18 Watt Marshall With Hammond Transformers - First Build, Voltage Help Needed

Discussion in 'The Workbench' started by Yamariv, Jan 10, 2020.

  1. Yamariv

    Yamariv Active Member

    Joined:
    Mar 19, 2017
    Messages:
    369
    Likes Received:
    230
    Location:
    Ottawa, Canada
    Just finished building my first amp, an 18 Watt Marshall build and need some suggestions and advice on the final voltages. I loved every bit of the build, and already have another 2 planned!

    I used the transformers from a Hammond built AO-50 which were supposed to be perfect for an 18 Watt Clone. The AO-50 came with the exact same tube compliment including the EZ81 as a Rectifier so I figured I was good to go. The transformer tested at 648V on the High Voltage line and 7.3V totally unloaded. I knew the tube Filament line had to be dropped down a bit so I have those resistors on order.

    I used the Tube Depot 18 Watt Layout and have attached a pic of my final testing results with what they recommend vs my actual results voltage wise in blue pen. Now, the instructions say their results were done on a 117vac line where my house on average sits at 123-124Vac at the plugs which is what I set my Variac to get real world results in my area.

    Now the B+ voltage coming out of the rectifier is what I'm worried about. With all the tubes installed, I was getting 383 VDC before the Cap Can and 360 VDC after it. From what I've read, this is going to be too high for the EL34's to handle correct?

    Is the only way to safely drop the B+ by using Zener Diodes? I was thinking of changing the Rectifier to an Octal style in hopes to drop it that way but I can't because this transformer does not have a 5V Heater line.

    Any help is appreciated! Cheers

    20200109_233313_resized.jpg 20200109_233044_resized.jpg
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    14,866
    Likes Received:
    8,725
    Location:
    Start the reactor... Free Mars!
    383 DC is good. I wouldn't worry over the B+ voltage. All the way up to 450V should still work good.
    Don't use zener diodes it doesn't work very well.
    Run it on 120V from the variac if the line voltage is too high.
     
    thetragichero, myersbw and Yamariv like this.
  3. Yamariv

    Yamariv Active Member

    Joined:
    Mar 19, 2017
    Messages:
    369
    Likes Received:
    230
    Location:
    Ottawa, Canada
    Very interesting, thanks!

    So wouldn't the bias be super high with that type of B+? I'm still a bit confused on how to check bias the correct way on a Cathode Biased amp. I have a couple bucking transformers that will drop the line voltage to lower levels that I made for my Vintage Hifi amp but I want to have this 18 Watt as my main gigging and practice amp so I was hoping to have it reliable at 123VAC from most wall circuits without having to lug a variac or transformer around with me.

    Interesting about the Zener diodes, what the issue with that way? Cheers
     
  4. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    3,654
    Likes Received:
    2,077
    The bias on these is set by the cathode resistor and its bypass cap. The resistor can be anything between 100R and 150R depending on how hot you want it. The bypass cap that goes with it will influence how much bass is passed but a typical value might be 50-500uF.
    What does your kit say?
     
  5. Yamariv

    Yamariv Active Member

    Joined:
    Mar 19, 2017
    Messages:
    369
    Likes Received:
    230
    Location:
    Ottawa, Canada
    My kit suggested a 10 Watt 150ohm resistor for the Cathode which I hear is good to cool things down a bit. Sorry for the newb reply on the bypass cap but is that the one that's right beside the Cathode resistor? Mine says 47uf @ 50volts..This cap is attached to the Cathode resistor on the board, would this be it?
     
  6. Yamariv

    Yamariv Active Member

    Joined:
    Mar 19, 2017
    Messages:
    369
    Likes Received:
    230
    Location:
    Ottawa, Canada
    Can someone clarify this for me.. When guys say the best B+ voltage level is 345 vdc for an 18 Watt build, where are they measuring this? Before the can cap or after?
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    14,866
    Likes Received:
    8,725
    Location:
    Start the reactor... Free Mars!
    Measure the B+ at the center tap of the output transformer.
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    14,866
    Likes Received:
    8,725
    Location:
    Start the reactor... Free Mars!
    You can measure the plate current and adjust the resistor if necessary.
    The current should be 90-100% of the rated plate dissipation.
    This is how class A amps are biased.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    14,866
    Likes Received:
    8,725
    Location:
    Start the reactor... Free Mars!
    Do you have a bias probe?

    The zener drifts with temperature it's not really the most ideal way to go.
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    14,866
    Likes Received:
    8,725
    Location:
    Start the reactor... Free Mars!
    OK this is how it goes:
    once you put the tubes in and bias it for 90% class A, the voltage will drop down.

    I think you have been measuring without the power supply fully loaded.
    When you get the power supply loaded up to 90%, the voltage will drop down.
     
  11. Yamariv

    Yamariv Active Member

    Joined:
    Mar 19, 2017
    Messages:
    369
    Likes Received:
    230
    Location:
    Ottawa, Canada
    Those measurements were taken with all the tubes in it last night. I'm referring to the 383v before the can cap and the 360v after it. Which one of these are used to calculate the "ideal" 345 vdc I'm shooting for. After the cap or before? If it's after then 360vdc isn't too far off
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    14,866
    Likes Received:
    8,725
    Location:
    Start the reactor... Free Mars!
    What schematic are you using?
    Can you post pictures of the insides?

    "I'm referring to the 383v before the can cap and the 360v after it."

    383 volts ac? or DC?
     
    Yamariv likes this.
  13. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    14,866
    Likes Received:
    8,725
    Location:
    Start the reactor... Free Mars!
    What is the voltage at the center tap of the output transformer?
     
    Yamariv likes this.
  14. myersbw

    myersbw Well-Known Member

    Joined:
    Oct 22, 2015
    Messages:
    984
    Likes Received:
    441
    Location:
    SW Ohio
    First things first. Let's clarify. You're instructions say "insert the two power tubes" and you state it's an 18 watt Marshall clone. That's EL84 and not the EL34 you mentioned (assuming a typo on your part).

    I assume you don't have a socket adapter to directly measure plate current and voltage. If you measure 383 with reference to ground, you're measuring the tube plate voltage + the cathode resistor voltage drop together. That's not reflective of the 'voltage across the tube' for typical power calculations or max voltage consideration.

    The cool thing about having that cathode resistor is you can get close to the current in the tube. But, you have to keep in mind you'll be calculating both the screen AND the plate current together. It would help if you'd actually measured that cathode resistor value so you can calculate accurately.

    Let's assume your cathode resistor is actually 150 ohms. From the photo in the instructions, that 11.55 volts measured is across the cathode resistor. 1.55V/150 is 84mA for both tubes. We will, for the moment, assume they're closely matched so we're looking at 42mA PER TUBE. Now...let's assume a screen current of about 4mA so that we're likely closer to (42mA-PLATE minus 4mA-SCREEN equaling 38mA estimated PLATE current)

    Next, we need an accurate plate voltage. Since you have a cathode resistor, we have to subtract that out to get an accurate plate voltage (anode to cathode voltage drop) to calculate the tube's plate dissipation. You're not showing enough information to be totally accurate about this...I'm only ASSUMING the 383 is the voltage from the rectifier and that you may be at 360 as it's feeding the EL84's (?)...irregardless, I'll show you the math, but what we want is the voltage at pin 7 on the EL84 socket as measured with respect to ground. Let's say 360 for now.

    So, the actual voltage drop (plate voltage) we want to know would be 360V-11.55V which brings the tube's plate voltage down to 348V. If closer to 383, it's higher.

    So, IF 348, then the tube's plate dissipation (watts) is (as Pwatts=VxI) or 348volts X 0.038amps equalling 13.2 watts.
    ____________

    If you want it accurate, measure that voltage at Pin 7 of the EL84 socket...then we'll have a baseline.

    The bigger issue here is...1 - you really should've measured the cathode resistor to get close. But, 2 - since the tubes are rarely ever drawing the same current PER TUBE...one is likely channeling a little more current than the other.

    Unless you're investing in a socket to make some measurements more accurate. If that amp sounds good, get a safe playing environment where you can clearly see those EL84's...turn off the lights except for a nightlight to see by...and play for a good 30 minutes and watch to see if either of those tubes' plates start glowing a dull cherry red. Yes? Then you could sub another tube, or increase that cathode resistor a bit to keep the dissipation manageable. Make sure that cathode resistor is not physically touching any other component or the board...it'll get hot!
     
    Last edited: Jan 12, 2020
    NSV-Andy, Yamariv and CoyotesGator like this.
  15. Yamariv

    Yamariv Active Member

    Joined:
    Mar 19, 2017
    Messages:
    369
    Likes Received:
    230
    Location:
    Ottawa, Canada
    The 383dc B+ voltage I was referring to was right out of the rectifier before the can cap. Voltage right after the cap is 360dc B+ voltage and is the number I should have focused on more as it's the ones going to the bias resistors then the EL84's as I do more research.

    The layout I'm using is the Tube Depot 18 Watt Tremolo

    20200109_233313_resized.jpg 1818Watt.jpg
     
  16. Yamariv

    Yamariv Active Member

    Joined:
    Mar 19, 2017
    Messages:
    369
    Likes Received:
    230
    Location:
    Ottawa, Canada
    Wow, very cool of you to help me with that! Yes that was for sure a typo meant to writer EL84 not El34 :facepalm: I'm gonna head out to the garage shortly and take a bunch more detailed measurements and I'll post the results shortly! Maybe we can once and for all figure this out when I can provide the right reading and numbers :fingersx: This Cathode bias thing is still pretty foreign to me but I'm slowly catching on..Thanks again for the help, more test results soon!
     
  17. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    14,866
    Likes Received:
    8,725
    Location:
    Start the reactor... Free Mars!
    "The 383dc B+ voltage I was referring to was right out of the rectifier before the can cap. Voltage right after the cap is 360dc"

    I still don't understand how you get that.
    Are you talking about voltage on one side of R30, and the other side of R30?
    How are you getting 23 volts of voltage drop?
    The can cap is 2 capacitors in one can.

    The voltage out of the rectifier is the same as at the capacitor C18.
    There should be no 23 volt difference.
     
    Last edited: Jan 12, 2020
    Yamariv likes this.
  18. Guitar-Rocker

    Guitar-Rocker Well-Known Member

    Joined:
    Mar 5, 2012
    Messages:
    2,746
    Likes Received:
    2,335
    Location:
    Indianapolis, Indiana
    There is a 2.2K dropping resistor across the can lugs, in lue of a choke.
     
    Yamariv likes this.
  19. Yamariv

    Yamariv Active Member

    Joined:
    Mar 19, 2017
    Messages:
    369
    Likes Received:
    230
    Location:
    Ottawa, Canada
    This ^^
     
  20. Yamariv

    Yamariv Active Member

    Joined:
    Mar 19, 2017
    Messages:
    369
    Likes Received:
    230
    Location:
    Ottawa, Canada
    Alright, so I printed the voltage test sheet off of the 18 Watt Forum and got all the readings and I've attached a pic. So what's the verdict, you guys think I need to drop the B+ or am I close enough? Any other issues with the readings you guys see?

    Note* The first row of readings on the EL84's and the Rectifier were through the dim bulb tester (300w bulb), the second row is direct from the wall at 123.5vac (ish) so that's probably the readings to go with.

    Any chance one of you guys could help me figure out the bias of the Power tubes with these readings? Should I drop the B+ a bit? Thanks for the help :dude:
    Chart1.jpg
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice