Completely Out of My Depth - Modulus 50w 69 Build

[NickKUK]

About the switches, make sure that up is open and down is closed, like in all Marshalls. At least, it would drive me crazy if it wasnt..

*looks at combo* forward/back main switches and left/right mod switches.. *prepares straight jacket*

 

[NickKUK]

Get a light bulb limiter built and a selection of bulb wattages sorted out ready for initial testing.
eg 25W for no valves, 60W with valves, 100 or 150W with signal.

If this is his first start up - I'd suggest he writes out a sequence plan here and we can critique it. Here's some points:

1. I would advocate don't use the front panel on switch. With the power to the limiter off, plug the amp into it and switch the amp to the on position - then switch the power to the limiter on.. when there's a problem switch off at the limiter and not the front panel initially. It gets you out of the habit of touching the amp metal work on the first start. Put the light bulb limiter switch away from the amp but close to you - that way you can pull the power quickly without leaning over or being close to the amp.

2. I like getting a probe pack - ones with 1000V clip on connectors (eg Silvertronic probe pack) - that allows you to connect the probes ahead of start up to monitor the B+.

3. Dummy load connected before start..

4. When you start first without tubes, the B+ voltages will go high without load. Next put in only the pre-amp tubes and test. Then ensure your bias pots are set to the lowest bias and then install the power tubes. All this on the light bulb limiter.


If you've treble checked the board using a multi meter before powering up, including checking for ground and shorts, then usually this is uneventful but assumptions are not good practice.

 

[Marcomel79]

*looks at combo* forward/back main switches and left/right mod switches.. *prepares straight jacket*

Even worse, down power on, up off of standby, brrrrrr......

 

[NickKUK]

Even worse, down power on, up off of standby, brrrrrr......

The standard for UK sockets and light switches - up for off, down for on

 

[PelliX]

2. I like getting a probe pack - ones with 1000V clip on connectors (eg Silvertronic probe pack) - that allows you to connect the probes ahead of start up to monitor the B+.

Certainly, though let's not forget the meter. Many cheaper meters appear to only handle a max of 250V (AC or DC), others list 400V DC as the max. At the risk of a shameless plug, check out Brymen meters. Affordable and most of them are really decent on the inside. Proper isolation, HSRC fuses, good update rates. Shaving a few tenners off that, Mastech offer some nice units. I have a couple in use regularly and really like them. They CAT insulation ratings are perhaps a little ambitious and most don't have HSRC fuses, but they're real workhorses.

The standard for UK sockets and light switches - up for off, down for on

Yes. A lot of places and engineers could do well to memorize that. The number of switches I've corrected in gear and walls.... geesh.

 

[NickKUK]

Certainly, though let's not forget the meter. Many cheaper meters appear to only handle a max of 250V (AC or DC), others list 400V DC as the max. At the risk of a shameless plug, check out Brymen meters. Affordable and most of them are really decent on the inside. Proper isolation, HSRC fuses, good update rates. Shaving a few tenners off that, Mastech offer some nice units. I have a couple in use regularly and really like them. They CAT insulation ratings are perhaps a little ambitious and most don't have HSRC fuses, but they're real workhorses.

Agreed - this is something I'm quite big on. The number of YouTube videos that use 300V meters or worse 30V croc clip leads intended for low voltage makes me shiver. The silvertronic leads are 4mm sheathed and so fit most multimeters etc.

I had two random cheapies - the first meter was 300V and the second 600V. I ended up getting a Brymen 869s (1000V) for a birthday present and after never used the other meters. Most I've had on it was 640Vdc at which point not having fingers anywhere near it with the clips was the way to go.

CAT ratings are really for where the device is used, but still nice to have seen the 869 be safe with the 5kV and 15kV tests even if it needed a diode repair. Overkill on a single build, but a named brand certified 600-1000Vdc will continue to be of use. What is nice s the AC-DC works above the voltage that my scope can tolerate (total 400Vpp) so for fast signal checks it's great without having to build in voltage dividers etc. But.. we're getting off topic

You can never have enough mutimeters

 

[PelliX]

The silvertronic leads are 4mm sheathed and so fit most multimeters etc.

Yup, a good set of silicon leads really makes life a lot more pleasant. That's my only gripe with the Mastechs, really. The stock leads have decent insulation and the probe tips are OK but they feel like crap. For just a tenner or two on top they could throw in some decent ones and offer an even better deal. One of mine has that 'auto sensing' thing for the leads so it flashes a socket when it's missing a lead and beeps if you have a lead in the wrong socket. Fine, but because of the LED ring and switch inside there, regular probes are a little too long and either stick out ~2mm or need the insulation cut short. Hmphh.

I might just fire up my UNI-T insulation tester and see what some of the el-cheapo leads handle later on...

What is nice s the AC-DC works above the voltage that my scope can tolerate (total 400Vpp) so for fast signal checks it's great without having to build in voltage dividers etc.

Well, that's 400Vpp per channel, so presuming you have two or more channels... you can do more measuring channel to channel. Not something I'd generally recommend, you seem to know your stuff. The real solution would be X10 probes (drops your 640V to 64V) or a nice differential probe. Those are expensive, but there are plenty of DIY options out there. I have a mix of X1/X10 and X10 probes for most stuff. I'm also the proud owner of a Tektronix 2430A among others, which *only* does probe auto-sensing. No configuration option like on my HP's and co. Drastic times, drastic measures:



The X10 probes I generally use with it have the 'sensing' contact, but they have 6.3K instead of 10K resistors, thus the scope thinks it's an X100. So I dutifully put a 10K resistor around that washer and plug in the other end into the ground terminal with a banana plug. It now sees 10K between the sense ring and ground and applies the X10 logic. The washer also insulates the sense pin on the probe, of course.

 

[kingfredward]

Oh wow lots of talk of multi meters here’s the one I have - it’s a max of 600v



The good news is I am done with my turret board.



The only thing is missing is a 10k resistor no more left in the package (I have emailed modulus) anyone know if I can use the 10k 3w resistor instead?

I have also enquired where on the board I have to attach the speaker outs to the board apparently he’s going to make a diagram for me so waiting on that

Any feedback is welcome like always. The closer I am getting to completion the more scared I am becoming tbh

 

[Marcomel79]

Oh wow lots of talk of multi meters here’s the one I have - it’s a max of 600v

View attachment 157930

The good news is I am done with my turret board.
View attachment 157931
View attachment 157934View attachment 157933View attachment 157932

The only thing is missing is a 10k resistor no more left in the package (I have emailed modulus) anyone know if I can use the 10k 3w resistor instead?

I have also enquired where on the board I have to attach the speaker outs to the board apparently he’s going to make a diagram for me so waiting on that

Any feedback is welcome like always. The closer I am getting to completion the more scared I am becoming tbh

You can but its gonna be big? Cant see from the picture, but the bias caps positive side needs to be grounded..

 

[PelliX]

What Marco said. Also, the solder joint on the wiper of the bias pot - maybe it's just a trick of the light - but that looks like a dry joint...?

Apart from that it's coming along nicely...

 

[kingfredward]

Here you go better photos of the bias cap also there is a ground running from it by the black wire

And here is where the 10k resistor is missing

 

[Phoenix1]

Yuo should look at several youtubes on building the amp ! Electric drills do the twisting for you and make it neat and tidy as possible. Try some spring clamps and cut up chopstix and a little silicone adhesive to hole the wires in place

 

[Phoenix1]

Oops you I guess are on the home stretch now. Good luck ! Many good hands in here and these guys will help !