Late generation Ampeg SVT-CL Schematics 2009-2013

[nevetslab]

I finally got around to replacing the shorted Power Xfmr on the Ampeg SVT-CL I took in early 2020, before the Pandemic shut us all down. I never got the amp powered up, as that's when I found the major fault with it. Then in late 2020, I obtained a replacement Power Xfmr for it, and yesterday I finally took the time to swap them out and see what I had to work with.

Turns out this model is the later generation unit when Ampeg revised their PCB's to Double-Sided, changed the from the 14 pin Ribbon Cables between the Preamp and Power amp to a single-in-line PCB connector with discrete crimped socket leads. The the 16-pin Ribbon cable between the Power Amp PCB and the Tube PCB changed as well.

When I powered this unit up, after getting all the Primary Leads connected along with the secondary leads into the front edge of the Power Amp PCB, I found the Bias 1 Control wasn't working correctly, Getting it corrected took multiple disassemblies before finally getting it right, though I never did find just WHAT was causing it....but it took removing the Bias pot, pulling it apart, clean/lubricating it then remounting it.

I found in this later generation, Ampeg installed a 22k 2W Metal Oxide resistor between the feed to the Plates of the Driver Tube Cathode Followers, dropping that 380VDC level down to around 288VDC. R4 is the PCB P/N, which doesn't exist in the earlier units. Those have 380VDC on the plates of that stage.

I've attached photos of this later generation PCB's, which have the PCB numbers/rev letters.





I did end up with a set of Core Bolts, Mounting Brackets and End Bells from the shorted Power Xfmr. I've yet to snap a core bolt, and hope I never do, but this later generation amp I often find their Power and Output Xfmrs seem to have loose Core bolts, while the mounting hardware threaded into the mounting brackets seem to stay tight.

Anyway, the Main Power Amp PCB Board Number: 2033854 Rev E-00
Preamp PCB 2009-2012 2033753-10 Rev C-03
Tube PCB 2033759-10 Rev D-00

I will have to pull the AC Mains and Output PCB's to get their board numbers and rev numbers.

All these boards are now Double-Sided, Lead-Free Solder, and loaded with RTV EVERYWHERE to make Servicing a pure joy.

Does anyone have Service Documents that mate up with this generation?

[nevetslab]

I've got the PCB Board Numbers on these last two boards:

AC Mains/Relay PCB 2010-2011 Rev K
Output PCB 2033757-10 Rev C-00



These last two boards are basically the same as before, only now being double--sided, and built using Lead-Free Solder.

[Jon Snell]

I have searched through my collection but nothing from Loud Tech, all older SLMs.
Sorry.

[nevetslab]

Thanks for checking. Meanwhile, having restored the bias adjust circuit for the Bias 1 Adj Pot, I had a look at the plate current values of the Sovetk Wing C 6550 Power tubes that came out of the amp, and also found a single Groove Tube 6550 in the set. After looking at the first pair, I set the Bias Voltage the same (-48.7VDC) so I could see what the current reading was thru the 10 ohm cathode resistors. It ranged from 14mA thru 27mA on the Sovtek tubes, with the GT tube at 25mA. I moved the tubes around to get the higher value tubes together and the lower value ones together, then did my best to balance the two halves. Not ideal, but got it landed and it remained stable over time. Avg current 21.4mA and 21.8mA on the upper/lower halves.

Everything works, so now I gotta track down the owner and see what he wants to do. I have a fair amount of labor into it, plus the cost of a new power xfmr. I suspect the repair bill without a new matched sextet of power tubes may scare him away. If nothing else, it works well enough to serve for matching power tubes for other SVT amps.