Tales From The Tone Lounge; Bassman Modificationshttp://tone-lizard.com/Bassman_Modifications.htm Fender Bassman 'heads' came in many colors, and circuits! here are a plethora of Bassman 'heads' out there, and just as many circuit variations. This leads to some poor soul complaining that his Bassman doesn't sound like the one used by his favorite guitar player. What is a fellow to do? Well, by following this guide, you can decide which Bassman head is best suited to you. The others can easily be modified to do what you ask, but the vintage re-sale value is greatly decreased. Remember, the vintage 'community' doesn't give a rat's ass what the amplifier sounds like; it strictly comes down to 'is it original'? Therefore, we want to modify the chassis as little as humanly possible, and save the replaced resistors and capacitors in a little baggie. Should you decide to fleece some other sucker via eBay with your 'original' Bassman head, it's easier to do if the 'old' parts are still around. Of course there are accepted mods (or 'upgrades') that are absolutely not going to ruin the vintage value of your Bassman head. Changing to a 3-prong AC cord is pretty much an expected upgrade or mod, as is replacing the filter capacitors. Changing the coupling capacitors to Orange DropsTM or some other boutique brand is also accepted, and often adds to the hyperbole in eBay auctions. Anything else (i.e adding a 'Soul Control' to the back of the chassis) is open to the whims and fads as set out by the vintage community. Your mileage may definitely vary. 1 of 7 1/18/2012 10:51 AM post 1976. I'll remind you again. The most common seems to be the AB165 (they were issued for almost twenty years). Odd 3rd gain stage common to both channels. As above. Odd 3rd gain stage common to both 'channels'. as we'll see in a minute. and these are not great for guitar players. Different circuits.htm A BRIEF CHART ere we'll have a little chart with the various Bassman circuits to be found today. Very odd inverse feedback circuit. I have worked on many AB165 Bassman heads (and personally own one) that 'feature' the AA165 tube chart. Keep this in mind as well. Also. with more local feedback than AA270. Bassman Circuit 6G6 (1961-1962) 6G6A (1962-1964) 6G6B AA270 (1968-1983) Tube Complement Notes Odd circuits aplenty. and we'll need to dig deeper to find out what we really have. 'Bass' channel has extra gain stage.com/Bassman_Modifications. See AA270. Most web sites will have us dating our Bassman head to the hour it came off of the assembly line from external appearances. starting about 1977. This means transformers. awful for guitar. Inverse feedback circuit boosts bass frequencies. not great for guitar. This would give the least amount of 'work' involved in order to have a very good guitar amplifier. make for unnecessary confusion. with linear-taper 'Volume' control in 'Bass' channel. DC coupled first gain stages. You must replace them before putting the amplifier up for resale. 4 x 7025 2 x 5881 As above. Increased output power. Very close to 'classic' circuit. with slightly different tone controls in 'Bass' channel. many classic Bassman derivatives and today's boutique amplifiers all follow the 5F6 circuitry. Tapped 'Treble' control in 'Normal' channel.Tales From The Tone Lounge. and what we can do to 'improve' them. whilst none I can name follow the AB165 circuit verbatim. 2 of 7 1/18/2012 10:51 AM . 3 x 7025. yet it can still be referred to as 'AB165' (as an example). First up is the input of the 6G6A. Local feedback at output stage. High amount of inverse feedback. and filtering circuits will change. 1 x 12AT7 2 x 6L6GC AA371 (1968-1983) AA864 (1964-1967) AB165 (1964-1983) AC568 (1968-1983) As above As above. with local feedback. with local feedback. This is probably because there is no easy way to tell. As above. cathode-driven 'Bass' control in 'Bass' channel. even if the circuit inside was a 6G6B. post 1976. The '6G6' series tends to come in a lovely 'Blonde' vinyl. power supplies. Increased output power. but not 100% accurate. The tube chart seen inside the 'box' is a good indicator. 'Treble' control is after 3rd gain stage. output power was increased on many Bassman heads from about 40-watts RMS to about 60-watts RMS. keep the 'old' parts in a little 'Zip-Loc' baggie. is still helpful and interesting. Increased output power. Not a great guitar circuit. For my money. the 'best' Bassman head would have to be the AA864. he chart above. As above. You can think about all of these opinions on various Bassman circuits this way. as an example. ow we'll look at a few close-ups inside the circuitry of these Bassman heads. These are also the only Bassman 'heads' to feature a 'Presence' control. The 'King' of screwball inverse feedback circuits. It was common to 'use up' 6G6A charts. As above. but few explain how to determine the internal circuit. post 1976. Odd 3rd gain stage common to both channels. has more inverse feedback. There is no real 'trick' in determining which circuit you have without taking the chassis out. and are highly collectible. and if it will suit our purposes. and find out why they may not be good for today's players. Bassman Modifications http://tone-lizard. The 'worst' candidate? I'll vote for the AA371. These are just basic guidelines. all called a 'Bassman'. although neither authoritative nor complete. As above. Blackface Bassman circuit has interesting possibilities.25uF capacitors in the tone stack. From the partial schematic above we may deduce that there is a lot of gain not being utilized. as we see below. and the . to more guitar friendly values.com/Bassman_Modifications. So. The 'Bass' control is very separate from the 'Treble' control. beyond the input and tone control stages. are you a collector or a 'working' player? There is more to tweak. But the 'Top 40' player needs a little more. Bassman Modifications http://tone-lizard. the DC coupled cathode-follower actually has a gain factor of less than '1'. hence the channel name. 3 of 7 1/18/2012 10:51 AM . I sometimes make these amplifiers into the typical 'classic' circuit seen in 'Tweed' Bassman and Marshall amplifiers. 6G6 Treble control was very separate from the 'Tone' control to avoid 'interaction'. The tone controls are also very 'Bass' oriented. replace the .htm Input of the 6G6A 'wastes' a lot of gain. The DC coupling is done to keep the phase shifting in the lowest frequencies at a minimum.1uF capacitor feeding the 'Treble' control. For the 'modern' guitar player interested in a 'raw' tone for classic Rock 'n' Roll or Blues. The sad part in all of this is the 'wasted' gain stages used to drive separate tone controls and as 'recovery'. We'd be correct. and using the cathode follower as a plate-loaded gain stage is definitely the ticket.Tales From The Tone Lounge. you could have the one-half of that 12AX7 to use as you desire. At the least. these heads are still hard to beat. you must decide. If the cathode follower is not to your liking. 4 of 7 1/18/2012 10:51 AM . as well as reduce the gain slightly (and more so with a lower gain tube). although not entirely undeservedly. What is not pointed out. I use my own AA165 for many gigs playing Classic Rock 'n' Roll and Blues. Use a bare piece of wire.001uF capacitor. Again. There are many versions of the Blackface Bassman out there. Lastly. the reality is that Fender made many 'improvements' to the Blackface circuit. the resistor values are altered to attempt a better 'balance'. but noteworthy nonetheless is the 12AT7 phase inverter tube. as well as the 'beefed up' Bass channel.047uF capacitor over to the plate side of the 100K plate-load resistor. Seen at point #2. 'Short out' the parallel combination of the 470K resistor and the . AA371 has the 'King of Screwball' inverse feedback circuit. With a simple A/B box. What gets overlooked is the local feedback provided by the 220K resistor located between the plate and the grid of the stage. perfect for raw Blues soloing.22uF capacitor. Below is a list of a few ideas I like to do.003uF capacitor in parallel with the 100K plate-load resistor. so there isn't really one single Blackface circuit.htm bove we see a partial schematic to the seldom-encountered AA165. Speaking of that . everyone likes to think they have the one with 'that classic Fender tone'. You can also look at 'bumping up' the grid return resistor from 220K to either 470K or 1Meg. Remove the . At point #1 we see the 3rd gain stage which is common to both channels. You now have the 'Normal' channel. By simply removing this little resistor. or move the . the overall gain is increased significantly.Tales From The Tone Lounge. using the long lead of the capacitor to take pin #8 to chassis ground. Bassman Modifications http://tone-lizard. You can make the circuit much closer to the 'Tweed' Bassman circuit. I will revoice the tone circuit. You may notice the 2000pF capacitors in the schematic above located between the control grid and ground of each 6L6. The circuit above is in the 'Bass' channel. Lastly. In the actual amplifier itself. but the noise level is improved. by substituting a 12AX7 in its place. making the lower frequencies have little or no feedback (the lower frequencies are only attenuated by the 47K resistor). and end up with two very distinct 'channels'. I sometimes parallel up the two halves of the 12AX7. that gave 'Silverface' amplifiers a bad name. The next example will demonstrate this quite nicely. all because 'distortion' is not desired by the engineers. Another curious circuit anomaly is the inverse feedback circuit. There are other 'improvements' to this circuit. and have the 'Deep' switch boost the mids by adding a series resistance to the 'Middle' resistor already in place. replace it with a . You do not get any more gain doing this.com/Bassman_Modifications. between pin #5 and pin #8. they will be on the tube socket. Remove the .022uF capacitor. which also has the other half to this 12AX7 available for many purposes. This feedback circuit couples only the higher frequencies. as a classic Fender circuit (if there is such a thing). it really is the strangest feedback I have seen in a guitar amplifier. You will also notice about this time Fender started using a 12AT7 for the phase inverter tube. Above is a very partial schematic of the AA371. and any classic Marshall. However.047uF capacitor. the 'adjustable' tube is balanced to the 'non-adjustable' tube. you may notice one side of the section is fed a constant negative voltage. Seen above is one of two circuits people pay some guru technician hundreds of dollars to remove.01uF seen in the AA371 seen earlier. At point #1 we see the local feedback has been reduced.htm The AB165 is a little better suited for guitar players. as opposed to the . Also not recommended are guru-types who 5 of 7 1/18/2012 10:51 AM . but unfortunately the bias circuit has been 'improved'. Bassman Modifications http://tone-lizard. that the tone ends up extremely neutered for guitar players looking for a 'raw' tone. and the grid resistance has also been increased. It's a long story. so good. don't miss it!). a welcome circuit 'upgrade'. At point #2 we see the inverse (global) feedback is also slightly 'improved' as well. Dealing with the bias circuit. so all frequencies get attenuated equally up to this point.Tales From The Tone Lounge. So far. as we see below. which is independent of the setting to the bias 'pot'. but still sounds neutered. which has a few 'improvements' over the AA165 seen earlier. The inverse feedback gets coupled via the . This gives a little more gain to the stage. The other part of the exercise is to make the phase inverter section identical to the Blackface-era circuit values. There is so much feedback going on here. Therefore. Overall the global feedback circuit works the guitar frequency range pretty well. you adjust the potentiometer until the hum heard through the speaker(s) is at a minimum.com/Bassman_Modifications. Above is a partial schematic of the AB165 circuit.1uF capacitor to the phase inverter stage. The theory is that residual hum will be canceled out in a 'perfectly' balanced push-pull output stage. Lastly. we see another stage of local feedback at point #3 (it is repeated on the second 6L6. but this system is full of downsides. In this case. There is no 'treble peaking' capacitor across the 47K resistor. and supposedly the tubes are biased properly. and is not recommended for guitar players. and we may even notice the 2000pF capacitors are 'missing' from the output tubes. 'Improved' bias balancing circuit is seen in many later-day Fender amplifiers. calling the operation 'Blackfacing'. You are free to experiment with resistor values here. Look over the drawing. as well as the phase inverter resistor values. You can experiment with these Plate load resistors. Of course you can try a 12AX7 as your phase inverter tube. The bias resistor is shown reduced to 10K. There is an attempt made to try and balance the phase inverter by reducing the overall gain. Reduce this until the overall tonal range is more to your liking with your specific guitar. The preamp circuit has a few of its own areas to 'improve'. Of course you are free to experiment with this resistor. but for a little more gain I suggest 100K/120K. Using larger values will give you an amplifier that has less headroom. The mods shown above will help put your AB165 circuit much closer to providing the classic Bassman tones. The Plate load resistors are shown as both being 100K. Do note that you reverse the Plate leads from the output transformer primary. usually only the bias supply is 'fixed'. Bassman Modifications http://tone-lizard. The final 'best' value will depend entirely on you. and then I will give you a few hints and tips for tweaking this circuit even further. try 120K/150K or 150K/180K for even more gain.1uF value.com/Bassman_Modifications. Here are a few hints and tips for those who want to experiment even further than what I have shown. elow is a recap of what I usually do with a typical AB165 circuit. 'Improved' AB165 circuit is much closer to classic Bassman circuit. The 'classic' Fender pairing is 82K/100K.Tales From The Tone Lounge. The feedback resistor is shown reduced to the 'classic' Fender value.001uF for certain applications. but these mods are easy to figure out on your own if you simply compare the two channels.htm 'Blackface' your Bassman head. The mods shown above put more gain in this section. I have gone as 'small' as . 6 of 7 1/18/2012 10:51 AM . you can go as low as 6800-ohms. The phase inverter input coupling capacitor is show as the stock . yet miss all the feedback stages. and find out what circuit we have. the guitar you play. Now you know why.To read about tweaking the bias. just having a 'Bassman' amplifier is not a guaranteed ticket to Tone Heaven.Tales From The Tone Lounge. citing the 'Bass' channel as having an awful guitar tone. Most players just use the 'Normal' channel. and the speakers we will be using. but the rewards can be very satisfying. see MODS AND ODDS.com/Bassman_Modifications. the final step in achieving 'The Ultimate Bassman'. Remove Ads + Increase Speed 7 of 7 1/18/2012 10:51 AM .htm s you can see. Bassman Modifications http://tone-lizard. It's a lot of work. and if it will suit our needs as guitar players. For more ideas on tweaking the gain stages to our liking after we've settled on an overall circuit. simply CLICK HERE. This is also determined by the music you play. We need to remove any rose-colored glasses.
Report "Tales From the Tone Lounge; Bassman Modifications"