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Messages - 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.
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.
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.
Amplifier Discussion / Re: 12w Marshall
January 19, 2023, 01:14:07 PM
Hey guzzis3, I'm glad you made it over here!  Lots of amazing, knowledgeable, and helpful people here.  Enjoy!
Great now I've got more work to do ... just when I thought I was done tinkering!  This actually seems like a fantastic option to make the taper more gradual at the low end.  I will have to try this and I'll reply with my results when I do.
Well it's been quite a while since I first posted this topic and I just wanted to provide an update for the sake of completeness.

I did try a few different values of audio taper pots to replace the stock linear pots and I found that they made the volume controls slightly less sensitive at the low end of their rotation as compared to the stock linear pots.  It was not as dramatic a change as I had hoped it would be, so I also tried using a reverse audio taper pot and this did not work well at all (being more sensitive at the low end of the rotation ... as it should).

Since the different values did not have any noticeable affect on the behaviour of the volume controls, I stuck with the stock value of 4.7k but used the audio taper pots in both the treble volume and bass volume controls.

I should also note that I am running this amp into a 16 ohm load rather than the rated 4 ohm load.  So, according to Marshall's senior service engineer, the amp should be putting out approximately 75 watts.  Even at this power level, I find that I have both volume controls set at (or slightly below) 1 out of 10 on the dial which still makes it quite sensitive to small changes.  But I have found a balance that I'm happy with and have no plans to make further changes at this point.  I will post a new topic if I change my mind and decide to start tweaking things again.

Thanks again to everyone who contributed their time, knowledge, and ideas.
Thanks Tassie ... very much appreciate your input!

I have decided to go the route of trying the log pots for the treble and bass volume controls.  You are correct, the pots are the 24mm Alphas.  I've ordered the three different values that were available in log taper (4.7k stock value, along with 22k, and 100k).  Based on the results of the circuit modelling, it seems there's nothing to loose by trying the 100k pots first.

I love the idea of the push-pull, but I'm going to keep this simple.  I'll be very happy if I can just get some finer control at low volume levels.
Hey Phil,

Thank you so much!  My understanding is improving slowly, but steadily.  If you'll indulge a few more, somewhat rambling, lengthy questions ... if not, I totally understand and thank you again for everything you've shared.

I tried modelling a few different potentiometer values (4.7k, 47k, and 470k) to gain additional understanding and found that the higher value pots produce the same range of sweep when the upper and lower limits are treated as a percentage of pot value.  For example, the total gain range is the same for the 4.7k pot swept from 5 ohms to 4.7k ohms as compared to the 470k pot swept from 500 ohms to 470k ohms.  This is as I would have expected since the voltage divider should operate the same as long as the proportions remain the same regardless of the absolute values.  Am i correct here?

One thing I did not expect is that the higher pot values seemed to have a slightly stronger low-bass response (from about 40 Hz to 200 Hz) as compared to the lower pot values.  See the attached "Overlay 1" where the blue traces are 4.7k pots, the green are 47k pots, and the red are 470k pots.  The plots are essentially identical above 400 Hz, but show that the higher value pots provide more bass extension at corresponding settings.  Is it correct to assume that this is due to the higher resistance to ground of the high-value pots is allowing more low frequency content to pass through rather than bleed off to ground?

Lastly, I tried modelling the same three pot values swept from an initial resistance value of 5 ohms up to their respective maximums (5 ohms to 4.7k, 5 ohms to 47k, and 5 ohms to 470k).  In these plots I did see that the higher pot values indeed provide greater range at the quiet end of their sweep, as shown in the "Overlay 2" attachment.  This again is as I would have expected, but my question is which sweep is more representative of what a real potentiometer would produce?  In other words is the lowest attainable resistance of a potentiometer some particular value regardless of the pot's maximum value or is it a percentage of the pot's maximum value?  Or (as I fear) does it vary from pot to pot and manufacturer to manufacturer meaning that this has all been an exercise with no practical value?!

Thank you again!
I very much appreciate everyone's input on this as I'm quite new to the world of modifications.  I have a couple of questions for those with more experience that myself (which is just about everyone!):

As my end goal is to make the volume controls less sensitive at very low settings, is there something preferable about adding resistance (either fixed or a pot) before the power amp versus changing the volume pots from linear taper to log taper?

I have found volume settings that I am quite happy with, but I would really like to have finer control in this area and it seems like log taper pots would accomplish this without adding components or making permanent modifications.

If I may ask another question to further my understanding of this circuit:  It seems like the two volume pots (VR10 and VR11) are not located between their op-amp's input pins and output pins, and so should not affect the gain of these op-amp stages.  Is it correct to say that these two volume pots control the signal passed to the power amp as voltage dividers rather than by altering the gain (and therefore clipping behaviour) of the op-amps?

Thank you again for all your input and insight.
Thanks Tassie, that's a nice solution in a single cable.  If the volume control in the effects loop had worked out, this would have made a very neat and tidy installation.  Unfortunately, since the boost and volume controls are after the effects loop in this circuit, this didn't work out as well as it would have in a circuit where the effects loop is located after all the preamp controls.

Earlier today I remembered that I had modeled this preamp circuit in Micro-Cap so I took a look at it again.  I used the stepping analysis to model the effect of sweeping both volume controls from 0 to 100% in 10% steps using both linear and logarithmic tapers.  I've attached the plots and it seems the linear taper produces a huge jump in the first 10% of the pot's rotation, where the log taper uses over 70% of the pot's rotation to cover the same range.

I don't have much experience using Micro-Cap, so I'd be happy to share the file with anyone who's more familiar with it and could verify whether my analysis is valid or find any mistakes in my use of the AC analysis tools or the model itself.

I've written to an engineer at Marshall to get their input on whether swapping from linear to logarithmic taper pots would make the low end of the volume controls less sensitive to small movements.  I'll post an update if/when I get a response.
Thanks Will!

I probably should have clarified in my original post, that while I would like to reduce the volume (or have better control over the volume at the quiet end), I DO want the preamp to be pushed into clipping.  I like the tone and overdrive grit that I'm getting with the first gain control at maximum and most of the EQ knobs boosted to about 3 o'clock.  So, while lowering the gain of the first stage would certainly reduce the overall volume, it would also not overdrive as much and that's less desirable to me than the too-sensitive volume controls.

I think I'm going to order the log pots for the volume controls and see how that works out.  I'll post back here if/when I try it.
Yup, my dumb mistake.  I realized after reading the first few replies that the transformer is in fact different between the 100W and 200W models.

Today I tried the "pot-in-a-box" in the effects loop, but it is quite finicky and also affects the way the boost control responds and hence the tone.  This amp doesn't have a conventional single master volume control.  It has separate treble volume and bass volume with a variable crossover.

I've attached the preamp schematic, and it seems there are three gain stages and their corresponding controls (boost, treble volume, and bass volume) after the effects loop.  Would it be correct to say that the placement of the effects loop within this circuit limits the effectiveness of the "pot-in-a-box" to control the overall output volume and causes it to affect other aspects of the amp's tone?

I also noticed that the treble and bass volume pots (VR10 and VR11) are linear taper rather than audio taper.  Would this tend to make them more sensitive at the lower end of their rotation as compared to audio taper pots?  If so, would it be worthwhile swapping these for audio taper pots?  I suppose this thread has become more about managing the quiet end of the amp's volume range rather than reducing the amp's power output.

Thanks again to everyone for your input.
I'm thinking about building a simple DIY volume control to put in the effects loop rather than buying a more complex (and expensive) volume pedal.  Is there an ideal potentiometer value if I go with a simple input->pot->output arrangement?  Any drawbacks to such a simple circuit?

Thanks again for talking me down earlier!
Thanks for the replies all!  This is the first amp I've owned with an effects loop ... never even occurred to me to try a volume pedal in the loop.
I've got a Marshall 3520 Integrated Bass System 200W head that I really like, but I'm finding that it is too loud for home use.  The volume controls (separate treble and bass volumes) are barely above zero and the slightest movement takes it from silent to way-too-loud.

Can I reduce the output power in half by eliminating two of the four output MOSFETs in order to make the power amp like that of the 100W 3510 model?  I've attached power amp schematics of these two models and encircled the differences between the 3510 and the 3520.

If so, would this be as simple as desoldering one leg of R17 and R22 while leaving everything else in place and undisturbed?