Express build progress & questions from a newbie...

[M Fowler]

MCK,

I have never used or installed a VVR, sorry! There are a bunch on this forum that have. Where the hell are they all when you need them.

Mark

[MCK]

MCK,

I have never used or installed a VVR, sorry! There are a bunch on this forum that have. Where the hell are they all when you need them.

Mark

Heh heh... Still recovering from the after-party effects I'm sure. No worries. I'm sure someone with the knowledge will chime in soon. I hope. Thanks for writing back.

[Gibsonman63]

R1 comes off of the input jack. I moved mine to the tube socket on my Liverpool build. I screwed a little two pole terminal close to V1 and used it for this resistor, plus the new cap. Search "Judy Jean" and my pics are there. For the new cap downstream of V1, I was able to squeeze it on the preamp board. When I ordered my parts from RJ, he included a couple of extra turrets.

I hope it helps.

I will be back into my Express soon. My VVR, can caps and external bias adjustment parts are sitting on my workbench staring at me.

[Tillydog]

I have built a home brewed VVR type circuit into an AX84 4-4-0 (Express preamp with low power triode output stage) - the schematic looks OK.

You don't have to add these parts straight away - the amp will work fine without, but the guitar & main volume pots will get scratchy at low VVR settings. You may find that this is only an irritation at levels you will never use (so no need for the mods).

If/when you decide to add these parts, you *do* need the extra 1M grid leak resistor after the volume pot, but you don't need the 1M resistor on the input jack - I fitted the extra cap on the jack socket itself (along with the 1M resistor now after it).

Andy

[MCK]

Guys, thank you very much for the references and confirmation of my understanding. I will follow your lead. My plan is to install this mod on day 1 and now I know what I need to do. Updates to come tomorrow. Today was a non-amp-building day... Thanks once again.

[MCK]

Totally new question... Given that I'm putting in a VVR and thus can reduce the voltages across the amp to about 10% of usual run time readings while keeping the heaters at 6.3 volts do I really need the Standby switch in there?

Think about it. VVR wound all the way down my amp will be idling over almost as if it was in Power On / Standby Off state. I'm seriously thinking of omitting the Standby switch. This article from Merlin has also been weighing in on this for me : http://www.freewebs.com/valvewizard1/standby.html

Thoughts? Pros / Cons? Thanks much!!!

[MCK]

duplicate post removed.

[MCK]

I have a new quick question...

My PT has a centre tap for the heater (brown/wht). The schematics call for this to be connected to chassis ground. The centre tab for the 300V windings is asked to be connected to the ground lugs near the power cable entrance point. Where should the CT for heaters be connected to? Same ground lugs near the power cable or is there a better location for this?

Thanks for your help.

[ampgeek]

Same ground lug at the mains entrance as you suspected.
Good luck,
Dave O.

[MCK]

Thanks. Progress being made. Wiring the PT & switches etc now. Lots of fun & learning. Thanks for all your support.

[MCK]

OT question... I have a Stancor A-3801 I am using on this build. This OT was a pull from a homemade amp of sorts I bought on ebay about 2 years ago. This was an old 6L6 based amp and it was working before I dismantled it so I always presumed the OT was good to go in my Express build...

Today I was getting ready to wire it in but the cloth on the primaries have dulled such that the red & brown are not 100% clear... blue is... and I wanted to make sure I get the leads right so I tried to use my DVM to measure the resistance and see where I get 6600 ohms. Problem is that I get 6600 nowhere... Here are the resistance's I see between each combination out of the 3 wires :

Blue to X -> 122.2 ohm
Blue to Y -> 63.9 ohm
X to Y -> 58.5 ohm

Based on the readings above can you tell if the OT is good or busted? Is there another way I should test it?

If its good then can you tell based on the above readings whether X is red (CT) or brown?

Thanks much for your help.

EDIT : thinking this through, I'm thinking that the primaries should be Blue and X so in other words I think X should mean Brown. My logic there is that the two sides of the CT should have almost equal resistance which I see between Blue to Y and X to Y so therefore my logic says X is the other primary. Am I on the right track?

I'm still wondering about the 6600 ohm vs what I'm seeing though. Perhaps I don't understand what the 6.6K delineation for this OT actually means. I appreciate any and all guidance you might have. Thanks!!!

[hans-jörg]

Hi,
you are right. Y is the CT and Blue and X are the primaries .
You cannot measure the Raa (6k6) in this way. You need a AC voltage (6,3V from the heaters), a calculator and a volt meter.

Hans-Jörg

[MCK]

Hi there. Thanks so much for your kind reply. I am almost ready to be able to fire up just the PT and get 6.3V for testing out of it. Could you kindly point me to the OT testing procedure? Would love to make sure this OT is up to the mark before I proceed further.

The fact that I am getting close but dissimilar resistance readings from both sides of the primaries worry me a little. Wondering if this means there might be a short in there on one side? Thoughts?

[pablogt]

I think this can help:
http://paulrubyamps.com/info.html#OTimpedance

Pablo

[MCK]

Thanks so much! Jumping in now head first! This build and the supportive, friendly TAG membership has been a great learning experience. Thank you so much.