too much gain on Ef86 Vox Ac15

[rafaelctt]

  I have a lot of amplification on the clean channel. I think that due to the Ef86.This causes that as soon as the volume goes up there is too much gain, I can not pass it from 2( out of 10). And therefore also a lot of noise. Any ideas?
Thanks

[kagliostro]

Revise components around ef86

Plate resistor, cathode resistor, bypass cap

or adopt a voltage divider at the exit of ef86

or adopt a split load resistor in place of the single plate resistor


Franco

[sluckey]

Show us your schematic.

[tubeswell]

A couple of EF86 combinations to try

Rp = 100k, Rg2 = 560k (with 100nF Cg2), Rk = 680R (with 10uF Ck)

Or (for lower gain)

Rp = 47k, Rg2 = 330k (with 100nF Cg2), Rk = 680R (with 10uF Ck)


YMMV

[rafaelctt]

I used the values ​​from the original schema building this version of Sluckey

[sluckey]

My AC-15 normal channel is definitely not too gainy on "2". The normal channel is considerably louder than the vibrato channel, but not to the point of calling it "too gainy".

Did you build the entire amp? Did you use a 500K-A volume pot? I suggest measuring some voltages and comparing to the voltages on my schematic.

[rafaelctt]

I recreated the circuit with a very good quality board with good turret spacing. I have measured voltages and they are all a little higher, but it is that the 290PX outputs 420 volts at no load. I can have a 2 or 3% voltage variation compared to the original scheme.
Now everything works perfectly but I have a very big gain. Possibly due to non-adaptation of input impedance. I'm trying it out with an electric bass.





https://photos.app.goo.gl/rYocXhewFUCZ46So6

[rafaelctt]

My AC-15 normal channel is definitely not too gainy on "2". The normal channel is considerably louder than the vibrato channel, but not to the point of calling it "too gainy".

Did you build the entire amp? Did you use a 500K-A volume pot? I suggest measuring some voltages and comparing to the voltages on my schematic.

yes, it is faithfully built based on those schematics

[sluckey]

With the amp in operate mode how much voltage do you measure on pin 3 of the EZ81 rectifier tube?

[rafaelctt]

362 V

[sluckey]

362 V

No reason to get excited.

Does your bass have active pickups? IOW, electronic preamp. And why are you running a bass through this wimpy 15 watt amp?

[rafaelctt]

It is not an active pickups. And I've tried it with a bass because that's what I have here. I imagine that the impedance issue should be the same for bass, guitar, etc. as long as they are the same type of pickup, single coil, double coil, etc... The issue is that the amplification of the EF86 is too great, I would like to lower the gain.

[sluckey]

OK, a very easy thing to do would be replace the green wire between the Brilliance switch and the NOR Vol with a 470K resistor. Use an even larger resistor to reduce the signal even more, or a smaller resistor to reduce the signal not so much. Doing this does not really affect the EF86 gain. It simply drops the output signal so you can turn the volume knob to a higher number.

If you really rather reduce the actual gain of the EF86, then follow Tubeswell suggestions in reply #3.

edited for grammar 

[kagliostro]

Following Sluckey council you can give a try splitting the plate resistor and feeding the following circuit with that signal (same effect of the added 470K resistor, without the intrinsec noise induced by that added resistor, end result depends also on how well Is acting your PS filtering)


Look to the first triode plate resistor arrangement

Franco

[rafaelctt]

I have made the following measurements and checks:
The 470k resistor replacing the green wire of the brill switch does not affect anything. I have shorted input of EF86(pin 9) to ground. This way I avoid input signal. I have made these voltage measurements and it seems very high to me that of pin 1 (130 V). With all this, when I turn up the volume there is a very big noise like an open signal circuit. That's the strange thing. From the volume to the inverter, there is hardly any noise. The same goes for the vibrato channel, there is no noise but the signal, as we know, is much lower.

[sluckey]

The 470k resistor replacing the green wire of the brill switch does not affect anything.

That is surprising! Should have cut the signal in half. Are you using the simple brill switch or the 6 position rotary switch? Measure resistance between the vol pot wiper and ground. Slowly turn the pot. Resistance should smoothly change from zero to 500K. Check the ground connection on the vol pot. Try replacing the vol pot.

[rafaelctt]

I use a simple brill switch. The measurements of both potentiometers (vol.normal and vibrato) are the same in their travel and are well connected to ground. I still think that it is an impedance issue, since I have no problem with the signal generator any. My generator is a Hewlett packard 3311A.

[tubeswell]

... shorted input of EF86(pin 9) to ... avoid input signal. ...With all this, when I turn up the volume there is a very big noise like an open signal circuit. That's the strange thing. From the volume to the inverter, there is hardly any noise.

Are you worried about the noise? Without having a sound-byte, its difficult to form an impression of whether it's 'normal' noise-floor noise or not. But if its significant hissing or humming, it could be a bad EF86, or a bad EF86 screen supply resistor, or plate resistor - or bad other load resistance (like the volume pot) in the signal path after the EF86, or a bad solder connection or signal ground connection.

[shooter]

'normal' noise-floor noise or not

my noise go-nogo check
Amp on 7 guitar on 1
Noise?
NO, keep playing
Yes, quit playing, start scoping
 

[sluckey]

I still think that it is an impedance issue, since I have no problem with the signal generator any.

Sounds like we've been chasing ghosts. 

[Ed_Chambley]

Not quite clear with your problem.  Do you mean too much signal gain (Ef86)?  This the question I see.  I also see mention of Active electronics. The EF86 is a British designed pentode tube (which provide enough gain for the small magnets in the needle cartridge to be amplified).  Regardless of how this if frequency constructed, suggested use of the tube, I concede Vox and Matchless have been successful.


I say this to remove the idea it may be a "gain structure" problem as the PROVEN build is.  You have an issue.  Solve the problem by first using a signal source designed for the Amplifiers Input! Secondly, insure each channel gain has within reason of the posted values.  The design works, keep that in mind.  It works very well.  It works the best of any club amp I have payed.  It is the only amp I always carry.


Know playing the EF86 Channel and 12Ax7, the EF86 knob will be higher at unity, normal!
Now, you tell me.  What you wanna do!

[rafaelctt]

I still think that it is an impedance issue, since I have no problem with the signal generator any.

Sounds like we've been chasing ghosts. 

it gives me the same impression
the signal generator has an impedance of 600 ohms while an electric guitar has an impedance of a few kOhms. That's what I meant as a differentiating factor.

[rafaelctt]

... shorted input of EF86(pin 9) to ... avoid input signal. ...With all this, when I turn up the volume there is a very big noise like an open signal circuit. That's the strange thing. From the volume to the inverter, there is hardly any noise.

Are you worried about the noise? Without having a sound-byte, its difficult to form an impression of whether it's 'normal' noise-floor noise or not. But if its significant hissing or humming, it could be a bad EF86, or a bad EF86 screen supply resistor, or plate resistor - or bad other load resistance (like the volume pot) in the signal path after the EF86, or a bad solder connection or signal ground connection.

I have measured all the components around the EF86 and they are all ok. I only have to try another Ef86 that I don't have at the moment.