Mark III HRG Blue Stripe -> DRG No Stripe Conversion

Jay Omega · Feb 2, 2024

I have a pristine Mark III HRG hardwood combo on hand that I don't get along well with. It's more compressed and squishy sounding than I care for. I'm not sure whether it's due to the wimpier power section or preamp differences, maybe both.. Even though it has the EV - 12L speaker, I don't use the hardwood cab so I want to change to a head. I also have in my possession an early Mark III DRG No Stripe with the 105 PT and SC-152019B output transformer.

I've been working on a Mark style amp kit for a while, I received prototype custom transformers yesterday that I designed based on the 105 and SC-152019B and I need to test them. There's probably no better way than to throw them in an amp and see how they run!

Wanted to share this mini project with y'all. I have a general idea of where I want to go but who knows what we'll run into along the way. Kind of breaks my heart to mod this time capsule, I'm sure some of you are cringing.. If I'm honest, my stomach is turning a bit but I'm diving in anyway. Here are a couple before photos:

Jay Omega · Feb 2, 2024

Pulled the chassis, no indication that this had ever been opened before. Pretty neat! Also realized that it's going to take some doing to get the PT out, more demo than I had expected but not a huge deal

Jay Omega · Feb 2, 2024

First order of business was to remove the power supply card and add more of the 220 uF caps

Jay Omega · Feb 2, 2024

Found something interesting..

Next step was to remove the output transformer. Here is the new simul OT (left) next to the removed 100W Boogie transformer.

The removed transformer was labeled 562100. The core was the same size as my IIB HRG OT that's labeled T100-152019. The primary impedance of the Boogie transformer with an 8 ohm resistive load was 2.08k at 1 kHz.

Here's the fun part that I've been curious about for some time.. Notice the screw on the chassis I found underneath the OT core:

It appears this was making contact with the OT transformer core:

I noticed the same screw on my IIB HRG when converting it to a IIC+, and on various IIC+ pictures I've studied over the years:

On my IIB, the chassis mounds up around the screw and there was also cardboard of some sort underneath the core. I assumed that they used the cardboard to isolate the core from the steel chassis and this screw to pull the chassis away. Now I'm wondering if they use it to ground some of the laminations, anyone know?

I reinstalled the Boogie OT to see if it contacted the screw, and although it's close it appears that it doesn't. The screw didn't seem to go to anything inside the Mark III but it's under the PCB so can't visually verify. I rolled the dice and took it out and it was too short to be doing anything.

If anyone knows anything about this I'd be interested! Going to fire up the HRG and drive it all the way in and out to see if I hear anything change.

GJgo · Feb 3, 2024

If you're going to test this, IMO the correct way to do it is to first start with recording a reamped a track that you can reamp to each iteration with fixed settings, a separate speaker & mic that's not touched so that the tests are truly ceterus paribus. Otherwise all you're testing is your emotion. If you do, here's my prediction once you level-match the recorded tones. (since I've done this).
- The tubes make no difference.
- The transformers make no difference.
- The black vs blue preamp makes a slight difference, in that the blue is a lot more aggressive. (The blue stripe is the most aggressive Mark preamp ever made.)

The hundred watt A/B pentode power section is more immediate & powerful than the squishier 75w Class A triode, A/B pentode blended Simul power section. Triode really softens it up. What makes you think the H is wimpier?

Also heads up (which you may already know), when you switch the PT on the MKIII (assuming the output is different) you will also need to replace this bias resistor to get the power tube bias back to stock.

Good luck!

Jay Omega · Feb 3, 2024

After some thinking and experimenting, I've convinced myself the purpose of the mystery screw is to offset the OT core from the chassis so that it doesn't pick up any flux transferred from the PT via the steel chassis. I don't believe it needs to make contact or ground some core laminations.

I got the demo on the power tube sockets complete and the PT out which took some doing.

Here is a comparison of the power transformers, as you can see the replacement has a considerably larger core.

Demo was the easy part, it's going to take some work to clean up all these terminals and wires so that I can re-terminate everything I had to remove so that the amp is reliable when I'm finished.

I disconnected the death cap connection to the switch, and I don't think I'll reconnect it when I'm done.

Joey1970 · Feb 4, 2024

Jay Omega said:
I have a pristine Mark III HRG hardwood combo on hand that I don't get along well with. It's more compressed and squishy sounding than I care for. I'm not sure whether it's due to the wimpier power section or preamp differences, maybe both.. Even though it has the EV - 12L speaker, I don't use the hardwood cab so I want to change to a head. I also have in my possession an early Mark III DRG No Stripe with the 105 PT and SC-152019B output transformer.

I've been working on a Mark style amp kit for a while, I received prototype custom transformers yesterday that I designed based on the 105 and SC-152019B and I need to test them. There's probably no better way than to throw them in an amp and see how they run!

Wanted to share this mini project with y'all. I have a general idea of where I want to go but who knows what we'll run into along the way. Kind of breaks my heart to mod this time capsule, I'm sure some of you are cringing.. If I'm honest, my stomach is turning a bit but I'm diving in anyway. Here are a couple before photos:

Joey1970 · Feb 4, 2024

it's the combo cab, I did not want to say it out loud.

Jay Omega · Feb 5, 2024

Welp, had my first meltdown with this thing. As you can see the fuse holder didn't survive the demo and prep.

Took this as an opportunity to order a replacement along with some other parts I'll need for the project. Going to be a few days before everything gets here so finding other stuff to work on in the meantime. Notice in the background, you can see I replaced the reverb and presence pots with push / pull switched pots. Going to incorporate two feedback network caps found in the Mark IV as switchable options. With the knobs pushed in, the amp will function as normal. If you pull the presence cap it will introduce the "Extreme Mode" cap, or the cap that's active when a Mark IV presence control is pushed in. When you pull the Reverb pot, the 500p bypass cap on the 3.3k will be active. This cap should cut some high frequencies, and be most noticeable at low presence settings.

Jay Omega · Feb 8, 2024

Got it wired up and fired up. I have not secured the wiring yet, but I'm pretty happy with the performance! The PT was putting out voltages dead nuts to the 105, and the OT sounds right on the money. Huge low end on the transformer just like the original. I have a couple minor things to talk with the MFG about but these will be ready for production release soon.

The extreme mode cap does its thing, and the 3.3k bypass is very subtle as expected. The effect is more pronounced at higher volumes. Although it technically cuts highs due to its arrangement, it sounds like a slight mid push when you activate.

I hardwired in this 100n cap shown in the Mark IV schematic below so I could experiment with it, it's the yellow IC cap sitting on the 8 ohm speaker jack in the photo above.

I think the reason Boogie added it was to avoid a pop when changing channels, but I always thought they used way too low of a value. In theory it will reduce low frequency feedback, which will result in a bigger low end. I took it in and out of the circuit by jumping across it with alligator clips and it does in fact have a big impact on the sound and feel. I liked the low end and the high gain pick attack the best when I bypassed it, this was the tightest and most articulate sound to me. Fun stuff to play around with, there are really a lot of tailoring possibilities.

I haven't touched the preamp yet, but I might start messing around over the next couple days. One thing I noticed when play testing is that my EQ Q seems lower than other amps I've played. I measured the resistance of the VTL5C4 when it was on and it was higher than others I've measured at 190 ohms. I may change this out for NSL-32SR2 which will have significantly lower on resistance in the circuit, and consequently higher Q, and add some resistance to get it down to 150 ohms or lower which seems more normal.

Henz · Feb 9, 2024

Jay Omega said:
Got it wired up and fired up. I have not secured the wiring yet, but I'm pretty happy with the performance! The PT was putting out voltages dead nuts to the 105, and the OT sounds right on the money. Huge low end on the transformer just like the original. I have a couple minor things to talk with the MFG about but these will be ready for production release soon.

View attachment 3195

The extreme mode cap does its thing, and the 3.3k bypass is very subtle as expected. The effect is more pronounced at higher volumes. Although it technically cuts highs due to its arrangement, it sounds like a slight mid push when you activate.

I hardwired in this 100n cap shown in the Mark IV schematic below so I could experiment with it, it's the yellow IC cap sitting on the 8 ohm speaker jack in the photo above.

View attachment 3196

I think the reason Boogie added it was to avoid a pop when changing channels, but I always thought they used way too low of a value. In theory it will reduce low frequency feedback, which will result in a bigger low end. I took it in and out of the circuit by jumping across it with alligator clips and it does in fact have a big impact on the sound and feel. I liked the low end and the high gain pick attack the best when I bypassed it, this was the tightest and most articulate sound to me. Fun stuff to play around with, there are really a lot of tailoring possibilities.

I haven't touched the preamp yet, but I might start messing around over the next couple days. One thing I noticed when play testing is that my EQ Q seems lower than other amps I've played. I measured the resistance of the VTL5C4 when it was on and it was higher than others I've measured at 190 ohms. I may change this out for NSL-32SR2 which will have significantly lower on resistance in the circuit, and consequently higher Q, and add some resistance to get it down to 150 ohms or lower which seems more normal.

Positive progress is what I love. Congrats! Dang, if I had your knowledge and testing gear my amp would be just the way I want it. But there'd be 25 mini switches everywhere! I know Ampo is tweakable to ones ideals but I just don't know where to start.

Fiveband · 2024-11-18T02:35:20-0500

To summarize, this conversion consists of swapping the OT and PT, and adding/swapping a couple of caps?
What power tube plate voltages did you get from the new transformer?

Mark III HRG Blue Stripe -> DRG No Stripe Conversion

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