18w Marshall Lite

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scstill
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18w Marshall Lite

Post by scstill »

Just finished a Hammond AO39 to Marshall 18w Lite.
Built it pretty much according to Sluckeys design except with 5U4
BUT it Squeals a lot with 12AX7 in PI position.
Much better with 12AU7.
Any suggestions to tame the build with 12AX7?
Also note the 500uf bypass cap.
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Bieworm
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Re: 18w Marshall Lite

Post by Bieworm »

Strange… have you tried other 12AX7’s?
While yor bias is on te higher side at 90% dissipation, it isn’t thát high. The plate voltage on the PI is kinda low otoh.
Maybe lead dress issues create parasitic oscillation and make the amp squal?

To tame the amp you can do some things:

- Try to lower the bias with a 180R bias resistor and see how tha affects the squeal.
- make the bias of the PI colder by altering the cathode resistor?
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scstill
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Re: 18w Marshall Lite

Post by scstill »

Good ideas thanks
BTW The DC voltages on the PI:
with 12ax7 are 198v plate 67v cathode 65v below 820 cathode resistor
with 12au7 are 146v plate 101v cathode 99v below 820 cathode resistor
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scstill
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Re: 18w Marshall Lite

Post by scstill »

Bieworm wrote:
Sun 02/02/25 3:54 pm
While yor bias is on te higher side at 90% dissipation, it isn’t thát high.
Is this dissipation calculation correct??
11.5v/150ohm cathode = 38ma for each tube
320v * 38ma = 12.2w / 17w = 72%

Maybe the plate voltage of 320v is high considering max spec is 300.
I stayed with the original Hammond power supply just replaced the electrolytics. The orig Hammond voltage was 315 on the plate
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Bieworm
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Re: 18w Marshall Lite

Post by Bieworm »

Use the tube bias calculator online. Saves you the worries.
https://robrobinette.com/Tube_Bias_Calc ... e_vignette
The plate to cathode voltage is the voltage you get when subtracting the cathode voltage from the plate voltage.
You’ll se you get 90% dissipation.

Don’t worry about the plate voltage over 300V. On an 18 watt we target for 340 to 345 V. Heck, I have had 18 watts with over 360 V on the plates.
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scstill
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Re: 18w Marshall Lite

Post by scstill »

Thanks for the calculator idea....
But this calculator assumes a max dissipation of 12w for the 6bq5. According to the 6bq5 datasheet for class AB operation at 300v its 17w and at 250w its 11w. This design operates closer to the 17w max.

http://www.r-type.org/pdfs/6bq5.pdf
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Re: 18w Marshall Lite

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scstill wrote:
Mon 02/03/25 7:09 pm
Thanks for the calculator idea....
But this calculator assumes a max dissipation of 12w for the 6bq5. According to the 6bq5 datasheet for class AB operation at 300v its 17w and at 250w its 11w. This design operates closer to the 17w max.

http://www.r-type.org/pdfs/6bq5.pdf
Why make such assumptions? The calculator is designed to choose the tube type. Then you add the plate to cathode voltage. Fill in the bias resistor value and the cathode voltage. The result is the dissipation. The wattage can be calculated based on the parameters, but this is in fact very subjective. You have to neglect theoretical output values. There are ways to determine the output exactly, but this involves more than some calculations. Load, OT specs ..for instance.
Volume is also dependent on other factors such as speaker sensitivity ,…
People tend to overthink the watts produced. I have built 10 watt amps with this circuit and they are almost equally loud as 18 watts. It’s a good attitude to care for the technical and theoretical aspects for education’s sake, but don’t loose yourself in that area. Worry more about what to do with all that joyful tone🤘
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Re: 18w Marshall Lite

Post by JMPGuitars »

18W is 67% as loud as 100W. Dropping to 10W won't blow your mind. 😉

Assuming identical speakers, every time you double the wattage, you only go up 3db. To double the perceived volume, you need an increase of 10db.
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scstill
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Re: 18w Marshall Lite

Post by scstill »

Bieworm wrote:
Tue 02/04/25 3:52 am

Why make such assumptions? The calculator is designed to choose the tube type.
The 12w is what the calculator assumes not me.
It cannot be changed once you pick 6bq5
Seems wrong relative to the tube sheet I attached earlier which is 17w at 300v.
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Re: 18w Marshall Lite

Post by JMPGuitars »

scstill wrote:
Thu 02/06/25 12:42 pm
Bieworm wrote:
Tue 02/04/25 3:52 am

Why make such assumptions? The calculator is designed to choose the tube type.
The 12w is what the calculator assumes not me.
It cannot be changed once you pick 6bq5
Seems wrong relative to the tube sheet I attached earlier which is 17w at 300v.
You're reading the datasheet incorrectly. Look on page 1 of the sheet you supplied under Maximum Ratings. Per tube max is 12Watts, same as EL84. The 7189 can go slightly higher, but the tubes you're referring to are 12W max per tube.

What you're quoting is the expected output of a PAIR of tubes in class AB, with 300V plate voltage. 17W total (8.5W each).

Thanks,
Josh
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