Mini Z Questions

[dorrisant]

I'm building another one of my "Recycled" amps... a Mini Z stuffed into a Bugera V5.

1 - Which triode gets the 1k5/22uF? Which one gets the 2k7/.68uF? I'm leaning towardes the standard 1k5/22uF for the input... just looking for confirmation. There are two schems on the web for this circuit, each one showing them opposite.

2 - Does anyone know the voltages? The tubes are the same for both circuits, (Bugera/Dr Z) so I'm assuming they are close. I want to adjust the voltages if they are off a bit.

Thanks for looking!

[sliberty]

The sachem I have shows the first stage as having 2.7k / 25uF, and the second stage as 1.5K / .68uF.

And the schem doesn't show any voltages.

[dorrisant]

Ok... at idle the amp is quiet and sounds close to correct. All of the stock values were installed. Only differences are the added screen resistor and the changed value of V2 cathode resistor.

V2 Voltages:
Plate = 305
Screen = 315 - I added a 150Ω Screen Resistor
Cathode = 11.89 across actual 326Ω (330Ω 5%) = 36.5mA

This is a tiny bit over 90% dissipation but close enough, at least for now. The cathode R value on the schem is 180Ω. I had to raise the value to get the proper current.

I am not that good here... the screen voltage looks to high. Is this to be expected? I mean I have heard it said that you should "mind your screens". Maybe I really don't need the screen resistor in this circuit. I don't know. Going by schems I've seen, some have them, some don't. Should I be concerned about the higher screen voltage? Dr Z isn't...

[Littlewyan]

https://farm5.staticflickr.com/4031/450 ... 84f1_b.jpg

According to this schematic its normal for the screen voltage to be higher than the anode voltage. The 150R Screen Grid Resistor you've added should help, especially under load. You might find when you start playing the amp the screen voltage sags.

The question is, how does it sound?

[dorrisant]

Sounds a bit compressed and spongy... and that is the schem I used.

Here are some closeup pics of the EL84... Looks like the screens are roasting.

[Littlewyan]

Ok its looking a bit toasty . Plenty of amps run with just a 100R resistor for EL84 Screen Grids though.

I am wondering your EL84 is duff as its running at 90% with a 330R Biasing Resistor. Try another one if you have one. The Epiphone Valve Junior even uses a 220R Resistor for biasing the EL84 and that schematic has a 180R. EL84s can be run quite hot. Obivously they won't last as long but plenty of amps run the voltage too high with them.

What Output Transformer are you using?

[dorrisant]

TX set and chassis is from a Bugera V5... Probably a higher unloaded voltage on the PT. I kept the attenuator that was in it. DC rectifier and filters are under the Mains PCB on the top right. The switch on the front changes the cathode R&C set between V1A & B. I forgot to mention these things...

[Littlewyan]

Well you said earlier the anode voltage for the EL84 is 305V which should be fine. Can you try another EL84?

It looks cool by the way

[JMFahey]

Hey, nice build

[pdf64]

Ok its looking a bit toasty . Plenty of amps run with just a 100R resistor for EL84 Screen Grids though

Yes, but in the case of a classic AC30, there will be 4 x EL84 pulling screen grid + pre-amp current through a ~500 ohm resistance (choke), so there will be rather more voltage drop and sag on the screen node.
My thinking is that if plate dissipation is too high in such an amp then it's better to drop the HT voltage, rather than bias it cold.
eg leave the bias resistor at 180 ohms and insert ~150 ohms into the HT.

[Littlewyan]

Good point.

When inserting a resistor into the HT between the OT centre tap and the rectifier its my understanding that it will introduce sag, unless you have a filter cap either side of it. Is that right?

[dorrisant]

Ok its looking a bit toasty . Plenty of amps run with just a 100R resistor for EL84 Screen Grids though

Yes, but in the case of a classic AC30, there will be 4 x EL84 pulling screen grid + pre-amp current through a ~500 ohm resistance (choke), so there will be rather more voltage drop and sag on the screen node.
My thinking is that if plate dissipation is too high in such an amp then it's better to drop the HT voltage, rather than bias it cold.
eg leave the bias resistor at 180 ohms and insert ~150 ohms into the HT.

Thanks for all of the replies!!

I can use a MOSFET B+ dropper to get the HT down and go back with the original value of 180Ω... I can adjust the dropper circuit until the bias is correct... Before I go to that much effort, are you thinking this will clear up the screen current issue? I'm not just fishing for pat answers, I am really trying to learn how to resolve this issue now and when it arises in the future. Just trying to wrap my mind around it.

[Littlewyan]

Have you tried another EL84 yet? Just in case its as simple as that.

[dorrisant]

Have you tried another EL84 yet? Just in case its as simple as that.

Yes... I tried 3, two different brands. Voltages are within 2v on the plate and screen for all 3. Same roasting screens for all.

[Littlewyan]

Right ok. Well bringing the HT down a bit as PDF64 suggested will help, or as its only the screens that are unhappy at the moment you could try upping the 150R screen resistor to 1K.

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