Coverting old pa head to Bass head

[blindboybenton]

Hi

Ive had a selmer PA100/4 here for a while that aint been doing much and was in a bit of a mess. So ive made the decided to turn it into a new bass amp
I want to use the chassis and all the Transformers and start again with the Guts!!

The thing has been stripped down and in bits so no going back now. The plan is to build a Marshall JCM800 super bass.? Do you guys have any ideas on this any better ideas on what to build?

http://www.drtube.com/schematics/marshall/1992u.gif marshall

Selmer PA 100 Amplifier Schematic selmer

Im going to remove the valve rectifiers and use solid state. The selmer HT is about 500v.
Using the chassis from the selmer and ill be making a new cabinet. I was hoping to put the volume/tone pots on the front of the chassis. Close ish to the preamp valves
Is that going to cause a problem?

Any opinions or assistance you can offer would be welcomed.

many thanks in advance

Blindboybenton

[bob p]

The thing has been stripped down and in bits so no going back now.

There's something to be said for gutting an amp so that there's no going back -- I recently did the exact opposite with a little EL88 HiFi amp that I had planned to gut to convert into an AC-15 or a Liverpool. I let curiosity get the better of me, so I made the mistake of repairing it and fully restoring the circuit, and now I like it so much that I just couldn't bring myself to strip it down:

http://music-electronics-forum.com/t32615/

Now I'm going to have to start my amp build from scratch. At least by gutting your amp you'll be protected from being stagnated by "modders remorse."

Im going to remove the valve rectifiers and use solid state.

A pair of GZ34 should give you a pretty stiff supply, but if you want to go SS that will open some doors for you. As soon as you make the decision to go with SS rectifiers that opens the door for you to use large filter caps at the input to your DC supply. Now that you're no longer constrained by the size of the first filter cap you have a lot more options when it comes to choosing your build.

Do you guys have any ideas on this any better ideas on what to build?

I'm sort of in the same situation, as I'm working on a 6x6L6 amp that I'm going to convert into a bass head. As I've been thinking about this, I came to a few conclusions that you've probably already thought about, but they're probably worth listing anyway.

1. I like to think of the power amp first, and design the power supply, bias supply, and the drive stages around the power amp's needs. Since you've got iron that's ready for 4xEL34 you have lots of options for tube choices. You can pretty much choose whatever set of tubes you want because a 4xEL34 PT will have plenty of current rating for the filament supplies. That means that you can pretty much pick any of the common tubes for your output stage. Have you already decided on staying with EL34?

2. When it comes to preamp choices for bass applications, most people tend to think about choosing between Fender and Ampeg style preamps. By "Fender" that usually means the BF/SF Bassman type of setup (like the Alembic F2B), or it could mean Tweed Bassman that uses a CF to drive the tonestack (like a Marshall). Then there's the Ampeg SVT type of preamp option with the selectable EQ. Do you have any preferences for one over the others?

3. You've got a lot of preamp tube sockets to work with. That Marshall schematic that you posted only uses a pair of preamp tubes. Your Selmer has enough sockets that you could build two totally independent preamp channels, so you'd have the choice of having the best of both worlds. That chassis gives you a lot of flexibility. If I had your chassis, I'd be thinking about building two amps in one chassis. The immediate thought that pops into my mind is JKB's Transatlantic Bassman.

http://www.blueguitar.org/new/text/t...ic-bassman.pdf

http://www.blueguitar.org/new/schem/...w/trans_at.pdf

[blindboybenton]

Many thanks Bob
I dont play 6 strings.. 4 is enough. So i just want a bass amp. Im using a Carlsbro PA100 at present with a 2 x 10" cab. I play classic 60s/70 rock. at shows i have just enough headroom if i get any louder i annoy the others so a few more watts wouldnt go a miss. Not sure if i could be a few more out of the o/t. Maybe two channels for bass one with some more grunt. with some extras like line out

Should i stick to EL34 or something else

John

[Enzo]

I appreciate the desire to just remove the entire circuit and start over, but the power amp section is probably going to wind up looking just about like what was torn out. Unless you go to a totally different phase inverter, you will likely build the same thing that was already there. And the power tubes are more or less universal already.

Just a thought.

[bob p]

Although you may think it's a guitar amp, that schematic that I linked to is based upon two Fender Bassmen: the Blonde version and the Tweed Version. The Tweed Bassman is also the basis for the Marshall Circuit you're thinking about. so even though you're only thinking about 4 strings instead of 6, my recommendations wouldn't change: you still have to choose between the 3 preamp flavors: Tweed Bassman (aka Marshall) vs. Blonde/BF/SF Bassman (Alembic F2B) vs. Ampeg. In your situation, I'm thinking that a hybrid amp that uses the preamp channels from any 2 of these would be the basis for a nice amp.

Like Enzo said, the output section can't change much. You're probably going to end up with an LTPI driving a quad of whatever. Take your pick.

When it comes to output tubes, it all boils down to what you want. For bass I'm a fan of 6550 or 6L6 but there are people who prefer EL34. It's like choosing vanilla vs. chocolate ice cream. Both flavors are good.

If you want to have a line out, then you'll need CF drive. The SVT (V5) and I think the Boogie 400 are examples.

[bob p]

I appreciate the desire to just remove the entire circuit and start over, but the power amp section is probably going to wind up looking just about like what was torn out. Unless you go to a totally different phase inverter, you will likely build the same thing that was already there.

Look at the bright side, Enzo -- now that he's gutted the amp all of those crappy old CC resistors are gone. Chances are that he'll rebuild the amp using MF and all of the nasty crackling noises will be history.

[blindboybenton]

Hi enzo

All the tube bases in the output stage were tired and in need on replacement. The chassis was thick with wax and dirt.. Its already been done. This is something for me to do inbetween fixing amps for customers. I get bored easy lol

BBB

[blindboybenton]

Well we here in the good old uk.. Love the EL34. But i have a 100watt amp with thoses and as im going to have to buy some new tubes.. If EL34 were Vanilla and 6550 were chocolate
EL34 it is .. So the bright channel on the amp you recm'd is a bass bright channel ?. Ive never owned a fender or a ampeg. I do love the 60s over driven sound.. Big fan of Free and The Who
and andy fraser both had tones to die for.. This is why ive picked the Marshall

BBb

[blindboybenton]

And Boy was i getting issues with this old amp.. I gutted it to put it to rest. The other reason why i want to build my own amp is to show my customers... look what i can do lol

BBB