NAD - Ampeg SVT 3 Pro

[JapanAxe]

I have had a hankering for yer actual Ampeg for some while, so having recently parted company with my TC Classic, I was pleased when an SVT 3 Pro came up for sale on BC at a very reasonable price. (Yes I know the 'real deal' is an all-valve job but they weigh a ton and they're too wide to sit on top of my cabs without looking a bit ridiculous.)

Anyway, said head turned up on Wednesday so I have had a couple of little goes with it. Btw none of the following should be taken as criticism of the BCer who sold the head to me - it was used, the price was right, and (as with used cars) you do expect to spend a few bob on tweakage to get a used amp how you like it.

The seller had wisely suggested checking that the valves were still correctly seated after shipping, so before powering up I unscrewed the top and tested each valve in turn on my Orange Valve Tester(TM) (which has probably paid for itself already). The valves are a mix of Fender/Sovtek, GT/Sovtek, and EH - three 12AX7 in the preamp, and a 12AX7 and 12AU7 to drive the power MOSFETs. The tester failed a couple, apparently because of the mismatch between the two triodes. This is not necessarily a problem, but having studied the schematic, the 12AU7 seems to work like a phase splitter, so that one needs to be well balanced.

I also noticed that the toroidal transformer is slightly loose, so I need to locate a suitable box spanner to tighten this down.

I connected the amp to my BF Midget to check out the sound. the fan is a little noisy so I may look to replace it. I reckon the bearings have become worn over time. At gig levels you wouldn't notice it at all, but I wouldn't want it to fail in use!

Considering that the bass and treble controls are passive, they have a good range and are smoothly graduated. I am likely to use the 5-way mid frequency switch in position 1 (220Hz) or 2 (450Hz), to control the low mids. I had thought I wouldn't be bothering with the graphic EQ, but it is very musical-sounding. I dialled in a gentle low-mids hump, and really liked the effect on my sound, plus the extra volume available.

Neither the Bright nor Hi switches are of much interest to me, but I was pleasantly surprised by the effect of the Lo switch (which scoops the mids and slightly raises the bass) as it gives you a different voice to play with.

I didn't hear much difference from the Tube Gain knob until I connected the head to my full speaker setup (Compact plus Midget) and wound the wick up. This control actually adjusts the plate voltage in the last valve stages, so it goes from compressed/distorted at 0 to full headroom at 10. At gigging levels you can create the impression of a valve amp at full tilt without actually running the amp at full power.

As many have remarked, you do have to turn this amp up to get the most out of it. (Okay you may get the same volume from another head with the master at 4, but by 5 you may have hit the buffers.)

There seems to be an element of overdriven sound that won't go away, regardless of gain structure. If it was a MOSFET bias problem I would expect to hear it as the note faded away, but I don't, so my money is on one of the valves, most likely the 12AU7 with mismatched triodes. I am confident I can track it down by monitoring the signal at different points in the chain, so no biggie there. I have some 12AX7s in my spares box but I'm going to have to order another 12AU7. All part of the fun, and once again, no reflection on the BC seller.

I am currently chilling at my sister-in-law's, but I will post some pics once I am home, and report on my findings at a later date.

[hamfist]

Coolio. Enjoy ! Fiddling around with amps is fun.

Never played a 3Pro myself but the design of the tube gain control seems very odd and counter-intuitive. Good old Ampeg !

[JapanAxe]

[quote name='hamfist' timestamp='1397941620' post='2428961']
Coolio. Enjoy ! Fiddling around with amps is fun.

Never played a 3Pro myself but the design of the tube gain control seems very odd and counter-intuitive. Good old Ampeg !
[/quote]

It is a misnomer. It should be called 'Plate Voltage'. I will try measuring the voltage range at some point. In the meantime, here is my rig pictured next to Mr Axe junior's new [url="http://basschat.co.uk/topic/235006-ashdown-mibass-20-1x10-combo"]Ashdown MiBass 2.0 combo[/url]. Note Mrs Axe's excess cushion stockpile. How dare she go on at me about having too much kit!



EDIT: Sorry, this is possibly the blurriest wonkiest pic I have ever taken. Will try to do better!

Edited April 19, 2014 by JapanAxe

[goingdownslow]

Regarding the bias, it is worthwile measuring the voltage across the resistors just to rule it out. Costs nothing.
I checked mine and found...
Initial readings, (mV)
1.6 - 3.0
3.7 - 2.8
3.6 - 2.4
3.2 - 3.2
average
3.025 - 2.85

and after adjustment...
14.9 - 26.1
30.4 - 23.5
27.3 - 20.1
22.5 - 23.4
average
23.775 - 23.275

It sounded ok before but was like a different amp after adjustment.

[JapanAxe]

[quote name='goingdownslow' timestamp='1397947137' post='2428998']
Regarding the bias, it is worthwile measuring the voltage across the resistors just to rule it out. Costs nothing.
I checked mine and found...
Initial readings, (mV)
1.6 - 3.0
3.7 - 2.8
3.6 - 2.4
3.2 - 3.2
average
3.025 - 2.85

and after adjustment...
14.9 - 26.1
30.4 - 23.5
27.3 - 20.1
22.5 - 23.4
average
23.775 - 23.275

It sounded ok before but was like a different amp after adjustment.
[/quote]

Cheers Pete that will be incredibly helpful. I take it those are the voltages across the eight big chunky 0.47ohm 5W resistors that sit in the middle of the heatsink? I have read that those voltages should be around 25mV (although elsewhere on the Interweb I have read that the current through them should be 25mA, making the voltage 11.75mA!)

Did you set the bias yourself using a 10kHz signal and an oscilloscope, or did a tech do it for you?

[goingdownslow]

[quote name='JapanAxe' timestamp='1397948064' post='2429005']
Did you set the bias yourself using a 10kHz signal and an oscilloscope, or did a tech do it for you?
[/quote]

[quote] I take it those are the voltages across the eight big chunky 0.47ohm 5W resistors that sit in the middle of the heatsink?[/quote] Yes.

I set it myself giving the pot slightest turn and checking the mV each time with a mutimeter. At one stage I had 40mV, so shows how sensitive the pot is.
My 3Pro had a sizzle from new, could only hear it at low volume, I thought it was part of the sound till I read the posts on [url="http://www.talkbass.com/threads/svt3pro-bias-setting.437406/"]HERE[/url].
I hesitated for a while before doing it as I wasn't sure if I was going down the right track as it being a new (USA) amp I didn't think it would be the bias, then I thought no harm in checking the voltages. I was surprised but chuffed at what readings I found, knowing then that this was the cause. Armed with a small screwdriver and being wary of what I had read about the pot being sensitive I took my time. Did it with amp cold with no signal and no load on. I stopped when I got the average reading as posted above as I thought it being close enough, set the amp on my cab plugged in bass and the amp was clean as a whistle, and with what I thought was more punch on gigs.
Reading up the SVT Bias threads on TB gave me the confidence to have a go.
It is also possible that a batch had been set wrong at the factory as it hard to think that it drifted that far out.

Good luck with whatever you do.

Edited to say, whatever others have posted, I turned the pot clockwise to increase the voltage.

Edited April 20, 2014 by goingdownslow

[Guest]

I could here the fizz on my 3 at all volumes, moreso when quiet but it was still there when it was cranked. Having the bias done sorted it out and the amp sounded sweet after that.
I never had any problems with it not being loud enough either.

[JapanAxe]

[quote name='goingdownslow' timestamp='1397984575' post='2429125']
Yes.

I set it myself giving the pot slightest turn and checking the mV each time with a mutimeter. At one stage I had 40mV, so shows how sensitive the pot is.
My 3Pro had a sizzle from new, could only hear it at low volume, I thought it was part of the sound till I read the posts on [url="http://www.talkbass.com/threads/svt3pro-bias-setting.437406/"]HERE[/url].
I hesitated for a while before doing it as I wasn't sure if I was going down the right track as it being a new (USA) amp I didn't think it would be the bias, then I thought no harm in checking the voltages. I was surprised but chuffed at what readings I found, knowing then that this was the cause. Armed with a small screwdriver and being wary of what I had read about the pot being sensitive I took my time. Did it with amp cold with no signal and no load on. I stopped when I got the average reading as posted above as I thought it being close enough, set the amp on my cab plugged in bass and the amp was clean as a whistle, and with what I thought was more punch on gigs.
Reading up the SVT Bias threads on TB gave me the confidence to have a go.
It is also possible that a batch had been set wrong at the factory as it hard to think that it drifted that far out.

Good luck with whatever you do.

Edited to say, whatever others have posted, I turned the pot clockwise to increase the voltage.
[/quote]

Huge thanks again Pete, particularly on the bias pot direction. I have read some 3 Pro bias threads on Talkbass, but not the one you linked. As you say, it costs nothing for me to check those voltages. I have an electronics-savvy mate with a signal generator and an oscilloscope, so I may well draft him in if it seems like bias adjustments are needed. If that bit ain't broke then I won't try to fix it, but there is also the option of fitting a multi-turn bias pot that is easier to set accurately, and less prone to drift through vibration etc.

[quote name='kennyrodg' timestamp='1397985951' post='2429145']
I could here the fizz on my 3 at all volumes, moreso when quiet but it was still there when it was cranked. Having the bias done sorted it out and the amp sounded sweet after that.
I never had any problems with it not being loud enough either.

[/quote]

OK that puts bias well and truly ahead in the running as a possible issue. Enough waffle from me, off to the amp and multi-meter!

[JapanAxe]

Well I did it and surprise surprise, the amp was biased very cold. Initial readings in mV across the source resistors, looking at them from the front of amp:

5.1 - 3.9
5.1 - 2.1
4.8 - 8.2
[b]5.6[/b] - 7.1

Average = 5.24

I clipped my DMM leads to the resistor whose reading is shown in bold (just above the average reading) and gently turned the trim-pot until it read close to 25mV, then took the readings:

23.6 - 20.9
25.2 - 14.7
22.5 - 33.7
23.6 - 30.6

Average = 24.4

So far so good, no readings over 37.5mV (i.e. 25mV + 50%)

I connected a speaker, plugged in a bass, and lo and behold, no muddy distortion. For good measure I turned off, disconnected the speaker and bass, turned the amp on, and took the readings again:

24.3 - 21.4
24.8 - 14.7
22.9 - 35.8
24.9 - 31.2

Average = 25.0

I'm not sure why they've all gone up a bit, but that trim pot barely needed touching!

I also tightened down the transformer (11mm Halfords socket did the trick), but looking at the construction of the amp I think the fan can wait for another day! I'm still going to order a balanced 12AU7 from Hot Rox, then all should be well

Here's some pics:

General view of the interior with heatsink channel roughly centre rear, MOSFETs either side, source resistors along the bottom -



The notorious blue trimpot -



Preamp valves -



Driver valves - 12AU7 is nearest the camera -



Mid frequency selector (green body) next to tapped inductor

[goingdownslow]

Nice one.
Hope you enjoy the amp as much as I do mine.

[JapanAxe]

[quote name='goingdownslow' timestamp='1398116142' post='2430703']
Nice one.
Hope you enjoy the amp as much as I do mine.
[/quote]

Yes, looking forward to gigging it now

[JapanAxe]

Just checked out the effect of the Tube Gain control on the plate voltages in the driver section. Measured with respect to the chassis earth, the voltages at pin 6 of the 12AX7 (marked J11 on the schematic and rear circuit board) are:

63V with Tube Gain at 0
133V with Tube Gain at 5 (12 o'clock)
173V with Tube Gain at 10

I have just installed a new JJ 12AU7 in the driver section, and with the Master set high you can hear a hard distortion with Tube Gain at 0. From about 2-3 onwards it just sounds pushed, with maximum headroom at 10 as expected.

EDIT: The bias has drifted again. Temperature seems to affect it. Off to Maplin for one of [url="http://www.maplin.co.uk/p/10k-andohm-cermet-square-22-turn-preset-potentiometer-uh25c"]these[/url] I think:

Edited April 26, 2014 by JapanAxe

[Jonesy64]

Mine arrived yesterday, very pleased so far. I have bookmarked this thread for future reference, loads of great information here.

Good luck with yours JapanAxe :-)

[JapanAxe]

Finally got round to replacing the trim-pot today.

First removed the driver valves from the rear circuit board. Then undid the rear panel connectors and ribbon cable from the DI board and popped it out:



Ddi the same with the main rear circuit board:



Removed the five fixing screws from the board. Had to move the transformer to make enough room for all the sockets to clear the back panel. Twisted the board up, removed the old trim pot, and cleaned up the holes:



The new trimpot was a bit small so I put the outer pins in vertically and left the middle pin sticking out:



I salvaged a leg from the old trim pot, soldered it in place, and joined it up to central pin on the new trim pot:



After everything was back together except the top cover, I set the bias again. I had noted the resistances between the pins on the old trim pot, and set the new one up the same, so it didn't take too long to set it right. The voltages still drift a bit, but no more than a millivolt or so, and it's much easier now to tweak the bias without it suddenly going into panic territory.

While I was in there, I realised the cooling fan would be much easier to replace than I had thought, so I noted the details for future reference. Looks like you can only get it from America anyway, but I've found a close equivalent on eBay - a low noise job for linear amps.



A couple of hours well spent

Edited May 9, 2014 by JapanAxe

[goingdownslow]

I have not been back inside mine since I first adjusted it. While it is sounding ok I'll leave well enough alone, but if I had reason to adjust it again I will also go down the route you have gone and change the trimpot.