Finally Building JTM45: Stupid Questions As They Arise

[M Fowler]

I like the Mojo chassis but you have to realize they are advertising it as a JTM45 when it is really a JTM50 using EL34's. I went through that experience already and you must use the correct sized transformers on this chassis too.

I ended up building two amps one with Mojo JTM45/50 chassis and their Heyboer transformers EL34 and the other a Triode steel chassis JTM45 with MC iron using KT66's. I liked the Mojo EL34 version the best for tone. That amp is now blasting away on the stages of Nashville.

Good job on your build Steve looks great.

Mark

[The New Steve H]

Thanks for the support. If it wasn't for this forum, the garage probably would have burned down. I still have to get out there and tighten things up and make the guts prettier.

I think I'm done with prefab chassis. I expected the Mojo box to help me avoid the aggravation I had been through with my aluminum channel Bassman, but it was actually worse, and the result, while slicker, doesn't please me as much. I don't like the turret board, either. The ID of the turrets is tiny, and the turrets are way too far apart, in my opinion.

I believe it's time to get a new piece of aluminum and move ahead with the 18-watt 5F6A idea.

I'm also thinking about different materials for cabinets. I'm wondering if there is a market for classic circuits in imaginative, hand-built, one-off enclosures.

[M Fowler]

Well Dynaman was building classic circuits in tool boxes so anything is possible.

I saw a guy use an old retro looking metal space heater enclosure for a amp.

Yeah Marshall boards are usually wide and you better have resistors with long leeds to use them. I usually use 1w or 2w metal film or NOS carbon comp resistors to reach all the way across the board. I don't like the forked turrets either. I would rather just make my own JTM45 board and fit components in the circuit my way.

Well you provided an exercise in how to adapt and overcome as we used to say in the Marines.

[The New Steve H]

Here's the newest roadblock. I have never biased an amp before. It's time to bias this one.

I am reading up on it. A bias calculator at ax84.com gave me a list of current levels for KT66s. Apparently I am shooting for 40 mA. Here is the thing: how do I measure the current without taking the socket apart?

Also, I don't have an ammeter, so I guess I know what I'll be shopping for tomorrow.

[Phil_S]

Measuring amps is kill you dead dangerous and I suggest you should not do that. When you put an amp meter in the circuit, it is live and part of the circuit, carrying the full amperage. It is much safer to measure the voltage drop across a resistor and calculate the amps. Simpler, too.

What most folks do is solder a precision 1 ohm 1% resistor to each power tube cathode. Then ground the resistor. Put your volt meter on that resistor, one probe on each side of the resistor. (Or, one probe at the cathode pin and the other at ground, same thing.) Use clips, don't hand hold it. Your voltage is the current. It is Ohms Law at its best: I=V/R, where R=1, so it is I=V! You should make a bit of allowance for the screen current, which is included in the number.

If you don't have/can't find 1 ohm resistors, use 10 ohm resistors, and then remember to divide by 10 for the correct result. You can leave the resistors in circuit. They have no effect.

Google for this. You'll find tons.

[jelle]

+1.

http://www.duncanamps.com/technical/lvbias.html

[Reeltarded]

Let's consider it for a moment, 10R resistors with a 1% rating will always be slightly better than a 1R for shooting in the shadows near the darkness, and possibly one order better, on average.

Steve, go to Metro's Wiki (www.metroamp.com) and look at the JTM-45 build instructions. (.pdf)

It has a section on how the bias check resistor is mounted, and at the end of the build, it has first power-up, and biasing instructions. It also has expected voltages at all pins for their transformer setup. Your's will probably be different, but ballpark.

Use fast-blow fuses for startup, bias, and change to slo-blows Before you blow one and think you have a terrible problem.

Since we are cavemen, we don't use perfection this time. You don't have to wring the last .00034mV (on DC setting on your multitester with bias check resistors) for the amp to work well, or sound best. Within a couple mV is fine, as long as it's low. lol

When you increase bias current, tubes will show lower voltage by a bit, use that new voltage, rebias, use the new voltage, rebias, use that new voltage and rebias. Within about 5 trys from start, you will see the tiny bias change barely moves the voltage, and then you are done.

Relax. Be aware, but relax, take your time. The tubes aren't going to melt with a cold bias start, you have forever. Take your time. It's ok. Make sure you have lots of light, a clear path, and a steady hand. Relax. I was too freaked out to touch the guts of an amp up til about 5 months ago, and now I swap bias 10 times a day when I am swapping tubes.

Clear path, lots of light. Clear head. Relax.

Do pay more attention to your connection probe to resistor than to the meter. Place the meter on something just behind the upturned amp so you can see the probe and the meter at the same time. I use probes with a little ring of smaller diameter about a half inch from the tip so it kinda locks on the resistor leg in a positive way as long as you hold light pressure. No slipping, no worry.

Clear path, lots of light, clear head. Take forever. Relax buddy.

[overtone]

+1
and to help keep you relaxed you can use clip probes like Phil mentioned.
Below is a picture of the Porsche of clip probes.
These are German overkill, but really great to use and a great investment.
When you push the red knob a sprung-wire-jaw-thingy pops out to clip on.
By clipping everything in before powering up you have less chance of slipping with your probe...

I get curious-nerdy when biasing, so before firing up I clip in three meters:
one across the 1 Ohm resistor to read the bias
one to read the Plate DC V
one to read the grid DC V
That way my best hand is free to adjust the bias pot (the other is playing pocket billiards), I can watch the numbers as the power comes up + keep a beady eye on the Plate voltage, all at the same time. The grid voltage is not vital info, but interesting to know and impress the girls with later at the pub.

Best, tony

[The New Steve H]

Thanks for the help.

I had an idea for a gadget. You mount a socket on some kind of base, and you solder wires to the bottom, leading to pins which fit in the amp socket. You rig it up so one wire goes to your ammeter and then back to the amp, so you can measure the current while biasing. You put the tube in the new socket, and you plug everything in to the old socket and the ammeter.

Bad idea?

[Reeltarded]

Well, they make that already, but you see how many of us are using it.

I do that too. Think of ways to keep myself busy.

The clip-on probes and more than one meter is a great way to go! How obvious is genius? Right there in front of me all the time, just not in any mirror I have seen.

[The New Steve H]

I don't have an ammeter that works in the appropriate range. I hate to shell out for another Fluke.

[Reeltarded]

Just run to Radio Shack and grab a pair of those $14 cheapies. They work fine enough. They have clip-on probes at the Shack too.

[The New Steve H]

Thanks. I'll give that a shot.

I'm a little confused about the whole biasing thing. When I saw Gerald Weber, the Infallible Oracle of All Things Tube , talk about it, he said it was a voltage thing. Moving the signal around so it doesn't clip. When I read about it now, people keep talking about current. It seems like it's all about getting a good tone without frying the plates.

After I watched the Weber video, it seemed like an oscilloscope would be the way to go, but if all I care about is current, I guess it has to be a meter.

On top of that, I don't see why my 5F6A doesn't need biasing, since it's pretty much the same thing as a JTM45. Looking around, I can see that the 5F6A is sometimes built with a pot, but the originals were not, and as far as I know, the tubes didn't explode.

[John_P_WI]

Thanks. I'll give that a shot.

I'm a little confused about the whole biasing thing. When I saw Gerald Weber, the Infallible Oracle of All Things Tube , talk about it, he said it was a voltage thing. Moving the signal around so it doesn't clip. When I read about it now, people keep talking about current.

The tubes have a given "safe" dissipation rating. I have not seen GW discuss this, he may be referring to the negative voltage, or bias voltage applied in a "fixed" biased amp.

In a very simplistic description:

When measuring the voltage drop across a 1 ohm or 10 ohm resistor, ohms law V=IR or V/R = I is used to find the idle current.

Using the found current and for an example.

IF from above we determined to have .035 amps (35 milliamps) and know the plate voltage to be 425 volts - THEN Power = V x I or P = 0.035 x 425v and we have approximately 15 watts dissipation at idle through the tube.

If the tube is rated at 25 watts, then (15 / 25) x 100 = 60% of the rated dissipation.

Of course there will be a little current used by the screens (less than 10%) that is not accounted for or separated for using this method (it is "summed" with the plate current using this method) therefore the true plate dissipation will be slightly less.

Hope this helps, remember your rant months ago something about hating engineers and math...... ???

Good luck.

[John_P_WI]

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