AB165 Bassman Mod Choices

[Tapp]

Hi all. For those of you who have modded the AB165 Bassman heads, can you describe the tonal differences between the various mods? I'm probably going to start out with doing the Andy Ruhl AB165/AA864 hybrid mod. I've heard it really opens up these heads. It's just something in me that hates to leave that unused triode alone LOL! I'm researching the 5F6A Bass channel mod and also the parallel triode mod. Many choices with this particular circuit.

Thanks for any thoughts/comments!

[billyz]

Here you go.
http://www.billyz.net/Fender%20Bassman% ... 20mod.html

I have found 500pf to be the ideal coupling cap to the phase inverter.
The .02 circled in red , I now use 500pf.

[Tapp]

Thanks Billy! Have you made a comparison between the parallel triode vs using the extra triode as a cathode follower?

[Stevem]

Those amps need all the help they can get, that PI section is the poorest sound in all the fender line, even the silverface PI section from the latter amps with reverb sound better!

[billyz]

I have to confess that I am not in the cathode follower camp. Unless its needed for long runs or other special needs.
I prefer the plate driven circuits. The parallel triode as I understand it does lower the output impedance a little as well . It gives a hair more
Output and is a little quieter too.

I have been doing this mod for a while and IMO blows the others away. The clean channels also becomes much better due to the reworking of the pi and power amp section. The worst
Thing about the bassman for guitar is the feedback network , that one thing alone is well worth doing.

Sometimes I convert the deep switch to a switchable spilt plate load , but with the normal channel I don't feel it's really needed.

[Tapp]

Thanks Billy and Steven. Billy, I was thinking about doing maybe some sort of mid-boost for the deep switch; something maybe kinda raunchy there, don't know. Split load is interesting!

[billyz]

Thanks Billy and Steven. Billy, I was thinking about doing maybe some sort of mid-boost for the deep switch; something maybe kinda raunchy there, don't know. Split load is interesting!

Look carefully and you will see I up the middle resistor to 10k . Too much and you might just as well lift the tone stack (aka PAB) , which is cool too.

[Tapp]

I did notice that Billy and I also noticed the increased feedback tail resistor that's usually 100r, sometimes 1K5 in some Fenders. Now, will I need to flip the OT primary leads when I change that feedback circuit?

[billyz]

I did notice that Billy and I also noticed the increased feedback tail resistor that's usually 100r, sometimes 1K5 in some Fenders. Now, will I need to flip the OT primary leads when I change that feedback circuit?

Most likely, everyone I've done , needed to be reversed. You'll know when you fire it up the first time if it squeals .

[Tapp]

Billy, etc... Do you have any problems running a Bassman head into an 8ohm load? In my experience with other Fender's they have been able to handle 100% mismatch load with no problem. It's not that I particularly want to stress the amp or looking for a tonal change; most all of my cabs are 8ohm.

[billyz]

8 ohm is ok. But does sound much better into 4 ohm.

[Tapp]

Thanks, I may have to do a little speaker switching then. Do you have a preferred speaker for this head? I'm leaning toward a good ceramic Celestion, maybe V30 or G12H30 or a combo of both.

[billyz]

Those speakers are fine. Speakers are like beer, different flavors and styles, there is not one right kind. Personally I prefer alnicos or a mix. I've got two old hepners in a 2x12 front ported theil cab and I like them with this amp . A little darker and lower efficiency. But I have used scum backs , ASW, and various celestions , all good.

[Tapp]

Thanks Billy for the help! Well, I've finally been able to fire this Bassman head up. I've been so busy and no time to work on it. I got all the filter caps changed and the bias cap. I'm leaving the bypass caps for now. This bassman head is an early silver trim SF so it's got the good Mallory blue tubes and cloth wiring. When I bought this Bassman, it had no tubes and didn't even know if it worked; well it works now! I tubed it with OS RCA 7025's in the first 2 positions and a 12AX7 in the 3rd. A JAN Sylvania 12AT7. Right now I have current Tungsol 6L6GC's for power, but may play around with OS in here too. Before I fired the amp up sans cap job, I went ahead and changed the bias balance to "true adjust". I installed a 30K or so resistor in series with the 50K pot. I fired the amp up and was only able to get about 29ma per tube (OT shunt method), and ran out of range. I guess I'll increase the 470R bias board resistor to give me some more range in my 50K pot. Although 29ma/448Pv didn't sound too bad.

When I first fired the head up I grabbed my Strat and noticed the Normal channel was pretty much standard Fender clean although nothing to write home about; of course I wasn't expecting much anyway! Now the Bass channel was way dark/dull, but I will say on the bridge pickup of the Strat and cranked a little, it gave a decent "James Gang" kinda dirty tone.

Back to the circuit, I now pulled the 220K 6L6 plate feedback resistors, and pulled all of the plate load parallel caps. I can't believe what a difference this made! No more dark Bass channel now! It's on the thin side to my tastes, and so I'm really curious how the BillyZ mods are going to improve this now!

Billy, what do you normally bias these heads for? Do you scope them or just set a current value?

[billyz]

I usually set the bias to something around 40ma per tube . do the math. 30w divided by the plate voltage x 70% .
I change the 15K resistor off the 10k bias pot to 27k to ground. That has always given me plenty of bias voltage leeway. After converting to AB763 style bias of course.

I think you have too much bias voltage. Try jumping your 30k resistor and dropping the bias voltage. piggy back another 30k to it, use jumper till you get it right then solder one in.
At 448vdc you could go for 46ma per tube, or a little less.