Ampeg V4b Rework

we worked on a V4 a while back, now we have a V4b in the house,

the "b" probably means good for bass, which it is due to the nice chunk of hernia  iron,

we are going with loose caps instead of the 3 ea. forty dollar cans, we can get some Nichys in there for about twenty bucks,

lets knock out the plate and screen filters on a piece of moth proof cedar, tie warp and hot glue gun, no terminals required,

increase voltage divider Resistance for less heat>

what about the inverter and preamp tubes filters ?

well we used some doug fir to keep out the termites,

plug in the new JJ 7027's and stand back because of that 545 B+, 

amp works great! dead quiet after you get past V1 and V2 12AX7a preamp stages,

you can check this by inserting a signal into the ext amp jacks and listening for hum,

the hum pot was good which is a rarity on these 70's amps, tubes short and take them down like Mohamad Ali

problem is when you turn up the funky midrange or hit the ultra sw it sounds like a buzz saw coming thru the attic,

we cut traces on the control board, splice in three gnd wires at various points in the circuit and tried every permutation possible at different chassis locations all to no anvil. 

we already have reworked the pwr supply gnd wires, soldered them to chassis and got rid of the silly diode to ground and the standby switch, so everything is dead quiet except the first two preamp stages for both 1 and 2 inputs, this was checked by shorting ac to chassis via a .47 cap on pin 1 and 6 for both tubes, so we have isolated the problem to the area between the preamp stages, the input circuit from the phone jacks is fine as they use coax,

so we are going to rip out the control board and go point to point, look at the inductor waiting to pick up every radio station east of the mississippi, well maybe not, it is kind of low z so maybe chassis hum might be a possibility,
we can put it in a can, can we not?

and while we are in there what about that PEC6470000 tone module?  lets go discrete and wait for big tone,

here is the preamp section, why not ditch the 8 zillion parts in the ultra bass circuit (circled in red) and put a single cap in there (what value) ?

most of the bass boost comes from switching in the cathode cap anyway, (blue)

this will make hard wiring the sw easier,

notice erroneous wire shorting signal out (red cross)

and the missing gnd from tone cap circuit (green)

some dude named Hangman was selling PEC replacement boards a while back at the V4 forum, do not know current status of that situation,  cap values can be tweaked in various locations to match up closer to SVT circuit, and hi freq caps can be adjusted also to make ultra hi circuit more usable.

note that regular V4 has the sensitivity sw instead of the bass boost,

rippage complete 

inductor specs, don 't know the turns, probably about 1200,  looks like about #34, probably about 2" per turn,= 2400"=200 ft,

1000 ft / 200 ft = 5,    so 5 x 52 ohms DCR = 260 ohms/ 100 ft = #34 Heavy Build (used for torroid winders to prevent scraping thru insulation by traveler hook)

Mercury Mag sells these for about 100 bucks, better of buying junk amp for parts or designing new mid control if cloning this amp,

here is the PEC6470000 circuit values for a build out>

LOVE V4Bs! My back absolutely HATES them. Baby SVT alright! How much does a baby elephant weigh?  :

making progress on this beast, tone has been improved by replacing the PEC module with regular caps and resistors, and by upgrading coupling caps,

we cased the inductor in a steel can which dropped hum/fizz a bit but it is still a problem

these amps use linear volume pots which is just crazy for an amp this loud, so we replaced those with some CTS  audio taper pots but had to file a flat spot on the round shaft in order to use the stock knobs,

we notice the scope patterns for the screen supply match the induced fizz hitting the preamp tubes, we also noticed that the screen resistors are mounted on the preamp board and are very close to the midrange pot, we also notice a vacant pin on the octal sockets, so we will move the screen resistors to the octal sockets which will take away the hi voltage from the preamp board, and we may as well ditch the silly 3.6 ohm plate resistors which are probably ready to cause problems anyway, this will eliminate another 3 miles of wire, get two 540 volt circuits off the preamp board (plate and screen supply wires and resistors)  and hopefully get the noise level down on this fine sounding amp,

other mods might include a partial self bias resistor to drop some plate voltage and move the power down to large club level,

and there are provisions on the preamp pc board for an rf filter on the input circuit. the holes are there but the components were not stuffed for reasons unknown, these amps have a rep for picking up wolfman jack from across the border,

coax has been replaced from the input jacks, speaker jacks have been grounded at the chassis instead of the fiber washer and gnd lead system, star grounds have been used for the preamp circuits,

in the future, the best way to tame this amp would be to rip out the pc boards, mount tube sockets to the metal plates that bolt to the chassis, and use a Fender ground scheme, this sounds like a lot of work but considering the time spent trying to polish a you know what, the non pc board approach would be a serious upgrade. and would also make future servicing a cinch.

surprised somebody has not done a turret board for this amp,

well we ripped out the pc boards so we can do a Fender/Marshall type build with some new ceramic sockets and shields, got the new chunk of steel over the cavity,  got a ceramic 12 pin compactron socket on the way,

we can now string the tubes in order instead of jumping back and forth between stages to the 12 pin board, the V4b does not have reverb so this will be an easy build, we might even develop a turret board, who makes a cheap crimper for G10 boards?

we already have the tone circuits in place and have decided to put back in the filter for the hi-lo switch,

strange thing is they knock the bass down between stages 1 and 2, then build it back up by connecting the cathode cap when you are in bass boost mode to boost bass from stage 2 to 3.

maybe they do not want to overload the second stage,

did some filter simulation, changing the 1.2 meg to 1.5 meg made no difference, so Ampeg was probably using up excess surplus inventory,

simulation also shows that we do not need the  second RC pi section, this saves 4 parts on our new build,

here is the plot of the bass curve being rolled of prior to stage 2, where it will be boosted again,  showing no difference between leaving out the second RC network>

got rid of board #1 and stuck a plate in there,

sockets came in, we install the tubes prior to soldering because

#1 -  it heat sinks the pins so the solder don't run,

#2  the solder can not clog the tube pin holes if it does run,

now we can get rid of board #2 and put the filter caps there,

this will save some wire and move the grounds to a better place,

we need a steel plate, the doctors are removing it right now, 

a while back we changed from pc board tone to point to point tone, saves more wire,

installed the inductor in a steel cage,

my VT-40 is one of my favourite amps, i really like the eq. some previous owner had added a master volume on it, but it was useless to drill the original chassis, because there is a jack on the back where an external master volume can be connected, ampeg sold an unit precisely for this purpose.
it's not the quietest amp. this project inspired me to take a look at it and will be trying some of the improvements. maybe not as audacious as ripping completely apart the thing as you did, but at least trying to improve this beast. ampeg really made top notch amps, it's a shame they are not as easy to find in europe

we finally finished this job, now it is as quiet as a church mouse,

start with the heater wires, otherwise it is a pain to add them to a P to p project,

using fender pickup wire for the low amperage tubes,  using b and w to keep polarity sorting easy,

we are wiring inputs 1 and 2 to the first tube instead of splitting them into V1 and V2 like Ampeg did originally, this makes wiring a bit more logical and  easier on the brain,

got some Eurofoil caps from W. Germany, whenever that was,

got a start  on the Compactron socket which gets pretty busy,

working our way on down the line, soldered grounds, and we now have a good ground plane compared to the previous PC board

this is when you pray that you left enuff room between sockets,

getting there,

got some coupling caps installed and some midrange caps, original circuit says use .33 uf but .22 uf should be fine, 

also upped the wattage on the pwr supply screen resistor to a wirewound ceramic after the 5 watt model started smoking during a tune up, (ultra-sonic osc)

turns out we have increased the frequency response of the amp by ridding ourselves of excess wire, so we have a Ham radio on our hands, but all was needed was was a 100 pf on the grid of the Compactron (input triode pin 11 circled in blue)

we also upped the second B+ resistor (bottom center) to keep things reliable over the long haul,