Brass grounding plate

[Jarvini]

Thanks again Mike can't thank you enough.
I take it that means the shield gets grounded to the top lug of the input jack rather than an earth tag on the 9 pin tube socket bolt

[mikepukmel]

Also, Scott, if you haven't yet done so, please check out the many great tutorials on draining the filter caps. It's a matter of life or death (or severe injury!!) Can't wait to see photos of your build, and hear some sound clips!

[mac dillard]

You may like to take a look at this for grounding a Fender AB763. http://sluckeyamps.com/misc/AB763_Deluxe_Lite.pdf

[mikepukmel]

Mac, nice work, thanks for the link.

[Jarvini]

Thanks for the heads up on the filter caps Mike don't worry I've watched numerous you tube tutorials on how to safely discharge the caps and safe working practices with tube amps I did stumble across a guy called roy Blankenship whilst trawling the net and his amps looked fantastic! I'll post some pics of the build in due course I'm building the amp as a head instead of a combo as I have a few different speakers I'm going to build into cabs as well for different impedance options so it might take a while but Rome wasn't built in a day and all that
Thanks again
Scott

[soundmasterg]

I've added a picture of the layout that I'm using so I can ask you all another question ?
Should the wire from the 1.5k on the far right of the board go to the top lug of the input jack and have a wire connecting the middle lug to the top lug and should there be a wire connecting the top lug to the brass grounding plate as there is no mention of this in the build notes but the earth symbol leads me to believe the top lug should go to earth ?

The 1.5k resistor in your linked layout is a cathode resistor. The two caps next to it are connected to a switch in order to switch to different gain levels. All of these go from pin 3 of the tube (the cathode) to ground. If you use a random chassis ground method like Fender's with their brass plate then grounding it at the same place as the rest of the preamp stages is fine, or you can ground it through the jack contact like they show in the layout. If using a bus bar method with isolated jacks, then you need a wire to go from the top lug of the jack on your layout diagram to the ground buss. It all depends on what ground scheme you use.

As Mike mentioned, the 68k works best as a grid stop if it is connected directly to the tube socket with a short lead. That resistor also serves to isolate the inputs from each other if you have dual inputs like on a vintage Fender. It will function fine on the board also as the many vintage amps working fine all these years can attest to. It just works better if mounted directly to the socket. The 'grid stop' function of that resistor becomes more critical in later stages, but keep in mind that most vintage amps didn't use grid stops except for the 68k on the input, and they mounted them everywhere except right at the socket, so it isn't something that is completely necessary in all designs on every stage in the amp. There should always be one on the input however or your amp can oscillate or start to amplify radio.

@Mike, the pots usually ground to the chassis with their star washers and nuts. All that is grounding at that point is the pot casing btw. If you are going to go to the trouble of soldering a buss to the back of the pots then isolating them helps, but it isn't really necessary to go to all that trouble. The pot casings ground fine to the chassis as long as the nuts are tight and you use the star washer, but grounding the pot casing is just helping with shielding. The circuit grounds are what really matter and any point on the pot lugs that is grounding should be controlled rather than random. Most of the pots casings today are not easy to solder to and for something that isn't really necessary, it is a lot of trouble to go to in order to make it functional. Remember that any buss to the back of the pots is so the casing will help in shielding, but it has nothing to do with your circuit grounds.

Greg

[mikepukmel]

...

@Mike, the pots usually ground to the chassis with their star washers and nuts. All that is grounding at that point is the pot casing btw. If you are going to go to the trouble of soldering a buss to the back of the pots then isolating them helps, but it isn't really necessary to go to all that trouble. The pot casings ground fine to the chassis as long as the nuts are tight and you use the star washer, but grounding the pot casing is just helping with shielding. The circuit grounds are what really matter and any point on the pot lugs that is grounding should be controlled rather than random. Most of the pots casings today are not easy to solder to and for something that isn't really necessary, it is a lot of trouble to go to in order to make it functional. Remember that any buss to the back of the pots is so the casing will help in shielding, but it has nothing to do with your circuit grounds.

Greg

(Scott, don't mean to hijack your thread, this is a great topic hope you don't mind a few related questions . . .)

Greg, thanks! I am still not very nimble with the big soldering iron, and was afraid to melt a bunch of stuff, so in my recent build, I didn't solder anything to the back of any pots (ground, etc). Anyplace that needed a ground, I routed a wire. Sounds like Im not quite there yet since I routed ground wires onto the brass plate (of a AB763 Deluxe type frankenstein) but the grounds are not not all in the same spot, and there are quite a few. (6 total)


Re http://www.thevintagesound.com/ffg/s...763_layout.gif there are 2 grounds one on the vol and one on the bass pots, and then the shielded wiring will produce a 3rd, for each channel. If these ground wires are all run down to the brass plate, but not at a star, will this cause a problem? Its not a bus type, but not fender type either. I could never figure out why they'd solder something to the back of a pot that was a ground in the schematic. If the pot got loose, then all kinds of crackle and hum could occur (had that happen on a really ancient blackface princeton I had ages ago). Maybe its a "Danger Will Robinson" to tell you to go tighten the pots?

[Jarvini]

hijack all you want the more i can learn the better and as ive had some great help so far i might as well post this here too

this is what ive currently sourced for my build
all my signal caps are tube amp doctor mustard caps
the filter caps are sprague atom as are the 25uf/50 electrolitics
500pf silver mica cap for the tone to volume pot
switchcraft jacks
allen bradley 1 meg audio pots
classic tone usa 40-18031 15w output transformer
ceramic valve sockets
mini boost switch
1.5mm grp sheet which i will make my own turret board from (i have the turrets too) i know fenders use eyelet board but i think turret boards look neater and will be a bit easier to work with
im going to opt for carbon comp resistors (allen bradley) im going to purchase these in one hit if i can as the postage from the usa will kill me if bought seperately

ive also bought lots of different wire various gauges and colours but its all stranded not sure if solid core would be easier to work with though?

so whats left well im going to build it in a much larger chassis than a champ probably a jtm 45 size which i will buy blank and have my engineer friend do all the hole cutting for me (handy friend to have) im going to make it so the valves come out the top not the back

now for the questions
1) the output transformer has three output options 4/8/16 ohm can i build the amp using all three options with three output jacks instead of two (only ever using one) as i have 4/8/16 ohm speakers that im going to build in cabs

2)what would the best power transformer be as the suggestions in the book seem to be hard to locate the specs are as follows:-

240v primary
secondary 310v-0-310v to 330v-0-330v (rated for at least 70mA-80mA)
center tapped 6.3v ac filament supply at 4A
5.0v ac rectifier filament supply at 3A

what do you think so far ? i know you guys could probably build this amp with your feet in the dark but as its my first i welcome any advise
Dave Hunter was right though it is a minefield of contradicting opinions and preferences and very very addictive and i love it even if it is costing me small fortune in postage from the states

[soundmasterg]

(Scott, don't mean to hijack your thread, this is a great topic hope you don't mind a few related questions . . .)

Greg, thanks! I am still not very nimble with the big soldering iron, and was afraid to melt a bunch of stuff, so in my recent build, I didn't solder anything to the back of any pots (ground, etc). Anyplace that needed a ground, I routed a wire. Sounds like Im not quite there yet since I routed ground wires onto the brass plate (of a AB763 Deluxe type frankenstein) but the grounds are not not all in the same spot, and there are quite a few. (6 total)


Re http://www.thevintagesound.com/ffg/s...763_layout.gif there are 2 grounds one on the vol and one on the bass pots, and then the shielded wiring will produce a 3rd, for each channel. If these ground wires are all run down to the brass plate, but not at a star, will this cause a problem? Its not a bus type, but not fender type either. I could never figure out why they'd solder something to the back of a pot that was a ground in the schematic. If the pot got loose, then all kinds of crackle and hum could occur (had that happen on a really ancient blackface princeton I had ages ago). Maybe its a "Danger Will Robinson" to tell you to go tighten the pots?

I've never bothered to solder to the back of pots. I've done it in guitars but in the amps there is no point as if mounted correctly with the serrated washer and the nut, it will ground the case of the pot to the chassis. Grounding to a brass plate at multiple points is the same as a random chassis ground, since the brass plate is contacting the chassis at many points. It isn't even necessary to ground the signal ground to the chassis in order for the circuit to function. The chassis is a convenient reference point that can help with shielding but the amp will function without being grounded to the chassis. You still always need the safety ground to the chassis....that is required by code. Sometimes a piece of equipment will have a ground lift switch, especially studio recording equipment that is meant to connect to multiple chassis. When ground is lifted, then you have a particular sized resistor and capacitor that ground goes through on its way to the chassis which grounds it at AC.

Grounding randomly to the chassis will work....most vintage amps were done this way. There are better ways though, which you are discovering. Instead of using a brass grounding plate, connect all of your grounds to a buss bar that is on your board. The connect that buss at one end only to the chassis....say at the input of the amp. Use isolated jacks and connect all their ground points to the buss also, along with any other grounds in the amp...BUT, connect them in the proper places on the buss bar. In order for this to work properly, do not locate all your filter caps in one spot like Fender does...put them near the circuits they supply so you can have local stars where each circuit supplied by the cap will connect.

I'll have to look at your link and comment later...gotta leave work for the day.

Adding to prior comments:

The link you attached is just a vintage Fender, which used random chassis grounding. It works ok for lower gain circuits like that particular Fender but hums more than it should and as you get more gain, it doesn't work very well at all. Adding a shielded cable doesn't really have any detriments as long as you only ground one end of the shield. If your grounding scheme is the random chassis ground, then adding your shield to the back of a pot or to the brass isn't going to make a difference. Its no ground loop because only one end of the shield is grounded.

Fender made their amps with the grounds to the back of the pots because they could wire up the pot outside of the amp and just put the pot in and not have to solder another wire. It took less time and money basically. Also, back in the day, most of the general population was somewhat familiar with tubes and tube circuits since they were still in use in general consumer products, so they were aware of the pitfalls, and if a pot came loose they might have had an idea that they may have to tighten up a pot.

Greg

[mikepukmel]

hijack all you want the more i can learn the better and as ive had some great help so far i might as well post this here too

this is what ive currently sourced for my build
all my signal caps are tube amp doctor mustard caps
the filter caps are sprague atom as are the 25uf/50 electrolitics
500pf silver mica cap for the tone to volume pot
switchcraft jacks
allen bradley 1 meg audio pots
classic tone usa 40-18031 15w output transformer
ceramic valve sockets
mini boost switch
1.5mm grp sheet which i will make my own turret board from (i have the turrets too) i know fenders use eyelet board but i think turret boards look neater and will be a bit easier to work with
im going to opt for carbon comp resistors (allen bradley) im going to purchase these in one hit if i can as the postage from the usa will kill me if bought seperately

ive also bought lots of different wire various gauges and colours but its all stranded not sure if solid core would be easier to work with though?

so whats left well im going to build it in a much larger chassis than a champ probably a jtm 45 size which i will buy blank and have my engineer friend do all the hole cutting for me (handy friend to have) im going to make it so the valves come out the top not the back

now for the questions
1) the output transformer has three output options 4/8/16 ohm can i build the amp using all three options with three output jacks instead of two (only ever using one) as i have 4/8/16 ohm speakers that im going to build in cabs

2)what would the best power transformer be as the suggestions in the book seem to be hard to locate the specs are as follows:-

240v primary
secondary 310v-0-310v to 330v-0-330v (rated for at least 70mA-80mA)
center tapped 6.3v ac filament supply at 4A
5.0v ac rectifier filament supply at 3A

what do you think so far ? i know you guys could probably build this amp with your feet in the dark but as its my first i welcome any advise
Dave Hunter was right though it is a minefield of contradicting opinions and preferences and very very addictive and i love it even if it is costing me small fortune in postage from the states

Im too much a newby to tell you what to buy, but your parts look pretty similar to what I got, with a few exceptions.
The spragues are really expensive, ended up with F&T brand. Specs looked good and quality reviews are good as well.
Classic tones have a good rep. Only reason I got hammonds is that they fit the chassis I got on Ebay.

Ask around the gurus here about carbon comp resistors. I think you're better off with metal film most anywhere in your circuit.
I got stranded wire due to suggestions on this blog. A few of the more experienced builders says that if you bend back and forth, which we'll probably do as new builders, the solid will break more easily, and really tough to find when a break is inside the insulation.
Awesome idea to put your build in a big chassis in a head box. I fought too hard to wire up this thin fender style with my still weak soldering and general building skils. Wish I had used a head style box and bigger chassis.

Re the transformer specs, 3a seems fairly standard for 5v sec. since a 5AR4/GZ34 use 1.9A on the 5v line.
Seems like you have a lot more current on your 6.3v line than necessary. A 6V6 takes 0.45a, so two take 0.9a, 6 12AX? take 0.3a each or 1.8a, so a deluxe reverb with tremolo needs about 2.7a filament, so 4a is quite a bit higher than you need (as far as I can tell).

Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Power Transformers

The single ended OT's have about 2a on the 6.3 sec, the two 6V6 (e.g. deluxe) have 3a on the 6.3 and bigger, e.g. vibrolux 2x 6L6 have 4a.
So, you would probably be able to get away with a smaller power transformer, unless you're planning on adding lots of stuff later.

Oh man, I wish I could build one of these that easily. Ive been months on my first build, still not done. But .... I haven' blown anything up yet!

[mikepukmel]

Thanks Greg.

[soundmasterg]

this is what ive currently sourced for my build
all my signal caps are tube amp doctor mustard caps
the filter caps are sprague atom as are the 25uf/50 electrolitics
500pf silver mica cap for the tone to volume pot
switchcraft jacks
allen bradley 1 meg audio pots
classic tone usa 40-18031 15w output transformer
ceramic valve sockets
mini boost switch
1.5mm grp sheet which i will make my own turret board from (i have the turrets too) i know fenders use eyelet board but i think turret boards look neater and will be a bit easier to work with
im going to opt for carbon comp resistors (allen bradley) im going to purchase these in one hit if i can as the postage from the usa will kill me if bought seperately

ive also bought lots of different wire various gauges and colours but its all stranded not sure if solid core would be easier to work with though?

so whats left well im going to build it in a much larger chassis than a champ probably a jtm 45 size which i will buy blank and have my engineer friend do all the hole cutting for me (handy friend to have) im going to make it so the valves come out the top not the back

now for the questions
1) the output transformer has three output options 4/8/16 ohm can i build the amp using all three options with three output jacks instead of two (only ever using one) as i have 4/8/16 ohm speakers that im going to build in cabs

2)what would the best power transformer be as the suggestions in the book seem to be hard to locate the specs are as follows:-

240v primary
secondary 310v-0-310v to 330v-0-330v (rated for at least 70mA-80mA)
center tapped 6.3v ac filament supply at 4A
5.0v ac rectifier filament supply at 3A

what do you think so far ? i know you guys could probably build this amp with your feet in the dark but as its my first i welcome any advise
Dave Hunter was right though it is a minefield of contradicting opinions and preferences and very very addictive and i love it even if it is costing me small fortune in postage from the states

The TAD mustard caps will work fine, but so would Mallory 150's, Sozo's, Jupiters, polyester Orange Drops like the 6PS series, etc. As long as the caps aren't leaking DC and are polyester then the amp will sound fine...if you use polypropylene caps, then sometimes they can be a little too good for a guitar amp and the highs will be a little harsh. For hifi polypropylene is great though. But cap differences are very subtle so it isn't a huge difference from brand to brand or type to type with like values.

Sprague Atom electrolytic caps are VERY overpriced and shouldn't be used anymore these days. They are old technology and are often a modern cap inside the very large Sprague case. At three times the price that is stupid. They will work ok though.....but in the future, get some F & T, Xicon, Rubycon, Nichicon, or Panasonic's. Stay away from Illinois caps...they are not very good.

The silver mica makes things a little smoother in that position...sometimes the ceramic sounds better there also. Those caps are cheap so you can try either one.

Switchcraft jacks are great, though if you got the standard ones, you can't fit the isolating washers on them. They sell longer shaft ones for that purpose....so with those ones you have, you'll have to use the random chassis grounding or any other method that does not use isolated jacks.

Allen Bradley pots are fine but no need to go 1M....the usual 1/2 watt are fine.

Those transformers are fine.

The ceramic sockets will be ok as long as they were good quality. Ceramic sockets are harder to tension, and they won't arc like the plastic types will, but the tension part is what I don't like. McMurdo, Cinch, Belton all made good sockets and I know Belton is still around...not sure about the other two. The McMurdo and Cinch were UK based companies I believe so you might find more of them over there in surplus places.

Allen Bradley made good carbon comp resistors, but those type of resistors are noisy and their values change with applied voltage, heat, age, etc. They cause distortion that can be pleasing to the ear but only if used in positions where there is lots of voltage swing, such as the plate resistors in a phase inverter. Otherwise, good quality metal film will make for a much quieter hum and hiss level in your amp and they sound fine. Just make sure to get good quality metal film and not cheap crap from China. One other caveat with metal film resistors and a lot of modern resistors is that they don't have long leads when compared to vintage stuff so you have to redesign the layouts in the amp to fit the modern parts. Carbon comp resistors are also more expensive these days than metal film in most cases. Carbon Film is a middle of the road choice but they are cheap and sound fine also.

Stranded wire is a better choice. Solid core is easier to put in place where you want it, but the wire is often nicked when you strip it and then eventually it will fail. Teflon wire will stay in place with stranded but it is hard to strip. Not all wire is created equally either btw. The wire that Weber sells comes from China I think and it is pretty lousy....but Hoffman sells good stuff...AES/CE Dist sells good stuff. I get mine locally at a supplier for Intel for a great price for PVC and Teflon. For the vintage style cloth covered I get it from Hoffman. There is also Apex JR for some great surplus stuff. Word of advice...pick a color code to use for your wiring so it is easier to spot what is what when you look in the chassis. For instance, red is B+ and plates, orange is screens, green is heaters, brown is cathodes, purple are grids, etc.

Just put three jacks on the back and label them accordingly and use whichever one you want at a time. Example: plug your 8 ohm cabinet into the 8 ohm jack. Next week plug your friend's 4 ohm cabinet into the 4 ohm jack, and the next time plug your 16 ohm cabinet into the 16 ohm jack. Just be sure to match your load up and you are fine as long as your speaker cab can handle the power.

I'm not sure as far as the power transformer. Look around a bit. Classic Tone, Heyboer, Weber, Hammond....all of them are fine and reasonably priced. Mercury Magnetics are fine but they are WAY overpriced. I like Hammonds for power transformers but don't like them much for outputs. In the UK there are likely many surplus around but maybe you can purchase Drakes or Radiospares or something like that. I use Heyboer a lot for my home brewed stuff, but I often have them make me custom stuff.

Greg

[mikepukmel]

re ... Teflon wire will stay in place with stranded but it is hard to strip. ...
Greg

More newbie stories:

Oh yeah, found out the hard way! I bought this high rolling no-nick wire stripper. Tried it out on a bunch of stuff around the house, worked great. Then I bought a bunch of mil spec stranded, silver coated copper, teflon insulation for the amp build. Aaaand, the no nick stripper won't strip the dang stuff. it peels the insulation off one side, but won't strip it. So, it was back to the awful ordinary strippers, but still really hard to strip. I finally figured out to first setup the stripper for a little bit bigger gauge wire than Im using, so the blade doesn't touch the wire, then grab the wire with the stripper, then rotate the stripper around the wire a few times to score that tough teflon, and pull hard. About 50% of the time only one side of the wire tears, but the wire doesn't get nicked, so I can peel the teflon off the wire, and cut it with a clipper. Really wish the high rolling strippers worked.

A benefit of this wire is that it takes a HUGE amount of heat. The insulation on the transformers starts to melt back a bit if I hold the soldering iron on just a second too long, but Ive unsoldered, moved and re-soldered the mil spec wire and the insulation is still perfect. Even bumped the iron tip into the insulation a few times and it just leaves a little waste solder mark on it, it doesn't burn it. Amazing stuff that way. I think I'll still use it on the next build and try to find a better stripper made for this stuff.

[Jarvini]

Thanks for all your help guys so glad I found this forum and that people are so willing to offer such solid advice really helpful things are starting to make sense now
Scott

[soundmasterg]

More newbie stories:

Oh yeah, found out the hard way! I bought this high rolling no-nick wire stripper. Tried it out on a bunch of stuff around the house, worked great. Then I bought a bunch of mil spec stranded, silver coated copper, teflon insulation for the amp build. Aaaand, the no nick stripper won't strip the dang stuff. it peels the insulation off one side, but won't strip it. So, it was back to the awful ordinary strippers, but still really hard to strip. I finally figured out to first setup the stripper for a little bit bigger gauge wire than Im using, so the blade doesn't touch the wire, then grab the wire with the stripper, then rotate the stripper around the wire a few times to score that tough teflon, and pull hard. About 50% of the time only one side of the wire tears, but the wire doesn't get nicked, so I can peel the teflon off the wire, and cut it with a clipper. Really wish the high rolling strippers worked.

A benefit of this wire is that it takes a HUGE amount of heat. The insulation on the transformers starts to melt back a bit if I hold the soldering iron on just a second too long, but Ive unsoldered, moved and re-soldered the mil spec wire and the insulation is still perfect. Even bumped the iron tip into the insulation a few times and it just leaves a little waste solder mark on it, it doesn't burn it. Amazing stuff that way. I think I'll still use it on the next build and try to find a better stripper made for this stuff.

You could try a thermal stripper for the teflon. I've got one and it works well, but I usually just use the regular PVC stranded with a normal stripper. Once you get good with your soldering skills you will rarely melt the insulation. Make sure to use a temp controlled soldering station rather than just a soldering iron that just plugs in. They work MUCH better and allow your soldering to improve 100%. Also pre-tin your tip with leaded solder before you touch it to your joint. It will transfer heat better and quicker to the joint so you spend less time with the iron heating the joint and other things up. Make sure your tip is clean too...every time you go to solder, clean it then pre-tin. Another benefit to the soldering stations is that the tips last MUCH longer.

Greg