Fender Pre and Bassman Volume questions

[Structo]

I suppose another alternative you could use is a Dumble clean channel preamp.
It doesn't have the huge mid scoop tone stack and is fuller, richer sounding.

Here is how it would work.
The lower right section is the second half of V1 or V1b. I didn't have enough room to draw it to the side.

[Subjecttochange]

Agreed- whats the easiest way for me to draw it out? Do you know of any good software that's free? I think it'll be impossible for me to explain it- and a lot easier to draw the whole thing and just show you.

Once you guys see it- I'm sure you'll make light work of my preamp problems.

[Structo]

Most software for it is expensive.
I have a Visio which is a Microsoft package but it is better for layouts.
The other one guys use is the Express PCB and SCH.

It's freeware but it has a pretty good learning curve unless you have experience with CAD software.

http://www.expresspcb.com/ExpressPCBHtm ... ftware.htm

When I am drawing simple schematics I use MS Paint.

What I do is copy a schematic that is already drawn and then erase stuff I don't want and draw the new stuff. It is tedious but it can work pretty good once you learn your way around it.

[Subjecttochange]

Yep- thats what I was trying to do with paint... but using old and kind of unclear schematics was a bit annoying... I figured doing it over would make me rethink everything I've done- and allow me to communicate it better to you guys.

Software doesn't look too difficult- Can't say it'll be a work of art when I'm done though.

[Subjecttochange]

Here's my first legitimate computer drawn schematic. Hopefully it's not too bad- and will give you an idea of what I am doing.

This is what I currently have (+ or - some random caps or resistors that are slightly different values).

Also- like some other bassman models I was considering a 12AY7 for the cleaner pre. I have an extra 12AY7... and I just want to keep it squeaky clean.

Let me know what you think and if you can see the drawing or not.

[Structo]

Looks like you did a good job on it.

Did you draw that with the Express program?

I'm a little curious why you used the 6 position tone switch vs a tone stack?
That isn't part of a Rocket is it?

Then you have the cathode follower and what appears to be a static tone stack on the follower portion minus the pots.

It looks like the mixing are is fine.

Pretty standard LTP PI.

So it still does not work and this is how it is wired?

Does either side work well or both are too quiet?

I think what some of the guys were saying is that you need a 12ax7 stage after the mixing resistors to combine the signals before the PI.

This amp could have real possibilities if you made it a channel switcher with one input.
Have a foot pedal to switch between the two channels and it would seriously rock!

This is what I have for the Rocket preamp.

[M Fowler]

You do not have a recovery amp following each tone stack at junction of 220k before the PI.

[Subjecttochange]

Thanks Structo- The express program was pretty easy once I got the hang of it. The few modifications to the Rocket circuit are adaptations from the EF86 channel I had before. Since I don't have enough holes/space on the front panel- I opted to use the switch as the tone shaping and set the tone stack with fixed values. If I was just beginning this project- I would def. use the pots instead. Next time!

The plan is to do a channel switching relay out to a footswitch. At which point should I be switching? Input (before the first triode)? After the tone stack? Never used a relay before- don't know where the noise problems will be depending on the location.

I do think this amp will be killer when it's done. Thanks to you two! I might have to build a 6V6 version for myself when I am done.

M Fowler- yeah I know I don't have one. I drew it up exactly as I currently have it. I'm still not seeing it though... I tried to wire up yesterday with what I thought was a mixing stage- but it failed. How would the recovery stage look?

Thanks guys

[Subjecttochange]

Oh and both preamps in it's current state, are operating successfully. They're loud. Just not quite overpowering a drummer loud. They're also both at similar relative volume which made me think that it's the mixing into the PI that was the problem in the first place.

Would it be best to put the tone stack immediately following the first triode on the rocket pre- move the tone switch after the second- then make the third triode a recovery?

Then use the last available triode for the recovery for the fender channel?

[Structo]

I can't really help with the Rocket side of things since I haven't built one or played through one.

What do you have the 6L6 biased at? And what is their plate voltage?
Most P/P 6L6 amps are very loud! So maybe there is something going on there.
I suppose you could try a bit higher on the PI plates like 100K/110K to see if that adds or takes away from the flavor of the amp.
I have 110K/120K on my amp and it just feels more alive with those compared to the lower values I have had in it.

I see you only have a 10uF filter on the bias circuit.
I know that a lot of Fenders used 8-10uF but more modern amps typically use a 100uF cap there.

Not sure of the proper way to use a relay in your amp either.
My experience is with my D'Lite which is a Dumble clone.
It cascades the OD section after the clean section so it isn't a true channel switcher.

I would try to find a schematic of an amp with two channels that are switched to compare to and borrow from that.

I found these looking around.
Does your PT have any extra taps, like a 5v tap?

You can tap off of the 6.3v heater winding but you will have to isolate everything from chassis ground or it will short out your supply.

I used a 8v regulated supply for my relay setup and used 5v relays.
I haven't had any problems with this setup at all.

Here is the way one guy switched his Marshall.

[Subjecttochange]

Yep Got a 5V tap on the PT. I actually sacrificed the tube Rectifier for it. I never really like tube rectifiers anyway- hence the sag switch if my friend Bo really wants that sound. I guess it's more of a feel than a sound anyway.

I've got a little tiny board and a low voltage relay all ready to go or at least... ready to try. I just want to make sure that the amp is completely healthy before I go ahead and make more changes.

So with the mixing stage- should I just run the two signals from the 220ks into pin 7, some simple resistor to the ground on 8 and power it up on pin 6 with that common DCV that I am using for the rest of the preamps with a 100W resistor- then run that out to the PI?

I threw it into the drawing and updated my last attachment.

Just a few questions:
On the cathode resistor... value? I am assuming no bypass.
And re-entering the PI... should I change the coupling cap? Do I need the 220k- bigger/smaller?

[Structo]

Not sure about that third 220K, you may not need it and it might kill too much high freq.

As for the cathode resistor, not real sure but I think you just want unity at that point but I am not sure of the value.
I've seen 820R used on other amps so possibly a value between 820 and 1K.

[Subjecttochange]

Me neither. Do I need a resistor at all and/or does this recovery stage even look correct?

I just threw 5k on there. 1k sounds much better to me.

I had a hard time finding anything running two switchable channels. Maybe I'm just looking in all the wrong places.

[Structo]

That's a good question about the resistors.
Because if you only have one preamp in circuit at a time then you don't need to mix them.
Now in that Marshall schematic I posted there was another link between the two channels I didn't really understand but I don't know why you couldn't simply use a DPDT relay to switch one hot signal wire with the the other.

You could have one channel on each side of the relay and have one on the normally closed and the other on the other side on the normally open.
Then when the relay is energised it will switch to the other channel.

Here is a diagram of the relay I am talking about.
1 and 16 are the coil lugs.
# 4 and 13 are the common lug of the switching and 6 & 11 are the normally closed. That is the state it is in when not energised.
Lugs 8 & 9 are the normally closed part. So when the relay is energised the common lug is connected to that lug.
You used to be able to buy a little printed circuit board at Radio Shack that would take a relay socket. I think it was made for an IC but the relay sockets fit it perfectly.
If you can't locate those then

It's common to put a diode across the relay coil to prevent spikes.
We use a 1N4007 for that.

Here are the Mouser part numbers for the 5v relay, socket and diode.

653-G5V-2-H1-DC5

575-199316

621-1N4007

This the circuit board I was thinking of. Kind of big but unless you feel like etching your own.
You may be able to find a smaller one but this will give you an idea.
http://www.radioshack.com/product/index ... Id=2103799

[Subjecttochange]

I have got a few of these laying around- I figured I'd use them.

http://www.mouser.com/ProductDetail/Tyc ... 1osBUvQ%3d

Pretty close to the specs you gave me. I wired it up like a normal dpdt though... no wonder that didn't work.

I am going to put in the last stage today and try to wire up the relay- see what happens.