50W Build Questions

straycat · Post by **straycat** » Tue Dec 25, 2007 1:15 pm

1. I am using PEC pots and the buss will run thru the eyelets of the pots that require grounding. The board and input grounds will be tied to that buss. Where should that buss be gounded at the chassis? I see some that use a lug at the choke bolt for the gound. Should the presence pot be tied with the rest of the pots or have a seperate ground tied someplace else?Do not want to create any gound loops. It looks like the alpha pots in the instructions use only the pot as the connection for ground.

2. Where should the NFB connection be ? The instruction have the NFB connected at the speaker jack. The schematic has it connected to the 8ohm lug on the ohm selector. Where should this be connected and is there a difference ?

Thanks for any help!!

toner · Post by **toner** » Tue Dec 25, 2007 2:36 pm

I haven't built the 50w kit but I can help with your NFB question:

I *think* it was standard to put the NFB on the speaker jack in 50 watts. That's probably why the instructions show that way. When you connect it there, the amount of negative feedback will vary at different impedance settings. You'll have more NFB when set to 16 ohms than when set to 8 ohms. You'll have the least NFB on 4 ohms.

When you connect the NFB to a specific impedance switch tap, you'll get the same amount of NFB, regardless of the impedance setting. If connected to the 16 ohm tap, you'll always get that amount of NFB, no matter how the switch is set.

How do they sound?
more NFB = cleaner, tighter bass, "smooth"
less NFB = more power tube gain, looser, "raw"

Earlier amps like JTM 45's, '67 plexi's, etc. have more NFB and later amps like 70's metal panels have less NFB. That should give you an idea of the effects.

neikeel · Post by **neikeel** » Thu Dec 27, 2007 4:56 am

toner wrote: I *think* it was standard to put the NFB on the speaker jack in 50 watts. That's probably why the instructions show that way. When you connect it there, the amount of negative feedback will vary at different impedance settings. You'll have more NFB when set to 16 ohms than when set to 8 ohms. You'll have the least NFB on 4 ohms.

When you connect the NFB to a specific impedance switch tap, you'll get the same amount of NFB, regardless of the impedance setting. If connected to the 16 ohm tap, you'll always get that amount of NFB, no matter how the switch is set.

How do they sound?
more NFB = cleaner, tighter bass, "smooth"
less NFB = more power tube gain, looser, "raw"

Earlier amps like JTM 45's, '67 plexi's, etc. have more NFB and later amps like 70's metal panels have less NFB. That should give you an idea of the effects.

All of the explaination is correct except the bit about std + jacks.

Most lead and bass 50 watters of the era George based his on had NFB on 8ohm tap as std. The organ amps had it on speaker jacks. I prefer 8ohm tap myself as I only ever use one 160ohm 4x12 cab.

straycat · Post by **straycat** » Thu Dec 27, 2007 5:57 pm

Thanks toner and neikeel. I think I am just going to tie the NFB to the 8ohm tap as the 1987 schematic from Marstrans site shows. Thanks for explanation.

Is it OK to tie the preamp grounds along with the presense pot to a solder lug on the choke mounting bolt ?

neikeel · Post by **neikeel** » Fri Dec 28, 2007 7:23 am

I would follow the standard grounding scheme (ie all pre amp earths and input jacks to the pot based bus rail, or inyour case with PECs on the rail you are using on the lugs) and then run a black wire from the presence pot to a star earth back near the PT either on a PT mounting bolt or the filter cap grounding point.

I used PECs on my 45/100 like Flames early build thet work well, no grounding issues.