Carlsbro TR60 non-adjustable fixed bias EL34

Discussion in 'The Workbench' started by StingRay85, Dec 5, 2019.

  1. StingRay85

    StingRay85 Active Member

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    Hey guys, doing my best to learn a couple of things so don't shoot me or assume in an expert (because I'm not). I have recently bought a Carlsbro TR60 combo (actually two but one is used for parts), and found out the power section is somewhat different than what I've already seen in JMP Marshalls. Most importantly there is not a bias pot. I use a TAD tool to check cathode current (which I know is different from plate current), dropped in a fresh pair of TAD tubes, and a bit concerned with the 46-48 mA it's pulling in idle at 470V. According to the calculations and assuming 8W from the screens (correct me when this is wrong), it calculates to just about 70% of max. plate current, which is supposed to be the real limit. I read a very nice and long post on 70% vs. 90% so I understand what this means more or less. So digging a bit deeper to see how this TR60 could be described, I found the term non-adjustable fixed bias (where people also say that doesn't exist as it is still adjustable by changing the resistors). So when looking at the schematics, please correct me when I'm wrong but the addition of a 10k bias resistor in series with the 47k resistor should resolve that.

    Be gentle on me, I'm new to this stuff and eager to learn ;-)

    60tr-&-60tr-twin.jpg

    That seems to be case with the TC100 amp

    100tc.jpg

    So is it really as easy as that, adding the adjustable 10k resistor, and then lowering cathode current by increasing the value of the 47k resistor to e.g. 50 or 52k ohms? Or should I take higher resistance, like 27k which is used in the super lead? Or am I still missing something fundamental here? At this point of my learning curve I have no interest yet in measuring on the plates themselves.
     
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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes if you put another resistor in series with the 47K, it will make the bias cooler.
    You could make the bias adjustable and you can look at the bias adjust circuits on other schematics to do that.
    Yes it seems you are on the right track.
     
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  3. StingRay85

    StingRay85 Active Member

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    Thanks. That's what I will do for now. I also wondered why the TR60 doesn't have a real diode rectifier bridge, while it would probably have been easy to do so as the 4 diodes are there. The TC100 does have the bridge.
     
  4. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    @StingRay85
    You should us a higher value resistance for the bias adjusting potentiometer/trimmer like the 27k of the SuperLead or maybe a 25k or even a 20k.
    10k may get it lower but maybe not low enough and does not leave a range for even lower values if wanted.

    The 60 and 100 models use two different power transformers. The 60 has a center tap with double the voltage at the HT secondary whereas the 100 does not.
     
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  5. ampmadscientist

    ampmadscientist Well-Known Member

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    The diodes in both 60 and 100 are doing basically the same thing.
    It is a configuration difference but with a similar result.
    The diodes will help to turn AC into DC for both type configurations.
     
  6. StingRay85

    StingRay85 Active Member

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    Thanks I start to understand. The grounded center tap makes the difference here. The filter caps smooth out the signal.

    Still the concept of the ground is something I don't fully grasp yet. If I understand correctly, it is the 0V DC line that gets interconnected throughout the amp. So something completely different than earth ground coming from the AC plug, which hopefully the chassis is connected to.
     
  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    In a nutshell, lots of smaller amplifiers can use lower power transformers so they get manufactured with a center tapped HT secondary because they use thinner limited current carrying wire. Why, I do not know because a non-center tapped HT secondary transformer can be used for lower power amplifiers as well utilizing the same bridge rectifier circuit. This type does use less wire but in a larger diameter for more current carrying capability.

    By the way, the center tapped HT secondary transformer is usually used with a normal full wave rectifier design. Both the full wave and the bridge type rectifiers are tied to ground. The full wave is tied at the HT center tap and the bridge is tied at the low side of the rectifier bridge.
     
    Last edited: Dec 6, 2019
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  8. StingRay85

    StingRay85 Active Member

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    Thanks for all the solid info guys.

    As I need to work closely to the filter caps, I want to make sure they are safely discharged when working. When looking at schematics, the 47 µF is in parallel with the 47k resistor, so that should be safe to touch. I wonder about the other caps. On one side they are connected to ground, other side there is a 10W 25k resistor. Is it's function to bleed the caps after disconnecting the amp?

    Also, the amp was modified with a standby switch, I didn't dig deeper into where it is actually position, will do so in a minute. Is it safe to say that best thing is to have the standby off when working on the amp? I read some things about the circuit getting blocked and the caps cannot bleed from their own.
     
  9. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The 47uF is in the bias supply and is not the one(s) you should worry about.
    The 50uF and 20uF of the B+ rail (HT) are the ones that hold high voltage and nothing is installed to quickly bleed them off in this amplifier.
    The 25k 10W is a B+ dropper feeding the reverb/solid state circuit. It does not bleed off high voltage.

    You can strap a 100k 2W from B+ to chassis ground to bleed things.
    Put your meter VDC on the there to see it at or near 0 VDC.
     
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  10. ampmadscientist

    ampmadscientist Well-Known Member

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    The 0 volts is the earth ground, same as the power socket ground, that is connected to the chassis. It's all the same ground / earth connection.
     
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  11. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes in most designs the resistor bleeds the voltage down when the amp is shut off.
    The resistor usually bleeds all the caps down.
    Then disconnect the mains lead from the amplifier when you are working on it.
    (disconnect the power)
    After the power is disconnected it's safer to work on.
     
  12. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    This is the scene right before he gets frozen for 1000 years. Great show.


    [​IMG]
     
  13. StingRay85

    StingRay85 Active Member

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    Gotcha. I made a light bulb setup to check all caps but didn't see a thing. Now I know for sure. Connect between B+ and ground and see what happens
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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  15. StingRay85

    StingRay85 Active Member

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    Ok on topic again ;-)

    One of the issues I noticed with the bias was that the EL34B valves I put in them were just running very hot from themselves. Put in a pair of old Siemens tubes and it's cold again. So I'll stick to those for now.

    Also, I found out the standby switch is in the same line as the choke. When switching in on when turning off the sound, it gives a loud pop to the speaker. I disconnected the amp to see what was going on. Two of the filter caps were empty while two where still holding 420V. When I switched again the standby, the other two drained too. When looking at the schematics it makes sense that the right two keep their charge. So, is that really a good place to put the standby switch? I don't mind the caps holding their charge, but also want to remove the pop when engaging it. Any thoughts on this?

    Cheers
     
  16. StingRay85

    StingRay85 Active Member

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    The easy solution of course would be to only use the standby switch when turning on the amp, and switching it off with the mains switch
     
  17. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    That is one of the standard places to locate a STANDBY switch but it could be located prior to any B+ power filter capacitors or at the power transformer HT center tap in line with the HT fuse.

    The series 50uF power supply filter capacitors would benefit having balancing resistors parallel to each one, total of four. 150k 1W power resistors would be good. That makes sure that any one capacitor does not over drop too much voltage especially if they are lower rated for voltage as well as bleeds off any VDC standing in the B+ supply.
     
  18. StingRay85

    StingRay85 Active Member

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    Thanks for the solid advice. Will do that when making an adjustable bias trimmer. Learned a lot on power supply last few days, very cool.
     
  19. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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  20. StingRay85

    StingRay85 Active Member

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    I have quite a crazy question, because I'm troubleshooting my Carlsbro TC100 for a continuous crackling/popping sound. The schematics you can find in the first post. I found out the capacitor on the bias circuit is REVERSED in my amp, and it sure looks like a crappy DIY job. It's rated for 160V. I'm still learning this stuff, but to me it doesn't seem like a very good idea to have an electrolytic cap in the reserved position connected to ground. Would that result into (1) bad biasing control and/or (2) bad ground performance resulting into this continuous crackling? I wonder what the experts would think :)

    reversed cap.jpg
     

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