Re: JTM45 -- Am I done?
- From: "Phil S" <psymonds_no_spam@xxxxxxxxxxx>
- Date: Wed, 2 Nov 2005 10:13:13 -0500
"Ether" <ether@xxxxxxxxxx> wrote in message
news:1130910566.552700.23200@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> Phil S wrote:
>> It took hours more than I expected.
>
> Welcome to the club!
>
>> Nevertheless, this is a hobby and I
>> enjoyed every minute. I suspect I'll uncover some layout problems, but I
>> think I soldered the last wire tonight. It's too late for me to fire it
>> up.
>> I've got concerns about do that and need a bit more advice and
>> encouragement.
>
> Go Phil! (Ok...now for the advice.)
>
>>
>> This is my first amp with a grid bias circuit. I'm concerned about what
>> happens when I first power up.
>
> KABLOOOEY! (Just kidding.)
Humor is appreciated!
>
>> I only have one set of (EL34) tubes.
>
> Man! You're gonna need dozens! (Not really.)
Yes, I've got the gist of this ;~}
>
>> I have the bias pot/resistor combo set to the stock value of the
>> resistor indicated on the schematic.
>
> That may be fine, but the only way to tell for sure is to check your
> bias feed voltage. Does your schematic give you a typical operating
> figure for the bias voltage? (A Super Reverb is in the ballpark of -50
> volts or so. Not sure what EL34's would expect in your circuit. Around
> -40V from what I could find.)
>
>> Both tubes are marked Ip = 33.0; one is 5.3 gm, the other 5.4gm.
>
> That just tells you that they're matched in whatever test circuit. (Who
> knows what voltage was used, but they should be close enough for your
> purposes, anyway.)
>
>> Just plug 'em in for starters and make sure they don't red
>> plate?
>
> No! Test your bias feed to make sure you've got negative voltage in a
> reasonable range. When you're satisfied with that--and your satistfied
> that the rest of your wiring is correct--you can put the output tubes
> in.
>
Yes, I see that Lord Valve's instructions are quite specific on this matter.
>> I figured that first power up should be only with the GZ34 rectifier and
>> then I should check various plate, grid and cathode voltages.
>
> Excellent idea. Check your bias feed voltage, too.
Yup, this is where I expected to feel a little weak in the knees. Thanks
for propping me up. You know, with a cathode biased amp, this is
irrelevant, so now I'll be prepared.
>
>> I realize some of these will change when I put the tubes in.
>
> Yep.
>
>> Provided nothing looks too odd, I'll put the tubes in and check again?
>
> Yes. But you should get an idea of what your bias voltage should be
> first. (If it's on your schematic, great. If not, maybe someone with a
> JTM45 can help here.) Adjust your bias voltage to a larger negative
> number than you'll need--that will keep the dissipation lower to start
> with. You can adjust the voltage to taste later.
>
>>
>> For speaker load, I'm using a small pair of 8z old stereo speakers that I
>> can wire in parallel or series. Does it matter that the wattage on this
>> amp is way high for these speakers?
>
> No problem. Just as long as you've got a load, all is good. Just don't
> crank the sucker when you're playing!
>
>> I won't be running any signal through it while on the bench. I will
>> probably twiddle the volume and other knobs a little bit and will plug
>> a cable into the input. This just wasn't an issue on the 5E3 and I hope
>> it isn't here.
>
> You can go blind with all that twiddling, but the amp will be fine.
As long as I don't get caught twiddling, I don't think anything bad will
happen. You only go blind when you get caught.
>
>>
>> Finally, there is the bias procedure. I'll be using the cathode resistor
>> method. Any tips other than just carefully following Willie's infamous
>> instructions? I've got 10 ohm resistors (actually they meter 10.2) on
>> the
>> cathode pins. I always seem to be math challenged on whether to divide
>> or
>> multiply. Too bad I couldn't find any suitable 1 ohm resistors ;~{ so a
>> little confirmation of how to handle the math would be appreciated.
>
> A 1-ohm, 1% resistor would have made your life easier, but what you've
> got will work. Remember that Voltage is proportional to Current times
> Resistance. So:
>
> V = I * R .
>
> When you're testing the current using the 1-ohm resistor method, you're
> actually measuring *voltage*, but because R=1, your V = I * 1, so the
> current (I) is the same as your voltage measurement. With a 10-ohm
> resistor, this is your formula:
>
> V = I * 10 .
>
> So the voltage measurement on your meter is TEN TIMES the actual figure
> for the current. In short, divide the number on your DC voltmeter by
> 10, and that will give you your Current reading.
What I thought. I guess this would have been apparent as soon as I saw the
reading.
>
> Here's another guide, should you need it:
>
> http://www.vtl.com/pages/bias.html .
>
>>
>> Thanks.
>>
>> Phil
>
> No problem! Let us know how this thing sounds.
>
> --E
>
It may be a day or two until I get back to it. Work is getting in the way
;~{.
Thanks.
Regards,
Phil
.
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