Ampeg V4

this one seems to be popular,

one of those "cult" amps with dedicated websites,

some folks even say that they have had spiritual experiences while playing through this amp,

but hey,  give Hendrix a cigar box and some rubber bands...jus sayin..wtf, over?

complaint is red plating, has some Sovtek 6L6 tubes, OEM tubes were 7027's,

RCA says you can squeeze 76 watts out of a pair in push-pull AB class,

but you need 540 volts and 100ma of idle current to get there,

a 6L6 will take a little punishment, but not for long, especially at 540 B+,

what do we do? get the guy to spring for a quartet of JJ 7027's at 22 bucks a pop?

they plug right in, 6L6 and 7027 share about the same pinout, Ampeg wired this amp to take both,

only way to drop the B+ is by changing to choke input which does not sound that great, or dump the voltage as heat thru resistors which is a primitive fix at best,

there is another solution....

cathode bias! 

through in a couple of 270 ohm 10 watters in parallel and you can make the tube think that it is seeing 40 volts less B+, or around there, you get a nice compressed sound that can make a solid state rect amp sound like it has a tube rect, which is what we need to get the 6L6 to have as much mojo as the 7027,

ask somebody why they don't use cheaper 6L6 tubes in this amp and they say if they wanted that sound they would have bought a Fender,  but have they heard cathode bias? probably not, Fender quit using that back in the 50's on that 5881 equipped Bassman,

another benefit of cathode bias is that these amps are notorious for getting the sound guy to ask that you turn the amp down, so everybody runs them with the volume set on 3,

with cathode bias, we can get the club owner to let us set the volume control to maybe 7?

we will see...

here is the schematic>  ;D

lots of small bottles on this chassis, and some with weird numbers,

like 6K11, 12DW7, 6CG7,

reverb pan is mounted on the chassis lid, coax is used for low level signals, not something you see a lot of in old tube amps,

this chassis is a pain to work on, tubes and capacitors sit higher than the iron,

it has some mounting plates you could sit the amp on had it not been for the rubber grommets that are used to shock mount the chassis,

so we have to go out back and chop down a 2 x 4 and use wood screws to get it stable,

weakest point of these old Ampeg amps seem to be grounding,

Leo F could have taught them a few things in this area,

maybe solder was more expensive in Linden New Jersey,

Magnavox was associated with Ampeg, so some of the tubes may be labeled accordingly,

schematics here>
http://unofficialampeg.com/schematics.html
http://ampwares.com/amplifiers/ampeg-schematics/
http://www.prowessamplifiers.com/schematics/ampeg/

parts>
http://www.fliptops.net/technical-information

misc>
http://www.ampegv4.com/index.php
http://www.triodeel.com/7027.htm
http://vintageamps.com/plexiboard/viewforum.php?f=52

some people like the V4 for bass also,



putting down a few grounds>

Those reverb tanks suck.

Best thing you can do for a V4 is turn it upside down to keep the heat from cooking the board. 

I converted mine to EL34, that's a bitchin' option. 

Residual B+ in these has bit me worse than any other amp.  Be damn careful in there. 

Cool old amp.  I totally agree about the weird construction, though.  I recapped and checked out a friend's old GU-12 combo a couple of years back.  Once we replaced all of the old electrolytics it sounded really good.  It had two 7591s with cathode bias, the 12dw7 phase inverter and a 6u11 compactron thing which was used for the reverb drive and pickup plus an extra gain stage.

His had a bad reverb squeal if you turned it past about half way.  His isolators were dried out and cracking so he had to pay through the nose for new ones.  The reverb still squealed, so he ended up buying a fender style reverb bag to solve that.  It was about as loud as my mid-70s Deluxe Reverb which has a pretty efficient speaker in it.  Cool little amp, but what a pain to work on.  Same cover plate over the top of the PCBs and all that extra jazz that makes it hard to work on.  What do the pot date codes tell you about the age?

I have a V4. (And a Reverberocket 2, and a Reverbojet 2, and a Gemini I, and a Portaflex B-15.)

It was a GREAT sounding amp. Loud as hell, but crunchy in a great way.

I was recording my friends' band and their lead guitarist was using it. We went and got dinner, and when we got back he fired it up and it went PFFFFFFT. It was sad.

This was 20 years ago. I should probably fix the amp.

-a

Or you could sell me your broken one!

It should be run with 6550, factory option after the 7027 starter-tubes burnt up.

On "my" amp the shorted 7027 also burned up the PT. Which was perhaps a good thing. I rigged a 40W Bassman PT to get 400V (be careful, you have to hack the rectifier or you get 800V!!).

With 400V you can actually run cathode-bias effectively. With 500+V the self-bias will get choppy at high output. But IMHO anything is better than the TOO LOUD TOO HIGH VOLTAGE condition they were made at.

thanks for all the replies! 

yes we had an arc welding experiment going on yesterday,

got the B+ probe on pin 3 too close to heater pin 2 and a nice half inch silver arc started up between pins for about 3 seconds before i hit the power switch,

that pwr xfmr is pretty hefty, don't want to get across that,

6550's run at 700 volts in the Music Man amps, so that might be a good choice,

the guy down at the shop said he left his V4 on and went out for a break, came back and it too was also sizzling, don't know what breaks, maybe the OPT or tubes, or both,

we wired up the mains and front panel pwr and stand by sw to a jones strip,
pretty dark and scary down in that corner with wires going everywhere and mounted to a flimsy  terminal strip (red box)>

I have heard a rumor about a repair shop here in Brooklyn with a stack of V4s.  Seems the transformer goes and nobody wants to spend the $$$ to replace it.  You wanna offer your services CJ? 

I was able to sub my V4 in for a dead SVT during a recording session and on the same speaker cabinet was able to match the sound exactly.  50 lbs lighter too!

emrr said:

Residual B+ in these has bit me worse than any other amp.  Be damn careful in there.

Click to expand...

Yep. Or outside of there.

To get the voltage rating high enough on the first filter cap, Ampeg put two can caps in series with a voltage divider in parallel across them.  If you replace the can caps with new ones, be sure to also put a new cardboard sleeve on, because the exposed can sits at ~1/2 the B+ voltage out of the rectifier. 

I got a V4 back from a tech like that once, and it was certainly not a nice surprise.

The Compactron tube (6k11) is used for the midrange control, not reverb.

Be sure to remove that ground cap, they like to explode, in my experience.

+1 on the 6550s

good news, the OPT is bad!  ;D

scrubbing up for surgery as we speak, no testing to do because it don't work,

this amp had some hum so i was bridging filter caps with a good one,

put the good one away to discharge by itself,  just did a human discharge so the ol ticker is still good,  , however, the pliers i was holding have gone missing as they flew away during the spiritual experience, 

pri leads run thru a tunnel, sec leads go to an impedance sw,

out of circuit, looks like 11 lams of 150 EI plus 2 keepers,