Time for my first recap - Advice sought

[freepressright]

I've done my share of solder-in mods to tube amps over the last couple years. I've learned my way around them fairly well and am abreast of all the safety measures I should take before embarking on my journey to do my first recap of a Rauland-Borg 2010 PA amp that I've converted over for guitar.

Already I have switched out the old Ampenol mic input for a 1/4 inch jack, and have added a 1/4-inch 8 ohm speaker out. Right now, the amp has demonstrated great potential to sound good, but has hum clearly indicative of bad filter caps.

This amp is a little 10-watt amp driven by a single 6L6, has a 12AX7 in the preamp side and a 6CA4 rectifier. It sports a mic volume (now main guitar volume), a phono volume for an RCA input and a tone control.

I have removed the wiring to the RCA inputs to make room for the mini caps I will add. I am going to use Sprague atom caps of the correct capacitance and voltage. Right now the amp has an old Mallory FP style can on top. I will leave it there strictly for cosmetics but will run all the respective positive connections to the positive ends of mini caps.

My question, though, concerns where to ground the mini caps. Can I use what I suspect to be a grounding tab off the original FP can? I have attached a picture of the inside of this amp.

Please note that the wiring you see going from the RCA inputs to the pot in the center no longer exist. I have cleared them out of the way as a place to put the mini caps.

It is also important to note that I will be replacing coupling caps, adding a three-wire grounded cord and new resistors as well. In the end, I will probably even replace the pots.

Any advice you can give me on the proper grounding location for the new capacitors would be appreciated. As I said, this is my first project. I have no money in this amp so it seems like a good launching point.

Thanks so much!

-Adam

[booj]

Can I use what I suspect to be a grounding tab off the original FP can?

sure. but will it prevent hum? I don't know. The thing is a rats nest. (I mean that in a good way....??). If it's low gain you may not have a problem. I'm thinking the layout may cause hum to be transmittd to the preamp stage. Also you may do well to replace the multi-section cap with induvidual caps.

[stokes]

As long as you are replacing the can cap with individual caps it is best to ground the preamp filters over near the preamp tube grounds and the main and screen caps by the power tube cathode resistor and the heater center tap.A problem with can caps is that they ground all the caps together,it is best to seperate the higher current main and screen tap away from the preamp grounds.

[freepressright]

Yes, I am using individual caps to replace the can. I can't find a can anywhere with matching values.

This amp breaks up very nicely when turned up, and ultimately, I am going to adapt a BitMo Trio Kit, as used in the Epiphone Valve Junior, which includes a three-way voicing switch and a pull-for-boost knob. I have an official artist endorsement from BitMo and have their kits in my Blackheart and EVJ. In fact, that's what sparked my interest in tweaking tube amps.

Stokes, what you told me sounds like solid advice, however, I must admit my handicap in translating your words to an actual location. Talk to me like I'm a 12-year-old, if you would please, and explain to me where these grounding locations are. I have my idea, but I want to make sure it jives with yours, as it sounds like you're far more experienced than I.

Thank you, by the way, for taking the time to reply. I find your reply helpful and thoughtful.

-AT

[Enzo]

SOme thoughts. Not trying to disuade you from swapping out the caps, but I want you to do it for the right reasons. Hum comes from many sources. it is not generic all in one noise. And hum from the filter caps and hum from the grounding scheme will have different cures, and the cures for the one will not affect the other. An analogy is aches and pains in your body. You might have a sore muscle or the flu, and either one could make you ache, but for different reasons. And wrapping your knee with an Ace bandage won;t make your arm feel any better.

SO: FIlter caps dry out and allow ripple to increase. You can measure your B+ with a DC voltmeter. If you set the meter to AC volts and re-measure, you will read the ripple voltage. How much you got?

Your amp has a single 6L6, so it is single ended, not push pull. Push pull amps have a natural hum cancellation, and that makes them inherently less hummy than single ended amps. Some of your hum could be a natural result of single endedness.

There are ground currents through any circuit. if the ground of a preamp circuit winds up sharing a current path with power supply currents, hum can be introduced.

And there are other hums, such as from heater interaction with signal elements in preamp tubes. There are grounding and shielding related hums.

Each of these has its own separate cure. One tip is this, power supply related hums will be at 120Hz, twice the mains frequency (100Hz in other parts of the world) grounding related hum and hum from poor sshielding or magnetic coupling will be 60Hz hum.

[freepressright]

Enzo,

Appreciate the insight. And thanks for taking the time to reply.

Speaking of grounding, I mentioned that I want to add a three-wire power cord to this amp. Any suggestions or advice? I've read to run the ground wire underneath one of the transformer lugs/bolts. This amp doesn't have a traditional fuse holder arrangement, but does have a soldered in fuse off the switch. What's the best way for me to approach adding the new power plug to ensure I have properly covered my bases.

I understand that any and all liability for this project is my responsibility. I hold no one else responsible for the actions I choose should they result in an unintended consequence.

-AT

[freepressright]

And just FYI, this amp was free, and while I truly believe it's exhibiting capacitor hum, I'm basically working on it mainly for experience. If I make a mistake along the way, I'll learn from it and I'm not really out anything more than the cost of caps and resistors.

Any advice on my project is highly valued and much appreciated.

-AT

[booj]

>Green - earth ground, or the center prong, goes to either power transformer lug or to a chassis location where the amps other grounds are on the other side of the transformer. or as far from the amp circuit as possible.

>Black - HOT lead. This is the lead that carrys the voltage that changes all the time. That needs to go to a fuse right away. Fender amps generally have the on-off switch before the fuse.

>White - This is neutral, and it is electrically connected to earth ground, but not at neccessarily at the outlet. That is an interesting subject in itself.

That's American color coding. European wiring uses different colors.

[JHow]

What a cool picture. Thanks for sharing! Is that an extra AC outlet on the back of the chassis?

[stokes]

Okay,your main and screen caps should get grounded where you see the green/yellow wire from the power tranny grounded.The other one is the preamp filter,ground that on a ground connection over by the preamp tube.Another source of hum that Enzo pointed out comes from your filaments,this can be lessened by removing that green/yellow wire ,this is your filament center tap,from ground,where it is now,and connect it directly to pin 8 of the 6L6.This will "float" the filament current above your signal ground keeping that hum out of your signal path.

[freepressright]

Okay,your main and screen caps should get grounded where you see the green/yellow wire from the power tranny grounded.The other one is the preamp filter,ground that on a ground connection over by the preamp tube.Another source of hum that Enzo pointed out comes from your filaments,this can be lessened by removing that green/yellow wire ,this is your filament center tap,from ground,where it is now,and connect it directly to pin 8 of the 6L6.This will "float" the filament current above your signal ground keeping that hum out of your signal path.

Stokes,

A big thanks for the advice. I'm about an hour away from starting this project.

I'll let everyone know the outcome.

-AT

[freepressright]

I want to thank everyone who helped contribute advice to my first recap project. It is done and the amp works, WITHOUT the annoying hum that was present originally.

It took me about two hours from start to finish, mainly because I was careful, checking and double checking solder joints and paying very good attention to instructions. I also checked the values on all the existing resistors, and found that only one of them significantly drifted from its original value.

I did make one stupid blunder along the way. When I installed the three-wire electrical cable, I paid no attention to the fact that some moron yanked the grounding pin out of it. I didn't notice until I got home from the workshop at my place of employment. That said, I'll tear it back apart tomorrow morning and install a new cable.

I didn't bother replacing the coupling caps just yet. I wanted to make sure the filter cap fix was what it needed.

I swapped the 12AX7 with a 7062 Sylvania JAN SQ (Special Quality). It has around the same gain factor as a 12AV7. This allows the amp to have a shimmering clean, with only a very slight breakup.

I derive my overdrive channel from a Behringer VT-999 Vintage Tube Monster, 12AX7-driven distortion box with a BitMo Monstrosity kit installed. It's an excellent match to this amp.

This amp has no LED to indicate if it's on or off. How can I power up a 12-volt rated LED from Radio Shack, assuming household current is what I would be tapping at the switch. Do I need a resistor, a voltage regulator or a stepdown transformer of some kind?

Any advice would be appreciated. Thanks!

[freepressright]

Here are some before and after pics of the amp.

-AT

[stokes]

The easiest pilot light would be one of those smallish 120 vac numbers they sell at radio Shack,not the 12 volt one.

[olddawg]

Why don't you remove the phopn volume pot and mount your pilo lamp there? If you want to be fancy, move the other pots and put the pilot lamp on one side or the other.