Some guidance needed for the Hammond Meteor

[Mark Hammer]

Redeveloping on old Traynor 10G practice amplifier, using Doug Hammond's Meteor as the front end, and a TDA2030 amp from the General Guitar Gadgets site as the power section.  The power comes from the original power transformer, and is delivering  about +/-17.5VDC after rectification.  I used a rather unusual approach, since the largest suitable caps on hand were 1000uf/35V.  I did the usual FWR-into-two-caps, but fed the + and - voltages from the caps through a single diode to a second set of 1000uf smoothing caps.  Takes a bit longer for them to discharge after power is removed, but I'm okay with that.

I've tried setting it up under the assumption that each FET should be fed 1/2 the supply voltage for maximum headroom, but I'm not able to get anything completely clean.  I'm also not able to dial in 1/2 V+ for the first two FETs, only the last 3.

So, any pointers here to provide a clean sound?  Should I change the values of some of the source resistors if I can't get the trimmers to provide clean at that voltage?  Or should I maybe drop the 17.5V down to something lower?

[Eb7+9]

 

maybe it's just me, would it be a good thing to recommended that those who lack basic knowledge of electronics refrain from playing with jFETs ... I mean, the chances of blindly landing on your feet using double parameter devices, times x devices, involves lottery level luck in many scenarios ...

the methodology that design engineers learn and use wasn't derived for nothing // why ignore it here ?! can DIY survive on blind device swapping alone ?! ... only to blame the device or the technology when things don't work out

I think this forum has revealed its reluctance to make use of this methodology and instead tries to fall back on the use of trimpots almost exclusively, relying and thus developing a potentially misleading form of communal consensus ... whether it's about bogus jFET device matching, in dealing w questionable (foreign made) jFET devices, VCR applications, or even making basic jFET gain stages work as required ... indeed, jFETs provide a special area of DIY darkness so-to-speak // until proper methodology is adapted

hope 20/20 will push some of you to do some studying ... and solving

Good Luck

[composition4]

That's helpful

[Eb7+9]

several years ago I posted a sort of "visual key" to help understand how to bias jFET devices for class-A use (whether in mu-stages or plain single ended) ... in the process paving a way to playing a trick on the transfer specs to produce non-linear transferring ... this can be found on my Triode Emulation page

using the graphical idea presented there, while fooling around with the Vp model parameter value inside SPICE, is really all one needs to gain a full jFET design picture - that is, for common class-A gain stage applications ... this then can also help decide the ordering of devices, say, within a gainy preamp/od circuit, ie., to obtain certain overall dynamic behaviour (dirt early/late etc) ...

the spec values are more important here than the actual device listing as many of our favourite device types have overlapping spec limit areas

[Gus]

Mark you have a +- supply so maybe the following might help
The Meteor uses +9VDC
You also need to be aware of the drain to gate voltage difference with JFETs

https://www.diystompboxes.com/smfforum/index.php?topic=103492.0

https://www.diystompboxes.com/smfforum/index.php?topic=93817.msg807917#msg807917

[PRR]

> not able to dial in 1/2 V+ for the first two FETs, only the last 3.

J2 and J3 are biased *exactly* the same: 33k, 2.7k. So why are yours different?


And as said, transposing JFETs to a different voltage is non-trivial. And the description say wide range of tone controllable from *guitar controls*, which won't scale with supply voltage. Why don't you derive a 9V supply for this?

[Mark Hammer]

I think I may just do that (i.e., derive a 9V supply).  When it comes to JFETs, I am sadly a paint-by-numbers guy.  I need to work on that.  But in the meantime, I'd just like to get this working and cross it off the to-do list.