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Thread: Fender Harvard 5F10

  1. #1
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    Fender Harvard 5F10

    Time to build another amp using as much as possible parts I have on hand. Transformers I have would lend themselves well to a Fender Harvard, which I also understand makes the basis for a very sweet harp amp. A few questions though, directed especially to anyone with experience with these amps (Goose?):
    1. How much of the Harvard's character comes from the 6AT6/6AV6 preamp tube? Some have maintained that these are basically 1/2 of a 5751/12AX7, and since I have plenty of the latter but none of the 7 pin tubes or sockets, I may opt for this if there's no tonal reason not to do so. Also, having 2 input triodes would let me use grid leak biasing on one, with the other set up in the normal Harvard fashion.
    2. The Harvard schematic shows no grid load resistor to ground on the input. I would think this would be a problem for mic elements that want to see a
    high impedance, esp crystals. Anyone with any experience here?
    3. Anyone with experience or thoughts on preamp plate voltages for a Harvard harp amp? Stock values are 160, 170 and 207V on the 1st and 2nd preamp stages and PI, respectively. Normally I'd go a bit lower, 100-125V on the first stage.
    4. Coupling caps--I have lots of .1 on hand, which is my default value for a harp amp. Any reason not to use these to couple all stages?
    5. The schematic shows a 25uf cathode bypass cap on the 1st preamp stage, none on the 2nd. I'm inclined to add a switchable 50-100uf bypass cap to the 2nd stage, to provide the option for more punch. Again, anyone with experience/thoughts here?
    Any other comments from people with experience with this circuit will be appreciated.

  2. #2
    Old Timer soundguruman's Avatar
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    Quote Originally Posted by allende View Post
    Time to build another amp using as much as possible parts I have on hand. Transformers I have would lend themselves well to a Fender Harvard, which I also understand makes the basis for a very sweet harp amp. A few questions though, directed especially to anyone with experience with these amps (Goose?):
    1. How much of the Harvard's character comes from the 6AT6/6AV6 preamp tube? Some have maintained that these are basically 1/2 of a 5751/12AX7, and since I have plenty of the latter but none of the 7 pin tubes or sockets, I may opt for this if there's no tonal reason not to do so. Also, having 2 input triodes would let me use grid leak biasing on one, with the other set up in the normal Harvard fashion.

    The 6AV6 is not the same as 1/2- 12AX7. This tube is used for automatic volume control (compressor type) circuits. It has two extra control grids that are used for a special purpose audio limiting gizmo. The preferred substitute is
    6BK6. It definitely has a unique construction, although it also is a hi mu triode, like a 12AX7. 7 pin sockets are easy to obtain and install.

    2. The Harvard schematic shows no grid load resistor to ground on the input. I would think this would be a problem for mic elements that want to see a
    high impedance, esp crystals. Anyone with any experience here?

    It does not work out that way. It can go without a resistor as long as it's connected to something and the wire is shielded, like a mic element, or shorted to ground.

    3. Anyone with experience or thoughts on preamp plate voltages for a Harvard harp amp? Stock values are 160, 170 and 207V on the 1st and 2nd preamp stages and PI, respectively. Normally I'd go a bit lower, 100-125V on the first stage.

    Whelp, there's always more headroom to be had with higher plate voltages.

    4. Coupling caps--I have lots of .1 on hand, which is my default value for a harp amp. Any reason not to use these to couple all stages?

    Using .1 for all the stages will result in motor-boating low frequency oscillation.

    5. The schematic shows a 25uf cathode bypass cap on the 1st preamp stage, none on the 2nd. I'm inclined to add a switchable 50-100uf bypass cap to the 2nd stage, to provide the option for more punch. Again, anyone with experience/thoughts here?

    It will certainly punch more with a bypass on the second stage, but no need to go higher than 25uf. I might use 2.2! This may also cause the audio to overload a bit too much, and sound muddy, but try it anyhow.
    I mean there is a master volume? I think you need one if you are planning on doing that modification.

    Any other comments from people with experience with this circuit will be appreciated.
    The stock values are the best place to start, instead of trying to redesign

  3. #3
    Old Timer soundguruman's Avatar
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    Quote Originally Posted by allende View Post
    Time to build another amp using as much as possible parts I have on hand. Transformers I have would lend themselves well to a Fender Harvard, which I also understand makes the basis for a very sweet harp amp. A few questions though, directed especially to anyone with experience with these amps (Goose?):
    1. How much of the Harvard's character comes from the 6AT6/6AV6 preamp tube? Some have maintained that these are basically 1/2 of a 5751/12AX7, and since I have plenty of the latter but none of the 7 pin tubes or sockets, I may opt for this if there's no tonal reason not to do so. Also, having 2 input triodes would let me use grid leak biasing on one, with the other set up in the normal Harvard fashion.

    The 6AV6 is not the same as 1/2- 12AX7. This tube is used for automatic volume control (compressor type) circuits. It has two extra control grids that are used for a special purpose audio limiting gizmo. The preferred substitute is
    6BK6. It definitely has a unique construction, although it also is a hi mu triode, like a 12AX7. 7 pin sockets are easy to obtain and install.

    2. The Harvard schematic shows no grid load resistor to ground on the input. I would think this would be a problem for mic elements that want to see a
    high impedance, esp crystals. Anyone with any experience here?

    It does not work out that way. It can go without a resistor as long as it's connected to something and the wire is shielded, like a mic element, or shorted to ground.

    3. Anyone with experience or thoughts on preamp plate voltages for a Harvard harp amp? Stock values are 160, 170 and 207V on the 1st and 2nd preamp stages and PI, respectively. Normally I'd go a bit lower, 100-125V on the first stage.

    Whelp, there's always more headroom to be had with higher plate voltages.

    4. Coupling caps--I have lots of .1 on hand, which is my default value for a harp amp. Any reason not to use these to couple all stages?

    Using .1 for all the stages will result in motor-boating low frequency oscillation.

    5. The schematic shows a 25uf cathode bypass cap on the 1st preamp stage, none on the 2nd. I'm inclined to add a switchable 50-100uf bypass cap to the 2nd stage, to provide the option for more punch. Again, anyone with experience/thoughts here?

    It will certainly punch more with a bypass on the second stage, but no need to go higher than 25uf. I might use 2.2! This may also cause the audio to overload a bit too much, and sound muddy, but try it anyhow.
    I mean there is a master volume? I think you need one if you are planning on doing that modification.

    Any other comments from people with experience with this circuit will be appreciated.
    The stock values are the best place to start, instead of trying to redesign

  4. #4
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    Quote Originally Posted by allende View Post
    ....
    The schematic shows a 25uf cathode bypass cap on the 1st preamp stage, none on the 2nd. I'm inclined to add a switchable 50-100uf bypass cap to the 2nd stage,... .
    There's NFB to the 2nd stage cathode.

  5. #5
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    Just a little FYI, 'cuz I know this personally: if you are a U2 fan, you've heard the 5F10 on tons of their recorded material. It usually what Edge records with.

    Keep it stock I say. Those are great amps as they are. Not THAT much different than a 5E3, but the lower cap values and NFB yields a narrower power-bandwidth and slightly stiffer feel (of course, the 10" speaker helps too), which is why guitars sound so nice and twangy through them. Should work brilliantly (no pun intended) for harp.
    John R. Frondelli
    dBm Pro Audio Services, New York, NY

    "Mediocre is the new 'Good' "

  6. #6
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    Here's one example of a 5F10 build:

    Green Eagle Amps - 1959 Tweed Harvard 5F10

  7. #7
    Junior Member Winnie Thomas's Avatar
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    I've built a couple 5F10 circuits. The only change I made was in adding a bias pot in place of the 6K8 resistor.

    It is a great little amp. I used a weber Deluxe PU, the lower violtage taps, and a Weber Deluxe OT.

    Winnie

  8. #8
    Supporting Member loudthud's Avatar
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    Crystal mics like the high impedance that you get with grid leak bias, and you automatically get low plate voltage. I've done this on a couple of 5E3 based builds. Installed a switch in one of the extra input jack holes to select one or both inputs. Typically you ground the cathode on grid leak bias so you can't get any treble boost from the cathode bypass. But, you can still get the treble boost, just hook the grid leak resistor to the cathode instead of ground.

    I like the spongy feel of cathode bias for the 6V6s. Maybe you could make that switchable.
    Attached Thumbnails Attached Thumbnails grid_leak.gif  
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  9. #9
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    Question on the Harvard. Is it a 2 x 6v6 SE amp or a standard push - pull amp. I'm asking in regard to transformer purchases. Thanks.

  10. #10
    Supporting Member Jazz P Bass's Avatar
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    Fender Harvard 5F10 Schematic

    The Harvard output section is a standard fixed bias, grounded cathode, 6V6 push pull configuration.
    The class is AB.
    Attached Files Attached Files

  11. #11
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    I frequently find that these old Harvard amps exhibit an ultrasonic oscillation right at about 75% to 90% full output power.... must be the wiring layout... if you remove the negative feedback, it goes away.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  12. #12
    Junior Member Winnie Thomas's Avatar
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    Quote Originally Posted by Bruce / Mission Amps View Post
    I frequently find that these old Harvard amps exhibit an ultrasonic oscillation right at about 75% to 90% full output power.... must be the wiring layout... if you remove the negative feedback, it goes away.

    Bruce,

    It's been a long time since that day in Cheyenne at the airport. I'm living in AZ. How are you?

    Winnie

  13. #13
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    gtownblues

    Thanks for the help.

  14. #14
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Winnie Thomas View Post
    Bruce,

    It's been a long time since that day in Cheyenne at the airport. I'm living in AZ. How are you?

    Winnie
    Doing well... oh yes, of course I remember flying up there from Denver to meet up.
    I don't fly as much as I used to, but still have it and we try to get some time in when we can.
    Where in AZ are you now?
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  15. #15
    Junior Member Winnie Thomas's Avatar
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    I live in the town of Cochise, in Southeastern AZ, 5 miles South of exit 331 on I 10. Much nicer weather here than in VT.
    Last edited by Winnie Thomas; 06-11-2013 at 11:52 PM.

  16. #16
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Wow, that's darn near Mexico brother....what shock from VT!!
    I flew down to Carlsbad, NM two summers ago, a couple hundred miles east of you.... hot as all heck.
    That is about the furthest I've gone in 4-5 years.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  17. #17
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    Bruce, how would you go about removing the negative feedback from the 5F10 circuit?

  18. #18
    Supporting Member Jazz P Bass's Avatar
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    There is a 56K resistor that takes the output transformer signal & injects it into the cathode of 1/2 of the 12AX7.
    http://music-electronics-forum.com/a...5f10_schem.pdf
    That is the NFB resistor.
    If you do not want NFB, remove the connection.

  19. #19
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    There is a 56K resistor that takes the output transformer signal & injects it into the cathode of 1/2 of the 12AX7.
    http://music-electronics-forum.com/a...5f10_schem.pdf
    That is the NFB resistor.
    If you do not want NFB, remove the connection.
    Or get a 50K to 100K pot to put in series with the 56K resistor, wire it as a variable resistor and mount it somewhere so you can increase or decrease the amount of NFB.
    I don't think those amps had two speaker jacks so.... you have to be selective on where to and how to mount the pot so as not to wreck the vintage value.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  20. #20
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    My Harvard copy is my favorite guitar amp.
    http://i422.photobucket.com/albums/p...s/Harvard1.jpg

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