Fender Bassman 100, dating and stuff.

[sibo]

Mostly right but it does depend on the overall design of the amp. Most modern high gain amps do distort first in the preamp stage but some designs are setup to distort the power amp stage before the preamp starts to distort.


No. The gain is produced in the tube stage not the volume control potentiometer. The volume pot is there to throttle the full gain to a lower value when you turn it down. Think of it like the accelerator pedal in your car. The engine produces the power to make the car go. Not the pedal. The pedal is just how you control the engine. The volume control in a guitar amp is actually a variable attenuator. It does not "produce" any gain.

Also, good preamp distortion sounds are usually produced by adding extra gain stages that have attenuation and frequency response shaping circuitry between those gain stages.

Interesting! And how would this be accomplished in my case? Would you say I'd be taking on a project that is too difficult for me to pull off? I don't mind spending alot of time reading and learning as I really find this intriguing.

[eschertron]

Also, good preamp distortion sounds are usually produced by adding extra gain stages that have attenuation and frequency response shaping circuitry between those gain stages.

Interesting! And how would this be accomplished in my case? Would you say I'd be taking on a project that is too difficult for me to pull off? I don't mind spending alot of time reading and learning as I really find this intriguing.

Hi, Sibo! I have been a member here for a few months, come from an electronics background but knew very little about vacuum tubes until last year. I have learned a lot, I'm sure you will too. There are some great teachers on these boards. Reading and learning should be a good time!

A thing I've learned about tube amps is that there's been a steady evolution towards more gain stages (roughly equivalent to the number of tubes, not exactly) as guitarists wanted more distortion or more control over the quality of distortion that they got. So in each age, guitarists put pedals in front of their amps to get that sound, then designers modified amps to get that sound with pedals, and the cycle repeats...
Your amp is a good example of the 'first' iteration of the cycle. Don't hesitate to slap any (or all) pedals in front of it to hear what it sounds like. Fact is, for argument's sake, putting a pedal - especially a clean boost as mentioned above - is *just like* adding another tube stage to the preamp. The signal gets beefier right into the input of the amp, and all the preamp tubes get a chance to be overdriven and show their true colors.

Having said that, the amp could be full of tubes that are all tired and a little past their prime. Don't be afraid to pull them out, mark which came from which socket (could be important later), and identify each by number. Do they all match the tube complement sheet? The easiest 'mod' anyone can do is to buy a new tube and swap out some of the old ones to see if there is a tonal or volume difference. Plus is gets your hands dirty with tube technology.

Keep us posted on your progress!

[sibo]

Hi, Sibo! I have been a member here for a few months, come from an electronics background but knew very little about vacuum tubes until last year. I have learned a lot, I'm sure you will too. There are some great teachers on these boards. Reading and learning should be a good time!

A thing I've learned about tube amps is that there's been a steady evolution towards more gain stages (roughly equivalent to the number of tubes, not exactly) as guitarists wanted more distortion or more control over the quality of distortion that they got. So in each age, guitarists put pedals in front of their amps to get that sound, then designers modified amps to get that sound with pedals, and the cycle repeats...
Your amp is a good example of the 'first' iteration of the cycle. Don't hesitate to slap any (or all) pedals in front of it to hear what it sounds like. Fact is, for argument's sake, putting a pedal - especially a clean boost as mentioned above - is *just like* adding another tube stage to the preamp. The signal gets beefier right into the input of the amp, and all the preamp tubes get a chance to be overdriven and show their true colors.

Having said that, the amp could be full of tubes that are all tired and a little past their prime. Don't be afraid to pull them out, mark which came from which socket (could be important later), and identify each by number. Do they all match the tube complement sheet? The easiest 'mod' anyone can do is to buy a new tube and swap out some of the old ones to see if there is a tonal or volume difference. Plus is gets your hands dirty with tube technology.

Keep us posted on your progress!

A thought on that, again from a beginners perspective. Don't you have to re-bias tubes when you change to new ones? I mean if I'm changing I might aswell go for a model with lower headroom, earlier breakup point or whatever. Does the bassman even have bias settings at all?

[eschertron]

A thought on that, again from a beginners perspective. Don't you have to re-bias tubes when you change to new ones? I mean if I'm changing I might aswell go for a model with lower headroom, earlier breakup point or whatever. Does the bassman even have bias settings at all?

Good question! If your amp is indeed the one depicted by the schematic posted by Jazz P in post #2, then it has what they call a 'balance' control instead of a bias LEVEL control. I'm guessing (didn't do the math!) that the amp is biased cold enough that rebias should not be an issue. Someone with more experience might bear this out...

I was thinking about the preamp tubes myself, as that is where the distortion that you're looking for should be generated. As a rule, preamp tubes are cathode-biased and do not need to have their bias checked when replacing. Some amp designs also cathode-bias the power tubes - my 5w champ comes to mind - so they are immune to the need for rebias as well.

Cathode bias does not prevent gearheads from changing resistors to change the bias ANYWAY. Happens all the time. Cathode bias simply means that the topology of the stage allows the tube to "find its bias" by how much current it pulls through the cathode resistor and, per Ohm's law, how much bias voltage is applied to the tube. "Hotter" tube -> pulls more current -> drops more voltage across the cathode resistor -> makes the bias more negative (colder bias) -> reduces the power that the tube is dissipating -> cools down the "Hot" tube. Self-adjusting.

So replace away on the preamp tubes. IIRC the 7025 has just a wee less gain than the 12AX7 commonly used in guitar preamps. If the amp doesn't already have 12AX7s where that tube chart shows 7025s, you might be pleasantly surprised by a swap. It is possible to also put a 12AX7 in the 12AT7 spot. I recommend that you do a thorough search on these boards and others to see what others say about these changes. Know as much as you can about what can happen for even the most minor amp modification.

[sibo]

Okay I think it's time for me to me to show myself in this thread again as I still intend to continue working to improve my sound.

What's happened since a month ago is, I got myself a MXR Micro Amp M133, basically a boost pedal with a single gain knob and footswitch.
This, as Steve Conner suggested really did the trick with adding some grit and dirt to my sound. At the same time I started playing with my other pedals to find something that I find somewhat different than most other guitarists sound like. I am missing one ingredient for some songs though, reverb.
I know this is very easy to get reverb in pedal form aswell but I wanted to check with you guys how hard it is to install a reverb in the bassman head? Is it considered very hard to wire it though a footswitch jack in order to use a footswitch to it?
This is just because I'd rather do it myself than just buy myself a pedal. I like the struggle!

Again, if anyone has any mod schematics or guides to share, I'd be delighted to read.

Thanks.

[Jazz P Bass]

Honestly, it would be best to use a reverb pedal. (easy enough to turn on & off)
A tube reverb will require a tube (duh), a reverb transformer & a reverb tank.
For a solid state reverb you will have to come up with a low voltage power source.

[sibo]

Honestly, it would be best to use a reverb pedal. (easy enough to turn on & off)
A tube reverb will require a tube (duh), a reverb transformer & a reverb tank.
For a solid state reverb you will have to come up with a low voltage power source.

Tube would be neater, I really love fenders vintage reverb sound (I know, should've gone for that from the beginning). However I guess it's hard to fit all those parts in the head? Plus it's probably very expensive? The vintage fender rev is tube though, right?

Adding a transformer for powering a solid state reverb is not a problem for me. I guess that's exacly the same as buying a reverb pedal though.. Except the opportunity to choose where in the loop it should be jacked in.

Conclusion is, I guess - Eighter I do it, and do it right (tube) or I don't do it at all.

[Leo_Gnardo]

Okay I think it's time for me to me to show myself in this thread again as I still intend to continue working to improve my sound.

What's happened since a month ago is, I got myself a MXR Micro Amp M133, basically a boost pedal with a single gain knob and footswitch.
This, as Steve Conner suggested really did the trick with adding some grit and dirt to my sound. At the same time I started playing with my other pedals to find something that I find somewhat different than most other guitarists sound like. I am missing one ingredient for some songs though, reverb.
I know this is very easy to get reverb in pedal form aswell but I wanted to check with you guys how hard it is to install a reverb in the bassman head? Is it considered very hard to wire it though a footswitch jack in order to use a footswitch to it?
This is just because I'd rather do it myself than just buy myself a pedal. I like the struggle!

Again, if anyone has any mod schematics or guides to share, I'd be delighted to read.

Thanks.

Sure thing sibo, got a couple ideas to pitch to you. The "Fender" tone has quite a deep midrange cut, even with the mid turned all the way up, in those amps that have one. It's a 10K pot. Change this to 25K or 50K and you can dial in a lot more mid, and that may get your ears happier. In those Fender preamps that have no mid, I "steal" the #2 input, mount a pot there, and use it to replace the (usually) 6K8 stock mid fixed resistor. Mind, that when you have the mids cranked way up, the treble & bass pots will do less.

Another mod I like to offer, and done hundreds of times, is swap out EQ stack components to match those of the 5F6A Bassman. Marshall "borrowed" this stack and - they did well didn't they? A couple of 0.022 uF caps, a 470 or 500 pF treble cap, reduce the 100K "slope" resistor to 56K or even 33K for a darker tone, and you're off and running. Just do one channel, then you have a choice of colors. Stick a mid control in #2 input if there already isn't one.

There's tons of room inside those 100W bass heads. If you really want to, you can duplicate the Fender reverb circuit. And you can alter the dry/rev mix circuit to add gain & tone to your taste. One little problem is, if you find your tone with the master dialed way down, the reverb could get lost in the sauce.

FWIW the Bassman 100 was a favorite of mine, mid 70's. Didn't own one until many years later, but every time my band played one of the local colleges (SUNY Albany), one of our fans lent me his, and I got a taste for it. Mostly because it HAD a mid control, and I could dial it all the way down. That and a Guild Starfire II bass, and/or Jazz Bass, pair of EV 15's, it was sonic heaven.

[Chuck H]

Since Leo is offering up mods... Altering the mids and tone stack is an easy way for a novice to get into amp modding. The effects are dramatic and gratifying. But that still won't get you into lower volume, natural amp distortion. So I would suggest an attenuator. Along side the mid boost it's the best way I know of to get vintage, cranked amp tone at reasonable volume.

As to the reverb... There is entirely too much credit given to tube and pan reverb circuits. In fact I will go out on a limb and say that most of the recorded reverb you hear for guitar tracks since about 1975 was done with an outboard unit that includes transistor devices. A much easier way (than tube and pan) to add reverb to your head would be a transistor unit with a digital reverb module. It can be added to the amp right before the phase inverter and would have very little affect on the dry tone. One huge advantage to this method is that it's much less sensitive or prone to noise and hum problems. Another advantage is that it would be small enough to manage easily. The tonal difference is really small enough that it becomes apples vs. oranges, not good vs. bad. And I promise it would sound way better than a reverb pedal. Those things sound so bad used at amp inputs that they should stop making them. JM2C

EDIT: Here's another idea!!! Get a small practice amp (if you don't already own one) and add a "speaker driven line out" to your amp. Then you can plug the line out into any reverb unit (even a pedal) and from there into the practice amp giving you ten or twenty watts of reverb. One nice thing about this idea is that you can locate the reverb dedicated amp anywhere! Ambiance at it's best.

EDIT 2: I have a simple resistive attenuator design that is about as effective and affordable as these things get if your interested.

[sibo]

Wow those last two posts really made me google a lot!

First, yes I am now going to go with Leo_Gnardo's idea of changing mid pot and EQ components to the 5F6A types. TYVM for that Leo! A Leo in this thread can't go wrong I suppose haha..
Questions that come to mind concerning those two mods are, would you suggest 25k or 50k mid pot? What type of capacitors should I use for best results? I understand the "slope" resistor alters eq for mid but is it like a special resistor or just a regular one with a given value of resistance? A regular resistor to me is the tiny one with colored rings on it..
Do you have any reliable walkthrough for the eq mod or would you say I can manage it through your supervision in this thread? Know that when I open the amp to mod it it's gonna be the first time I lay eyes on the circuitry. I'm not lazy and want you to do the work for me, I just want to plan things through so that I know what I'm doing, before I do it.

Chuck H, I think you might've nailed it, turns out the attenuator thingy seems to be exactly what I'm after (never heard of it until just now, at least didn't know exactly what it did).
I would be very interested in trying that design, TYVM.

These three projects will be the beginning of my project, the reverb idea will have to wait for its turn. However you got me curious Chuck H, that idea of using a small amp for reverb seems so cool.. I'd love to try it later. Turns out I found my crappy 25W Fender transistor combo amp from when I was like 12 in the garage, might come in handy at some point.

[Chuck H]

You can handle the tone stack mods. And your right about the slope resistor altering the knee frequencies. Using the Marshall cap values and keeping the 100k slope resistor for Fender topographies usually gets good results. I would go with a 50k mid pot. You only get another 4dB of mids between five and ten on the knob compared to a 25k but the point IS a boost, right? Use an audio taper pot and it'll help smooth out the control. An unfortunate side effect of the big mid pot is that it limits how low can turn the bass. So it's possible that with the mid control up high the preamp will have too much bass for good overdrive. This can happen even with the bass knob at zero. So here's another mid boost for Fender type amps that works really well. Replace the treble pot with a push/pull pot. The "push/pull" is a switch. You can use that switch to parallel another capacitor with the treble cap. So, for example, if you have a 470p treble cap and the switch parallels a 250pf cap with it you now have a total of 720pf for the treble cap value. And this lets in more mids, but not the lower frequency muddy mids like the normal mid control. But do both mods so you can jostle between the two and experiment.

You can make a speaker driven line out for your amp very easily. Gimme a day or two and I'll post circuits for both the attenuator and the line out.

[sibo]

20 minutes of typing and the "token expired"..

Anyhow..
The plan is to boost mid, yessir!
After a few more hours of reading and planning I also decided that later on, I will be adding a presence setting. Not the one where you boost the treble but the "real thing" where you supposedly block some feedback from going to the ground and instead ending up in your cabinet and making your ears cry of joy. So far that's hocus-pocus to me but soon it won't be! Thoughts on that?

I tried putting some components together on ELFA, the only Swedish electric component shop I know where you can buy stuff without being a corporation. Turns out they had about half of the components I needed and also they seemed a bit over-priced. For example 50kOhm pot (linear) was a bit over 20€. They didn't even sell push-pull pots bleh..

Is there a point in all the default capacitors being 600V rated when the voltage in is 230V? If so I will have a hard time finding capacitors with good tolerance and some kind of heat tolerance. This being a tube amp that's supposed to be pushed I guess I should aim for about 100 degrees Celsius at least?
Oh and should I aim for any specific type of caps? I mean there are like 6 categories, all with sub-genres. Plastic or ceramic etc..

Any idea of a good supplier that ships to Europe (Sweden)?

[Leo_Gnardo]

20 minutes of typing and the "token expired"..

Anyhow..
The plan is to boost mid, yessir!
After a few more hours of reading and planning I also decided that later on, I will be adding a presence setting. Not the one where you boost the treble but the "real thing" where you supposedly block some feedback from going to the ground and instead ending up in your cabinet and making your ears cry of joy. So far that's hocus-pocus to me but soon it won't be! Thoughts on that?

I tried putting some components together on ELFA, the only Swedish electric component shop I know where you can buy stuff without being a corporation. Turns out they had about half of the components I needed and also they seemed a bit over-priced. For example 50kOhm pot (linear) was a bit over 20€. They didn't even sell push-pull pots bleh..

Is there a point in all the default capacitors being 600V rated when the voltage in is 230V? If so I will have a hard time finding capacitors with good tolerance and some kind of heat tolerance. This being a tube amp that's supposed to be pushed I guess I should aim for about 100 degrees Celsius at least?
Oh and should I aim for any specific type of caps? I mean there are like 6 categories, all with sub-genres. Plastic or ceramic etc..

Any idea of a good supplier that ships to Europe (Sweden)?

I think Mouser has a European presence, which should save you on shipping & duty. I'm a fan of Mallory 150 caps - they have 'em - and house brand pots I think made by Alpha which are perfectly good for what you're doing. I'd pick 50K Audio taper for your mids. I'd select 630VDC rated caps although true, 400V would probably work just fine in your EQ circuits. For a few pennies, some extra insurance against breakdown. The only heat problem I ever had with Mallory 150's was from keeping the soldering iron on 'em for too long. There are also fans of "orange drops", also good, tough to break, I think their manufacturer is now Vishay instead of Sprague.

For a true presence control, I recently worked out a variation on the old 5F6A/Marshall which uses a 1K pot and 4.7 uF film cap. Then you don't have to change the circuit values in a standard Fender. It's the same old circuit with values changed to work with the standard Fender feedback & output drive. Only one thing keeps it from being terrific - wish I could find a revers taper 1K pot then it would be the bees knees. All the "action" is crammed into one end of a linear pot. I s'pose one could use a 1K audio taper pot and reverse the pot connections, then max bright would be all the way down.

Slope R is nothing special, any kind will do fine. The lower you go, the less highs & more mids & lows. Sunn used to put in a slope pot so players could make their own choice on the fly.

Antique Electronics has lots of useful parts and I'm sure can ship to Sweden but it will cost some. There's a good music-gear parts supplier in the UK too, just can't remember their name right now. I'm sure someone will remind us - - - please do!

I'm likin' Chuck's idea of using the SS amp as a reverb. Could also use it as a preamp - then you could get some savage tones out of your Bassman 100 and maybe skip lots of mods on it, save those ideas for another amp that may be a better candidate, or a build from scratch project. Lots of directions you could go. One thing worries me about your old 25W SS Fender - it's possible that the speaker output is "floating" in other words, neither side of the speaker out is at ground, and if you do ground it, kaboom goes the output chip. If there's a line out or FX send, you could use that. Don't discount the idea, it may sound surprisingly good.

[sibo]

For a true presence control, I recently worked out a variation on the old 5F6A/Marshall which uses a 1K pot and 4.7 uF film cap. Then you don't have to change the circuit values in a standard Fender. It's the same old circuit with values changed to work with the standard Fender feedback & output drive. Only one thing keeps it from being terrific - wish I could find a revers taper 1K pot then it would be the bees knees. All the "action" is crammed into one end of a linear pot. I s'pose one could use a 1K audio taper pot and reverse the pot connections, then max bright would be all the way down.

Slope R is nothing special, any kind will do fine. The lower you go, the less highs & more mids & lows. Sunn used to put in a slope pot so players could make their own choice on the fly.

Thanks. Is there any way I could view your schematic on that presence wiring? I don't really understand how to do the presence-control wiring..
Reversing the pot legs and using it "upside-down" is no problem for me. Seems like a good idea to use a pot as slope R as I don't know yet what value sounds good to my ears. Good idea there!

I think I might wanna make one order with all the components so I'll buy the presence components now too and do it when I'm satisfied with the prior stuff.

[Chuck H]

Hey sibo. Sorry for the delay. I've been having trouble with my browser and attachments. Here is a diagram for an attenuator and a line out circuit for your amp.