Lowering the gain in my Wreck

[gnugear]

I've got so much gain in my Wreck that I rarely go above 1:00 on the volume. What would I need to do to lower the gain slightly and achieve more clean headroom?

Also, what do 6V6s do to the sound? Is it just a matter of swapping tubes and rebiasing?

[Matt72]

Easiest fix first? You can really reduce gain by swapping out one or more of the AX7s for AT7s.

[gnugear]

How about another option. I got three NOS Mullards that I'd hate to not use!!

[Roe]

try this:
v3: ecc81/12at7
v2: 5751 or 12at7
v4&5: kt88 or 6550

[brownnote]

try this:
v3: ecc81/12at7
v2: 5751 or 12at7
v4&5: kt88 or 6550

You might want to check PT specs before you try that...6550 are some hungry hogs, HV & FIL current demands will probably exceed PT capabilities.

[brownnote]

I think you want to look at the V2 grid leak resistor, what do you have in there now?

You might consider a trim pot in series with the .002 coupling cap.

[gnugear]

Is there a simple way to keep the amps volume from ramping up so quickly?

[UR12]

I have had good luck with substituting a 250k trim pot in for the 150kresistor and adjusting the amount of signal sent to the 3rd gain stage. I ended up with a 100k in my liverpool as this sounded best to me.

[riscado]

how about adding another resistor on top of that 150k resistor, wouldn't take make a nice voltage divider that would limit the signal going to the third section?

[sliberty]

My Express has a very simple MV sort of control added. I replaced the 1M resistor on the leading edge ofthe PI witha 1M pot instead. I keep this control down to about 1/3 most of the time, which reduces gain and volume considerably. I then kick the amp with a clean boost pedal when I want the "little extra". And, I can always dime the MV to get back to standard Express specs.

[riscado]

you can also add a local negative feedback around the third gain stage, much like a blackface bassman...

[Roe]

I tested a bit today. here's what I would try:

1) ecc81 in V3
2 ecc81 in V1

[hairyandy]

I noticed a good bit more clean headroom in my Express when I tried some new JJ KT77s. They sound great (Dan from 65 amps described the KT77 to me as what an EL34 wants to be when it grows up! and they bias just like an EL34. That way you could keep your Mullards in the front end.

Andy

[gearhead]

you can also add a local negative feedback around the third gain stage, much like a blackface bassman...

Doesn't the Express already have local NFB on the third stage already?

[Trace]

I've got so much gain in my Wreck that I rarely go above 1:00 on the volume. What would I need to do to lower the gain slightly and achieve more clean headroom?

There are numerous ways to do this and all will yield varying results, meaning they will all lower gain and increase headroom but at what cost? What I am suggesting is that some changes will effect the tone as well as the feel of the amp. Some may be more desirable than others all depending on the transformers, layout, wire used, voltages, component type, etc.

Some ideas to experiment with;
Let me preface this by saying I am going by the circuit that is in the schematics section of this forum (http://www.undustrialdesign.com/trainwr ... erpool.jpg). You could experiment with lowering the B+ dropping resistors (IE; raise voltage and increase headroom). Experiment with the feedback (82K/100K on the 4ohm, 39K/47K on the 8ohm). Add an input resistor to V1A’s grid (you can start with a typical 68K value). You could increase V1A’s cathode resistor up to 4.7K. Increase V1B’s cathode resistor. You could remove the R/C from the cathode of V1B and then jump V1B’s cathode to V1A’s cathode (they will effectively be sharing V1A’s R/C cathode values. You can hang a resistor off V1B’s grid to ground (180K to 220K). Insert a resistor in series after V1B’s .001uF coupling cap to add more resistance 220K to 1M), you can also place a cap across the resistor to add top-end. Increase V2A’s cathode resistor (possibly place a resistor in series with a cap to bypass the cathode resistor). Lower the PI’s grid resistors and/or plate resistors, you can convert the amp to adjustable bias, etc.

Again there are a lot of options but again everything will change and effect the amp’s overall tone as well as the feel. Trying several things will let you know what will best work for you and nothing beats hands-on experience.


Hope it helps
Trace