Ampeg V4 Rebuild

The Ampeg V4 guitar head is a 100 watt amplifier using 7027 power output tubes. Since these tubes are not available anymore with the exception of some of the Russian manufacturers, a good 6L6GC tube can handle the 540 volts on the plates. This specific amp I restored already had the Svetlana 6L6GC and were in there for 4 years. One of the tubes was weak and one new matched pair was ordered that had the same bias rating as the other tubes. These amps weigh a ton, so make sure the carrying strap is in good shape.

The first thing is to remove the amp from the case and then remove the side mounting rails which connect to the chassis using rubber stand off's, so the chassis is essentially floating on rubber feet. To work on this amp ,requires you work on it vertically since the whole circuit is built on a couple of circuit boards that need soldering from the bottom and removal of the components from the other side. I simply wedged in a few pieces of small wood under the power transformer and it easily stands up on it's own leaving you 2 hands to work on it.

The first thing is to replace all the old Power Supply Capacitors. The manufacturer used a couple of can caps for the first section of 100uf/450 in series with 100k resistors in parallel to achieve 50uf at 900v. One section of the original can cap doesn't get used since the the ground on the first can doesn't go to ground but connects to the second section only.

What I used here was a clamp mounted single 100uf at 500v and then a second can dual cap at 100uf at 500v. The single can connects to the second can and then there is an available second section to be used for the next part of the power supply rail. The second section of the power supply then uses the second half of the dual section can plus another in series to increase the voltage above the 500v rating of the can cap. I measured and this section section was at approx. 535v. I installed a 20uf at 500v in series with the 100uf to give a voltage rating of 1000v and approx 60uf which is higher than the stock 20uf total, The next can cap at the other end of the chassis is a 3 section can cap rated at 50uf at 500v. I used a dual section 50uf at 500v and then installed a 47uf at 450v for the last section which has about 325v. The 47uf cap can be installed right on the terminal strip.

The next step was I replaced all the power supply rail resistors which are the 470 ohm 7w with a 10 watt, the 3k with a 10 watt .All the plate and screen resistors along with the bias network resistors were also replaced. All the rest of the bias circuitry caps were replaced as per the original values. The .33uf coupling caps feeding from the 12AU7 tube were replaced with solens fast caps.

This makes for a good dependable power supply and bias.

The amp after being fired up had some static and was caused by loose preamp tube sockets. So I went through and dexoit all the pins cleaned them with a tube pin socket brush and tightened each socket. This helped eliminate all the static and noise.

The tubes in this amp were running about 30ma per tube. This should play good for many more years now.