Ceriatone OTS not loud

[Bob-I]

.... It may have a high frequency oscillation that's robbing the amp of power. Very hard to track down without a scope.

How can you see high freq oscillation on a scope - how to do that??

It can be pretty easy at times, and a little tricky but here's the gist of it.

Start by turning the amp on, with speaker connected and connect the scope. Look for any wave forms that shouldn't be there. Sometimes it'll look like a HF sine wave. If you don't see this, you're not clean yet.

Next connect a dummy load, input a sine wave at 500-1000HZ and look at the sine wave at the input and output of every stage. If there's oscillation you'll see squiggles at the peak of the sine wave.

Adjust the tone controls thoughout the range, adjust the volumes up to clipping. The sine wave should flatten on top and not become squiggly.

[glasman]

I had one on my bench about a week ago. The most it would do at 8 ohms was about 31 watts. This was with two different different sets of output tubes (one that is known to do at least 48W). I didn't go into too deep, but probably the OT. The voltages were about right.

Gary

Curious how you are measuring this Gary? Quiescient power with a plate voltage of about 440V and a bias current of 35mA will give you 31W for any dual 6L6 amp. (As I'm sure you know, once you start measuring with a signal, then you need to specify the frequency and the distortion at which you measure.) IOW, 1% THD vs 0.1% THD yield *very* different powers, and are not necessarily significant in a guitar amp, except on a relative basis. Also not sure what you mean by power tubes "doing" 48W....did you re-bias the amp to take advantage of that capability (set them to 55mA @ 440V)?

The OT is pretty substantial, so I'd be surprised if it was a bottleneck in the power transfer, but I am curious as to your measurements.

To the OP, you still haven't really said if the amp gets loud as you turn up both the master and the vol.....so I'm still not sure if you are complaining about the taper of the pots or the ultimate volume levels. With the MV maxed and the vol maxed, your ears should be bleeding.... I don't find setting everything at 12:00 to be especially loud in a band setting at all, and that goes for any of the D-type amps I've owned.

The measurement techique is pretty simple.

1. Insert 1 1khz into the return jack on the back.
2. Adjust the generator output level until the signal at the 4 ohm tap just starts to clip.
3. Measure the P/P signal. (I verify with a Keithly true RMS meter)
4. Divide by 2.8
5. Square the result and divide by the load impedance.

Result is the RMS power at clipping. I use exactly the same technique on my own amps, typically I see between 44 and 48 watts using this technique on 50 watt amps.

FWIW, I had a second Ceriatone here yesterday and it measured about the same as the first (33W) on JJ 6L6s. Supply at idle was 440v.

To me it is one of two things, the OT has the wrong impedance for the voltages, or the power supply lacks the headroom needed. Both of which as pretty easily to check and if I have another one here I will check both.

FWIW, both of these amps were factory wired and personnally I thought that they were pretty well executed. Neat wiring and decent parts. I also noticed that Nik is now riveting the corners of the chassis (this amp was delivered two weeks ago). The only adjustments I made were to the PI trimmer and changed the first divider resistor to drop the V1 and V2 plates down a bit, the amp was a tad harsh. After taming it down we were able to turn up the internal trimmer a bit (it it VERY low as delivered)The owner was very happy and the cost for this service ... nothing, he is buying a GW 2x12 cab for his OTS.

[fullerplast]

The measurement techique is pretty simple.

1. Insert 1 1khz into the return jack on the back.
2. Adjust the generator output level until the signal at the 4 ohm tap just starts to clip.
3. Measure the P/P signal. (I verify with a Keithly true RMS meter)
4. Divide by 2.8
5. Square the result and divide by the load impedance.

Result is the RMS power at clipping. I use exactly the same technique on my own amps, typically I see between 44 and 48 watts using this technique on 50 watt amps.

FWIW, I had a second Ceriatone here yesterday and it measured about the same as the first (33W) on JJ 6L6s. Supply at idle was 440v.

To me it is one of two things, the OT has the wrong impedance for the voltages, or the power supply lacks the headroom needed. Both of which as pretty easily to check and if I have another one here I will check both.

FWIW, both of these amps were factory wired and personnally I thought that they were pretty well executed. Neat wiring and decent parts. I also noticed that Nik is now riveting the corners of the chassis (this amp was delivered two weeks ago). The only adjustments I made were to the PI trimmer and changed the first divider resistor to drop the V1 and V2 plates down a bit, the amp was a tad harsh. After taming it down we were able to turn up the internal trimmer a bit (it it VERY low as delivered)The owner was very happy and the cost for this service ... nothing, he is buying a GW 2x12 cab for his OTS.

Thanks for the description. That sounds perfectly reasonable, but unfortunately power ratings of tube amps are one of those things that have *a lot* of leeway either direction, depending on who is measuring and what the THD is defined to be where the measurement is taken. At the onset of clipping is fine if everybody was consistent, but often some degree of distortion is considered acceptable in a guitar amp and that's where the variations come in....nobody really specifies what their particular power rating is measured at. Five or 10% THD is often considered very acceptable in a guitar amp and makes a huge difference in the rated power. (Not defending or condemning any builders in particular....power "optimism" is common practice in the industry ) We would absolutely find huge descrepancies were we to take 20 amps off the shelf from various manufacturers and compared their power ratings in a consistent manner.

I don't know at what THD level Nik (or anybody else besides yourself) measures their power, but going back to the original poster's issue.....the difference in volume between a 44W amp and a 33W amp is just a bit over 1dB. That's considered the lowest perceptable change in volume that one can hear, so as your measurements apply to his particular problem, I doubt that is the issue. Most speakers have a larger variation than that.

Thanks for your input on the V1 and V2 plate voltages....do you recall what voltage you dropped them to? I find the useable range of the trimmer in this amp to be in the 9:00-10:30 range. Anything higher tends to be harsh sounding.

[bluesfendermanblues]

.... It may have a high frequency oscillation that's robbing the amp of power. Very hard to track down without a scope.

How can you see high freq oscillation on a scope - how to do that??

It can be pretty easy at times, and a little tricky but here's the gist of it.

Start by turning the amp on, with speaker connected and connect the scope. Look for any wave forms that shouldn't be there. Sometimes it'll look like a HF sine wave. If you don't see this, you're not clean yet.

Next connect a dummy load, input a sine wave at 500-1000HZ and look at the sine wave at the input and output of every stage. If there's oscillation you'll see squiggles at the peak of the sine wave.

Adjust the tone controls thoughout the range, adjust the volumes up to clipping. The sine wave should flatten on top and not become squiggly.

Thanks, I'll check my amp just to be on the safe side.

[glasman]

Thanks for the description. That sounds perfectly reasonable, but unfortunately power ratings of tube amps are one of those things that have *a lot* of leeway either direction, depending on who is measuring and what the THD is defined to be where the measurement is taken. At the onset of clipping is fine if everybody was consistent, but often some degree of distortion is considered acceptable in a guitar amp and that's where the variations come in....nobody really specifies what their particular power rating is measured at. Five or 10% THD is often considered very acceptable in a guitar amp and makes a huge difference in the rated power. (Not defending or condemning any builders in particular....power "optimism" is common practice in the industry ) We would absolutely find huge descrepancies were we to take 20 amps off the shelf from various manufacturers and compared their power ratings in a consistent manner.

I don't know at what THD level Nik (or anybody else besides yourself) measures their power, but going back to the original poster's issue.....the difference in volume between a 44W amp and a 33W amp is just a bit over 1dB. That's considered the lowest perceptable change in volume that one can hear, so as your measurements apply to his particular problem, I doubt that is the issue. Most speakers have a larger variation than that.

Thanks for your input on the V1 and V2 plate voltages....do you recall what voltage you dropped them to? I find the useable range of the trimmer in this amp to be in the 9:00-10:30 range. Anything higher tends to be harsh sounding.

As far as THD and absolute power, in this case it really doesn't matter (to me) as I have a technique for measuring amps that I have used for years. The same technique, same person evaluating the results. On most amps this yields measurement taken at approximately 2% (and yes I do have the equipment to check THD). Ceriatone to me seems a little under powered as compared to my amp builds.

As far as final voltages, V1 is 182 and 185, V2 is now 193 and 194. PI plates were at ~285. V1 plates were over 200 volts as delivered. Both the customer and I both agreed that is sounded much better at these levels. I also had to readjust the PI trimmer and this probably helped the tone as much as the voltage drop.

[Fischerman]

Gary,
I just have a scope, generator, and DMM. I never really know how much distortion is acceptable when I try to measure power...the waveform slowly gets chubby at the tips but I don't know how chubby 1% or 5% or whatever is. What does the waveform look like at 'max clean power'? Is there any noticeable chubbiness on the waveform? Thanks.

[glasman]

Gary,
I just have a scope, generator, and DMM. I never really know how much distortion is acceptable when I try to measure power...the waveform slowly gets chubby at the tips but I don't know how chubby 1% or 5% or whatever is. What does the waveform look like at 'max clean power'? Is there any noticeable chubbiness on the waveform? Thanks.

I normally raise the signal to clipping and then back off the generator just to the point where the top starts rounding off.

Next time I am adjusting an amp I will post some pictures.

Gary

[fullerplast]

Gary,
I just have a scope, generator, and DMM. I never really know how much distortion is acceptable when I try to measure power...the waveform slowly gets chubby at the tips but I don't know how chubby 1% or 5% or whatever is. What does the waveform look like at 'max clean power'? Is there any noticeable chubbiness on the waveform? Thanks.

You've actually hit on both the reason power measurements at a particular THD are both subjective *and* the reason they are not really critical for tube guitar amps. Tubes don't clip like SS devices....it's more of a soft compression/waveform distortion, and really it's what makes tubes more pleasing to the ear. As you indicate, it's difficult to precisely quantify the distortion by eye with a scope. The distortion is best seen by observing the harmonics via a spectrum analyzer or by using a notch filter to filter out the fundamental. Any delta from the fundamental is distortion.

From a guitar player's point of view however, it would be extremely difficult to tell the difference by listening, and in fact more distortion would likely be more pleasing to the ear....At that point the whole issue of where you measure power becomes simply an exercise in relative comparisons between specific amps, to the extent you can consistently identify the distortion point at which you wish to measure power. That is what Gary is doing here, and how accurately he can distinguish "clipping" from amp to amp (which I think is your question ) will determine how accurate the relative power measurements will be.

[Structo]

I found this chart the other day which kind of displays what you are saying about the difference between a tube amp and solid state amp is.

[img:448:293]http://users.chariot.net.au/~gmarts/pix/dist-decay.gif[/img]

[martin manning]

Great thread here; lots of useful stuff!

...To me it is one of two things, the OT has the wrong impedance for the voltages, or the power supply lacks the headroom needed. Both of which as pretty easily to check and if I have another one here I will check both.

Can you elaborate a bit on this? Re the OT, do you mean just checking the Z ratio or something else? And, I'm very interested in how you would test the power supply.

Thanks,

MPM

[Boboscat]

I had one on my bench about a week ago. The most it would do at 8 ohms was about 31 watts. This was with two different different sets of output tubes (one that is known to do at least 48W). I didn't go into too deep, but probably the OT. The voltages were about right.

Gary

Gary,

How would you rectify the problem of the OT having the wrong impedance for the voltages or the power supply lacking the headroom needed?

thanks,

[Boboscat]

I had one on my bench about a week ago. The most it would do at 8 ohms was about 31 watts. This was with two different different sets of output tubes (one that is known to do at least 48W). I didn't go into too deep, but probably the OT. The voltages were about right.

Gary

Gary,

How would you rectify the problem of the OT having the wrong impedance for the voltages or the power supply lacking the headroom needed?

thanks,

[Boboscat]

I had one on my bench about a week ago. The most it would do at 8 ohms was about 31 watts. This was with two different different sets of output tubes (one that is known to do at least 48W). I didn't go into too deep, but probably the OT. The voltages were about right.

Gary

Gary,

How would you rectify the problem of the OT having the wrong impedance for the voltages or the power supply lacking the headroom needed?

thanks,