Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups darrenriley.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Fender LTP Values?

Discussion in 'Amp Tech Center' started by Bill Moore, Sep 14, 2018.

  1. Bill Moore

    Bill Moore Tele-Holic

    I have started on a Bassman 10 project, I couldn't find a layout anywhere, but after contacting MojoTone, they supplied one. Then of course, I felt obliged to let them build a turret board, and it is nice, (but blue).
    My question is, why the odd, (100/91 ohm), resistor value in the LTP? Why not stay with the common 100/82 used in most Fender circuits, (that I have experience with)? I did find the 91 ohm value, and ordered some, but just wondering why the difference.
    Bassma 10 Layout.gif
     
    Last edited: Sep 14, 2018

  2. archetype

    archetype Fiend of Leo's Ad Free Member

    Jun 4, 2005
    Williamsville NY
    As I am ignernt... what's an LTP?
     

  3. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    Those don't appear to be the original fender values on the layout, the schematics have 47K 47K.

    Why 91 instead of 82?...well the reason for having different values was the thought that it would compensate for the differing amplifiication from the two stages of the PI, and the 91K used instead of a 82K in combination with the100K, will, when used with a 12AX7, actually gets you closer to this unity, but I believe the 91K value was not a standard value for regular cheap, carbon compound resistors.
    I recall reading the math behind this somewhere, and will see if I can find it. Interesting to see if it works out as well for a 12AT7.

    But why change the values on the layout? Maybe to make it more suitabe as a modern guitar amp, rather than a bass amp. I'm surprised that they did not change the 12AT7 tube to a 12AX7 where these resistor values would be more normal, sort of a half-hearted decision.
     
    Last edited: Sep 14, 2018

  4. Bill Moore

    Bill Moore Tele-Holic

    I have not really compared the values to the schematic, but it too is difficult to read!
    I am building it as a bass amp!
     

    Attached Files:

    Last edited: Sep 14, 2018

  5. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    There is also this version of the schematic
    upload_2018-9-14_19-7-32.jpeg
     

  6. VintageSG

    VintageSG Friend of Leo's Ad Free Member

    Mar 31, 2016
    Yorkshire
    A type of phase inverter.

    Compare how V3 is used in the Bassman schematic to how V2, particularly V2b is used in the attached schematic using a cathodyne.
     

    Attached Files:


  7. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
    28
    Feb 22, 2009
    New York
    IMO, if you're going bass amp, you want the current driving properties of the 12AT7 LTPI, over the AX7. For this, it's usually preferable to use lower plate values like 47k-51k. Also, with the lower output impedance of the PI with the AT7, you'll get a bit cleaner and punchier bass response. If you want to take it a step further, you can add cathode follower stages between the PI and output tubes like on the Fender Studio Bass and Super Twin 180 amps, which will further bring the output impedance down. This will also limit, or eliminate 'blocking distortion' when the amp is being pushed hard with a lot of low end through it, so you can keep a wide bandwidth, through the amp, without 'farting out'
     
    robrob likes this.

  8. Bill Moore

    Bill Moore Tele-Holic

    Thank you for the nicer schematic, Bendyha!
    I was going over my parts according to the layout, but now I will have to correlate with the schematic. I guess I will have some odd value parts for another project!
     

  9. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    Just remember, with 47K/47K plate load resistors, the V3 12AT7 pulls more current than it would with the traditional 100K/81K plate resistors.
     
    Bill Moore likes this.

  10. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    ^^ which means you need to use higher rated plate resistors, probably 2W would be your minimum, 4W would be better, and hardly any bigger.
     
    robrob, Old Tele man and Bill Moore like this.

  11. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Age:
    66
    Mar 2, 2003
    Lawndale CA
    The caps are also different on the schematic - .1uf, not .022uf.

    You can't build it from the layout. The layout doesn't show the filter caps, the filament wires, "real" ground connections nor important lead dress details.

    Layouts can be useful as "quick reference documents" when performing service, but they are not meant to be used in amp construction. Even when combined with a schematic you're often missing the filament wiring - and there are specific conventions that need to be followed in all areas of lead dress, none of which are drawn on either. Layouts often lack dates, making it difficult to determine if the version was a very early one or came several iterations later.

    Some have assembled tweed amps from kits using a layout - but that's akin to building a plastic model kit.

    As far as differing parts values in the Bassman 10, it was not very useful as a bass amp and like other amps there were several interim changes not documented by a model code change. @Bendyha made a good point about the layout shown possibly being thought of as more of a guitar amp, or a "crossover". While I don't recall parts values on the ones I've worked on or the couple I've owned, with the right speakers and some tweaks they could be fun low-gain, chunky-sounding amps for rhythm guitar.

    If you intend to use it for bass - have you ever played through one at the volume level you need? If not, I strongly suggest putting off building it until you've heard what they sound like. In my experience (including as a working bass player) I've never encountered a bass player that would use one voluntarily.:eek:
     

  12. wanderin kind

    wanderin kind Tele-Meister

    Age:
    63
    253
    Apr 24, 2018
    Olympia WA
    the resistor values are not that critical. you could even use two 100K resistors and would be hard pressed to hear any difference. most dual triodes do not have the best matching as far as gain anyway. the output transformer will make up for any differences in power tube signals being different due to inverter errors.

    any blackface Fender will have plate resistors that have drifted upwards of 15% anyway.

    heck, this 1961 Silvertone 1472 used 20% tolerance resistors, add 55 years to that and they are more like 30%. that is the beauty of tubes, it really don't matter that much.

    https://groupdiy.com/index.php?topic=69965.0
     

  13. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    Really?...more than doubling, from 47K to 100K...I believe that is more than enough to notice.
     

  14. wanderin kind

    wanderin kind Tele-Meister

    Age:
    63
    253
    Apr 24, 2018
    Olympia WA
    looks like he has the "blackface" version...


    r.jpg
     

  15. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    Ah yes....with the introduction of the Bassman 10 in 1972, it was a Silverface. The ca. 1977 change over to the Ultra-Linear output transformer for this model, (and others) was around the time when they started blacking-up again..............not real Blackfaces as such.............. #cultral (mis)appropriation.

    The "real" schematic for the UL 10 is something I have not seen, only some not completly trustworthy alterations to the original...and they show 2 X 47K...whereas the layout above of the UL version, most definitly also an altered version of something else does have a 91K, 100K mix....which would sound no different from a 100K, 100K set-up.
    Maybe versions of both exist, not uncommon for a Fender....there are probably versions built with other slight anomalies.
     
    Bill Moore likes this.

  16. the fatch

    the fatch TDPRI Member

    Age:
    73
    76
    Dec 19, 2017
    LEEDS UK
    In Merlin’s book (page 162), he writes that the 5F6 Bassman was designed according to hi-fi principles prevailing at the time, and he includes the calculations which Fender would have used to find that for best balance when a 100k plate resistor (Rp2) is used for the non-inverting triode, a plate resistor (Rp1) of 84.25k would be required for the inverting triode, so the closest preferred value of 82k was chosen. The calculations include the internal plate resistance (rp) of the triodes and their amplification factor (mu), the value of the tail resistor and the value of the grid leaks on the power tubes. These calculations have presumably been performed with the values of the components in your amp and resulted in a value close to 91k for Rp2.
    Merlin states that for tail resistors greater than 18k, the balance is worse with the 100k/82k plate resistors. He shows a design for a cathode biased LTPPI using a 12AX7 with a 68k tail resistor and 100k plate resistors. If it is driving 220k power tube grid leaks, the calculations show that a 97.75k plate resistor for the inverting triode would be required for perfect balance.
    Out of interest, substituting the tail resistor in this circuit with E6 preferred values shows:
    Closest
    Tail Ideal E12 E24
    10k 84.25k 82k 82k
    15k 89.05k 82k 91k
    22k 92.41k 100k 91k
    33k 94.97k 100k 91k
    47k 96.56k 100k 100k
    68k 97.75k 100k 100k
     
    Bill Moore likes this.

  17. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    It seems reasonable to expect not much difference between 100/100 and 100/90 and 100/80.


    But what is the affect of going from 100/100 to 50/50?


    On the DC side of things, the plate voltage would go up, due to less loss in the plate resistor.



    On the AC side,


    It was pointed out above that less plate load resistance would mean more (signal) current thru the plate load resistance.



    There is an ohms law relationship in there, probably among other influences,



    V = IR


    The current is more but the resistance is less, so what happens to the signal voltage?



    Less plate load resistance would end up lowering the available signal voltage to pass on to the power tubes, I would think. Is that right?
     

  18. Bill Moore

    Bill Moore Tele-Holic

    Thanks all for the replies, Bendyha's schematic is the best one I've seen for a non-UL circuit. When I contacted Fender, they sent me a '77 UL schematic, and said they have no layout! I suppose I need to study them all, and decide what differences there are between them. (I know I should stick to something that has more precise documentation!) But folks here on the forum, (that are much more knowledgeable than I am), always seem willing to advise/critique those of us with less electronic backgrounds!
    This amp is being put together with a lot of spare parts on hand, and will be overbuilt with the Twin RI PT, and OT, but I think it will supply deep-clean bass, which is the goal. (One friend even suggested using KT88 output tubes!)
    Silverface, I am aware there are more components involved in the circuit, just as there were on my other 3 Fender type amp builds. As for the amp's sound, I started to build a '71 circuit, (like my wife's first bass amp), but on advice from others I consider knowledgeable, I'm going with the Bassman 10 circuit. It will be plenty adequate for the "venues" where we will be jamming. I'm also refurbishing a 4-10 Hartke speaker cab for this project. If she needs more power, she still has the BG-250, with the folded 18/12 cab that has worked in the past in large venues.

    Here is the schematic Fender sent:
    Bassman 10 (1977).pdf
     
    Bendyha likes this.

  19. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
    28
    Feb 22, 2009
    New York
    Yep. But IMO, the PI shouldn't be viewed as a 'gain stage' especially in a bass amp. The two earlier gain stages would provide plenty of voltage gain. Higher current driving properties are preferable for big power and lots of lows, so sacrificing a little voltage gain is not really a big deal.. With a LTPI it's still WAY above unity in most cases you still have a gain amplification factor above 20. (I'm getting gain factor of 23 w/ 47k and 26 w/100k, on a calculator, so we're talking minimal differences, as well)



    As far as the bassman 10.. You may find a UL output transformer to be preferable for bass performance. Might be able to snag someone's UL twin OT for cheap, because guitar players generally hate UL 70's Fenders. (Funny though, no one complains about UL Marshall Majors, Sunns, Hiwatts.. UL isn't the issue, that's why *Wink*)

    Also, I'd tighten up the filtering in that amp, a lot, in all the stages, and including the bias supply. You want the power section pretty stiff, for bass, IMO.
     
    Wally likes this.

  20. wanderin kind

    wanderin kind Tele-Meister

    Age:
    63
    253
    Apr 24, 2018
    Olympia WA
    they use an interesting approach in the Blues Jr, tapping off a voltage divider, looks like this would keep even DC plate voltage on both triode sections,

    blues jr PI.png
     

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.