Vox Cambridge Reverb V1031 non-polar electro caps

[DrGonz78]

I am repairing a V1031 Vox amp and have a question in regard to the non-polar electros. The customer had replaced the output transistors as a way to restore the output signal on the amp. They installed those correctly and still had no significant output. I scoped out the signal path and discovered a bad C19 4uf non-polar electro and replaced it with a 10uf non-polar electro. This has restored the output signal and the amp sounds pretty good. The only problem now is turning the bass pot up creates a nasty overdrive distortion. It is something in regard to a germanium fuzz sound, but it is not consistent making a pop here and there at lower settings on the bass pot. At first I thought it was the speaker, but it happens on any speaker. I remember seeing a strange effect on the scope that it is emanating from the preamp area that is triggered by turning up the bass pot as I was tracing out the signal path. I am looking at C9(1uf NP electro) as a possible culprit and I am thinking about just replacing all the electros in the amp for good measure.

Now would replacing the 4uf cap C19 with a 10uf cause any kind of foreseeable problem? I am having trouble finding crossover 4uf non-polar electro's and am wondering how exact my replacements need to be? Can I use other types of non-polar caps besides electros? There are only these two non-polar electros (C9 and C19) in the amp so kind of a pain ordering from this one place online that I found...>>> Capacitors

I have no idea what this amp is supposed to sound like, but reading about it I gather that it has a gainy type drive to the preamp. So, would that be a clipping germanium type gain sound? So as I increase the lower frequency response (bass pot) it is trying to work correctly, but perhaps the electro that sits right across the bass pot is going bad too. I measure the capacitance reading at 2.2uf where it supposed to be 1uf. At first the cap was not reading very well and was shooting around nanofarad readings. I wiggled the cap around and eventually got the 2.2uf reading. I did not have time to recheck the signal path at that point, but I am pretty sure these electros are all on their way out, after all the amp was made in 1966. Thank you to anyone with some advice or insight that might have worked on these amps.

Here is a link to the v1081 as it is the same as the v1031, but one less speaker.
http://music-electronics-forum.com/a...y-ii-v1081.pdf

[soundguruman]

Put @X10uF in series for = 5uF. Or use 4.7 uF
The bass is going too subsonic maybe from using 10 uF.

For sure electrolytic goes bad with age.

There is actually 3 Vox...
one made in England which is the real Vox
and one made in California, which is fake Vox.

and one made in China / Vietnam, which is nothing like either of them.

The California Vox (Thomas Organ Vox) is well known and famous, but many people do not realize it's not the real Vox.

It's easy to get it mixed up, and many people do confuse them.

[52 Bill]

The first question is what are the output transistors in the amp right now? Did the owner replace germanium ones with silicons? If he did, did he rebias the output stage to work with the silicons?

As far as the cap value, the larger value will extend the low frequency input to the power amp, but should not cause any real problems.

[tedmich]

The C9 looks to be a cludge added to the original schematic drawing, maybe because of a similar issue its also a 1uF, I'd not 10X it!
Plenty of NP caps are available, might as well use same values!
How old is this thing? I replace all ELs after about 15 years.

[DrGonz78]

It's all original from 1966 as per the only parts changed being the output transistors. The outputs are germanium, so no problem with rebias of the amp for silicon. I will have to trace it again on the scope later tonight and see if replacing C9 is the culprit. I have a 1uf NP electro on something else that I can use so I will give it a shot.

[drewl]

I've rebuilt/repaired several of the ss Thomas Vox amps, and if you search RG has an excellent site on restoring these amps.
In short, all of those cheap black caps should be replaced. It will save time and prevent the amp from coming back with another problem soon.
It is fine to use more traditional values like 2.2 or 4.7uf etc.
And the non polarized caps are readily available from places like Mouser or DigiKey.

[R.G.]

In the interest of public service, I will translate into information usable by real humans.

Put @X10uF in series for = 5uF. Or use 4.7 uF
The bass is going too subsonic maybe from using 10 uF.
For sure electrolytic goes bad with age.

This is correct-ish. Two 10uF NP caps in series or one 4.7uF NP will be a great substitute. I have personally done this many times.
The bass **may** be "going too subsonic", which translates as "the stability of that amplifier section may have been upset by the extra bass response". It could be a solder ball or a wrong lead placement on the very difficult-to-work-on PCBs, too.
And it is true that electrolytics do degrade with age.

It is here that the misinformation starts.

There is actually 3 Vox...
one made in England which is the real Vox
and one made in California, which is fake Vox.
and one made in China / Vietnam, which is nothing like either of them.
The California Vox (Thomas Organ Vox) is well known and famous, but many people do not realize it's not the real Vox.

Vox was started in England, and the early English tube Voxes have become classics. The owner of the UK Vox setup never had enough money to set up production well and to get to the USA market. He licensed the Vox brand to Thomas Organ in the USA, thinking they would buy his amps and ship him royalties. Instead, they used the license to design and build their own amps under the Vox name.

Note that on all levels, Thomas Organ Vox amps are legally "Vox" amplifiers as recognized and supported by the originator of the UK Vox amps. In fact, The designer of the UK Vox amps worked on location at Thomas Organ to get the Thomas Vox amps to an agreeable level of quality. They were different from the UK Vox amps, but they are real, no-fooling Vox amps no matter what SGM may have decided about them.

UK Vox went broke, was sold, made more amps, was sold again a couple of times, and eventually was bought by Korg. They were made in various places and by different manufacturers in the UK before Korg took the Vox brand to manufacture in Asia. All of these are as "Vox" as any other.

SGM is once again spouting confidently things of which he has no real knowledge, and attempting to make his opinions look like fact by saying them confidently.

SGM, I'd welcome a detailed discussion of Vox history with you. Want to talk about it? If you don't I will take that as your agreement that what I've just said is correct on all levels and that you agree that you are wrong.

[DrGonz78]

Okay all things SGM aside, I am still trying to figure what is causing this buzzy crackle noise in louder/lower frequencies. SGM was of help in the regard to putting 2 10uf NP caps in series to make a 5uf rating. But changing that C19 back to 5uf did not change the symptom of the amp. I also changed the 1uf C9 NP cap for good measure just to make sure and still the problem persists.

I am tracing the signal out and studying the schematic further before taking off the board to replace any other caps at this point. One MAJOR detail I left out, which I should have mentioned, is that there is no reverb tank connected to the amp. Still I don't believe that should matter very much as the amp should function with reverb connected or not. The only component of the reverb circuit that is connected is the transformer and of course the circuit board components. The leads to the reverb tank are not connected.

What I just discovered tonight makes me suspect the main volume pot as a possible culprit. I scoped the signal and it is really difficult to know what you are seeing without hearing it at the same time. I did hook up a speaker but the scope really shows me better what the sine wave is really doing. Anyway I notice that when turning the pot into the 12-noon to 2 o'clock position it generates a whacked out sine wave that is not normal behavior. So I measure the pot on both sweeps and across to check the resistance. I measure 2.7k across the pot and at that 12 noon point to around 2 o'clock there is a rise to 3.3k that is not in linear fashion measuring the wipers. The behavior of the pot is the same on either wiper side and I don't think it is caused by the 0.001uf cap in paralell to legs 1 and 2. The scope output is all crazy looking lines bouncin around between that presumably bad part of the pot. Also you can feel that there is rough feeling in the pot at that point too. I tried cleaning the pot but that did not change anything. The pot still has that rough feeling too around 12-2 o'clock. I will have to try swapping out the pot to see if that is the culprit or not. Will report back...

[R.G.]

Debugging the Thomas Vox amps is hugely helped by the fact that all but a couple of the pots are DC-isolated from the circuits by caps. Measure the DC voltage on the pot terminals and see if there is DC leaking into them. DC on a pot will do very odd things, not only to the sound.

The pot impedance from wiper to both ends peaks in the middle of the electrical travel. You're describing a situation where the pot position modifies the action of the amplification, and that suggests that the pot is modifying the DC operating conditions. Go take a look at that.

If it doesn't help, let me know and I'll go eyeball the 1031 schematic and think about it some more. I went through some detailed refurb with my Cambridge a few years back.

As an aside, the Cambridge reverb was my very first amplifier. I mourned it for decades after selling it until I could get another.

[Jazz P Bass]

For a learning experiance, take the pot apart.

[R.G.]

Upon reflection:
Either look for DC on the tone/volume controls, or just replace C5, C9, and C11. C9 is a NP electro, but use a modern $0.50 1uF/63V film cap, and it will remain stable forever, not drifting like electros do. C5 and C11 block DC from the controls.

I started to say "if you find DC on the controls, replace them", but that's silly. Just replace them. They're very likely leaky, and if they aren't they will be soon. This is the reasoning that says replace all the electros. They're nearly 50 years old. If you replace the one or two that are bad, the customer may be back next week for warranty because the next one failed. If the customer won't go for that, explain that reasoning to them, and make them sign a written statement that they understand why you can't warranty it unless you replace them all.

No, what you're hearing is not a consequence of a germanium fuzz sound. or the design of the amp.

Now would replacing the 4uf cap C19 with a 10uf cause any kind of foreseeable problem?

No. Full stop.

Can I use other types of non-polar caps besides electros?

Yes. For all the cases where you can get film (usually 1uF and 2uF) do that. They're cheap and small now, like they were not when the amp was made.

One MAJOR detail I left out, which I should have mentioned, is that there is no reverb tank connected to the amp. Still I don't believe that should matter very much as the amp should function with reverb connected or not. The only component of the reverb circuit that is connected is the transformer and of course the circuit board components. The leads to the reverb tank are not connected.

Probably no hurt, but it would be better to short a 1K across the connectors. Mutes the return line to keep noise off the recovery amp, and terminates the driver transformer to keep spikes down.

Thomas used an odd reverb circuit. It can oscillate and do other strange things if C13 is not good. Put in a new 220uF for C13. Just do it.

What I just discovered tonight makes me suspect the main volume pot as a possible culprit. I scoped the signal and it is really difficult to know what you are seeing without hearing it at the same time. I did hook up a speaker but the scope really shows me better what the sine wave is really doing. Anyway I notice that when turning the pot into the 12-noon to 2 o'clock position it generates a whacked out sine wave that is not normal behavior. So I measure the pot on both sweeps and across to check the resistance. I measure 2.7k across the pot and at that 12 noon point to around 2 o'clock there is a rise to 3.3k that is not in linear fashion measuring the wipers. The behavior of the pot is the same on either wiper side and I don't think it is caused by the 0.001uf cap in paralell to legs 1 and 2. The scope output is all crazy looking lines bouncin around between that presumably bad part of the pot. Also you can feel that there is rough feeling in the pot at that point too. I tried cleaning the pot but that did not change anything. The pot still has that rough feeling too around 12-2 o'clock. I will have to try swapping out the pot to see if that is the culprit or not. Will report back...

New pot. If the rotation feels funny, it's got bad places on the element.

[DrGonz78]

So I took apart the pot and had a look... These are great pots!! I wish all pots were made like these ones! Anyhow I clean it lightly one more time and cleaned of the wipers too while I had it apart. Put the pot back together and it does not have that rough spot anymore. The pot functions just as a pot should function and it read 9.4k before the finally cleaning. After the cleaning it read 9.6k so it actually got even better. So now I just don't think it is the pot.

I am gonna have to just order those electros and try that approach. I have not had Ah Ha! moment since replacing C19 and getting the sound levels back. Hopefully I am just dealing with really leaky caps. BTW no DC on the pots, well 4mv at the worst but that does not seem too bad. Also, I will have to figure out where to put that 1k resistor to cut off the reverb circuit. Thanks for all the info so far!

[R.G.]

Also, I will have to figure out where to put that 1k resistor to cut off the reverb circuit.

Just clip one 1K across each of the plugs going to/from the missing tank. The output cable is then terminated in 1K, which is an OK-ish match to most reverb tanks and won't drive your driver transformer or its transistor crazy, and the 1K across the input keeps hum and noise down. You can short the cable from the output of the missing tank, but then you'd have to know which cable was which; with two 1K resistors, you can treat them equally.

[DrGonz78]

Just clip one 1K across each of the plugs going to/from the missing tank. The output cable is then terminated in 1K, which is an OK-ish match to most reverb tanks and won't drive your driver transformer or its transistor crazy, and the 1K across the input keeps hum and noise down. You can short the cable from the output of the missing tank, but then you'd have to know which cable was which; with two 1K resistors, you can treat them equally.

So there was only one cable connect in the amp for the reverb circuit. The other plug is missing and so is the actual tank. The cable connected that I removed is the one appearing in this part of the board layout(the return).


So that would be P5 on the schematic and I think that is the return. The send would come right off the tap on the transformer. So all I have to do is connect 1k resistor between those points, I think?

[DrGonz78]

Well I replaced all the electros on the board and still this same problem persists. Either way it will be good for the customer to have all those caps replaced, so not a bad thing. Also, now while I am hunting down the problem I can get those electros out of my way. The buzz saw sound is best I can describe the symptom. Lower notes cut in with a distorted buzz when played hard and volum turned up higher than 2-3. The signal sounds stronger and more responsive after changing the electros. I am wondering if this is a bad transistor. I am going to try some tests by sending the line signal (preamp area) to another amp to hear exactly what is going on. Perhaps even this is related to the reverb tank missing, but I just don't know yet.

Edit: I just sent the signal from the audio pot output to another amp and it was fuzz saw distorting even worse than it does through the amp itself. All the strings from D up higher sound good but any note played lower generate that old fuzz sound. What is happening in the preamp circuit as I turn up the volume? I have scoped the individual Q1 and Q2 only to see my test sine wave in good form. Turning up the volume does not change the signal arriving or leaving at those transistors. So I connected the other amp to where the treble pot has the incoming signal connected and it sounds good without that nasty fuzz distortion. Then I connect the other flow of signal leaving the treble pot and it is fine sounding. That signal flow is across a 1nanofarad cap in parallel to the volume pot. Signal flow into the volume pot is fine, but then after the pot is where the fuzz sound is present. Not that the fuzz sound is not dependant on the volume being turned up actually. So still everything to me is now pointing back at that pot as the the culprit. This is like a merry go round...


Note that C10 and C9 have been replaced new.