Sundown tuneup...

[ic-racer]

My Sundown had been acting up, so up on the bench to get a little tuneup. In the past, one of the 12AX7 tube sockets had given me trouble and it looked like the same thing again.

I used a little pin to tighten up the pin holders in the socket and swapped a few tubes in and out and that got it going again. I suspect some day I may need to replace that socket.

While I was in there I decided to go ahead and convert the A36 in to an A50.

Here is a letter I got from Dennis Kager that explains the differences between the A36 and the A50. If you note the date, you can see I just never got around to doing the conversion.

The boost mod was pretty simple. Just removing a 25uf cap and a .2 uf cap and putting in a new .2uf cap. I could not re-use the existing .2uf cap because the new cap needed long leads to fit on the PC board.

[ic-racer]

Here are some shots of the Sundown innards. It is a 50w amp with two 6L6s and 5 preamp tubes. It has relays for channel switching. There is a clean channel and a distorted channel.

In addition to the Master Volume, there also is a phase cancellation control between the PI and the output tubes. He also included a rheostat on the cathodes of the 6L6s to further limit output.

The gain boost switches a bypass cap in-and-out of the cathode of the first gain stage.

There is a mid boost that lifts the mid pot from ground.

[ic-racer]

Here is the schematic.

[ic-racer]

Here it is getting a sound check before being put back in the cabinet.

Here is the sound check, I think its a keeper!

http://soundclick.com/share?songid=6667124

[DennisF]

Thanks for posting this.
I have one of these as well cool amp, is the schem you posted updated with the mid boost mod?

-D

[ic-racer]

Thanks for posting this.
I have one of these as well cool amp, is the schem you posted updated with the mid boost mod?

-D

Yes. That schematic shows what it would look like after the changes to make it an A50.

After I looked more closely at the schematic, I saw two other areas (other than what Dennis mentions in the letter) on the A50. So, I went ahead and did all the changes to convert it over.

If you have an A36, then the complete list of changes (or at least all I could pick up so far) are as follows:

1) After V3b there is a 470k/270p in the signal path that are not in the A50 schematic. I just jumped them with a small wire, keeping the original components in place.
2) There is a .022uf and a resistor in the signal path for the governor control (on the back of the pot) that are not in the A50 schematic. Again I just jumped them with a small wire.
3) After the first input tube the signal splits to go to either green or red channel. There is 1M in series for the red path on both A36 and A50. On the way to the green channel, there is a 1M on the A36 and a 470k on the A50 schematic. I swapped out the 1M and added a 470k. (I think this brightens up the clean channel).
4) V3 cathode bypass Boost changes to a .022uf bypass with boost on (and no bypass cap with boost OFF). The A36 had a 25uf bypass with boost ON and the .022uf bypass with boost OFF.


Overall I like all the changes.

[Bob-I]

Dennis' shop is a bout 10 minutes from me. He's a talkative guy when he's not too busy with repairs. His show is as big a mess as mine.

I was never too impressed with the Sundown's. I liked the stuff he worked on at Ampeg better but they can be made into nice amps with some work.

[Bear]

Ah, the amps from the other N.J. amp guru. Never got their due, but that partly be because they were kind of notorious for having issues. The first time I heard them live, the guy playing carried two so he'd have a back-up. Great tone, though.

Bear

[DennisF]

Thanks for the clarification on the mid boost.

Mine is an A-50 s/n:901010 It has 6L6's in it.

-D

[ic-racer]

Ok, so I got it all packaged in the cabinet again and I was hearing a faint crackle! Before it was a loud crackle (which it had done many times in the past, and it was associated with moving the tube in the loose socket).

So, not the V3 tube socket is all nice an snug, I was getting a very faint crackle. I narrowed it down to 7 components just by noting that the Gain had NO effect on the loudness of the crackle, and the Master caused it to get louder or quieter.

After lifting some components off the board I narrowed it down to either the 100k plate resistor or the .022 coupling cap of V3b. I even removed V3 and it still crackled, so I knew it was not the tube. I could not tell which of the two components was bad because lifting one, or the other caused loss of all signal. So I replaced them both and .... Still it crackled!

Then, on a whim I connected the two lifted ends of the 100k and the .022 coupler (ie bypassing the circuit board and the connection to the tube socket) and NO crackle, so it was either in the socket or the PC board.

Here is a look at the PC board. I had just cleaned the connections of V3 in the middle of the picture. You can see V4 has yet to be cleaned. There was lots of old hardened residual flux all over the board. I cleaned it with pure isopropyl alcohol.

WOW!! No more crackling. So this grunge on the PC board really can affect the signal. So I went ahead and cleaned the whole board the best I could. I'm also glad I didn't have to replace the PC tube socket.

So, now thinking back to when I got the amp in 1996 I have a probable cause for it's behavior. I think it started the low volume crackling and the prior owner or a tech kept swapping out tubes on V3. Thus eventually causing very loose tube pin holders. They got so loose, that when I got the amp, it was non-functional because V3 was not making useful connection in the socket. But I think the reason they kept swapping tubes was the underlying crackling from the crust on the board.

Anyway, now it sounds great. Better then I have ever heard it since I have owned it.

[Wayne]

I remember being taught that "solder guck" absorbs water and can lead to problems down the road. I've never had something fail because of it, despite the fact that 95% of the time I'm too lazy to clean up after a small re-soldering job.

When I do clean up after soldering my weapon of choice is balls of coffee filter paper held in a set of forceps/small needle nose pliers, and contact cleaner. These are things I always have on hand.

W

[ic-racer]

After cleaning this thing up and being amazed at the sound of the amp, I wonder how many other Sundowns have a similar problem. This amp had always been a somewhat mediocre sounding amp, but I kept it in my collection for historical reasons, and because it had some cool circuitry designed by Dennis.

[Bear]

Sundowns are famous for problems, and I had particularly heard of hard-to-track crackle issues. I don't know how much of it has to do with compromising tone. Makes you wonder if a non-PCB clone might fare better on the reliability and tone fronts. I imagine the new problem is working out the lead dress to cope with the high gain.

Bear

[ic-racer]

Sundowns are famous for problems, and I had particularly heard of hard-to-track crackle issues. I don't know how much of it has to do with compromising tone. Makes you wonder if a non-PCB clone might fare better on the reliability and tone fronts. I imagine the new problem is working out the lead dress to cope with the high gain.

Bear

Agree. Doing a clone of this amp circuit might have some promise. I'll admit I have not seen everything, but Dennis' OD circuit looks unique to me. Might appeal to the person tired of doing the same old circuits.

[Structo]

First time I have heard of flux casuing problems.
Of course I am not that experienced in tube amps.
I wonder if that stuff conducts small amounts of current?