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April 02, 2023, 09:03:31 AM

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Danelectro Nifty Fifty Modification (with schematic)

Started by Dino Boreanaz, March 16, 2023, 07:08:32 PM

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Dino Boreanaz

I've had a Danelectro Nifty Fifty for over 20 years now and recently I hadn't been playing it as much as I used to.  I quite enjoy the tone, but only in the first quarter of the gain (dirty/sweet) control's sweep; above that it becomes too fuzzy and fizzy for my taste.  I also found the volume (level) control to be VERY sensitive to the slightest movement near its minimum setting which is where I use it most as I am an at-home-only player.  So, I looked into making some modifications to make this a more useable and enjoyable amp to play.

The quick summary is I changed the volume pot from linear to logarithmic taper, reduced the maximum gain by changing the value of the gain pot, and changed from symmetrical clipping LEDs to asymmetrical silicon diodes resulting in a more useable amp for my at-home use.

Warning, the following is a MUCH more long-winded explanation of my tinkering:

I replaced the stock 100k linear volume pot with a 100k log pot and this alone did wonders for making the amp more enjoyable at home.  I cannot understand why the stock volume pot is a linear taper rather than a logarithmic (audio taper), but I would recommend this change to anyone wanting to use this amp at home.  I can now smoothly control the volume from off to a reasonable in-home level without fear that the slightest touch of the volume knob will cause it to become either silent or unacceptably loud.

I then went about making the gain control more useable.  I started by creating a schematic of the circuit (attached below) up to the volume control from the board (because I could not find one online) and then modelled the circuit in Micro-Cap to get a better understanding of how the components contribute to the gain and tone.  I found that the gain is primarily controlled by the value of the gain pot (VR1) and the values of R3 and R4.  I also found that if the sum of VR1 and R3 is kept constant, the overall "tone" (shape of the frequency response plots) and minimum gain do not change while the maximum gain is determined by the value of VR1.  This seemed like the right place to start since I only wanted to lower the maximum amount of gain available in order to make more of the knob's sweep useful to me.  The stock values are 100k for VR1, and 10k for R3, so I tried using a 10k VR1 pot and a 100k R3.  This did keep the overall tone the same as well as the same fully clean character at minimum gain while drastically lowering the breakup at maximum gain.  This works fine for me since I only use this amp for clean to cleanish tones.

Another area that I experimented with was the clipping LEDs.  The stock circuit uses two anti-parallel red LEDs (LED1 and LED2) hard-clipping to ground.  I tried replacing these with just about every imaginable combination of different colour LED as well as plain silicon diodes as well as removing the clipping diodes altogether.  I found the stock red-red arrangement did not produce very much breakup and most other LED colours did not change this much, except blue-blue which resulted in very little noticeable breakup even at maximum gain.  I then used the plain silicon diodes and this produced noticeably more breakup.  I tried a variety of asymmetrical arrangements as well and found that my favourite breakup was produced by two silicon diodes in series clipping one half of the wave and no clipping on the other half.  The two silicon diodes in series result in a DMM-measured forward voltage of around 1.1 V (about halfway between that of a single silicon diode and a red LED).

I did also experiment with different values for R3 and R4.  I found that changing R3 raised or lowered the amount of gain (and breakup) without a noticeable change to the overall tone, but I was satisfied with the previously-mentioned 100k in this position.  The gain of the second gain stage is controlled by R4 and here I tried a variety of values from 100k to 2Meg.  The lower values in this position reduced the overall gain and made the tone brighter, while higher values increased the gain and made the tone darker.  In the end, I found the stock value of 680k to be the best compromise for me.

I also tried swapping the 10k VR1 gain pot for a 50k pot in order to get a broader range of gain control and experimented with the value of R3 to again try to find an acceptable compromise between the inherent tone and the overall minimum and maximum gain.  In the end I found myself not using the upper end of the control with the 50k pot, so I went back to the 10k pot.

This involved a lot of back-to-back testing and repetitive playing (apologies to my family), but I've now got something that is much more useful to me.  If anyone is interested, I'd be happy to share the Micro-Cap file if it's of any use or interest to anyone.


Nice work, but I think VR1 must be wired up as a rheostat, like the honeytone here:  https://www.ssguitar.com/index.php?msg=29583  ?

Dino Boreanaz

Thanks g1!

I've attached a photo of the board along with overlays of the traces and components connected to VR1, and I think I've shown it correctly on the schematic.  Please let me know if something needs to be corrected and I'll gladly make the changes.

I just rechecked the connections with a meter and the wiper of VR1 is not directly connected to either of its two outer terminals as it is in blackcorvo's Honeytone schematic.  But again, if anything is amiss or I've misunderstood please do let me know.  I'm not an expert, and I've enjoyed learning what little I do know.


Your drawing is correct.  I haven't seen it done this way before, or I just didn't notice.
Essentially, the value of R3 is being changed as the pot is adjusted.
The part of the pot in the feedback loop is standard.

Dino Boreanaz

Thanks for getting back to me, I was worried that I had missed something as I've never drawn a schematic from a board.

Your description is exactly how I had thought about it while considering my modifications ... part of the pot is in the feedback loop of the first stage controlling its gain while the remainder adds to R3 controlling the signal to the second stage.