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

[Raymond P.]

Doesn't look like there's significant change in B+ voltage to account for the screen voltage change. It looks like R6 varies the amount of current into the screen grid though (the difference in current through R8 and R7). I wonder where that current ultimately flows to, as it doesn't seen to be going out of any of the other pins of the 6SJ7.

Despite the mysteries, it's great your amp is working!

[EricS]

Looking at the schematic again and the associated voltage drops that appear between B+ and Ground in the driver circuit, adjusting R6 seems to have little impact on achieving the 170v target plate voltage.

Total noob question here:
Is there any immediate danger associated with me adjusting the 91k value of R9?  If I increased the resistance of R9 to something larger than 91k, would it drop more voltage, thereby decreasing the plate voltage and allow more voltage to develop across R6?

Does this sound reasonable, or am I missing something fundamental here...

[EricS]

I spent some time making measurements with some meters, a signal generator, and scope with the hopes that someone can either guide me toward making a change to elevate the driver cathode/adjust the plate voltage to the intended voltage, or just leave it alone and start enjoying the amp.  Overall, the amp with the PSVane 300B tube can drive about 8v into an 8R load with an input signal of 0.8vAC before distortion becomes evident on the scope.

Here is a description of the tables I've attached:

The first attached chart is gain measurements of the 6SJ7GT driver stage, measured at the "input side" of the coupling cap with an 8R load attached.  Looks pretty constant at about 37dB of gain for the first stage.  Don't know if this is where it is supposed to be or not...

The second chart is the overall gain of the entire amplifier into an 8R load as well as input signal level sensitivity. Overall gain for the entire amp into an 8R load looks pretty constant at about 21dB.  This seems a little low to me, I was expecting somewhere closer to 27dB based on some comments PJ posted some time ago on another forum.  It also took close to 0.75vAC input in order for the amp to reach maximum clean output on the scope.  PJ's prior posting also indicated an input sensitivity of about 0.35vAC, so this figure looks a little "off" to me...

The third chart is an experiment with different values of the Driver Plate Resistor R9.  I didn't know how safe this experiment would be, so I only powered up the amp briefly enough for the B+ to stabilize and make a reading, then I shut it down.  Changing R9 seemed only to impact the plate voltage and didn't do much to the cathode elevation. 

I also swapped out R10 for one of Mike's BPC-16 Ni chokes.  As PB predicted, this was a bad idea.  It both introduced noise (hum pickup) and absolutely KILLED the bandwidth of the amp.  With R10 set to 250k as per the schematic, the point at which output magnitude was reduced to 70% relative to the magnitude of a 1kHz sine wave is about 22kHz (this seems a bit low to me - I was expecting a tube amp to be somewhere near 200kHz bandwidth, but perhaps I am wrong here).  When R10 was replaced with the grid choke, the 70% output mark happened at 10kHz!  I put the 250k resistor back in.

I also have images from the scope for all of these readings.  I figured I didn't need to post 50 images until someone asked for something specific...

So, is there tweaking to do to adjust plate/cathode voltage levels?  A few of these readings seem to deviate from my expectations...

[Raymond P.]

Looking at your 3rd table, it does look like the current into the screen grid flows out of the cathode. (Previously, I was just looking at the voltages marked in the schematic.) If you could raise this current, the cathode voltage should rise. In turn the driver tube should draw less plate current, which would drop the plate voltage. You could raise the screen current by lowering R8 or increasing R7.


Anyway, all of that is just conjecture on my part, so proceed cautiously. 

[Paul Birkeland]

If you're going to put a pot in for the cathode resistor, you will want to adjust it very, very slowly to get the target plate voltage.  You are looking to be very, very close to your original value.  If you're off by more than a couple hundred ohms in one direction or the either, you will fall off a cliff so to speak in terms of operation. 

[EricS]

Ah, I think I'm beginning to understand a little better now.  Given the relatively large size of the bypass cap and the very small mA draw of the tube, I think I was adjusting too quickly to see the effect that I was looking for.  I'll experiment again tomorrow and be a little more patient as I adjust things.  I'll be sure to get a meter on the plate as well.  Somehow, I think I may have missed the obvious 

[Paul Joppa]

If you're going to put a pot in for the cathode resistor, you will want to adjust it very, very slowly to get the target plate voltage.  You are looking to be very, very close to your original value.  If you're off by more than a couple hundred ohms in one direction or the either, you will fall off a cliff so to speak in terms of operation.

I've been out of town for the holiday. Fortunately, this post by PB covers what I was going to say.

[EricS]

The 6SJ7 does seem to be out of spec.

You nailed it, Paul!  The tube I was using sure is out of spec!  Like most problems in life, this one is another a case of user error  .  While I was adjusting R6, I was also measuring voltage drop across R6 expecting to see a much larger voltage change.  I had assumed voltage drop across R9 and R6 would somehow be constant and would co-vary, so I only put a meter across R6, not R9 that leads to the plate - duh...

So, while measuring the PLATE voltage at pin 8 (thanks for the kick, PB), adjusting R6 actually makes a big difference!  It was easy to dial in 170vDC at the plate once I started measuring correctly!  The problem is that until I was satisfied that the amp was working properly, I was using my most-used-looking driver tube and it seems that it is furthest out of spec - it takes a value of 873R to hit 170v on the driver plate.  The remaining 10 tubes are all clustered between 1200R and 1500R (average of 1327R) in order to hit a plate target of 170v.  Looks like individual tubes are just as variable as solid state transistors...   

I'll re-run my gain and bandwidth measurements and see where things stand next.  I'll also post an updated voltage map of the amp shortly.

Very happy now!    Many thanks for the help and support, guys!  I hope everyone had a nice holiday weekend!

Question: Is there any value to placing a pot in series with R6 so that adjustments can be made as any individual tube ages? 

[Paul Birkeland]

Pentodes are a fair bit touchier than triodes are.  You can expect the plate voltage to be a little different for each tube you use, there's nothing wrong with that.  I think you could stick with 1.4K cathode resistors and call it good. 

I wouldn't bother with the pot, good working tubes should all bias up to an acceptable plate voltage.

[EricS]

Now that I've got a better handle on the driver plate voltage and it is dialed in, it dragged down overall B+ by a few volts, which also reduced the output of the 6.3v secondary winding for the tube filaments.  Adjusting the driver tube back to ~6.3v was cake.  Adjusting the 300B back to 5.0v-5.2v range that Jac recommended is taking some more experimentation.

The original configuration was diodes --> 154B chokes (3mH, 0R15) --> 10,000uF cap --> 300B filament, but now my voltage is too low.  I see two paths and was hoping someone could provide a sanity check for me:

1) Put a smaller cap (1,000uF 35v) between the diodes and chokes to make a CLC filter.  This worked well and I was able to hit a filament voltage of 5.1vDC with 122.5v (pretty typical) coming out of the wall.  Using my variac, I was able to experiment with the impact of different sized caps and get an acceptable window of filament voltages over the typical range of my AC mains voltage.   I was pleased with this result until I measured ripple on the 1,000uF cap at 1.5vAC.  With an ESR of 0R016, this seems like a ripple current of 1.5v/0.016R = 93 AMPS ?!?!?  Is this calculation correct?  If it is correct, this poor little cap surely won't last very long before it pops....

2) Use two equal size caps (6,800uF ?) in the CLC (which results in filament voltage closer to 6.0v) then add a power resistor on each leg of the filament supply (because of the center tap hum pot) to drop the filament voltage down to the desired target.  This seems like a more stable approach, but I also want to do some additional measurements to make sure that it doesn't delay warming the filament relative to when the B+ arrives at the tube.

Any thoughts or reactions?


 

[Paul Birkeland]

1) Put a smaller cap (1,000uF 35v) between the diodes and chokes to make a CLC filter.  This worked well and I was able to hit a filament voltage of 5.1vDC with 122.5v (pretty typical) coming out of the wall.  Using my variac, I was able to experiment with the impact of different sized caps and get an acceptable window of filament voltages over the typical range of my AC mains voltage.   I was pleased with this result until I measured ripple on the 1,000uF cap at 1.5vAC.  With an ESR of 0R016, this seems like a ripple current of 1.5v/0.016R = 93 AMPS ?!?!?  Is this calculation correct?  If it is correct, this poor little cap surely won't last very long before it pops....

The cap input filter is what should be done to bring the voltage up.  Depending on how much additional voltage you need and what rectifier diodes you are using, you could also use more efficient rectifier diodes to bring the voltage up.  While the 1,000uF cap has an ESR of 0.016 ohms, it's capacitive reactance at 120Hz is 1.3 ohms.  Still, for this reason, you'll commonly see a few caps in parallel to share this abuse. 

2) Use two equal size caps (6,800uF ?) in the CLC (which results in filament voltage closer to 6.0v) then add a power resistor on each leg of the filament supply (because of the center tap hum pot) to drop the filament voltage down to the desired target.  This seems like a more stable approach, but I also want to do some additional measurements to make sure that it doesn't delay warming the filament relative to when the B+ arrives at the tube.

That will work and is an approach we use.

You can also rectify the 6.3V winding on the PGP 8.1 with a choke input filter and a big cap.  1N5820 diodes and the Hammond 155B got me to 4.95V in my own 300B project. 

[Deke609]

  While the 1,000uF cap has an ESR of 0.016 ohms, it's capacitive reactance at 120Hz is 1.3 ohms. 

I don't mean to derail the thread, but for those of us newbies trying to follow along, does this mean that what we need to look at to avoid filter capacitor "abuse" is the ratio of ripple to capacitor impedance (reactance + equivalent series resistance)? And that, in the case of very low ESR caps acting as reservoir or filtering caps in a power supply, capacitive reactance dominates and is all we really need to look at for practical purposes?


many thanks, Derek

[EricS]

Thanks for the confirmation, Paul!

You can also rectify the 6.3V winding on the PGP 8.1 with a choke input filter and a big cap.  1N5820 diodes and the Hammond 155B got me to 4.95V in my own 300B project.

This is where I was until I adjusted the plate voltage on the driver tube.  Then my 300B filament fell from 4.95v to 4.50v.  Moving to the 154B chokes only got me back to ~4.75v.  Still too low.

I'll continue to experiment with the CLC approach with similar sized caps and a few dropping resistors.  I was hoping to avoid using any electrolytics at all - oh well...

Derek: I'm not quite sure how/where Paul determined capacitive reactance at 120Hz to be 1.3 ohms.  I don't fully understand the implications of this measure.

[Paul Birkeland]

Reactance is AC impedance.  Capacitors and inductors have impedance that varies with frequency.  The impedance of a capacitor is usually (hopefully) higher than its ESR.

I'm surprised you didn't get more voltage from the 6.3V winding, especially if you are using a 154B.  You are using 1N5820 diodes?  (and your bog standard testing 300B?)

[Deke609]

Derek: I'm not quite sure how/where Paul determined capacitive reactance at 120Hz to be 1.3 ohms. 

I think it's the textbook equation for cap reactance (that I've read a million times but never used): Xc = 1 / (2*pi*f*C) = 1 /(6.28*120Hz*0.001F) = 1.32Ohms

Edit: And in quoting the formula for cap reactance, I think I've answered my own question above about relative importance of reactance versus ESR. Since reactance varies inversely with capacitance, the smaller the capacitance the higher the reactance. This suggests to me that for low ESR caps, reactance will dominate unless the cap is insanely large - e.g., tenths of a Farad.