Fender Frontman Mod.

[OldSolder]

Hi all. First post from long time reader of this forum.

I have a little Fender Frontman 15. It was cheap and almost works for a practice amp.
This one was assembled in Mexico so has a upc1188h for the output amp.
Checking the data sheet the 1188 is rated at 20 watts yet is only rated at 15 watts in this amp.

Does anyone have a solution as to how I can unlock that other 5 watts?
The supply voltage is the same. Could it be as easy as changing the NFB resistor or is that just wishful thinking?

Thanks for any ideas. I'm playing an archtop through it and have changed the speaker in the pursuit of a bigger warmer sound.

[Jazz P Bass]

My take?

It is what it is.

Unlocking another 5 watts will hardly be discernable.

And no, the feedback path is not the way to go.

You did the correct thing by changing to a different speaker. (not specified what the change was to though.)

What the amp needs is a good 12".

[g1]

Agree with Jazz that the extra 5W may not be noticeable. But I guess at low power levels, every watt counts.
You said the supply voltage is "the same". +/- 22V? When putting out full power? If not, it's probably limited to 15W due to the power transformer.
What speaker did you put in? High efficiency? Try it through a bigger 8 ohm cab and see what you think. (two or four 12")

[J M Fahey]

Like Enzo says (roughly): "the amplifier is the thingy between the power supply and the speaker" meaning the supply actually drives the speaker, the amplifier is just the "faucet" which lets more or less pass, but does not create power by itself.

If you keep the same supply, no tweaking will rise power appreciably ; you might get fractions of a watt by splitting hairs here and there, not worth it.

The BIG improvement comes from a better speker(s).

[OldSolder]

Thanks for the replies guys.

I've measured the voltage across the secondaries, 17.8 Vac. The label on the back of the cabinet reads 38 watts.
If I get a tranny With 15 - 0 - 15 and 4amp rating, could I sub that assuming that the extra current would stop voltage sag under load?
I plan to add a heatsink to the pwr chip and replace the u1 -u2 4560 with an OAP 2134 or TLO72.
I replaced the speaker with a 8" hifi woofer. The sound is better than before, better bottom end.
So now I want to get as much "clean headroom" as I can. I'm doing this because I need a small amp for back reasons and old age.

[Jazz P Bass]

Waste of time.

Buy something that will do what you want.

Peavey Rage 158 Transtube Solid State Guitar Amplifier | eBay

[olddawg]

If you like the SS Fender Frontman series. Look for a 212R. I picked one up with the foorswitch for $35 off of CL. Had to fix some bad connections. Looks brand new. Looks like a blackface Twin. 100 watts. Usable reverb. Very gigable. Even the dirty channel isn't awful if you aren't playing metal.

[Enzo]

You are obsessing over 5 watts. If you DOUBLE the output power, the thing will only be 3 decibels louder. 3 decibels is just about enough to hear side by side. Replacing the power supply for a few extra watts is not worth the effort.

You say the data sheet claims 20 watts for the IC, but in your amp, is the heat sink and cooling up to the increased need?

If you just want a little louder, get a new speaker. Other have said that.

I mentioned doubling the power nets you 3db louder. The exact same 3db louder can be had by getting a speaker with a 3db higher efficiency rating.

[Steve A.]

The 15W is measured with a specific maximum THD figure in mind. If you overdrive it I suspect that it would put out the full 20W (is this correct, Enzo?) For more drive you could increase the gain of the preamp.

Is this one from the 90s with a solid back? Nice little practise amp...

Steve Ahola

[OldSolder]

Not sure of the year but it has the closed back.
I put a opa2134 in and a new woofer and I like the sound.
Thinking of putting a 50 watt pwr amp circuit board in and just using the pre amp.
I like the idea of something small with a big sound. Ala mesa.

[Steve A.]

There should be a PR number on the back to help us identify the amp. Do you have a link to the schematic?

Steve Ahola

[OldSolder]

Its a PR241. Mexico model with UPC1188h.

I have the schematic but can't upload it because it's a .pdf file.
There are 2 different schems, the one at Fender has the Tda2050.
The other one has Mexico in the title.
Thanks Steve.

[Enzo]

Steve, I have no idea how they rated it, but I'd assume the IC was good for clean right up til it clips, any distortion coming from the preamp.

Scope the rails, and scope the output run up to clip. See if the signal clipping is limited by the rail voltage or preamp.

[Enzo]

You want it to sound big? Disconnect what is inside it, and connect the thing to a 4x12 cab.

[OldSolder]

Ok gents point taken.

No further mods will be undertaken.

Have just put a layby on a 30watt Kustom acoustic amp.
Might be a bit better.

Thanks.