HRM Preamp Project

[Luthierwnc]

Hi Guys,

For the last couple years I've been circling the idea of creating preamps to mimic the various Dumble generations that would work with a single power amp. Lately that has turned more towards introducing discrete preamps that would work with a complete amplifier. The heavy, expensive parts are pretty similar in these amps and a working alt preamp would give options without the heft.

The schematic below is a winter project that is designed to work with a 2nd gen 50-watt head. This particular preamp is an HRM. I have a 100-watt 102 as well but that has the loop built in and wouldn't align with this box.

On the schematic, the input to the preamp goes to one of two relays in parallel. In the normally closed position that feeds right back out to the head. In the normally open position it goes through the preamp. The other DPDT relay controls the output. The normally closed position takes the send from the amp and sends it to the Dumbleator. The normally open position takes the output from the OD relay to the loop. I am using two relays to ground the unused preamp but that might be overkill.

The rest of the preamp is pretty familiar. I didn't put a bright on the clean master in this schematic but that is an option with this circuit since the #2 lug isn't grounded. I plan to build this in a 2.5" high rack box. It would fit in a single-height chassis except for the extra HRM tone-stack knobs clutters up a single row of controls. If I used a daughter-board or put them on the main board 1U would be fine. I've had excellent results with the Antek toroidal power transformers. They have one at 0-280VAC with two 6.3V windings that is perfect for this application.

I haven't lurked much here lately but over the years I haven't seen a gizmo like this one. I'm concerned about introducing ground loops. The plan is to use the same power strip but there will be a lot of wire between the chassis grounds. Not so touchy at the pre/power amp junction but susceptible to hums and squeals early on.

If anyone has an experience or opinion they want to share, I'd be glad to hear about it. If this project is a "go", I'll catalog the experience and post it as I go.

Thanks for reading, Skip

[Bob-I]

I'm fairly convinced that some of the magic bloom is caused by the power amp pulling the B+ rail down briefly. I've tried a preamp like this, exact voltages as my amps, and run into one of my amp's power section. It felt totally different.

[Luthierwnc]

Hi Bob,

I wondered about that but thought that of all the amps in ampdom, this one has the least sag. The amp in question has the older-style power supply.

In the day I know organ amps had HV and heater wires that went to the preamps so they could leave the heavy iron at ground level. That is a little too adventurous for me but it would get the bloom if there was any to be had. It would also make the thing cheaper and lighter.

My theory (such as it is) is that future generations of these might have the power amp and loop in the main head and then as many distinct preamps as you wanted in the rack. Wouldn't have to be just "D" amps either. You could have the 5F6-A, plexi, Soldano and/or blackface loaded into one or two 1U spaces along with your HRM/102.

Food for thought.

Thanks, Skip

[Bob-I]

Hi Bob,

I wondered about that but thought that of all the amps in ampdom, this one has the least sag.

Thanks, Skip

It's not about sag, it's about bloom.

What I've observed, very unscientifically, is that lower preamp voltages result in a slightly darker tone. I've also noticed that when you hit the amp hard the preamp voltages drop quite a bit. I ran a signal generator into an amp with a dummy load, with 200V on V2B as I increased the signal it went as low as 160.

So my theory is the initial attack is bright because the preamp voltage is ~200, then the tone gets darker and weaker while the voltage drops to 160, then gradually gets brighter "blooms" as the caps recharge and return to full 200.

Through all of that, the power output remained fairly constant.

Conclusion, you don't get the bloom without tying to the power amp rail.

Yes, several hammonds have the power rail distributed throughout the organ! including running through the cord to the external tone cabinet. The system seems to work, I have a 1964 A100 that's 100% original! not as much as a tube has been replaced and it works perfectly, and sounds amazing.

[ic-racer]

Just this week I have been studying my #124 (6L6x4) and HRM (EL34 x4) amps by plugging the output of one preamp into the other to hear the differences in the power sections. I observed that some of the 'crunch' of the HRM amplifier is gone when the HRM preamp is amplified by the 6L6x4 power amp of #124.

[Luthierwnc]

Bob, I haven't ever looked at how the voltages change when you run a transient through the amp. Curious -- but it makes sense.

IC, in this case, the amp is a 2nd gen (at heart) so a modern preamp would sound very different, even with 6L6 finals. IICR, the B+ is around 450VDC and it has the older power supply with single caps after the OT. Here is the amp in an older thread although it now has a matching rack cab with the lator and an FX processor.

https://tubeamparchive.com/viewtopic.ph ... ht=lindley

[Luthierwnc]

I've been fiddling with the project. It has morphed into a high-plate Skyliner clean with the choice of a non-HRM overdrive, an HRM overdrive or the non feeding the HRM. I could add the NFBL on V1b but it isn't drawn on the attached schematic. On the layout diagram the HRM tone stack is on the front panel which sports two rows of knobs. It will be a 2U rack size. The choice of whether to use the original amp preamp or the rack preamp is a 4PDT mini switch on the panel. It is shown in two sections on the schematic.

Also included is the footswitch and a block diagram of the relay switching. Not including the PAB, I got it down to two stomp switches for all four modes (clean, OD, HRM, both). To do the LEDs I put the resistor ahead of the green LED because the OD relay has to be active in three of the four modes but should only be illuminated for two of them.

This project isn't any further along than what you see so it could change several more times before it even gets off the ground. Any more comments or suggestions are welcome. Cheers, Skip

[Luthierwnc]

Any bites?

[John_P_WI]

Skip,

Personally I think this is great and wish I had the time to contribute / collaborate and work up a pcb to incorporate the relays etc. Sometimes life and work get in the way.

I wonder IF the 1k power resistor at B+4 could be increased? As shown it will be a fairly stiff rail. Could this be increased to add a little dynamics / note flip?

Curious and will continue to watch.

Thanks, John

[Luthierwnc]

Thanks John, My policy on rail resistors is to get a bunch of sizes then mix and match. I'm wondering if a choke first and splitting the two OD lines might not be a bad idea too.

I think the switching will work pretty well. That stuff doesn't come intuitively to me but I put on Paint and draw the lines for each setting. This one came out OK. Where I'm least sure of myself is the entrance from the non to the HRM when using both ODs. I knocked the signal down from the pole of the "both" relay going back to the HRM with a trimmer and a .01 cap but that is a shot in the dark. Again, extra values and solder to dial it in.

Don't know when I'll get to this one but I think it could have practical applications for any amp that has an FX loop. I've seen other types but none that give the player the option to choose whether to use the rack effect or use the one in the amp with one flick. Ground loops could also be an issue. Maybe a separately-fused convenience outlet on the effect might lessen the potential between the ground plates. Food for thought,

sh

[John_P_WI]

Van Weelden used (still does?) to make killer preamps. Soldano has the XL88, these all could be used for inspiration IF a mental block occurs...

I'll try to read through this thread when I have time, good luck.

John

[talbany]

Van Weelden used (still does?) to make killer preamps. Soldano has the XL88, these all could be used for inspiration IF a mental block occurs...

Here is another cool HRM Preamp..VVT Amps
http://www.vvtamps.com/portfolio/x-model-guitar-preamp/

Tony

[Luthierwnc]

Hi Tony,

That is a great little unit.

My project hatched a couple years ago when I thought about replacing an HRM I sold to a friend. I've got a 102 (kinda) and a 2nd gen. Then it occurred to me that I already had the heavy, expensive parts. A rack device that would fit in the same case as the Dumbleator and an Intellifex wouldn't increase the lug-factor either.

I was looking for something that would give me the choice of the internal preamp or the rack preamp with a single switch. I have it as a 4PDT on the panel but it could also be a foot-switched relay. Guitar goes to the input jack and the normally closed position sends it into the amp input. The same switch position also routs the amp send to the loop. Flip it up and the internal preamp is grounded and the rack preamp drives the loop. No cables have to be moved.

Except for finding space for all the knobs and switches, all this would easily fit in a single rack chassis with room for a loop -- or a plexi preamp for that matter. The kinda part of the my 102 is that it has an internal loop so I'll have to punch in and out shorting jacks to make this compatible.

I don't know if there would be any commercial demand for something like this but there is something to be said for not needing to haul an extra 40-lb head for HRM stylings. Even if this gets greenlighted, it will be a month or two before I can even think about soldering.

Thanks again to all for your comments and experience, sh

[Luthierwnc]

PS Here is a copy of the board etch with the wires. I do these on MS Paint to actual size and print them on a laser printer using slick but thin magazine pages. Then I iron them to the clad board and etch them in ferric chloride for 10-20 minutes. Works great -- as long as you remember to drill the holes after you etch them (else the etchant eats into the holes under the melted plastic toner).

The trimmer is a little 10-turn with the pins in a row chosen more for size than anything. I like having the knob on top and use a barbeque skewer with a couple of knife cuts sharpened into a screw driver (also great for biasing live amps). Relays are Panasonic DS2Y-S-DC5V.

FWIW, sh

[Luthierwnc]

Hi Guys,

I've got this thing up and spark-tested. The clean, OD and HRM OD all work although they will need a little tweaking. What doesn't work is the OD into the HRM OD. My first fix will to bypass the .01 cap (circled).

If anyone has a better idea or experiments to recommend, I'd love to hear them. Thanks, sh