First Amp Build - Express

[shaunf]

Hi all,

A few months ago, I managed to rebuild the turret board in my '69 JMP50 as the old phelonic board had become conductive, and I enjoyed the process so much, I decided to build another amp from scratch.

I have very little electronics experience, but I've been reading as much as I can. I received my parts kit from RJ last week and I thought I'd share some photos as I progress in my build.

Here's where I am as of last night:

[IMG1488]http://img.photobucket.com/albums/v206/ ... G_1605.jpg[/img]

[IMG1362]http://img.photobucket.com/albums/v206/ ... G_1610.jpg[/img]

[IMG1665]http://img.photobucket.com/albums/v206/ ... G_1606.jpg[/img]

I have a question regarding the OT Secondaries. Do I have them the right way round? I'm using a Heyboer HTS-9384 OT. I've left these leads at a length that either one can reach the furtherest power tube, and just tucked the Purple lead under the PS Board, and towards the back corner by the IEC socket. Is this likely to cause any problems during initial startup and testing?

Also, does the Mostfet placement for the VVR look ok?

Finally, what about my speaker jacks? I added the black jumper between the respective grounds on the jacks, instead of just relying on the chassis connection on the second jack. Is this ok to do? Also the first jack is a shorting jack, but I wasn't 100% sure on how this works, so I don't think I have it working correctly at the moment. In order to get it to short when not connected, do I just jumper the lug at the top to the ground lug?

Otherwise, please offer any critisims you may have regarding my work so far, and if you can spot anything wrong at this point, I'd appreciate any feedback you may have.

Sorry if some of my questions are a bit dumb, but I am a rookie and I'd rather ask them and learn, if you are prepared to answer them.

[Bob S]

Hi Shaunf,
leave the OT secondaries long until fire up. If she squeals, reverse them.
The VVR placement is fine - same place I put them & no probs so far.
Personally I don't use shorting jacks for speakers. There's pros & cons either way. If you want to use the shorting pin, jumper to the ground pin.
Connecting the speaker jacks ground together is a good idea. If one works loose, the other one will still have a good ground until you can re-tighten.
On the cap cans I would split up the grounds of the main resevoir and the screen/pre-amp can. I put the main ground closer to the incoming mains chassis ground & the screen/pre ground towards the front panel pots.
Good luck with the build. Looks good. Keep posting & have fun,
Bob

[M Fowler]

Nice looking amp, I like rj's boards they look vintage.

[John_P_WI]

Sometimes "remote" mosfets like shown need a gate stop (100k) resistor right at the mosfet gate if oscillations occur. It can also be a good idea to twist the wires between the board and mosfet. Finally, you do have the spacer between the mosfet and chassis?

[shaunf]

Thanks for the responses guys.

John, if by spacer you mean that transparent plastic film thingy that came in the packet with the MOSFET, then yes, I did install it between the chassis and the MOSFET.

I haven't seen other examples of similar amps with twisted lines to the MOSFET, and the reference pics seem to indicate that their lines were longer than mine, but will bear it in mind if I have a problem. Please could you explain this gate stopper resistor thing to me, since its way over my head, and again I haven't seen it before, nor was it mentioned in Dana's instructions that came with the VVR.

Thanks again, will keep the photos coming if you're interested, and I'd love you to keep the comments coming. As part of my research over the last few months before starting my build, I must have read nearly 70 pages back on this forum, and what strikes me is how much activity there was in older build threads. I hope mine is active since I enjoy the interaction.

Regards,
Shaun

[John_P_WI]

Shaun,

Yes, the plastic insulator is what I was referring to.

Mosfets are able to work at very high frequencies, into the RF (Radio frequency) range and when used as voltage followers are prone to oscillations. I do not know the exact VVR schematic, but it is common place to add something like a 100k gate stop at the mosfet when it is not mounted on the pcb or near the associated voltage control circuitry. Similar to a grid stop resistor used on output tubes that are able to operate at high frequencies, the resistor is mounted to the tube socket.

Just an FYI.

Regarding the twisting wires, it is something I learned many years ago and generally twist all associated power wires.

Good luck.

John

[JazzGuitarGimp]

Shaun,

As to the clear insulator for the MOSFET (these used to be made of mica, back in the day IIRC - probably showing my age here). Anyway, just sandwiching the insulator between the transistor and the chassis will not transfer as much heat as you will get with the use of heat sink compound. Here's a link to a rather large tube at Mouser:

http://www.mouser.com/ProductDetail/Tec ... XBsLQbR1oi

It used to be available in very small tubes for a reasonable price. I just tried digikey, but there website appears to be down.

Use a thin, even coat of this stuff on both sides of the insulator, and you will get the maximum heat transfer to the chassis.

Good looking build, by the way....

My best,
Lou

[markr14850]

Please be very careful when using grease. If you use a grease that is at all conductive and accidentally bridge the back of the MOSFET to the chassis, something will blow.

[Bob S]

Shaunf - an Express for your first amp build...
Very challenging for us mere mortals.
Post Often - everything matters with an express.
Some of the more adept guys will help for sure.
They can be a humbling experience.
Good luck with yours.
Bob

[JazzGuitarGimp]

Please be very careful when using grease. If you use a grease that is at all conductive and accidentally bridge the back of the MOSFET to the chassis, something will blow.

Hi markr14850,

I am curious, has this happened to you personally, or is this something you've heard from someone else? I have never heard of heat sink compound being conductive. I've used it many times, and have never had an issue like this.

Regards,
Lou

[shaunf]

Shaunf - an Express for your first amp build...
Very challenging for us mere mortals.
Post Often - everything matters with an express.
Some of the more adept guys will help for sure.
They can be a humbling experience.
Good luck with yours.
Bob

I'm unfortunately one of those types who doesn't like to take too many steps to get where I want to be. If I want something, I will do absolutely everything necessary to ensure that I can succeed with whatever my goal is. That's not to say I'm hasty or I don't believe in paying my dues. I'd just rather do all the learning upfront, and then take the step to be where I want to be. Always been like that.

I suppose I could have tried to build some easier amps first, but then what would I do with them when I actually don't want them, and want a Trainwreck Express?

A year ago I was given a faulty and abused '69 Marshall JMP 50 head, with a conductive turret board. With absolutely zero electronics knowledge and no decent local tech available, through reading, asking questions and a lot of care managed to rebuild it into an absolutely awesome amp, so I reckon with care, patience and a little assistance from you gentlemen, I will most definitely succeed. Failure is not an option in my book. I'm far to stubborn for that once I put my mind ot something.

[shaunf]

Please be very careful when using grease. If you use a grease that is at all conductive and accidentally bridge the back of the MOSFET to the chassis, something will blow.

Hi markr14850,

I am curious, has this happened to you personally, or is this something you've heard from someone else? I have never heard of heat sink compound being conductive. I've used it many times, and have never had an issue like this.

Regards,
Lou

I've read a lot of posts on the forum, including many from Dana who makes the VVR's who strongly discourage the use of those compounds, due to the fact that many of them are conductive, so I think I'm going to avoid this. If I have heat issues, I'll look for a better heat pad or fit a heat sink. Fortunately I thought ahead too and got hold of several spare mosfets.

[John_P_WI]

Please be very careful when using grease. If you use a grease that is at all conductive and accidentally bridge the back of the MOSFET to the chassis, something will blow.

Hi markr14850,

I am curious, has this happened to you personally, or is this something you've heard from someone else? I have never heard of heat sink compound being conductive. I've used it many times, and have never had an issue like this.

Regards,
Lou

I've read a lot of posts on the forum, including many from Dana who makes the VVR's who strongly discourage the use of those compounds, due to the fact that many of them are conductive, so I think I'm going to avoid this. If I have heat issues, I'll look for a better heat pad or fit a heat sink. Fortunately I thought ahead too and got hold of several spare mosfets.

The heat transfer compound should be used with the mica spacers and the trick is to spread it very thin and to "squeegee" off the excess. The compound should only fill the microscopic voids in the chassis and mosfet. If you tighten the mosfet down and compound oozes out, you have used far too much.

[Tillydog]

I am curious, has this happened to you personally, or is this something you've heard from someone else? I have never heard of heat sink compound being conductive. I've used it many times, and have never had an issue like this.

Regards,
Lou

Me neither, but see here:

https://tubeamparchive.com/viewtopic.ph ... c&start=32

I can see Dana's point of view.

Andy

[redshark]

Shaunf, congrats for starting with an express to be your first build. I did the same with almost NO electronic knowledge but it helps my roomate is an electronic technician. Anyway I'm sure you can succeed!!