Dr Z EMS JCM800/JTM50 style amp

[RobBozic]

Hi,
I saw this pic of the Dr Z EMS amp from this website. http://www.proguitar.de/ems.html


It's a JCM800 circuit with some features, one of which is a simple HI-LO switch which cuts the gain down to JTM50 type gain levels.


The switch adds a 33K resistor to ground after the 68K input ressitor, as well as lifting the 0.68uf cap from the V2a 820 cathode.


Regarding the 33K input resistor to ground in parallel to the 1m (31K), is this any different from the traditional Fender 'Hi-Lo' input jacks? I suppose you could use 68K and 33K and get the same response? The Hi would go to input would parallel 68K & 33K for 22K.


http://www.proguitar.de/images/ems-insidejpg-1600.jpg

[pdf64]

I don’t like the concept of loading the input down, as the guitar-amp interaction will be significantly different depending on whether there’s a buffered pedal between them.

[tubeswell]

The switch adds a 33K resistor to ground after the 68K input ressitor, as well as lifting the 0.68uf cap from the V2a 820 cathode.
... is this any different from the traditional Fender 'Hi-Lo' input jacks?...

Fender Hi/Lo input has either 34k in series and 1M to ground, or 68k in series and 68k to ground

[RobBozic]

Hi,
Yep, a 6db voltage divider.


Idon’t know why he didn’t just do the 68k/68k.


Thanks gents.
Rob

[pdf64]

Hi,
Yep, a 6db voltage divider. ...

If there's a straight connection between guitar and amp, compared to the regular 'high' input, due to its increased loading, the effect of the 'low' input will be more than just a 6dB attenuation. ie with instrument vol up high, the pickup's treble resonant peak will be damped.
If that's novel info, have a read of http://www.buildyourguitar.com/resources/lemme/

[JB]

Hi,
Yep, a 6db voltage divider. ...

If there's a straight connection between guitar and amp, compared to the regular 'high' input, due to its increased loading, the effect of the 'low' input will be more than just a 6dB attenuation. ie with instrument vol up high, the pickup's treble resonant peak will be damped.
If that's novel info, have a read of http://www.buildyourguitar.com/resources/lemme/

Yes but it can be useful tonally depending on the guitar and situation.  Lot's of photo's of Hendrix on stage with two or three Superleads and he's plugged into the low input on the bright channel and daisy chaining from the high to the low on the next amp.  Even early on, there's a photo of him at the Marquee in London (66, 67?) with a single stack, JTM45/100 head,  and he's on bright channel low input. He sounded pretty good!  I appreciate he had a wah and a fuzz face, not straight in, but he must have preferred that tone rather than going in the high input and turning the volume and treble down.

[pdf64]

Hi,
Yep, a 6db voltage divider. ...

If there's a straight connection between guitar and amp, compared to the regular 'high' input, due to its increased loading, the effect of the 'low' input will be more than just a 6dB attenuation. ie with instrument vol up high, the pickup's treble resonant peak will be damped.
If that's novel info, have a read of http://www.buildyourguitar.com/resources/lemme/

Yes but it can be useful tonally depending on the guitar and situation.  Lot's of photo's of Hendrix on stage with two or three Superleads and he's plugged into the low input on the bright channel and daisy chaining from the high to the low on the next amp.  Even early on, there's a photo of him at the Marquee in London (66, 67?) with a single stack, JTM45/100 head,  and he's on bright channel low input. He sounded pretty good!  I appreciate he had a wah and a fuzz face, not straight in, but he must have preferred that tone rather than going in the high input and turning the volume and treble down.

Tonally, the low input will be roughly similar to turning the instrument's tone control down a bit, ie such that the total parallel resistance of the tone and vol controls is about 140k (the tone cap's impedance may be regarded as a short up in that freq range, compared to such high resistances).
So it might be handy for calming down a Strat's 'tone controlless' bridge pickup.
Note that both the Fuzz Face and original wah had rather lower input impedances than a low input, and hence an even greater damping effect on pickup resonance.