Need Help with First Tube Build - WE91 300B Parafeed Derivative

[EricS]

Someone suggested that I insert a resistor and see if that reduces the noise level. It did reduce the noise, just not enough to make me happy.  Any resistor greater than 10k results in the meter not moving at all - it just buzzed. Smaller resistors in the range of 1k to 7k actually allow the meter to move and certainly reduce the noise, but still not to a level that I find acceptable.

Bummer. I think I'll need to look into digital meters and find a way to mount one inside the housing for the mechanical meter since all of the digital ones have a smaller diameter face plate. 

Thanks for the links, Tubejack.

[EricS]

OK, I feel like I cheated by replacing a period hour meter with a new digital one, but it is much more quiet than the original one that I chose.  I think it's time to call this one done and move onto other projects 

[EricS]

When I first built this amp, I wired the output transformer for 8-Ohm output.  I am contemplating adding another output terminal and a DPDT switch to the top plate of the chassis so that I can switch back and forth between 4R and 8R speakers.  Is a 6A/125v switch sufficient for this function?  Is there a reason that I’m not considering why I should not implement a speaker output switch like this? 

This is the specific switch I was thinking of using:
https://www.onlinecomponents.com/en/gc-electronics/35072bu-11403409.html

Thanks for any input and Happy New Year!

[Paul Birkeland]

That is super duper overkill.  You are talking about a place where there's roughly 8V RMS appearing with about 1A of current, and you're unlikely to throw the switch while using the amp, so contact current ratings are far less important than doing something like switching mains!

[EricS]

Gotcha.  The primary attributes that caught my attention are that it was black, so it would match my overall color scheme, and it would fit in a hole that already exists in my top plate. Maybe I could mount a smaller switch under the hood…

[Paul Birkeland]

In order to feed your potential for absolute OCD, some find it's nice to pay attention to contact plating material.  Here's a black switch with gold contacts.  Maybe it matters, maybe it doesn't, but you never know till you try.
https://www.mouser.com/ProductDetail/Marquardt-Switches/18236101?qs=4EOvy6wCNnuu0isWrGp0Lw%3D%3D

[EricS]

Thanks for the link and the laugh, Paul!

Happy New Year 

[EricS]

As it turns out, my mains voltage varies way more than I though it did.  Jac at EML specifies a pretty tight voltage window for the 300B heater, so I ended up using two different bucking transformers and implementing a switch to move between them as needed.   

Using half of a 10VCT transformer drops about 9VAC from the AC mains and is ideal during the winter months when the AC grid stays pretty close to 125vAC.  During the summer air conditioning months, the grid droops quite a bit and the 10V bucking transformer drops too much voltage, taking the 300B filament below the 5.0vDC minimum.  Half of a 5VCT transformer drops about 5VAC from the AC mains and works great during the summer. 

To keep an eye on the filament voltage, I picked up an LED voltmeter from Amazon.  I like this one because it has a calibration pot on the back:   https://www.amazon.com/bayite-Voltmeter-Motorcycle-Polarity-Protection/dp/B00YALV0NG/ref=cm_cr_arp_d_product_top?ie=UTF8 

The display is a bit bright, so I put a strip of LED dimming tape across the display:  https://www.amazon.com/dp/B00CLVEQCO/

The tape tames the brightness and makes it much more subdued, but still easy to read.  Mounting the LED to the inside edge of the chassis bottom makes it practically invisible unless you squat down and view it from directly in front.  While I didn't do any output FFT measurements yet, I cannot hear any adverse effects such as hum or distortion as a result.  Now it's super simple to keep the filament voltage in the recommended 5.00v to 5.25v range.