EL84 Davoli amp

[saint gillis]

Hello I'm giving a refresh to a nice little Davoli guitar amp, here's the schematic

I did a recap of course, and it still has a good amount of hum.. it doesn't seem to come from the preamp as it is still present when I remove V1 tube.. any thoughts ?

 

[musipol]

I’m not familiar with that brand but being single ended output it will hum more than PP versions. The el84 plate supply doesn’t have much filtering.

 

[saint gillis]

A fender champ has far less filtering, and has less hum..

 

[abbey road d enfer]

What happens if you remove the other ECC83? Could it be something sent by the tremolo circuit?
Also it could be that it runs too hot; waht is the voltage acroos the 120r cathode resistor?
Are you sure the negative of the 80uF is grounded at the same place than the 120r?

 

[saint gillis]

No difference in hum when removing also the tremolo tube.
Voltage accross 120r is 8,7V

The 120r is grounded at a random point, not far from the Tone pot

 

[rock soderstrom]

any thoughts ?

Yes, I recently had a Vox AC4 TV on the bench with a similar problem.
It may be that the power transformer is magnetically leaking directly into the OPT. With the Vox, the hum was completely gone after I flipped the OPT 90 degrees.

 

[saint gillis]

Thanks, let me check

 

[rock soderstrom]

If the hum is there instantly after power up, I would test varying the position of the OPT..if the hum intensity changes, you know what's going on

Edit: yes, check it

 

[saint gillis]

Hey, so taking the OPT away from the power transformer helped a bit, but not much..

 

[rock soderstrom]

Hmm, orientation can be more important than distance. See my photos. Did you check that too?

 

[saint gillis]

Yes

 

[rock soderstrom]

Ok. Whats the frequency of the hum? 50 or 100Hz?

 

[saint gillis]

More 50Hz I'd say, with a bit of 100Hz

 

[saint gillis]

Are you sure the negative of the 80uF is grounded at the same place than the 120r?

Should it be ?

 

[Matador]

I've had good luck reducing noise with heater elevation in Champ-like circuits (which is connecting the filament supply secondary center tap to a 50V supply tapped from the preamp high voltage rail). You can easily tell if the hum is heater related by running the heaters from an external 6V DC source - a few lantern batteries in parallel work great for testing like this. You can also get away with inexpensive 6V regulated wall-warts for testing, as it's easy to find them capable of several amps output meant for driving LED lights.

 

[abbey road d enfer]

Should it be ?

It's good practice.

 

[scott2000]

Maybe something in here to look over. Or not.. Elevating heaters is pretty great too as mentioned. Although I've only ever tried with AC...
And turn off that fan next to you...lol

https://www.aikenamps.com/index.php/grounding

 

[CJ]

how many mili-amps are running through that poor power tube?

i would connect transformer hot to B2, and 80 uf is a bit high, going to affect tone especially if the speaker is challenged,

those amps are noisy from the get go, maybe lift the heater center tap and make a hum balance control with a pot,

have any other 6BQ5's? try tube sub,
notice low B+ and pi filter>

 

[saint gillis]

Hey guys, heater elevation did the trick, I mean it works so well that its almost 100% quiet, thank you!

Now the 1st preamp stage brings a lot of noise and hum, when the volume is "0" = no noise , but when I raise the volume, even when grounding both inputs there is a hell of a noise, I must say the chassis is open, but it is too much noise though.

Should I connect the heaters with twisted wires fender style instead of pcb tracks as it is now ?

 

[rock soderstrom]

Good news! Maybe you just solved an existing ground (contact) problem when you elevated the heater voltage?

About the noise, are you using shielded wire from the input to V1 and to the gain pot? Noisey tube?

External twisted heater wireing ist better but with a PCB version you can also get an extremely noise free amp if the layout is good.

The Vox Ac4 TV mentioned above was so quiet after the mod I never expected it to be.
I always thought that AC heated single end amps on PCB can't be really quiet. Reality showed that it absolutely can. I was really surprised. Dead quiet...