VOX Powered Rocket with VVR2

[RJ Guitars]

Here is something for the masses to contemplate...

As I read over tube data sheets I see that the output load impedance changes with the plate voltage. An El84 has a plate resistance of 5.2K at 300V and 4.5K at 250V.

I know sometimes there is this thought that a certain tube has a certain load resistance when in fact it's constantly changing. So there really isn't any transformer that is perfect for any tube, just transformers that operate well across a broad enough range to accommodate whatever is thrown at them.

I don't know how linear the change in resistance behavior is and I guess I'm not sure how to find that on the tube charts... maybe it is there. If it's negligible at the extremes then no harm no foul and play on. However, I am curious as we "Power Scale" to that extreme low voltage is that load impedance very low and we are stretching the transformer outside it's operating load ideal?

Gerald Webers book states that operating an out put transformer within 100% of it's ideal speaker impedance will have no negative impact on things (poorly quoted). Can we also stretch that transformers effective range 100% on the input load resistance?

I might suggest that we more often than not operate Output transformers way outside of the perfect impedance match and in many cases that very fact is part of the mojo of some really cool amps?

I've got a few days out of town but I'll be curious what kind of discussion this generates... I am quite curious about it myself.

rj

[Stanz]

RJ

I did build KOC's AC-15, which was basically an AC-30, with two power tubes and SS rectification. Great amp. I ended up using Hammond PT and a Vox AC-15 OT. I wanted smaller power. I don't really see the need for huge amps as I am not gigging anymore.

I just finished up Ron's kit that he sold me from your group build. Sounds very nice, very similar to the AC-15 I built. The one issue I had with the AC-15 was it got dirty pretty quickly at around 9 o'clock, and it was too much distortion (unusable, or undesirable) with the volume past 12 o'clock. Seems to be the same case with the Rocket. That is why I wanted to build the Rocket, more usable clean volume.

[RJ Guitars]

I did build KOC's AC-15, which was basically an AC-30, with two power tubes and SS rectification. Great amp. I ended up using Hammond PT and a Vox AC-15 OT. I wanted smaller power. I don't really see the need for huge amps as I am not gigging anymore.

I just finished up Ron's kit that he sold me from your group build. Sounds very nice, very similar to the AC-15 I built. The one issue I had with the AC-15 was it got dirty pretty quickly at around 9 o'clock, and it was too much distortion (unusable, or undesirable) with the volume past 12 o'clock. Seems to be the same case with the Rocket. That is why I wanted to build the Rocket, more usable clean volume.

Hey I am glad that you got the amp together... they seem like really great amps and they are pretty simple to put together.

I am not entirely pleased with the AC-30 tone stack or how fast the volume pot seems to come on. I'm sorting this out a little bit at a time and will probably build a couple more Rocket iterations.

I find that the Rocket has a really cool clean tone and even though it does break up at some point after 9 o'clock, the tone stays pretty sweet and is very useful when I've got it cranked. It is pretty stinking loud for anything indoors unless maybe it's a big concert hall or a band that just plays too loud for the venue.

I did find that I had to chose my speaker cabinet and speakers carefully. I have a couple Carvin "British Series" 12's in a closed back cabinet and it sounds great clean but definitely goes into fuzz-out when it's cranked. Ultimately I found that a 2x10 open-back cabinet with a blue alnico and an Eminence Ramrod sounds great and lets me crank it all the way up with good results.

I am just starting to look into a different volume pot or altering the taper somehow so that it's easier to adjust in the clean regime. I found that it actually has a lot of clean headroom but you have to have a feather touch in that first quarter turn to dial it in.

Does anybody have any suggestions on altering the pot characteristics? I used both Alpha and Bourns pots and they behave the same.

rj

[fishy]

Won't the bright cap effect the operation of the volume pot, particularly at lower resistances?

I was also wondering about the cathode resistor on V2, how much of a change is there between using a 56k and 68k?

Pete

[Fischerman]

FWIW, I put a 3-position bright switch in mine and used 47pF and 100pF caps and it usually stays on the 47pF setting. From reading about Vox AC30s and mods...a 68pF bright cap is supposedly the concensus 'best' value. The 100pF cap just makes it a little too 'brittle' at lower volumes IMO. IME the bright cap seems to have less of an effect in this circuit than it does in other circuits but it's key for getting the top end just right at lower volumes.

On the cathode follower I originally had 56k but that is borderline overdissipating the tube so I increased it to 68k. It didn't seem to affect tone very much.

Also, after playing mine and learning a little about this circuit, it seems that the compression is a crucial part to getting it just right. One direction sounds too plinky/squishy and the other direction 'barks' too much.

[angelodp]

rj, why did you refrain from using the additional caps and resistors on the input and volume pot as recommended by Dana?

best ange

[RJ Guitars]

Here is where this amp ended up... https://tubeamparchive.com/viewtopic.php?t=27888