New to forum.

[dan stanton]

Hello to all,
I'm trying to build a practice bass amp, 25-30 watts. Like the guitar practice amp @ememag. How would I modify it for bass? Thanks for any help.

[dan stanton]

Oops, that's epemag.

[joecool85]

Could you post a link to the schematic?

[J M Fahey]

Hi Dan.
Cool project !!!!!!!!!!!
And it is EPEMag indeed!!
The link is: http://www.epemag3.com/index.php?option=com_docman&task=doc_download&gid=184&Itemid=38
You do not need to modify much, as it is it's better for bass than for guitar.
1) use a slightly beefier power transformer: 12+12VAC, 25 to 40 V.A.
2) use a good audio woofer, 8" or 10" (or 12" if you dare), 4 ohms
3) that heat sink/dissipator is skimpy, try to mount there a somewhat larger, finned one
4) the left side of the board is supported by the jacks and pot; fix something for the right side.
A larger sheet aluminum heat sink, bolted to the front panel, can do double duty as extra support
It's so cool that I'll build one just for fun.
The microphone idea is amazing. British ingenuity at its best.

[dan stanton]

Thanks JM! I'll be using 12" Jensen(s). See anything on the preamp stage to take out/add for better bass? I have to redraw the board to move the 2030 to a side heat sink. Might want to put that website so others can see it. Let us know how yours goes.

[dan stanton]

So, a 3 amp transformer is OK?

[J M Fahey]

A 12+12V 2A transformer is fine, a 3 A is better.
Use a TDA2040 or 2050 instead, the pinout is the same but they are beefier.
Something I often see in practice amps, is to add a stamp-size mini-board, ,just with the TDA20xx pins , so the power chip can be mounted away from the front board, usually bolted to the chassis or to a better heatsink.
You can use the EPE board and only add the little adapter.
You might even solder wires to the "floating" pins but it's unprofessional .
In that case heat shrink insulate chip wiring or sparks will fly.
Keep the chip close, if possible wires no more than a couple inches, four maximum, for stability.
I'll build one as-is to test, and if good (I think so) I'll adapt it for a 30W TDA2050 , separate volume controls and a simple tone control.
Will also use a TL072 (dual op amp) instead the single TL081, the price is the same
Anyway the project is great as-is.
Those speakers you mention will turn it into a serious amp.

[dan stanton]

Thanks again for the response. I really appreciate it. And I'll do nothing unprofessional :tu:
I'll check out the tl072. I was thinking about a '50 myself. The speakers are 50 yrs old, and still crank!

[J M Fahey]

Hi Dan.
First of all, pamper those Jensens.
If they are 50 year old Alnicos, original never reconed, they are worth a lot.
Sell them on EBay for a good price, buy a couple higher power new ones, and pocket the change.
Problem is, the original ones stand 15W maximum, they won't stand 50W between them.
Well, *maybe*, but why take unnecessary chances?
Just to be sure, post whatever is written on them, Jensen used paint and rubberstamps to mark various codes on the frame, at unusual places.
The idea of a future TL072 (dual op amp) is to add some tone control, another channel, whatever.

[dan stanton]

The Jensens are ceramic mags. C12rl. 25 watt rms, 50 peak.

[joecool85]

The Jensens are ceramic mags. C12rl. 25 watt rms, 50 peak.

I have a 10" woofer from a JVC powered sub that I gutted.  It's in near mint shape, 50w RMS, 100w PEAK - 4 ohms.  I'd let you have it for the price to ship it out to you.  I was going to use it, but now I need to get rid of some of my projects.

[dan stanton]

Where are you at, Joe?

[joecool85]

Where are you at, Joe?

I'm in central maine, pm me if you want me to get you a shipping quote.