Ampeg V4b Rebuild

[cjenrick]

got tired of the 7 different types of noise this amp makes depending on tone settings and rocker switch positions, no time to try a bunch of different ground schemes, bummed out dealing with brittle pc boards with lifted trace pads and questionable water stains,

so we ripped out the boards and started from scratch with a Ham radio style build.

now we can put the preamp tubes in order instead of jumping from board to board (6K11 is on a different board)

ceramic sockets ready to wire>

[cjenrick]

now we have a ground plane to work with, and a ceramic socket for the Compactron is even available off evilbay, and we have shields which will help retain the tubes, the 6K11 seems to be firmly anchored without a shield via the 12 pins in the new socket,

[cjenrick]

wiring complete, quiet as a mouse, even with the mid turned up in middle rocker position, even with the lo boost on,

mods:

replace expensive multicans with loose Nichicon's
470 > 1K 5 W screen resistors
20 watt screen dropper in B+ supply
run preamp filters from screen supply instead of OPT B+ for more filtering
put inductor coil in a can and pot with polycrystal wax
dump PEC tone chip for discrete components
change noisy heater tube (12DW7) with 12AU7a and change plate resistor from 220K to 82K 1W
this lowers power a bit but takes away 60 Hz hum
ceramic pwr tube sockets
remove gremlin prone standby circuit and ground diode
remove tired out flyback diodes
remove .047 mains cap as we have a three pronged cord
remove coax feeding the never used preamp output jacks on the back for less hum pickup
used Eurocaps from W Germany

[cjenrick]

we also used soldered grounds,

this was a bit of work but the results are worth it, should provide years of service, at least until the OPT insl breaks down,
those PC boards were really tired and would have been a constant source of strife and grief.

used some NOS 2 watt CC's for the PI plates,

you could easily pop in some 6550's in place of the JJ 7027's which might handle the 550 volts of B+ a little longer,

[g1]

Not meaning to criticize, but if you went as far as making a discrete PEC, I'm surprised you didn't make a discrete 6K11 with something in current production, like a couple 12AX7's ?
Do you not have trouble getting good quality 6K11's at a reasonable price?

[Justin Thomas]

Maybe the one in it was still good? Stupid Ampeg... I know that it makes sense that Magnavox would use Compactrons, since they were TV manufacturers. But Ampeg started using the stupid things long before Magnavox owned them. And we've been cursing them ever since for the 6K11 & 6BK11. The 6U10 is still affordable, at least... I actually have a 6K11 & 6AV11 that came in a Jet. I'll use them in <SOME>thing eventually.

Justin

[Leo_Gnardo]

you could easily pop in some 6550's in place of the JJ 7027's

I tried that some 25 years ago when there were no currently made 7027's, and the price on NOS were jumping high. When I found the glass on 6550's was jam up to the transformer, gave up on them. I'm sure they would work electrically. But couldn't stand the prediction they would rattle themselves into shards, banging against the nearby transformer while being transported.

[Enzo]

Maybe the one in it was still good? Stupid Ampeg... I know that it makes sense that Magnavox would use Compactrons, since they were TV manufacturers. But Ampeg started using the stupid things long before Magnavox owned them. And we've been cursing them ever since for the 6K11 & 6BK11. The 6U10 is still affordable, at least... I actually have a 6K11 & 6AV11 that came in a Jet. I'll use them in <SOME>thing eventually.

We curse them because we cannot buy them any more. But in those days they were just another tube, and why not use them. Fewer tube sockets, fewer tubes. Sure Magnavox didn't yet own them, but hey Fender wasn't owned by RCA, but used a ton of their tubes. They had no way of knowing that 20-30 years later the tubes would be rare. Any more than the solid state amp makers in the 1970s knew germanium transistors would become hen's teeth.

[Justin Thomas]

I wasn't there to see it, but the way I read the few official docs I could find, Compactrons were invented first to enable manufacture of an AA5 radio with only one tube (they came close) and as a last-ditch effort to pack as many functions as possible into as small a space as possible, at the same time transistors were getting better and better... almost like the last-gasp effort to prove that tubes were superior, or maybe to say, "hey, we're tubes - remember us? We are still valid! Please don't make us go away, please?"

I can see where the venerable 6L6 & 12AX7 may not have been going anywhere, and there were a bazillion devices already using them, but it almost seems to me that Compactrons were introduced at a time when the design of NEW circuits for tubes were already going on the way out - okay, maybe not in 1966, but when a 1980 Super Champ or a 1978 Ampeg needs one, I mean, tubes were already dead in the water, as I see it. By 1975 it would have seemed silly to me as a manufacturer (Magnavox) to be designing with things that haven't been around long and are not exactly in plentiful supply compared to the Old Standbys. I mean, were they still building new tube TVs in the mid-70s? Maybe they were extra-dirt-cheap at the time because they were already obsolete when they initially hit the market? Maybe the engineers fell for the sales hype - some of the press releases did feel like that a little bit.

I love Ampegs, and I really like 12-pin tubes - I think some of them are the coolest looking little widgets around. But man it sucks trying to retube a J-12D or a V4B w. 6BK11s & 6K11s!

Justin

[Leo_Gnardo]

By 1975 it would have seemed silly to me as a manufacturer (Magnavox) to be designing with things that haven't been around long and are not exactly in plentiful supply compared to the Old Standbys. I mean, were they still building new tube TVs in the mid-70s? Maybe they were extra-dirt-cheap at the time because they were already obsolete when they initially hit the market?

Some other things to think about re Compactrons and similar items. Late 50's early 60's production engineers were trying to streamline the production line. Want to increase line efficiency by reducing the number of tubes that needed to be installed, bingo use compactrons! (Then they were wiped out by transistors, no contest.) Also compactrons were intended mostly for TV's, the #1 civilian electronic consumer item, meant to last 5 years or so then be replaced. Same time line for guitar amps. We're sort of lucky amps were mostly built better than TV's of the time, many still around, sounding great as long as they're maintained. Imagine if cars of the 60's were all so well built. Detroit had that planned obsolescence thing down, didn't they?

[Enzo]

Don't forget the Nuvistors.

[cjenrick]

thanks for all the replies! yes the 6K11 was in good shape,

here is a link to Vacuum Tube Valley

page 22 has a Compactron article by John Atwood>

https://web.archive.org/web/20131122.../VTV/VTV20.pdf

[Chuck H]

I'm wicked impressed I'm also a little surprised that there were no unexpected noise or tonal issues with the new build since the layout was altered so dramatically. IIRC there is a switched inductor as part of the tone shaping circuit for that preamp. A coil of wire in a preamp can cause all manor of trouble at typical guitar amp gain levels. Since these amps were sold by the thousands the PCB was obviously suitable. Changing the component and lead proximity so dramatically, well, I might expect a different result. I have to wonder... Have you tested the amp at any plausible gain levels for the possibility of noise or instability? In other words, can you crank the amp to full output, or even power tube clipping without stability or noise problems? What was the nature of your final performance testing?

[cjenrick]

the reason for the rebuild was that the PC boards had what looked like water stains on them, and they smelled funky when we soldered on them, gets pretty moist up here in Olympia WA, especially this year, fearing conductive PC boards we decided to get rid of them, they were warped pretty bad and traces were starting to lift off. especially in the hi heat areas like the screen resistors, this probably would have caused additional problems down the road,

chassis layout is a crap shoot, you can do certain things with grid leads and plate circuits, lead dress, but if you have zero problems then you got lucky. Word has it that there was quite a pile of failed chassis layouts out back at the Fender factory, so they went through their share of trial and error on certain models before they were happy,

we tried to use similar ground schemes as your typical Twin Reverb layout, there were still problems, since we soldered directly to the tube sockets, chassis wire footage was greatly reduced, this resulted in increased bandwidth, which led to some hi freq oscillation when the mid was dimed, this was traced to Pin 11 of the Compactron which is a grid, so a small cap was added from grid to ground, the smallest value being used which stopped the problem so that fidelity was not altered to any audible difference, about 100 pf,

there was still some heater hum which was traced to the 6K11, this is the tube with the dissimilar triodes (12AX7/12AU7) we do not know if the hum is due to the fact that the triodes have different amplification factors, or if it is because the tube was not designed for low noise, so we used a 12AU7 in it's place which got rid of the problem in a big way. overall gain was reduced about %20 but the amp is plenty loud already, some folks even pull two tubes to tame the beast a bit, the loudness factor is probably a result of the generous lam stacks of the pwr and output transformers,

part of this perceived loudness is also due to the unconventional use of linear volume pots, so we replaced those with audio taper pots so you do not have to play the amp on 2 anymore, Ampeg was probably trying to out blast Fender on the showroom floor as price was probably not a consideration here. the shafts of the CTS pots had to be filed into D shafts to fit those expensive $36 a piece knobs,

we put the torroid mid coil in an old input transformer can and sealed it with wax just in case it was picking up hum, but it sits in a low Z part of the circuit so it is unclear if there was any need to do this,

moving the rectifiers and screen resistors off the preamp board seemed to help get rid of a bit of noise also. the 7027 tubes do not allow soldering of resistors to the sockets as they have duplicate pins for screen and grid.

now that we have a reasonably stable layout, next time we can transfer the build to a turret board setup to make it easier to solder and easier to service.