Bassman Mods

Bassman Style

There 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.

A BRIEF CHART

Here we’ll have a little chart with the various Bassman circuits to be found today. There is no real ‘trick’ in determining which circuit you have without taking the chassis out. The tube chart seen inside the ‘box’ is a good indicator, but not 100% accurate. It was common to ‘use up’ 6G6A charts, as an example, even if the circuit inside was a 6G6B. I have worked on many AB165 Bassman heads (and personally own one) that ‘feature’ the AA165 tube chart. Most web sites will have us dating our Bassman head to the hour it came off of the assembly line from external appearances, but few explain how to determine the internal circuit. This is probably because there is no easy way to tell. These are just basic guidelines, and we’ll need to dig deeper to find out what we really have, and if it will suit our purposes. Keep this in mind as well; starting about 1977, output power was increased on many Bassman heads from about 40-watts RMS to about 60-watts RMS. This means transformers, power supplies, and filtering circuits will change, yet it can still be referred to as ‘AB165‘ (as an example).

Bassman Circuit
Tube Complement
Notes
6G6
(1961-1962)
4 x 7025
 2 x 5881
Odd circuits aplenty. DC coupled first gain stages, cathode-driven ‘Bass’ control in ‘Bass’ channel. ‘Treble’ control is after 3rd gain stage. Tapped ‘Treble’ control in ‘Normal’ channel.
6G6A
(1962-1964)
As above.
As above, with linear-taper ‘Volume’ control in ‘Bass’ channel. Also, has more inverse feedback.
6G6B
As above.
As above, with slightly different tone controls in ‘Bass’ channel.
AA270
(1968-1983)
3 x 7025, 1 x 12AT7
 2 x 6L6GC
Odd 3rd gain stage common to both ‘channels’, with local feedback. Very odd inverse feedback circuit. Not a great guitar circuit. Increased output power, post 1976.
AA371
(1968-1983)
As above
Odd 3rd gain stage common to both channels, with more local feedback than AA270. The ‘King’ of screwball inverse feedback circuits; awful for guitar. Increased output power, post 1976.
AA864
(1964-1967)
As above.
Very close to ‘classic’ circuit. ‘Bass’ channel has extra gain stage. High amount of inverse feedback.
AB165
(1964-1983)
As above.
Odd 3rd gain stage common to both channels, with local feedback. Inverse feedback circuit boosts bass frequencies; not great for guitar. Local feedback at output stage. Increased output power, post 1976.
AC568
(1968-1983)
As above.
See AA270.
 Different circuits, all called a ‘Bassman’, make for unnecessary confusion.

 

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p style=”text-align: left;”>The chart above, although neither authoritative nor complete, is still helpful and interesting. For my money, the ‘best’ Bassman head would have to be the AA864. This would give the least amount of ‘work’ involved in order to have a very good guitar amplifier. The ‘worst’ candidate? I’ll vote for the AA371. The most common seems to be the AB165 (they were issued for almost twenty years), and these are not great for guitar players, as we’ll see in a minute. The ‘6G6’ series tends to come in a lovely ‘Blonde’ vinyl, and are highly collectible. These are also the only Bassman ‘heads’ to feature a ‘Presence’ control. You can think about all of these opinions on various Bassman circuits this way; many classic Bassman derivatives and today’s boutique amplifiers all follow the 5F6 circuitry, whilst none I can name follow the AB165 circuit verbatim.

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p style=”text-align: left;”>Now we’ll look at a few close-ups inside the circuitry of these Bassman heads, and find out why they may not be good for today’s players, and what we can do to ‘improve’ them. I’ll remind you again, keep the ‘old’ parts in a little ‘Zip-Loc’ baggie. You must replace them before putting the amplifier up for resale. First up is the input of the 6G6A.

6G6A_inputInput of the 6G6A ‘wastes’ a lot of gain.

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p style=”text-align: left;”>From the partial schematic above we may deduce that there is a lot of gain not being utilized. We’d be correct; the DC coupled cathode-follower actually has a gain factor of less than ‘1’. The DC coupling is done to keep the phase shifting in the lowest frequencies at a minimum. The tone controls are also very ‘Bass’ oriented, hence the channel name. The ‘Bass’ control is very separate from the ‘Treble’ control, as we see below.

6G6B_treble6G6 Treble control was very separate from the ‘Tone’ control to avoid ‘interaction’.

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p style=”text-align: left;”>The sad part in all of this is the ‘wasted’ gain stages used to drive separate tone controls and as ‘recovery’. I sometimes make these amplifiers into the typical ‘classic’ circuit seen in ‘Tweed’ Bassman and Marshall amplifiers. At the least, replace the .25uF capacitors in the tone stack, and the .1uF capacitor feeding the ‘Treble’ control, to more guitar friendly values. If the cathode follower is not to your liking, you could have the one-half of that 12AX7 to use as you desire. For the ‘modern’ guitar player interested in a ‘raw’ tone for classic Rock ‘n’ Roll or Blues, these heads are still hard to beat. But the ‘Top 40’ player needs a little more, and using the cathode follower as a plate-loaded gain stage is definitely the ticket. So, you must decide, are you a collector or a ‘working’ player? There is more to tweak, beyond the input and tone control stages.

AA165_preampBlackface Bassman circuit has interesting possibilities.

Above we see a partial schematic to the seldom-encountered AA165. The circuit above is in the ‘Bass’ channel, which also has the other half to this 12AX7 available for many purposes. Below is a list of a few ideas I like to do, and end up with two very distinct ‘channels’. With a simple A/B box, I use my own AA165 for many gigs playing Classic Rock ‘n’ Roll and Blues.

  • ‘Short out’ the parallel combination of the 470K resistor and the .001uF capacitor. Use a bare piece of wire, or move the .047uF capacitor over to the plate side of the 100K plate-load resistor.
  • Speaking of that .047uF capacitor; replace it with a .022uF capacitor.
  • Remove the .22uF capacitor.
  • Remove the .003uF capacitor in parallel with the 100K plate-load resistor.

I sometimes parallel up the two halves of the 12AX7. You do not get any more gain doing this, but the noise level is improved. You now have the ‘Normal’ channel, as a classic Fender circuit (if there is such a thing), as well as the ‘beefed up’ Bass channel, perfect for raw Blues soloing. Lastly, I will revoice the tone circuit, and have the ‘Deep’ switch boost the mids by adding a series resistance to the ‘Middle’ resistor already in place. You will also notice about this time Fender started using a 12AT7 for the phase inverter tube. There are many versions of the Blackface Bassman out there; everyone likes to think they have the one with ‘that classic Fender tone’. However, the reality is that Fender made many ‘improvements’ to the Blackface circuit, so there isn’t really one single Blackface circuit. The next example will demonstrate this quite nicely.

AA371AA371 has the ‘King of Screwball’ inverse feedback circuit.

Above is a very partial schematic of the AA371. At point #1 we see the 3rd gain stage which is common to both channels. What gets overlooked is the local feedback provided by the 220K resistor located between the plate and the grid of the stage. By simply removing this little resistor, the overall gain is increased significantly. You can also look at ‘bumping up’ the grid return resistor from 220K to either 470K or 1Meg. Another curious circuit anomaly is the inverse feedback circuit. Seen at point #2, it really is the strangest feedback I have seen in a guitar amplifier. This feedback circuit couples only the higher frequencies, making the lower frequencies have little or no feedback (the lower frequencies are only attenuated by the 47K resistor). There are other ‘improvements’ to this circuit, that gave ‘Silverface’ amplifiers a bad name, although not entirely undeservedly. You may notice the 2000pF capacitors in the schematic above located between the control grid and ground of each 6L6. In the actual amplifier itself, they will be on the tube socket, between pin #5 and pin #8, using the long lead of the capacitor to take pin #8 to chassis ground. What is not pointed out, but noteworthy nonetheless is the 12AT7 phase inverter tube. You can make the circuit much closer to the ‘Tweed’ Bassman circuit, and any classic Marshall, by substituting a 12AX7 in its place. Lastly, the resistor values are altered to attempt a better ‘balance’, as well as reduce the gain slightly (and more so with a lower gain tube). Again, all because ‘distortion’ is not desired by the engineers.

AB165The AB165 is a little better suited for guitar players, but still sounds neutered.

Above is a partial schematic of the AB165 circuit, which has a few ‘improvements’ over the AA165 seen earlier. At point #1 we see the local feedback has been reduced, and the grid resistance has also been increased. This gives a little more gain to the stage, a welcome circuit ‘upgrade’. At point #2 we see the inverse (global) feedback is also slightly ‘improved’ as well. There is no ‘treble peaking’ capacitor across the 47K resistor, so all frequencies get attenuated equally up to this point. The inverse feedback gets coupled via the .1uF capacitor to the phase inverter stage, as opposed to the .01uF seen in the AA371 seen earlier. Lastly, we see another stage of local feedback at point #3 (it is repeated on the second 6L6; don’t miss it!). There is so much feedback going on here, that the tone ends up extremely neutered for guitar players looking for a ‘raw’ tone. Overall the global feedback circuit works the guitar frequency range pretty well, and we may even notice the 2000pF capacitors are ‘missing’ from the output tubes. So far, so good, but unfortunately the bias circuit has been ‘improved’, as we see below.

AB165_bias‘Improved’ bias balancing circuit is seen in many later-day Fender amplifiers.

Seen above is one of two circuits people pay some guru technician hundreds of dollars to remove, calling the operation ‘Blackfacing’. The other part of the exercise is to make the phase inverter section identical to the Blackface-era circuit values. Dealing with the bias circuit, you may notice one side of the section is fed a constant negative voltage, which is independent of the setting to the bias ‘pot’. In this case, the ‘adjustable’ tube is balanced to the ‘non-adjustable’ tube, and supposedly the tubes are biased properly. The theory is that residual hum will be canceled out in a ‘perfectly’ balanced push-pull output stage. Therefore, you adjust the potentiometer until the hum heard through the speaker(s) is at a minimum. It’s a long story, but this system is full of downsides, and is not recommended for guitar players. Also not recommended are guru-types who ‘Blackface‘ your Bassman head, yet miss all the feedback stages; usually only the bias supply is ‘fixed’, as well as the phase inverter resistor values.

Below is a recap of what I usually do with a typical AB165 circuit. Look over the drawing, and then I will give you a few hints and tips for tweaking this circuit even further. The preamp circuit has a few of its own areas to ‘improve’, but these mods are easy to figure out on your own if you simply compare the two channels.

moddedAB165‘Improved’ AB165 circuit is much closer to classic Bassman circuit.

The mods shown above will help put your AB165 circuit much closer to providing the classic Bassman tones. Do note that you reverse the Plate leads from the output transformer primary. Here are a few hints and tips for those who want to experiment even further than what I have shown.

  • The phase inverter input coupling capacitor is show as the stock .1uF value. Reduce this until the overall tonal range is more to your liking with your specific guitar. I have gone as ‘small’ as .001uF for certain applications. The final ‘best’ value will depend entirely on you.
  • The feedback resistor is shown reduced to the ‘classic’ Fender value. Of course you are free to experiment with this resistor. Using larger values will give you an amplifier that has less headroom.
  • There is an attempt made to try and balance the phase inverter by reducing the overall gain. The mods shown above put more gain in this section. You are free to experiment with resistor values here. The bias resistor is shown reduced to 10K; you can go as low as 6800-ohms. The Plate load resistors are shown as both being 100K. The ‘classic’ Fender pairing is 82K/100K, but for a little more gain I suggest 100K/120K. You can experiment with these Plate load resistors; try 120K/150K or 150K/180K for even more gain.
  • Of course you can try a 12AX7 as your phase inverter tube.

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p style=”text-align: left;”>As you can see, just having a ‘Bassman’ amplifier is not a guaranteed ticket to Tone Heaven. Most players just use the ‘Normal’ channel, citing the ‘Bass’ channel as having an awful guitar tone. Now you know why. We need to remove any rose-colored glasses, and find out what circuit we have, and if it will suit our needs as guitar players. This is also determined by the music you play, the guitar you play, and the speakers we will be using. It’s a lot of work, but the rewards can be very satisfying. 

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