hum and squeal - hand wired Blackheart Little giant

[mrwilson]

howdy folks
I a new bee to amp building and this is my first attempt

I have a BH5 with a hammond OP trans i followed a BH5 schematic with alnicomagnet stage 2 british tweaks and created my own layout

very excited that it works and makes a great sound
unfortunately it has huge hum and it makes a squeal as you turn up the vol with the master on full (the master vol was an addition when i bought the amp second hand)
the squeal happens on the way up and stops when the max vol is reached

it has DC heaters who's wires i have double checked to insure i didn't flip things around

touching the chassis cuts the level of the hum a little but not entirely.
I know part of the problem is likely to be an earth issue but i cant see where i went wrong

when there is no Gat plugged in and i wave my finger over the input socket inside the amp the hum get really pronounced a little like the sound you get touching the tip of your lead when its out of the Gat.

there is a power scale board which i have bypassed for now as i shorted the amp of first start up and blew the fuses now the PS doesn't work.

the amp is also wired for a tube recto and an optional 6v6gt output tube but they are not in or on when i have this problem…..i figured id try and sort this out before i opened any other potential can of worms.

any suggestions greatly appreciated as i am gagging to play it but the hum is too much to handle

[Jazz P Bass]

The squeal may be the output transformer primary connections.
Try reversing the leads that go to the output tube plates (blue/brown?)
As to the hum. Do any of the controls change the hum.
If this is a high gain amplifier, that may explain the finger waving at the input jack.

[olddawg]

Is the input jack a self terminating type? Is the wire running from the jack to V1 shielded and only grounded at one end? Does the amp still hum if you pull V1?

[mrwilson]

thanks for the fast response fellas thats awesome

i am a real newbee so i'm going to have to ask a few dumb questions

Jazz p bass - the V1a cat resistor is a 1k5 with a 2.2uf cap
v1b cath is 680r with a 2.2uf cap
both have a 100k plate resistor
does that make it a high gain amp? it sounds like one but to be honest i don't know how to work it out. i've just been following the schematic with the alterations. i'd love to know how to do the math if you have a link to point me at..?

and im confused about the output tube plates you mention…doesn't the op tube only have 1 plate? (an EL84 by the way)

Olddawg - the hum disappears completely without the 12ax7 preamp tube. i changed the tube when i replaced it with an old mullard and got some extra noise for my trouble …about the same squeal but some awful crackle like a tv when the channel isn't tuned in.

the wire from the jack to the v1a isn't shielded so i'll fix that as soon as i can. grounded at both ends right?
and no the jack isn't self terminating … as i understand it a self terminating jack makes a connection between the two input leads when the jack is out right? in my case
the jack is a standard mono jack

thanks

[olddawg]

thanks for the fast response fellas thats awesome

i am a real newbee so i'm going to have to ask a few dumb questions

Jazz p bass - the V1a cat resistor is a 1k5 with a 2.2uf cap
v1b cath is 680r with a 2.2uf cap
both have a 100k plate resistor
does that make it a high gain amp? it sounds like one but to be honest i don't know how to work it out. i've just been following the schematic with the alterations. i'd love to know how to do the math if you have a link to point me at..?

and im confused about the output tube plates you mention…doesn't the op tube only have 1 plate? (an EL84 by the way)

Olddawg - the hum disappears completely without the 12ax7 preamp tube. i changed the tube when i replaced it with an old mullard and got some extra noise for my trouble …about the same squeal but some awful crackle like a tv when the channel isn't tuned in.

the wire from the jack to the v1a isn't shielded so i'll fix that as soon as i can. grounded at both ends right?
and no the jack isn't self terminating … as i understand it a self terminating jack makes a connection between the two input leads when the jack is out right? in my case
the jack is a standard mono jack

thanks

I believe that your BH5 is nearly the same as an Epiphone Valve Jr? Single ended with one EL84 right? There are whole forums dedicated to VJs. I have no idea why you would even try to run a bass through one. Oh well.... A self terminating jack does just that, it grounds the input when you unplug so you don't get hum with the amp unplugged. The shieldled wire passed the jack should only be connected on one end. That may be your whole problem, that and where you are grounding your input. You're picking up hum in the first gain stage like an antenna.

[mrwilson]

thanks
yep very similar to the VJs but with a full tone section in between the two preamp stages in the one 12ax7
i'm not running a bass through it...sorry if i gave that impression. cant imagine that would sound to hot.
will sort out the input with a shielded lead and ground it at one end and see how that goes.

thanks again fellas