Harley Benton GA15

[flintan]

Sure. But i've messed some with it now. I've cut off the part of the pcb with the low voltage part and built the fender tone stack in between the first tube. Changed some coupling caps and tried to sort out the grounding.

The overall tone is a lot better now but it sometimes has a sort of farty distortion. The hum problem is still there and now it also has a bit of oscillation on higher gain settings (about two o clock and above) in the preamp.  :-\

Measured the DC resistance of the output transformer and the CT is not really centered.. 107 ohms on one side and 127 on the other. Mybe this is the reason for the power amp hum. There is also some blue glow from the EL84 tubes. But i guess that's normal?

 

[flintan]

Some pictures of the mess:


   
http://web.comhem.se/linusandersson/ga15_2.JPG

http://web.comhem.se/linusandersson/ga15_3.JPG

http://web.comhem.se/linusandersson/ga15_4.JPG

http://web.comhem.se/linusandersson/ga15_5.JPG

http://web.comhem.se/linusandersson/ga15_6.JPG

http://web.comhem.se/linusandersson/ga15_7.JPG

{EDIT- changed over-large inline images to links - PRR}

 

[flintan]

Thanx for the pictures. Looks very nice to me. If all Fenders would be like this!!
I was also interested in the bad soldering but I see that's to much trouble now.

Well maybe it isn't that bad. What i mainly didn't like was the use of several tiny jumpers in the ground plane and that the overall pcb design seem to be just more or less random placement of components. I'll take some more pictures next time i remove the pcb.

I've also noticed the Sovtek tubes. Is it an improvement?

The EL84's are Sovtek and the ECC83 chinese, originally.

draw a schematic so we can help or is it the schematic from the previous posts?
How did you do the grounding?

It's almost the same:

I have one separat wire from the psu ground on the pcb to chassis star ground, one wire from the "star" point of audio ground on the pcb to chassis star and one wire (which probably is unnecessary since there is no NFB) from the speaker ground. The heaters are separate traces on the pcb so i kept that as is. Heaters are connected to psu ground via two 100R resistors.

The ground for the tone stack and the volume controls are taken of a trace on the pcb which is quite nearby where V2 has its ground on the pcb.

measure AC voltage with the power on the plates of V4 & V5.

remove V3 than listen. What is happening to the hum? Put it back in. do the same with V2.
What if you turn the volume down?

Will do. The hum is still there with both volume controls at zero. Then it decreases some with volume controls at about nine o clock. Then at higher gain the hum increases again to a higher level.

Thanks a lot for your effort!

 

[flintan]

measure AC voltage with the power on the plates of V4 & V5.

remove V3 than listen. What is happening to the hum? Put it back in. do the same with V2.
What if you turn the volume down?

With V3 out it's dead silent. With V2 out there's hum, independent of volume setting, or maybe a tiny bit less at zero.

DC at the plate of V4 is 316V. DC at V5 is 314V. But while measuring V4 i got a lot of nasty oscillation.

Could not get any AC measurements. When putting the probes in place it started at about 9V AC and then fast (half a second) going down to 0V. Same for both V4 and V5 but with oscillation at V4. Dont know if it's maybe my multimeter that's not good enough. Had the range at 20V.

 

[abbey road d enfer]

Sure. But i've messed some with it now. I've cut off the part of the pcb with the low voltage part and built the fender tone stack in between the first tube. Changed some coupling caps and tried to sort out the grounding.

The overall tone is a lot better now but it sometimes has a sort of farty distortion. The hum problem is still there and now it also has a bit of oscillation on higher gain settings (about two o clock and above) in the preamp.  :-\

The oscillation is the first thing you havE to sort out. I reckon when you solve it, the hum will go away too. Lead dress is important. I think the plate transformer connections are too close to the tone stack.

Measured the DC resistance of the output transformer and the CT is not really centered.. 107 ohms on one side and 127 on the other.

Absolutely normal for cheap construction xfmr. Same number of turns but one layer is wrapped around the other so has different length. No reason for worry.

Mybe this is the reason for the power amp hum.

Definitely no.

There is also some blue glow from the EL84 tubes. But i guess that's normal?

I don't totally agree with reanimator. I worry when I see blue glow in power tubes. It may go away when you sort out the oscillation. Have you checked the voltage across the cathode res (120R)? It may not be harmful but can introduce severe distortion and lack of power.

 

[flintan]

Ok. What about putting a copper screen between the tone stack and pcb? And perhaps shielded cables to and from the tone stack. It's only oscillating at high gain in the preamp, higher than what was possible in original configuration.

Will measure the cathode res volt later.

My first impression is everything after volume2  is ok when it comes to hum.
[...]
You could inject audio in pot 2 with V2 removed and see what happens.

With V2 removed there's still hum, even with the second volume control at zero.

 

[abbey road d enfer]

The actual input of the power amp is high impedance because of the 82k resistor that is dangling from the pot wiper. The wire is very close to the plate connections.
Put the resistor on board, as close as possible to the grid and shield the wire from the pot. Anyway, why did you put this 82k res? Miller effect?

 

[PRR]

> the CT is not really centered.. 107 ohms on one side and 127 on the other. Mybe this is the reason for the power amp hum.

No. It is an AUDIO transformer. Do you even care about the DC resistance? Maybe you care 5%: the DCR should be much lower than the working impedance, 5% is a ballpark. The 1% "unbalance" is utterly beneath notice.

> There is also some blue glow from the EL84 tubes. But i guess that's normal?

Any glow BETWEEN cathode and plate is very-bad, and the tube will die very soon.

Glow on the inside surface of the glass is quite harmless and fairly normal. Most glass has some phosphorescent impurity. Electron tubes have wild electrons.

Why do you have your speaker-output wire routed over your preamp socket?

OMG. That's the OT -primary-, isn't it? I'm shocked it plays at all. Was it routed like that from the factory?

> didn't like was the use of several tiny jumpers in the ground plane and that the overall pcb design seem to be just more or less random placement of components.

Or maybe the layout was carefully adjusted to work well, rather than to look neat or minimize the offensive jumpers. (Which are often place where alternate parts could be installed for a 2010 model.)

 

[reanimatorstudio]

thanx abbey road d enfer  &  PRR, I'm learning. Please continue with this project.

 

[flintan]

The actual input of the power amp is high impedance because of the 82k resistor that is dangling from the pot wiper. The wire is very close to the plate connections.
Put the resistor on board, as close as possible to the grid and shield the wire from the pot. Anyway, why did you put this 82k res? Miller effect?

That resistor is just hanging on there from the original design. I'll try to remove it.

> There is also some blue glow from the EL84 tubes. But i guess that's normal?

Any glow BETWEEN cathode and plate is very-bad, and the tube will die very soon.

Glow on the inside surface of the glass is quite harmless and fairly normal. Most glass has some phosphorescent impurity. Electron tubes have wild electrons.

The glow is from inside the tube not the glass surface so i guess it's bad then..

Why do you have your speaker-output wire routed over your preamp socket?

OMG. That's the OT -primary-, isn't it? I'm shocked it plays at all. Was it routed like that from the factory

Yes it is and it was made like that from factory. Would it help to twist the wires, shield and run them in different direction more over the psu?

> didn't like was the use of several tiny jumpers in the ground plane and that the overall pcb design seem to be just more or less random placement of components.

Or maybe the layout was carefully adjusted to work well, rather than to look neat or minimize the offensive jumpers. (Which are often place where alternate parts could be installed for a 2010 model.)

Maybe, but it didn't work really well from factory because it had this hum from the start.

Thanks a lot for you help!

 

[flintan]

I've cured all oscillations and now it sounds really nice. The "farty" distortion and the blue glow has disappeared. But it still hums like before.

Here's a picture of the pcb with ground and heater trances marked:

 

[flintan]

Regarding the pcb grounding i have not really changed anything yet. I have removed the headphones output and the ic's and cut that part off the pcb.

 

[flintan]

Found this and this. Interesting. Too bad the pdf is in french..

 

[audioforge]

is it this pdf?
http://www.transartistik.net/mikka/spip/IMG/pdf/Modifs_GA15_Harley_Benton.pdf

audioforge

 

[flintan]

Ok thanks.

Ah, you ment about the oscillation.. I rearranged the wiring with shielded cables and better positioning. I'll take some pictures next time i open it.

 

[flintan]

Thanks, yes it seems like that but i'll probably fiddle some more with bias resistors and capacitor values. I have not listened to it with my "real" guitar yet, fender jazzmaster.

I have already removed the transformer connections for the 12V part. And the high voltage part is easy to disconnect with the bypass switch. Believe i've done that already but don't remember the result now, think there still was some hum. Will test to remove the heaters also. Thanks for the tip! But it will take a week or two now until i can find time to finish this (and some other stuff waiting to be built).

 

[flintan]

Finally found some time to continue with this..

This is what i've done so far:

I connected the preamp tubes to the 12V supply and it seems to be ok. It surely cured most of the hum. I have to use the amp some more before saying anything more about the sound or if there's any other issues.

Some pictures:

 

[flintan]

Have finally found some some time to try the amp some more. There some annoying resonant noise at some notes, especially e. What do you think is the reason? It's some type of resonance. I don't know if i have to change some coupling cap valus or if it could be the chinese preamp tubes being microphonic?

Here's some fooling around sound files..with a "BaCH" Czech Telecaster copy and a reverb unit i've recently built based on the digital Belton rverb chip.

http://www.strandband.se/linus/clean.mp3
http://www.strandband.se/linus/crunchy.mp3
http://www.strandband.se/linus/overdrive.mp3
http://www.strandband.se/linus/resonant ****.mp3

What do you think is the reason for the resonant stuff?

 

[flintan]

No one that recognizes this type of ringing stuff heard here: http://www.strandband.se/linus/resonant ****.mp3?

 

[flintan]

I will remove c16 next time i open it and se what it does. I have tried it with the stock speaker as well as this one and if i remember correctly now it was the same. But i will test with another speaker again. Thanks!