plexisaur sighted

rexracer · Post by **rexracer** » Sat 03/19/11 2:17 am

finished a 36 watt plexi. like the amp. the dual volume controls give you some nice choices. good gain and nice harmonics. not very much clean in this amp. "purple plexisaur"

Post by **zaphod_phil** » Sat 03/19/11 2:07 pm

Based on which design(s)?

rexracer · Post by **rexracer** » Sat 03/19/11 11:45 pm

It is the one in the download section matched to a 36 watt power section with diode rectifier. I have too much voltage. 370vdc on pin 7 of the EL 84s and 342vdc on pin 9. The pre amp tubes are about 20% high too but I am not sure since I'm comparing to the standard chart. Can someone aim me towards a voltage chart for the plexi?

http://www.18watt.com/modules/Gallery/a ... ayout4.jpg

Post by **zaphod_phil** » Sun 03/20/11 9:03 pm

I'm not aware of a voltage chart for the Plexi, although there might be information on voltages in this thread - http://www.18watt.com/modules.php?name= ... ic&t=20330 In any case the PI and power amp voltages should be about the same as for a regular 18W or 36W. You will probably just need to put some zeners into the B+ line to knock around 30V off.

rexracer · Post by **rexracer** » Sun 03/27/11 10:22 pm

So I throw in a couple of 12 watt zeners from radio sap and I only lose 2 volts. Put them in after the first filter cap in series before the resistor. wrong zeners? or what? thanks in advance.

Post by **zaphod_phil** » Sun 03/27/11 11:52 pm

Sounds like you need to turn them the other way round. Zeners are meant to work reverse-biased.

Also, what resistor do you mean? I would have thought the OT centre-tap connection would be the next point right after the zeners.

rexracer · Post by **rexracer** » Mon 03/28/11 12:41 am

thanks phil

Yes you are right the ot center tap is next

phsyconoodler · Post by **phsyconoodler** » Fri 09/16/11 6:57 pm

To drop B+ put the zeners between the power transformer center tap and ground.Not in the B+ line and not in the OT center tap.

Post by **zaphod_phil** » Sun 09/18/11 10:36 am

I'm afraid I have to somewhat disagree with that recommendation. To get the rated voltage drop through any zener, it needs to be placed where the DC is smoothest, which in most amps is after the first filter cap. When you place a zener in the PT centre-tap connection it is being fed chopped AC, so for over half the time is operating below the threshold current required for correct operation. It apparently also puts more stress on the zener.

I have built a lot of amps with zeners and I always put them in the B+ line, because of these factors.

But I agree that you definitely don't place zeners in the *OT* centre tap connection.

Post by **CurtissRobin** » Mon 09/19/11 12:26 am

Placement of zeners between the CT and ground also elevates the winding above ground. As a rule, that's best avoided even if it's arguably harmless.

KennyO

phsyconoodler · Post by **phsyconoodler** » Mon 09/19/11 10:34 am

Reverse zeners between the PT center tap and ground has been done for many,many years with zero bad effects.It depends on how much voltage you want to dump.
Funny I've never seen zeners used in a B+ line,but that's just me.
I've even used a large resistor between the center tap and ground with no ill effects.I just put a 20uf/450v cap across the resistor with the positive end to ground as it does indeed elevate the CT.
I did that in a Trainwreck build I did and I successfully lowered the voltage 30v and the PT is fine.It doesn't get any hotter.
I really think it depends on how much voltage you need to dump and how stout the PT is.
If it's better in the filtered B+ line then why haven't more amp gurus suggested that before?
I'm not trying to be argumentative,just curious.

Post by **CurtissRobin** » Mon 09/19/11 12:33 pm

Reverse zeners between the PT center tap and ground has been done for many,many years with zero bad effects.

I'll admit that my experience with zener diodes in this application is limited but my experience with ground problems is extensive and the universal rule in grounding is to make the path to ground as close to zero ohms as you can. That's the basis of star and bus ground topologies. You certainly won't harm the PT by wiring zener diodes on one side vs. the other side because the load on the PT is the same in both those configurations.

KennyO

Post by **zaphod_phil** » Mon 09/19/11 6:57 pm

If it's better in the filtered B+ line then why haven't more amp gurus suggested that before? I'm not trying to be argumentative,just curious.

Depends who you define as an "amp guru". A number of the ones I've come across have turned out to have quite a few holes in their knowledge of electronics. If you simply check the characteristic curves of zeners, you can clearly see why they work better with filtered DC. I myself have measured the voltage difference you get in real life. Or if you rate someone like "Valve Wizard" Merlin Blencow, he's very clear on this point.

....my experience with ground problems is extensive and the universal rule in grounding is to make the path to ground as close to zero ohms as you can.

+1

katopan · Post by **katopan** » Mon 09/19/11 7:51 pm

+1
Amp gurus...

phsyconoodler · Post by **phsyconoodler** » Tue 09/20/11 10:48 am

Yes I admit my knowledge on the use of Zeners is limited but grounding does not need to be an issue even with a PT center tap elevated.the rest of the grounds are not grounded to the chassis where the center tap is anyway unless you use a star ground system,which I and many others do not use anyway.
I would love to hear the reason why it would have anything at all to do with a ground loop.
I have used a zener in the CT of a couple of amps and used a large wattage resistor and capacitor in another.Neither amps have grounding issues of any kind.Now obviously it's still best to choose the proper PT when designing an amp,but this method of lowering B+ does not have any issues that I've been able to see.
I do like your idea of NOT using the center tap.Also many amps used to lift the center tap of the OT as their method of of a standby.Traynor being one.They have lived for many decades like this will no ill effects at all.
So tell me again why it's an issue?

Donations

Links

Welcome

plexisaur sighted

plexisaur sighted