New Project Amp Day: Ampeg J-20

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Igorilla
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New Project Amp Day: Ampeg J-20

Post by Igorilla » Tue May 07, 2019 3:39 am

I have always wanted a 20-25 Watts brownfaceish combo amp but with the extra of some added clean headroom.
During the last year I started thinking of building one myself but the options for getting parts for a reasonable price here in Europe are pretty scarce.

Last year I learned about Ampeg J-20 which is supposed to be very close relative of the brown 6G3 Deluxe. Handwired amp with good quality transformers, CC resistors and quality caps.
Finally, I have managed to get a J_20 a couple of weeks ago for a very good price. It is mint and sounds pretty good in part (see the comments below).

Comparing the schematics of both amps the main differences are:

1. Normal channel of J-20 is like the bright channel of 6G3. Sounds good with my JM Jag but is just a tad too bright.

2. Bright channel of J-20 is like bright channel of 6G3 with added bright cap (!). Unusable with Jm & Jag for my taste.

3. Both channels of J-20 go into a single volume pot. 6G3 has two separate volume and tone pots. But this is no big concern for me. The simpler the better!

4. J-20 has a little non-adjustable presence circuit which 6G3 does not have. This is a big plus for me as I like the presence knob on my blonde Bassman.

5. Some minor changes in component values to achieve more clean headroom as compared to 6G3. I like that.

6. According to Obeid Khan, designer of the amp, both the power as well as the output transformer are over-designed with 6V6s AND 6L6s in mind. This is pretty cool.

The stock alnico speaker in J-20 is okay when played clean but falls apart when the volume gets urgent. So a speaker swap is due in the future.

The amp needs new electrolytic caps as the original ones are still in there. When the parts arrive I am planning to do the following changes:
0. Filter/power supply caps with higher voltage rating (500 V as opposed to 450 V what is there right now) to accommodate higher voltages
when I try 6L6s + SS-Recto.
1. instal bias pot
2. Convert the normal channel of j-20 to normal channel of 6G3. Essentially adding one cap parallel to R7.

3. Remove the bright cap in the bright channel of j-20 converting it closer to bright channel specs of 6G3.

4. I'll make the non-adjusable presence circuit adjustable by adding a pot and installing it in the extension speaker jack. But this is optional.

5. I'll need to replace the speaker with something efficient + big sounding+lightweight. I've tried the amp with an 1x12 cab with a JBL k120 speaker. This was a massive improvement over the stock speaker, but the JBL is just too heavy. Any suggestions?

My plan is to use both channels with an ABY-switch.

Here it is:
Image
Last edited by Igorilla on Mon May 13, 2019 3:42 am, edited 1 time in total.

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redchapterjubilee
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Re: New Project Amp Day: Ampeg J-20

Post by redchapterjubilee » Wed May 08, 2019 5:50 am

I am on the make for one of these myself as I am wanting in on the brown Deluxe vibe.

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Re: New Project Amp Day: Ampeg J-20

Post by wproffitt » Thu May 09, 2019 9:30 am

These have always sounded like great amps to me. I have yet to play through one, the online demos all sound great. Keep us posted on the mods you’re planning!

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Re: New Project Amp Day: Ampeg J-20

Post by Igorilla » Mon May 13, 2019 2:20 am

A little update after I have had some time to work on the amp:
I have changed all electrolytic caps as they were at least 10 years old and the previous owner
didn't use the amp in the last couple of years because he switched to acoustics completely.

The only circuit changes I did are adding a bias pot and slightly revoicing the channels.

This are the sections on the board that are involved:


Image

I have done the following to the lower part:
1. Removed the bright cap (450 pF). This removes the shrill/ice pick & loads of hiss from the bright channel and makes it really twangy with a JM.

2. Added a 0.003 uF cap parallel to R7. This makes the normal channel more mids focused in a good way. Great with Jag and JM.

Next, I have to try the whole setup with an ABY-pedal.

Here the pic of the result:

Image

I have changed the speaker to Eminence GA-SC64 and like the result very much. No ice pick, great balanced tone across the spectrum. I think this speaker is a great match to this amp.

At the moment I have TAD 6L6WGC short bottles and a solid state rectifier installed. The 6L6s dont't give much more volume, maybe 25 Watts instead of 20 Watts with 6V6s. But they give a richer and bigger sound and they increase the clean headroom.

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Re: New Project Amp Day: Ampeg J-20

Post by s_mcsleazy » Mon May 13, 2019 6:21 am

i'm keeping an eye on this one. i've always thought these old ampegs were super interesting. never had a chance to play one though
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Re: New Project Amp Day: Ampeg J-20

Post by Bert Camenbert » Mon May 13, 2019 6:41 am

The Ampeg J-20 isn't that old: it came out 10 years ago or so, I think. Made in Vietnam or Korea, if I recall correctly.
Last edited by Bert Camenbert on Mon May 13, 2019 8:21 am, edited 1 time in total.

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Re: New Project Amp Day: Ampeg J-20

Post by Igorilla » Mon May 13, 2019 6:47 am

Yes, this particular J-20 is from 2008/2009 and this one is made in Korea.

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Re: New Project Amp Day: Ampeg J-20

Post by Bert Camenbert » Mon May 13, 2019 8:22 am

Fixed. Thanks!

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Re: New Project Amp Day: Ampeg J-20

Post by s_mcsleazy » Mon May 13, 2019 10:24 am

learn something new everyday.
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Re: New Project Amp Day: Ampeg J-20

Post by Igorilla » Thu May 23, 2019 5:13 am

A little update on the amp.

I took the J-20 to two practices so far and like the amp very much. It is not that big sounding as my blonde bassman with a 15'' tone ring box, but it is very usable for me as a backup, practice amp or small grab and go amp.

The speaker (Emi Ga SC64) is really a great match to J-20.

You can hear the amp and a Jazzmaster in this little impro that we recorded yesterday evening that we will evolve into a spooky instrumental song eventually.

ATTENTION: surf dork content!!! ;D

Left : Ampeg J-20 bright channnel + Jazzmaster
Right : 63 Vibroverb RI bright channel + Jazzmaster

https://soundcloud.com/igorilla-1/it-ca ... ze/s-barDG

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Re: New Project Amp Day: Ampeg J-20

Post by wproffitt » Fri May 24, 2019 3:14 pm

That sounds great! Well, both amps do, but I really liked the sound of the J-20.

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Re: New Project Amp Day: Ampeg J-20

Post by øøøøøøø » Sat May 25, 2019 8:51 am

If changing from 6V6 to 6L6, also change to a speaker with half the impedance for best results.

If the stock speaker with 6V6s is 8 ohm, use a 4 ohm speaker with 6L6s, for example

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Re: New Project Amp Day: Ampeg J-20

Post by Igorilla » Sun May 26, 2019 4:02 am

wproffitt wrote:
Fri May 24, 2019 3:14 pm
That sounds great! Well, both amps do, but I really liked the sound of the J-20.
I was surprised too as I didn't expect that outcome. The result is, to my ears,

-Vibroverb RI leans sonicaly toward the bigger brownface amps, i.e. slightly more bottom end, slimmer mids, slightly bigger sounding.

- J-20 twangy with good amount of mids and a better treble response.

But to be fair the Vibroverb needs some fine tuning at the moment. We are in the process of trying different preamp tubes to optimize the tone. I've added bias pot and adjusted the bias of the output tubes of the Vibroverb already, which was a big improvement.

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Re: New Project Amp Day: Ampeg J-20

Post by Igorilla » Sun May 26, 2019 4:18 am

øøøøøøø wrote:
Sat May 25, 2019 8:51 am
If changing from 6V6 to 6L6, also change to a speaker with half the impedance for best results.

If the stock speaker with 6V6s is 8 ohm, use a 4 ohm speaker with 6L6s, for example
Ah yes, I didn't mention that part of the trial. I did this two weeks ago. The outcome with 6L6s and a 4 Ohm speaker was suboptimal to my ears (loss of character and slightly louder) compared to a 8 Ohm speaker. I don't know why as I couldn't get the specs of the OT by now.
I'll write Ampeg in the course of the next week and ask if it is possible to get the service manual or alike with the specs of the parts. Or at least the specs of the power and output transformers.

Anyway, the OT of J-20 is slightly larger than the OT of Vibroverb/Vibrolux/Tremolux, maybe they just slammed a generic 6L6-OT they used at that time for other amps into J-20 and used it with 6V6 tubes...

What I have found out is the statement of the designer of the amp that, as far as he remembers, the OT is 6.6 kOhm into 8 Ohm.

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