VTL Stereo 50

I recently picked this up at my local audio store as it came in on a trade in.

I haven't  really heard too much about VTL, till I started looking on the internet and found about about David Manley's accomplishments.

This was built around the mid 80's and is a Stereo Tube Power Amp, rated at 50 watts/ch, using 6L6/5881 style power output tubes and 12AT7 predriver and phase splitter tubes.

I brought the unit home, hooked it up and checked the bias. It has a bias pin jack and bias pots for all 4 tubes...this is a real plus if you have  unmatched pairs, you can bias each one indivigually.

The bias was almost bang on the .29v, so time to have a listen. It sounded ok,,nothing to jump up and down about and the image seemed more pronounced to the right.

Well I thought it was worth giving it the once over being now over 25 years old and nice tribute to the late David Manley.

I started googling the internet but didn't come up with much info other than a few pictures of the front, nothing under the chassis. Thanks to SDS labs who did have a schematic drawn up.

So there is NO factory schematic and little info, so I thought I would share everything I have put together.

 When I opened the bottom cover, I noticed some work had been done to the amp. It didn't look good, as the odd part was replaced just to do a repair.

The below pix shows the underside. It has 2 circuit boards. One sits on the 4 filter caps and has the main B+ power supply diodes and with the DC heater bridge(which is gone) and filter cap underneath.

The other circuit board is the main board with the Neg Bias supply diodes/filter caps and adjustment pots/pins plus the whole front end driver circuit, input and output tube sockets.

The power transformer actually has 2 sets of high voltage secondary windings for the B+. So esentially 2 seperate power supplies, one for each channel.

The Grey and Purple wires are the 2 windings. In the middle of the board something looked like it got hot and some temp wires were run.

Further investigation found that there  was actually a bridge rectifier mounted on that board and the heatsink was the bottom chassis with a screw attached. This bridge rectifier was used to provide DC heaters for all the driver tubes  plus the Power Tubes.

I have never seen an amp  have DC heaters for the power tubes and it really isn't necessary.

 The person that did the fix mounted a high value bridge on the back chassis and still used DC heaters to all the tubes.

Eventually I found a fellow in one of the news groups who had the same amp send me a pix of the underneath. I then saw that his had been either been converted or came that way from the factory with DC heaters for the driver tubes and AC heaters for the Power tubes. I guess there were many different versions of this amp and it looks like they used this same chassis and board to build quite a few amps back then.

 So I decided to convert it to AC heaters for the power tubes.

Below is a pix of the board all redone with  AC heaters and  installed new 3 amp diodes for the DC heaters.

 The Green transformer wires are the 6.3VAC tap and feed the pins 7 and 2 of the 6L6 tubes. I had to do some cutting of the circuit board traces as pin 2 of the power tubes connected to pin 4/5 of the driver tubes and pin 7 of the power tubes connected to pin 9 of the driver tubes.

So you have to seperate the traces and have only the  Pin 2 of power tubes and Pin 7 of the power tubes for each channel used for AC. 

  For the DC heaters a jumper then has to be run to connect the 2 pin 4/5 of both driver tubes and the 2  pin 9 together. + DC goes to pin 9 and -DC goes to pins 4/5.

 On top of this major upgrade the 68uf/450v and the 330uf/300v filter caps were replaced. The main 1000uf/300v tested perfectly using a Sprague cap tester.

I also replaced all the screen grid 1K resistors with 5W Kiwame and use Dale 5W 10 ohm for the cathode resistors. I replaced all the signal path resistors with Kiwame carbon film and also used them for the plate resistors. While I was upgrading the parts I noticed the one channel input tube cathode was wired wrong compared to the other, probably the cause of the imbalance of the one channel.  The Wima coupling caps were replaced  with K40 PIO caps and all other little electrolytic caps were also replaced.

 I installed new gold input rca jacks and better quality speaker binding posts.

I then reinstalled the main board back in the chassis wired the unit all back up. The output transformers actually have pins that solder onto the board.

I fired the unit on the bench and all voltages were fine. Not much to reference to but the B+ on this unit is around 560VDC wow..and tubes running 29ma. Quite the design.

This is an ultralinear design, running very high plate voltages, low current through the tubes providing 50w ch.

Hooking the unit up to my system actually suprised me. It had lots of power and the detail was amazing. You could hear the difference in the sound right away.

After a few hours of running , it's a very nice amp to listen to, has lots of detail, the bass is tight not muddy an has nice soundstage.

I compared it to my Golden Tube SE40 and the GT is still king but this amp is right up there.

 The mismatches in the parts in the input tubes between channels, plus the new caps and resistors have really taken this amp to a new dimension.

I then installed some better 12AT7 tubes -don't use any of the JJ or crap russian new version tubes...(used 12AT7 are very common, use a GE, Sylvania, RCA)the change in the 12AT7 improved the detail a bit more ..definetly a nice amp.

 A 2 part .pdf schematic is available below, one for the power supply and the other for the power amp section.