Upgrading a GA15 Harley Benton

[Mikka]

Hi !

Here is how I upgraded a friend GA15.

It's here.

It's in french but I can answer to your questions.

There are three clips, the first was done before the mods (amp in stock), the second during the tweak and the last after all the mods were done.

Now the amp sound realy nice.

There were some mistakes on the amp conception.
The power supply had not well thinked and caused some hum.
The tube voltage was totaly out of the datasheets.
The resistor cathode BIAS value wasn't correct.
The préamp component values were realy bad and there was no gain pot.
The amp just had three EQ pots and a master pot.
And it was totaly impossible to have some préamp or power amp overdrive.

[sstavrin]

Hi. I am new to the forum. I have also changed the tubes of the amp with JJ Tesla ECC83S and the speaker with eminence the copperhead. The sound improved very much but I want to fix the humming problem. What did you do about that? Can you give me some instructions. I don t know a lot about electronics but I think I can do the mods in my university lab. And unfortunately I don t speake french.

[Mikka]

Hi !

The first hum cause is the magnetic saturation of the PT because the bad power supply structure and value and 'cause the wrong value of the resistor on the cathode BIAS system.

If you just want to change the cathode bias resistor you have to draw the loadline of your amp but it's a hard way because the datasheet don't give technical informations for a so high plate voltage. The EL84 tubes are for 300V max voltage.

The better way IMHO is to do like I propose in my modds.
In th PDF file I give the schematic for the modds.
If you know enough in electronic laws and parts you'll we understand how to do that.

I used a resistor of 330ohms 25W to drop the plate Voltage to around 300V before the first CR filtering stage.
That reduce the voltage pic in the PT and save it of the magnetic saturation and after I biased the power stage with a new cathode resistor of 150Ohms 11W.

It's the first thing to do IMHO.

Nice to help you... you're welcome.

[sstavrin]

thank you for your help...

[Loztboy]

I'm really interested in the gain mod.
Could you explain it in english, step by step?

[Mikka]

Hi,

I can try yes !


But before I would like to say to you that just add a gain pot don't totaly change the gain level. It's a the combination of the entire of the mods that you can find on the PDF (see the link in my first post) that really help to achive a good average of gain.

Here is the way to add the gain pot.

First you have to make a hole to add the gain pot somewhere near the TS pots

Desoldering R4 (1M) and R5(16.9k)

Around R4 -> Replace R4 by a 470k resistor, just solder the connection with C2 and solder on the other strap of R4 a wire to the strap 1 of a 1M log pot.
Solder a wire in place of the free hole of the PCB (it's the grid of V1b) and solder the other end of this wire on the strap 2 of the pot (the slider or cursor). You better do that with a shielded cable.

At last, solder a wire between the ground (one of the R5 connection) and the strap 3 of the pot.

For more gain you can try the same mod but without the 470k resistor.

If you want, you can add a bright cap on your pot.

Hope this helps you !

[Loztboy]

Thanks.
I will try it some day
I just ordered the "Alnicomagnet Mod Kit" I'll check it out before i do the gainpot mod.

[Mikka]

Hej,

Du er en normann !

Hvor er du i Norge ?
Jeg snakker litt bokmål. Min far var norsk.
Det gjorde meg enda mer glad for å hjelpe.

Ha det bra.

[awl67]

Hi !

Here is how I upgraded a friend GA15.

It's here.

Hi,

I'm new on board. Thanks for the modification. I did it this weekend and the GA15 sounds great, I changed also tubes and speaker for Eminence Legend 1058 but i feel it was louder before modification. Can that be true?

best regards,
Adam.

[Structo]

Could be from the new speaker.
If the efficiency rating sesitivity (SPL) is lower, it will take more power to reach the same volume.

[d95err]

EL84s are run well over 300V by most major manufacturers, so that's not really an issue.

[Mikka]

Hi !

Here is how I upgraded a friend GA15.

It's here.

Hi,

I'm new on board. Thanks for the modification. I did it this weekend and the GA15 sounds great, I changed also tubes and speaker for Eminence Legend 1058 but i feel it was louder before modification. Can that be true?



best regards,
Adam.

Hi,

Yes it's true, running power tubes with less voltage get down power with more class A sound result, but in the same time the sensitivity of your new speaker is not particulary high.

[awl67]

Hi,

Yes it's true, running power tubes with less voltage get down power with more class A sound result, but in the same time the sensitivity of your new speaker is not particulary high.

Thanks for quick answer. In a few words, what can I do to get more power?

regards,
Adam.

[awl67]

Could be from the new speaker.
If the efficiency rating sesitivity (SPL) is lower, it will take more power to reach the same volume.

Hi,

Do you suggest the stock China speaker has better performance than Eminence?

[Mikka]

Probably yes.

Hi,

Yes it's true, running power tubes with less voltage get down power with more class A sound result, but in the same time the sensitivity of your new speaker is not particulary high.

Thanks for quick answer. In a few words, what can I do to get more power?

regards,
Adam.

Change for more efficient speaker like Eminence Governor for exemple.