New AC30 Build -Questions on Interaction between channels

[wittyjeff]

...any thoughts on the voltage question above?

[sluckey]

Don't touch!

 

[pdf64]

The HT to the power stage seems a bit higher than ideal, 320V seems a happier place to me.
What’s the cathode voltage and cathode current?
And the HT to the vib/trem preamp seems bit low.

[wittyjeff]

The HT to the power stage seems a bit higher than ideal, 320V seems a happier place to me.

I can knock it down a bit easily enough but it will pull down everything downstream a bit.

What’s the cathode voltage and cathode current?

I'm assuming you mean the cathodes off the power tubes... I will check and report back.

And the HT to the vib/trem preamp seems bit low.

This is the "E" HT tap on my schematic.  From what I could see in some other schematics that had voltages listed (although I could only find two) they seemed to think that 165V was adequate.  I'm running at 177V.  The whole VIB channel and all the oscillators and such are rather complicated and I have no idea what voltages really should be.  Steve Luckey built at least one of these (probably more) and I used his schematic for this channel, but he didn't include measured voltage readings on the one I used.  He seems to think I'm within a reasonable range.  If there's a benefit to increased voltage, I'm all for it and that particular voltage reducing resistor is easy to access and change.  Just curious to know your thoughts and further details before I dive in.

[wittyjeff]

Ok, I put my multimeter in series with one of the power tube cathodes and got a consistent  current reading of 0.041~0.043 A.  I got a voltage reading (as compared to chassis ground) of 10.8~10.82 VDC.
How does that look?  Remember that these are tied to two sets of resistor/bypass cap units by pairs (so that one pair of power tubes can be pulled and the remaining pair will be biased correctly (so half values I believe).
Does this look ok?  I'm not 100% sure what to expect for readings.

[wittyjeff]

..any final thoughts on voltages before I consider this amp "finished and ready for testing?"

[pdf64]

Your EL84 seem to be idling a bit cooler than I expected, given the HT voltage. But that's fine, it's not a concern, I suppose that the 1k screen grid resistors are helping out a bit there (what's the VDC across them?).

Here's a schematic with some voltages https://el34world.com/charts/Schematics/files/Vox/Vox_ac30volt_factory.jpg
Not sure how valid it is or what era it's from but suspect it's the late 60s, perhaps toward the end of the Birch-Stolec ownership; the VAC noted for the PT HT winding (180-0-180) must be in error.
Whatever, the voltages noted there for the Vib/Trem channel seem a bit more in line with what I'd expect than those of your amp. But if your amp is playing fine and you're not getting nasty early overdrive on that channel (?) then it's a moot point.
My concern was that as the vol control for that channel is right at the end of it, it can be susceptible to harsh overdrive if the input signal is much above 'vintage single coil' level, eg using my R8 Les Paul with stock vintage type 'Burstbuckers', when plugged into that channel on a typical old AC30, I have to use the 'low' input or turn the guitar vol down a bit.

[sluckey]

I'm surprised you didn't use the same power supply as the AC-30.4 used. If you had, you would be able to easily change the voltage for any node (C, D, and E) by changing a single 22K resistor, without affecting the voltage of the other nodes. I suspect your tube voltages would be a bit different if you had followed the original schematic.

[wittyjeff]

But if your amp is playing fine and you're not getting nasty early overdrive on that channel (?) then it's a moot point.

I haven't experienced any early overdrive on that channel, but I only have a cheap guitar with single coils to test with.  I do notice that the VIB Channel is not as loud as the other two channels, but other than that it sounds ok.  Sounds real nice when blended with the other channels though, so I'd say it's acceptable.  On the other hand, those voltages in the schematic you sent make sense and maybe I should get mine just a bit closer to those and see what happens.

[wittyjeff]

I'm surprised you didn't use the same power supply as the AC-30.4 used. If you had, you would be able to easily change the voltage for any node (C, D, and E) by changing a single 22K resistor, without affecting the voltage of the other nodes. I suspect your tube voltages would be a bit different if you had followed the original schematic.

A LOT of things would have been much simpler if I had followed your original schematic, that's for sure.  In this case I wanted to implement some of the Kevin O'Conner ideas (distributed filtering for one) in the power supply just to see if there's a benefit to it.  I will say that the amp is very quiet (low noise floor).  In any future AC30 style builds, I'm definitely going to make things a LOT simpler.  the ability to fine-tune the voltages on each node in your design makes a whole lot of sense and will definitely be used in any future iterations.  Thanks again for publishing that and for your help.

[sluckey]

That's not my design. I just copied the original Vox schematic, but untangled it so that it's easier to see the structure. The original Vox schematic has those filter caps scattered all over the drawing. Troubleshooting the Vox schematic is like an easter egg hunt! 

[wittyjeff]

That's not my design. I just copied the original Vox schematic, but untangled it so that it's easier to see the structure. The original Vox schematic has those filter caps scattered all over the drawing. Troubleshooting the Vox schematic is like an easter egg hunt! 

Your schematics are fantastic.  I've studied (well at least looked at anyway) most of the ones on your website.  They have been invaluable in my learning process.  I've built a couple of Marshall 18W designs, and the All American Duo, and used parts of others (I especially like the "Warbler" concept).  It's amazing how simple or complex (headache inducing) the same schematic can be depending on who is drawing it.  We all owe a lot to how well you communicate your depth of experience in amplification.  I, for one, appreciate it greatly.

[wittyjeff]

With thanks to pdf64 for the VOX AC30 schematic with voltages notated (see above) I've made some adjustments to fine-tune the voltages somewhat.  Please take a look and let me know if you see any problems with where my voltages ended up after swapping out the various main power supply resistors (see chart attached).  Thanks in advance, any comments are welcome.

[pdf64]

Seems good 1st look over
With the screen grids up over 350V, I’m still confused why the EL84 aren’t drawing more current 

[wittyjeff]

Seems good 1st look over
With the screen grids up over 350V, I’m still confused why the EL84 aren’t drawing more current 

Thanks!  I have no idea either, but I'm confident in my measurement (i.e. I disconnected the cathode wire and ran my multimeter in series with it to get the amp reading, amp was at idle, but I think that's ok). 
Everything seems to be functioning quite well.  The VIB Channel is still about 25% less volume than the other two channels but I'm scared to even go down that path.  The channel sounds really nice when blended with the other two so I think that will allow the VIB channel to have a quality life and sense of purpose.
I think this amp is going to be considered, dare I say... complete!  Unless anyone else has something to add... there's still time.

[sluckey]

The VIB Channel is still about 25% less volume than the other two channels

That's just the way it is. Nothing to do here. The AC-15 acts the same way.

[HotBluePlates]

... The VIB Channel is still about 25% less volume than the other two channels ...

The VIB Channel is still about 25% less volume than the other two channels

That's just the way it is. Nothing to do here. ...

The Vib/Trem channel is what I play through 95% of the time on my vintage AC30.

I never bothered to pay attention to whether I have to turn the knob a few degrees more.  As long as I don't turn the Volume control full-up and still say, "It's not loud enough," then there's no problem.

[wittyjeff]

Thanks for your help and comments.  Looks like this thing is ready to rock!