Inside the Fender Greta (thread II)

[LightningPhil]

The Inside the Fender Greta thread was rather interesting - since I have one of these - obviously...

Attached is a schematic found online and comes from ToneBuster.org - thanks for drawing it up.

A modified version is included also. The goal was to stop it sounding like a fart machine.

To achieve this, bass gain was limited everywhere and rolled off a little on the input and between stages.

The result works well enough to stop meddling with the electronics. Tone goes from clean and rather quiet up to quite silly and surprisingly loud levels of saturation - with very little bass, but not a disappointing absence, considering the size of the amp.

 

[LightningPhil]

The case was modified with a handle - why this came without one I don't know. The main purpose of the amp is to pick it up and slap it on a table. A handle really helps there...

 

[LightningPhil]

And the final mod - replaced the speaker with an 8 ohm Visaton FR12. Took a bit of hunting to find something slim enough to fit.

The original speaker had a resonance above 200 Hz!! Way too high. This one is below 100, where it should be for a guitar.

To improve the loundess and mechanically roll off the bass a bit, a backing magnet was added. This decreases the electrical Q of the speaker, making anything around resonance a little quieter - and, due to the increased magnetic field through the voice coil, increases mid and high output a little.

It didn't do much, but was noticeable enough to leave the magnet there. After-all every little helps. Note - installing the magnet the wrong way around has the opposite effect - a little more bass and less mid range and high. You'll want the magnet opposing (pushing away) the back of the speaker.

Here's a link for the speaker:
(link removed)

And a link to purchase it. It's not expensive:
https://cpc.farnell.com/visaton/fr12-2061/speaker-full-range-130mm-40w-8ohm/dp/LS01672

The magnet was a 20mm thick, 25mm diameter neodymium found on eBay. Cost ~£3.50 ish.

To fit the speaker in, a pplastic bracket had to be 3D printed. I've lost the file as this bit was done a year or so ago. Wasn't hard though and anyone with a 3D printer can draw a round disk with a chunk cut out and a couple of screw holes. Card or thin wood would also work.

Lastly, and very importantly, foam was squished between the speaker frame and the grill to stop it vibrating and making everything sound like a tin of wasps.

 

[Shuster]

Brother, that's pretty good at your rip old age

 

[LightningPhil]

Now realise I should have replaced R23 with a short - in order to make the gain mods function as designed... Where is the screwdriver?

Schematic above has been edited to reflect this correction.

 

[Scottz]

It would be great if you could post a sound file of the new configuration.

 

[Wally]

Welcome, Phil....

 

[JustABluesGuy]

Those are some pretty drastic mods there!

 

[JustABluesGuy]

What’s the reason for the speaker bracket?

 

[LightningPhil]

It would be great if you could post a sound file of the new configuration.

Lack of recording gear. Will see what I can bodge together over the next could of days...

 

[LightningPhil]

What’s the reason for the speaker bracket?

The speaker is a little bigger than the original. Which is great as bigger is better and whatnot.

However, the moving part is larger than the original hole and the hole is covered with the metal mesh. That would cause the cone to hit it. The bracket is more a spacer than much else.

 

[LightningPhil]

Those are some pretty drastic mods there!

Yup. Caught myself modding my AC30 and decided to mess with something less expensive first.

The AC30 has very similar cathode bias mods to smooth out the distortion. Essentially the top boost channel is much more of a high pass channel - as it actually amplifies the bass frequencies more than the treble - then bins off the bass frequencies. Seemed silly, so I reduced the gain there. Result is a bit of a tighter sound with a smooth increase in gain going up the notes. Not finished meddling with it yet. Can post about that later too.

 

[LightningPhil]

In lieu of a sound clip - here's a couple of plots.

They were generated using LTspice. http://www.duncanamps.com is well worth a look should anyone wish to have a go at modelling a valve amp.

Seems I can't upload the files for this project - "file extension not allowed". (why can't I upload virus.mc.virusface.exe?)

Plots have the same scale and the frequency range is from 84Hz to 5kHz - which I consider the area of interest for a guitar that's not in a drop tuning.

The first plot is from a simulation of the stock amp with volume and tone half way (restively - they're 500k pots, so a 250k, 250k split). Since they're analogue pots, this represents a turned up most of the way position.

Anyhow, you can see it's a fairly flat response - raising about 2dB over the fundamental range of frequencies on the fret-board.

The second is the modified gain curve (from guitar to speaker -measured in exactly the same place). It's not flat at all... The top remains the same, but through the range of notes, it increases in gain by about 13 dB going from the lowest to the highest note on the guitar.

Whether it's the best gain curve for the amp and speaker - who knows? It did make me play with it rather than leaving it on the shelf. And the farty low notes which put me off have been eliminated.

Running it through a pair of 12" Celestion Golds was loud - surprisingly so for an amp that has a 12AT7 output tube. They quite accentuated the highs! The built in speaker is quite sufficient for messing about with though.

 

[LightningPhil]

Right, should have been 82.4 Hz not 84 Hz...

 

[a wobblin]

Very impressive modifications. I am assuming as a practice amp you no longer need an external cab ?
How much of the 'farty' sound is/was down to the speaker and how much to the electronic circuitry ?

 

[LightningPhil]

Sorry for late reply.

It's a tiny amp and the desire to turn it up to make up for its size is huge. This meant that it was distorting in the gain stages. Reducing the size of the cathode biasing capacitor reduced the gain of the bass but leaves the highs alone. That way it can sound loud, but distort the bass less and thus be less farty. The original speaker hid much of it as its resonance was so high that bass simply didn't exist. Now it at lease has a hint of bass. More than before and without any notes sounding pronounced.

It's an amusing little amp and fun. Though usually use my also modded VOX AC30C2 for practice. Plays clean nicely, but once in a while I make use of living in a detached house with really old deaf neighbours - then it shines and rattles the windows.

The best thing about the old deaf neighbours not hearing my noodling is - while I may have many years of experience with electronics, I've only been playing a guitar for a few.

 

[Wally]

Observation... I like to have not just a treble cut pot on my guitar but also a bass roll off pot. That allows me to have full range of iciness when playing clean and also to be able to remove low end when driving an amp hard. This will prevent that loose low end mid from happening in grainy situations....and it allows for versatility with fingertip control on the guitar.

 

[radiocaster]

Observation... I like to have not just a tell cut pot on my guitar but also a bass roll off pot. That allows me to have full range of iciness when playing clean and also to be able to remove low end when driving an amp hard. This will prevent that loose low end mid from happening in grainy situations....and it allows for versatility with fingertip control on the guitar.

The more you turn the tone knob to the right, the more bass you cut and the more treble you add. I noticed because I usually play recorded music through it, not guitar. Also I use a speaker meant for an old tube radio, and I sometimes need to cut the bass because it won't handle too much.

 

[arielyitus]

Observation... I like to have not just a treble cut pot on my guitar but also a bass roll off pot. That allows me to have full range of iciness when playing clean and also to be able to remove low end when driving an amp hard. This will prevent that loose low end mid from happening in grainy situations....and it allows for versatility with fingertip control on the guitar.

Hi Wally, I had never thought on the bass rolloff from the guitar, what a great idea. I've been using an eq pedal just to reduce bass for my 5e3, do you have any recommended circuits/values?

(Sorry to OP for using this thread)

Thanks

 

[Tele5284]

Hey I just bought one of these amps, and I’m interested in upgrading the tubes. Any suggestions? Thanks.