Build Log - #102 - So it Begins

[xtian]

On your 5v switching supply board, those two 1000uF/50v caps sure look like they have no solder on the +ve eyelets.

[PicknStrum]

I cant hear anything wrong. Transformers can buzz a little, and buzz louder when you switch on the high voltage. Did you leave the nuts on the transformer when you inslalled it? (The nuts that are on the bolts of the transformer need to stay on and sit between the transformer and chassis. You need additional nuts to bolt the transformer on to the chassis)

As for your voltage reading: i don't know how accurate your meter is, but to read the diffrence in voltage over the transformer you need to connect your meter accordingly. The positive lead to the centertap (B+) and the negative lead to the anode(s) of the tube(s) you want to measure. The meter will read more accurate that way on a low voltage setting.

Okay - hope I'm just being a bit sensitive to it. It probably is just a normal PT buzz - I mean it's pretty light and was definitely hard to hear in that video. I don't think I'd even be able to hear it once it's closed up in a cabinet.

I left the nuts on the transformer itself and used separate nuts to mount it to the chassis.

Makes total sense what you're saying about my meter. I was checking things versus ground so my meter probably wasn't sensitive enough to pickup a difference in voltage.

I definitely can only hear a very light background noise and the switches clicking. I hear no noise at all.

The only other area that 'might' be problematic is the OT leads going to the impedance selector switch are somewhat close to the OD tube. It might have been better to route them underneath the jacks and behind the PI tube instead, and that can induce hum into other signal path areas pretty easily...

Not sure it's related, though since you've mentioned it's in the power section, not the OT.

~Phil

I'll keep that routing in mind for my next build. Yeah, it's definitely in the power section.

On your 5v switching supply board, those two 1000uF/50v caps sure look like they have no solder on the +ve eyelets.

Yes, good catch - I have those routed under the board as well and soldered in the correct spots. I'll tack them down to the eyelets as well.

Appreciate everyone's time checking that out - proves I'm being way to sensitive about what is probably a normal sound. Like I said, there's probably no way I could hear it through a cab anyway. I'm going to consider this amp done other than throwing in that new 12v PT.


The only other thing I'm still a bit confused about is my bias. It's probably user error but here's what I've got:

Power tube plate voltages = 447vDC

OT CT method
-Reading a voltage drop of .128V (using Richard's method) from OT CT to the plates
-Reading a resistance of about 13.5ohms from OT CT to the plates
- 0.128V / 13.5ohms = 9mA

Reading Plate Current as Voltage Through 1ohm dropping resistors at Cathode
-Reading 6mV = 6mA


Now I know my bias pot is turned all the way to cool but the highest I can even get my plate current to is 20mA (read as mV). If I'm understanding Rob's calculator correctly (which I haven't had an issue with in the past), 70% max dissipation would be 48mA for 6L6GC. Not that I would bias the amp there but this seems massively under-biased!

Ugh, can't figure out what I'm doing wrong.

Thanks again to everyone for all the help!

EDIT: may have figured something out. I followed the 'adapted layout' for setting up the bias pot which shows a 47k resistor off the bias pot to ground. Talbany's shows a 27k resistor. Wonder if that's my issue. When I get a chance, I'll go back through that area and make sure it's setup like talbany's layout.

[xtian]

This is a 100w amp, right? (Four power tubes?)

So with the OT resistance/V drop method, the reading you get is for two tubes. With the cathode shunt method, that’s the current for one tube.

[Richard1001]

Before anything, make sure you have a good volt/multimeter. If you are using a cheap meter buy something decent.

measure the grid 1 voltage to see what is going on. If it is -58 volts, it is correct the idle current is low. If it is -47 volts, you should have around 30 mA per tube.
Draw a schematic of the bias voltage supply and measure the votages and many people here can tell you what to change to get the right range for the biasvoltage to be setup correct.

[PicknStrum]

This is a 100w amp, right? (Four power tubes?)

So with the OT resistance/V drop method, the reading you get is for two tubes. With the cathode shunt method, that’s the current for one tube.

Thanks for clarifying that - makes sense to me now!

Before anything, make sure you have a good volt/multimeter. If you are using a cheap meter buy something decent.

measure the grid 1 voltage to see what is going on. If it is -58 volts, it is correct the idle current is low. If it is -47 volts, you should have around 30 mA per tube.
Draw a schematic of the bias voltage supply and measure the votages and many people here can tell you what to change to get the right range for the biasvoltage to be setup correct.

Although I don't have a Fluke meter, I do have a a $60+ Klein meter that has been mostly accurate for me. I got rid of my $7 Menards meter on my first build

I ended up doing a little digging on this forum and did not realize that the resistor coming off the bias pot may need adjustment. I found a great post on here by RG that ran through the entire circuit and a few other threads indicating that the resistor may need to be adjusted to get the correct range of plate current. So I clipped a resistor in parallel to drop the 47k down to 17k and it got me right in the range I needed! Soldered that baby in and I'm sitting at about 45mA. The amp sounds great! I did not anticipating liking the cleans as much as I do - actually think I prefer them over the OD channel, with my strat anyway.

I was planning on picking up an EMV12 but I really really like what I'm hearing out of the Texas Heat speaker I have in my cab here.

You guys are fantastic - thanks for your time, knowledge, and patience with me!

I'll update you guys once I get this in a head - hopefully won't have too many more silly questions for you.

[PicknStrum]

A couple updates:

I had a successful transplant of the 12V transformer, so my OD, FET boost, and PAB are back! $20 lesson learned on that one.

Also, I could not handle the buzz from the chassis. I don't think it was really coming through the video I posted. It was loud enough that I could hear it a few feet away. Since Xtian had mentioned the fish pump buzz, I couldn't stop thinking that was it. When I would move the ceramics on my rectifier board, there was a change in tone of the buzz. I decided to try a set of high voltage films. I found some rated for 1kv DC and 500v AC. Although 500v AC doesn't seem like that much wiggle room with 325V being fed, my understanding is they would share the AC burden being that there are 2 in series. So, I threw these new capacitors in and VOILA! The annoying buzz is gone.

THANK YOU FOR THE SUGGESTION XTIAN! It was driving me crazy.

After some research, I guess those ceramics were "singing capacitors." Apparently ceramics are/can be piezoelectric which is what causes singing capacitors. Someone can absolutely correct me here if I'm wrong. Here is a shot of the new caps:

IMG_4905.JPG

If anyone is wondering what caps I switched to, here they are - they fit pretty flawlessly into Martin's rectifier board:

https://www.digikey.com/en/products/det ... GYCQBdAXyA

[xtian]

Awesome! Yes, "singing" ceramic caps.

In my experience, snubbers on the rectifier diodes are not needed at all.

[PicknStrum]

Thought I'd post a few pictures of the head building process. Finally built the bullet and bought a box joint jig to make nice joints. My previous jigs were homemade and I could never get my joints quite right. This jig from Rockler is awesome though! I still have some filling and routing to do of course. I saw an old video of Taylor @ Amplified Nation where he had built a cab with a crossbar. I really liked it so I added one to mine.

I purchased white tolex as well as salt and pepper grill cloth. This will be my first time adding tolex to cab. I'm going to try water-based contact cement as I prefer to use the non-toxic stuff. I'll keep you guys posted on how things go.

Note, the photos are basically of the cab upside down (i'm guessing you already figured that out) because I have yet to locate the chassis screws.

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[xtian]

Point to your Rockler jig, please. I had a miserable time trying to set up my table saw for finger joints.

[PicknStrum]

Point to your Rockler jig, please. I had a miserable time trying to set up my table saw for finger joints.

So I've only ever used my router / table for making box joints. Here is the router jig I used from rockler:

https://www.rockler.com/router-table-box-joint-jig

Although a surprisingly small jig, I was fairly easily able to get great results on my first time using it after I spent a good amount of time on setup to get the joints nice and tight. I used a 1/2" spiral up-cut bit from Harbor Freight (https://www.harborfreight.com/3-piece-s ... 66733.html) which worked a lot better than a straight bit. Some day I may invest in a better bit, but this works great for now.

[professormudd]

Point to your Rockler jig, please. I had a miserable time trying to set up my table saw for finger joints.

I have this one and I like it quite a bit.

https://www.amazon.com/PORTER-CABLE-421 ... B000HGL59M

[PicknStrum]

Point to your Rockler jig, please. I had a miserable time trying to set up my table saw for finger joints.

I have this one and I like it quite a bit.

https://www.amazon.com/PORTER-CABLE-421 ... B000HGL59M

That's a nice looking jig! Definitely looks like it does the job!

Just a couple quick updates - I ended up getting my sides and corners routed. It went pretty well but I did have to do some filling on them. The bottom picture is my current back panel. I'm not sure I'm completely satisfied with the cutout. My straight lines are really good but my rounded cutouts aren't the best. How do you guys go about cutting the rounded edges in your back panels? I used a jigsaw with a general purpose wood blade. Thinking about picking up a scroll blade to try and get more accurate rounding.

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[Todd Hepler]

How do you guys go about cutting the rounded edges in your back panels?

I gave up trying to cut a consistent line with a Jigsaw a long time ago. These days I use a circle jig. You can buy a commercial one or easily make your own.

Here's an example on amazon: https://www.amazon.com/dp/B01K86VSSI/re ... ljaz10cnVl

Good luck!

-T

[PicknStrum]

How do you guys go about cutting the rounded edges in your back panels?

I gave up trying to cut a consistent line with a Jigsaw a long time ago. These days I use a circle jig. You can buy a commercial one or easily make your own.

Here's an example on amazon: https://www.amazon.com/dp/B01K86VSSI/re ... ljaz10cnVl

Good luck!

-T

Thanks for the tip! I ended up making a circle jig and routing out the rounded corners. It worked much better! I definitely still need to practice this a bit more to get it closer to perfect but it works for now.

Finally pretty close to done with the head cabinet. I went with a cream tolex with salt and pepper grill cloth. I added some piping to cover the gap under the chassis.

Still waiting on a handle and corners before I do some final photos but here's where I'm at for now.

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[Todd Hepler]

I ended up making a circle jig and routing out the rounded corners. It worked much better! I definitely still need to practice this a bit more to get it closer to perfect but it works for now.

Very glad I could help refine a process for you - the back panel turned out great - nice work!

-T