Sunn Model A

[benoit]

Got a friend's Model A on my bench. Overall it's in decent shape considering it's around 40 years old. Few things I want to try to fix though.

1. V1 is super microphonic. Switching to a known good tube did nothing to fix it. I'm inclined to thing it may be a failure in the spot where the wire harness clips onto the preamp board for channel one, because I get a loud pop when I hit those wires or the chassis near v1. You can see the wires in question in an attached photograph. Not totally sure what to do about it, but I'll figure something out.

2. Plate voltage is really high. Schematic shows 525V, I read 573V. Heaters read 6.1V so I doubt it's simply due to mains voltages being higher than they were 40 years ago. I guess the 6550s (newer EH in this case) can probably take it but I'm curious as to why the voltages would be that high.

3. The fet boost. Yuck. it's a 2n4304. I don't know if those are prone to noise but with the boost on low, the amp is dead quiet. With it on high it's really hissy. If it were my amp I'd probably just rip the thing out and be done with it but the owner wants it kept original as possible. Anyone know of a magical substitution that would drop in and fix all my problems?

Other than those 3 things she's a sweet sounding beast. Nice verb, no where near as aggressive as blackface verb on 10 but for most applications that's a good thing. Nice full sounding cleans, cool amp. A monster to carry though, glad it's not mine.

Anyway, if anyone has any sage advice I'd be glad to hear it.

[selloutrr]

WIRE FIX.... Solder the wires directly to the PCB instead of the clip harness. You might want to try alligator clips first to see if that really does solve the problem. Also check all grounds you might find that something is loose to ground... make sure all the screws and bolts are tight. Clean sockets and tighten pins, make sure no residue is making an intermittent trace.

HIGH VOLTAGE... power filter section may be giving up...

NOISE BOOST... besides cross referencing the part in question and finding a suitable replacemnt with low noise characteristics try looking at the pot that controls the volume it might need a cleaning / replacement.

Looks like a cool project keep us posted!

[benoit]

First, thanks for the reply..

WIRE FIX.... Solder the wires directly to the PCB instead of the clip harness. You might want to try alligator clips first to see if that really does solve the problem. Also check all grounds you might find that something is loose to ground... make sure all the screws and bolts are tight. Clean sockets and tighten pins, make sure no residue is making an intermittent trace.

I will try the jumper idea. It's pin 1,2,3,6,7,8, so it'll be sort of a pain to test, pin spacing is not very wide, but I do have 6 alligator leads. How does one tighten a pin? Denatured alcohol work ok for cleaning sockets?

HIGH VOLTAGE... power filter section may be giving up...

Roger that, I forgot to mention it but a recap was first on the list. So would that be because of a cap whose capacitance was dropping?

NOISE BOOST... besides cross referencing the part in question and finding a suitable replacemnt with low noise characteristics try looking at the pot that controls the volume it might need a cleaning / replacement.

I'll do that. Cleaning all the pots was another thing I planned on. The pots do seem to be in great shape. Nothing acts weird, and there's no scratchiness in any of them, but it certainly can't hurt. Tomorrow.

Ben

[Sonny ReVerb]

How does one tighten a pin? Denatured alcohol work ok for cleaning sockets?

From the TDPRI forum:

It's pretty easy to clean and retension tube sockets.

First, discharge the filter caps.

Second, get a tool about the size of that little poker that the dentist uses. The smallest jeweler's screwdriver may work too. I have a set of tools that I got from radio shack that work well for poking wires around and are perfect for the job.

The springs in the tube sockets are simply two pieces of metal that fit on either side of the tube prongs, so you just stick your tool into the socket and start wanking on the sides of these things to bend 'em back together.

To clean, you get some electric contact cleaner, put a little squirt into each element of the socket, then jam your tube in and out a few times. An old ham radio guy told me that works as well as anything else you can do.

[JD0x0]

if the voltages are a little high it's nothing to worry about, But like i always say, KT88's are always an option. IMO they sound alot better than 6550's

[benoit]

if the voltages are a little high it's nothing to worry about, But like i always say, KT88's are always an option. IMO they sound alot better than 6550's

Right, I'll see where things sit after the recap. The owner had talked about trying some different tubes.

[benoit]

Tightened the socket, cleaned with contact cleaner, reflowed all the joints on the socket, the connector, and the preamp board. The tube itself is no longer microphonic but the plate and grid wires of half the 12ax7 are. Maybe that will be ok when the amps buttoned up, I don't know. Also, the unshielded portion of the channel two lead from jack to preamp board also makes loads of noise when I tap it.

Caps should be here within the week, which hopefully will take care of voltage issues.

Just noticed that someone cut the parallel speaker jack out and then attached 3 long wires which go absolutely nowhere. What a useful mod!

Found a replacement for the FET: NTE133.

Hoping to get this wrapped up this next week.