What do you guys think about this?

[gui_tarzan]

I want to build a low-wattage amp that has a chimey (Fender-style) preamp but will break up smoothly for a nice blues tone for small venues. EL84s seem like a good fit so if you don't mind I'd like some critique of this design. It's a take-off on a Vox power amp with a Fender pre-amp. It's going in a small case so five tubes is all I want to deal with. I know I could use a SS rectifier but I like the feel of the 5AR4 in my other amps.

So the question is did I miss anything or do any of you recommend any changes in parts values?

[edit] The PI actually has a 12AT7.

[David Root]

I would change the PI values a bit if you're going to use a 12AT7 as the PI. 47K/51K plates and a lower value tail resistor, maybe 22K, to start.

Also with a 20H choke just feeding the screens, PI and preamp you don't need 100mA capability, that will save a lot of $$, and I would be inclined to back off a bit on those two 22uF PS caps as the choke will do plenty of filtering.

I'm not knowledgeable about Vox circuits, they are obviously "different"!

I assumed 300Vp on the PI B+ and looked at the 12AX7 PI in TubeCad, and as you have drawn it it's pretty clean and well balanced, so if you want a bit of hair in the PI (nooo pun intended!) to transfer into the EL84 grids I would lower the PI tail resistor. That may detract from the overal Voxiness of the power end, I don't know.

If you use a 12AT7 instead without changing the cap/resistor values it will be a bit harder to distort the PI output. If you change to 47K/51K and a 22K tail it will draw 2 mA instead of 1mA, at the same Vp of abpout 250V, which is also not a bad thing I think, again a caveat on "Voxiness".

[sluckey]

You don't really need the .02 and 220K in front of the PI. And 50Ω shared cathode resistor on the EL84s will cause them to run hot. I usually see 120, 130, and 150Ω in that position.

[Gaz]

Just do a SS rectifier. It does not matter as much with hot cathode bias. I have an AC30 build with it on a switch and the only difference you can hear is the preamp voltage browning out. Feel is the same. People will probably argue with that statement.

[Jana]

What, us argue? Surely you jest!

[Jana]

I would add a mid pot.

[Gaz]

I would add a mid pot.

Agreed.

[gui_tarzan]

This is exactly why I love this forum. Thanks guys, I will get started on it with the changes and see what happens.

[gui_tarzan]

I figured the AT would drive the EL84s more than the AX but that's opposite of what it would actually do since the AX has more gain, right?

And by the 47/51K are you talking about changing the 100K resistors on the PI plates?

hair in the PI (nooo pun intended!)

LOL! That made me laugh!

[matt h]

(deleted)

[David Root]

gui_tarzan, yes PI plates to 47K/51K. I would try both, you might prefer the 100K. After all, the BF Fender AB763 PI is 82K/100K. Not Voxy though.

As you lower the plate resistors the gain drops, as you lower the tail resistor the current goes up. In the same circuit the 12AT7 will have less gain, higher clean voltage swing. That's an oversimplification of course as any change usually makes several changes in both the DC and AC voltages. It's a moving target!

If you want to see this quickly without plate curves, rulers and graph paper etc. I highly recommend a copy of TubeCad software. I have used it for years. It was designed by a tube GEB (Giant Earth Brain) and it is not expensive.

[matt h]

(deleted)

[gui_tarzan]

I'm not necessarily looking for a Vox tone, just something that speaks out with some attitude without being brash. I used that portion of a Vox schematic (minus two power tubes) because it was the quickest EL84 amp to find at the time. I'm also very open to other '84 power amp designs if you guys don't think this will result in a smokey blues tone.

Yes, there are other factors such as the speaker, guitar, etc. so maybe this will help - I have a Jensen 12" speaker out of a 1960 Hammond M100 in an open box. My guitars are a Fender Telecaster MIJ with "Texas Specials" and an Epiphone LP Jr. with a Duncan SH-1. I occasionally play through a Tube Screamer TS-10 and a Rocktron Big Crush compressor/sustainer along with a basic CryBaby pedal so that's the makeup of my standard guitar gear.

My main axe is the Jazz in my avatar, though obviously that goes through a completely different setup. I'm an occasional guitar player but our band is re-configuring and I may end up sharing bass/guitar duties with someone for more song diversity. If we do that I will need something like this.

[tictac]

Looks exactly like the AX84 Voxy Lady schematic......

[dvnator]

This is very close to the Tremolux 6G9.
http://chasingtone.com/schematicheaven/ ... 9_d-fa.pdf

You might need to adjust the coupling cap on the input to the PI as in make it smaller. Somewhere in the .002 to 500p range like most of the best Blackface amps.