Cathode bias & Bias Trem & Post PIMV

JJman

Member
Messages
994
My project head has cathode bias and bias vary trem. Having both is not new and I was able to reference different schematics. Including a post PI MV was always part of the plan and I used the common approach of a dual pot to ground. It worked and sounded good but the trem effect would also reduce when the MV was reduced. So no trem was possible at low MV settings.

Full scheme:

Pseudo-Express-scheme1.jpg




I considered the "crossline" MV but it seems to me that one would need very good balance between the "lines" for this to work well. I've read that some say it sounds weird but I think Matchless uses them and they don't sound weird to me.

I figured I'd try the type where the MV pot replaces the grid (not stopper) resistors. To me this looks the most logical since it's "pure" especially when maxed. A non-MV schematic doesn't really change by using this type if the grid resistors are the same values as the dual pot.

I got some dual pots and started with the 100k audio. I had settled on 82k grid resistors during a crossover-distortion-prevention project and 100k is pretty close. I just finished the install and the trem is now fully strong at all settings on the MV. Scheme:

Pseudo-Express-Scheme2.jpg




All looks good on the scope. It' pretty cool watching the trem with a clean signal. I haven't noticed a difference in tone yet between the setups other than the trem. Need to play more. The taper/slope of the volume change on the audio/log MV is good and I think linear would probably come up to soon/fast on the dial.

I figure it's a good idea to install some resistors from the MV wiper to the trem line in case a wiper looses contact. This way the tube won't drift way up from having no reference to dc ground. I haven't installed them yet but they are shown on the above scheme. If the harm the trem I won't use them. I did some testing and ~500k's will make the dual pot a dual 85k, or so, getting me back close to the original grid resistor values. There's very little crossover distortion when cranked at this time anyway.

To bad I waited so long to try this.
 

Keyser Soze

Member
Messages
1,472
Nifty thinking. I could have pondered that for a decades and never hit upon it.

Also, I like your adjustable cathode bias set-up. Is that meant to indicate a stereo jack set up as your test probe point? If so, another great idea.
 

JJman

Member
Messages
994
Yes I have a stereo female jack in a few amps for probing the idle or scoping. In a BF/SF Fender it's a good thing to load in the "polarity" switch spot.

I went with 2.2meg resistors on the wipers of the MV to minimize the change in the net grid resistance value. No change to the trem and the outputs just barely reach a hair of crossover distortion when cranked.

When I was testing the new pot it wasn't mounted. I saw a fuzzy oscillation thing during cutoff on one of the tubes when driven hard. At 1st I thought I would need to scrap the approach but when I grabbed the pot the fuzzy thing stopped. So it was the lack of shielding from not being mounted yet. Now it's fine.
 

TweeDLX

Member
Messages
3,763
I'm interested in the coupling caps in your PI. Interesting choice of values. What can you tell me about them? I'm guessing to offset some of the treble from the NFB set up? Is that a snubber on the PI plate resistors? Mike
 

JJman

Member
Messages
994
Input to the PI is the largest value orange drop I could find from mouser. I also picked up some 2.0u couplers (forget the brand) but the 0.68u OD is adequate and matches the other ODs look. I changed to this high value when I added the parallel loop. I didn't want DC on the loop's mixer pot so there is an "extra" coupler cap in line from the preamp to the PI. Each cap in a series reduces the overall value, hence the "big" one.

The coupler’s values on the PI's outputs are common for Fender and the Trainwreck Express which was the starting point for the schematic. Grid stoppers are larger than common to reduce crossover distortion, although they weren’t the solution that worked. (Lower grid leaks did.)

The NFB is minimal based on the 10k and 1k values in it and I'm not using any "Presence." I really don't notice a difference in tone with the NFB vs. w/o it so it's staying in to make me feel a little "safer."

Snubber across the PI plates also added since it's on some of the Express schemes. Since it's within the NFB loop, and it's a small value, it doesn't reduce treble noticeably. Also there to keep things under control (make me feel better.)

I got to play it more yesterday with the new MV. It definitely sounds much better and some quirky things are no longer happening. I can max the trem and the preamp with the volume low and it still sounds very cool. Plenty of different tones on tap. It even seems like the loop is working better where I use a reverb or delay pedal. That could just be wishful thinking.

I still need to record some to see if the “buzzy/staticky” distortion is gone that I used to hear, when recording. I had confirmed that it was not from the speakers or mic or software. We’ll see.

Here’s the latest pic of the rat nest.

2010-7-DSCF0001.JPG
 




Trending Topics

Top Bottom