Express build progress & questions from a newbie...

[MCK]

Ok. I'm still alive!!! Testing completed! Wow. If I get this much buzz just by powering up my PT & OT can't imagine how it'll be to power up the finished amp for the first time...

So I had just finished wiring my PT, pilot light, On/Off switch & Movistor and so I was ready to test the PT. After double checking all connections, snipped wiring (260V, 5V etc) and the heat-shrink insulation etc I connected the 300V secondaries to my DVM and connected the power cable and gave the switch its first powered flick! Check!!! 600V reading on the DVM. So far so good.

Then I insulated the 300V using some electricians tape and repeated the same experiment on the 6.3 heaters. Check. Reading 6.81V... A little high compared to the 6.3V but this is no load reading so I'm thinking it'll go down a little.

Now we move to the OT test prescribed in Paul Ruby page. 6.81V connected to the OT primary, DVM connected to the common and each one of the secondaries in turn. Readings below. No smoke. Happy. Colors are all faded so I'm using alphabetical identifiers again...

OT Secondary X -> 0.320V
OT Secondary Y -> 0.154V
OT Secondary Z -> 0.087V

Based on the above can someone kindly help me deduce which one is the 4 ohm tap and which one is the 16 ohm tap? 8 ohm is self deducing as Y since its the one in the middle... Right?

Then I move on to the calculations. Here's what I get after the division and square for each tap

X = 452.90
Y = 1955.48
Z = 6127.11

So I have to final two questions on the OT health...

Based on the above data would you consider this OT to be in spec for an A-3801?

Recalling that I am seeing 64.2 ohm resistance on one side of the primaries (Blue to Red) and 58.8 on the other side (Brown to Red) I am worried about the imbalance in the push/pull setup which might lead to hum or other issues I'm not aware of yet. The difference is a good 10%! Can anyone let me know whether this level of difference is ok to proceed with or whether this might indicate a short inside the coil and thus might lead to larger problems down the road?

What's the verdict on my OT? Thank you so very much for going through my observations and giving me a piece of your mind. All the best.

[hans-jörg]

Hi,
the imbalance is normal. Don´t worry.
What is 4 or 16 Ohm you can see with your ohm meter on the secondaries. But please without any voltage! You will read something like 0, ... The higher is the 16 Ohm, the lowest is the 4 Ohm. The 8 you already indicated. To find this out you need only the Ohm meter. So whats your calculation for the Raa? Lets say with the 8 Ohm tap.

Read this in addition to the Ruby site:
http://www.marstran.com/Measuring%20Pri ... edance.htm

its more easy understanding!

Best

Hans-Jörg

[MCK]

Thanks for your confirmation. So in my data set this means Sec #1 is 16 ohms and Sec #3 is 4 ohms.

The last step in the calculation was what was missing in my previous effort :

Step 9) Take the impedance ratio and times it by the secondary tap rating (4, 8 or 16 which ever one you chose).

So this means my Raa is as follows :

16 ohm tap - Raa = 7246.27
8 ohm tap - Raa = 15643.82
4 ohm tap - Raa = 24508.44

Do these make sense? Did I get something wrong along the way? Thanks for all your help.

[MCK]

She's alive!!! Completed the build today. Went through the startup checks and all went ok. No smokes or fires. Very happy. In goes the tubes and connected to a speaker & guitar and away we go! All is good up with VVR at 40% or less but above that I have some oscillations, whine, shrill noises etc. Presence set to max seems to help a little. Treble set to near max seems to also reduce the issue. Anyways. I will close this build thread and start a new one for the troubleshooting after I collect some data and check my lead dress once again.

Thanks so much for all the help. Could not get this far without all your support. As the saying goes... It takes a village to raise the barn. Keep well.

edit : reversed OT... Hint from Cliff's build at : https://tubeamparchive.com/viewtopic.ph ... scillation

Once I reversed by blue & brown (A3801) sonic bliss has arrived. Its quite late now so I will do my tuning and tube rolling tomorrow but off the get go the amp sounds very very nice.

How did I make this mistake? Assumptions... Build guide talks about blue & brown OT colors for Heyboer. I had read that the Heyboer was based on A3801 so I wrongly assumed the colors would be the same... Wrong. Wish I had seen Cliff's problem thread earlier on. Anyways, means there will be at least two expresses with cut OT wires that needed to be re-extended.

Build photos promised.

[MCK]

Here are some photos from the as built. I had one minor fix after these photos were taken so other than that this is what it looks like now. Oh and the OT reversal of course... Any critique, suggestions, warnings etc most welcome. Thanks in advance.

A lot of build photos here : https://picasaweb.google.com/email.kere ... directlink

Some specific shots :

[img:800:600]https://lh6.googleusercontent.com/-dAKF ... G_0831.JPG[/img]

[img:640:480]https://lh4.googleusercontent.com/-XbgV ... G_0832.JPG[/img]

[img:640:480]https://lh3.googleusercontent.com/-8LxC ... G_0837.JPG[/img]

[MCK]

I need to lower the Bias range. I have a 25K bias pot but for some reason the amp is very hot. I am following the VVR instructions which is to set the VVR bias pot to its max and use the amp bias pot... With VVR set to Max and amp bias pot at its lowest I can barely hit 70% on the tubes so I need to get a little lower.

[MCK]

Ok. Short update. Changed the 220k to 150k feeding the bias circuit and now I can hit the range but this is with the bias pot on VVR set at its lowest!!!. So somehow my bias circuit is off its rockers. I have no idea what might be causing this much deviation. Since the VVR instructions call for setting one or the other pot at its max I feel I need to lower the bias resistor even more. I will try something lower and see what happens... Now for the good news...

As I biased the tubes at about 60% almost all stability issues went away!!! I am so over the moon. I can max volume at max VVR and max Bass & Mid as well. The Treble starts whistling after about 80% and so does Presence. I chop-sticked my way around but could not see any difference in this issue. I have not done any tube rolling yet and the amp has no bottom cover so these issues could disappear as I look for a better fitting tube or place a bottom cover on.

On a related note, there is just a bit of hum. Not a disturbing level at all but its there at all times. Interestingly it is there even when the amp is on Standby. What does this tell me? I am surprised about this and have no clue.

EDIT : Hum is a non-issue. I was testing the amp on its side (vertically) with the PT touching my work surface which is a large kitchen table. The PT vibration was getting amplified by the work table and hence the slight hum... That also explains why the hum was there even when the amp was on standby... Mystery solved. Amp has no hum. Yippeee!!!

Thanks as always for all your help & support. All the best.

[geetarpicker]

There is a very good chance that once you get the bottom cover on it the amp will be much more stable or even 100%!

[MCK]

There is a very good chance that once you get the bottom cover on it the amp will be much more stable or even 100%!

Agreed. Either that or the tube shields or other selected tubes etc. I'm thrilled to see how good this thing turned out. Mine is a definite beginners luck. That and a lot of following in the tried and true footsteps.

[MCK]

Ok. Made a base for the amp from a plank of pine. Found a thick aluminum disposable baking tray (shhh, don't tell my wife) and cut it to the appropriate size and glued it on the wooden base. I have myself a makeshift bottom cover + safe working base until I have the head cab sorted out.

Fire away and guess what... I can now max out all controls without a guitar cable plugged in. With the cable plugged in but no guitar on the other end if I move the end of the cable (which would normally go in the guitar) over the amp, especially the trannies the whistling comes back. I guess I might want to cover the inside of the complete cab with aluminum shielding to reduce this kind of interaction.

For all intents and purposes the amp is 100% tamed now. Lets roll a few tubes in and see how it turns out. I am so very happy with this. Can't believe it actually. Beginners luck strike big time. Thanks everyone.

[MCK]

Ok. Made a base for the amp from a plank of pine. Found a thick aluminum disposable baking tray (shhh, don't tell my wife) and cut it to the appropriate size and glued it on the wooden base. I have myself a makeshift bottom cover + safe working base until I have the head cab sorted out.

Fire away and guess what... I can now max out all controls without a guitar cable plugged in. With the cable plugged in but no guitar on the other end if I move the end of the cable (which would normally go in the guitar) over the amp, especially the trannies the whistling comes back. I guess I might want to cover the inside of the complete cab with aluminum shielding to reduce this kind of interaction.

For all intents and purposes the amp is 100% tamed now. Lets roll a few tubes in and see how it turns out. I am so very happy with this. Can't believe it actually. Beginners luck strike big time. Thanks everyone.

[leadfootdriver]

Congrats on the build!

I'd be interested on your thoughts on the PPIMV/VVR tones compared to full volume tones.

Let us know what you think.

[MCK]

I use an Aracom attenuator and rely on a mix of VVR + attenuator to bring volume down to a home-friendly level. I think using a little of this and a little of that to achieve the ultimate goal gives me a more balanced tone compared to going all out with just one tool. VVR is very transparent down to about 40% for me and after that the amp gets a little flubby. Mind you the amp is very new and has not been tubed ideally so some of this could perhaps be tuned out. Having said that, cutting the Express volumes down with VVR is a reality. I am very happy with the ease of install and the quality of manufacture from Dana. I also used VVR in Lil' Devil from Dana and in that case it worked wonderful even down to the minimum VVR levels. Hope this helps.

[statorvane]

Found a thick aluminum disposable baking tray (shhh, don't tell my wife) and cut it to the appropriate size and glued it on the wooden base.

Our little secret. Next time however, get some double stick tape and heavy aluminum foil. Works great and just may save you from unspeakable horrors.

[MCK]

Great tip. Thanks much.