Ampeg svt 4 pro

[clintronics]

Anyone know the correct plate voltages on the 3 tubes?

[nevetslab]

just looking thru the schematics, I forgot that Ampeg didn't bother giving those details. Though in the Power Amp schematic, they did give the B+ Supply voltage at Idle of 288VDC. But, as you 've no doubt seen, there aren't any details, on the three tubes, such as cathode voltages, from which current thru each would allow the voltage drop across the plate resistors calculated and arrive at the Plate Voltage.

My SVT4-Pro is over at the CenterStaging Guitar Dept at present. I'd have to open it up to get that info, though I may have that in my SVT4-Pro Service Notes....would have to dig thru them.

Lemme see if there's some details in the other SVTx-Pro schematics for that. In the SVT3-Pro, they show the B+ supply for that to be 300VDC. They give plate voltages at numerous Test Points on that model. If you have those schematics, that might give you approximate values to go by, though not all the circuits are the same as in the SVT4-Pro. I can post those schematics in the morning.

I just scanned thru my service notes spanning 2010 thru 2019, but seemed to be mostly concerned with the MosFET Output stages and screening/batching IRFP240's and IRFP9240s. I didn't find any DC Voltages recorded of the three Preamp tubes in those notes.

[Enzo]

Per the schematic B+ is +288vDC.

But in general, the voltages on the tubes are more likely to be REAL high or missing, rather than "wrong".

In a typical triode stage, if the tube is not conducting, the plate will measure the same as the B+ supply and the cathode will read zero. If the cathode sits real high and plate real low, then th tube is conducting too hard.

SO stop and think what voltages you expect. Some small voltage difference is not likely the cause of some problem.

[nevetslab]

I stopped off to pick up my Ampeg SVT4-Pro, and opened it up to measure the DC Voltages on the three tubes:

V1
Pin 1 173VDC
Pin 2 0VDC
Pin 3 1.42VDC
Pin 6 137VDC
Pin 7 0VDC
Pin 8 1.27VDC

V2
Pin 1 106VDC
Pin 2 0VDC
Pin 3 1.1VDC
Pin 6 270VDC
Pin 7 106.0VDC
Pin 8 108.8VDC

V3
Pin 1 179VDC
Pin 2 4.0VDC
Pin 3 5.5VDC
Pin 6 106VDC
Pin 7 15.3VDC
Pin 8 16.8VDC

B+ on this amp is 270VDC, as is seen at the plate of V2 Pin 6

I don't have Adobe Illustrator on this computer, and wasn't able to find a way to enter the text using Adobe Acrobat 5.0. I'll see if I can modify the Preamp Schematic at home and add these readings on the three tube stages and post that later.

For reference, here's the Preamp Schematic for the SVT3-Pro, which does list plate voltages (for reference)

Preamp PCB Schematics (534XXHB).pdf

[clintronics]

I stopped off to pick up my Ampeg SVT4-Pro, and opened it up to measure the DC Voltages on the three tubes:

V1
Pin 1 173VDC
Pin 2 0VDC
Pin 3 1.42VDC
Pin 6 137VDC
Pin 7 0VDC
Pin 8 1.27VDC

V2
Pin 1 106VDC
Pin 2 0VDC
Pin 3 1.1VDC
Pin 6 270VDC
Pin 7 106.0VDC
Pin 8 108.8VDC

V3
Pin 1 179VDC
Pin 2 4.0VDC
Pin 3 5.5VDC
Pin 6 106VDC
Pin 7 15.3VDC
Pin 8 16.8VDC

B+ on this amp is 270VDC, as is seen at the plate of V2 Pin 6

I don't have Adobe Illustrator on this computer, and wasn't able to find a way to enter the text using Adobe Acrobat 5.0. I'll see if I can modify the Preamp Schematic at home and add these readings on the three tube stages and post that later.

For reference, here's the Preamp Schematic for the SVT3-Pro, which does list plate voltages (for reference)

[ATTACH]n966818[/ATTACH]

Thank You! exactly what I wanted. was just wondering if my v2a plate voltage of 113vdc was lower than normal. Evidently thats normal. but it does seem that some of my cathodes are lower than yours by about 20-30%

[Enzo]

Always remember, these are just guitar amps, not precision lab equipment. Voltages can be "way off" and the amp still work fine.

[nevetslab]

You mentioned in your PM that swapping out the Preamp Tubes didn't restore the character that appeared thru the amp when it was newer or in the recent past. One of the negative aspects of the SVT4-Pro, if you want to call it that, is all of the wiring and circuits between the input jack and the Power Amp INPUT jacks. Take a look at the Block diagram in the Owner's manual, as well as all of the signal jacks used in the Signal Inserts/Returns, the large degree of IC op amps in the signal path to provide Graphic EQ, Crossover functions (which have to be patched at the rear panel), signal routing before you get to the power amp stages.

And, on the Preamp PCB, this amp IS known to develop solder joint fractures on all of the Front Panel pots, the pushbutton switches can become intermittent so they need exercising on a regular basis as it ages, as do the pots. In doing full service to these amps, I remove the front panel PCB to address all those. Often removing the pots, removing the covers to apply contact cleaner (with brush) and exercise them that way, then put the pot back together and solder back into the PCB.. Exercising the multipin connector cables at the preamp board and the other end of them helps restore the conductivity of the power, ground and signal paths. Exercising the phone jacks on the upper PCB assy helps. Getting at the Normal switching contacts of those phone jacks is a PITA.

Often exercising those Send/Return jacks, as well as the Preamp OUT/Power Amp input jacks help restore the 'character' the amp had.

I've attached a lot of the service documents to give you a real idea of what you're up against in restoring order to what seemed to have changed over time in the tonal character of the amp.

The Graphic EQ, in my opinion, is on the hard side of tonal character. I haven't tried changing the caps used in the Graphic filter stages of that circuit to see if that helps.



SVT-4PRO Owner's Manual.pdf

Bass Preamp PCB Schematics (53905SCH_0 - for d and e versions only).pdf

1600 Watt Power Amp PCB Schematics (428XXSCH_1).pdf

Bass Amplifier Assembly Drawings (00-252-XX, Rev 8).pdf

Hardware gets loose in these amps. When I pull one apart, first place I start is removing the Preamp PCB assembly, and replace the front panel handle hardware (#10-24 PHMS) with Socket Cap screws and split-lock washers, cinch them down tight so that ends the panel handles from loosening up. Power Xfmr mtg hardware loosens, the PCB mtg screws that hold the Power amp assembly to the chassis loosens, sometimes come out. Preamp PCB mtg screws sometimes loosen. Fan mounting hardware, as well as the Bridge Rectifier mtg hardware loosen. The 25A Bridge solder terminals are known to develop solder fractures and stop the amp from working.

The Power Amp PCB is not what I would call a great quality PCB, and layout, trace widths, solder pad sizes chosen lead to trace failure, pads pulling up during service, etc. So there's a lot of hidden 'gotchas' on that board. Access to the MosFET's on the Power Amp PCB side below the power supply PCB assembly are a nightmare to get at for active bias adjustments.

But, when all has been sorted out and restored to how it should be, they're a great sounding amp. I was lucky enough to have been given one, and just paid shipping from PA to Burbank, CA, which was around $100. Had a number of parts to replace, and all well worth it.



In this setup, I have the Symetrix Parametric EQ patched in, with it's output fed to the Symetrix 501 Compressor/Llimiter. It is a lot more complex than just plugging into the Hiwatt DR201 and dialing up a solid tone and letting it rip, now having built one of those (less the real power and output xfmrs from the Hiwatt).