Announcement

Collapse
No announcement yet.

5F2A with a 6L6 prebuild questions

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • swaptronics_tech
    replied
    Build another amp and call it a 5F2A?

    I would stick with a NOS 5Y3FT and NOS 6V6GT tube, RCA 12AX7, Magnetic Components PT and OT and stock Fender schematic for this build.

    When built correctly in a tweed covered pine cabinet with 10" alnico speaker (yes alnico--try Magnavox 10" alnico) it sounds period correct.

    I have built several of these in the past and they sound fantastic. Measure your 470-ohm cathode bypass resistor and sometimes you need to substitute a 500-ohm or higher one if your bias voltage and current are too high--e.g. over 15 watts of peak power.

    Once you start changing tubes (stay away from solid state rectifier) you do not have a tweed princeton amp.

    Unless you want to "red plate" your 6V6 tubes or burn up your power transformer, there is no need to change this to a 6L6 based amp--build a 5E3 instead if you want more volume.

    Best regards,

    Dave Adams (a.k.a. "swaptronics")




    Originally posted by Gerard View Post
    Hi, I have just about all the parts on hand to build a 5f2a with a 6l6. I have some questions about the bias resistor and O.T. I will be running the 6l6 at about 400 volts with a 5y3 and about 450 volts with a gz34 recto. The P.T will handle the load of the tubes, so thats not an issue. What O.T would be a good choice? I had my eye on a Hammond 125ese 15 watt with 5k primary.Also should the bias resistor be a 5watt or 10watt at around 270R or so depending on the voltage? and should I raise the value of the filter caps , if so, what to values?


    Thanks again for your time. This is my third build, my first one was a Mission 5e3 that I kept stock and love the sound. This forum has been a great help to me over the past years and I appreciate all the help.

    Leave a comment:


  • Gerard
    replied
    Thanks Guy's

    Thanks for all you suggestions! I rotated the 12ax7 which made me loose about 3" of wire, I installed a shielded wire from the volume pot to pin 7. rotated the .022 uf cap from the board to the 6l6 and installed the 220k to ground at the tube. No hum or oscillation at all, after the repairs!
    This amp is way louder than I expected! Almost at loud and my mission 5e3 but with a little less thump! That is what I was looking for. This thing screams with a blues driver in front of it.
    Now I need to build a cabinet! Thanks for all your help!

    Leave a comment:


  • Gerard
    replied
    All great points MWJB

    I will follow your recommendations and rotate the 12ax7 , move the cap to shorten the grid wire to at 6l6, and install the 220k to ground at the tube. also shield the volume wire.

    This might take me some time, but I will get back with you.

    Thanks, Gerard

    Leave a comment:


  • MWJB
    replied
    Yes shield the wire from volume pot to 12AX7 pin 7, route it (and any other grid wires) away from plate wires (12AX pins 1 & 6). The orientation of your tube sockets seems to encourage long parallel runs of plate & grid wires? Would it be feasible/easier to rotate the 12AX7 by 180 deg?

    Also the wire running from the 220K grid load to the power tube grid is rather long, you could also shield this, or what I would do is remove the coupling cap that connects to 12AX7 pin 6 and see if it will reach from the turret where it meets the 100K plate resistor straight to the power tube? Keep it away from the heater wires, mount a new 220K directly on the power tube pin 5 & ground to PT bolt.

    What about the PT centre taps & grounds are these still soldered to the chassis?

    A bypass cap at pin 8 of the 12AX7 will shunt a large proportion of the signal there to ground, lessening the effect of the NFB loop (it will still have some effect). It will also increase gain, not something I think you want until your oscillation issues are sorted.

    Connect a true RMS AC meter across your speaker terminals, play the amp. Voltage will jump about some but try and determine an average, square the average then divide by speaker load in ohms...I'd expect about 8W?

    Leave a comment:


  • Gerard
    replied
    working better

    I took your suggestions and did the following:

    corrected the input jack 1m resistor wiring

    grounded the tone cap at the pot


    removed the ground wire from the input to the output jacks

    grounded the volume pot to the input jack ground

    changed the cathode resistor to 680 ohms, now idling at 51ma

    changed the input resistor from 68k to 22k (is that ok?)

    the hum has always been very quite even at wide open throttle. no problem with 60hz hum.

    the other problem I was having with the buzzing distortion with the NFB circuit hooked up was the O.T wires. I just reversed them from tube to B+. That took care of the buzzing distortion.

    When I built this amp, I thought it would have more output (loudness). I have built a mission 5e3 and that to me is quite loud! I know this is a small practice amp , but I thought it would be louder.
    I have tried all types of tubes in it , 5u4gb, gz34,5y3gt, 6v6,6l6gc,12ax7,12ay7, 12au7,12at7, I havent got a favorite yet, and yes I will watch the voltage when switching tubes.

    The only problem I have now is some sort of parasitic oscillation . When the voulme is past 5, there is a slight vibrato effect, just slightly noticable. The amp is still usable but I need to tace that problem down. Which wires should I look at first .Which wires would I need to shorten or reroute?

    Would you recommend using a shielded wire from the volume pot to pin #7 of the 12ax7?

    Any thoughts on putting a cap on the second cathode resistor of the 12ax7?

    Thanks for all your input !!

    Leave a comment:


  • MWJB
    replied
    Lose the black ground wire that runs from your input jack to the speaker jacks, the speaker jacks ground through their connection to the chassis at the socket nut. If you have insulated them, don't.

    Leave a comment:


  • big-daddy-59
    replied
    with a single input the grid resistor should be 33k not 68k. I can't tell from your pictures what the value is. Preamp voltages are a little high still, might want to increase the value of R11.

    Leave a comment:


  • MWJB
    replied
    Go bigger on the cathode resistor, 510ohms is borderline at 436vdc. 680-800ohms.

    No, you shouldn't need to change the NFB loop. Something else is probably wrong.

    Grid stopper can go from pin 6 to 5. Connection to 220K grid load connects to pin 6. Though, this shouldn't really be necessary.

    Your input jack doesn't look right? 1meg should go from hot to ground, with a jumper from ground to the switch.

    You seem to have all your grounds daisy chained, separate the preamp & power amp grouns, run the preamp grounds to the input jack, power amp to a PT bolt. That 0.0047cap should be grounded to the back of the tone pot.

    Leave a comment:


  • Gerard
    replied
    Up and running (almost)

    Hi, I just finished my 5f2a with a 6l6gc. When I first started it up, it sounded like garbage! rattling/buzzing distortion. So I disconnected the negitive feed back wire. That made it sound alot better. (do I need the change the value of the negitive feedback resistor? or is something else wrong?)
    2nd, When the volume is turned up past 9, the sound cuts out (oscillation?). What pins of the 6l6 do I need to install the grid stopper resistor on?
    I am running this amp with a gz34 , 6l6gc and 12ax7 . hammond 123ese O.T. It has 440v on the B+ . 510 ohm cathode resistor
    How do I make this amp less muddy with H/B pickups?
    I have not adjusted any voltages yet, I could use some help! here's the voltages

    6L6GC
    #3-436v
    #4-395v
    #8-31v

    12AX7
    #1-212v
    #3-1.70v
    #6-215v
    #8-1.66v
    Attached Files

    Leave a comment:


  • Gerard
    replied
    All great points!! Thanks. Now it is off to my build.

    Leave a comment:


  • tyru007
    replied
    MWJB - I may try subbing differnet rectifier some time if I can get my hands on a few types just to see. I just wanted to know if a diode + "sag" resistor = GZ34 rectifier, which it appears it does for an SE amp. I have left mine with the solid state rectifier.

    Gerard - One thing I found out worked well for reducing the hum from the filements was to raise the ground voltage reference for the 6.3V power supply. This along with good power supply filter made my amp almost silent at full volume.

    If your 6.3V supply has no center tap, create a artificial center tap using two 100 ohm resistor then tie the center tap to either the cathode of the power tube (at about 20-25V) or create a 30-40V supply using a voltage divider with 270K then 22K resistor to ground off the B+ (450V) supply (also acts as your capacitor bleed resistor. The voltage at the connection of the 270K and 22K should be about 35V.

    I have mine tied to the cathode of the power tube. For some reason I get a slight hum at the speaker when the amp is in standby, which goes away when the standby switch is turned to play.

    Leave a comment:


  • MWJB
    replied
    "I compared a rectifier of 4 UF4007 diodes and a 220R 25W inductive wirewound resistor to a GZ34 rectifier so that they both gave about the same B+. Granted that the GZ34 does not produce much sag and the 220R resistor does add resistance to change in voltage. I did not detect much diffence the the method of rectification for the map." By trying to emulate the voltage drop of the tube, with the diodes & resistor, you have muddied the water somewhat regarding a comparison between tube and SS rectification. Diodes alone don't drop the voltage, it would have been a better comparison if you had A/B'd the diodes vs the GZ34 without the dropping resistor. What you did was similar to Weber's copper cap recto & some folks feel that these sound just like tubes, so I'm not surprised that there wasn't much difference.

    Leave a comment:


  • tyru007
    replied
    Originally posted by Gerard View Post
    All great points Tubeswell and Tryu007. I thank you for your comments. What will adding a grid resistor to the power tube do? and the difference sound wise from a 1.5k to a 5.6k? grid resistor would be?
    Used to prevent parasitic oscillation and blocking distortion. It will not effect tone. Fenders typically use the 1.5K on 6L6s while marshalls use the 5.6K with El34s. I'd go with the higher 5.6K.

    For more info go to:
    http://www.aikenamps.com/InputRes.htm
    http://www.freewebs.com/valvewizard/gridstopper.html

    MWJB - I compared a rectifier of 4 UF4007 diodes and a 220R 25W inductive wirewound resistor to a GZ34 rectifier so that they both gave about the same B+. Granted that the GZ34 does not produce much sag and the 220R resistor does add resistance to change in voltage. I did not detect much diffence in these two methods of rectification for the SE amp.
    Last edited by tyru007; 09-29-2009, 02:25 PM.

    Leave a comment:


  • MWJB
    replied
    "The circuit does not appear to sag. Except for a voltage drop, I get the same response using a tube or diodes to rectify the voltage." - That isn't typical, I normally find that the envelope changes with each different rectifier. On a regular champ with 1x6V6 you might not get a huge change in idle voltage from one recto to the next because current draw through it is not that great.

    Leave a comment:


  • Gerard
    replied
    All great points Tubeswell and Tryu007. I thank you for your comments. What will adding a grid resistor to the power tube do? and the difference sound wise from a 1.5k to a 5.6k? grid resistor would be?

    Leave a comment:

Working...
X