Fender Bassman 100 rebuild

[Mark]

This is more or less a New Years resolution and a gift for my bass playing brother in September.

The amp is pretty messed up as someone tried to turn it into a Mesa Boogie Mark 1. I have a new faceplate for it and I'm wondering what mods would best suit this amp for bass playing. The normal channel with a master volume seems a bit silly to me. I will double value of the filter caps in the first stage for less sag and implement a better bias circuit.

I wondered if putting a cathode follower between the PI and output stage would be a good thing?

Let me know your thoughts.

https://www.thetubestore.com/lib/thetub ... ematic.pdf

[norburybrook]

Mark,

Is he going to be gigging with it? if so a transformer balanced D.i. output would be very useful. would be great for recording too.



M

[Stevem]

A big big need for a good bass tone is a mid boost and a frequency filter sweep , not just a mid cut which is all you can get out of either channels stock tone stack in that amp!
The only way to get any resemblance to a mid boost is to back down on the bass and treble settings and then 50% your volume is gone!

I would do away with one channel in that amp so you can have those extra tubes to build / have a active preamp tone section as in a Ampeg SVT tube head, or build in a graphic EQ section like a Sunn 300T bass head.

Power supply wise besides fattening up the filter uf values I would remove those 6 old rectifiers and run only two modern Fast diodes with the needed reverse voltage handling and this will make for less noise hash in the amps output and pick you up a few more volts of V+ for more head room.
Be sure to leave the new diodes leads long as they act like a heat sink.

Also power supply wise those 1 watt 470 resistors on each output tube are just too small for all the heat that gets generated below them, no less there electrical needs!
Up them to 5 watt ciramic types and increase the 1.5k 1/2 watt resistors there to 2 watters.

[Mark]

Thanks for the replies Marcus and Steve.

The only mods I have in mind are getting rid of the master volume as I've found they adversely the tone on the amps I came in contact with. Increasing the filter cap values as previously mentioned is a cert.

Steve what FAST diodes do you have in mind?
How do these diodes differ from UF4007?

I'll post pictures of the chassis etc tomorrow.

[Stevem]

A uf2007 IS a fast recovery type , but it's only rated for 2 amps and more importantly 1000 volts in reverse flow.
If you have these just run 2 in series on each ac leg of the PT and you will be more then fine.

[Mark]

Thanks for your advice, this is basically what I'm starting with.

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[Mark]

I have been thinking of using a slightly different P.I. stage, as shown in the link below. I suppose I could implement a balance pot even out the output of the P.I. stage.

https://ampgarage.com/forum/download/fi ... &mode=view

[Stevem]

What would be your reason for going that route?

[Mark]

The feedback network and the tail resistor in the PI seems a bit weird to me. I certainly don't want the 2000pF caps hanging off the grids of the output valves.

https://cdn.shopify.com/s/files/1/2325/ ... 0715640943

[Mark]

With any of these old Fender amps with the black fibre boards, I'm concerned that they are conductive. In testing this board out I decided to put 384VDC (rail voltage of a Champ amp) on one side of the board and earth on the other side.

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I then thought I would try and measure voltage on the eyelets.

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Does my method add up, thus I need to replace the board as there will be crosstalk in the power supply?
(I imagine the circuit card will measure much the same.)

What are your thoughts?

[martin manning]

That board is definitely not an ideal insulator, but it could be worse. There are many reports by people trying to save the original cards by cleaning, baking them dry, and applying some hydrophobic substance. Results are mixed, though, and often temporary. New G-10 boards will fix it permanently.

[Colossal]

New G-10 boards will fix it permanently.

Completely agree. Get rid of that garbage. There is no utility whatsoever in using something with a known fault. It will cause problems at some point and when it does, there is nothing to be done but swear and rebuild.

For my Bassman build, I am using 1/16" thick black G10/FR4 sanded to a smooth matte to make them look like the old boards.

[Charlie Wilson]

You can clean the original boards with brake cleaner. If you are going to replace them, I like Garolite XX. It is WAY easier to cut and drill and it looks a lot like the vintage board material.
CW

[Mark]

I have heard about brake cleaner, but does it really work?

The brake cleaner does have a reputation for being quite toxic.

I can get another set of fibre boards made by Mojotone.

I'd prefer to have the Garolite boards made up as it is difficult to cut and drill. To make matters worse it does have a reputation for being bad for your health if breathing protection isn't worn and it isn't cleaned properly after cutting.

[Charlie Wilson]

No, no, no. Do not get fiberboard from Mojotone! That stuff is the worst. It will be more conductive than the original fiberboard. Ok I vented. Brake cleaner is Acetone and wood alcohol. I wouldn't drink it and you can wear gloves but it is ok. It evaporates very quickly and does not leave a residue. You can use a something like a glass baking pan. Spray a half inch or so of the brake cleaner into the pan. Then take a toothbrush and scrub it. You will be amazed at how much junk comes off of them. It does kind of dry the board out little. You can take a heat gun(after it has dried, very flammable) and re blacken it. Don't over heat or you can cause it to warp. Just so you know, pure water is not very conductive. It is the dissolved minerals in the water that make it conductive. Once those are deposited in the board there is no way to get rid of it. If the problem is all the dirt and burnt flux on the outside of the board, cleaning will help, not such much with mineral deposits inside the board.
CW