Weird A$$ Vox Thing

[Ledzepp007]

Would the attached schematic work as an EF86 preamp headed to a standard fare LTP?

Thanks!

[sluckey]

Probably. Be prepared to maybe knock some gain down. Easy with a resistor between the .022 cap and top of the gain pot. Might need a grid stopper between the wiper of the gain pot and grid of next tube.

[Ledzepp007]

Probably. Be prepared to maybe knock some gain down. Easy with a resistor between the .022 cap and top of the gain pot. Might need a grid stopper between the wiper of the gain pot and grid of next tube.

Extremely helpful.

What value resistor might be a good starting place to knock gain down? Also, are we talking like a standard fare grid stopper (i.e., 33k-ish)?

Thanks!!!!!

[tubenit]

Have you seen Geezer's  HoSo56?    There are layouts available and versions with different power tubes also.

http://el34world.com/Forum/index.php?topic=6176.0

You might borrow some ideas from it?

With respect, Tubenit

[sluckey]

Good find. That looks exactly like what LZ wants.

[Ledzepp007]

Literally exactly what I was after. I was looking at some higher gain circuits earlier (e.g., the SLO) and saw a similar use of gain reduction/treble boost with the 470k resistor/.001 cap. It's a very clever design.

Thanks for the help.

[Ledzepp007]

So, I built this(ish) twice now. First one worked, had audible hum that became louder with volume. Irritating. Figuring it was the usual mess of mine, I just rebuilt it. Still works. Still sounds awesome. Still has hum. 60-cycle I think. Voltages are good AFAIK. Chopsticking the heater wires change nothing. Pulling the EF86 reduces the hum by about half. Last time, pulling the PI tube cut the hum.

Ideas? It's a really good, gainy amp. Very compressed and nice. Just what I was after. Hate the hum, though.

Thanks!

[kagliostro]

If you can, first give a try with DC on heaters (a DC PS or a 6V Battery)

so you can establish if there is a problem with heaters

or, if you have a CT on heater windings, you can go with elevated heaters using a voltage divider or connecting the heaters CT to the cathode of the power tubes

if you haven't a CT on heaters try a virtual CT using a pair of 100R or 220R

Franco

[Ledzepp007]

I do have a center tap for the heater supply. Might try elevated heater voltage. Thanks for the suggestion

[terminalgs]

make sure the internal shield (pin 7 or 2) is grounded.  This seems unlikely.  if you have 60 cycle hum.


does the hum go completely if you turn the volume to zero?


If you want to see if its the EF86 related, swap your input to the grid of the grid stage of the top boost 12ax7. (in other words, bypass the ef86).  you'll need to disconnect the ef86 output from the input of the next, or just pull the tube out..


what does your power supply circuit look like?  how many filter cap nodes in the chain,  and do you feed B+ to the ef86 and the 12ax7 with the same filter cap?

[Ledzepp007]

I have both 2 and 7 going to ground.

I elevated the heaters by tying the center tap to the 6V6 cathodes. Unless I am imagining things, it did seem to help. But the hum remains. Also, the hum reduces but is still audible when the volume control is zero.

PS is 40-20-20-20 with 10k and 27k dropping resistors. The EF86 gets its own filter node.

Is this likely to be a lead dress issue? Thanks

[tubenit]

Can you please post a photo of the chassis interior.  As an FYI,  I never had a problem with the HoSo56 builds I've done using a 5879.

With respect, Tubenit

[terminalgs]

the hum reduces but is still audible when the volume control is zero.

If noise exists when volume is zero, then that hum is introduced to signal path to the right of the volume pot (looking at a schematic).  noise that appears and increases with increases in volume control is introduced somewhere to the left.  so, you could be looking at multiple problems.

power tube heaters need to be wired so the heater hum is out of phase and gets cancelled (pin 4 to pin 4, pin 5 to pin 5)

[Ledzepp007]

I mean, it's ugly in there fellas. Ugly. I'm embarrassed. Gut shot attached.

[kagliostro]

There is always someone whose works are better than ours

but i've seen works that were a lot (but really a lot, you can't imagine how much) worst than your and some of those had good performances despite all

Franco

[Ledzepp007]

I'll try and prod around with my chopstick tonight. Otherwise, I suppose I can truly and clean things up in there.

[Ledzepp007]

By the way, I think the noise is acceptable now. Not perfect, but not a big deal. My unshielded control cavity on my Les Paul makes substantially more noise.

[tubenit]

So, what resolved it and lowered the noise? 

With respect, Tubenit

[Ledzepp007]

Elevating the heaters (referenced at the power tubes cathode resistor) and being less neurotic

[sluckey]

and being less neurotic

I've fixed a lot of stuff like that!