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Amp minimum input level/sensitivity issue?

Discussion in 'Amp Tech Center' started by Five Aces, Nov 26, 2020.

  1. Five Aces

    Five Aces TDPRI Member

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    I was doing a comparison of a few old SS amps (use for practice mainly but work for small gig/party situations, too) and I noticed something that kinda surprised me.
    I had a Fender Frontman 25R & 65R side by side. For guitars, I used a Fender Standard AmSe Strat (2004), a Gibson LP Studio (2003), and a Gretsch Pro Jet (2015). My choice of guitars was mainly for comparing the amps with the different pickups of choice (Single vs HB vs FilterTron)
    The 65R (quite loud, BTW) was actually great on the clean channel with all my guitars. (Don't care about distortion channel... don't use it!)
    On the other hand, the clean sound with the smaller 25R was real good with the Strat & LP, but it didn't like the Gretsch (FilterTron PUs) as it was noticeably distorted.
    Thinking the 25R had a problem, I actually got a 2nd 25R and the same thing happened!
    OK.... it's possible the speakers in the 25R may be sub-standard so I disconnected the 10" speaker and connected it to the 12" in the 65R for comparison... same problem so it seems to be nothing related to a speaker difference.
    Would it be caused by a difference in input sensitivity between the two amps (25R vs 65R)?
    The only other thing of significance (maybe?) is that the FilterTron PUs ohm out at aprox 4.6k & 4.7K vs my other guitars that measure out greater than 5.8k each on any of their pickups.
    Doubting the pickups are the issue, I'm thinking more like the input circuit of the 25R is significantly different. Is it something that can be fixed or changed? Sensitivity? Impedence?
    Thinking both 25R & 65R amps are the Frontman series, I would think the circuitry would have somewhat similar specs?

    (please, no SS amp sarcasm... just looking for a more technical explaination as to what's going on).
     
  2. EsquireBoy

    EsquireBoy Tele-Afflicted

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    IME impedance shifts the resonant peak of your guitar circuit, but I do not see how it could cause distortion as you experienced.

    The lower impedance of Filtertrons does not mean these pickups have low output. The Filtertrons you tested may have a higher output than your LP humbuckers.
    Maybe the pickups are adjusted closer to the strings on that guitar too. Anyway I would think the distortion you heard with the smaller amp comes from overdriving the amp, and not on the bigger one because it’s got more headroom.
     
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The input resistor is larger on the 65R. Probably easier to turn down the guitar volume than to muck about changing the resistor in the 25R.

    Input resistor on 25R = 15k.

    The two jacks of the 65R have a different circuit than the 25R. One jack would have an equivalent resistance of 65k. The other jack would be different than the 25R circuit arrangement with two 130k resistors making a voltage divider, dropping the voltage by 50%.

    Disclaimer: The schematic for the 65R I found on the internet is very small and difficult to read. I am assuming the resistors are 130k.
     
  4. Five Aces

    Five Aces TDPRI Member

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    You are correct....65R has 130K resistors on the input configured such that input#2 is aprox 6db less for use with active or higher output pickups (as per the manual). For comparison, I was only using input #1 on the 65R.
    Would make sense about the possibility the FilterTrons are higher in output. They like to be close to the strings! But cutting the volume on the guitar to at least half , I can still detect a lack of clarity from the 25R....
    So, as an added experiment, I fed the preamp out from the 65R into the Aux.In of the 25R and wow! ...nice and clean! And might I add, that 25R puts out serious volume for its size!!!
    Summary & Question.... would it be that simple a fix to just change the 25R's input resistor to 65K or greater? Its easy to do and I can handle that easily as long as it doesn't drastically effect anything else. Would that mod also effectively give it more headroom? That 25R has more than enough power to handle the input cut IMO...
     
  5. Rockhead

    Rockhead Tele-Meister

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    I have a Frontman 25 and a Gretsch 5220 with Broadtron pickups and although they are 4.6, 4.7ohms they are hotter than my Tele with GFS mean 90's. I didn't have the Gretsch before I swapped the original speaker with a Celestian 10-30 but I don't hear any distortion playing clean. With my OD pedal I have to turn the gain down significantly compared to my Tele's.
     
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    We would be making an assumption that the rest of the preamp gain structure is the same in the two amps. So I will say there is a good possibility reducing the signal voltage with that resistor will work.
    I would either try a 130k like in the 65R amp or put a 500k pot temporarily replacing the resistor. Dial in the resistance that works, then measure the resistance of the pot and use a resistor of that value as a permanent replacement.
    The perceived headroom might be different because the volume knob would be pointing at a higher number. In actuality it should attain the same volume before breakup.

    The 65R schematic I have found on the internet is small and not easy to read so making comparisons between the circuits of the 25R and 65R is difficult.
     
  7. Five Aces

    Five Aces TDPRI Member

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    Well, another comparison that may shed light.....
    I got my 5120 back from my other place (its upgraded with BT FilterTrons like whats in my Pro Jet) so i gave it a try. It is also, but to a lesser degree, has audible distortion, too.
    So it seems clear that the common factor has to do with the input drive level. And Gretsch owners will tell you the FilterTrons need to be close to the strings to wake up the sound. So this gives me something to work with...... I think I'll experiment with changing the input resistor in the 25R to either 65K or 130K and compare the results. If the results are positive, it would make the 25R more versatile with a variety of guitars. Ya think?
     
  8. PhredE

    PhredE Tele-Afflicted

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    I used to have one of the predecessors to the Frontman 65R -- the Princeton 112+ (reverb, 2 channels, effects loop, 65WRMS, etc). It may not be identical to the FMan65R, but is very similar (but had only 1 input, IIRC) ..Thought I might include a link to a schematic in case that helps you all out:

    No affiliation nor knowledge of the site, but was glad I could find the schematic out there on the net somewhere..

    http://www.electronicstudio.net/schematics/Fender/Princeton_112_Plus_Sche3BB.pdf
    (great clean channel on that thing FWIW)
     
  9. Five Aces

    Five Aces TDPRI Member

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    Thanks.... I took a look and on initial observation the input circuit of the 112 schematic look identical to the 65R except the 112 had 33K on the input as compared to the 65R that has 65K (by using 2ea. 130K which is R1 & R4 in Parallel) on the primary (#1) input jack and 130K (R4) on the (#2) input jack (meant to lower the drive for active pickups).
    The 25R input schematic is also identical to the 65R with the exception it uses a 15K at the input.
    So it seems obvious that the input drive level of the 25R can be reduced by swapping a 65K resistor (more or less) in place of the 15K resistor and should yield positive results. I know the 25R is putting out more than enough of its rated 25Watts where it shouldn't hurt it at all.... hey, its a practice amp after all. Now, just got to try & find time to do the mod and prove it! I'll let you know how it turns out when I get the time to try it!
     
    Last edited: Nov 27, 2020
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  10. Five Aces

    Five Aces TDPRI Member

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    OK... hopefully this may be good info for others having similar issues as this.....
    My evaluations and experiments are done and as it turns out, there are several options and ideas that can be done, depending on ones' individual issues.

    #1) To reduce the input drive level, the 15K resistor on the 25R input can be easily accessed and replaced. I choose a 33K because it drops the aprox. input level aprox. 6db (just trying to be consistance like the intent of the 2nd input jack on many of the other modern Fender SS amps). Unless you do a side-by-side A/B comparison, you probably won't even notice it once it's done. The 25R's can take it & pack a pretty good punch and you're hardly cutting into you max output! What you may notice is a little more controled volume at the 0-2 vol. range because of the resistor reducing the input level. It also helps in raising the vol level before distortion (clipping) starts. If you want to have the option to switch resistor values (therefore switching input sensitivity level) you could either mount a switch or a 2nd input jack on the rear panel. Some SS amps (like my G-DEC Series amps) did just that & had the 2nd input jack on the rear. it's a rather easy mod.

    *Note: Although reducing the guitar volume pot will reduce the input level to the amp, its not quite the same results as the resistor change in the amp. Depending what caps you may have on the guitars circuitry pots and how they interact with the tone pot.
    (I felt the amp mod produced consistancy for me without changing the characteristics of my guitars. You may feel otherwise.)

    #2) I had the luxury of having 2 identical 25R's so I could compare results side by side. So it made it easier to make changes and compare.
    One thing I didn't expect (or realize) that the FilterTron pickups in my Gretsch guitar can indeed be hotter. And if you set them up close to the strings like Gretsch players (and myself) all do to "wake them up!" you will get alot of drive out of them! That is enough to cross into that slight distortion level. If you're running anything in line other than a clean input , you likely may never really notice it (as in dark rock / metalheads).
    I found that lowering my BT FilterTron pickups by aprox. 2 to 3/64' of an inch, and then raising the individual poles by the same amount, will keep the output balanced but also gave a more distinct clarity to each string. So my original thought of denying that pickup height was a problem was definately flawed to a point. But in my defense, my 65R amp didn't have the problem so I tended to focus on the 25R as the sole cause of the problem. My bad!

    A final note:

    1) This supports the fact that the matchup of guitars & amps is as individual as the players using them. What works for me doesn't necessarily apply to all. Remember that when you comment on others!

    2) Sound is truly subjective because we all interpret AND communicate what we hear differently. So again, what I defined as a bad sound may be desired by others. So discussions of what sounds good or bad can indeed get lost in the process.
     
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