Harmony H 400A

[korby]

I just bought one and I'm wondering about putting a three prong cord on it . I read this and now I'm not sure.

"RE: Harmony H400A amp?"

"Ooops! I just looked at that schematic. It's set up almost like an AC/DC amp, with one conductor of the mains line hooked to the chassis. You CAN'T ground that. You'd get an intense blue flash and pop when you plugged it in, then your circuit breaker would trip and your lights would go out. The only way to run that completely without risk of shock is to plug it into an isolation transformer. However, you can make it a lot safer by replacing the cord with a polarized two-prong one arranged so that the neutral wire of the socket is always connected to the chassis. As long as somebody didn't wire a socket backwards, you're OK with that.

You can identify these things at a glance by the half wave rectifier. (35W4) "

What do you think ? This might help .
http://harmony.demont.net/documents/...rmony_400a.pdf
Thanks

[korby]

Here is what it looks like , this isnt mine . The one I have has a water stain on the front , I have not taken pics yet .

[korby]

This amp sounds great very warm . Bit of a Stones sound . It hums for about 5 minutes after turning on then quiets down to about a fish tank pump hum volume, I've got tubes on the way , these are original.

[gmoon]

I have a similar small amp with the heaters in series on the AC mains, etc. It has a transformer for the plate voltage, though.

New tubes may very well remedy the fading sound. You might consider recapping while you're at it.

The isolation transformer idea wouldn't hurt. The Triad N-68X is a good choice for a small amp (up to 50 watts total power consumption.)

[52 Bill]

If this amp is wired correctly as it is shown in the schematic, the chassis/signal ground is separated from the AC line. So some of what was posted from the other source does not apply here.

I do agree, that a isolation transformer is the safest way to use the amp.

[korby]

isolation transformer Yep I was just ordering one , thanks .
Here is the guts .
This is the one I got .
TRANSFORMER - LINE ISOLATION, HAMMOND, 175 VA

P-T169VS

Hammond 169 Series Line Isolation Transformer
175 VA capacity, 1300 secondary current ma.
Primary 115 VAC, 60 Hz.
Secondary 115 VAC
Electrostatic shield between primary & secondary
Connection by flexible leads, 8" minimum length
Perfect for circuit isolation or "Classic" bias power supply
CSA certified (# LR3902)


RoHS Compliant
I put the new tubes in before taking it apart and it was way louder . Cap is shot and is what was causing the hum .

[pdf64]

'If this amp is wired correctly as it is shown in the schematic, the chassis/signal ground is separated from the AC line. So some of what was posted from the other source does not apply here.'
52 Bill, I hate to contradict, but this is incorrect.
The chassis of this amp, and therefore your guitar strings/metalwork, is connected to the mains power supply when it's plugged into the wall and switched on.
Don't use this amp until an isolation transformer / chassis grounding has been permanently installed by a competent person.
Cut the plug off in the meantime. Peter.

[korby]

52 Bill, I hate to contradict, but this is incorrect.
The chassis of this amp, and therefore your guitar strings/metalwork, is connected to the mains power supply when it's plugged into the wall and switched on.
Don't use this amp until an isolation transformer / chassis grounding has been permanently installed by a competent person.
Cut the plug off in the meantime. Peter.

Yep already done .

[korby]

Gotter' done .
Caps changed killed the hum , new old tubes made it twice as loud ,installed a fuse ,isolation transformer and grounded cord keeping us from getting killed and replaced puke green ,water stained grill cloth .

[Cygnus X1]

Bumping this one up to say thanks for the info, and I did nearly the same upgrades for a friend.

Two ceramic caps on there went to nowhere.
None of the original caps or anything else except the electrolytics were changed.
I bundled the replacements up and put a heavy ziptie through to hold them firmly in place.
The new transformer is mounted on the back along with the other updates.

Machinists will recognize the 1-2-3 block I'm using to hold it up.
For those searching, here is a pic of the one I did: