AC30 clone Vibrato Channel issues and noise

[skeeterbuck]

I built a clone of a Vox AC30. I used the Turretboard Layout and everything works OK with the following exceptions.

One: The amp is rather noisy. I've been told this is normal for the AC30's in general, but not having access to an original, I'm not too sure my noise level is similar to an original. I didn't use the heater ground connection located next to V1 and instead have 2 100 ohm resistors grounded to the cathode of one of the el84's.

The other issue and one that I find more perplexing is that the power from the vibrato channel is no where near the power of either the normal or brilliant channels. This amp has the added on top boost circuit with the rear facing panel. Where as the normal channel on about 1/2 way up starts getting rather loud and of course the TB channel gets extremely loud set the same. The vibrato channel almost all the way up still isn't nearly as loud. I checked and rechecked the circuit and can't find anything amiss. I have changed out the 12ax7 for this channel for several other tubes and this doesn't make any difference.

Any ideas on what is going on or where to check first?

Thanks!

[skeeterbuck]

FYI: http://turretboard.knucklehead.dk/wp-co ... alrev7.pdf

[pdf64]

I think that's pretty normal.
If you didn't have the Top Boost circuit, the Brilliant channel would probably have a similar gain level to the Vib/Trem Channel.
To even things up a bit, and also reduce hiss at idle, I suggest that you try removing the cathode bypass from the top boost common cathode stage.

[pompeiisneaks]

I've not built a top boost version, but I did build the EF86 version and the EF86 channel is way louder than the tremolo side. I think the top boost channel has an extra gain stage as I recall it, meaning you get more gain out of it than the tremolo side.

Do you have a schematic of this layout anywhere? I can't really read layouts that well for circuit analysis. I mean, I can, but it takes probably 6x longer than just having the schematic. As for noise you mentioned using two 100 ohm resistors, Did you then remove the green wire from ground connection? You do not want center tap of the heaters grounded AND a virtual center tap. Noise can be caused by groud loops, or other leads getting to close to signal path wires. You can use a wooden chopstick to try and move around wires or tap the board and components to see if it changes the noise.

~Phil

[skeeterbuck]

When I first built the amp, it was just a normal version AC30 with 3 channels (vibrato, normal and brilliant) the normal and brilliant channels had the same amount of gain but obviously voiced differently. The vibrato channel was much weaker than the other two. To give an example on a scale of 1 to 10 with 10 being almost painfully loud, I would estimate the following

Vibrato channel: 4
Normal Channel: 8
Brilliant channel: 8
Brilliant channel with TB add on: 10+

The power transformer I used was a Mojotone version which I think is made by Heyboer and there was no center tap for the heater circuit, therefore I added the two 100 ohm resistors for a virtual center tap.

Unfortunately, I don't have a schematic for the AC30 with the add on TB circuit. I don't ever remember seeing a JMI Vox schematic for that version. Only the schematic for the TB where the extra Bass and treble pots are already in the control panel.

pdf64, when you suggest removing the cathode bypass from the common cathode stage, I not sure what you're referring to. Is this the cathode bypass cap for the top boost 12ax7?

Thanks for the suggestions!

[sluckey]

I have a 1960 AC-15 clone and the vib channel is much quieter than the normal channel. My vib channel is probably identical to yours. I just accept it as the way it is.

[pdf64]

…
pdf64, when you suggest removing the cathode bypass from the common cathode stage, I not sure what you're referring to. Is this the cathode bypass cap for the top boost 12ax7?
…

Yes, I thought what I wrote was pretty clear

try removing the cathode bypass from the top boost common cathode stage

But nevermind, if it wasn’t clear to you, that’s what matters. If in doubt, ask. So fair play

[skeeterbuck]

I have a 1960 AC-15 clone and the vib channel is much quieter than the normal channel. My vib channel is probably identical to yours. I just accept it as the way it is.

I would expect that considering that your dealing with a EF86 preamp tube in the normal channel vs. a 12ax7 in the vibrato channel.

With the AC30, since both the normal and vibrato channel have a 12ax7 preamp tube with the vibrato off, I would expect similar gain.

Thanks for the input.

[skeeterbuck]

…
pdf64, when you suggest removing the cathode bypass from the common cathode stage, I not sure what you're referring to. Is this the cathode bypass cap for the top boost 12ax7?
…

Yes, I thought what I wrote was pretty clear

try removing the cathode bypass from the top boost common cathode stage

But nevermind, if it wasn’t clear to you, that’s what matters. If in doubt, ask. So fair play

Exactly, better to be safe than sorry. Thanks for the suggestions.

[pdf64]

…
I would expect that considering that your dealing with a EF86 preamp tube in the normal channel vs. a 12ax7 in the vibrato channel.
…

Off the top of my head, a typical EF86 stage will only have about 5dB more gain than a typical 12AX7 stage. That’s less than the difference between the standard hi and lo input jacks. It’s not a night and day difference.

Whereas with the Vib/Trem channel and all its signal processing stages, a design choice was made to put its vol control at the end of the channel, ie after all the signal processing. That was probably done to maximise the signal to noise ratio, ie the vol control reduced hiss along with the signal.
But that then made the channel vulnerable to being overdriven by louder pickups, and given the cathodynes in the signal path, that overdrive sounds somewhat nasty. Try a clean boost to see for yourself. So to reduce the risk of that, the input stage of the channel uses a split load on its anode to reduce gain
So that, along with all the signal processing, results in the channel having the lower max available gain (than the normal channel etc) that you’ve noticed.