Oh, 1 more thing missing on the Rocket..(+ KF flyer & cl

[soma_hero]

Clyde, why do you think it's not class A? I read over the Aiken article on why the AC30 isn't really class A and he proposes some changes to the circuit to get it into class A. The changes are essentially lowering plate voltage to around where the Rocket is at, and biasing real hot.

[rawnster]

Hey Rooster, nice clips! Sorry I haven’t commented earlier; I’ve been out of town. Yep, your recordings represent what I remember of your amp. I notice that even around 9 o’clock, with the guitar dimed, your tone’s got a touch of hair on it. My amp does the same thing. In my clips, all of the settings were around 12 o’clock…including the volume. To get the clean tones I recorded, I simply potted the guitar down till it cleaned up. It does also help to play with a bit more finesse. Having the express build around a while before building the rocket helped that quite a bit. In that, the express is UBER touch sensitive, which forced me to change the way I play. Anyway……

Rooster, you’re right; I wish the rocket did have a bit more headroom. It’d be cool if it didn’t start breaking up till around 12 o’clock or so. I haven’t had the chance to swap out cathode resistors yet, but I aim to for sure. Actually, am thinking about just adding a 10 ohm resistor in series to the 50 ohm that’s currently installed and see what that does.

Thanks for making the clips, Rooster. It’s really not a trivial endeavor!

[Clyde]

I'm familiar with Aiken's article, in which he states that 250V is max plate voltage for class A with a 31 ohm cathode resistor. As well, the preamp gain is reduced to prevent overdriving the output section. No one here is even close to those specifications. The Rocket was never designed to be class A.
There just aren't many class A amps around, although some claim to be.

[rooster]

Clyde and anybody else - Yes, the Aiken article is interesting. But PLEASE remember that he is also the guy who will tell you the the CARBON 1 meg resistor on your input jack will add noise to the signal. No, give me a break.

What I do see out there are pockets of knowledgable amp guys, Ken Fischer, David Funk, and Gerald Weber to name a few, who say - simply - that the AC30 is a Class A PP amp. Gerald Weber actually said, in reference to the Aikens statements. 'Well, whatever someone wants to call it, I am calling it an amp that is Class A PP by design.' And I kinda thought that was a good thing to say. I mean, in Aiken's world where a 1 meg carbon comp resistor on the input jack can cause noise in the circuit, then I think he may have a point about the AC30.

Any, I thought a lot about this because I got in a heated debate about this one time. And this is what I came up with. If a power tube is biased to 100% of its duty cycle, then its running in Class A. And to prove my point, all I had to do was plug one of the the EL84s into my bias probe and my DVM and pound on my gtr. IF the voltage (ma voltage) strayed 10-40 ma like it does with Class A/B, then I would agree with Aikens. BUT it doesn't. The only way you can change that bias voltage is by increasing the voltage going to the tube - and this isn't going to happen.

So Aikens says it does fluctuate (a bit) - and it does a few ma, as does a Champ - but nothing like an actual Class A/B amp does. So therefore, he has his 'Aha! moment and declares it a Class A/B amp. ?????????? Really?

OK, and that's all I got on that subject. For me it breaks down to understanding the man. This is a guy who says that a 1 meg carbon comp resistor to ground on an input jack can create noise in the first preamp stage. Really????

rawnster, you are a gentleman, thank you! You too, Clyde, thank you, as well.

[Clyde]

Rooster, with all due respect, GW (whose initials resemble someone else of his ilk), has spread more BS and half-truths about amps around than anyone else I know.

[Zippy]

OK, and that's all I got on that subject. For me it breaks down to understanding the man. This is a guy who says that a 1 meg carbon comp resistor to ground on an input jack can create noise in the first preamp stage. Really????

That is not what he is saying, Rooster. Here is the quote:

"Since noise is proportional to resistor value, the 1Meg to ground resistor on the first stage of a guitar amplifier will create much more noise than the 68K grid resistor, because the value is 14.7 times larger. This means that carbon comp vs. metal film is more noticeable and important for the 1Meg resistor than the 68K input resistor. However, when a guitar is plugged in to the amplifier, the pickup resistance/inductance and cable capacitance is in parallel with the 1Meg grid resistor, so its effect on noise is greatly reduced. When the guitar is unplugged or turned all the way down, the 68K series grid resistor becomes the predominant noise source. Depending on the tube type and input stage topology, the resistor noise may be greater than the tube's referred input noise. To reduce the noise to a minimum, use the smallest possible input grid resistor value that still provides RF suppression."

[Aurora]

Gentlemen

The statement about noise is unfortunately very true. Guitar amps still have to obey to the same laws of physics as any other amp, and any resistor will produce a noise voltage proportional to the square root of its value, times a set of given parameters and a physical constant. If you look at the schematics again, you will notice that the 1M is short circuited when there's no jack plugged in. This will reduce the noise to the level set by the series grid resistor. With the guitar plugged in, this resistor is parallelled with the impedance of the PU/PUPS in use, which will be the dominant impedance to set the noise level, along with the internal noise of the input tube section. It is generally necesseary to have a DC path from grid to ground, to funnel away stray electrons which may hit the grid, and which could otherwise lead to a negative potential at the grid which would block the tube. 1M seems to be the "universal value" for guitar amps.....
The series resistor from input to grid, is to give som protection from RF interference. In other types of amps, one would probably include a small choke of 20-100 uH as well, but I have never seen this in a guitar amp
OTOH I don't pretend to be a guitar amp oracle - this is just basic electronics.

EDIT : It is generally believed that CC resistors produce more noise than metal film. This may be so, but the resistor value itself will be the dominant source, whatever type of resistor.
And- the Aiken pages are actually very good, although I have not "screened" them looking for errors. There's a lot of myths floating around the net - and by mouth. There's probably some in there too.
It is actually 36 years ago thses days, since I read tube theory at college.
If there are errors, it will surely take me a long time to find them
I've spent most of my professional life dealing with "sand".....and some tubes

[JoeCOOl]

I think the whole "class A" debate boils down to semantics.

Loose definition of class A = both push pull tubes always conducting and flowing current, they never go into cutoff.

If you called a 2 tube marshall a 20W class A amp, I think you could bias it hot enough to get 20W at 400 plate volts and conduct full cycle. By definition this would be a 20W class A amp.

The same amp pushing 45W would be AB as you would have hit tube cutoff at some point.

I think all of this stuff gets down to specifics, but in any case, wheny ou are pounding the output section of any amp including a low plate volt hot biased amp, I think you will hit cutoff to get output stage distortion. So for guitar amps why even have the arguement as you are intending to distort these and all of the amp class defintions were done with an implied low distortion level.

[2tone]

The original Rocket ( mine anyway) didn't have a 25/25 bypass cap on V1 2nd stage, nor the 220k to ground. These were put in some amps by Ken for more of a true AC 30 sound ( tricking the amp into thinking there is another channel like an AC 30. These 2 items will give more breakup and grit like the AC 30. Without these 2 parts, there will be increased clarity and bottom definition. I put a switch on my Rocket to switch in or out this "mod". I usually prefer the sound without the extra parts, because the Rocket has more fidelity than the Vox, and it sounds cleaner without the second bypass cap and the 220k .. Also, the output cathode resistors will make a difference in sound and tube dissipation. Early Rockets had output tube voltages of from 300 to 315 volts. Now some old AC30's and the Matchless run higher because the old power trannies were for 115 volts, not 120 volts. Makes a big difference. I had a one off power tranny in my Rocket with a high and low voltgae tap. It came with the low voltage( 300). I later hooked up the high voltage tap and got about 350 volts. I had to change the cathode resistor to 68 from the 50 ohm stock. Now it has even more punch, bottom, and bolder mids. It lost a a little sparkle on top and delicate chimey top, but overall I like it better. Ken always did say the Rocket was an improved AC 30 TB. He had his power filter changes and tranny changes, but otherwise it's an AC 30 TB. He was a huge AC 30 fan....

[Clyde]

An original Rocket? That's pretty cool if that's what I'm understanding. by the 220K resistor to ground, do you mean the one, or 2 100K resistors in the power supply?

[M Fowler]

2tone,

Good information and you had or still have a Rocket, nice. The higher voltage surprises me. So why do most assume that the Rocket did not run that high a voltage? The whole subject is confusing and I have gone back to the beginning of this forum several times reading about which trannys where in what amp vs. what brand name and the works. Now it all makes sense that Allynmey ran 300v secondary on his Rocket. Using the 245-0-245 I don't have problems with clean, chimey tone and kicks in at about 1 to 2.

Mark

[rooster]

2tone - Hey, thanks indeed for checking in here. But I have to clarify what you said. The 220K resistor you spoke of is the 1/2 watt coupling resistor that connects to the .047uf coupling cap that ultimately goes to the PI. It might seem like it does, but it does not go to ground. And, yes, it is in the original TB model like you say, and because of the two channels. In other words, the Normal and Brilliant channel each have their own 220K resistor that join at the PI .047 coupling cap.

So you created a switch to remove this along with the cathode cap at pin 3? Yes? I heard this but I wanted to make sure.

One thing to note here is that, where it is not necessary to isolate the Rocket single channel from any other channel, it does a bit more than that in the circuit - it knocks off a bit of the highs. Not much, but obviously KF could hear it and thought he needed it in his amp.

I think that you are saying that your amp has more 'clarity' without the resistor in place, and this would make sense then. But it also serves as slight buffer - so without it the PI is driven a bit harder. So you are getting more treble response but you are also getting a bit more signal as well. Eh, apparently it sounds pretty good so that's the main thing.

Yeah, but how cool to have an original. You Rule, 2tone!!!! And thank you for sharing.

[2tone]

Hi Guys.. My Rocket was a unique one in that it had an extra high voltage tap on the power tranny. When I bought it, it had the low voltage tap hooked up and ran just over 300 plate volts. Other Rockets ran up to maybe 315V. Ken used several different power trannies in the dozen Rockets he made. The 220k resistor is an optional one joined to the 220k in line with the .047( >05) cap feeding the PI. It isn't the one in line with the 220k attaching to the wiper of the treble pot. It's an additional 220k (that joins the 220k feeding the PI coupling cap ), and in the Rocket this optional 220k would go to ground. In the AC 30 it was the 220k from another channel, tied in with the non optional 220k. I hope that clears it up. All Rockets came with the 220k in line with the PI coupling cap....It's just an option to put a second 220k that ties in between the 220k and the .047 cap feeding the PI, and it goes to ground. I put in one switch to have both the 25/25 cap and the 220k to ground to simulate a more voxy sound(not as fidelic as the Rocket's sound)..

[rooster]

2tone - Woah!! That is really interesting then, and not what I thought you were saying at all. So then, are you saying that you bought it with both 220K resistors in place and YOU came up with the switch idea? Or did Ken offer you this mod and somebody else performed it? Or did Ken do the mod for that matter? (Too, I guess I should ask if you bought it from Ken directly, and, if that's the case, do you have any stories about him to share. They always play well here, of course. )

Man, 2tone, I have got to try this. And again, thanks for sharing. BTW, did you say that you thought the number of Rockets Ken built was under 20? I have no idea and I would be curious to a count actually.

Thanks again for explaining things.

[2tone]

I bought the amp from Ken directly. Back when Trainwrecks were almost unheard of. I used to talk with him weekly for years and years. I put in the switch mod myself per Ken's info. Also wired it ti high voltage years later when he told me to try it. Also, I have jumpered in different output trannies in the past. They gave the amp a whole different character, but none as balanced as the original tranny. A Bassman output gave it even more low end, but shaved off the upper end mojo. Marshall output was not as open and more congested. I had thought the number of Rockets made was around 12, but Ken would never tell anyone the total number of his amps made. It may well be less than 12, as I had only heard mention of 5 different ones..