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

[Paul Birkeland]

If your gear will do a graph of frequency response, that may be useful to look at too.

It is interesting that the HF rolloff is so pronounced, but a wheezy 6SJ7 isn't going to be happy driving much in the way of capacitance!

[Deke609]

With the 250k grid leak resistor: -3dB bandwidth points are 20Hz to ~17kHz - not really sure that I can hear above 17kHz ;-)
With the "Nickel" BCP-16 Grid Choke: -3dB bandwidth points are 21Hz to 11kHz - wow, what a nosedive!
With the "High Nickel" BCP-16 Grid Choke: -3dB bandwidth points are 23Hz to 11.5kHz - same dramatic dropoff
Not sure if the sound quality from the chokes is better, but based on bandwidth, the 250k resistor will stay for sure!

@Eric - Before committing to the 250K R, it might be worthwhile experimenting with different (higher) interstage coupling cap values.  For example, going to 1.5uF will reduce the impedance by a third, and going to 0.2uF would knock it down by half -- that might bump up your high frequency response? Maybe PJ and PB will provide some expert guidance on this. I seem to recall that playing with the coupling cap value may require changes to the grid resistor value.

[Edit] - I see that PB has already suggested this may not work:

...  a wheezy 6SJ7 isn't going to be happy driving much in the way of capacitance!

But maybe there's a bit of wiggle room for bumping up capacitance.

cheers, Derek

[EricS]

I'm a little confused by the bandwidth with the 250k resistor.  I thought I made a previous measurement where I got closer to 22k - but this was with my 6SJ7 tube that was out of spec....

Paul -  The figures that I measured were eyeballed with my scope and a signal generator.  I used a 1kHz frequency as a baseline, then adjusted frequency up and down until the output magnitude was ~70% of the 1kHz signal.  It is my understanding that the 30% down mark represents the -3dB point (but perhaps I am incorrect here). 

I have a PC with a sound card and have both REW and ARTA software installed.  I'll see if I can use some combination here to do a signal sweep with the amp.  Is this what you are looking for?

Happy Turkey Day to everyone in the US - it's time to put ours in the oven.

[Paul Birkeland]

@Eric - Before committing to the 250K R, it might be worthwhile experimenting with different (higher) interstage coupling cap values.

Bigger coupling caps will lower the LF response a little bit, but they won't restore the missing treble.

The bigger issue is that the gain of a pentode is approximately its Gm at whatever operating point you're using times the loading impedance.  With a plate loading resistor and a grid leak resistor, the loading impedance will be resistive until you get down to where the coupling cap start to roll things off.  With a grid choke instead of a grid leak resistor, that loading impedance will move around a bit.

With a triode, the plate impedance of the triode in a properly designed circuit is much lower than what's going on with the grid choke, so those frequency response aberrations are likely invisible. 

[EricS]

I tried a different method of measuring bandwidth earlier this evening.  Instead of using a signal generator and my scope and doing a manual signal sweep with a dummy load attached, I used a PC with a sound card and Room EQ Wizard software.  I was able to produce the attached image of a frequency sweep.  This shows essentially flat response from about 13Hz all of the way out past 20kHz (with a small bump around 10kHz) using the 250k grid leak resistor.  I can't make REW sample out past 20kHz.  I don't know why my signal generator and scope showed more reduced bandwidth, though both are quite old...

So, then I started to compare the 250k grid leak resistor with the grid chokes and something went wrong but I don't know what.  I removed the 250k resistor and inserted the Nickel Grid Choke with alligator clips.  I powered up the amp again and the 1/2A fuse flashed immediately.  I checked the wiring and didn't see anything wrong, I didn't find any inadvertent shorts or lose wires.  I removed the choke and restored the 250k grid leak resistor.  I powered up again and the new 1/2A fuse began to glow red as the B+ came up (after about 20 seconds) and then it blew.  Before the fuse blew, B+ voltage on the 300B plate and the 6SJ7 plate were right where they should be.  Somewhere, something is drawing extra current...  Not sure what is going on, I'll have to check more carefully in the morning.

Anyhow, bandwidth looks better with this measurement technique.

[Paul Birkeland]

If you unplug the 300B during any of this, it's easy to accidentally put it in the socket improperly, where the fat pins aren't in the fat holes.  If you do this, you short B+ to the cathode resistor through the 300B filament and things get crispy in a hurry.

[EricS]

Paul - That is a danger of running the prototype with the tube sockets upside down for sure, but I didn't remove any tubes during my test.

I'm trying to track this fuse problem down by isolating as much as I can and bringing additional parts of the circuit in one-by-one.  Here is what I have found so far:

- With no tubes in, the fuse remains intact - this tells me the transformer is fine.
- I put the tubes in one by one and powered just the filaments (with B+ disconnected) and this worked fine - no problems with any of the three filament supplies.

Before I go any further, is there any danger with powering up the amp with B+ ONLY on the driver tube?  Or with B+ ONLY  on the output tube?  These are the last two parts of the circuit to test.  All of my resistors in the circuit check out...

Edit to add schematic.

[Paul Birkeland]

When you have a cap coupled amp like that, you can run one tube and not the other.  Without the 300B in, the B+ will be quite a bit higher than it should be.

[EricS]

Thanks for the confirmation, Paul!  I was thinking it might work out ok (specifically because of the cap that you pointed out), but didn't want to screw something up if I was wrong...

[EricS]

Figured it out- another self-inflicted problem that does not really exist.   

It looks like I blew the first fuse by poking around with my volt meter probes.  I think this had something to do with connecting one lead to the 300B plate and the other lead to the "high side" of the cathode bypass cap C4/R4 combo.  I should have measured each with respect to ground and then subtracted, oops...   

The original fuse that was in place was a 1/2A slow-blow.  My best guess is that subsequent fuses were 1/2A fast-blow, because they always toasted as the B+ was ramping up and approaching 400v.   I was unable to tell by the markings on the fuse whether they were the slow- or fast-blow type.  After making lots of measurements, using my variac, and blowing through a string of additional 1/2A fuses, I finally replaced the fuse with a 1.0A fast-blow and it seems to be surviving now.  I've rechecked all of the voltages, comparing them to the original voltage map that I made and everything seems fine now. 

My best calculation for the power consumption of this amp is about 50w steady state (throw in the additional heat from a few hot resistors and it appears that a 1/2A fuse was right on the edge: the slow-blow survived, but the fast-blow didn't.

[Paul Birkeland]

You may find that you still blow the 1A fuses occasionally.  You could just toss a 3A fast in there instead.  It will allow things to get a little crispier if there's a fault, but it will still blow.  You could also look at using an NTC thermistor to mellow out the startup surge. 

[EricS]

Just measured total current draw for the first time (duh...).  I pulled the fuse and put my meter across the terminals.

Power on surge: 0.52A  (64w)
Drops to 0.34A (42w) after the filaments are warm
Steady State 0.76A (93w) after B+ is up, so a 500mA fuse was definitely too small. 

Paul, you are right (as usual!) - it just might open a 1A fast-acting fuse from time to time.

[EricS]

With a few days of unstructured time finally on the horizon, I'll be starting to prototype my build on a 1/8" aluminum sheet.  The tube sockets that I have are NOS Johnson ceramic sockets that mount from the underside of the top plate. 

Since I imagine it's tough to make a hole larger after discovering that I didn't include enough clearance, how much additional space should I include for clearance around the tube base?  If the base of my 300B tube is an actual 35mm in diameter, is a 38mm hole sufficient, or should I go with a 40mm hole?  The smaller hole will look neater though the larger hole will potentially provide more cooling.  If I go with the smaller hole, is it necessary to make a perimeter of smaller holes around the tube hole for cooling?

[Deke609]

Eric - I don't have an answer to your question, but I just had to comment on the beauty of your metal work! Those look like very precise and clean holes. Are you sending it out, or doing it yourself?  If doing it yourself, can you describe your method?

cheers and thanks, Derek

[Paul Birkeland]

I'd recommend looking through a few tube datasheets to see what the base diameter is listed as.  You may find a little variation, but I would imagine 3mm is good enough.

With your mounting plans (which are good), you'll want a rubber washer in the stack so you can tighten down the socket without cracking it.