'66 Fender Blackface Bassman - Cost of Mods

[bassman333]

Hey guys,

I recently dug out my old Bassman AB165 circuit and was wondering if the mods are "accurate" mods.

For example, I know you can do a JTM45 thing in the Bass Chanell, does that mean it's virtually the same circuit and sound as an original JTM45?

Also, this is important, do any of you know roughly how expensive it will be to modify the Bassman?

One last question, can you mod a "gain" switch on the Bassman so I can control the gain on the amp and not have to crank it to get distortion?

Thanks.

[Zippy]

For example, I know you can do a JTM45 thing in the Bass Chanell, does that mean it's virtually the same circuit and sound as an original JTM45?

The circuit can be replicately down to the type of resistors and capacitors used and it will still not sound exactly like a JTM45. There are other issues - transformers, voltages, etc. that have a significant impact on the final product.

[bassman333]

Ok, thank you.

I was thinking of putting a 12AX7 in the phase inverter stage instead of the 12at7 that is currently in there just to experiment, do you know if I can just swap them out or do they need to be biased?

[Dr-Joned]

You can try any of the 12a*7 tubes. The 12ax7 is the highest gain of the group. They are all in a self biased circuit so there is no biasing required.

[bassman333]

Again, thanks.

If anyone can inform me of a master volume mod, that'd be great!

[selloutrr]

a PPIMV will help you achive the gain and control the ouput volume of the amp.

You would have to convert the amp to EL34 to get closer to the traditional "marshall" tone.

IMHO i'd add a PPIMV using a hole that is not in use, ext speaker? I would not drill any holes!


http://www.tylergrund.com/images/tylers_PPIV.jpg

http://metroamp.com/wiki/images/9/93/Lar-Mar_PPI-MV.jpg

some marshall / KF mod history - techy stuff

http://www.lynx.bc.ca/~jc/marshallAmps.html

[bassman333]

Guys,

I just plugged my Bassman in and fiddled around with it cranked (earplugs and all) and I was kind of taken back by the lack of gain on the amp. It has 12ax7 pre amp tubes for more gain, but still...

It sounds NOTHING like this....and this is pretty much the exact same as my amp, AB165 circuit and all. Please take a look.

http://www.youtube.com/watch?v=_N8wNmi9B40

How can there be such a disparity in "overdrive" or "distortion," whatever you want to call it, when I have the same amp as the guy in the video?

Anyone have any ideas?

[Phil_S]

There can be all sorts of good reasons for that. Are the tubes used up? Maybe you need a cap job? Both? Something else out of spec? I think you would benefit from a competent tech looking it over. We can only speculate.

Where are you located? Maybe someone will have a suggestion. I wouldn't want to bring an antique to just anyone.

[bassman333]

I put new 12ax7 tubes just before I fired it up.

The power tubes are old, however.

It might need a cap job, though I do not know for sure how to tell if it does or doesn't.

[Structo]

First clue for expired filter caps is excessive 60Hz or 120Hz hum.

[selloutrr]

second clue waxy looking bubbles or bulging on the case. or crystalized powder leaking out of the vent hole.

Are you preamps tubes, just new? or Known to be new and strong?
It's possible the video is running a high gain preamp tube.
output tube life would signifcantly lower your output volume.

Are you speakers wired properly?
correct impedance?
all the same phase?

IMHO it's not worth basing an amp on what another amp sounds like, it's better to judge your amp by how it sounds on it's own.

i.e. Do you have the same recording device used to make the video?
the same guitar and playing technique?
standing in the same place wearing the same clothes? chewing the same gum...
the same cord and if used pedal(s), same battery life?
the same amp? ( sort of, though every model change did matter to the circuit design)
with the same knob settings and the same resistance on the potentiometers?
the same tubes with the same amount of life?
the same room, temp / climate, elevation, time of day...?
Was the amp warmed up for the same amount of time?
Where the speakers the same?
Where the speakers broken in the same> ( same amount of hours of use? ) using the same notes to form then the same way...

I know this seems like over kill but it all played a role in making that tone.

In the studio one night i'll have magic the next day nothing moved and the magic is gone. the amp was warmed up to that special point and everything just clicked.

if volume/break up is a concern check
tubes ( test them and see how much life is left, new or old )
test your speakers and wiring make sure the phase is correct and nothing is blown.
make sure the speaker imp is correct.
if the electrolytic caps are over 15 yrs old it's probably time to replace them, if over 20+yrs for sure! even if the amp is quiet the circuit will benefit from new caps.

[greenbottle]

Just as an aside the Very First JTM45 was a Fender 410 Bassman. Same valves,same components in a chassis turned up sidedown which put the inputs at the wrong end. Like al large amp makers Marshall is cost driven.

6l6 output tubes were imported in England so Marshall switched to a locally produced Mullard EL34. Over a period of time JTM was made brighter and brighter and then they upped the power 100 Watts hence
the Plexi.

Fender amps of that era were meant to be played CLEAN and as such 50 watt ams were provided with 50 watt speakers. If play heavilly distorted signal through those 50 watt speakers you will severly shorten their lifespan. You can't buy replacements. The closest you will get are made by Weber.

So if you must have a JTM sound, restore your Bassman and sell to someone who wants a Bassman and build yourself a JTM clone. Chassis,labels transformers are available from Weber and Mojo. I don't know if Metro is still making these available. Circuits and layouts are widely available on the net and you already know where to come for help and advice.

Alan

[vibratoking]

I'd bet the output tubes are weak. Check the output tube bias. Then try new output tubes. That would be my first move.