Randall RG75 G3 Combo amp

[andywragg]

Hi There,

Looking for some advice on the speaker voltages I should be seeing on this amp when at full chat. I have the amp connected to my trusty 300w GLX dim bulb current limiter, and when I open up the master volume once it hits 7 on the dial the bulb starts to emit light. Not the full 300w of nitesun light when there's a short but I'd say between 25 and 50% brightness.

All I have is the schematics courtesy of DrGonz78 (thanks again chap). These don't show any test voltages other than the HT supply to the 12AT7.

Have observed the clean channel on my crusty old scope and once the bulb starts to glow the clean 400Hz wave I'm injecting into the amp front end is clipped to the point where the wave is almost trapezoidal at full beans (Observed at the dummy load).

User is complaining that the amp noise cuts out at 7 on the MV dial but using an 8ohm DL I dont see this on my scope. This could be the Dim Bulb absorbing some of the load. I dont really have the option of running the speaker at full beans as I live in a house joined to 3 others.

Any help greatly appreciated.

[J M Fahey]

the amp noise cuts out at 7 on the MV

Sorry, but, what does this mean?

Also: if the amp works and you want to test some possible problem which happens at high power, you must plug it straight in the wall outlet, not through a lamp bulb limiter.

And a 300W bulb is way too large , unless you are testing a 3000W RMS power amp or similar load.

[Jazz P Bass]

Yup.
Ditch the limiter.

And build an 8 ohm 'dummy load'.

[andywragg]

Hi JM Thanks for responding.

To answer your specific question - According to the user, noise from the speaker stops when your turn the master volume control past 7 on the dial. As I said I'm not in a position to test this right now (using the speaker) as I know the neighbours are at home. Also my family has now come home too so I'll have to wait until no on is around.

I'll remove the dim bulb device from the set up and re-test and record the voltage measurements from the scope using the dummy load.

As for the size of the bulb this is what I was recommended by YouTube's UncleDoug. It's what he uses, here in the UK a 300w bulb will draw 1.25 amps which is lower than the mains fuse in the amp. I'd prefer to use a variac in conjunction with a voltmeter and ammeter, but my budget doesn't extend to getting one just yet. Fixing amps for friends is just my hobby at the moment.

[andywragg]

I have one - it's how I observed the scope

[andywragg]

OK further analysis.

Using an 8ohm dummy load and no current limiter. 400Hz test tone with all channel levels at max.

The clean channel has an undistorted sine wave up to 7 on the master vol dial. This gives 78v peak to peak on the scope. Can you confirm whether this is to be expected.

From 7 to max the output/ampltude does not increase in magnitude but clips assymetrically until the wave is almost trapezoidal.

The Gain channels both show distortion from 0 to max. At no time is there a loss of signal at the dummy load.

I'm going to try a speaker test tomorrow using a know good 8 ohm speaker.

[J M Fahey]

78Vpp=100W RMS into an 8 ohms load.

I still have a problem with your "noise" cutting above 7, what kind of noise is it? : hiss/hum/buzz/motorboating/radio pickup/beeps?

As of "uncle doug" internet lessons, let's say that we had our own resident "uncle doug" short time ago.

In favour of the real "uncle doug" , at the very beginning of his video (00:15 seconds) he says that for SS amps the bulb must be "less than 100W for adequate protection" .
https://www.youtube.com/watch?v=lp6KxU4Bv1g

On the contrary, his take on the death capacitor and hot chassis amps is dubious (being polite) and many other of his ideas are on the kooky side, so beware.

He checks for grounding by touching the chassis with a wire and watching the sparks !!!!!!!!!!!!!!!!!!!!!!!!!

Kept watching his videos, now I see that he tests properly working amps (testing hum and such) *always* plugged through his 300W bulb.

The limiter is NOT intended to be used that way, period.

It limits current in broken amps until you repair them, then it's out for good.

[andywragg]

Hi

So far I have yet to reproduce the users observed behavior of the amp guitar tone (noise) cutting out at 6+ on the master volume pot setting. I thought I'd made that clear from my later posts. I can detect a signal on the dummy load at all MV pot levels past 0.

100W RMS is beyond the capability of the amp according to the manual - it should be 75W RMS at 8ohms 1% THD. So either my CRO is faulty, my measurements are off or the amp is drawing too much current and producing too much power. I did check the 1V test point on the CRO before I began, to align the trace.

I will be testing the amp with the stock speaker and a known good speaker (in one of my own cabs) tomorrow once I'm sure the neighbours are out and I can make 'noise'/sound without fear of disturbing others.

[andywragg]

Stock speaker checks out at full chat, it was very loud even with ear plugs. Amp switching pedal works also. Will be testing it at a local guitar shop over the weekend as they have an iso booth.

Unless this is a time related issue, I'll put this down to external influences.

Dunno what might have caused this.


Thanks for your help guys.

[J M Fahey]

Problem is 5 minutes loud is no problem at all.

Even better than the iso booth at the shop, have him pick both of you up, amp and tech, to his rehearsal studio and you listen him playing for a couple hours, full blast, battling a drummer and a bass player (and even another guitar player).

I bet the problem is thermal.

Tell him to have some kind of fan available, even if a small desktop type.
When amp cuts off, blow fresh air towards it, if problem diasppears then thermal is confirmed.

If not, at least you could also check some other stuff, as:
* does the amp fully turn off or lights remain on?
* it mutes but you still hear faint hum/hiss in the background?
* you bang it with your fist: does sound come back?

and so on.

Sometimes a stupid detail is the key, such as propping the amp against a wall and killing ventilation, or an outlet or extension breaker tripping, or one pedal or cable or even his guitar dies after some time,and "volume on 7" is just a coincidence in time, etc.

[andywragg]

OK now I have been able to reproduce the fault. i've had the amp (fully assembled in the cab) running for 1/4 hour at low levels with no issue, using the test tone generator app on my Galaxy S5.

Fault only occurs on Gain 1/2 channels, not clean. the only control I'm changing is the MV pot.

Speaker output dies suddenly at MV dial setting 6 and above. The lights are still on (no fuses blown), and if I lower the MV back down below the point at which the fault occurs the speaker output returns.

I checked the heat sink at the rear afterwards and it did seem pretty cool considering the amp had been run on full beans.

[km6xz]

The master gain control VR9 could have an open on the resist material in the pot but that does not fit all the symptoms. If the pot was open at lower settings the open would have to be between the wiper and top of the pot. So it would happen. If it was open below the 6 o'clock position it would be loud in all positions. So for your symptoms I would check the pot wiper to resist trace with an ohm meter to see if it opens up at some positions yet the total end to end resistance of the two outer pins of the pot are steady, and read about 50k ohms. A possible cause could be some dirt or debris on the resist trace that prevents wiper contact with the resist trace. That part of the circuit is very simple so it should not be hard to trace using the schematic.

[andywragg]

I'll give it a check but the clean channel doesn't crap out past 6 - it goes all the way without issue - so this would lead me to believe that the pot is fine as it's common to all channel circuits.

[g1]

Perhaps it is breaking into HF oscillation.

[km6xz]

I'll give it a check but the clean channel doesn't crap out past 6 - it goes all the way without issue - so this would lead me to believe that the pot is fine as it's common to all channel circuits.

Cleaning being OK was the first thing that caused me to suspect the master pot before I dug out the schematic. The Clean channel bypasses most of the preamp and has its common tie point with lead channel at the output of the first fix gain input opamp IC1 section A, pin 1. The lead channel goes through more gain stages and diode clipping before the master gain and before the lead eq section. It is easy to verify, just look at the output of IC1 b section(pin 7) and pin 1 of IC2 at the same time. I bet pin 7 of IC1 stays constant and pin 1 of IC1 cuts out over 6o'clock on the master. There are only 4 components between those points, coupling cap C31 and the Master vol control, R41 and the IC2 A section. The only one expecting to have possible defect related to level or pot position would be the pot itself.
Most trouble shooting has little to do with checking a lot of things. It has to do with taking a few telling readings and looking for a theory of failure that could cause all the symptoms observed. For example a single test reading at some mid point eliminates most of the circuit as a cause, either before or after that point. A lot of guys get into trouble by probing around a lot and getting a confused impression of what is happening and they start replacing parts. That is the worst way to approach a problem, any problem. If one understands the circuit, that approach is a waste of time and often introduces more problems. Tech induced problems are the hardest to find since they are not naturally occurring circuit failures, and the cause is often because something was put in wrong or in the wrong place that deviates from a logical signal flow. A rule of thumb I taught all techs who worked for me, no matter how much experience they had. Never make a measurement until you determine what you should expect. Taking a voltage measurement means nothing unless it is significant in deviation from expectations. If you do not already know what it should be, the time was wasted measuring it. If it is off from predicted, either it is an important clue or it means they do not understand the circuit well enough to make a solid diagnosis and are probably more of a threat to the gear than the failure itself. Diagnosis is the valuable thing that a tech can sell. Replacing things is monkey work. A good diagnostician is rare in every field so they always are in demand. For a shop it means the shop can charge less and make more. Customers think they can get a better deal from a shop that charges less but usually they pay more, often a lot more than a shop that does not resort to shotgunning, can diagnose and repair efficiently and turn it around quickly. A shop charging $100 an hour is sometimes twice the bargain as a $25/hour shop, getting it back faster, for a lower price and with less damage caused in the repair. Of course many who charge $100/hour are hacks and hucksters and every shop owned by a Youtube Guru charging $150/hour is a con-man from what I have seen
I had a large shop that repaired all sorts of pro audio gear(54 warranty brands) and most repairs were $65 flat rate so other shops and even factories were sending large volumes of things like ADATs and power amps so they could mark them up to $150-250. Yet at that low price, we were making a lot due to efficiency. Net income from that $65 repair was probably $35 each and doing 25-50 a day, which is more than low efficiency shops which might take all day to do one. Diagnostics is the product sold of value, replacing parts is monkey work as I said.