Replies to This Discussion

Permalink Reply by eric gittins on April 29, 2013 at 3:30pm

that amp looks really good fella, well done.

hum

your earth bus may well be giving you problems with noise.

for my earth bus order i pretty much follow the signal from input to power supply. take a look at the schematic (not the layout), and earth the points in order from left to right on the schematic.  ie. input jack, 5meg res., 1/3/5 on the 6sj7, volume control, cathode res/cap for 6v6, then 3 power supply caps, then to a screw on the power transformer mount to chessis.  this keeps the noisy and high current parts of the circuit away from the input circuitry.  (remember, any noise induced into the front of the amp gets amplified, noise at the tail end not so important as you have a bigger signal to compete with it).

some folks use a probably better system, which runs the bus as listed above, but stops before the filter caps.  so all the signal earths are on the bus bar which goes to the trans. screw on the the chassis, then the filter caps are earthed in order on their own bus which goes to the chassis earth point of the HT centre-tap.

i don't tend to use a star-ground system, though lots of folks swear by it, i don't know that its necessary in a 4W amp.

that being said, it looks like you haven't used a chassis at all in the new cabinet, so have no shielding for the amp at all? the standard earthed chassis system goes a long way to keeping out the hum of the world.

you could perhaps, start with stick-on shielding (like under a fender pickguard) on the timber panels under the tagboard and valve panel - and earth that.  aluminium flashing might suit - just make sure it cant lift up and touch anything live.

i would run a shielded lead to your volume pot too.  a shielded stereo cable works well from the pot, with one colour from the preamp, one colour to the 6v6, and the shield to the earth on the right section of the bus bar.

glad to hear there's no voltage on the speaker anymore.

and that g15 is a great find. now you need to build a Vibroverb to put it in! lucky you're coming out of winter so you can keep up with your shed projects.

cheers

e

Permalink Reply by GARAGE HERMIT on April 30, 2013 at 11:10am

thank's eric for that detailed answer, i'll do the change's to the earthing asap, i've some aluminium tape i could use to sheild it too, not bragging but i've 2 of the G15's, bought them a few year's ago for no particular reason, i just thought they looked good, and would come in for something one day, as you do, i'll have to look up a vibroverb to see what it is, if it's not too complicated, i'll have a go,
yes, the weather's picking up a bit here, i dont have to pour boiling water on the lock's on the garage to get them to work now,
thanks paul.

Permalink Reply by eric gittins on April 30, 2013 at 1:13pm

the blackface viboverb is famous as the stevie ray vaughn amp.  it came first with 2 x 10" speakers, then a single 15". SRV's were seriously modded, and there's heaps of stuff on the net about them.

not really a simple amp build, but you gotta aim high!

i think marshall used the g15 in some of their PAs?

e

Permalink Reply by GARAGE HERMIT on May 7, 2013 at 12:09am

well, messed about with it for a bit, still got some hum, so i've gutted it and decided to use the s/s chassis that was in it,  made a smaller tag board to fit, and moved the OT transformer to get everything to fit,

going to do the same with the bigger cab s/s chassis too, but on that one the transformer i'm using has no 5v output, so i'll use diode's for rectification,

bought some hole punche's too, make's doing the valve base hole's a lot easier,

that "vibroverb" look's a little bit too busy for me, at the moment,

 

Permalink Reply by GARAGE HERMIT on May 7, 2013 at 12:30am

here's the grounding system i was thinking of using, can you see any problem with them,?.

 

 

Permalink Reply by GARAGE HERMIT on May 7, 2013 at 12:40am

the transformer i'm using has'nt a dedicated CT for the 5v and 6.3v heater's, just the factory hard wired CT  shown in the pic, would i have to use a false CT on both the 5v & 6.3v output's,?,

 

Permalink Reply by eric gittins on May 7, 2013 at 4:02pm

wow, all change.

your signal ground shouldn't earth to the chassis at the input jack end, only at the other end after the 8uf cap node of the 6v6 screen. 

run the input jack earth straight to the earth bus, not via the volume pot either. 

the volume pot earth should be down the line after the preamp earths, just in front of the cathode resistor and bypass cap for the 6v6.

the speaker doesn't need to be earthed to the bus bar. it can be earthed at the speaker jack straight to the chassis.

i would lift that link between the HT centre tap and one leg of the 6.3v winding.  that earths one side of your filament supply, which will very likely be noisy, as the filament supply would be unbalanced, and things like twisting the filamant wires around each other wouldn't work to cancel any AC hum. 

the HT CT should go straight to earth, not via anything else, unless you are using a zener to drop the voltage a little or putting a standby switch there.

yes, create an artificial CT for the 6.3 supply after you remove that earth link, with a couple of 100ohm resistors.  if heater hum is still a problem, you could float the artificial CT on top of a higher DC voltage by running the joined ends of the 100 ohm resistors to the cathode of the 6V6 rather than to ground. 

don't make an artificial CT for the 5v supply for the rectifier.  with a directly heated tube like the 5y3, the heater has the whole B+ on it too.

check out merlin's site for lots of info.http://www.valvewizard.co.uk/heater.html

e

and aren't chassis punches the best thing ever?

Permalink Reply by GARAGE HERMIT on May 10, 2013 at 10:48am

top tip's eric, thank's,

the artificial CT, does it go to pin 8 on the 6V6GT, if floating the CT there is better than just earthing it to the chassis, i might as well do it now while i'm wiring it up,

that valvewizard certianly get's into it, very informative,

i've cut the link out between the HT CT and the 6.3v, wonder why it was like that from the factory,

here's a pic of the revised grounding system, hope i've got it right this time,

 

 

Permalink Reply by eric gittins on May 12, 2013 at 1:36pm

yes, still run the the two 100 ohm resistors from either side of the 6.3v winding, and run a wire from where they join together to pin 8 of the 6v6.

some early amp designs ran one leg of the filament supply to the chassis, and then had to run just one filament wire to the valves (earth the valves other filament tab to complete the circuit). it's noisy though, and we would rather have good than cheap!

your earth bus is probably fine, but i would put the volume earth in between the .005 preamp screen cap and the 6v6 cathode bits.

e

Permalink Reply by GARAGE HERMIT on May 13, 2013 at 10:59am

well, finished wiring it up, damn hum is still there at anything above 3/4 volume, put the 100 ohm resistor's on to pin 8 of the 6V6, moved the volume earth too, think i'll try another volume pot as this old one i'm using is a bit notchy, does it have to be 1m or could i use a 500 ohm one,?

Permalink Reply by eric gittins on May 13, 2013 at 2:30pm

is the hum 50hz or 100hz?  50 is filament, 100 is from rectified B+ supply.

i imagine that your 15" speaker would happily go down to 50Hz, a smaller 6" may not.  does the hum go away with as smaller speaker?

input jack isolated from the chassis? best to insulate it.

earth bus line earthing at one point on chassis, filter caps and CT at another? this is also best.

some layouts around the net had the OT feeding from the first filter cap, which is wrong, so check that yours comes from the second powersupply node, and the screen (6v6 pin4) from the third node.

are your transformers orientated at 90deg to each other, if not hum can be induced in the OT.

many people use a choke to filter the power supply in this amp to help with hum problems.

have you tried changing valves?

and hopefully you have not put in the 'death cap' that is in the schematic.  that's the .05 on the AC side of the PT.  you have an earthed mains, and don't need it.

e

Permalink Reply by GARAGE HERMIT on May 21, 2013 at 7:18am

hard for me to tell if it's 50 or 100hz,

big and little speaker's hum,

i've insulated the input jack,

earth bus one point earth, filter cap's and CT at another,

OT feed's correct,

transformer's at 90 to each other,

i have a few choke's, what value woud i need,?

changed valve's, still hum's,

no death cap,

what i've found is the wire from the 6V6 pin 5 to the volume pot is the one making the hum, swapped it for a screened one, still hum's, if i move it about i can reduce the hum, but it end's up in a unuseable position,

any more thought's ?

 

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