Guitar Gear Forum

So I've been thinking for a while that a 5150 would be the tits for my new project. I nearly pulled the trigger on a kartakou colossus, but then I stumbled across a guy in the UK who is selling preamp PCBs for all sorts of cool shit, including the 5150 (https://aurora-audio.co.uk/)



There it is. No clean channel. I don't need one The smaller board is an opamp gyrator eq that is pretty much identical to the 5 band on a mrak. This is going on the end for a bit of extra sculpting.

Spec wise I can build this more or less to the evh 5150iii stealth spec, just without the cathode follower for the tonestack as I'm all out of triodes. I don't think it makes a huge difference. More on that in a later post.

I'm just waiting on a new high voltage smps power supply, then I can start on the build properly. I'll populate the board with what I have in the meantime.

Stoked for this. I'll be running it into a Peavey classic 120 mono power amp. Kinda want to try out the Palmer 1u ss power amp too, but that's a way off.

Looking forward to this dude.

_ej_ wrote:Looking forward to this dude.

+1

These are my favorite kind of threads.

Thanks guys. It's nice to have the itch to play and build again after a few years away from it.

These boards and the HV smps I'm trying out could be a great way for people to get their feet wet with amp building. Building power amps is expensive

I bet that will be sexual

I still have that smp power supply you recommended from earlier this year. Been too busy to get started. Look forward to your build.

Hell yeah!

Right on!!!! Looking forward to following!!!

In!

Following this, get some sir.

So today I got my work space in order ahead of the build. The first stumble was that my friend who is fitting me a new bathroom broke my wire strippers

Zozobra wrote:So today I got my work space in order ahead of the build. The first stumble was that my friend who is fitting me a new bathroom broke my wire strippers

Hahahaha damn

Keep him away from the shitter pipes!

To be fair they were like 15 years old and cheap shit

So that's as far as I got with the resistors I have in stock. Tomorrow I'll do the caps then place an CPC/Mouser order for the missing parts.
I may swap some of the 1/2W metal film 220k plate resistors for 2W versions as that will improve the noise floor. For now I'm not super bothered The 5150iii S uses 1/4W though out and 1/2W for the plates so whatever

As I said earlier, I'll be building this more or less to Stealth specs, with a few changes, well what does that even mean?



Here is a side by side comparison of the Stealth (top) 3rd channel and the regular 5150/6505 (bottom). The key differences have been highlighted and numbered:

1) The stealth has some extra tone shaping in the input stage as part of the first stage plate load. I'd need to sim this in LTSpice to know exactly what is happening, but it looks like a partial plate bypass, which will roll some of the top end of right at the input.
2) The stealth has an additional 1M resistor between the wiper and ground of the gain pot, changing the taper
3) The OG fiddy has a much larger gridstopper. This acts with the Miller capacitance to form an RC lowpass filter. Larger R values push the corner frequency of the filter down to roll off more top end.
4) The Stealth has a smaller coupling cap, rolling off more bass to tighten the amp. I'll be ignoring this
5) The Stealth doesn't use the famous 39k cold clipper found in the SLO, OG fiddy and rectos. It uses 5k, which is still a cold bias, but not super cold clipper. This eliminates some of the buzzy/harsh 5150 tone by not clipping like an absolute bitch.
6) The Stealth uses a bypassed plate resistor to trim some high end. The OG bypasses R16 to a similar effect. They work in different ways, but do the same thing.
7) The Stealth has a larger bass cap in the tonestack for more lows. The Stealth also has a cathode follower before the tonestack, the OG doesn't. It wont' make much difference as the cathode follower isn't being hugely overdriven like in a Marshall circuit.


Many of these things are there to achieve the same goal but in different ways.
The biggest tonal impact by far will be #5.
#1 Probably helps with ice picky tones by scrubbing the very harshest top out of the signal before it propagates into horrible intermodulation distortion hash, so I will certainly try tacking that in once it's built and debugged.
#3 may be the same as #1 in the grand scheme of things. If I keep #1 from the stealth I should also keep it's #3 or it may wind up muffled.
I'll keep #7 and ignore #4 because I'm a slut for low end.
#6 probably doesn't matter either way.

Nice job breaking down the theory behind those designs.

i really need to get into building one of these days, looking forward to watching the progress

Populated most of the caps today. I need to put an order in for the missing parts, but that's a job for later. So far this PCB has been really nice to work with Good space to work in and the pads will take a little bit of abuse while I try a few mods out.

This is looking good so far! I am eager to see the finished product, as well as hearing it.

Cole

The high voltage SMPS arrived. I'm just waiting on a parts order for the pieces I'm missing. They should arrive by the end of the week hopefully.

Work has been kicking my ass this week, but I've managed to find 15 minutes to test that the SMPS works. I adjusted to 350VDC. With the trimmer maxed it'll hit 367VDC. An extra 17V isn't going to make a huge difference so I'll keep it at 350.
The SMPS has a soft start on the B+ to allow the heaters to come on before the B+ to minimize inrush currents. The stock unit has a 30s delay, which would annoy me and is unnecessary for a preamp. I asked them to reduce this and the B+ fires up after 12s, which is fine.

All in all it looks good. I don't have a working scope currently, so I can't check the outputs for noise but I'm sure it'll be fine. If there is any noise I can slap a MOSFET choke on it before the preamp board.

Awesome to see this.

Your build is looking good. I still haven't gotten around to trying the focal point with the 5120 boards but I bet it will sound awesome.

Ah, I didn't know that you were on here

I'm a lazy shit and still need to order the bits I'm missing work is a bit brutal at the moment so I keep forgetting.

Yeah I've been in here for a good while.

I always forget to check forums these days.

If you don't know of them switch Electronics are a good place in the UK for parts.

Currently working with them to do 'kits' including my boards

Oh that would be sick and would take a lot of the pain out of it. I have drawers full of most of the common garden parts, but not quite everything I need. I usually order from RS/CPC or bitsbox

I'll check them out. I was struggling a little bit to find the caps I wanted from one place for the eq board. Well, at least if I didn't want to buy 10 of them at a time anyway.