Heathkit EA-3 conversion to guitar amp - diyAudio
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Old 9th August 2007, 06:46 AM   #1
Jozer99 is offline Jozer99  United States
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Default Heathkit EA-3 conversion to guitar amp

Hi! I'm new to both this forum, and to the world of tube amplification. I'm a guitarist and tinker. I build guitars and effects pedals, but this is my first foray into tubes and amplifiers. I know my way around with a soldering iron and a multimeter, but this is the first time I've done high voltage electronics.

I just got a good deal on a Heathkit EA-3 amplifier. Schematic here. I'm thinking that it would make a great little practice amp. I mostly play blues and classic rock type music, so I don't need massive overdrive, but a nice driven blues sound would be good. As this is my first amp project, I would like to keep things relatively simple.

Here is my basic plan for modifications, in order of execution:

0. Fix amplifier (if its broken, haven't received it yet)
1. Install 3 pronged cord with ground.
2. Install 1/4" jacks for mag-phono input (which seems to have an extra preamp stage, which would be good for guitar, right?), and for speaker outputs.
3. Remove input switcher knob, leaving only the guitar input.
4. Standby switch.
5. Guitar appropriate tone controls, remove RIAA equalization.
6. Build custom cab/combo amp enclosure.

Do you guys think this is a good progression both in terms of easiness and usefulness? Do you have any other suggestions for the amp?

To wire in a 3 pronged power cord, all I need to do is attach the additional ground wire to the chassis, correct?

The amplifier currently has a 2 knob tone control (Treble and Bass). Do you think I should stick to a 2 knob control, or do a traditional 3 knob (Treble, Mid, and Bass) control?

What safety precautions should I take when working on the amp? From what I have read, I plan to do the following:

1. Wait 5 minutes after unplugging the amp to work on it.
2. Connect the + terminal of every capacitor to ground through a 10k 1W resistor for 2 minutes. Measure capacitor voltages before working on them.

Did I miss anything?

Thanks for all your help!
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Old 9th August 2007, 01:17 PM   #2
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This is a hi-fi amp, and I think you'll find that a lot of circuit changes are required. The mag phono input is equalized to the RIAA frequency response curve, and you want a "flat" response. The tone control rolloff frequencies are wrong for guitar - they'll have little effect. The split-load phase inverter is great in its linear range, but you might not like the way it sounds when overdriven. Speaking of overdrive... many guitar ams use small output transformers, and transformer saturation is part of their sound... this one has a good size transformer, and might be more like an Ampeg than a Fender...

Consider this a chassis on which you can build your guitar amp circuit of choice. The hard part is done - you have a chassis with all the major parts in place. Enough tubes for PLENTY of gain. Or sell it and get a small tube PA amp, which will be closer to what you want for guitar.
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Old 9th August 2007, 03:15 PM   #3
Jozer99 is offline Jozer99  United States
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Since this is my first project, I would prefer to keep it simple and small. A PA system would really be too large for me, if not in wattage, then the chassis size. I do kind of have my heard set on converting this amp.

I'm not looking to make a perfect 18W replica, just something that sounds OK, and is a good beginner project (I can make something nice once I know what I am doing!)

Does anyone have any suggestions or diagrams for some simple circuit mods (like removing the RIAA equalization and subsituting a Marshal tone circuit)?
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Old 9th August 2007, 03:27 PM   #4
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How about a Marshall 18W? http://www.drtube.com/marshall.htm#JMP18W

Leave out the tremelo channel and you'll only need two 12AX7s.
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Old 9th August 2007, 04:01 PM   #5
Jozer99 is offline Jozer99  United States
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For my first tube project (I am a complete tube/high voltage newbie) I would prefer to simply modify the existing circuit, rather than create a completely new amp out of the existing tubes.

One reason I bought this particular amp is that in the future, when I know what I am doing, I will be able to rewire it to something very similar to a Marshall 18 watt. In the mean time, how do I get rid of the RIAA EQ, and substitute my own tone circuit?
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Old 9th August 2007, 04:22 PM   #6
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I am still fairly new at this as well, however, I do not see anything wrong with your plan if you are simply using the amp for practice. I have done these kinds of modifications a few times to some old hi-fi and PA amps, and the results have usually been decent if the equipment wasn't shot to begin with. Instructions on how to do such a modification can be found here . Remember that many of the first electric guitarists simply plugged directly into old hi-fi amps. Best of luck!
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Old 9th August 2007, 11:28 PM   #7
Jozer99 is offline Jozer99  United States
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Thanks! That is indeed a great website (I already have it bookmarked), it goes over lots of the general theory, and some rough layouts, but does not provide any actual circuit diagrams. I am going to attempt to create my own diagram and post it, to let you guys critique it.
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Old 10th August 2007, 12:58 AM   #8
Jozer99 is offline Jozer99  United States
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Here is a link to the original schematic of the amplifer:
Original

Here is a link to my proposed modifications:
Modified

I really don't have any idea what I am doing. Does that modification look workable?

Here is what I did:
Removed input selector switch
Added volume control before 1st gain stage
Added EQ and volume control between 2nd and 3rd gain stages

How do I get rid of the RIAA equalization? Where in the circuit is that? If I actually solder all this up, will it work, or will it explode?


Thanks a ton!
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Old 10th August 2007, 01:12 AM   #9
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Remove C6 to remove RIAA EQ. Be prepared for a TREMENDOUS increase in gain for the mag input...

For a start, leave the tone controls - do this in steps, so if you make a mistake, you can fix it without a lot of searching... Your very first step should be to verify that it works as-is. Voltage measurements would be good too - the values are marked at the tube pins on the schematic.. If you don't have a DVM, get one at Harbor Freight or the like ($2.99 on sale right now...)

Have fun!
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Old 10th August 2007, 06:12 AM   #10
Jozer99 is offline Jozer99  United States
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By "remove" do you mean replace with wire, or completely cut it out of the circuit leaving a gap?

If I remove C6 completely, doesn't that mean I can take out R5 and C7 as well, as they are no longer connected to anything?
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