Results 1 to 22 of 22

Thread: First Time Biasing my Original '68 Fender Deluxe Reverb

  1. #1
    Member
    Join Date
    Jul 2019
    Location
    Sacramento, California
    Posts
    36
    Thumbs Up/Down
    Received: 7/0
    Given: 1/0
    Rep Power
    0

    First Time Biasing my Original '68 Fender Deluxe Reverb

    I'm learning (from Rob Robinette's excellent site) to bias one of my older amps and just need to bounce this off of someone more experienced to reinforce what I did. I'm using JJ 6V6's, measured V7 plate@420vdc, V8 plate@419vdc. B+@ 425.5vdc a difference of 5.5 & 6.5, respectively. I measured the plate to OT center tap resistance, V7@230.8ohms & V8@226.3ohms. Calculating the V7current, 5.5v/230.8=.0238a or 23.8ma, V8current, 6.5v/226.3=.0287a or 28.7ma

    If my calcs are correct, are these idle currents too hot? too cold? Just right?

    I noticed that when I raised or lowered the bias pot, both the plate and the OT voltages increased or lowered concurrently. How then does the bias get adjusted?

    0 Not allowed! Not allowed!

  2. #2
    Senior Member
    Join Date
    Jul 2011
    Location
    Adelaide, South Oz
    Posts
    738
    Thumbs Up/Down
    Received: 68/0
    Given: 4/0
    Rep Power
    9
    Calcs are good.
    For V8, 28.7 mA x 425.5V = 12.2 Watts Anode dissipation in a tube which has a data book rating of 12 Watts anode dissipation.
    So it is a bit hot but not that unusual.

    The culprit here is the B+ voltage which is 25V above the "typical" 400V

    I would be inclined to back the idle current off - ideally both tubes (if perfectly balanced) should have around 20mA with a B+ of 425V

    You could leave the bias a bit hot but back of the bias pot to get say 24 mA in V8 (the hottest tube) which is about 5.4 volts where you currently see 6.5V.

    Of course I 'm a bit obsessive and have a heap of 6V6 so I would be looking for a better balanced pair of 6V6 first but that is in "nitpicking" territory.

    Cheers,
    Ian

    1 Not allowed! Not allowed!

  3. #3
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,693
    Thumbs Up/Down
    Received: 434/1
    Given: 408/2
    Rep Power
    18
    Just to note that the JJ6V6S has a 14 watt plate limit. I assume thatís under the design max rating system commonly used for 6V6GTA info, rather than the design centre system used for 6V6GT info. The actual plate dissipation ratings of 6V6GT and 6V6GTA are the same, eg the GTA limit is 12 watts under the design centre system.

    Note that itís perfectly normal for HT / plate voltage to change according to the bias setting, as the bias voltage used adjusts the idle plate / cathode current, which is the main thing that loads the HT power supply.
    In just that same way that your mains wall voltage will probably dip / sag a little if all the big appliances in your house were working at full power simultaneously.

    The only way it could be avoided totally would be to have regulated power supplies in the amp. Folks that have tried that (well, steve conner at least) advise that it has a negative effect on how a guitar amp responds.

    Are you sure that you a getting a benefit from such a high idle plate current? I suggest that you see how it sounds / responds at 20mA. The lower the plate current the longer the tube life.

    1 Not allowed! Not allowed!
    Last edited by pdf64; 08-25-2019 at 03:03 PM.

  4. #4
    Member
    Join Date
    Jul 2019
    Location
    Sacramento, California
    Posts
    36
    Thumbs Up/Down
    Received: 7/0
    Given: 1/0
    Rep Power
    0
    Thank you, these are JJ6V6S tubes so I'll take your word on the 14W PD. I will assume that means that I can run the tubes at a little higher current than I would the 12W 6V6's. As mentioned by my Australian friend, Gingertube, the key seems to be dropping the B+, to the 420V spec. Is that an adjustment that I can make?

    0 Not allowed! Not allowed!

  5. #5
    Senior Member
    Join Date
    Jul 2011
    Location
    Adelaide, South Oz
    Posts
    738
    Thumbs Up/Down
    Received: 68/0
    Given: 4/0
    Rep Power
    9
    I looked at the '68 Custom Deluxe Reverb Service Manual and it says +415V for B+ whereas the original 1968 Deluxe Reverb said +398V so probably leave it alone.
    If you MUST trim it back a bit, the place to do this is small series resistors in the two (2) red wire feeds from the power tranny to the 5AR4 rectifier anodes (pins 4 and 6), say 33 or 47 Ohms 2W resistor in each feed - BUT that may change your sound (More compressed).
    Cheers,
    Ian

    0 Not allowed! Not allowed!

  6. #6
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,379
    Thumbs Up/Down
    Received: 131/0
    Given: 10/0
    Rep Power
    13
    Also the original schematics are marked "Voltages read to ground with electronic voltmeter. Values shown + or - 20%". That's some variance. In the UK at least these original amps are nearly always running higher voltages than the design spec of the tubes, and they've been doing this for decades. Usually they're close to 440v. My go-to tube for this situation is the 6V6S. Previously I'd been using Phillips JAN NOS 6V6 but these had become too expensive. If the dissipation is lowered as per Ian's suggestion, then the voltage isn't an issue.

    I can't recall ever lowering the B+ in a Fender amp and many get some serious usage in the hands of full-time musicians.

    0 Not allowed! Not allowed!

  7. #7
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,693
    Thumbs Up/Down
    Received: 434/1
    Given: 408/2
    Rep Power
    18
    Quote Originally Posted by Perkinsman View Post
    Thank you, these are JJ6V6S tubes so I'll take your word on the 14W PD. I will assume that means that I can run the tubes at a little higher current than I would the 12W 6V6's. As mentioned by my Australian friend, Gingertube, the key seems to be dropping the B+, to the 420V spec. Is that an adjustment that I can make?
    No need to take my word, see https://tubedata.altanatubes.com.br/...163/6/6V6S.pdf
    But the logic of idling a design max 14 watt 6V6 hotter than a design centre 12 watt 6V6 seems flawed. Until it’s revealed what rating system JJ use, I suggest to treat it as any other 6V6.
    If you wish to cool off conditions in an amp and you’re in a 120V mains region (including that in your profile helps avoid pointless suggestions), the brownbox looks a good solution https://www.amprx.net/
    I think you are idling the 6V6 too hot and ~20mA is more normal / reasonable; you understand that their plate dissipation increases when passing signal?
    As you’re learning why not experiment with various cooler idle settings?

    1 Not allowed! Not allowed!

  8. #8
    Senior Member nevetslab's Avatar
    Join Date
    Apr 2010
    Location
    Burbank, CA
    Posts
    1,733
    Thumbs Up/Down
    Received: 615/1
    Given: 1,430/1
    Rep Power
    11
    Quote Originally Posted by pdf64 View Post
    No need to take my word, see https://tubedata.altanatubes.com.br/...163/6/6V6S.pdf
    But the logic of idling a design max 14 watt 6V6 hotter than a design centre 12 watt 6V6 seems flawed. Until itís revealed what rating system JJ use, I suggest to treat it as any other 6V6.
    If you wish to cool off conditions in an amp and youíre in a 120V mains region (including that in your profile helps avoid pointless suggestions), the brownbox looks a good solution https://www.amprx.net/
    I think you are idling the 6V6 too hot and ~20mA is more normal / reasonable; you understand that their plate dissipation increases when passing signal?
    As youíre learning why not experiment with various cooler idle settings?
    Recently, working on a Fender 65 Deluxe Reverb reissue, that came in with the comment of it sounding a bit distorted, the plate current on both tubes were running around 27mA. I forget where the plate voltages were....in the 390-400V range. I looked at the amount of harmonic distortion I was getting (8 ohm dummy load). Minimum harmonic distortion (2nd & 3rd harmonics) occurs around 13-14mA. Above that, you're adding harmonics, or color to the sound of the amp to some degree. I usually keep the tubes with a bit of that, and around 20mA, they do sound really nice. They do sound a little cleaner at lower plate current, though it seems like you loose a bit of the character the amp offers.

    An easy way to measure the plate/screen current is to add 1 ohm cathode resistors between pin 8 and chassis on each of the power tube sockets. 1 ohm/2W metal oxide would be a good choice. Then, you read the voltage across those, say 22mV = 22mA. Just a thought.

    1 Not allowed! Not allowed!
    Logic is an organized way of going wrong with confidence

  9. #9
    Member
    Join Date
    Jul 2019
    Location
    Sacramento, California
    Posts
    36
    Thumbs Up/Down
    Received: 7/0
    Given: 1/0
    Rep Power
    0
    My amp circuit schematic (see atttachment) AB763 says that my plate voltage should be 415vdc and the B+ should be 420vdc. My previous voltages were 5v too high so today I adjusted the grid bias again to lower the plate voltage and current. I dropped it more in line with the schematic voltages and got my plate voltages to 416.5vdc, which gave me about 19.5-20ma, in line with recommendations here. The power calcs ran about 7.5Watts and the plate dissapation ran 67-70%. Is that good/bad/ugly? I'm still a little confused & hopefully someone can clarify a few things. When I adjusted the bias pot, ie control grid voltage, BOTH the plate voltage AND the B+ went up or down together. Isn't the B+ supposed to be fixed allowing the grid adjustment to vary plate current? Also, in using the OT Resistance Method of calculating bias, the resistance from each plate to the OT center tap (B+) is supposed to also be fixed but I found there to be a small difference (5-6 ohm) in measurements a few days ago. Really excited to hear the amp tonight!
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	AB763Schematic.jpg 
Views:	8 
Size:	488.8 KB 
ID:	54927  

    0 Not allowed! Not allowed!

  10. #10
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    6,186
    Thumbs Up/Down
    Received: 1,110/1
    Given: 880/1
    Rep Power
    16
    As a general rule, any unregulated power source (B+ in this case) will fluctuate with current draw on the supply. More current = less voltage and visa versa. That is normal.

    0 Not allowed! Not allowed!
    ďYeah, well, you know, thatís just, like, your opinion, man.Ē

  11. #11
    Member
    Join Date
    Jul 2019
    Location
    Sacramento, California
    Posts
    36
    Thumbs Up/Down
    Received: 7/0
    Given: 1/0
    Rep Power
    0
    Ohhh, so if I want to lower the bias, I have to raise the plate voltage & visa versa?

    0 Not allowed! Not allowed!

  12. #12
    Stray Cap DrGonz78's Avatar
    Join Date
    Oct 2011
    Location
    Scottsdale, AZ
    Posts
    2,087
    Blog Entries
    1
    Thumbs Up/Down
    Received: 294/5
    Given: 206/0
    Rep Power
    10
    Hence the term voltage sag when you're really jamming hard on an amp. The current is flowing and thus the B+ sags. Ebb and flow. Of course just setting bias shows the idle condition of this relationship. It's good to learn these concepts in real time while servicing an amp and then you really start to understand what an entire circuit is doing.

    1 Not allowed! Not allowed!

  13. #13
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,069
    Thumbs Up/Down
    Received: 2,114/4
    Given: 2,403/0
    Rep Power
    30
    Quote Originally Posted by Perkinsman View Post
    Ohhh, so if I want to lower the bias, I have to raise the plate voltage & visa versa?
    Not exactly. You wouldn't use bias current to adjust voltage. That could leave you with a dangerous bias condition. What's being said is that voltage will change with current. So, for a given bias condition there will be a corresponding voltage that will be different for every bias condition. It's incidental to the circuit with bias current being the only adjustable parameter.

    2 Not allowed! Not allowed!
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  14. #14
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,910
    Thumbs Up/Down
    Received: 1,433/1
    Given: 837/2
    Rep Power
    4
    I found there to be a small difference (5-6 ohm) in measurements a few days ago
    First of all a difference of 6 Ohm with respect to 231 Ohm/226 Ohm total means < 3% and is negligible.

    But to answer your question:

    1) The DCRs of the OT primary halves may differ up to 20%, depending on transformer construction.

    2) You may get somewhat different DCR readings at different times. Reason is that copper resistance increases with temperature (by ca. 4% per 10įC) and a cold transformer has a lower DCR. For very precise measurements DCR and voltage drop readings should be taken at the same (warm) transformer temperature (means you have to be quick).

    1 Not allowed! Not allowed!
    - Own Opinions Only -

  15. #15
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,608
    Thumbs Up/Down
    Received: 1,516/23
    Given: 3,837/11
    Rep Power
    22
    Quote Originally Posted by Chuck H View Post
    Not exactly. You wouldn't use bias current to adjust voltage. That could leave you with a dangerous bias condition. What's being said is that voltage will change with current. So, for a given bias condition there will be a corresponding voltage that will be different for every bias condition. It's incidental to the circuit with bias current being the only adjustable parameter.
    This is very important. Bias must never be used as a method of adjusting B+ voltage. It is used to adjust idle current and the effect on the supply voltage is incidental.

    If you think your B+ is too high, you must consider a few things. One is that all voltages are specified as + or - 20%.
    Also that the voltages are given with a specific line voltage that you may not be applying.
    Line voltages have greatly increased since these amps were made. In my area we are running about 125VAC from the wall, many of these schematics were drawn up with 117VAC power line.
    So all voltages in the amp may be high if the AC line is higher than schematic shows.
    High B+ can be changed by a few methods, (zeners, 'sag' resistors, etc.) but if it is caused by high AC source, the better option is to install a 'bucking' transformer in the primary side of the power transformer to drop the AC primary voltage to the vintage specified level.

    2 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  16. #16
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,608
    Thumbs Up/Down
    Received: 1,516/23
    Given: 3,837/11
    Rep Power
    22
    Also,

    Quote Originally Posted by Perkinsman View Post
    My previous voltages were 5v too high so today I adjusted the grid bias again to lower the plate voltage and current. I dropped it more in line with the schematic voltages and got my plate voltages to 416.5vdc, which gave me about 19.5-20ma, in line with recommendations here.
    Idle current and plate voltages are inverse, increasing one decreases the other. So if you decreased your idle current, your plate voltage and B+ should have increased.
    I thought your idle current was higher than 20mA before, so I'm not sure how you could have decreased idle current and B+ voltage at the same time. You should recheck your numbers.

    1 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  17. #17
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,069
    Thumbs Up/Down
    Received: 2,114/4
    Given: 2,403/0
    Rep Power
    30
    I've wondered at times why no one has ever used the zener across a cathode resistor thing to reduce "working" volts in amps like the DR. Just add a cathode resistor that biases 21V or 22V and put a 20V zener across it to fix the voltage. You could just use the zener, but resistors are good, durable buffers for such a circuit. So if you have 430V and add such a circuit your "working voltage" (the figure that matters to the tube) is now 410V. Take up the bias slack with the existing bias circuit, perhaps slightly modified to get a lower -voltage. Easy Peazy.

    0 Not allowed! Not allowed!
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  18. #18
    Member
    Join Date
    Jul 2019
    Location
    Sacramento, California
    Posts
    36
    Thumbs Up/Down
    Received: 7/0
    Given: 1/0
    Rep Power
    0
    My mistake, I rechecked today and you are correct, my plate voltage went up to 431.5v. The schematic states that plate voltage should be 415v, but because of this inverse relationship, I had to increase it in order to get the current down. +/- 20% would allow from 373.5 to 456.5, so I'm good! Thanks for the assistance, my amp sounds a little quieter but it's got a great blackface tone!

    2 Not allowed! Not allowed!

  19. #19
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,693
    Thumbs Up/Down
    Received: 434/1
    Given: 408/2
    Rep Power
    18
    Whenever you put an amp on the bench to check its operation, the first measurements to be made should be the mains and heater Vac; the ratio between the actual mains voltage and the nominal voltage that the transformer's primary was intended for will be applied to all secondary voltages.
    If the precise intended primary voltage is unknown (eg USA may be 110, 115, 117, 120), the reciprocal of the ratio between the actual heater voltage (with tubes fitted!) and its nominal, when multiplied by the actual mains voltage, will provide an accurate indication.

    2 Not allowed! Not allowed!
    Last edited by pdf64; 08-29-2019 at 06:38 PM.

  20. #20
    Member
    Join Date
    Jul 2019
    Location
    Sacramento, California
    Posts
    36
    Thumbs Up/Down
    Received: 7/0
    Given: 1/0
    Rep Power
    0
    Quote Originally Posted by pdf64 View Post
    Whenever you put an amp on the bench to check its operation, the first measurements to be made should be the mains and heater Vac; the ratio between the actual mains voltage and the nominal voltage that the transformer's primary was intended for will be applied to all secondary voltages.
    If the precise intended primary voltage is unknown (eg USA may be 110, 115, 117, 120), the reciprocal of the ratio between the actual heater voltage (with tubes fitted!) and its nominal, when multiplied by the actual mains voltage, will provide an accurate indication.
    Is the reason for that to understand why the measured voltages may be different than the schematic? I noticed on the AB763 schematic that there wasn't a stated mains voltage. My home voltage tested at 120.5VAC, which I'm sure was a bit higher. Btw, I liked your song, "Unlucky in Love", nice job!

    0 Not allowed! Not allowed!

  21. #21
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,693
    Thumbs Up/Down
    Received: 434/1
    Given: 408/2
    Rep Power
    18
    Unfortunately the intended mains voltage isn't always noted on every vintage Fender schematic or layout (might be on either, both or neither); however, by checking others contemporary to that of interest, we can be reasonably certain what it was. In this case, where noted, from the late 50s to the late 60s, most all Fender have 117Vac as the nominal mains, eg see the non reverb Deluxe AB763 https://el34world.com/charts/Schemat...763_layout.pdf

    120.5/117 indicates that all PT secondary voltages (and directly related Vdc) will be about 3% high, all else being equal; hence expect ~433Vdc at the standby switch / reservoir cap / OT CT HT node.

    The high idle current that your power tubes were pulling will have dragged that down, hence you measured ~425Vdc; but of course your mains may not have been 120.5 at the time of that measurement, hence the importance of checking it each time other measurements are being made.

    Thanks for the kind words

    2 Not allowed! Not allowed!
    Last edited by pdf64; 09-01-2019 at 10:12 AM.

  22. #22
    New Member
    Join Date
    Sep 2019
    Posts
    1
    Thumbs Up/Down
    Received: 1/0
    Given: 0/0
    Rep Power
    0
    First post here so Hello to all.

    One thing I've done over the years to lower the plate voltage in vintage (or any) amp is to substitute a 5R4 rectifier. In the DR for example it has a 5AR4 rectifier which has little DCV drop across it. By substituting the 5R4 you effectively lower the plate voltage by around 20VDC IME. The DC drop across the 5R4 is a lot higher than the 5AR4 so you will get sag but I generally like the effect on my amps. I use them in PR and DR circuits all the time. Since they are not used much you can purchase NOS ones very cheaply.

    1 Not allowed! Not allowed!
    Last edited by 67super; 09-01-2019 at 07:39 PM.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Replies: 27
    Last Post: 04-07-2017, 06:06 PM
  2. Rebiasing Original Fender Blues Deluxe
    By TheGrandWazoo in forum Maintenance, Troubleshooting & Repair
    Replies: 10
    Last Post: 12-23-2010, 12:40 PM
  3. QUESTION: Biasing a '65 Reissue Deluxe Reverb
    By Mango Moon in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 10-09-2008, 02:30 AM
  4. Troubleshoot BF Deluxe Reverb with original parts
    By jlatrace in forum Vintage Amps
    Replies: 1
    Last Post: 07-02-2007, 05:39 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •