Dewald stereo integrated PP EL84

[cubby01]

Hey all,

I got this Dewald in a trade from another AK'er. I did start to bring it up with a variac and monitored for anything disastrous. One channel got sound. The other channel was very faint. I know I need to replace a bunch of caps but I have some more basic questions about some of the circuit before I start replacing a bunch of parts I'll end up ripping out in the future.

Just a couple chassis shots here. I'll post the schematics and the questions in the second post.

 

[mhardy6647]

yes those capacitors (the coupling caps and the electrolytics) are evil and must be exorcised.
That poor rascal looks like its had a hard life, near the seashore.

 

[century tek]

Your faint channel could also be a dirty switch.

 

[cubby01]

Ok, to the schematic and questions. I do not have any compulsion to try to do any kind of historical restoration, ie my goal is not to have it looking and operating just as it did when it was new. My goal is to make use of the best of it and eliminate things I don't need and get rid of potential sound quality robbing bits.

The schematic is from a Lafeyette LA-235 which I've read is the same amp just a different silkscreen on the faceplate. I only show one channel in the schematic.

Questions:

1) From what is show can you guys tell if the phono section looks reasonable? Good? Bad? Tweeks I should want to do?

2) There is a rumble filter switch circled in green. I know rumble filters are useful but now well is this one implemented. Is the fact that it's there robbing sound when it's not in use?

3) There's a couple bits marked up in blue - a "blend" control that moves from stereo to mono. And there's a "Normal-Reverse" switch that just swapps channels. I'm just going to remove both of those. I don't see any use in them.

4) There are bass and trebble controls. What I'd like to do there is put in a bypass switch that totally avoids any EQ. Can I just go around that section or what do I need to do towards that goal. (this eq stuff is where I get lost)

5) I need to put an order info for caps. Which ones should be priority?

If there are any other suggestions I am open to them. Thanks much!!!

 

[cubby01]

yes those capacitors (the coupling caps and the electrolytics) are evil and must be exorcised.
That poor rascal looks like its had a hard life, near the seashore.

So basically looks like anything that's not a ceramic disc cap will need to go eh?

Yes it has quite a buildup of rust and gunk. It will get a good cleaning though :thmbsp:

 

[wa2ise]

So basically looks like anything that's not a ceramic disc cap will need to go eh?

That's it. But when you start replacing capacitors, change one at a time, so you don't forget where it goes. Others will say :yuck: but I avoid trying to remove all of the old cap leads from tube socket and switch terminals. They are fragile. I usually do a "J" hook connection between the old clipped off cap lead and the new cap lead. Try to position and route the caps and leads similarly to where the old leads and caps were.
More info on recapping tube electronics: http://www.antiqueradio.org/recap.htm Though, as I mentioned above, I wouldn't try to remove old cap lead wire from terminals.

If you decide to remove from the circuits some of the switches, physically leave them there so you can later do things like create a tone control bypass and such. But save that for later, after you have the amp up and runnning after recapping.

 

[zenith2134]

Sweet amp, dude:thmbsp:

I had a mono DeWald once, also PP EL84, that was extremely well built. Think it was like a 1958 model.

That unit'll look fine with some TLC...

 

[cubby01]

Thanks all. I have a parts order about to go so will be a delay before I do any recaps -- will give me some time to work on the cosmetics.

Quick question. The little electrolytic at C3 (left side of 3rd schem page above). Schematic says 20uf / 10v. I have 50v versions of 22uf and 10uf but not 20. Any harm in a 22uf there? Or better to gang two 10s?

 

[wa2ise]

Quick question. The little electrolytic at C3 (left side of 3rd schem page above). Schematic says 20uf / 10v. I have 50v versions of 22uf and 10uf but not 20. Any harm in a 22uf there? Or better to gang two 10s?

The 22uF will work fine. Back in the day, cap makers routinely made caps -20% to +50% of their ratings anyway. So 10% high won't matter.

 

[cubby01]

Just a progress update.

Recapped all 'lytics and coupling caps. Playing as I type

I have yet to go through and really check out all the voltages, etc. I'm sure there are some out of spec resistors I will need to get to. Phono section seems ok but a bit of noise I will want to look into as well.

The faceplate was brass plated steel and there was some pitting so I wet sanded it fairly clean down to 800 grit, I was going to paint it but then decided to just apply some adhesive backed faux metallic sheeting from the craft store. Doesn't look too bad for now. I still need to repaint the case when the weather warms up a bit.

Cleaned up quite a bit of the chassis, enough so that it won't stink when heating up anyway. I left some of the rust on the PT end bell as a reminder that rust never sleeps and it's the sound that matters more than the looks .

The Rumble Filter cuts out too much from the low end. Thankfully it can be totally switched out.

I'm not real happy with the tone controls, still giving them a chance and looking for any recommendations on how to add a bypass. Treble seems ok but bass adjustment seems to add muddiness.

pics:

 

[cubby01]

Well, flipped it over and started checking voltages, among other things
B+ source voltages:
..schem....> measured
....340v....>..337v
....300v....>..316v
....220v....>..193v
....200v....>..153v

Seems I must have some resistors out of spec. Since 337v is darn close to 340v I think if I just replace all the PS resistors with spec values then I should end up with close enough to the correct supply voltages.

Once my B+ source voltages are in order I will then recheck other pin voltages and see what needs to change next.

...Sound like the right approach at this point???

 

[jaymanaa]

Looks like you are doing just fine to me.:thmbsp: