Mini Z Questions

[martin manning]

Tony, do you know what the OT primary impedance is? Rule-of-thumb is Va^2/Pamax, 7k5 for 300V Va.

[dorrisant]

Tony, do you know what the OT primary impedance is?

13.74vac into primary
0.422vac out of secondary

1/.442=2.369
2.369x13.74=32.55
33:1
Running an 8Ω load...
33x33=1089
8Ωx1089=8.7k

Can you measure the voltage drop across the 150R Screen Resistor so we can see where the screen dissipation is at?

0.496vdc across 150Ω at idle...

[Littlewyan]

Ok I think I've got this right.

0.5VDC / 150Ohms = .003A

.003A * 315VDC = 1W (.945W)

This will go up of course when you're playing, so its a good idea to measure whilst you're playing. First however....

Try running that amp at half the impedance so the EL84 is seeing roughly 4.35K.

What Speaker do you have connected? These amps state on the back 4Ohm minimum which leads me to believe that OT is purposely roughly 5K at 4Ohms. Have you been using an 8Ohm Speaker with it?

[dorrisant]

I put leads across the 150Ω screen resistor. Roughly 0.5VDC measured.
Playing hard, volume at noon it reads a little over 3VDC.

Changed screen resistor to 1K. At idle it measures a tad under 4VDC.
Playing hard with this value peaks out at a little over 19VDC. So, with either value for the screen I am seeing about 3-4mA screen current at idle and about 20mA when playing hard.

I used the MOSFET B+dropper to get the plate voltage down to 266V with the screen at 277v. The 330Ω cathode resistor was biasing the tube too low so I changed it to 220Ω. I realize these voltages are a bit low but I can adjust this by changing one zener diode. I will do this when I get done typing.

BTW... I checked the plate and screen voltages at every step of the way. Before the B+ drop, after, before the cathode resistor change and after... I have noted roughly the same difference throughout testing.

I have both 4Ω and an 8Ω test speakers and dummie loads... It doesn't seem to affect the voltage measurements I'm taking.

[John_P_WI]

Hey Tony, I'm just throwing out food for thought. When I was active on the now defunct powerscaling.com forum, KOC always liked a healthy amount of screen resistance and he also talked about using a regulator to drop the screen voltage on SE EL84s about 30 v under the plate IIRC. Maybe you could reconfigure your mosfet, moving it down to the screen node, instead of the plate as it is now.

Andy, lazy mary amps, also played with a voltage divider at the screens to always keep them a few volts under the plate. Hope this helps.

[dorrisant]

John, I like the idea of a regulator for the screen. I just don't know how to use the MOFSET B+ Dropper for just the screen... it chops the voltage out of the ground connection for the main rectifier.

Why couldn't I make a separate rectifier just for the screen and feed the ground through the dropper? Would it matter having two rectifiers for the plate and screen with different ground references?

I may be making a mountain out of a molehill with this project... Couldn't I just raise the value of the first dropping resistor and adjust the preamp dropper down a bit to at least get it close enough to make the screen stop glowing? I have been keeping my eyes peeled for any info that covers how the dropping string values effect the operation and tone of an amp... still trying to get my head wrapped around that subject to start designing better from the beginning.

Just a note... DC resistance across the primary of the OT is 413Ω.

Could it be the attenuator?

[Littlewyan]

You could replace the 500R Choke resistor with a higher value. Some amps use 4.7K here but that might be a tad extreme.

I've not heard of an attenuator causing screen glow before. Can you try it without? I'll take a proper look at the schematic for the attenuator later.

Not sure about the OT primary resistance.

[Rober Medvesek]

I used the MOSFET B+dropper to get the plate voltage down to 266V with the screen at 277v.

At idle your plate voltage should be a few V higher than your screen voltage. Ether it was a typo or you connected screen and plate wrong.

Kind regards,
Robert

[Littlewyan]

It can be normal for the screen voltage to be higher than the anode voltage. Depends on the OT primary resistance and the dropper resistor/choke resistance.

[pdf64]

See 'conditions with continuous sine wave drive' at top of p3 of http://www.mif.pg.gda.pl/homepages/fran ... e/EL84.pdf
note 3k9 screen grid dropper, 130 ohm bias resistor.

[Littlewyan]

Funnily enough I just looked at the schematic for the Bugera V5 and they used a 4.7K screen dropper resistor with a 22uF Filter Cap on the screens. Looks like they read that page you quoted in the datasheet PDF64.

So there we are, up the choke resistor to 3.9/4.7K and you'll be alright. Don't up the screen grid resistor as that won't make a huge difference as the screens draw such a low current. Once you've done that you may be able to decrease the value of the bias resistor.

Don't worry too much about the voltages in the preamp either, just worry about how it sounds . Marshall only had 200V after the last dropper resistor in some of the 50W lead amps and I'd say they sound pretty dam good.

[pdf64]

Shock! Horror!
After 6 decades, tube guitar amp designers think on to check tube manufacturer's info.

[dorrisant]

Shock! Horror!
After 6 decades, tube guitar amp designers think on to check tube manufacturer's info.

Yes, lol! I don't want it coming back. Thanks for the help.

[dorrisant]

I removed the B+ dropper.

I tried the first string resistor at 4k7 and 3k9. The screen/plate voltages looked better at 4k7 (lower than the plate). Still looked good at 3k9 and was closer to the schematic so I left it there.

I removed the 1k screen resistor and replaced it with 100Ω.

Initially I restarted with a 180Ω cathode resistor but ended up at 270Ω.

No more glowing screens... Sounded good but I will wait until tomorrow to give it a loud test.

Thanks for all of the help to everyone who did... good lesson here.

[M Fowler]

Just went through a noisy Mini Z seems to have been the volume pot, cleaned it and back to normal.

Pretty cool little amp.