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

[Paul Birkeland]

There will indeed be some ripple.  PJ will certainly offer some insight into how concerned to be about the 4.75V.  FWIW, Jac's website says 5V +/-4% the last time I looked at it, which is 4.8V minimum.  If you can look at the output of that supply with a scope, you should be able to discern what's going on a bit better.

[Paul Joppa]

Yes, the choke filter reduces the AC but does not eliminate it. IIRC, the hum goes down 10dB or a little more.

You can add a capacitor at the power supply output (across the filament) to reduce the ripple if the hum is audible - 10,000uF at 6.3 to 10 volts would be reasonable. You can also increase the voltage if necessary by using a (much smaller) cap across the input (upstream side of the chokes). The high voltage power will alter the filament voltage, so don't get carried away until the rest of the amp is built.

Also, check your power line voltage. If it is low, then all the other voltages will also be low.

[EricS]

Ah, it didn't occur to me that what I was seeing was ripple - I'm used to mV ripple figures in solid state stuff...  Makes perfect sense that a cap after the chokes will help reduce this.  I have a 10,000uF 50v cap in my parts box that I can attach across the filament as an experiment then I can check the waveforms with my scope.  I can also play around with various configurations of LC or CLC or adding small (0R1 or 0R22) resistors to hit the target voltage in a completed amp.  The Duncan Amp simulation provides some useful guidance on what to expect with different size caps in this domain. 

In my correspondence with Jac, he recommended that 5.0v with 0% tolerance is best for the EML 300B tubes, but 5.0v to 5.2v is a good target for a warm amp.  He also indicated that the tubes are more forgiving of slightly over-voltage than slightly under-voltage and advised not to run them below 5.0v.

Also good to know that total draw on the transformer might have some impact on individual secondary windings - I was wondering if things might behave differently once the entire amp is completed (more current, more flux, more heat, etc in the mains transformer).

Thank you!

[2wo]

In your setup, the filement choke is reducing the ripple but not eliminateing it. This is likely to be fine. Many people use  straight  5v AC with  acceptable hum, it depends on how  sensitive your speakers are. I  would try it this way before making changes and if necessary you could go back and add additional stages of filtering. Also when you add the hum pot and associated resistors you may need to readjust the voltage at the filliment ....John

[EricS]

That makes sense to me, John.  I was just surprised when the AC ripple was half of the DC voltage.  Thanks for the insights.  Just as a quick experiment, addng the 10,000uF cap in conjunction with the 154B chokes reduced ripple to 80mV and provided a DC voltage of 5.1 after a few minutes.  The cap has the added benefit of providing a nice & soft ramp up curve for the filament.  This seems highly worthwhile to me.

I'm working on wiring up the rest of the amp now.  Given my schedule, this will likely take another few days to complete. 

[EricS]

Trivia question - how do people prototype the wire connections in their amps?  Is it OK to use these "European-style" screw terminal blocks to tie things together? Or should I be soldering all of the connections?   

[grufti]

WAGO connectors are perfect. You could actually build your final product with them for most connections. Flip open - insert wire - flip closed - again - and again - and again.

[Deke609]

WAGO connectors are perfect.

Thanks for this! Those things look great. Just ordered some.


cheers, Derek

[Paul Birkeland]

The Euro blocks are OK.  Often when I'm prototyping on wood I have a lot of components with really long leads, so I'll just solder those and end up cutting off the soldered part when I build the final amp.

The 5 lug terminal strips that we use are also super easy to screw into a piece of plywood with a wood screw.

I've attached a photo of a plywood prototype I made a while ago.  It also shows the little 9 pin breakout boards that I use that can be screwed to a piece of wood and wired up pretty easily.

[Deke609]

500V bench power supply. Nice!

[EricS]

Those WAGO connectors look pretty cool!  I should pick up a set for other projects as well!  Looks like both these and the Euro blocks are fully insulated, so they don't leave high voltage exposed to the touch or accidental shorts.

Paul, you've gotta provide some details on that awesome looking tube on your wooden breadboard!  My prototype is much smaller - I am trying to proof the idea with the same physical layout that I'm planning for the final chassis.  Here are some images of where I am now - lots of flying leads at present and I made a crude aluminum sink for the big power resistors.  And, yes, those a custom-made, precision cap clamps made from folded layers of duct tape and held down with a screw & washer for the time being.  I guess I used up all of my plastic plumbers strap because I couldn't find it.

Question: Is it OK to leave just the filaments powered up on their own for 30-45 mins to see how things behave as the PSU warms up?  I was thinking this might be a good step before moving onto the high voltage wiring...

[Paul Birkeland]

Oh yeah, you'll want some old clapped out tubes to use when you're getting all this going.  Someone's random single 300B where the other matching 300B died an early death (that's how I got my prototyping 300B).  That way you don't have to worry so much about damaging an otherwise expensive tube.

[EricS]

I had a thought about an old/cheap tube a short while ago as insurance during the build process...  I'll post something and see if anything turns up.

[Paul Birkeland]

https://www.ebay.com/itm/Sovtek-300B-Tube/113835500802?hash=item1a811fe502:g:9SYAAOSw01pc9ycr

https://www.ebay.com/itm/New-Psvane-300B-HiFi-Series-Vacuum-Tube-Plate-Current-Tested-GOLD-PINS-Hi-Fi/153017349251?epid=22010308575&hash=item23a08b7483:g:dvUAAOSw0Pxa9I2p

Those are the only options I see at the moment.

[EricS]

Thanks for the link - just ordered a spare tube.  I feel better about experimenting with a $60 tube until I know the amp is working and stable!