JTM45 Build In Progress, All Done! Thanks For The Help!

[ToneMerc]

Go build an engine and a firearm...



Then you can drive to where they are and have them for free.

Yep Guns and Gold......barter and then just take the rest.

TM

[NickC]

Oops. Looking at close-up of the photos I just noticed a solder joint I missed. After rehearsal tonight (time permitting) I'll photocopy the schematic, trace every circuit and mark it on the schematic with a highlighter. Then hook up the light-bulb limiter for first power-up, and check voltages (sans tubes).

[Reeltarded]

No fair! You aren't supposed to find your own faults. I am old and need coffee before looking for such things, and you just jump right in and solve the puzzle without buying a single vowel.... "I'd like to solve the puzzle! Syzygy!"

I am better at finding burned spots anyhow..

[NickC]

..............

I am better at finding burned spots anyhow..

You may get your chance after I power it up.

[Reeltarded]

I doubt it, and don't mess up something just to patronize me.

Let's just hear the thing makeanoise.

[NickC]

It lives! Played it for an hour, and now my ears are ringing mildly. It sounds good, and not noisey even maxxed out.

But there is an issue with the bias supply that I could use some advice on. I changed out the bias range resistor several times. I'm trying to get the range in the safe zone so that it will be in the safe zone at one extreme and can only go cold (yield more negative voltage) at the opposite extreme setting.

Running a pair of KT66s I'm looking for about -42 volts (plates are about 400V loaded), and wanted the range to go from about -40V to -46V (or close to that). I figured, within that range, there's no way it could be biased too hot by someone carelessly twiddling the bias control (mounted so accessible on top of the chassis near the outout tubes).

I tried bias range resistor (the one right before the diode in the bias circuit) of 165k ohm (actually two 330K in parallel) at 3 watt rating, and two 220k paralleled for 105k at 2 watt rating. Neither got me in the right zone. So I popped in a 180k 1/2 watt. Now the entire range, from one extreme sweep to the other is -44 to -42 volts. A bit on the cold side but "safe".

So my question is ......... is a 1/2 watt resistor in that spot okay or should I replace it with a 2 or 3 watt?

Also, is that negative bias range too narrow?

Thanks for any advice.

Best Wishes!

[Reeltarded]

Awesome!

About the bias, I think the range is too short, and another set of similar tubes would make you have to change the range one way or another to stay in that narrow one you have. I am not all about sticking a pot outside to adjust the bias like that on an open Marshall style (right side up, not like those pesky Fenders!) the other thing to consider is you are getting 400 at the plates, but then you change the bias, and now you aren't sure of your plates from outside.. Which is a minor consideration if it's your amp, you get a feel for what you are dealing with after a few runs at biasing.

I would prefer a +/-10mA setup, that is wide enough to swap in similar tubes of many brands/vintage. The RFTs I found go up by 5-8mA over all my current production stuff in my 50. Just thinking.

RAWKIN'! Congrats!

Oops. 1/2w fine in bias.

[Phil_S]

Nick,
Half watt resistors in the bias circuit are more than adequate. You have almost no current x ~40V, so the wattage flowing here is very low and well under 1/2W.

I think you will get the most action by adjusting the 56K at the other end of the circuit. See if you can find a 50K or 100K pot to use temporarily, so you can dial in the voltage. Then you will be able to select the correct resistor. Otherwise, you'll want to swap for the 56K: 27K (maybe), 33K, 47K, and maybe 68K. I don't know what your voltage is at the junction of the 180K and the diode. From that you can pretty well estimate the result.

You might also consider testing 10K and 22K instead of the 15K.

I don't think you'll see nearly the range of adjustment by swapping out the 180K dropping resistor before the diode.

Phil

[NickC]

Thanks Reel!

Okay, looking at it in terms of mA ratio to plate voltage (using Weber Bias Calculator) there is plenty of range (as it turns out) with the 180k bias range resistor (I get lost trying to figure the relationship of negative bias voltage to mA current).

The KT66s are a few volts apart in plate voltage, so I split the difference and biased the KT66s at 41.5mA with plates at 425V and 428V.

It sounds good and there's no red-plating.

I couldn't have done it without the fine folks at TAG. Thank you! Thank you! Thank you all!

Awesome!

About the bias, I think the range is too short, and another set of similar tubes would make you have to change the range one way or another to stay in that narrow one you have. I am not all about sticking a pot outside to adjust the bias like that on an open Marshall style (right side up, not like those pesky Fenders!) the other thing to consider is you are getting 400 at the plates, but then you change the bias, and now you aren't sure of your plates from outside.. Which is a minor consideration if it's your amp, you get a feel for what you are dealing with after a few runs at biasing.

I would prefer a +/-10mA setup, that is wide enough to swap in similar tubes of many brands/vintage. The RFTs I found go up by 5-8mA over all my current production stuff in my 50. Just thinking.

RAWKIN'! Congrats!

Oops. 1/2w fine in bias.

[Reeltarded]

Yes, this place is an amp farm, and damned good farmers!

[NickC]

Next job is to mount it in it's head-cabinet. Picture to follow.

[NickC]

In the cabinet

[Reeltarded]

Yep! Looks like a winner!

[statorvane]

Sweet lookin' amp there!

[NickC]

I decided to try this as a combo amp in a Bluesbreaker style cabinet: