Re: [Harp-L] Harp amp mods: Weber

[MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>]

Most of these mods are not easily applied to the Pro Jr. Gerald's amp has little 
in common with the Pro Jr and his amp was voiced from the ground up. If you like 
the Pro Jr (as some do, especially with a moderate output mic to counter the 
amp's high gain) as it is,  don't know if these mods would be best for you.

#1 - Pro Jr preamp voltages are already pretty low at 150vdc...you could try 
nearer 110-120v...but that 30v drop is probably the smalest increment drop where 
people will hear a difference. If you go too low the amp will become somewhat 
mushy. R33 is the resistor to change, try 39K to 56K @ 2W or better?

#2 -  I have used .1 coupling caps in larger amps with good results but the 
.01uf caps in the Pro Jr should be adequate, I'd be skeptical as to whether caps 
bigger than 0.022uf would be of any advantage.

#3 Self balancing PI would require rewiring the amp because of the PCB  
construction and ribbon wires connecting the tube sockets to the board. Gerald's 
innovation was to unbalance the PI deliberately via the Waveform Symmetry 
Control...you can do this in the Pro Jr by using a 12DW7/JJ ECC832 in V2 (12AX7 
next to the EL84s)...I'd give it a go & see what you think.

#4 - Gerald's amp is cathode biased, the Pro Jr is fixed bias, very different 
designs...I would actually go the other way and possibly reduce plate current, 
maybe see how things sound at 25-30mA per tube (Pro Jr's are usually underbiased 
& running lots of current anyway)? Also note that later production Kendrick TC35 
Harp Amps run idle currents around the mid 50's mA mark, not 70mA as in the 
original article.

The first things that I would try with the Pro Jr are try disconnecting C2 (22pf 
bright cap accross volume pot) & remnoving C3...both these caps are hard to get 
at on the control panel PCB. Also maybe try a smaller value negative feedback 
resistor (R27), maybe try parallel resistors to bring the value down to 20K, or 
50K & see how it sounds. I haven't actually tried any of these 3 mods I just 
offer them in the spirit of experimentation.

Regards, Mark.



________________________________
From: Robert Hale <robert@xxxxxxxxxxxxxx>
To: harp-L list <harp-l@xxxxxxxxxx>
Sent: Thursday, 23 June, 2011 18:59:30
Subject: [Harp-L] Harp amp mods: Weber

I play a Fender Pro Junior and like it live, with line out to the PA.
Gerald Weber's article here:
http://www.harpamps.com/weber/Guitar51.htm
comments on amp mods for harp generally, so I'm interested in what I
can do with my Pro Junior.

Your experience?

*increase the value of the power supply dropping resistor that feeds
the plate resistor of the preamp tubes

*Larger than normal coupling caps: .1 uf  best for harmonica

* self-balancing paraphase inverters rather than fixed

*High idle plate current in the output stage

Stay well
Play well

Robert Hale is the DUKE of WAIL
Distance Learning via Webcam
http://www.dukeofwail.com
Gilbert AZ (Phoenix)