1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Mojotone Studio One inspired build

Discussion in 'Shock Brother's DIY Amps' started by reggiepe, Oct 2, 2020.

  1. reggiepe

    reggiepe TDPRI Member

    Posts:
    36
    Joined:
    May 12, 2015
    Location:
    Pennsylvania
    Hey all,

    I am building a low watt amp that will take features from the Mojotone Studio One. Both preamp channels will pretty much be built as Mojotone had laid it out. I will remove the channel switching relay and have the channels mix with a pair of 220k resistors feeding the output section. However I have a pair of 12BH7's that I would like to use, so there will be a little more output from the cathode biased output section.

    The question I have is when adding a second 12BH7, do I still send the B side of the triode grids to ground on both of them when they run in parallel?

    It begs a 2nd question as well....when adding the second 12bh7, does the reflected impedance go down? Therefore running an 8 ohm load off of the 16 ohm secondary?

    I have attached Mojo's Schem for the studio one.
     

    Attached Files:

  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,910
    Joined:
    Dec 30, 2019
    Location:
    california
    Yes.
    Yes. the tubes together would want to see half the primary load of the original design.
    If I am looking at it correctly, operating without the switch will have the two channels out of phase.
     
    reggiepe likes this.
  3. reggiepe

    reggiepe TDPRI Member

    Posts:
    36
    Joined:
    May 12, 2015
    Location:
    Pennsylvania
    Thanks @Lowerleftcoast

    I see many many low power amp designs with 12BH7's and it confuses me with Mojo's using a 10k primary OT with a 4,8, and 16 secondary and others I see using 22k primaries. I have heard Mojo's studio one and it sounds great with their 10k. If I cut that in half with Mojo's transformer, it will be 5k with 2.4.and 8 ohm secondary's. Or with other's with 22k, down to 11k with 2.4. and 8 ohm secondary's.

    You mention the channels being out of phase...if I am thinking right, that shouldn't be an issue as long as I don't use the channels together. The 220k I mention was just copying what fender does to mix their channels in a Deluxe Reverb for instance. I believe the normal and vibrato channels are out of phase in that amp, if I'm not mistaken.
     
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,910
    Joined:
    Dec 30, 2019
    Location:
    california
    The micro amps are interesting. I am not sure why the primaries are recommended in the 22k range. Maybe it is because a reverb trannie is used as the OT? Maybe it fits a graph?
    I would listen to it with ~10k vs less and try to hear a favorable difference.
    That would require another input jack. You could put it on a switch or leave it as a relay. Not much difference in time or money for whatever you decide.
     
    reggiepe likes this.
  5. reggiepe

    reggiepe TDPRI Member

    Posts:
    36
    Joined:
    May 12, 2015
    Location:
    Pennsylvania
    I think I will build it in an 18 watt TMB chassis and then can split the channels with their own inputs. That will take care of the phasing. Is 220k the recommended size for the mix resistors for a reason? I would think that the lower the size of resistor, the lower the amount of cut of the signal.
     
  6. reggiepe

    reggiepe TDPRI Member

    Posts:
    36
    Joined:
    May 12, 2015
    Location:
    Pennsylvania
    This paralleling 2 12BH7's is driving me bananas. Anyone have any experience with wiring up in this manner? I duplicated the 12BH7 in the above schematic and I get low frequency oscillation. I pull one or the other out and it is fine, but in as a pair causes this. I tied them together with pin 1 on one to pin 1 on the other and pin 6 to pin 6. Brought the signal into 7 on both.
     
  7. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    Hey, are you trying to parallel 2 sets of 2 for a total of 4 valves in the power amp? Just want to make sure I understand the game plan before offering an opinion.
     
  8. reggiepe

    reggiepe TDPRI Member

    Posts:
    36
    Joined:
    May 12, 2015
    Location:
    Pennsylvania
    @andrewRneumann

    2 Valves total.... 4 triodes. I described the oscillation incorrectly. It is a loud hum that sounds like a guitar cord lying on the ground.....open circuit. I put a single 12BH7 into either socket and it sounds fine. Put a pair in and it starts up after the tubes warm up. When playing them individually, I have swapped the tubes to make sure it wasn't the tubes. I also did this with a pair of EC99's and have the same result.

    Both sockets work....check
    V4 and V5 are tied together via Pin 1 and Pin 6....check
    Signal input comes into pin 7 on both tubes....check
    Pin 2 grounded....check
    Cathodes are tied to 470R to ground...check
     
    andrewRneumann likes this.
  9. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    Thanks, I understand now.

    And you have jumpers on each individual socket? 1 tied to 6, 2 tied to 7, and 3 tied to 8?

    That cathode resistor should be halved. Do you have a 3W 240R? This is probably not the solution to the hum problem, but your bias voltage will be off if you don’t change it.
     
  10. reggiepe

    reggiepe TDPRI Member

    Posts:
    36
    Joined:
    May 12, 2015
    Location:
    Pennsylvania
    @andrewRneumann

    Forgot to add that I have 470R's on each cathode. I don't have a jumper from 3 to 8 on the sockets. I get ~10v across each of the 4 cathode resistors.
     
    andrewRneumann likes this.
  11. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    Ahh, I thought you might have created 2 “super-triodes” by jumpering right on the socket. I think I understand what you have done from your description. The last question for now is how are the anodes wired? Do those have jumpers?

    The easiest thing for the group would be to see a photo of how you wired it. And a schematic to go with.

    I don’t know much about the 1580 OT. Did you confirm it can handle twice the current and twice the wattage?

    If you have 4 individual cathode resistors, then each should be 1K. I would combine 2 pairs of cathodes to keep using the 470R resistors. 2 cathodes per each 470R.

    EDIT: All cathodes should be combined, and then send the whole thing through 2 470R in parallel for an effective resistance of 235R.
     
    Last edited: Oct 11, 2020
  12. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    The cathodes need to be all at the same potential before hitting any resistors. You have them all free to do their own thing.
     
  13. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    image.jpg
     
  14. reggiepe

    reggiepe TDPRI Member

    Posts:
    36
    Joined:
    May 12, 2015
    Location:
    Pennsylvania
    @andrewRneumann

    Anodes are tied together V4 pin 1 to V5 Pin 1 and V4 Pin6 to V5 Pin6. I am using a Hamond 1760F for this build. It is a 15 watt transformer with a 6950 primary to a 4,8,16 ohm Secondary. How did I end up using this? Well the 1580 was out of stock and has been for quite some time. So, I used an old Blues jr. transformer that put me in the ballpark for impedance needed for 2 tubes.....and it sounds great. So I ordered the Hammond so that I can have multi taps and can change the reflected impedance.

    For grins and giggles, I have tried other unsuccessful methods of wiring the plates too:

    V4 Pin1 gets the brown OT wire and V5 Pin 1 gets the blue OT wire with a jumper on each of them over to Pin 6. Barely any audio, but no hum.
    I have crossed over the jumpers for V4 and V5. Sending V4 Pin 1 to V5 Pin 6 and V5 Pin 6 to V4 Pin1. Barely any audio, but no hum.

    Here is something I haven't tried yet, but with everything I have tried it seems it may be the source. Pin 2's (Grid) on each of the sockets are tied to ground. (I was told that this is needed due to the way self split works) I have desoldered the input wires (pin7) to have the 12BH7's totally isolated (no change) So, it would seem that that hum is being injected from ground into one of the grids V4 Pin 2 or V5 Pin2. AND...the signal injection only happens when one tube is used. Drop in both...the hum returns.

    I will draw up a schematic this evening and try to get a good pic of the sockets as well.

    I will also tie the cathodes together to see if there is any difference.

    Thank you!
     
    andrewRneumann likes this.
  15. dan40

    dan40 Friend of Leo's

    Posts:
    2,100
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    A redplating power tube will also cause the same kind of hum through the speaker. Are you seeing any signs of redplating on the plates of each triode?

    Is this a single ended output stage or push pull?
     
  16. reggiepe

    reggiepe TDPRI Member

    Posts:
    36
    Joined:
    May 12, 2015
    Location:
    Pennsylvania
    @dan40 This is a dual self split power section. Essentially 4 triodes.

    I don't see any signs of redplating. Turned all of the lights off to see. From my calculations and Robrob's bias calculator I am running at 93% for a 12bh7 and 65% for an ECC99. That is with 304v on the plates and 10.1 volts across the 470 cathode resistor. This only happens with a pair of 12BH7's or a pair of ECC99's. Individually in either V4 or V5 socket there is zero hum.
     
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,910
    Joined:
    Dec 30, 2019
    Location:
    california
    Are you wiring the heaters out of phase? On SE parallel they should be out of phase.
     
    dan40 likes this.
  18. dan40

    dan40 Friend of Leo's

    Posts:
    2,100
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    I may have this wrong but I believe that Rob's bias calculator assumes the cathodes are tied together. Measuring each cathode separately (because of individual cathode resistors) means that you will actually be drawing twice as much current as the calculator is telling you.

    Edit; I am only assuming this because most of his Micro designs show the cathodes tied together with a single resistor for both triodes.
     
    andrewRneumann likes this.
  19. dan40

    dan40 Friend of Leo's

    Posts:
    2,100
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    I believe this is a self split/push pull output. I was wondering the same thing about the phase relationship.
     
  20. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    Awesome on the OT. I think it should work. Getting the right impedance speaker on the right tap can be confusing, so we could also talk through that again if you have questions.

    Here’s my drawing built out. I think you are very close to this, but without the cathodes all tied together. *I think* the cathodes being tied together is an essential part of this “differential amplifier.”

    I looked up the 12BH7 and it has a 1M grid leak maximum for cathode bias, so I drew that in as half the amount for two tubes.

    The cathode resistor size is based on the mojo schematic you provided. After you solve the issue at hand, you may need to adjust for the plate voltage you are running. I assume this amp operates in Class A so you should be looking for center bias.

    image.jpg

    Realized I left out the cap between OT primaries. You still get the idea...

    Good luck, I really hope you can get this to work!
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.