Quote from: Tassieviking on October 21, 2024, 02:13:31 PMI presume you are measuring either the positive or negative side of the amp, one 0R6 resistor is on the positive rail and one is on the negative rail.
If you use a 2 channel cro and run the cro on both resistors you would find one half sinewave on each resistor.

Interesting...and good to know. Thank you.

So, I'm guessing that if I were to scope both speakers outputs, I would see the other half of the sign wave on each of them...?

The amp speaker output.

Also, my clean channel isn't as clean as it should be...🤷 If this helps in the troubleshooting..?

If I have hooked up the oscilloscope correctly...this is the waveform I'm getting.

It appears to be missing half the waveform....

Just FYI: I am getting the same reading across both  .6 ohm resistors.  (e.g.: 20.4mV across both .6 ohm resistors with the trim pot almost maxed) I have noticed that the amp sounds better with the higher readings.

Anyone have any idea why the trim pot is not adjusting the bias as it should? It worked in the past.

UPDATE: For some reason. When I plug an additional speaker cab in. The amp actually sounds better. The tone improves. Not so flubby, tighter bass. The volume does drop some when the second speaker cab is plugged in.
If this is the case. What could the issue be that is causing half the waveform to be missing?

Well, as of next summer she'll be 40 years old, and still looks like the day I got her.

It was a summer day in 1985 in the Bay Area, and I was a huge DOKKEN fan. So, if you're gonna buy a new amp, what do you get?? Yep...a Randall RG100-ES 'FULL' stack. Why not??

So, I went to K&K Music in San Francisco on Columbus Street. When I first walked in, there it was. A Randall half stack. After test driving it, and hearing everything I was hearing on the 'Tooth and Nail' record. I just had to. And here we are almost 40 years later, and none the worse for wear.

She's like brand new.

So, feast your eyes!!!🤘🏻🤘🏻🤘🏻🤘🏻

PS: And no...she's not for sale. (-;

First off...thanks for the in depth reply.

As far as the HT filter caps are concerned. I never assumed they would give me any additional power. That would be absurd. (-; I was wondering if they might tighten up my bass response a little. Nothing more.

As for the amp. I'm not running any effects. The PUP I'm using is about 16K. The speaker I'm using is straight out of one of the 4x12 Randall cabs that came with the amp. Which is a G12M-70. I'm also using a V-30 that improves the tone a bit.

With regard to the biasing of the amp. I've read in a few posts here in this forum that the way you bias these amps is by measuring the voltage drop across the .6 ohm 10 watt resistors. There are two of them. It would be nice if someone could confirm this method for everyone.

I have my doubts about the bias method posted in this forum. The reason why is, I measure anywhere from zero volts with the trim pot turned counter clockwise, and as high as 25mV with the trim pot about noon, one o'clock position. The instructions say to bias the amp up to 40/50mV. The problem is, mine is only going as high as 25mA. So, I don't really think this is the correct biasing method.

I do have a scope. Which I am not completely proficient at using yet. So, if someone would like to give me some detailed instructions on how to use it to bias my amp correctly. I would be forever grateful.

Anyway, any suggestions/instructions would be greatly appreciated. I would really like to learn how to bias my amp the correct way.



-J

Ever wished you could have the RED or GREEN channel LEDs lit on your RG100-ES when you don't have the footswitch or just don't want to use the footswitch...?

Well...here's how...
[See photos attached below.]

On the rear panel where you would normally plug the footswitch in...you're going to install a small jumper wire. For the RED channel the jumper wire goes from the center right to the top right, and for the GREEN channel the jumper wire goes from the center right to bottom right.

OH!!! And here's a BONUS!!! If ya wanna completely shut off the green channel. Just jump the top left to the middle left (see last photo), and the green channel is completely shut off. NO MORE BLEED!!!!

That's all there is to it!!


ENJOY!!!

No. That was perfect. I totally got the battery analogy. Thanks for the clarification.

But, I'm still left with an amp that doesn't sound like it should, and I may be trying to bias this thing incorrectly.

Am I supposed to be reading the voltage drop across the two .6 ohm 10 watt resistors? If so, what is the correct bias reading range I should be seeing?

And also, why does my amp not sound correct no matter where I set the bias trim pot. It used to sound just like the tone in the Dokken 'Just Got Lucky' (guitar only) video.

Video---> https://youtu.be/Jl9Oqgzv0fU?si=lJBaMxde4CvG9Yew

But it doesn't anymore for some reason. It's kinda flubby...and too round sounding (the 10000uF HT FILTER caps didn't change anything.)

Anyone have any ideas what might be going on here???

Quote from: saturated on October 20, 2024, 02:38:50 AMI'm gonna say yes it's correct as it looks like a bipolar power supply but we better wait for verification

And I would think it is the capacitor on the left

You are correct. I followed the schematic back to the HT caps, and it was in fact the cap on the left. Now that the issue with the caps has been remedied. I'm having issues biasing the amp.

Based on a thread here in the forum. I should see 40/50mA across the two .6 ohm 10 watt resistors. However, I'm only seeing as high as 20mA with the bias pot nearly dimed clockwise.

However, it doesn't seem that the amp sounds like it should regardless of where I set the trim pot.

What am I missing here??

Any bias experts here in the forum regarding the Randall RG100-ES??

Hey guys??

It shows one of the HT filter caps with its + to ground. Is this correct??

If so, which cap is it with + to ground??

Here is a much clearer/sharper version of the schematic for the Randall RG80-ES/RG100-ES heads.

I didn't see a site or folder for Randall amps. So, I'm just posting it here for someone to place it in the correct location.

Here is the schematic for the Randall RG100 head preamp and power sections all rolled into a single schematic.

Well, if they come with just a tab bit tighter bass response, and I mean just a tad. They will be perfect.

Now, I'm familiar with the effect and feel of caps on a tube amp, but on a SS amp...not so much.

What would slightly smaller caps do to my tone and feel?? Say, 4700uF or so...

I'm assuming with the SS amp that a lower cap value may not result in a pleasant sag in my tone...or am I wrong??

Well, I was able to find a couple of 6800uF caps that are 100 volts and measure 35mmx70mm on Amazon for a whole $9.95 for 2pcs.

However, I would still like to know what the advantages or disadvantages would be if I used the 10000uF filter caps in the power section.

I'm assuming maybe a little bit better and tighter bass response if anything??

Thanks again in advance.

-J

Thanks for the replies.

Here is pic I found online (since I wasn't smart enough to take some while I was at it.)

The cap in the photo appears to be about 35mm if I'm not mistaken.

I did find a couple of 35mmx70mm 100V 10000uF caps in my forgotten stash. Is there any reason I would NOT want to use a 10000uF cap in the power supply positions??

Thanks in advance.

-J

Hello everyone!

I've been trying to find a couple of 6000uF 50 volt electrolytic caps for my RG100-ES, but I cannot find them anywhere.

Based on photos I'm assuming these cap cans are about 37mm to 40mm diam. cans??

If I cannot find 6000uF. Do I want to go little below or a little above the original 6000uF filter cans? Also, what would the impact be on tone if I were to go a little over or under the original 6000uF caps? I'm assuming that any voltage rating above the original 50 volts is acceptable?

Thanks in advance to all that reply.

-J