vox berkeley super reverb

[cbass]

Cool and if you get to drunk to walk you can just scoot along on your amp walker.

[Zephedone]

Cbass , The family cat already took a ride in the vox gocart ....no likey .

I am working on replacing the power supply filter caps and have come across some wiring that does not correspond with the schematic . I am wondering if it could have something to do with my elevated voltages . The problem is that the grids of the output tubes are tied to the wrong node per schematic . As it sits the grids (#9 pins) are tied to the first node along with the OT at 270 vdc and should be tied to the 240 vdc node per schematic . I stopped to get some opinions before I go any further . My thoughts would be to rewire as shown on the schematic . Also , should I be calling these grids the control grids ? Iwill try and upload the schematic with this post as well
Thanks ...

[Structo]

Wow!
That cleaned up nice!

Is that baffle stock?

I think my cab is plywood but it just has a couple boards top and bottom on the back.

Like I said, I have had that thing since I was around 18.

Finally is working great for me with the EV's.

[Zephedone]

Thanks for the compliment . The baffle is a replacement that I fabricated . The original baffle was pressed wood and swollen or falling apart , crumbling . The bottom half was not salvageable but the top part with the tolex held up well . I used a little jig saw to remove the damage and then a router to clean up the edges . I am a junk collector so I had a 10" baffle from an old console stereo to use as a guide for the speaker holes . Screwed that down to the new baffle and again used the jig saw / router technique . The cleats on the inside holding the baffle in place came from of all things a Thomas organ that I scrapped . Just so happened that the screw holes lined up where I needed them to be .

[Zephedone]

Power supply caps have been replaced . I could not find a drop in replacement for the old can cap . It's the actual physical size that I could not find as it is about 1" in diameter . On a terminal strip near the rectifier I was able to move some connections and free up a few lugs . I built the filter supply in a corner of the chassis and replaced one voltage dropping resistor that had drifted up around 15k higher than it's original value . I also replaced the old AC cord with a grounded 3 prong cord . I went ahead and moved the output tube control grids to the second node of the filter supply . Fired it up with just the rectifier and voltages were still a little high . Went ahead and installed the output tubes and checked voltages and came up with 315 vdc , 305 vdc , 285 vdc first second and third node . With all tubes installed voltages measured 305 / 293 / 183 . So the DC voltages have dropped down but I am not sure if I need to get them down a little more . I also don't know why the initial DC voltage dropped . My AC line voltage remained the same at 284/0/284 vac .

[Zephedone]

BTW the schematic shows these voltages in the power supply , 270 / 240 / 150 . Also the restster I replaced was supposed to be 56k but all I had was a 52k .

[Zephedone]

So I had a little time to fire it up and it sounds good . I don't like the look of the el84 outputs . They seem to be running hot with an intense glow . I'll get some detailed measurements on the rest of the outputs , grids , plates , cathodes , etc . I have a little room where I could install a dropping resister between the rectifier and the first filter cap . I figure if I could drop about 40 volts there I would be close to the voltages on the schematic . I forget the process of figuring out the voltage drop though . I'll do a search and and get the results posted so maybe someone might help me check my math .

[Structo]

How is the power tube bias setup on that amp?

[Zephedone]

Structo I am not sure I know what you are asking . The tubes share a 120 ohm resistor to ground from the cathodes and there is a 25/25 bypass cap across the cathode resistor It should have 260 vdc on the plates and 240 vdc on the screen grids if I am using the right terminology . The actual measured voltage is 305 / 293 plates and grids . Is any of this info useful ?

[Structo]

Since your plate voltage is high, you may want to increase the cathode resistor.

I would shoot for less than 40ma (80ma for both tubes) on the cathode current.

[drew]

The schematic depicts a 280-0-280 PT going into the EZ81, and 270 vdc coming out. Can that be right? Shouldn't it be making something like 1.1 or 1.3 x 280? Smart people, chime in here please.

[Zephedone]

Drew I did'nt realize that until you brought it up . I use 1.1 (Rect tube) and 1.4 (ss) for ballpark figures when scrounging PTs for homebrew builds . In fact my voltages 284 x 1.1 = 312 which is close to my 305 vdc at the first node .

[drew]

Another guy wondering why his voltages are higher than the schematic: http://music-electronics-forum.com/t10123/

According to this, these amps were "interim products," only produced for a period of months. Perhaps the kind of situation where errors cold be made in the official schematic, and never bothered to be corrected?

You have to think the amp was used for a lot of years as-built, before being retired to the porch for baby alligator housing. You might want to forget about trying to conform to the schematic, change the power tube grid connections back to how you found them and revert any other changes you might have made (other than the drifted value resistors), and see how the thing sounds.

[Zephedone]

Good idea ! Thanks Drew . I think I'll try that before trying to "fix" it . I'll take more in depth notes on voltages once I restore the circuit to "as built" .

[rp]

I'll again note that getting up to 340V on vintage Vox EL84 amps is normal and fine. As its a US made Vox I'd say some nice NOS GE 6BQ5s are in order. BTW I don't recall what the exact value was for my replacement can but it was something like 40/20/20 which I figured was close enough and it looked right. I had found a 1" one no problem, but it was years ago. You could stick this in there if you're tight for space, as you are, and at least externally it would be a clean looking repair:

http://www.tubesandmore.com/products/C-EC20-20-450

use the 20/20 for the 30/10 or parallel for the 40.