Page 4 of 4 FirstFirst 1234
Results 106 to 116 of 116

Thread: Looking for Supro Thunderbolt 6420 Build info

  1. #106
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,333
    Thumbs Up/Down
    Received: 133/0
    Given: 2,106/0
    Rep Power
    4
    Quote Originally Posted by Helmholtz View Post
    I would at least twist the grid wires. As the grid signals are opposite phase/polarity twisting will provide some shielding and produce a little capacitance (maybe 10 to 20pF) between the wires which may help to avoid oscillation.
    Thanks, will do. Should these leads also run along the chassis, or is it better to make them shorter 'flying'?

    0 Not allowed! Not allowed!
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  2. #107
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,333
    Thumbs Up/Down
    Received: 133/0
    Given: 2,106/0
    Rep Power
    4
    Quote Originally Posted by tubeswell View Post
    Will hum less if it’s connected directly to the ground lead of the reservoir cap. I suggest following the grounding layout in Merlin Blencowe’s grounding article. (Especially Fig 15.12 and Fig 15.14) http://www.valvewizard.co.uk/Grounding.pdf
    Thanks, reading now!
    <edit>

    OK I have the main ideas, but run into something Im not sure about (and probably get wrong). I got the "local star" concept. A filter cap that feeds a section of the amp, should have its negative lead tied to the 'grounds' in that section. Not sure what to do about what Merlin calls 'daisy chain', though. I think this is what I got wrong in the Fender Deluxe Reverb I hacked together.

    So, if I put a few lugs on the chassis, for this amp I think 3, would that work like Merlin's drawing? One for the preamp, one for PI one for power tubes, and one power supply + first reservior cap. There would be no wire between the local stars, the chassis would serve that purpose?

    0 Not allowed! Not allowed!
    Last edited by mikepukmel; 07-13-2019 at 12:25 AM.
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  3. #108
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,333
    Thumbs Up/Down
    Received: 133/0
    Given: 2,106/0
    Rep Power
    4
    So, if there were 3 lugs, one PS, one PI and one for the 'quiet stuff', with the quiet lug mounted at the far end of the chassis, ...

    Click image for larger version. 

Name:	suprothunderbolt_hand_drawn_layout.jpg 
Views:	4 
Size:	88.7 KB 
ID:	54297

    0 Not allowed! Not allowed!
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  4. #109
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,333
    Thumbs Up/Down
    Received: 133/0
    Given: 2,106/0
    Rep Power
    4
    One last question This amp will go close to 500v with no load, which is close to the filter cap rating. Ive seen bigger amps, like Fender Twin, use two caps in series, and 'balance' resistors. The AB763 used two 70uf, 350v. caps. They put two 220k 1W resistors across each cap.

    Question: how did they calculate that they needed 220k 1w resistors, and won't those resistors suck power from the PT that would not be used in an amp that didn't have these resistors? I.e. they had to up the windings a bit to account for these losses (to heat)?

    Thanks!

    0 Not allowed! Not allowed!
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  5. #110
    Supporting Member tubeswell's Avatar
    Join Date
    Feb 2007
    Location
    Wellington NZ
    Posts
    4,177
    Blog Entries
    1
    Thumbs Up/Down
    Received: 102/0
    Given: 33/0
    Rep Power
    18
    Quote Originally Posted by mikepukmel View Post
    Thanks, reading now!
    ... Not sure what to do about what Merlin calls 'daisy chain', though. I think this is what I got wrong in the Fender Deluxe Reverb I hacked together.

    So, if I put a few lugs on the chassis, for this amp I think 3, would that work like Merlin's drawing? One for the preamp, one for PI one for power tubes, and one power supply + first reservior cap. There would be no wire between the local stars, the chassis would serve that purpose?
    Not quite. In the 'daisy chain' system Merlin describes, all the grounds are daisy-chained together into one ground buss/wire, but only the input stage end of this daisy chain ground wire is attached to the chassis. The output tube end of the ground wire is floating (because it it weren't, you'd end up with a ground loop. Similarly, if you split the daisy chain up into components, and grounded each section separately, you'd be more likely to get ground loop hum, depending on whereabouts the chassis mains ground was situated w.r.t. each of the other signal stage grounds in the split-up ground).

    The idea behind the daisy chain is that the most sensitive pre-amp stages get a ground that is as-close-as-possible to chassis ground potential, and the other higher-current signal ground returns are further away from this, so that the daisy chain acts like one long (albeit low-resistance) voltage divider, and therefore any micro-rises in ground potential along this voltage divider caused by the high-current ground returns, will be smaller nearest to where the chain is connected to the chassis, so that there will be less interference (through filter cap reverse-shunting noise in) to the power supply rail of the most noise-sensitive ground returns in the input end of the pre-amp.

    1 Not allowed! Not allowed!
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

  6. #111
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    2,106
    Thumbs Up/Down
    Received: 419/4
    Given: 242/0
    Rep Power
    16
    Quote Originally Posted by mikepukmel View Post
    One last question This amp will go close to 500v with no load, which is close to the filter cap rating. Ive seen bigger amps, like Fender Twin, use two caps in series, and 'balance' resistors. The AB763 used two 70uf, 350v. caps. They put two 220k 1W resistors across each cap.

    Question: how did they calculate that they needed 220k 1w resistors, and won't those resistors suck power from the PT that would not be used in an amp that didn't have these resistors? I.e. they had to up the windings a bit to account for these losses (to heat)?
    500V across 440k is just over 1mA. The PT won't notice that.

    The value of the balance resistors is chosen to make the current through them much greater than the capacitor leakage current. If the cap leakage current is 10uA then it's only 1% of the resistor current and won't affect the voltage division and 1mA won't waste too much power so there's no need to go below 220k.

    2 Not allowed! Not allowed!

  7. #112
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,615
    Thumbs Up/Down
    Received: 1,203/1
    Given: 721/2
    Rep Power
    4
    Problem with leakage current is that there is wide variation and often little information available.
    Leakage current depends on capacitance, rated voltage, working voltage, operating temperature and the cap's operating history.

    For a general purpose 100/500V E-cap the formulas in the Siemens/Epcos literature give a typical leakage current of 53A at 20C. Operation at 50C increases the value by a factor 4.
    The same cap is considered acceptable as long as its leakage current is < 590A @ 20C or < 1470A @ 35C measured at its rated voltage. Operation below rated voltage strongly decreases leakage, e.g. at 50% of rated voltage leakage is lower by a factor 7.

    After longer discharged periods initial leakage currents may be up to 100 times higher than normal for several minutes.

    Of course in a series arrangement only leakage mismatch really matters.

    Still, 220k may not always be low enough for divider resistors to ensure equal cap voltages.

    Marshall amps often use 56k/2W dividers.

    2 Not allowed! Not allowed!
    - Own Opinions Only -

  8. #113
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,615
    Thumbs Up/Down
    Received: 1,203/1
    Given: 721/2
    Rep Power
    4
    Quote Originally Posted by mikepukmel View Post
    Thanks, will do. Should these leads also run along the chassis, or is it better to make them shorter 'flying'?
    I consider it more important to run the OT leads close to the chassis and keep the grid wires distant from the OT wires.
    Also the OT wires connecting to the plates should be as short as possible and be twisted as long as it makes sense to minimize field radiation.

    1 Not allowed! Not allowed!
    Last edited by Helmholtz; 07-13-2019 at 05:18 PM.
    - Own Opinions Only -

  9. #114
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,333
    Thumbs Up/Down
    Received: 133/0
    Given: 2,106/0
    Rep Power
    4
    Quote Originally Posted by Helmholtz View Post
    I consider it more important to run the OT leads close to the chassis and keep the grid wires distant from the OT wires.
    Also the OT wires connecting to the plates should be as short as possible and be twisted as long as it makes sense to minimize field radiation.
    Thanks!

    0 Not allowed! Not allowed!
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  10. #115
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,333
    Thumbs Up/Down
    Received: 133/0
    Given: 2,106/0
    Rep Power
    4
    Quote Originally Posted by tubeswell View Post
    Not quite. In the 'daisy chain' system Merlin describes, all the grounds are daisy-chained together into one ground buss/wire, but only the input stage end of this daisy chain ground wire is attached to the chassis. The output tube end of the ground wire is floating (because it it weren't, you'd end up with a ground loop. Similarly, if you split the daisy chain up into components, and grounded each section separately, you'd be more likely to get ground loop hum, depending on whereabouts the chassis mains ground was situated w.r.t. each of the other signal stage grounds in the split-up ground).

    The idea behind the daisy chain is that the most sensitive pre-amp stages get a ground that is as-close-as-possible to chassis ground potential, and the other higher-current signal ground returns are further away from this, so that the daisy chain acts like one long (albeit low-resistance) voltage divider, and therefore any micro-rises in ground potential along this voltage divider caused by the high-current ground returns, will be smaller nearest to where the chain is connected to the chassis, so that there will be less interference (through filter cap reverse-shunting noise in) to the power supply rail of the most noise-sensitive ground returns in the input end of the pre-amp.
    Awesome, thanks, reading and re-reading. I think I got it.

    (Sorry, have to get used to a single "thanks everyone" rather than one for each post to avoid so many thank you posts. Read all and re-read, great info.).

    0 Not allowed! Not allowed!
    Last edited by mikepukmel; 07-14-2019 at 09:05 PM.
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  11. #116
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,333
    Thumbs Up/Down
    Received: 133/0
    Given: 2,106/0
    Rep Power
    4
    I found this transformer, closest to the HV winding voltages recommended. The others were either too high hv voltage, or way undersized re HV secondary current rating. Is there any problem using this transformer and tying off the 5v winding? Would this work ok for the weatherford schematic?

    Size: 236 VA
    Dual Primary: 115 VAC, 125 VAC, 60 Hz.
    A.C. High Voltage Secondary RMS: 600V.C.T. @ 287mA
    Filament Winding #1 5 V C.T. @ 4 A
    Filament Winding #2 6.3 V C.T. @ 8 A

    https://www.tubesandmore.com/product...0-300-v-287-ma

    0 Not allowed! Not allowed!
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. 1960's Supro Thunderbolt scratch build copy
    By mort in forum Guitar Amps
    Replies: 45
    Last Post: 06-03-2019, 01:52 AM
  2. Replies: 2
    Last Post: 01-01-2019, 11:19 PM
  3. Supro 'BIG STAR' S6451 TR guitar amp NEED INFO/PRICE
    By Kemaelgymaeyth in forum Vintage Amps
    Replies: 0
    Last Post: 11-05-2014, 10:03 PM
  4. Supro Thunderbolt 6420 question
    By retrorod in forum Maintenance, Troubleshooting & Repair
    Replies: 3
    Last Post: 05-21-2014, 10:33 AM
  5. Supro Thunderbolt cabinet info?
    By jaydawg in forum Cabinetry
    Replies: 0
    Last Post: 09-23-2009, 03:29 PM

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
  •