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Thread: Silvertone 1484 Restoration - almost there, but need assistance!

  1. #36
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    Yeah - it is strange. If I take a reading from the voltage doubler it is also only pumping out around 450v when it should be 480v. I did replace all the old caps in here with 100uf / 450 caps. Will need to double check my work as seems odd that it’s 30v shy of where it should be.

    All of the original diodes in the doubler tested ok using the diode test on my DMM so left them in there. Could a degraded / poorly functioning diode cause this? I could always replace them with IN4007s I guess.

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    Stray Cap DrGonz78's Avatar
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    What's on schematics doesn't always match up perfectly. Typically it's higher B+ readings since line voltages were lower back when these amps were made. Maybe there were versions of the amp that had lower B+ with certain batches of transformers than what's on the schematic. Not sure if the diodes could drag down the B+ voltage. How accurate is your bias probe?

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  3. #38
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    They might be rated at 30 watts and not 25 watts.
    Good point, indeed.

    Would like to add that there is absolutely no good reason to bias a 30W 6L6 hotter than a 25W one. Bias determines idle current and idle current (not idle power dissipation!) determines performance (crossover distortion etc.) As all 6L6 and 5881 types can be expected to share more or less the same operating characteristics (otherwise they wouldn't be 6L6/5881 types) independent of limiting data, optimum idle current should be the same for all of them.

    Major difference is that a 30W 6L6 can be used with a higher plate voltage than a 25W tube at the same idle current. But within the 25W PD limit operating conditions and electrical performance should be essentially the same.

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  4. #39
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    If I take a reading from the voltage doubler it is also only pumping out around 450v when it should be 480v.
    I wouldn't worry about a difference of 6%. That's well within the daily variation of line/mains voltage. What was your mains voltage during measurements? What exactly is your heater voltage?

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  5. #40
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    Helmholtz

    Heater voltage is 6.2v and wall voltage 122v.

    I’m not too worried - just wanting to be sure it’s all operating ok before I put it back together.

    Noting the 28ma bias - should I be reducing the value of the bias resistor to try and get it closer to 70%, maybe around 36ma?

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  6. #41
    Stray Cap DrGonz78's Avatar
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    As far as this internet lore of 70% of max idle dissipation, I don't follow along. Most traditional tube amps that I bias are somewhere between 40-60% max idle dissipation. I am all about tube lasting longer rather than running them hotter for a shorter life.

    In the case of the old silvertone amp being discussed. If the amp was on my bench I would be more interested in installing adjustable bias capability, so I could easily install an array of different output tubes and bias them easily.

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  7. #42
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    You’re singing my tune.

    I’d just ordered a 50k linear taper pot just for that purpose.

    I’m doing some reading I am lead to believe these amps were generally biased cold from the factory. Which is why the power tubes last in them so long - and many still have the stock power tubes going strong.

    I’m not religiously about the 70% I generally use it as a guide. I usually bias largely by ear once in the ballpark to what sounds good to me.

    The bias pot will enable this to be done a little easier.

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    Lifetime Member Enzo's Avatar
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    Me too, forget the 70% internet thing. Most amp makers and many techs bias amps cooler than that, there is no right and wrong. Well... So cold you get massive crossover distortion is wrong one way, and so warm the tube get red hot is wrong the other way. Between those, whatever sounds good IS good. it is just a guitar amp, nothing precision about it.

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  9. #44
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    Heater voltage is 6.2v and wall voltage 122v.
    Now this is just perfect.

    Yes, 28mA cathode current looks a bit cold, meaning around 25mA real plate current. You may verify using the OT primary DCR biasing method (but pull the PI tube before measuring plate voltage to avoid oscillation).

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  10. #45
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    Once I receive the pot I’ll install and experiment and report back on where I settled in terms of bias.

    In the meantime I’ll recheck the voltage doubler circuit to ensure all connections are where they should be and are solid. May consider replacing old diodes in there with IN4007s but generally don’t like replacing parts for the sake of it. Though I suppose there is the argument these are now 55 year old diodes and putting new ones now is a good opportunity and good insurance. Something to cogitate on while wait for the bias post.

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