EL84 Davoli amp

[Tubetec]

The Ac-4 takes its HT after two smoothing caps and the 1k resistor , thats going to be a good bit quieter than directly off the rectifier/80 uf cap like in the Davoli .
Agreed that magnetic induction also is playing a part here , the residual on the HT line will be 100hz, the induced noise will be 50 hz ,both of which are not attenuated like in a PP amp . As previously mentioned transformer orientation and placement can make a big difference to induced magnetic hum ,

I've found it beneficial in terms of noise to trade some loss in HT volts for better smoothing in tube gear destined for recording , 220-250 volts on an EL84 is plenty .

Going down the rabbit hole of addition/subtraction of noise via injection before eliminating the root causes to the largest extent possible first ,that sounds questionable to me . By the same token the the humdinger pot may provide an additional fine tune after you've attended to the induced and HT noise , especially with AC heaters .

Ive used DC heater supplies on the EL84 also , a matched pair of 100 ohm resistors forming the centre tap ,that can be grounded or elevated to what ever potential you like .

Interesting results Rock , its amazing what a fine tune can do ,

 

[abbey road d enfer]

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.

Again it elicits checking grounding, particularly between the 1st stage cathode, the volume pot and the 2nd stage cathode. And also grounding of the filter caps.

Now I'm curious about bias.
According to your measurement, the EL84 runs at 75mA. With an estimated B+ of 330V, that would mean 24+ W of dissipation, where the tube would glow hotter than yellow.
Can you measure B+?
I came at the 330V figure by simulation, but I ignore the DCR of the xfmr secondary.

 

[saint gillis]

B+ is 315V and votage through 120r is 8,5V right now.
At the other side of the OPT's primary, voltage is 305V.

Otherwise you are right there is a ground loop, there is a straight ground trace on the whole bottom of the pcb connected to the chassis in two points, the left side (filtering caps side) is connected to the chassis through the pedal jack, and the right side (input signal side) is connected to the chassis through the input jacks.

Originally the center tap of the 6,3V is wired to the center tap of the HV that goes with a wire to the 80µ negative side, which is is connected to the chassis as I said through the pedal jack.

I can easily cut the trace quit anywhere to open the ground loop, but what grounding scheme would you advise?

 

[abbey road d enfer]

B+ is 315V and votage through 120r is 8,5V right now.

That means the dissipation is more than 20W, for a tube that's rated at 12!

I can easily cut the trace quit anywhere to open the ground loop, but what grounding scheme would you advise?

You need to experiment. I would establish grounding near the input and telescope ground from there up to the reservoir caps and cathode of EL84.
However, the output is grounded, in order for NFB to work, so check that the output jack is not grounded.

 

[saint gillis]

That means the dissipation is more than 20W, for a tube that's rated at 12!

Yes and plate voltage is also beyond the recommended limits.. But the power xformer seems correctly fed as heaters supply is 6,44VAC
And good old Philips EL84 seems always nice and sturdy

 

[Matador]

I can easily cut the trace quit anywhere to open the ground loop, but what grounding scheme would you advise?

I explored this topic a bit here: Optimizing old tube guitar amp's routing of analog 0V

Yes and plate voltage is also beyond the recommended limits..

As someone who has repaired over a hundred Mesa Boogie amps....don't do this!

You need to tweak (e.g. increase) the cathode resistor in order to get the dissipation down below 12W idle if you want the amp to not eat a new EL84 every few months. And I doubt the tone will suffer very much in trade.

 

[CJ]

you could add a tube rectifier like the 6CA4 that runs on 6.3, and if that is not enough, add a dropping resistor like the AC-10 for some serious sag.
raising the cathode resistor has not worked for me in the past, current did not seem to change much, and the operating point was less than, lowering the screen voltage is another option, but you might lose a little tone that way also.

OT i am am building a princeton reverb with el84 output, sounds good, am using a lorlin 6 pos switch for the vibrato which switches in different capacitors to vary the peed instead of the pot, thus keeping the modulation attack and decay more constant for different speeds.

 

[vintageampfixer]

Is the preamp stage only noisy with nothing plugged into the input jack or is it the same when you have guitar or something plugged into it?

 

[saint gillis]

Is the preamp stage only noisy with nothing plugged into the input jack or is it the same when you have guitar or something plugged into it?

It is the same when you have a guitar or nothing plugged in.

 

[saint gillis]

Hello thank you all, I'm working on all of this.
A 1K between diodes and B1 brings B1 to 234V (286VDC before the diodes)
Now we have 6V on the 120r
B3 is now 220V

Hum is noticeably lower

 

[saint gillis]

So I ended with 2 x 1K in series to reach a bit less than 70% of 12W

In fact heater elevation did nothing, I guess I was confused when I found the ground loop, or maybe when I connected the OPT's ground directly to the pcb rather than through the chassis as it was (the connection seemed loose) , or maybe I just switched off my fan as scott2000 said.

The readings about tube amps grounding were good.

And I had a last problem in the preamp stage , some strange noises like a strange drum-machine brrt-bzzt-tzzz-brrp , and I got rid of them with putting 1KV 1n ceramic caps across the 2 diodes.

It now sounds very nice and quiet.

Thank you everybody !