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Vintage threads from the first ten years

Hotrodding a Harmony H400A

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7/27/2000 2:35 PM

Hotrodding a Harmony H400A

I've got one of these, it's the little bugger without an isolation transformer and the tube compliment of 5OC5, 12AU6, and 35W4 and all the versatility of a tone AND a volume control.
I'm not at all hesitant to re-invent this altogether.
Any suggestions? Pull a tube for crunchy distortion at bedroom levels? Ok, that was a joke.
For the record, I really, really like the trainwreck tone.
Any and all suggestions appreciated.
- John

7/27/2000 8:11 PM

Dean Hazelwanter

I'm not sure if this qualifies as hot-rodding, but I just picked up a 'Regal' combo with 12AX7, 50L6 and 35Z5, for $2.00 'as is', at a thrift store. As I expected as soon as I saw the tube compliment(?), it gave a noticable shock when touching the chassis, no matter which way the linecord was plugged in. I left the filament string connected directly to the line, but used an isolation transformer for rest of the circuit. I also installed a 3 wire (grounded) linecord.

BTW, a good cheap place to get an isolation transformer capable of driving B+ voltages for an amp like this (but not the filaments), is an old razor outlet plate. These are regularly available at garage sales for less than 1$.

BTW2, the amp is safe now and sounds good, (even with the hum), but I'll probably use it for 'trading fodder' at the local 'vintage friendly' music store.

7/28/2000 3:18 PM

I have tried to mod a number of these low voltage guys and had always ended up putting them back together the way they were because whatever you do to them they sound like they have a blanket thrown over them to me. From what I understand the low voltages are not able to do much top end and this results in an amp that sounds a bit dull on clean sounds and more and more muffled as you make increases to the gain to get more distortion.

I have found the key that unlocks these amps is the Z Vex Super Hard On, which offers a very high impeadence to your pickups with a FET that does not load them down. So if you want to get these little guys to sing I would suggest either getting the Z Vex which is expensive but a great overdrive on many amps or building a similar sort of Fet stage as an effect or into the amp itself.

JM

7/28/2000 6:52 PM
Adrian	Ding! JM, that is precisely how i goose my Harmony H400. I use the Minibooster mu amp device. The sound "opens up" a bit. Really my Harmony is a one trick pony of the horsiest stripe - cool for what it is but i suspect no sort of platform for rewarding modification IMHA.

7/28/2000 7:37 PM

Thanks for your replies.
I have a SHO that I cannot stop using regardless of what amp I'm using at the time. I love this thing.
But,
from the replies here looks like it'll be a rebuild (cannibalize). I've got a few other quarterhorses that can handle this one trick better (IMHO) than the Harmony.

So I'm guessing people will suggest doing a AX84 P1 type thing, but I'd like a little more, something more akin to the hi-octane November. Suggestions?

Like I said before, I'm not against mods to this beater (though I'm not interested in effects loops or master volume, and, despite my attempt at humor in my first post, I do like the simplicity of just a volume and tone knob), and I wouldn't lose a bit of sleep over hacking this thing into something it was never meant to be.
thanks,
John

7/28/2000 9:20 PM

Once you've got the isolation tranny you can easily add a voltage doubler/tripler. Believe it or don't many of the electronics mags of the 1950s offered plans for "hi fi" amps made with 50L6s (50C5 is a miniature version) using voltage doublers. The higher voltage ratings aren't in the tube manuals but you can double the plate voltage safely - watch the plate dissipation and run the screens at 150v or less.

GW

7/28/2000 7:27 PM

I'm not familiar with the H400A circuit, but many of these "radio amps" have the preamp set up in grid leak bias. This type circuit has high gain, for max amplification of small signals, but will saturate too soon with a large signal. If the amp has a grounded cathode on the 12AU6, and a large (maybe 3meg) grid leak resistor and a coupling capacitor, it's a candidate for switching over to cathode biasing. Some of these amps have input jack resistor networks that can be altered to get a more conventional high/low senstitvity setup for more flexibility. I had a Harmony H303A that had the same tube lineup, but it had an isolation transformer built in, so there was no shock hazard as with a hot chassis.

Doc

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