Cross-line master volume problem

[The Radium King]

the best pot for a post-pi mv in a higher power amp tends to be a 100k or 250k audio no load, since the response occurs in the first 100k - 200 k or so of pot response. no load is good, as when running wide open you don't want a 200k resistor bridging your pi outputs - the pot disconnects from the circuit when at max (note that you have to disconnect one end of the pot for th eno load to work; wiper to one side of the pi, pin 1 or 3 to the other). you can make a no load pot by opening the pot up and scratching the carbon off one end of the trace.

[cgiff]

200% better?

HTH,
I have not tried a pre-PI master volume, I'm sure you have better range on the volume side . Do you notice any difference in gain structure / drive relative to the post PI control. Just curious without the extra PI stage did it clean up noticebly?

Cgiff

[HTH]

HTH,
I have not tried a pre-PI master volume, I'm sure you have better range on the volume side . Do you notice any difference in gain structure / drive relative to the post PI control. Just curious without the extra PI stage did it clean up noticebly?

Cgiff

I'd say the amp opened up tonally with the pre-PI mv - the crossline seemed to muffle the amp's tone.

I wouldn't say the gain structure changed at all, the tone just became punchier.

One think I HAVE done that changed the tone of the amp alot was to install a cathode/fixed bias switch. The amp sounds noticeably gainier & thicker in cathode bias setting. Haven't really wound the amp up high enough to see if fixed bias is louder.

[Humbucker]

I tried the cross-line master volume on my 1970 Traynor YBA-1 BassMaster 50W amp last year and I didn't like it at all. Then I started reading up on the ppimv and got some good info from Bruce at Mission Amps. I followed his schematic and ended up using a dual-gang 500K pot, I think that I used a pair of 33K resistors too.

The verdict? I really like it for slightly lowering the volume of the amp to listenable levels. Highly recommended. Thanks Bruce, much appreciated!
8^)

[hasserl]

I've built amps with three different common MV types, and added all three different types to different amps. In a few amps I've found a crossline MV to work very well. In others it has not worked so well. When it has not worked well the problem has been very poor tone at low volume setting; harsh, brittle, thin, yuckkk, just awful. However, on some it has worked just fine. I pulled the stock Fender pre-PI MV from my SF Super Reverb and added the Crossline style post-PI MV, and it works great, absolutely fan-freakin-tastic. I've got one in a Matchless Clubman style amp and it also works great there.

I typically use the dual ganged MV with four caps to isolate the pot form DC, and that has been the most successful design. I've used that in countyless amps. But I've also used the more simple dual ganged pot with no caps added, the so called Ken Fisher MV, also with great success. In fact, that is a very transparent MV, the only trouble with it is the remote chance of a loss of bias voltage if the wipers of the pots should loose contact.

But I'm a big believer is the use of a post-PI MV as a tool to greatly increase the versatility of an amp, with very little to no negative impact. And as I approach an amp build or modification I'll use the Crossline MV first, as it is so easy to install, and if it works well it is perfectly fine to use. If I don't like the result then I'll use one of the other two dual ganged pot types, depending on the application.

[Mouthurst]

I'm experimenting with a cross-line master volume (Type 3 of the Train Wreck pages) on a Fender Bandmaster, and something sure ain't right!

Here's what I did:

I used a 1M audio double pot (it's all I have handy) and just used the back 3 lugs.

From the FRONT of the amp, numbering the pot logs from LEFT to RIGHT, 1-2-3: Center lug (wiper) to the right-most 220k resistor (closest to the bias pot) and lug #3 to the leftmost 220k resistor. Lug #1 is not connected.

The master seems to jump in volume and become VERY LOUD almost immediately, say on a setting of about 2. Also quite noisy.

What have I done wrong here? It's sure not acting like a master volume! HELP!

Thanks,

Fred G.

Fred-That type of master volume is no good for fixed bias amplifiers-

William