trinityamps.com

Since I got the amp working again (see this thread: http://www.trinityamps.com/phpbb/viewtopic.php?f=7&t=3285&p=27509#p27509 ), I'm finally ready to tackle the VRM. Maybe I'll address the 6V6 vs. EL84 gain difference, but I'm not sure I'm ready for that yet. Based on what you provided, the VRM seems like a complete breeze!

The part I didn't highlight looks like a no-brainer. As I saw in that other VRM thread, I think I'll go with the pot in the back like the person that provided a picture. I really don't want to do the combo pot/power switch on the front. There's already a boost switch and an EL84/6V6 switch on the back, so I'm going to be reaching back there anyway. What's one more thing? Looks like I'm only really cutting into the one wire that goes from standby to the cap and inserting the VRM in there. Easy as pie. A ground wire and three wires to the pot are the rest. Nice!

The part I highlighted in red looks a bit more complicated, but I'm guessing the VRM kit will come with the proper documentation. How do I go about ordering a VRM kit?

_________________
Go Buckeyes!

Ordered the VRM yesterday. Since it comes with a switching pot, I talked myself out of a back-mounted pot and into going with the power switch VRM design. Looks like a pretty simple modification, so I can't wait to get started.

I haven't even bothered putting the chassis back into its cabinet yet since I got it working again (see post above). I figure I'll do the VRM first.


But looking ahead, the idea of modifying the circuitry to "heat up" the 6V6 mode has me intrigued. I've looked at Coco's post in the Resource section pertaining to driving the 6V6 side harder (in this thread: http://www.trinityamps.com/phpbb/viewtopic.php?f=12&t=763) a hundred times.


Coco wrote:
"Some builders have expressed a desire to drive the 6V6 harder in the v6 amp. Here is a way to do this. This will soon become our v6 Mk. II design.

The stock 18W Phase Inverter has a reduction in both gain and drive level, compared to a regular Marshall Long Tailed Pair PI. This was done in order to compensate for the high gain of EL84s relative to octal power tubes. To regain this for 6V6 tubes, you can reduce the PI tail resistor to around 20k (from 56k) and the PI cathode resistor down to 470 ohms (like in the Plexi). These two small changes are needed if you want to get the 6V6s "singing".

Once you make this change on a v6 amp, the higher drive level to the EL84s, once again, needs to be reduced and this is done with a simple voltage divider.

Replace each of the 470k resistors at the power tube grids with a resistor pair consisting of a 200k and a 270k resistor in series, give 470k in total. The 200k resistors are at the ground end, while the PI output and the 6V6 grids are connected to the 270k end. The EL84 grids are then fed the attenuated signal at the junction of the 270k and and 200k resistors.

Now, both EL84 & 6V6 should be driven about as hard. Thanks Zaphod!"


I've also printed out the old (mine) and new (Mark II) layout drawings to try and figure things out. I'm baffled. I just can't see a clean, clever way to take my older (Mark I???) layout amp and simply make those changes for the 6V6 tubes. Just gets uglier and uglier the more I think about it. There must be a "best practice" sort of way.

Changing the PI tail resistor from 56k to 20k is easy, as is changing the PI cathode resistor from 820 to 470. But the voltage divider part has me stumped. I can't think of a clean way to do the voltage divider part without taking the 8.2k resistors off the board and putting them on the EL84 tubes. I just can't see a clean layout in my head for doing this with my amp. Anybody have any ideas/diagram/suggestions? My head is spinning.

Also, what is the purpose of the two 200K resistors on the tube cathode switch?

_________________
Go Buckeyes!

There is no clean easy way of doing the mod without removing the grid stoppers (8K2) and relocating them to the tube socket. I think coco posted a picture or layout on this somewhere.

The 220K resistors on the tube switch are to prevent poping and probably not needed if you haven't noticed any excess noise from the switch to date.

_________________
http://pdfelectronics.com

Thanks for the clarification on those switch resistors. I never really had any popping issues, but I admittedly only switch tube settings with the amp in standby anyway.


I looked around for a layout. Finding one would be great. Or a picture.

_________________
Go Buckeyes!

Does this make sense (For each side, so times two):
1) Move the wire that currently jumpers the board to the EL84 tube from pin 2 to pin 8 (unused pin). But leave the other end of the wire in the eyelet where it currently sits (at the end of the 8.2K resistor).
2) Remove that 8.2K resistor from the board
3) Wire that 8.2k resistor between pin 2 and pin 8.
4) Remove the 470K resistor (that currently attaches to ground at one end and the junction of the 8.2k and the .01uF). Set this aside (will not be used).
5) Wire a 200K resistor between the former ground connection (center of board) of the removed 470k and the former power side eyelet of the now moved 8.2K. Now this 200K is between pin 2 and ground insted of the former 470K.
6) Wire one end of a 270K resistor to that power side eyelet that used to have the 8.2K, but now has one end of the 200K.
7) Wire the other end of that resistor to the control side eyelet (that used to have the 8.2K) to connect it to the .01uF.

Does this cleanly handle the voltage divider parts? Is it OK for the 200K and 270K to practically criss-cross the board, with the 200K actually passing over the top of the .01uF cap?

Of course, on the PI:
1) Replace 56K with 20K
2) Replace 820R with 470R


Does all of this sum it up?

_________________
Go Buckeyes!

VRM kit came in today's mail. I got excited, so I went ahead and soldered the resistors, diode and pot to the little circuit board. Knowing i can't complete the job tomight (too late/loud to drill the hole in the chassis), I wanted to ag least get something done.

Now the sanity check:

I PLAN TO REPLACE MY STANDBY SWITCH WITH THE SWITCHING POT. NOT MY POWER SWITCH. IS THAT WHAT'S USUALLY DONE?

_________________
Go Buckeyes!

No

Usually the other way around

_________________
http://pdfelectronics.com

Thanks kurtlives.

I realize now that the photo I've seen of the VRM implementation (IIRC, from the VRM thread on the Resource page) is clearly not an 18 watt. Looking at it closer, I guess it's a Tweed. That means (I think!) that the components going left to right are Standby - Fuse - Power - Bulb. That sort of messed up my thinking.

In that amp, the Power switch is in a nice, open location a tad further from the clutter of the transformer wires. In the 18 watt, it's right next to the transformer. But it looks like it'll work. It must, obviously, if that's what everyone else does!

I'll go ahead and do the power switch location for the VRM swap. I guess now I need to decide where to put the MOSFET. The power switch location makes it a really nice short run to the side wall of the chassis. That's how the Tweed in the resource section looks. But there's also that nice flat expanse of chassis floor near the standby switch close to the chassis grounds.

I wonder if it matters. I wonder if one location is quieter than the other.

_________________
Go Buckeyes!

I found this pic on my phone of an 18W I built with a VRM.

No difference in noise between standby and power location.

_________________
http://pdfelectronics.com

Wow, you soldered the MOSFET directly to the board. That looks so simple and clean. The only wires floating around would be the two B+ wires and the ground. Nice! I kind of wish I hadn't already soldered and braided up three wires.

When I give it a good looking over and see how easy it is to mount the MOSFET right there, I may have to take those wires out.

_________________
Go Buckeyes!

Happy to report that my VRM was successfully installed last night.

As recommended, I replaced the power switch, not the standby switch, with the VRM pot/switch. I decided to mount it like mentioned above, with the MOSFET right at the foot of the VRM board in that little space under where the power switch was and the transformer. The only drawback was having to remove those three wires that I braided up so nicely and had attached to the VRM board. Argh! So much for planning ahead! Getting all the solder out just right took longer than the rest of the install process!

Now, the only wires coming to or from the VRM board are the two B+ wires (one to cap, one to standby lug) and the ground wire. All are nice and short and rather neat. Heck, even the three wires that used to go to the former power switch and now go to the switch half of the VRM pot were shortened up and fit better.

Worked like a charm. I'm not noticing any scratchy sounds when playing with the VRM pot, so I haven't felt the need to do anything with the V1 input. Not sure what I should look at to tell me to do it or not. For now, the VRM works and I don't see anything wrong that needs fixing.




Next up, completing that 6V6 heat up process...

_________________
Go Buckeyes!

Followed up the VRM with the power tube balance mod. Works as advertised.

With this and the VRM, the amp is even better than before. Very pleased.


Only drawback is I'm noticing odd hiss ONLY with the normal channel. Not sure if relared.

_________________
Go Buckeyes!

Can see why it would be related to the VRM. Max the VRM out and that will tell you.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps