Volume Pot Mod Question

[jake155]

Hi folks,
I'm a newbie here (first post) and found this forum while doing a google search. The original thread I found is
https://tubeamparchive.com/viewtopic.ph ... 5498b6d392

I've decided to try adding a resistor to the volume pot on an old Fender Frontman 25R (SS amp). The volume gets too loud right around "1" and it's way more volume than I need. Can anyone predict or calculate what will happen with the power and speaker interaction after the mod?

I'm going to put a 22K 1/2Watt resistor between the middle and ground terminals on a 50K audio taper pot. I figured I'd start by going half the pot value and seeing how it works out.

The amp is 25W RMS (75 peak) with a matching speaker. So here are my questions for anyone who can and is willing to answer.

1. This will simply adjust the response of the volume pot to the point where the pot position matches the resistor, at which point it will act as if it's not there, right?

2. If #1 is Yes, would I be able to install a lower Watt speaker if I keep the volume level low?

3. If I install a 25K volume pot, would that keep the amp from reaching its maximum power rating?

Thanks in advance for any responses. As you can probably tell, my understanding of amp design is quite low.

[Sonny ReVerb]

First off, welcome!!

Sounds like you might want a VVR (power scaling) setup instead. Search for VVR.

[Or an attenuator or a smaller amp.]

This explains pot modification pretty thoroughly:

http://www.geofex.com/article_folders/p ... tscret.htm

1> Basically, yes. The extra resistor just changes the sweep. You can do a simple plot, if you do the calculations, to see the effect. Full volume will still be full volume.

2> You might try a less efficient speaker to lower the apparent volume (lower dB). You still need to allow for the full wattage of the amp.

3> Lowering the volume pot may load the signal more, but it may cause unwanted consequences. Others can probably address this better than I.

[PCollen]

Hi folks,
I'm a newbie here (first post) and found this forum while doing a google search. The original thread I found is
https://tubeamparchive.com/viewtopic.ph ... 5498b6d392

I've decided to try adding a resistor to the volume pot on an old Fender Frontman 25R (SS amp). The volume gets too loud right around "1" and it's way more volume than I need. Can anyone predict or calculate what will happen with the power and speaker interaction after the mod?

I'm going to put a 22K 1/2Watt resistor between the middle and ground terminals on a 50K audio taper pot. I figured I'd start by going half the pot value and seeing how it works out.

The amp is 25W RMS (75 peak) with a matching speaker. So here are my questions for anyone who can and is willing to answer.

1. This will simply adjust the response of the volume pot to the point where the pot position matches the resistor, at which point it will act as if it's not there, right?

2. If #1 is Yes, would I be able to install a lower Watt speaker if I keep the volume level low?

3. If I install a 25K volume pot, would that keep the amp from reaching its maximum power rating?

Thanks in advance for any responses. As you can probably tell, my understanding of amp design is quite low.

IF the amp is to loud at the beginning of the Vol control travel (0-4K ohms), then the 22K pot is not going to make any difference. It won't make much of a difference until you get to around 5K and above. If the 22K is 10X the parallel pot resistance value (wiper to ground) it will have little influence on the effective resistance value. At 5K , you will have an effective resistance of 4K under the upper 45K of the pot. At 10K it will be 6.875K under 40K , and at 22K it will be 11K under 28K. As you turn the pot up, the total resistive load on the prior stage will lessen and you will load down the prior stage. You may or may not like the result at higher volumes. Are you sure it's an audio pot volume control. If so, could it be a reverse audio, or perhaps wired backwards. 50K is pretty low for a volume pot. What amp is it ?

[labb]

I, too, want to see how this is answered. I just finished a clone of a Fender 5B3 and at about 1 1/2 on the volume knob it is really loud. Seems as though most of volume comes in immediately when you crack the volume knob.

[jake155]

Thanks for the replies.

The amp is a Fender Frontman 25R solid state (1st series). The volume pot is designated 50k 15A. My understanding is that the 15A represents the particular log plot of the pot, which is probably reverse log since the amp reaches high volume by about 3. In fact, I remember the original schematic stating that the amp was rated 25W at volume "3", though I no longer have it.

I'm looking for a cheap and easy fix for this. I've considered changing the speaker to a less efficient one but the cost doesn't work out. A new speaker will run around $40, which is about what I can get by selling the amp. New Frontman 15G or 15R (which are within usable volume level for me) run about $60-$80, and about $30 used. The resistor mod is about $1 and a possible volume pot replacement is about $6.

I don't want to get into adding extra parts like power attenuators or changing out the power supply since this is beyond my level of understanding of circuits and whatnot. This mod is a last option I'm considering before selling it. It's built like a tank and is super reliable for being 13 years old, so I'd like to hang on to it if possible but the volume issue makes it unusable at this point.

[Jana]

Buy 8 - 1 ohm / 10 watt resistors.

Wire them in series.

Connect the entire string to the amp.

Connect the speaker to one of the "taps" on the string of resistors.

It's a 25 watt solid state amp, it's not like you need a THD hotplate for it or anything of that quality.

Or, if you really want to hack around in the amp, get a pot with a pull switch on it. Same value pot as the original. 50K, right? Then, put a 220K to 470K resistor (1/2 watt) between the "hot" terminal of the pot and the original signal wire going into the pot. Wire the switch on the pot so that it jumpers the new resistor you just installed. With the switch activated the resistor is jumpered and it's like it isn't even there. Normal amp.

With the switch open, then the pot is like a 220K to 470k pot with all it's adjustment range in the first 50K.

[jjman]

I knew someone who had a SS Fender amp and had the same issue with the volume on the driver channel. It acted like the pot was linear instead of the correct log/audio taper.

[tubeswell]

Blurb on changing pot tapers with resistors (Note that the value of the overall resistance changes, as well as the taper of the pot)

[jake155]

I've been rereading the posts here and I'm confused about a couple of responses.

3> Lowering the volume pot may load the signal more, but it may cause unwanted consequences. Others can probably address this better than I.

What does it mean to "load the signal more"? Will it cause distortion?

Buy 8 - 1 ohm / 10 watt resistors.

Wire them in series.

Connect the entire string to the amp.

Connect the speaker to one of the "taps" on the string of resistors.

Where on the amp would I connect the resistor string? Wouldn't this change the output to 16 ohms? The speaker is 8 ohms.

I knew someone who had a SS Fender amp and had the same issue with the volume on the driver channel. It acted like the pot was linear instead of the correct log/audio taper.

A lot of online reviews for this amp say the volume is overly sensitive. The ones who don't complain tend to play at ear-ringing volumes at home. I believe the amp is set up this way to maintain that classic Fender clean sound and then allow for some overdrive as it's turned up. The manual has suggested settings and the Clean volume is always at 3. I really suspect it's a reverse log pot. The drive channel has very little overdrive range before going into crunchy distortion. The only linear pots (according to the schematic) are the Mid and Reverb. The Gain and Drive Volume are both 100K 30C audio pots, likely reverse log.

Blurb on changing pot tapers with resistors (Note that the value of the overall resistance changes, as well as the taper of the pot)

Any adverse effects? I just want to make the volume pot react slower as I turn it up.

[Sonny ReVerb]

I've been rereading the posts here and I'm confused about a couple of responses.

3> Lowering the volume pot may load the signal more, but it may cause unwanted consequences. Others can probably address this better than I.

What does it mean to "load the signal more"? Will it cause distortion

You would pick my area of weakest knowledge (impedance)

AFAIK - Generally, you want to drive a high impedance load with a low impedance source. The load here is the volume control. The source is the signal from the previous stage (probably tone stack). The impedance of the volume control roughly equates to its resistance (varies with AC tho).

The rest, I don't understand but I know the effect. As you lower the volume pot, the signal becomes 'loaded down' and at some point, loses its ability to provide enough signal to drive the following stage. This would be the opposite of 'overdrive'.

As you increase the value of the volume pot above a certain point, you won't notice much difference in perceived overall volume. But lowering the value of the pot will load the signal more and more. That may be worth a try for your desired result, but I believe it can cause other problems such as loss of dynamics, etc.

Hopefully, someone can correct my misunderstandings...

[jake155]

Thanks Sonny.

I think the best option is to try the resistor and leave everything stock since the amp is probably optimized for the parts already used. I can always easily remove the resistor if it doesn't work.