Express build progress & questions from a newbie...

[MCK]

Hi, I am about to start my Express build after much [personal] anticipation and delays. I literally began buying parts 2 years ago and got sidetracked many times by a number of life-changing events. Reading of others experiences here and getting the benefit of all the responses by those of you with great knowledge I have come to appreciate all you have done to get this community going and sustained! Thank you. I will be using Ron Worley's excellent build guide so thanks to Ron and all who contributed to that documentation set!

I will use this thread to document my progress and ask questions where needed. I hope you will have your usual patience with me.

First things first... checking BOM. I think I have everything I need. I am planning to use Dana Hall's excellent VVR. I have a standard chassis. The VVR seems to want to go where the the filter caps are supposed to go. I saw a post by Richie that says he and Dana sometimes use combined filter caps installed on the upper side of the chassis next to the PT (similar to old JMPs I suppose...

Question. Can I still fit the regular style filter caps and the VVR all in there or must I go with the combo cap option if I'm using the VVR? This will put me off track from my holiday building plans as I will need to order these caps and deal with additional chassis cut-outs etc.

Related question, any issues if I place the VVR facing the back?

Thanks much for all your help. I am looking forward to hearing the amp roar... Oh yes, it will be named Leyla. She is our pet Chihuahua and has the heart of a lion.

[MCK]

Read the build guide few times over to make sure my next questions are not covered there and don't think they are so here goes...

I am hoping to install external bias points and 2 bias pots so I can alter the bias for each tube individually. Searched the forum at length but could not get a hit on a suitable diagram for this little mod. Would love to get some input and help on this front.

Here's another one... How crucial is the backplate for the chassis? The chassis I bought did not come with one so I will need to track one down if this is a must have. Thoughts?

Thanks much in advance.

[dorrisant]

If you do the stack of caps you can just slide the stack a little toward the back of the chassis before you secure them. This should get you enough room for the VVR.

Here's one Gary did with the VVR in the back... Kind of similar...

http://www.ampgarage.com/forum/viewtopi ... highlight=

Keep in mind... You can probably mount the pot separate from the board so you can get the control to the front panel our wherever. I'm using the circuit published in Merlin's power supply book with the pot mounted remotely. I'm not absolutely sure about the other circuits, but don't see any reason why they didn't work this way.

Tony

[MCK]

Thanks for the photos and link to Gary's build. This helps answer many questions. Most appreciated.

[MCK]

New question. Making progress with mechanical install. Drilling missing holes on the chassis and making PT/OT installation. I noticed that my PT has cables coming from both ends and the build guide shows them all coming from one end. This is possibly because I have one of the international voltage PTs and hence the additional cables... This will mean I will have to deviate a little from the cable routing shown on the build guide which obviously is the 100% TW way. Does this deviation cause any concerns and if so how would you suggest I route my PT cables? Thanks in advance.

Edit : The PT in question is a HTS-5199-120/240 by the way which RJ kindly supplied a while back.

[M Fowler]

What your dealing with is rather common these days two PT wire holes not one. Plan ahead and proceed.

Because of the tightness of the PT and power board you will need to carefully plan your board mounting with counter sunk machine screws to at least one of them under the PT.

Mark

[MCK]

Thanks for confirming it'll be ok. Most appreciated.

Edit... Some Dremel scars on the chassis will certainly put my first build in the hack job category but so be it. I call it battle scars... Boy oh boy. Getting the iron on the chassis and cutting out the IEC socket hole took the better part of my day with my limited machine tools. Almost done with the mechanical prep.

I will now proof install all pots & switches to make sure I am good to go. Don't want to have to drill or file anymore once I start soldering.

[pablogt]

On the second page of this document http://www.ax84.com/october/AX84_Octobe ... 071004.pdf you can see an schematic for independent bias pots. Notice that the bias raw source has full wave rectification and not half wave as in the Express. Dual bias pos draw higher bias current than an single pot. I cannot do the maths for that but you could install the turrets for full wave, then install the standard half wave rectification. If you don't get enough negative voltage, install the extra diode and resistor (duplicate R29 and D11 and connect them to the other PT secondary). Or install full wave and tweak R30 till you are back in the expected voltages.

Pablo

[MCK]

Thanks for that Pablo. Since I also want to install Dana's VVR in there I may end up having to stay with single bias pot as opposed to double. I will study the material you point me to and see if I can understand how it might work. Thanks much!!!

[pablogt]

I'm working on a "wreackish" version of the October, with parallel input triodes, dual bias pots, power scaling and London Power's GMX. Attached is one of the possible layouts. You can also see the full wave bias rectification... The transformers will be in different places, so the final layout will be quite different. I'll post it when it's done

Pablo

[John_P_WI]

Pablo,

I will follow your project very closely, I am in the design process of a very similar liverpool with 6v6 output and GMX, which should allow the output to go from cream to mean.

On your layout, what is the single mosfet next to the power scaling board? Voltage clamp or other?

Cool! Post clips when you are done, I am very interested in how GMX works for you.

Sorry for the hijack MCK, we now return to the regular scheduled programming.

John

[pablogt]

It's a transistor, part of power scaling. The little board it's on scales the bias supply. I'm very curious about GMX too. I'll post the results. Back to MCK's thread...

Pablo

[John_P_WI]

Thanks Pablogt, yeah I had cathode bias on my mind and realized that was the bias tracking circuit after my post.

Again, back the regular scheduled program...

[MCK]

Guys no problems about the hijack. Pablo's project looks cool. Couple feets above my level of understanding but I hope to get there in time!

[MCK]

Almost done with the mechanical aspects of my build... And to think that I started off with a chassis that was 95% ready to use and ready made turreted perfboards etc... My hat truly goes off to all of you who do this for a living. Oh boy. There is so much labor of love involved. Thanks for all your hard work. Really.