The purpose of 6.6k Primary on Wreck OT's

[Zippy]

Make that "Express/Liverpool" - the topology of the Rocket is more closely associated with a tweed Fender Bassman.

I'm not sure what your point is. I think that most people here generally understand the phrase "TW preamp" to refer to that of the Express/Liverpool. I still maintain that the Rocket is nothing but an AC-30 clone.

Regardless, notice that both the AC-30 and Tweed Bassman preamp is identical in topology to the Plexi's.

My point is one of maintaining accuracy - "Most", "generally"... C'mon. Not all 'wrecks are Expresses. It really has nothing to do with what you feel about the Rocket.

No news about the similarity between the AC-30 and the tweed Bassman extending to the Plexi. That's part of why I prefer the Rocket among the 'wreck family.

[dave g]

This is a stupid thing to argue about. My intent is to show actual measurements and provide a technical explanation as to why guitar amps behave the way they do, in an effort to dispel a little bit of the mojo BS. I have no interest in nitpicking semantics, especially with respect to a TW model that comprises ~5-10% of the total number of TWs ever built.

On a more related note, I found a couple more plots on my laptop for anyone who might be interested. One is of a bogner ecstasy, the other is a high plate skyline non-hrm overdrive special. The bogner had the gain around noon, and the ODS was set to how I normally play (input vol 11:00, TMB around noon, drive 11:00, trimmer set 25k from ground, PAB off).

[FYL]

I still maintain that the Rocket is nothing but an AC-30 clone.

Yup, the Rocket is KF version of the AC30 top boost channel. Some minor component changes, a beefier power supply, a much simpler and better topology, different iron, better everything, but still a Vox clone.

"Since 1983,Trainwreck has been building a class "A" amplifier equipped with four EL-84 output tubes. Throughout the years many guitarists have asked if the Liverpool 30 was the same as the British built class "A" amp, made famous by four chaps from Liverpool, England. The Trainwreck amplifier was-- and is-- a totally different design with it's own unique sound."
From the Trainwreck catalog.

A totally different design ? Huh ?

[paulster]

"Since 1983,Trainwreck has been building a class "A" amplifier equipped with four EL-84 output tubes. Throughout the years many guitarists have asked if the Liverpool 30 was the same as the British built class "A" amp, made famous by four chaps from Liverpool, England. The Trainwreck amplifier was-- and is-- a totally different design with it's own unique sound."
From the Trainwreck catalog.

A totally different design ? Huh ?

Yep, the Liverpool is. The Rocket, on the other hand, isn't.

I built a stripped down AC30 Top Boost clone before the Rocket schematics were confirmed and, power supply design aside, it's a Rocket!

[Structo]

Dave I appreciate the trouble you have gone through to show these wave forms.
But, I guess I don't really know what I am looking at.

Can you break it down a bit more for me?

Thanks, Tom

[M Fowler]

The Rocket is a lot better

[RJ Guitars]

With a pentode, there is an impedance match which gives the most power. Say with an EL34, it's something like 3.5k (I'm approximating). There is also a very large amount of odd order distortion at this impedance, but not much second/even order. Increasing the impedance from this point gradually reduces the power output and the odd order distortion, but the even order distortion begins to increase. I think what Ken was going for was both reduction in power and odd order distortion (crunch). Even order distortion would be mostly cancelled out in the output transformer since the amp is push pull.

Using the above example, most hifi amps would decrease the impedance slightly, say 3.3k-3.4k where you get almost all the power, but much less distortion.

There's a great graph illustrating impedance vs power/2nd/3rd distortions in a book I have. I'll have to dig it out.

Also, the impedance affects the current in the tube, or the incline of the loadline. Higher matches make the loadline more level, lower matches are steeper.

Ultralinear, the screens are attached to the output transformer to get the pentode to act more like a triode, so it's probably not relevent to the express output section.

This goes right to the point of the thread... a good piece of info! I'd really like to see that graph if you can find it.

The HiFi community sorted all this stuff out years ago and they have a tremendous amount of knowledge to contribute to our builds... albeit sometimes hard for them to relate to our preference for a little bit of of dirt in the output.

One cautionary note - if you start talking about "MoJo" to the HiFi guys the next thing you know they will provide you a technical explanation and even some optional ways to achieve the same effect using a different set of parts and an entirely new layout... sometimes it's kinda like learning there is no Santa.

rj

[Zippy]

This is a stupid thing to argue about.

I have no interest in nitpicking semantics, especially with respect to a TW model that comprises ~5-10% of the total number of TWs ever built.

All I was doing was trying to keep it straight so someone doesn't come back later with citing YOUR posts and not knowing which is which. I love it when someone throws down a subjective statement, sustains it in an argument, and then calls it "stupid". Yeah, I've got nothing better to do - it's early and my kid woke me up. Coffee, please.

My intent is to show actual measurements and provide a technical explanation as to why guitar amps behave the way they do, in an effort to dispel a little bit of the mojo BS.

Cool. Regarding the tranny impedance selection, would you like to comment on the variation in effective output impedance as a function of operating voltage?

[M Fowler]

The purpose of the 6.6k primary on the Wreck OT was entirely to entice my ears tonally and it worked. Yes I prefer the 6.6k over the 5.2k and

Allyn Meyers and others were trying to tell us this a long time ago based on my early old post reviews.

Its all mojo not technical

Mark

[dave g]

All I was doing was trying to keep it straight so someone doesn't come back later with citing YOUR posts and not knowing which is which.

Really? Ok, what about your comment regarding the Rocket and the 5F6A? God, I hope some poor soul doesn't cite YOUR post as evidence that the Rocket is actually a Bassman clone...

Sorry, but "two preamp gain stages plus a CF driven tonestack" is where the similarities between the Rocket and the Bassman end. The coupling caps are different, the loading for each gain stage is different, plate and cathode resistors are different, the tonestack is different, the PI is different, there is no GNFB, and the output section is COMPLETELY different! Now, look at an AC-30TB schematic and ignore everything besides the top boost channel. The Rocket is a STRAIGHT clone!!! If you're as concerned about accuracy as you claim to be, then drop the Rocket/Bassman crap and call it what it is.

Cool. Regarding the tranny impedance selection, would you like to comment on the variation in effective output impedance as a function of operating voltage?

You can estimate the output impedance of the output tubes by squaring the plate voltage and dividing by the power output (which will be dependent on the bias current). So, for an express, 400^2 / 35 = 4571 ohms. However, this is just an estimate and as I'm sure you're aware, the output impedance changes drastically with frequency. Anything within the range of a 2:1 mismatch is usually safe.

I would be happy to make some more plots next weekend of an express running on the 5200 ohm tap versus the 6600 ohm tap if you're interested.

Structo, what you're looking at is the distortion spectrum of each of these amplifiers. I fed in a pure tone from a signal generator and used a spectrum analyzer to look at the output. Each peak that you see corresponds to a harmonic generated by clipping the input signal. By looking at the relative amplitudes of the harmonics, you can get a really good idea of the "character" of the distortion of each of the amplifiers. Of course, what you see isn't exactly what happens when you actually plug in a guitar because the signal from your pickups is the superposition of many frequencies to begin with, and linear circuit analysis fails when the amplifier is operating in a nonlinear region. That being said, in my experience I have found the results of this method of measurement to be positively correlated with what I actually hear.

[Richie]

I would be happy to make some more plots next weekend of an express running on the 5200 ohm tap versus the 6600 ohm tap if you're interested.

that would be great.

[Zippy]

I would be happy to make some more plots next weekend of an express running on the 5200 ohm tap versus the 6600 ohm tap if you're interested.

Yes, please.

You can estimate the output impedance of the output tubes by squaring the plate voltage and dividing by the power output (which will be dependent on the bias current). So, for an express, 400^2 / 35 = 4571 ohms. However, this is just an estimate and as I'm sure you're aware, the output impedance changes drastically with frequency. Anything within the range of a 2:1 mismatch is usually safe.

Given the interplay between frequency and effective impedance, can we then consider that transformer impedance will enhance an audibly significant range of frequencies?

[John C]

Why?

What's the point? what does this achieve? I like it, but I want to understand it more?

What are the reasons for it?

Is this a HiFi transformer spec?
Is the volume drop compared to say 3.5k Marshall OT the only reason?

etc.. etc.. real tech answers if possible.

Thanks guys! A great technical reference... say more and help us connect the dots back to the original post? Can you put this into context for understanding the 6.6K Ohm Wreck standard... how does this relate to what our ears hear, what is the impact on distortion, frequency range, volume, etc..

thanks again,

rj

I think you'll like this:

http://plitron.com/PDF/Atcl_3.pdf

[paulster]

Now, look at an AC-30TB schematic and ignore everything besides the top boost channel. The Rocket is a STRAIGHT clone!!!

+1. The only difference of any minor significance is the power supply, but that's only because it doesn't have the non-Top Boost preamp sections to drop the voltages.

[Noel Grassy]

Thanks Dave G for these fascinating examples. I'm sure few of us
will ever own a spectrum analyzer. The details you reveal are far removed from any "mojeaux" which is what Amp Garage is primarily
about. At least that's how I perceived the site back in 2005.
Keep the empirical data coming please,


Noel Grassy.