Other Amps

What happens to an amp that isn’t used for years?


You can use the variac in the mean time to bring the vox up. That serves the same purpose of a brimistor as anti-surge, but not for filtering which the brimistor is alsio involved in. As a preferred temporary fix you can add a 3-6 Ohm 5W wirewound or composite resistor in the place of the brimistor just to help the capacitor filter. However, it would be great if you pause the work until you get the brimistor or add the 5ohm 5Watt as things will not really work right until you have that installed.

With the short you now dont have an RC filter but the capacitor just acts like an AC shunt, which is ok but it s noisier. The RC filter cutoff equation is

f=1/(2 * Pi * R * C)

and if R tends to 0 you can clearly see that the frequency goes way past 50Hz to infinity, so technically no filtering at 50Hz. You then only rely on the capacitor to short AC as a shunt, which is not ideal.

So the resistance or brimistor is needed.

A shunt is usually sufficient, but not as effective reducing hum so you need the RC network of which the Brimistor is part of.

There are lots of shunt based power supplies which works perfectly well enough but they will have more hum than RC based power supplies. Denney tried to do it right.

Without at least some series resistance, The capacitor cannot properly get rid of the hum, it needs a series resistance to effectively filter the ripple. At the moment you have a short circuit which in effect makes the RC low pass filter only a capacitor in parallel. The Brimistor that used to be in there, supplied the series resistance and also gives you slow ramp up. The amp was designed like that for good reason.

I havent received the brimistor equivalents yet it is on its way and was shipped. It should be a nice solution. They are likely nos military surplus from 60s as they break down old equipment out of service here in the US and I always look out for what I need. I will ship it as soon as I received it and verified it. I sen it for free, no charge. It has been giving you trouble for decades without resolution when it was serviced, so a little bit of wait should not be such a big issue right ?

There are a lot of brimistors and surgistors in the old equipment, but people usually dont know what they are as they usually have no markings due to heat and aging and or usual custom components with unobtanium codes. Out of the handful I will match one for the AC50 and test it here. From what the person I got it from measured when I asked , they seem to be spot on. These seems to be spares that was kept with equipment as surgistors and brimistors took the brunt of faults and fail predictably as they should protecting the equipment.

The problem with the gap in the speaker is that if any filings or ferrous pieces go in there it is almost impossible to remove, as they are attracted to the magnetized side. The only way to remove that is to remove the magnet. I agree with his concern. That said, though it is far from the end of the world he will always give you a worst case outlook. Dont feel down by that. He is just correctly giving you the worst case scenario. I would have done the same. However, you can be sure that after a while there will be no scratches from that at all. They will align on the side.

Dust cap removal is what I expected. There you see what happens when someone not using the correct materials works on a speaker. Sounds like they used epoxy. There are three very specific completely reversible glues you need to use for coil, cap, damper and basket edges. You are in good hands, he knows what he is doing.


Yeah, I'm pretty serene about the speaker. It is what it is. It's clearly had a tough life and a fair bit of abuse, if I can save even part of it and get it working properly again I'll be OK with that. If we can save the cone and coil that would be a huge bonus. I know at least 2 studio owners that have been very pleased with this company, plus they were recommended to me by Celestion themselves so I don't think I could find anyone more likely to do a good job.

Yep, absolutely no rush at all. I sincerely appreciate all you've done and continue to do and my knowledge base is increasing daily so it's all fine by me. It's very kind of you to send that free-of-charge, but I insist on one day returning the favour some how.

Well, the new cap is in and it fitted a treat. I'm not sure there's any less hum, but as you say without the resistor in circuit the cap is not enough. Let's make a judgement when everything is present, and maybe even have another bash at the biasing once we know the components are all in place. Here's a link so you can here the hum. This is with all pots at zero:


Just for interest I left the camera running while I switched off so you can here a f@rty noise it makes. As with a lot of these maladies it's done it as long as I've had it.

Also, I know these problems have technically been here for decades, but don't forget I've not used this amp in earnest since about 1992. So it's not like it's been running kinda fine, but noisy; it was noisy when I put it to bed in '92 and it was noisy when it woke up last month - in the interim it's probably had no more than a couple of hours' running time, if that.

I say this because I don't want to give you the erroneous impression that it's somehow stood up against all the bad wiring and/or components for the last 30 years - it's simply that I haven't been using it.


Comparing it to your previous videos it sounds better to me. I can now hear the switch clicking much louder than the hum. I thought it was way louder. I am not there so it is difficult to gauge.

Lets just get it back to as close as possible to factory original first as it was designed to operate as. Right now it is not operating with components as intended.

In the next few days if you have time, just pull the phase inverter and see if the hum remains the same, which it should if the preamp and phase inverter supplies work properly. We just need to check if something else did not happen in the mean time.

Regarding switch crackle. If your amp is fully on with the variac, and you rotate the variac down to zero quickly, do you also hear the crackle ? If you do, then bring it up again and rotate it slower down to see if there is a point it happens at.


I suspect you’re right about the hum, I’m probably just focusing more on it. On the other hand this latest video is with only one speaker so that may make a difference.

I’ll try the variac test when I get home tonight.


Hum I just pulled the inverter and did some measurements using a rudimentary dB Meter app on my phone, 14" away from the front of the amp (measured for continuity). Again, not military-grade accurate, but it'll give us an idea.

  • Amp off and room as quiet as can be (boiler running in another room, but just audible) = c. 40dB SPL
  • Amp on with inverter removed - c. 50dB SPL
  • Amp on with inverter replaced = c. 65dB SPL

Power-Down noises Still happens if I down ramp the variac quickly, but not if I do it slowly. Doesn't happen with the inverter out, so it's either in the inverter circuitry or the pre-amp (I guess). I'll pull the inputs tubes later on and narrow it down a bit more.


I took out the input tubes and left just the inverter and power tubes and the power-down noise is gone. Replaced No.6 and the power-down noise is back, so it's in the tone circuitry of the Brilliant channel (tan on your road map). To confirm this I swapped the Brilliant tone circuit tube for the Normal one and the noise is gone again.

As it's a crackle that dies away at the same rate every time I assume it's related to a capacitor discharging or breaking down, and as it's on the Brilliant channel this may also be linked to the rustling paper noise we heard at the weekend.

Am I in the right ball-park?


There you go, now you get the hang of it to troubleshoot.

Re Tone Control on Brilliant Channel noise:

If it is a capacitor it is easy to check, but I first want to clarify the following. The only part I dont understand, is the noise that goes away when you swap the tone control tubes. The noise should stay in bright channel if it was the tone control in that channel causing the noise when you swap the tubes. If the noise goes with a specific tube then it is the tube causing it. Can you just explain it a bit more in detail ? I guess you can just update it in your previous post to save pages and I will edit this to reflect your changes if necessary.

Re Hum Do you mind giving me the name of the app, that way I can calibrate exactly what you hear there. Even if the app is inaccurate w.r.t the actual decibels it will be accurate if we both use the same app to compare. the inaccuracy in this case becomes a common mode error and is therefore irrelevant. Very clever of you, will help a lot.


Sorry, I didn't explain myself properly. I didn't mean I swapped the tubes, I meant I swapped which tubes - therefore which channel - I was listening to. So I removed the Brilliant tone tube then replaced the Normal tone tube.

The app is called Decibel X Pro.


Even if the app is inaccurate w.r.t the actual decibels it will be accurate if we both use the same app to compare. the inaccuracy in this case becomes a common mode error and is therefore irrelevant.

I would imagine different hardware (phones) may introduce some error factors.


Ok, thats good news, I misunderstood your post, sorry. In that case it is the cathode resistor/capacitor or tone control circuit causing the problem.

Before we look at each component individually and start desoldering or use a scope, in the tone control, do the following

What is the difference you hear doing the following?

(We are taking the series treble High Pass capacitor C21 in and out of the circuit)

  1. Turn bass control to zero.
  2. Turn amp on, Turn the Treble on the bright channel all the way up to 10, Switch off, Listen and remember crackle.
  3. Repeat above but with Treble control on 0.
  4. Now repeat 2 & 3 with bass control on 10

What is the difference in the crackle you hear on the 4 (audible) readings you took ?

You have to report back on:

  • B(0)T(10)
  • B(0)T(0)
  • B(10)T(10)
  • B(10)T(0)

TAG: Bright Channel Circuit Diagram

Here is the bright channel. And I marked numbered test points for future reference.


OK, I have the results.

  • 0 Bass 0 Treble = noise
  • 0 Bass 10 Treble = noise
  • 10 Bass 0 Treble = no noise
  • 10 Bass 10 Treble = no noise

So winding the bass control to MAX gets rid of the noise.


Set your Bass control to 10 and amp switched off. Place multimeter on Resistance, and connect black wire to chassis.


Measure the resistance at the Wiper (center pin) of the tone control.

It is almost zero ohms right ?


Option #1:

See if you can maybe get lucky with a BIC first: I doubt this problem will have a mechanical component, but lests check first. With the amp on and Treble=0, Bass=0, just check you still get avalanche when switching off, then switch on anagain and do the following. With the amp on and Bass&Treble as above, do the BIC on R40, R41, C15,C20,C21 and the cathode resistor of V6b I cannot make out the identifier of the resistor. See if you dont get a lucky crackle.

If you did not get any info with the above, move to the next.

Follow the circuit diagram I posted in previous post. namely "Bright Channel Circuit Diagram"

At Bass=10, R41 is shorted out so it has no influence, and at the same time the noise disappears. Thus the noise went away with R41 being shorted out by the potentiometer.

At Bass=0 R41 is in parallel with the 1M potentiometer at full, resulting in a total resistance of 9.9..k Ohm completely making the potentiometer insignificant. At the same time the noise is present due to the fact that R41 is in the circuit again.


  1. R41 shorted out ==> Noise disappears
  2. R41 fully added to the circuit (potentiometer fully shunted ouit at 100:1) ==> Noise Present.

So clearly R41 is a candidate as the noise follows it closely.

This is the first of a few options that can cause the avalanche noise. There are more. It is unlikely it is R41, but we follow the logic and eliminate the variables one by one. It is most likely one of the bass capacitors.

If it is not R41 creating the problem, It is sure triggering the noise by providing more current flow through the other 3 capacitors when it is in the circuit. That is clearly shown to be happening in the circuit diagram. If R41 checks out, they are most likely the culprits and our next stop.

  1. With Bass Potentiometer at 0, please measure resistance over outer pins of potentiometer. Compare with resistance you get over R41 and report back. We just check that the potentiometer actuall works at the 0 position.
  2. With Bass Potentiometer set at 0 again, Solder a jumper or use a cable with two crocodile clips across middle(wiper) and upper tab of potentiometer connected to C20. You basically just do what the potentiometer is doing anyway, but check if it is not the potentiometer end that has breakdown. After the jumper is connected, turn on and turn off. Is the noise still there ?
  3. Also with Bass at 1/4, 1/2, and 3/4 respectfully from full do you still get crackle and how does it change or is it just at Bass=0 that this happens ?

So, do you have any 5-15k resistors lying around or any value close? R41 is in a very low voltage and current circuit so anything from a 3/4W Power will work. Or maybe you have a potentiometer lying around you can set at 10k Ohm or anything close you can put in place of the resistor. ?

After this you can decide if you want to continue with a scope or with diagnostic removal of leads.

Expected time to do this is 5-10mins:


Well, today I took delivery of a bag of 100 or so NOS Erie resistors from the ‘60s so there’s a good chance I do.

It’s late here so I’ll pick this back up tomorrow and let you know what I find.


Just got word from the loudspeaker repair shop, the G12-H is alive and well!


Thats a great victory. Glad you got to the right people and not just recone it. A 1970 or earlier Pulsonic Bass Cone G12H is THE Celestion. I cannot find my references anymore and it is not on the internet, but I will look for them and send it to you once I find the discussions about the Pre Rola G12H bass cone and how and by who it was used. Show some photos when you get it back. As I mentioned before, I always palyed with Bass cabs. Sounded so much nicer with my JMP. Slightly less harsh, and handles hang-time better during overdrive.

Did you ask them to verify the impedance if it is 16Ohm ? If not maybe ask them beforehand so they can tell you what they found when you pick it up.


Sorry to interrupt your thread gents, but the technical discussion in this thread gets my engineer brain thriving.

Please keep it going.


Retnev and I have ratcheted up a notch or two off-board and we’re backing and forthing a fair bit and keen not to clog Bax’s server up too much with idle chit-chat.

This has a been a huge learning curve for me and judging from some PMs I’ve been getting it’s pretty popular out there too. We’ll bring you updates as and when we make progress, so stay tuned...


Deke, Just remember when you have time, to do a BIC test on the TAN section in the roadmap, Link

I know I mentioned it already, but it is necessary.

I attach a new image where you have to locate the components in the pink highlighted section within the TAN roadmap. Maybe the Avalanche noise has a mechanical component. Sometimes it does, but in this case I dont expect any. But lets see as it will be nice to know if there is a mechanical component or not. Just set the amp on the Tone Control settings which creates the Avalanche noise when you switch the amp off. Once you have your scope and function generator up, we can find the avalanche noise another way. Since you dont even use this channel, it is not crucial for operation of the amp for you, but we fix it anyway. Whenever you have time over the next few days. You have to locate them accurately in any case to do the scope tracing later. It seems we get a solution every 4 pages, so this thread should terminate at expected 20 pages.



To the right you see two images. The top one is the normal ripple of an RC filter as you have in your VOX too. The bottom one is what happens when you short out the resistor in the filter as what is currently the case in your AC50 as someone removed the Brimistor, and placed a short in its place. You can clearly see from my measurements what happens when you short out the resistance. The ripple increases a lot as it should in an RC filter with R shorted. I did the simulation on the rectified heater supply of the amp, as it is identical to your circuit except just scaled to a lower voltage. From the two oscilloscope images, you can see that the RMS voltages before and after the short are 79.4 and 128mV respectively. This results in an increase in audible hum of 162% if the R component is shorted, resulting in a 6dB increase in hum.

Here is a short video of the shorting process, so you see it is real measurements. Link

Never do shorting tests like this on High Tension lines.

Your case has exactly the same effect, except that your values are different in the filter. You should experience some hum reduction but not as much as my example. The reason for the 10Ohm resistance is to protect the capacitor until we have the Brimistor replacement and not primarily for hum removal.

Here is how it works. Human detection of audio signals are limited to -13dB approximately as I can remember from my Electronic engineering studies in reference to a standard reference signal. I stand corrected on the exact value, but as I can remember our visiual detection limit also curiously has detection to the same value of approximately -13dB. You have professional experience in auditory systems as I gather form other posts, so correct me if you find the value off, but the principle I am explaining is not dependent on a fixed Decibel level. You probably would use dbm/dbi or such instead.

So let us use a simple online calculator, which just calculates the cut-off point of an RC filter and enter the values of both your circuit and mine. The equation for the -3dB points of a filter is f=1/(2PiR*C). If you dont want to calculate this by hand, open two tabs (Control T in firefox) in your browser and open this link in each. Link .

In the first tab enter my values. Enter R=1.2 and C=4700u . In the next tab enter your values R=0.1 (short) C=40u. Now read off the -3dB point of the filter from the data calculated lower in the page. You will see a value for f_c stated. This value is inadequate for sensory hum rejection and we actually will have to go look at what happens at -13dB (granted number may be slightly different ) on the Bode diagram. Remember the xa nd y-axis are logarithmic and not linear when reading it.

For my case: R=1.2 and C=4700uF , the -3dB point is 28.21Hz and -13dB~100-120Hz (from Bode Blot) As you can see since the amp was designed for the US market, and I have full wave rectification, my ripple will be 120Hz and the -13dB audible threshold is clearly met by design in my amplifier if you look at the Bode plot. They did this 100% right in my amp.

For your Case: R=0.1 (short) C=40u, the -3dB point is 39kHz and -13dB~200kHz (from Bode Blot) That is no hum filter and filters nothing except above 200kHz ! (where only dolphins can hear). Your first supply capacitor now works as a shunt filter by means of using the capacitor series internal resistance and series capacitance to filter. This only works for Capacitors in the multiple 1000's of microfarads. So as it is now, it doesnt comply with a shunt filter or an RC filter. So in fact you have no filter.

So lets look at what your 10Ohm resistor does you will add. Again it is a temporary safety measure and will only reduce hum marginally and is just a placeholder. I will explain later why you cannot just throw any resistor in there.

For your Case: R=10 C=40u, the -3dB point is now 380Hz (compare with previous 39kHz!) and -13dB~1-2kHz (from Bode Blot) This is a waaay better situation, although we just used the 10Ohm to protect the supply capacitor and remove the flintstones short-circuit. You need the 10Ohm for many diagnostic uses in a HT line as a tool, but for now it will protect the capacitor and we can use it to track down the remainder of hum in my next posts so it is indispensable to have. From the bode plot, you will only expected to get 1dB hum reduction which is little. You can now, for your own exercise, play around with higher resistance and capacitance values in the place of the 1 Ohm in RC calculator I linked, and see where you get -13dB below 100Hz ripple (for UK full wave). You can then work out the voltage drop and power requirements for each resistance and capacitance entered and find a solution for yourself in place of a Brimistor, if no Brimistor arrives which is always possible. We will return to this later if necessary in that case. At least you wont work in the dark and can model solutions then before you buy components.

Based on above calculations you should get only marginal hum improvement and any improvement you get should be from the removal of the heating up of internal resistance of the Capacitor due to the shunt. If you switch a 70s power solid state amp on, you usually got a whoa-sounding swell of hum that would taper away and vanish within 2-3 seconds. That is the internal series resistance heating up at surge and switch on and as the resistance cools down inside the capacitor, its value decreases, and the hum subsides. Hum is strongly dependent on the series resistance of a capacitor when used as a shunt filter (shorted R in RC filter circuit) They didnt use any inrush on those amps, but sure does now. Shows you how far ahead of its time your AC50 MKIII was.

I will place this text in a document and link once above debugged so it doesnt take up so much space on GP. This should give you a good idea how to use Bode plots to analyze supply filters. It is easy to understand if you just work though my examples above.


Wow, that's some good detail there, thanks! Unfortunately the festive season is going to dominate things for a while as we have both sets of in-laws visiting, spread over the next 3 or 4 days. I'll get back to this as soon as I possibly can.

On another note I got the pre-Rola back today and it's working perfectly. The damper is re-affixed all the way round and the cone moves in and out freely with no noise at all (in fact it moves VERY freely so this cone is well and truly broken in and sounds creamy smooth). As I mentioned before he had to cut the old cap out the middle and replace it with a new one, so the fact that it’s a very dark grey rather than the pale grey of the rest is the only evidence he's even done anything. And he only charged me £25! I can't thank Paul at wembleyloudspeaker.com enough. Unfortunately it was a flying visit as he's coming under a seasonal bug and didn't want to stick around, so I chose not to bug him about the impedance, I'm pretty sure the values we measured at work were close enough.

Right now the amp sounds phenomenal. The Vox is very forward-voiced compared to Fender-style amps and it's the sound I've always had in my head, and my FT 6120 just rings like a bell through it. I've been using a Vox simulator on my Line 6 pedal for about 10 years so it'll be great to get the real thing back out and working again.

Also, once the amp is wound up to gigging volume the hum is negligible so it's certainly good to hit the road with as-is.


Deke, thats great,

If you know how to repair speakers it is not a big deal. If you use the wrong techniques and materials though it gets eeeexpensive. He clearly knows what he is doing as he finds it easy and charged little. I hope wembleyloudspeaker.com gets more business due to this thread as they should. I would have loved to fix it but the shipping would have been 5x the price to ship than what you paid.

Well I am not happy yet as I want to get that Vox as perfect and original as can be.
Please just add the 10 Ohm resistor asap so you can protect your Output transformer and first Supply capacitor. It will be sad if things go because of that with no standby switch or Brimistor.

FYI Good News. The Brimistors are on their way and I have a tracking number. As soon as I get them I will check how well they match as I have the original specifications of the Brimistor used in the Ac50. If there are small deviations I can correct with the addition of another component I will send that too with an explanation of why it is needed.

Also remember to find a resistor you can parallel with the existing 117Ohm (as I can remember) screen resistor to get it equal to the other one. I will post in detail why screen resistors are crucial in so many ways. I will also post an extensive section on how to troubleshoot your entire power supply sections so you can always do it from first principles in future. We are at the TAN section, but we can do the remaining sections in parallel. When you have time just work through the RC circuit examples I gave you in my previous post. You will need that to continue with the yellow, gray and purple sections as in the roadmap

I will keep posting about the other sections so that you have everything to get you up and running if you want to take it further and get rid of the remaining non critical avalanche noise and remaining hum and optimize the amp and more importantly restore it and not just fix it.

Let me know when you want to continue on remaining problems. Enjoy the holidays.


I just found the 10Ω in a package out on the driveway - damn’ amazon drivers too lazy to walk to the actual front door...

I can’t promise I’ll get it fitted in the next coupla days, but it will be fitted before I power it up again.

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