Hi,
My normal method is to use a 100w 16 ohm resistor. I find it a very convenient component because it can also be used as a dummy load to replace the speaker when doing osciloscope work. Its plenty adequate to absorb any power left in the filter caps. I just touch it in parallel with one of the caps and wait a couple of seconds. I then measure any remaining voltage accross each cap using a multimeter set to a high DC voltage range.

Having said all that, I have found that on my OTS simply powering the amp of in the normal fashion (turn to standby, wait 2 secs, turn off, wait 2 secs) will drain the caps. I always measure across the caps just in case even though it has never shown more than 10v when doing this, which is a perfectly safe level. Bear in mind that your OTS may behave differently and you should always, always, always check. "Measure twice, die never", thats my philosophy.

Same waiver as always, I cannot be held responsible for anyone doing this, your life is in your hands, not mine....

P.S. - Love the avatar, Roy is an inspiration!

Hi,
The resistor that I changed is on the bottom right of the main board on ceriatone's layout (labelled 150k). The layout is available on Ceriatone website. It's tricky to change without removing the board in my amp due to a large capacitor being in the way of the soldering iron. Obviously only mod your amp if you are very confident that you know what you are doing. Lethal voltages are present on this resistor if this is not done properly.  It may not be as simple as just turning the amp off as there are large capacitors in the amp which can store a lot of power after the amp is turned off. I cannot take any responsibility for this mod. Your life is in your hands! What I can say is, when done properly, I love the result.

Having got that out of the way, what I did was to replace this resistor with a 180k carbon film resistor rated for 2 watts. This works brilliantly in my OTS. I havnt looked at the hrm schematic, so can't comment on the suitability of this mod for that amp.

Hi exocet,
So it seems I have what you call a "high plate" amp as per the stock OTS config. I am deffinitely not trying to suggest that there is anything incorrect about the "high plate" config, nor the "low plate" config. Neither am I saying that the tonestack is incorrect in and of itself. When you say that the tone stack is based on actual examples do you mean that the combination of a "high plate" amp with a 150k slope resistor is based on an actual example?

There is always an impedance mismatch between v1a and the tonestack (assuming the guideline that "input impedance > 10 x output impedance" can be applied). In any case, this is never as high as 10 times, however, I feel its fair to say that the exact ratio should be preserved in all designs. After all, what reason would anyone have for deliberately changing this mismatch?

I know its a strong point of view, but the effect is so dramatic, and the theory seemingly sound, that I feel happy to assert that a "high plate" config requires a different slope resistor from a "low plate" config. I think the effect of taking this on board is so positive that I feel this goes beyond personal preference, and is in fact correct. I appreciate this is likely to be controversial in its lack of relativism.

Either way, whether this is a design flaw, or just my personal prefference I reccomend everyone first using this amp to experiment with this mod. I would personally love to see it incorporated into the OTS & S&M designs (maybe others require an adjustment too, I havn't checked all the configs...) I have tried this with a variety of loading scenarios (i.e. loads before the amp), guitars, effects pedals. There is no scenario which is not improved by the mod.

The effects of the mod are quite subtle in some scenarios (those which were not previously problematic) but dramatically better in other scenarios (those which were previously problematic). This further makes me feel that this is a design issue, rather than my personal preference. The change is not as simple making everything brighter or everything darker as might be attributed to preference, but seems to "correct" a number of issues

So,
I've been mainly happy with my OTS 100w and had about 6 months to get used to gigging it and to find the right speakers etc. I've found a startling discovery which has really made the amp work for me. I wanted to share it with everyone here in case they may like the improvement.

The issues I had were:
1 - Although the tone controls had a noticeable effect on the tone, they weren't as responsive as other amps I have had: the tone always seemed to have a slightly lo-fi quality for lack of a better word...
2 - The PAB was useless live. Too much bass was introduced unless I cranked the treble control, but this caused problems when PAB was not engaged.... This was especially bad on the clean channel, where the tone sounded flabby at all times with PAB on.

So I played with cathode and plate resistors. Even modded the amp to S&M specs. Also reduced snubbers to 270p. All these made a difference to the overal tone, but did not seem to impact the two issues above.

Today I was looking at a schematic of Dumble 124 and i got to thinking something. The cathode and plate resistors on v1a have been inreased in the ceriatone when compared to this dumble. I have read this helps warm up the sound somewhat. I thought, well I could change them to 124 specs, but would this really be the cause of all my problems? It turns out not.... the solution is more complex to explain, but simpler to implement:

The tone stack in 124 is the same as the ceriatone, so I realised that changing the plate resistor on v1a would have a likely impact on the impedance seen by the tone-stack, but this hasn't changed, so perhaps there is a mismatch, when compared to the original design. Turns out (assuming a 12ax7 output impedance of around 60k) that the change to the plate resistors cause the output impedance of v1a gain stage to increase from around 38k to around 47k... but the tonestack has not been modified, and still has the same input impedance. Guess what this does?  

So, I thought, can I re-create the same impedance match between v1a output and the tonestack input? Yes, I can! The slope resistor is surely the key.... So, what if we scale the slope resistor up by the ratio by which the v1a output impedance changed (47/38). Well, we get around 190k. Didn't have resistors to make up this value, but I had a 180k lying around. Replacing this one resistor made the amp behave beautifully in all scenarios! I couldn't believe what I was missing before I did this!

It seems that someone has introduced a well meaning mod (to change the v1a cathode and plate config) but forgotten to take account of the impedance impact on the next stage. When this is factored in, I think the amp behaves "as originally designed". I must stress that the plate and cathode have not been changed to 124 values, so the well meaning mod has been preserved, its just been taken to its full conclusion and implemented "correctly" (in my opinion obviously).

I urge all OTS owners to try this and welcome and views or comments on the above!

Cheers,
K

I've talked to nik and we dont know why I am getting the bleed that I am getting. Its something inside the amp causing it. Not sure what....

Just one thing nik menitoned which may be interesting. I wondered why you were seeing the same symptoms as me but on a different jack.... Nik made the point that a floating cable at the pre-amp in doesn't break the signal through the amp so your behaviour is normal and expected. I thought you may have the same problem as me because your master did not change the level of the "bleed" but of course, as your amp is HRM, the master doesn't impact the level of the distortion channel anyway!

I think this problem is either something unique to my amp or perhaps something to do with the non-HRM design...

Also, I have realised that this bleed in my amp is what is causing the bleed in the c-lator. With the c-lator not plugged into the mains and all capacitors drained (0v at all nodes in the c-lator) signal passes from guitar to speaker. The c-lator out control is connected to the PA in and therefore can be used to shunt the leak to ground (although the signal level still does not respond to the master volume! argh...

EDIT:
I have found an interesting bit of behaviour which may help narrow down the problem. It seems that with a cable in the PA in pin 2 (grid) of the PI becomes hyper sensitive, the cable leading to it makes loud noises when tapped with a chopstick. The same is true for the ground lead going to the 390 ohm resistor in the PI network. This is much more sensitive than pin 7 which suggests a huge PI imbalance (only when PA in has a floating lead!). This sensitivity does not occur when the PA in is removed and the amp is set to top volume so its obviously behaving abnormally in my scenario.... the search goes on.

Regards.

Hi,
I've been checking my relays just now actually and can see no probs. Orientation is deff fine too.

I have looked at the hrm layout. Its clearly different with respect to the relay switching config. I am pretty sure you are right about this causing the difference in our observations.

I think it be possible that whatever cross talk is happening in both of our cases is occuring within the relays themselves. I have tried swapping the two relays in the amp to make see if it might just be a leaky relay, but both behave the same, so chances are this is a design issue. I still can't explain why either of our amps leaks this signal (although in different ways). I have tried moving wires around to see if I can lessen the effect through lead-dress but not a single one has made any difference.

Because the HRM circuit is so different in the switching area I cannot see a direct way to change my circuit to closer match yours (basically, you seem to have your master connected before the relay hence any attenuation it provides also attenuates the cross-talk whereas my master is after the relay and hence any cross talk may be amplified?). I think i would rather have my amp behaving like yours in this respect as the bleed issue makes it harder to debug my C-lator (the bleed essentially running in parallel with the loop). I will have a think to see if I can come up with an improved switching scheme for the non-HRM which circumvents this issue.

Perhaps I shouldn't worry too much about this and focus instead on the noise that my Clator introduces (which I suspect may be a ground loop issue). Maybe others with non-HRM models can confirm my observations?

Once again, sorry about the pic it's very grainy. The mv connects to the connection between the preamp out tip and pa in break.

Hmm, interesting that you have the same effect on opposite jack. Even if our layouts are the same it could be a difference in switches (e.g. yours may break on the right-side looking at the top whereas mine may break on the left...) I don't know if nik might have changed supplier or something... My layout seems to match the photos on the ceriatone website. Anyway, I've attached a pic of my amp fx loop wiring. Its not the best quality as I only have my phone to take photos!

EDIT: When I look into the hole on my jacks, looking at the silver bands at the top, my break is on the left side. Is this the same with yours?

When I plug just a cable in the pre-out its like no change to the amp. It still passes signal as it with no cable in the pre-out. When I plug just a cable in the PA-in I get signal from the guitar to the speaker and it is really loud (turning the master volume has no effect on the volume of this signal, is this the same for you?). With floating cables in both pre-out and pa-in its just like a single cable in the pa-in. As soon as the something is plugged in to the other end of the pa-in cable the guitar signal no longer comes through and is replaced by whatever is plugged in (e.g. CD player). Because of this, when my c-lator is off, the behaviour is very similar to just having floating cables in both pa-in and pre-out (i.e. guitar signal bleeds through the amp but much more quietly though still not affected by the master volume control).

As for the C-lator I tried the supplied JJ tube as well as a sylvania and mullard I have lying around. All the same...

Thanks,
Yes there appears to be two problems:
1.) The amp effect loop is at fault for the bleed aspect. Say I have the distorted channel on with a low master volume setting. When I plug a single cable into the PA return (not connected to anything, i.e. floating) the distorted signal from the Preamp bleeds into the PA. The bleed somehow bypasses the master volume and in the above scenario the distorted channel bleeds very loudly (much louder than the master volume is set). The C-lator interacts with this effect hence my initial observation of bleed in the C-lator, which I think isn't at fault here.

2.) The C-lator introduces quite a lot of hum and hiss when used. This seems to be hugely exacerbated when turning effects in the loop on. When using these same effects in the amp loop with no C-lator no noise seems to be introduced.

EDIT
Regarding issue 1 above I am starting to feel this is a "feature" of the amp rather than something I have done wrong in building it. What I observed was the plugging in a longer cable to the PA in causes the bleed to reduce. It doesn't matter whether the cable is (i.e. pointing the floating end closer to the amp does not increase the bleed). This indicates it is not EM crosstalk to the floating lead but rather the effect of cable capacitance forming a path to ground from the floating PA in (more cable means more capacitance hence more signal shunted to ground). The reason the bleed is so loud is that the power amp is so strong (100w) and sensitive that it is exagerated. As soon as I plug a CD player into the floating cable the bleed is dwarfed and cannot be heard.

Can anyone else confirm that plugging a floating cable into the PA in whilst using their amp cause the guitar signal to come through the speaker bypassing the master volume?

Hi, thanks for the reply. Its a scratch build so it must be something I've done. Im just worried that I have damaged a component in initial testing. I will check the wiring again of course, but I'm pretty sure its OK. I'm really hoping for someone to give me a diagnosis based on the symptoms. My first guess is its a filter cap gone bad, but before I buy a replacement and rip out the board I would like to hear any suggestions.

EDIT

I've done some more tests to identify the problem and it seems to be something to do with the interaction between the c lator and my OTS100. It seems that the OTS bleeds signal from the preamp to the power amp when the poweramp in is plugged in but floating (e.g. just a cable in the poweramp in causes a large preamp signal to pass into the power stage). When the C lator is in standby it is essentially floating the amp's power amp in (the position of the out pot determines how much of this is bled to ground).

 For the distortion channel this bleed can be controled by the level control but the ratio and master do nothing. For the clean channel this bleed can be controlled by the master.

This is all a bit weird. I have tried identifying the leak in the amp itself but no luck. I've re-dressed all the wires and this makes no difference. The only thing I have determined is that grounding the floating power amp in causes the leak to stop.

Gregg,
Thanks for your info. How can it be a grounding issue if turning the amp fully on makes the hum go away? When on fully I cannot see any problems with the amp: no hums, no hiss, no oscillation untill gain and volume levels get ridiculous.

OK,
I've done some more testing and it seems the problem is nothing to do with the power tube section.

With the power tubes removed the hum still occurs in standby - this makes me think the hum is being induced in the OT itself or the leads to it (probably secondary because the hum volume or pitch do not change as I switch the impedance selector).

When I removed the preamp tubes as well (hence ensuring no heater current flowing through the amp (except the indicator light) nothing changed. Therefore I don't think the signal in the OT is being induced by the heater wires.

Whatever it is causing it I am starting to think that I shouldn't worry about it because the hum stops as soon as I fully engage the amp and its not very loud (about the same volume as my fridge!). I have read elsewhere that a standby hum could be a symptom of a rectifier problem, so I am still worried enough to hear any ideas anyone may have!!!

All the best.

Hmmm,
Checked the wiring (again) and I cant see anything wrong. The only thing I can see that may be wrong is that of the various AC taps from the power tranny I might have reversed polarity for some (this shouldn't make a different though, should it). Another thing is that I have twisted my 50v supply line, whereas on the pictures on ceriatone website these seem to be not twisted.....

I measured voltages around the power tubes in standby and the only thing I noticed which didnt seem right was that Grid 1 (Pin 5) on all the tubes measured -50V relative to ground. I cant see any wiring problem though...

Any ideas?

Many thanks!

I must have done something wrong when building my C-lator because I get a bunch of weird stuff hapening when plugged into my OTS100. I have poured over the schematic and can see nothing wrong (argh!) Obviously tried different tubes too as well as lifting the mains ground temporarily: no luck... Can anyone diagnose what the problem may be?

1.) When turning the unit into standby sound passes through the unit. This is before the unit is actually on.

2.) When turning from standby to on (after the reccomended 30 secs warmup) there is a lot of noise. As I turn the "in" pot to maximum some weird thudding sounds occur (thought it was a bad pot, but read on). After about 3-4 mins this seems to stop so the pot is not to blame, its something electrical! Generally, the unit seems noisy (hiss) when on even after the 3-4 mins and it seems to cause some very high freq oscillation in the rest of the amp when the presence is high up.

3.) When I turn the unit back to standby sound still passes through it for about 3-4 mins (is it a coincidence that this is a similar time period to issue 1?)

4.) When I turn the unit totally off (even unplug it) then sound still passes through it exactly like issue 2 above.

5.) When working on the unit I realised it keeps voltage in its caps much longer than my amp, it seems to never fall when turning off, so I have had to bleed the caps each time I work with it.

I am at wits end with this one? Perhaps I have broken something whilst experimenting with the unit. I can certainly perform a fix, if I have some idea of what needs fixing!

Any help would be much appreciated.

Hi all,
I've got my OTS 100 working beautifully. Really quite amazed at the sound, its a really special (!) amp. Just one observation, when in standby there is quite a loud hum. As soon as I turn the amp fully on the hum goes away completely (in fact the amp is super quiet!). Obviously this doesn't cause me any tone problems, but I wonder whether I have done something wrong in the build.

Has anyone else experienced anything similar or have any ideas as to the cause? It may be a pain for live use as I like to leave the amp on standby between sets, but the hum may make this difficult!

Many thanks,
BK