Harmony H303A

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Harmony H303A

Postby Trapper on Sun May 03, 2015 5:33 am

My wife got me this amp for my birthday! It’s a 1963 Harmony H303A, 5 watt tube amp, with the original 6” alnico Jensen speaker!! It’s a surprising loud 5 watts.
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Re: Harmony H303A

Postby Jack Hester on Sun May 03, 2015 9:39 am

Good score! I have an 'A', 'B', and 'C' model. Yours is one of the safe models. It has an isolation transformer, where my 'B' model just uses straight line voltage to the rectifier. I don't remember the 'C' model configuration.

It uses a 4 ohm speaker. I experimented with one of mine. I disconnected the internal speaker and connected the amp to two external 12" Fanes, in parallel. That little amp can move some air. It is a completely different amp. It sounds good with the little speaker, and sounds really great with the larger ones. They are sleepers.

Jack
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Re: Harmony H303A

Postby stratele52 on Sun May 03, 2015 11:47 am

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Re: Harmony H303A

Postby JJman on Mon May 11, 2015 12:08 am

I have a Silvertone with the same tube compliment. Added the iso tranny. It has 40u on the 35w4 which is the max on the spec sheet. That H303A scheme shows 60u on it.

The screen feeding resistor for the 12au6 shows it is grounded on that scheme too which I'm thinking is an error.

Anyone know if these really had 60u on the 35w4? I've seen mods that bring it to 100u but I assumed that was too radical. I would like to reduce the hum on the plate of the 50c5.
-If it says "Vintage" on it, it isn't.
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Re: Harmony H303A

Postby Stratmandoo on Thu Oct 06, 2016 3:54 am

I just scored on one of the h-303a models. Great amp! I have the two prong cord. How unsafe is it really? If you use a power strip with three prongs on it does it make it safer?
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Re: Harmony H303A

Postby stratele52 on Thu Oct 06, 2016 8:43 am

Stratmandoo wrote:I just scored on one of the h-303a models. Great amp! I have the two prong cord. How unsafe is it really? If you use a power strip with three prongs on it does it make it safer?


No, the amp have no ground wire inside , power strip or other gadget can't add a ground to a amp.

How unsafe, not too much if you use it alone in you house.
No other amp with microphone or guitar around it.

In the worst case , a short circuit inside the amp may hurt you

So easy and low cost a three prong, do the mod.
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Re: Harmony H303A

Postby stratele52 on Thu Oct 06, 2016 8:50 am

Trapper wrote:My wife got me this amp for my birthday! It’s a 1963 Harmony H303A, 5 watt tube amp, with the original 6” alnico Jensen speaker!! It’s a surprising loud 5 watts.


This amp use a 50C5 as power tube, and this tube is rated 2.3 watts maximum, and we don' t drive tube at is maximum
Yes it is loud 2.3 watts

https://frank.pocnet.net/sheets/049/5/50C5.pdf
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Re: Harmony H303A

Postby stratele52 on Thu Oct 06, 2016 9:01 am

JJman wrote: l. I would like to reduce the hum on the plate of the 50c5.


Hum is because
1- One side of the heater are at ground / chassis .
2- And input jack have no shorting contact to ground ; these are antenna to noise.
3- Need a 3 prong cord
4- A very low budget amp.

You may remove heater tubes pins and wire from transformer from ground and put wire to replace chassis ground.
After this mod, add a artificial ground with two 100 ohms resistor. One each side of heater transformer wire, other side of the resistors going to chassis ground

Schematic here;

http://bmamps.com/Schematics/Hamony/harmony_h303a.pdf
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