's amp, but the screen supply being only 308VDC makes me wonder about the supply caps. It's almost as though one of the rectifier diodes is open, or, the caps are just tired. It might be a good idea to replace at least the C9 100uF/450V, C8 100uF/250 & C10 47u/F/450V. Certainly no harm in replacing the rest of the HV supply caps, but if you're tight on funds, I'd see about getting the screen supply back up where it needs to be. You did say all the filter caps do look a bit tired.

I haven't independently varied the screen supply on one of these amps to see what the effect is if it's too low. The screens on the amp in my shop is a bit higher than the 340VDC called out on the schematic (earlier schematic). And, you haven't been able to get the bias voltage up in yours. It is suggesting power supply issues. All the other voltage you're reading jive with what I'm seeing, as you had pointed out.

In looking at the power supply schematic, I could lift out D11 and D14, which are the full wave rectifier diodes for the Screen supply (as well as the plate supply for V2B/V3B), and patch in my HP 712C HV Power Supply to see what the varying Screen supply voltage does on the performance of the amp. It may Saturday before I get a chance to do that, but I am curious.

I still haven't looked at the 2 ohm load conditions....obviously lower output voltage swing.....and C/S's SVT II amp doesn't reach the specified output voltage spec either.

[shzmm]

I'm wondering if changing the screen resistors to 220r --from a Ampeg service note-- is what is bringing down the screen voltage. I am not reluctant to change the caps just have to source them first!

Also, on the schematic I am using, I don't see D14. D11 appears to allow a draw for c9 after the doubler of d12,13 and c8,11.

Otherwise I can also jig up an external screen voltage and see what happens; I am also curious. However, first I will do some measurements again at the screen voltage at drive conditions. I would think if there is a huge loss there, then indeed that could be the culprit. If not, it is back to the drawing board!