Stand alone power amp build

[mrbadwr3nch]

Hey all.

I'm new here and need some help/advice.

I'm fairly new at building amps. I've built some Marshall and Fender clones from kits, Most recently I've scratch built a couple (one for me, one for a friend) of stand alone preamps that are basically the overdrive channel of a Soldano SLO from schem/layout created by Paul Edwards. When I say "scratch" build I mean, I drilled and punched the chassis, made the turret boards etc. The design and tone is killer with a switchable output for inst/line level so you can run it into the front end of an amp like a pedal or into the power amp in/effects return.

Now while it sounds awesome running into a JTM45 or a Shiva or a Winfield, or pretty much anything, I want to build a stand alone power amp to run it into. I want the first one to be the power section of a 5F4 Super.

I've got a decent knowledge foundation but not enough to know exactly how to get this done. I can look at a 5F4 schem and identify the different areas of the circuit but I'm not sure exactly where to cut off the power section and which transitional components need to stay or go. I know I've got to have the phase inverter in there and I want to keep the presence control but where in the circuit is the best place to put the volume? Which filter caps do I leave out? What is the best way to design the layout? Is there some "layouts for dummies" illustration software?

I attached some pics just 'cause everyone likes pics.

Any suggestions and all advice would be GREATLY appreciated!

[Stevem]

Well the first question is that even though you are building the amp that has a feedback circuit with a presence control, would you like the way the amp plays more without that circuit. For me unless I need very loud amp with super super clean tones than other wise I do not like feedback circuits in my amps, and here's why!

For all the good that they do in a amp the one big bad thing that I hate about them is that they make the output stage at a certain playing level jump from being nice and clean into distortion and in some amps there added frequency response on the top end takes the sweetness right out of a amp!
I just do not like the reduction in playing dynamics that too much feedback yanks out of most amps!

In light of this I would try out one of your current amps that have a feedback circuit with it disabled or stick in another resistor off of the speaker output to reduce it and see what you think, the first thing you will notice is a lot more gain so you will have to back the amps volume down some to have a good apples to apples check out session.

In regards to where crack a circuit open to just make a power amp just take a look at something like a Fender blues deville schematic and look where the power amp in jack is.

Filter cap wise the rule of thumb is that for every two gain stages, or one fully used 12AX7 you want one filter to decouple it from the other stages, you can push it to three stages but at high volumes and or gain settings you run a risk of oscillations happening.

By the way, real nice work on the amps there!

[mrbadwr3nch]

Thanks for the immediate response, wisdom and compliments!

I will probably make the NFB switchable for all the reasons you stated.

I have added effects loops to JTM amps so I don't know why I didn't think of your solution. Derp.

So on the 5F4 circuit, would I just run the input jack into a 220k res then into the grid of the PI? Or just straight into the PI grid?

Also, about the layout. This is the daunting part for me. I'm not looking forward to the hours of pencil sharpening and eraser worms. Is there some software or anything to aid in this process? I guess I can use Leo's diagram and just cut off what I don't need but I was hoping to do something more custom.

Now that I've been staring at the layout, it probably won't be that difficult but it would still be nice to do the custom layout on the magic google-box instead of with pencil and paper. Any suggestions for a program to get it done?

Another pic.

Thanks a ton!

[martin manning]

Something like this. I added screen resistors and a bias trimmer. Values for the bias trimmer resistors will depend upon what you have coming from the PT.

[mrbadwr3nch]

I can't express how awesome that is.

I plan on using this PT-http://www.mojotone.com/transformers/Fe ... OJO762.pdf

And this OT-
http://www.mojotone.com/transformers/Fe ... OJO769.pdf

The bias trimmer can be a 10k linear pot right? Or maybe a little explanation of your addition to the bias circuit would help me ALOT.

Also, I see a volume control at the input. Could you help me understand the .02uf cap and 33k res? Is the cap coupling/blocking? And the resistor is...well I'm not advanced enough to know.

What is the benefit of upping the filters to 22uf?

Also what is the AWESOME program you used to draw that schem?

I can't thank you enough for the help! If I'm asking bush-league questions I apologize. Just trying to wrap my head around all of this.

[martin manning]

The input cap is there to block any DC that might be on whatever you plug into it. The level control is a nice-to-have (it will be like a master volume), and the 33k "grid stopper" is there to get rid of RF noise that might be on the input cable. Along with the tube's input capacitance, it makes a low-pass filter.

With a typical 50VAC bias tap on the PT, the resistors could be say 1k from the transformer tap, the diode (1N4007), 47uF cap to ground, then a series combination of a 10k and a 10k pot in parallel to the cap. Adjustable bias is very handy to have when changing tubes. As long as you are adding it, you could add another stage of filtering, another series 1k and a 47uF to ground before the pot-resistor pair going to ground.

The values of the power supply filters are somewhat subjective, but 16u seems a bit low. Incidentally this circuit has its choke before the plate node, which is not often seen due to the large current capacity required.

Another power transformer option is Classic tone 40-18094 http://www.classictone.net/40-18094.html , which is an upright mount. No need to cut a large hole in the chassis.

The mark-ups on the fender schematic are just done in PowerPoint. Nothing fancy.

[mrbadwr3nch]

Sweet. That helps a ton!

I'd still be stuck wiring guitars if not for guys like you.

One more thing, after this power section build and a few more, I want to do a 25 watt, EL34 stand alone power section a-la Soldano Hot Rod 25. He uses 6L6's and some kind of custom PT that's only running like 300-325v to the plates to get that compressed, "Marshall on a turned down variac" sound.

Any thoughts on a standard PT that would get me in the same ballpark?

Thanks again.

[Stevem]

While you are building a nice adjustable bias circuit you should place a 3 watt 1 ohm 1% resistor in series from each output tubes pin 8 to ground as this will allow you to directly check and set the idle bias current with just a meter set for dc volts!
In fact if you bring these two points out to rear chassis mounted female banana jacks you can test and set it without pulling the chassis.
Nice mods on the Fender layout Martin!

[mrbadwr3nch]

While you are building a nice adjustable bias circuit you should place a 3 watt 1 ohm 1% resistor in series from each output tubes pin 8 to ground as this will allow you to directly check and set the idle bias current with just a meter set for dc volts!
In fact if you bring these two points out to rear chassis mounted female banana jacks you can test and set it without pulling the chassis.
Nice mods on the Fender layout Martin!

I will definitely be incorporating your external bias point/adjustment system in all my power amps.

I'm also considering a switchable tube/bridge rectifier. Maybe after I get the basic setup down.

Thanks again.

[tele_player]

While you are building a nice adjustable bias circuit you should place a 3 watt 1 ohm 1% resistor in series from each output tubes pin 8 to ground as this will allow you to directly check and set the idle bias current with just a meter set for dc volts!
In fact if you bring these two points out to rear chassis mounted female banana jacks you can test and set it without pulling the chassis.

A good idea, but why 3 W?

Robert

[Stevem]

I know a two watt is fine, it's just comes out of my mouth as the 3 watts are what I got deal on 5 years ago, 200 for 25 bucks! They must have fallen off a truck or something!

[JazzGuitarGimp]

I don't think this has been nentioned yet: the grid stopper resistor should be installed right at the tube socket, with the lead from the resistor cut as short as possible (basically, the resistor body should all but butt-up to the tube socket pin). This will give you the best radio frequency rejection. This is true for all the grid stopoer resistors in the amp.

[matt h]

(deleted)

[tele_player]

Or 1W. 1/2W would probably work for most common tubes/applications. Come to think of it, 1/8th watters would probably work a lot of the time. Typically common/available/cheap "precision" resistors tend to be 1watters though.

A 1/10th W resistor would handle 250mA for some perspective (which actually requires .0625W Or, uh, just over 1/20th of a watt.)


Using larger wattage values won't hurt anything, but cost more (monetarily and "geographically").

A 6v6 is rated for somewhere around 100mA maximum cathode current, using sufficiently small resistors could actually be used as a safety feature like a fuse. EL84's 65mA. I can't quickly find the max Ik for a 6L6GC but I'd figure in the 250mA range. I never really thought much about this before and probably wouldn't suggest this as the best safety feature ever but stranger things have happened... and made it into production amps.

Bingo!

[Smokebreak]

Cool amp Chris!!! I've just built a high gain preamp, so I'll be doing a power amp soon, too. I look forward to hearing how you like this one. Make sure and post some clips when you get a chance

Some things that struck me : Even though you will have different supplies for power and pre amps, I believe the power section will "sag" when hit hard, with the tube rectifier. This may not be desirable in a high gain amp. Depends on style.

Also, regarding the lower voltage power amp, I think part of the brown sound is lower voltages in the preamp, too. I'm not sure if a power amp ,alone, with lower voltages, could achieve what you are looking for(assuming "high" voltages in the pre). It certainly would be interesting to see.

Can someone explain the benefits of taking the signal off the divider in the CF, and how that seemingly small difference in the leg to ground (on the switch)would change the output level from instrument to line level?

http://www.pauledwards.co.uk/images/Schematic.png