Epiphone EA 28 RVT 120hz hum

[hereforever23]

I am driving myself crazy trying to get to the bottom of some 120hz hum in my recently purchased Epiphone EA-28 RVT. Its the same circuit as a Gibson GA 19 RVT

The hum follows the volume pot and goes away when the V1 tube is removed

Filter caps are new as are all the other electrolytics in the circuit
There looks to be some heat damage to the first dropping resistor in the power supply, R12. Would that be the source of any hum?

I've experimented with where the preamp filter cap is grounded with no success

Any ideas?

http://elektrotanya.com/PREVIEWS/63463243/23432455/oldies/epiphone/epiphone_ea-28rvt_schematic.pdf_1.png

[stratele52]

Do your test with NO guitar plug in the amp !

No hum with no V1 ; V1 is defect OR problem is BEFORE V1, You have V1 a and V1 b
 
Before V1 , check if shorting input jacks are working
Check wiring from input jack to V1 grids. The wiring could act like an antenna. You can replace by shielded wires.


Be sure heater have a center tap grounded , or artificial ground. And use 2 heater wire. Schematic is poor

[smackoj]

ditto to what the 'strat' man suggests. also, check all the ground connections and re-route if they don't comply with this setup or something similar.

One central star ground point for:
1 main filter cap grd
2 cathode circuit for output tubes
3 screen supply filter grd
4 the bias supply grd OR output grid return resistor circuit grd if self biased output
5 center tap for the B plus winding on the PT

Second central ground location for:
1 all preamp cathode circuit grds
2 all preamp grid return ciruit grds
3 all preamp filter caps
4 input jacks

If the epi 28 uses a steel chassis, double check any soldered grounds to the chassis with a really big 'HOT' solder iron

 

[hereforever23]

Hum is there when grid of V1a is grounded
The hum is not there when V1 is removed.
V1b is part of the reverb driver...I have the reverb switch engaged (grounding R41/C23) and have disconnect the lead to R2 (reverb pot). Same hum level
Heater center tap is grounded even though on the schematic it calls for it to go to power tube cathodes

Currently the main filter cap ground is with B+ center tap
I think the power tube cathode is with the preamp filter caps, preamp cathode which is grounded at one of the input jacks
I will check when I get to the bench again

Here is a better link for the schematic
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/gibsonamps/ea_28rvt-pathfinder.pdf

[stratele52]

Your Heater need a 100 ohms résistor each side to ground and center tap to power supply ground not 6V6 cathode.  Do like all Fender Blackface are.

Lead from R2 wiper to V1 b grid could give some hum . Disconnect it from the grid of V1 b not from the wiper side of R2

All grid's wire must be very very short or shieded for no hum.

Nice schematic ,thank's, it help

[HotBluePlates]

I am driving myself crazy trying to get to the bottom of some 120hz hum in my recently purchased Epiphone EA-28 RVT. Its the same circuit as a Gibson GA 19 RVT

The hum follows the volume pot and goes away when the V1 tube is removed

Filter caps are new as are all the other electrolytics in the circuit

120Hz hum seems unlikely, as long as you have good new caps and they are soldered into the amp properly (no cold solder joints or bad grounds).

The original schematic suggests can caps (A and B sections for each filter cap). Is that what you used?

The heater CT can be attached to the 6V6 cathodes, as it provides a free d.c. point to elevate the heaters. Which brings me to my last tip to try...

Do you have an extra cathode bypass cap handy? Say 25uF up to 250uF?

If so, try tacking it across the existing cathode bypass cap for V1. Hum may go away, which would indicate heater-to-cathode leakage of V1. At that point, you could either suspect a bad new cap for V1's bypass, or you could choose to replace with a tube exhibiting less leakage hum or you could permanently increase the size of V1's bypass (but that might make the amp sound muddy).

[stratele52]

HotBluePlate,

" The heater CT can be attached to the 6V6 cathodes, as it provides a free d.c. point to elevate the heaters. "
______________________
In my experience this is sometimes not quiet as center tap at ground with two 100 R

[HotBluePlates]

Either way, he's claiming 120Hz hum, which should be power supply related.

But we're debating sources of 60Hz hum.

Either he's wrong in the assessment of the hum frequency, or the new electrolytics are faulty or need soldering touchups (or aren't as new as we might assume).

[stratele52]

Either way, he's claiming 120Hz hum, which should be power supply related.

But we're debating sources of 60Hz hum.

Either he's wrong in the assessment of the hum frequency, or the new electrolytics are faulty or need soldering touchups (or aren't as new as we might assume).

We are not sure , we can't hear the hum and see the amp

[hereforever23]

120Hz hum seems unlikely, as long as you have good new caps and they are soldered into the amp properly (no cold solder joints or bad grounds).

The original schematic suggests can caps (A and B sections for each filter cap). Is that what you used?

The heater CT can be attached to the 6V6 cathodes, as it provides a free d.c. point to elevate the heaters. Which brings me to my last tip to try...

Do you have an extra cathode bypass cap handy? Say 25uF up to 250uF?

If so, try tacking it across the existing cathode bypass cap for V1. Hum may go away, which would indicate heater-to-cathode leakage of V1. At that point, you could either suspect a bad new cap for V1's bypass, or you could choose to replace with a tube exhibiting less leakage hum or you could permanently increase the size of V1's bypass (but that might make the amp sound muddy).

Yes I have spare 25uF to 50uF caps- I cant try that

I used individual filter caps.  I can try to reroute the heater CT.  Perhaps I should also try running them off a lantern battery.

I came to think it is 120hz by comparing it to the sound of my iphone running a sine wave at 120hz

[stratele52]

I came to think it is 120hz by comparing it to the sound of my iphone running a sine wave at 120hz

+1

[hereforever23]

Ok I have verified it to be 120hz with a DMM measuring speaker
I'm literally pulling my hair out
I rechecked all filter cap solder joints and hum is still there... !!!

[stratele52]

Can you send us some close up photo of you amp wiring , around V1  ?

Sure there is something you don't see , don't tell...or do well . You're not a tech.

[HotBluePlates]

Just checking. Your B+ rail does consist of:

5Y3 -> 20uF (OT center tap) -> 470Ω -> 20uF (6V6 screens) -> 4.7kΩ -> 10uF (phase inverter, reverb driver, trem oscillator) -> 10kΩ -> 10uF (V1, V2)

Correct?

Pictures of your amp's filter caps and grounding locations may help. You want the shortest possible loop of wire (or conductive path) for the PT center tap, 20uF caps and ground of the 6V6 270Ω cathode resistor and 20uF bypass cap. For other filter cap stages, you want to have a single path from cap - to cap -, and connect the grounds of preamp stages to the filter cap - that feeds them.

[hereforever23]

Just checking. Your B+ rail does consist of:

5Y3 -> 20uF (OT center tap) -> 470Ω -> 20uF (6V6 screens) -> 4.7kΩ -> 10uF (phase inverter, reverb driver, trem oscillator) -> 10kΩ -> 10uF (V1, V2)

Correct?

Yes.

I just realized that the secondary center tap seems to run back to the same ground point as the preamp and various cathodes (including 6v6 cathodes)
That would be the red/yellow wire
The heater center tap (green/yellow) also grounds here

Meanwhile the 2 20uF caps ground on a PT bolt along with a white wire...not sure what that wire would be but I assume it is not the PT center tap

I will post some pics the next chance I get if rerouting the center tap does not help

[Willabe]

Meanwhile the 2 20uF caps ground on a PT bolt along with a white wire...not sure what that wire would be but I assume it is not the PT center tap

Where does the other end come from?


              Brad     

[hereforever23]

Meanwhile the 2 20uF caps ground on a PT bolt along with a white wire...not sure what that wire would be but I assume it is not the PT center tap

Where does the other end come from?


              Brad     

sorry that wasnt clear
it also comes from the PT

[HotBluePlates]

Now this makes sense.

You have confirmed the hum frequency, which has to come from a full-wave rectifier. The only one of those in your amp is the 5Y3, so the hum has to be related to your B+ rail.

The biggest ripple current exists in the loop from PT -> 5Y3 -> 1st 20uF -> OT center tap -> 6V6's -> ground -> - terminal of 1st 20uF & PT secondary center-tap.

So if the preamp filters and/or preamp tube circuits share a ground with that initial 20uF filter cap, big hum/charging currents of the output stage could modulate preamp signal by virtue of the shared portion of ground.

I'd recommend reading the download available at Valve Wizard's site on grounding. You don't have to go crazy with star grounding or the like (as there are quiet amps that don't use strict star grounding), but I think the article explains the issue I'm describing, along with some diagrams to help you visualize the problem.

[Willabe]

sorry that wasnt clear
it also comes from the PT

Ok. It shows on the schemo you posted the PT has a wire to ground from the primary side. It's a shield in the PT.


              Brad     

[hereforever23]

Now this makes sense.

You have confirmed the hum frequency, which has to come from a full-wave rectifier. The only one of those in your amp is the 5Y3, so the hum has to be related to your B+ rail.

The biggest ripple current exists in the loop from PT -> 5Y3 -> 1st 20uF -> OT center tap -> 6V6's -> ground -> - terminal of 1st 20uF & PT secondary center-tap.

So if the preamp filters and/or preamp tube circuits share a ground with that initial 20uF filter cap, big hum/charging currents of the output stage could modulate preamp signal by virtue of the shared portion of ground.

I'd recommend reading the download available at Valve Wizard's site on grounding. You don't have to go crazy with star grounding or the like (as there are quiet amps that don't use strict star grounding), but I think the article explains the issue I'm describing, along with some diagrams to help you visualize the problem.

Earlier today I read - www.aikenamps.com/StarGround.html
He writes- "Do not connect the power transformer center-tap to the chassis and the first cap ground to the chassis at a different spot; this will cause heavy ground current flow in the chassis."

Well thats exactly what seems to have happened at some point in the amps life.  it's possible the 2 20uF caps were grounded somewhere other than the original grounding point when previous owner replaced those.

[HotBluePlates]

Current in the chassis is not a big deal until the heavy, buzzy current of the PT, rectifier and first filter cap is able to mix with gorund current for low level stages. Then you run the risk of having those low level stages pick up and amplify that hum.

What I really meant in my comment is some folks take star-grounding to a ridiculous extreme, but the fundamental concept is sound.

[hereforever23]

it's possible the 2 20uF caps were grounded somewhere other than the original grounding point when previous owner replaced those.

Connecting the center tap of the high voltage secondaries to the first 2 filter cap ground solved the hum issue

Thank you everyone for your suggestion

[stratele52]

Good news , congrat.

With some pictures at the beginning we probably see this . But you learn more by searching by yourself.
Many amps have these kind of desing problem. I see a lot from my customers .

[HotBluePlates]

Excellent! Congrats on the fix!