Never met this before

[Leevi]

I'm testing an AC30/DC30 based amp I got recently wired.

Preamp is grounded to a ground bus which is soldered to the pots and jacks.
The power amp is grounded to a star which is connected to one of the PT bolts.

There is a low level ground hum which can be heard even when the master vol is off.

I'm using choke and an extra filtering stage before I take the B+ to the OT.

When I test the amp with unplugged guitar the hum doesn't increase remarkable when i open the vol.

If I connect the bus ground and star ground together I get totally rid of the low level basic hum
but when I open the vol I get a very huge hum? Can the so called ground loop behave like this?

/Leevi

[Platefire]

I would start with some MM continuity checks on all ground connections. Try another guitar and cord. Have you got sheilded on grid input and to second stage preamp? Plate

[stratele52]

Leevi can you send the schematic ?  Do you do a clone of .... ?

Schematic of your amp as built could be very usefull . Also photos inside .

[Leevi]

Schematic of your amp as built could be very usefull . Also photos inside .

Little bit modified from this:
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/newamps/matchless_dc30_old.pdf

/Leevi

[stratele52]

" Little bit modified from this... " is enough to have some hum . You do not have sketch your mods ?

Without having the amp in front of me + no as bilt sketch..... . 


But more chance is a wire dress problem . I'll look at your photos

[Leevi]

You do not have sketch your mods ?

Some modifications
- No effect loop
- 500k master vol pot
- different pt 100 Ohm resistors in heaters instead of center tap
- power cathode including the half power switch implemented in different way (150R and 150R parallel when full power)
- EF86 plate with 220k and screen with 1M
- no master by-pass
- FAN with switch full-wave rectified from 5v
- stand-by in B+ with 10n cap

/Leevi

[Leevi]

I succeed to solve the problem.

I moved the start ground to the middle of the chassis quite close to the
cathode resistors. The preamp grounding  (bus) is still separated fro the power amp star.

Almost no hum at all can be heard even the master vol is fully open.

This time the PT bolt as ground was not a good solution.
Sometimes it works well?

Thanks
Leevi

[stratele52]

Chance are you have some bad wiring dress , but too hard to tell from our photo. There are many thing I have questions about , many thing I can't understand.

[stratele52]

This time the PT bolt as ground was not a good solution.
Sometimes it works well?

 

Good to know everything fine . Ground with PT bolt is very good IF contact with chassis is good . You move the ground and you do a good contact now.