***The Official Guitar Thread***

Soldato
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13 Jan 2003
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Ooh I like that, nice justification of the power valves.

The weirdest thing about these is they have the same valve count as a full 50W Marshall amp, pull the same crazy amount of heater current, same bulk/weight, but then have 2W output :D

The EF80 tubes output less heat compared to the EL84 allowing them to be placed closer. There is a little less current in the power side with the EF80s so you can use a smaller supply but that's what I like about it - closer tone to a full size Marshall. The output transformer itself make a lot of the tone as do the 12ax7s.

In terms of triode output tubes. I don't normally like 12au7s as they're not as linear as the 12BH7A-STR. The main thing is push-pull will cancel out symmetrical noise (ie reduce even harmonics and leave odd harmonics). With tubes with two triodes there's a cap on the combined heat dissipation although you could use one tube (two triodes) that would limit the power. Not that all the second harmonics from the triode would disappear as the triodes do have differences between triode sections and tubes so you'd still probably get a very slightly warmer sound.

The pentode is a different beast. The EF80 is an RF tube that works for audio.. for that reason it can amplify ultrasonics and RF so make sure that grid stopper resistor is on the pin of the grid. It also means you can use it ultra linear too but in the schematic they have at the bottom of the page it's running dropped from the B+ rather than the output transformer. The main thing is impedance and the relation with the transformer selection for tone.

What I like about that schematic is that it uses a tube rectifier which as you know means some appropriate sag in the B+ rails for that authentic JCM800 tonal distortion. Also they're using a 4H choke to reduce noise too so it should be deathly quiet. I may knock it up in LTSpce as there's lots of info for the hammond transformers etc for giggles. It always shocks me how much voltage is dropped across a tube rectifier..

I see they use a 12at7 on the phase inverter as to reduce the amplification gain for the smaller EF80s (it still has some but not in the same as the higher 12ax7). Also I did read the 12at7 is designed for grounded grid (ie with an LTP phase splitter for this purpose) but that's not an authoritative source.

Lastly a Celestron 12" Vintage 30 is 8/16ohm and about 100dB sensitivity and 60W max but the smaller 10" drops it's sensitivity to 97dB. Still plenty loud. I also have some birch ply left over that I could make a cabinet to suit the 12".

TL;DR - the EF80 2W clone has a closer sound to the JCM800 which would be good.
 
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Soldato
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Playing in working bands for the last 52 years I need a different sound for every song I play and using separates is virtually impossible to dial a tone in while the rest of the band are waiting for you so I'm done and dusted with them.
The Helix is just like messing with separate pedals and adjusting them on the fly. Yesterday I discovered I could put two effects (or more) on one stomp so for example playing a song then going into a solo where I need to go up 5db with delay, easy for the Helix, pain in the ass with separate pedals because I'd need to press the Boost then press the delay and then turn them back off after the solo.
My Helix now has presets for all my songs with a completely different sound on 90% of them and they sound awesome.

A fully loaded pedalboard is a thing of beauty but my lead guitarist has now altered the two sets so he can bend down and quickly dial in his tone with the least faffing about.

I do the exact same thing, except I play bass so I don't need the same variety of tones. I have 5 presets that I actually use: classic bass, something a bit more modern, something verging on metal, a really synthy sound for one song, and a Muse preset. That does everything I need really.

I also got one of these (from the UK) a while back and that has made my life a lot easier. FS1 and FS2 are up and down preset respectively, and FS3 activates and deactivates the tuner. So, I don't have to bend down between sets and if I'm using more than one preset in a song (that synthy sound), I just hit the button on the change. Strongly recommend this! It's midi, although I haven't got it working quite as I'd like as I can't get it to control a boost block in the Helix for some reason.

The only other pedal I have right now is a channel switch for my amp that gives more of a tube tone. I rarely use that tbh. I'm also building this at the moment, which will give me an HPF and LPF, and I believe also does a little bit of boost.

I do love the Helix, but there are some things I feel might be better done with a separate pedal. For example, noise gate and compressor.
 
Soldato
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I've copied the JCM800 schematic into LTspice, and a couple of things struck me about the small 2 watt build:
a) The JCM uses solid state diodes which gives the rails a stiffer rail and doesn't need to worry about the current draw for all the filter capacitors, the mini's schematic has a tube rectifier that will have a current limit - that makes the maximum capacitance you can have limited (due to inrush and ripple current overload). I see Rob puts a 40uF then some filter caps in the rails for each section after and shows a 71mA current on the rectifier tube. Not sure on the inrush spike but the choke should help a little with that.

b) The mini schematic is missing a complete gain section and uses low gain LTP then the EF80 is lowish gain too.. let's look at the tube mu (amplification)
JCM gain goes 100x 100x 100x 0.9x 100x 19x
mini-JCM goes 100x 100x 60x 7.4x

So the reason this was done is that power output needs high voltage swing (my 50W hifi into 8R has a voltage swing of 70+ volts peak to peak). So for our 2W we only need a smaller voltage swing. P=I^2 * R so sqrt(2watts/8R) = 0.5 amps, thus the voltage will be V=IR, so V=0.5*8 = 4 volts peak. \m/ Rocking hardcore power! So that means we only need to go from 100-300mV to 4000mV (ie from 0.1 to 0.3 volts to 4 volts) thus not as much as a 50W amp. We may need a little more depending on the speaker sensitivity chosen but 97dB seems ok..

Often power tubes don't have a massively high mu (essentially how many mA per a volt swing on the grid). As you can see the original marshal stages have a large mu coupled with a 100K resistors thus voltage = I*R that 100mA/grid volt gets multiple by 100K.. So it amplifies the voltage you put in at each stage. Power tubes often need a big swing input compared to the little 12ax7 hence you use a driver stage but they don't with the JCM so the LTP will create distortion driving the large grid capacitance of the EL84 etc.
The EF80 on the other hand has a small grid capacitance to cope with RF frequency swing speed needed. So we may have a difference in sound from the LTP. In theory we could compensate by adding a larger coupling cap to increase the gate capacitance and slow the tube down.. and load up the LTP to sound more like it's bigger brother.

What is interesting is that the JCM has a cathode follower in the middle of it so they obviously need more current to drive their tone stack... hmm food for thought as the mini schematic simply ploughs the first stage 12ax7 into the tonestack.. which would cause distortion but not the same as the JCM..
 
Soldato
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Actually - found this on Rob's site: https://robrobinette.com/How_the_Marshall_JCM800_Works.htm

So basically precisely as I said:
a) the cathode follower prevents the pre section from being dragged down by the tone stack and thus contributes to the JCM sound
b) the cold clipper (missing from the mini schematic) gives the rounded clipping tone that makes the Marshall sound. Hence rather important.
 
Soldato
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13 Jan 2003
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That was an awesome watch :) Really wish I had time to sit down and put some real practise in but I'm in the middle of a house move. It'd be a literal distraction if I tried to set up a space with an amp and a bass right now.

There's a series of these - the guy with the beard is the co-founder/CEO of Patreon too!
 
Soldato
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14 Dec 2005
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4,561
thought I'd share this...gave up trying to play the solo so many times because of the 2nd lick, just can't play it following tabs etc so made up a legato/3nps type thing to make it doable

 
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