Ampeg V4b Rework

[CJ]

changed the phase inverter to pwr tube coupling caps to some 1000 volt eurofoil vintage (red)

this amp was quiet when we tested it save for some heater hum form the 12DW7, the owner supplied a JJ ECC832 which is a modern version of the original dis-similar triode model, we tried a NOS tube to see if JJ was the problem but got the same results,

do not know if there is less hum cancel due to the different amplification factors of the two triodes,

but when we swapped the 12DW7 for a 12AU7a (dual triode with same gain for each) the hum went away, 

we upped the two 47K PI resistors to 2 watts instead of 1 watt, as the 1 watt jobs get pretty warm, used NOS carbon comp for any resisdual funk from voltage/linear distortion, which should troll  in some comments from the audiophoolery gang  ;D

but now we have less gain and a wrong value plate resistor for the 12AU7, so we swapped the 220K for an 82K so we would be on a better part of the loadline,  lost about 25% power at the same volume setting but so what, people yank two pwr tubes because this amp is so loud, and we could still get to clipping even with a 25 mv input signal with the volume maxed out,  the 82k is tucked under the blue cap across the tube socket,

plus we have a driver that is easier to source and much more quiet, 

 

[CJ]

so there ya go, about 3 weeks work the first time around, which should go down to 3 days now that we know where the parts go and all the mods,

we have no extra hum when switching in the low boost, and no awful noise when dialing up the midrange in various switch locations, switching noise from the pwr diodes is gone because we moved those, and the ground scheme seems to work fine.

we made a slight change to the pwr supply filters, dasiy chaining them like Fender instead of splitting off the main B+, this makes use of the screen filter section caps, which lowers ripple on the preamp and inverter tubes,

and we decided why rebuild the preamp and not change the pwr tube sockets so we have added some nice ceramic jobs in there for the 550 volt B+ on the jj 7027's,

 

[CJ]

lets put down some AC/DC

preamp (both sections about the same)

 

[CJ]

ac is red, dc blue

not much gain on the first stage of the "circle of three" midrange tube, must be a lot of NFB,

 

[CJ]

and the output section

forgot about another mod, changed 470 screen R's to 1K 5 watt,
kept the hi value 47K stoppers, in case that is part of the V4 sound,

also clipped out the silly flyback diodes (D7,8,9,10) which can cause trouble down the line, that OPT can handle anything you throw at it,  and the 3.6 plate R's are gone.

no room on tube sockets for resistor mounting as the 7027 has pin sharing, screen is 1 or 4, grid is 5 and 6, so watch out for that,