Help proof-reading a schematic

[shepdog]

I had been having some issues with my Proalve 60 red platting and after having a grid resistor blow after replacing the tubes, I've decided to simplify everything and just rebuild the power amp section. I got a copy of the schematic and right off the bat found some errors. Circled in red is what first tipped me off to the inaccuracies. The photo I'm posting is about half the schematic, let me know if you'd like the rest. Also feel free to offer suggestions on how to improve this design.

Thanks

[img:929:678]http://i50.tinypic.com/r6xx7t.jpg[/img]

[Structo]

I think that thing you circled is simply an effort to line up the line because the rectifier is a little lower on the drawing.
It is not a component.

The schematic is drawn a little unusual with the 6L6 screens shown on the left side.

I don't see any grid stoppers on the power tubes. This amp uses an interstage transformer to the power tubes.

Do you perhaps mean the 470 ohm screen resistors?

Can you post a link to the whole schematic?

[oldhousescott]

Shepdog is correct. There should not be a connection between that part of the bridge and ground. That would present a direct short to that half of the winding.

[Firestorm]

I suspect the negative side of the bridge isn't actually grounded in the actual build. Wouldn't that wipe out the bias supply? Maybe it's supposed to connect to the junction of that last pair of 470uFs. Annoying that the actual bias voltage isn't shown. I also suspect that the last 47uF in the lower right is shown with its polarity reversed.

[Structo]

Yeah, I didn't look past the squiggle much.

That is odd that they show the ground triangle there.
Maybe it is a floating ground or other point.

We need to see the whole schematic.

[butwhatif]

The fw bridge should not be grounded at that point !!!

[shepdog]

Thanks for the replies so far. Here is the schematic in full.

[img1600]http://oi51.tinypic.com/wisnif.jpg[/img]

[Ian444]

That power supply is +/-14V for the IC's, with a -47V rail for bias regulated by the 1N5261 zener. The 47uF cap next to the zener does show reversed polarity as Firestorm mentioned. The 4 x 470uF caps should all be grounded on the +/-14V supply, you can see a dot between the last pair but it isn't connected to anything, that's a typo.

The -47V bias is fed to the centre tap of the phase-splitting tranny which is driven by the TL072. If your tubes are red-plating, make sure you are getting the full -47V at the grids (pin 5) of the output tubes. You can check that with the tubes pulled. Are your output tubes actually 6L6GC, or some sort of equivalent? If you fitted 6L6G's (as opposed to 6L6GC's) or equivalent they might tend to red-plate.

[shepdog]

They were a set of Mesa 6L6GCs that initially started redplating. Soon after, one of the 470 ohm grid resistors blew. I replaced both of the resistors and put a set of brand new Groove Tube 6L6GEs in and one of the two started to redplate. My next move was to switch the tubes between the sockets and I found that the redplating followed the tube. I talked to the Groove Tube support and they sent me a new set and I have been reluctant to try them until I got everything rebuilt. Figured I would go ahead and replace all the electrolytics and high wattage resistors. Would two watts be sufficient for the grid and the 220ks that are paralleled to the filtering caps? How should I go about rebuilding the supply for the OP amps seeing as how I will only be powering a single Tl071? Do I still need to build in the third stage that provides the +/- 14, or is that necessary for the +/- 15 as well? Also, any recommendations on changing any of the values before I order parts?

Thanks

[shepdog]

In pricing out parts, I am having a hard time finding 100uf 450V axial capacitors. Could I use a lower farad capacitor such as a 22uf or 50uf? What would I be losing, if anything?

Thanks

[Kazooman]

I don't know where you have looked, but those are very common caps. Antique Electronic Supply has at least three brands to choose from ranging from about $5 - 10. Angela Instruments also has them. I am certain that any of the big suppliers such as Mouser would have them also.

[butwhatif]

what voltage do you have on the pin 5s, BEFORE you turn on the standby hi voltage? don't turn on the hi voltage unless it's at least negative 30 DC volts-pref higher. i don't think the scheme quotes a voltage.
if you had the amp on wired the way the print shows, you prob blew a diode or two, and likely on the side of the bridge that feeds the bias circuit--

[shepdog]

Yeah, I was hoping to put the order through on mouser, but the only 100uf 450v axials they have in stock are $25 each. I ended up spending a little more and ordering from tube depot. I have yet to rebuild, but I will be sure to check the bias voltage before putting the tubes in.

[Kazooman]

That's amazing! I checked Mouser and you are correct that the only axial lead caps that are in stock are $25. Just out of curiosity I redid the search for radial lead caps and they have Nichicons in stock for $3.24 each. I have never used high voltage Nichicon caps, but I have used their smaller caps in many effect pedals with good results. However, if it were my amp I think I would spring for the Spragues or something similar.

[shepdog]

I saw the radials too but I decided that I'd like to build this on a turret board. Thanks again to everyone who helped out.