Amp build for a first time builder?

[martin manning]

lack of a good manual is making me nervous.

i really want to make the 5f6a because i know i'll love the amp when i'm done, but i'm nervous that i'm going to run into problems since there really isn't any good documentation from what i can find.

It's a great amp; an icon, really. There is plenty of info available, starting with the original Fender schematic and layout below.

The '59 Bassman reissue schematic has lots of DC and AC voltages listed, plus some part values that were changed from the old (1957) schematic:
http://support.fender.com/schematics/gu ... ematic.pdf

I have some gut shots pinched from an e-Bay auction that will be a great help in understanding the original. Yours won't be an exact replica, but in fact you can make some improvements to the ground scheme and power supply wiring. I'd also recommend adding a trimmer pot to the bias circuit and 1-ohm resistors from the power tube cathodes to ground so that you can easily set the idle current. I can send or post a marked-up layout that covers these changes and additions. The extra parts are not expensive and can be ordered from Mouser.

[rogb]

lack of a good manual is making me nervous.

i really want to make the 5f6a because i know i'll love the amp when i'm done, but i'm nervous that i'm going to run into problems since there really isn't any good documentation from what i can find.

Dude, Martin and Firestorm have got your back, with these guys on board you have NO reason to feel nervous lol.

Between them they will have all the answers.

Work through the layout, relate it to the schematic, get an idea of the signal path, figure out what voltages go where, measure your components before fitting, do some careful wiring -paying attention to lead dress and filaments. There you go -one killer Bassman!

Enjoy your build!

[yoyohomieg5432]

I have some gut shots pinched from an e-Bay auction that will be a great help in understanding the original. Yours won't be an exact replica, but in fact you can make some improvements to the ground scheme and power supply wiring. I'd also recommend adding a trimmer pot to the bias circuit and 1-ohm resistors from the power tube cathodes to ground so that you can easily set the idle current. I can send or post a marked-up layout that covers these changes and additions. The extra parts are not expensive and can be ordered from Mouser.

Hey Martin,

Anything you want to send me would really be great.

I'm assuming idle current is the current when not playing? Why would you want to adjust this? Or is the idle current when the amp is off and a means of uncharging the capacitors? Or does the idle current refer to the proper current flow from the tubes?

[skyboltone]

I'm assuming idle current is the current when not playing? Why would you want to adjust this? Or is the idle current when the amp is off and a means of uncharging the capacitors? Or does the idle current refer to the proper current flow from the tubes?

Kind of important that you get a handle on idle current. Any tube manual and about 99% of tube theory discussions will get after this pretty good. It has to do with where you operate the tube on the curve.

[OneWithNot]

What Martin means is that the 5F6A Bassman doesn't have adjustable fixed bias. The bias voltage is set by a resistor in that circuit. Most amps now have adjustable fixed bias. You adjust the bias via a potentiometer when the tubes are idle (no input), thats what he means by "idle current".

A lot has changed since that amp has been designed, and a few modifications to the overall layout would produce a supererior outcome as far as performance and reliability are concerned, without changing any of the original design's character. Two of those changes would be adjustable bias and an improved grounding scheme.

A three-wire power cord and elimination of the ground switch and death cap are also necessary for safety reasons.

Have you considered just how freaking loud and heavy a tweed Bassman is? Maybe drop by a local Guitar Center and try out a reissue just to see.

[Firestorm]

Have you considered just how freaking loud and heavy a tweed Bassman is? Maybe drop by a local Guitar Center and try out a reissue just to see.

But lighter than a Bluesbreaker. THAT is one hernia waiting to happen.

[martin manning]

I'm assuming idle current is the current when not playing? Why would you want to adjust this?

Idle current is what most people call "bias." It's the zero-signal or quiescent current.

In preamp stages, the zero signal current is set about half-way between full-on and full-off so that the stage can amplify both positive and negative signal swings applied at the input with approximately equal effectiveness. This is a class A amplifier stage, where the tube is conducting current over 100% of a zero-positive-zero-negative-zero cycle. If the signal is a sine wave you can say it is conducting over 360 degrees of each cycle. If much power is needed, this is very inefficient, as the average current is the same at zero signal as it is at full power.

A big improvement can be made using a push-pull stage with two tubes, where one tube conducts current only on the positive signal swing and the other only on the negative swing. At zero signal, no current flows through either side. The two outputs can then be combined to reproduce the complete signal. This is a class B stage, where each device conducts over exactly 180 degrees of a sinusoidal input signal.

Most audio power amps are class AB, where a push-pull stage is used, but with some overlap so that both sides are conducting current when the signal voltage is near zero, or in other words each device conducts current for more than 180 degrees but less than 360 degrees of a sinusoidal signal.

When you adjust the idle current or bias, you are adjusting the amount of overlap by adjusting a DC voltage reference or "offset" about which the signal voltage varies. If there is too little overlap, distortion of the signal will occur around the point where the signal crosses over from positive to negative. If there is too much, the limit of the ability of the tubes to dissipate waste heat can be exceeded, resulting in shortened life. Production variation from one part to the next results in different idle current for a given applied voltage, so some means to make an adjustment of the bias voltage is useful.

Have you considered just how freaking loud and heavy a tweed Bassman is? Maybe drop by a local Guitar Center and try out a reissue just to see.

Good idea!

Here's my put at a "modernized" 5F6-A layout for critical review:

Edit: Updated layout and schematic is here: https://tubeamparchive.com/viewtopic.ph ... 321#226321

[skyboltone]

Good job Martin! What vector graphics program is that?

Dan

[martin manning]

Nothing fancy, just PowerPoint.

[M Fowler]

I agree the 5f6 is a large cabinet with those four 10 inch speakers. Since it is light weight pine lugging it around isn't all that bad but I never play my clone ever.

Mark

[skyboltone]

I agree the 5f6 is a large cabinet with those four 10 inch speakers. Since it is light weight pine lugging it around isn't all that bad but I never play my clone ever.

Mark

I have a speaker cab modeled after that. I use it with a head amp. It's loaded with Eminence Legend 10"s. Pretty nice.

[yoyohomieg5432]

I have a couple of questions with the schematic

1) in the original schematic, some capacitors have 2 different numbers. For example, one of them is .02 - 400. what does this mean?

2) why is the capacitor in the oldschool ground switch called a death cap?

3) in your modification file, what does B+1G mean? How is B+1G different from B+1?

4) some of the values of caps/resistors are changed. why did you change these?

5)one of the resistors has a value '6k8'. I havent seen this format before. what does the 8 after mean?

thanks for the help

[yoyohomieg5432]

one more: i was looking at the layout from the weber website:
http://taweber.powweb.com/store/5f6a_schem.jpg

this one uses 6l6, 12ax7, and wz34.
the schematic from you uses 5881, gz34, 12ax7, 12ay7.

why the difference in tubes?

[johnnyreece]

Weber uses tubes readily available. The WZ34 is their copper cap solid-state replacement for a GZ34. They frequently use a 12AX7 in place of a 12AY7 (the 5E3 clone I built from them was the same way).

[Reeltarded]

I have a couple of questions with the schematic

5)one of the resistors has a value '6k8'. I havent seen this format before. what does the 8 after mean?

thanks for the help

6k8 is 6.8k. It just gets away from using a decimal to make things less (and more) confusing.

6M8 would mean 6.8 meg, and so on.