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Harp Mods

3/10/2002 7:38 PM
Harp Mods
Anybody have success with mods for harp? I've been playing around with preamp configs and coupling caps. So far, I have a 5C3 preamp (cathode to ground) on one input and 5E3 on the other. I'm still playing with bypass cap and resistor values, any recommendations? Other feedback reduction mods? coupling cap favorites?  
Great job keeping the forum going.  
3/11/2002 1:55 AM
Bruce /Mission Amps

Here's some things I'm playing with... but I'm still in R&D too!!  
First off, use only the normal channel for harp and you don't have to do everyone of these suggestions.  
Increase the 1M resistor on the normal channel's input jack to 2m7. This depends on the mic you are using.  
In the power supply, use a 33K dropping resistor instead of the 22K.  
Try a 180K plate load for the normal channel with a small ceramic .001uF/500v to .0033uF/500v cap across the 180k plate load.  
Split the cathode RCs of V1. Do not share the same cathode parallel cap and resistor anymore.  
Use a 1500 cathode resistor for both bright and normal channel triodes but a 22uF to 68uF cathode bypass cap across the normal channel's cathode resistor and use a 4.7uF across the bright channel cathode resistor.  
On the normal channel, keep the .1uF coupling cap (even a .22uF seems to work OK) to the volume control and try a a .022uF to .047uF to the bright channel's volume pot.  
Use a 1/2 watt 360k to 1M resistor on the wiper of the normal channel's pot right to ground on the pot.  
Let's you turn up a bit more without over doing it in acoustical feedback.  
Lower the single 1m grid load resistor on V2 to 220k.  
Here's a weird one and it seems to work for some and not for others and is for harp only playing....  
try using a 120K grid load resistor on one 6V6 and a 220K on the other.  
This makes one power tube react differently then the other one and harp tone is a little more woody sounding to me.  
Speaker: P12N or slightly dark sounding ceramic of your choice.... the MOJO V12-30 sounds pretty good and isn't too much money.  
3/11/2002 3:48 PM

Thanks Bruce,  
On the wierd one...a guy over at the Weber board suggested a 12DW7 as a PI to create the unbalancing of the power tubes. Gerald Weber's article on Harp mods suggested a self-balancing paraphase inverter (5D3) w/ a pot in place of the resistor common to both circuits for an unbalanced signal.  
I've ordered a 12DW7 to test. I'll try out some of the values you suggest. I don't really want to rebuild the PI, but it would be nice to test 'em all.  
3/11/2002 4:38 PM
Bruce /Mission Amps

Hmm... I can't see how a 12DW7 would unbalance anything.  
The tube is like a 12AX7 and a 12AU7 in the same envelope.  
In the 5E3, the phase inverter is just a single triode, wired as a cathodyne driver (zero gain) so any triode in that spot would still do the same thing, albeit with different amounts of attenuation but it would still should be balanced.  
The AC mismatching of the power tubes does a mildly interesting tone thing though and it can be done with using different grid load values on the 6V6s.  
With all things, a singular mod is never the magic or silver bullet!!  
I read that G. Weber thing a few months ago and it is a bit confusing, in terms of what the pot does, other then upset the AC balance of the PI.  
You don't have to rebuild the PI of the 5E3, just  
strap another 180K to 220k reistor over one of the 220K grid load resistors and you'll get a slight mismatch of the AC signal going into the power tubes.  
IMHO, the darker grind comes from lowering the voltages to the preamp tubes and some of the harp tone comes from using less gain in the front end and subseqently turning the volume up louder.  
3/12/2002 1:41 AM

Oops, sorry for my poor interpretation of what the 12DW7 is doing. I guess the lower gain is what's making the difference. I popped one in and feedback is almost gone. I can crank the volume to almost 10. The tone is very thick, almost like a touch of reverb is added. I have the plates at 82V on the preamp and that made a huge difference in tone. I'm going to keep playing with all these options, it's hard to know when to stop. It's also difficult to keep track of which sounds best. I need a couple of extra amps for comparisons. Thanks for all the help.  
3/12/2002 7:08 PM
Steve A.

Hmm... I can't see how a 12DW7 would unbalance anything.  
    I guess that the guy on the Weber board was thinking of an amp that used a LTP PI...  
    So which triode is the 12AX7 and which is the 12AU7?  
Steve Ahola
3/13/2002 5:01 AM
Bruce /Mission Amps

The 12AX7'ish side is pins 6,7 and 8, while 1,2, and 3 is the low gain (12AU7'ish) side.  
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