JMI VOXAC30 TOP BOOST low headroom.

[Swamp]

Hi! Here Daniel from Portugal! I am reading this forum for a while and now I hope get some help if possible.

I have an VOX like this one:
http://www.voxshowroom.com/ct/amp/uk...ube_layout.jpg


It took some new resistors previously, and I just had to finish the recap and correct some wrong values in it.

All voltages seems OK now, and I put new sockets on power tubes and V1.

The problem is that the amp seems have low headroom. On 3, it sound loud but not so clean. I really believe that this amp have more headroom.

The only thing different from the original schematic is a 2x 32Uf can cap, that on the schematic seems be 2x 16uF.

With ECC81 on V1 I got more headroom but I don't think thatīs the way to do this.

Starting to believe that there is some problem with the OT.

I can check it on scope.
Thanks

[pdf64]

Do you know if the EL84 are all good and reasonably well matched?
Have you substituted a known good tube of the correct type into each socket in turn?
Try comparing the voltages in your amp to http://bmamps.com/Schematics/vox/Vox...lt-factory.jpg
OTs are tough; if it was made in the 60s, failing resistors and capacitors are far more likely.
In addition to the scope, have you got a good meter, signal generator and a dummy load for the amp?

[Swamp]

Do you know if the EL84 are all good and reasonably well matched?
Have you substituted a known good tube of the correct type into each socket in turn?
Try comparing the voltages in your amp to http://bmamps.com/Schematics/vox/Vox...lt-factory.jpg
OTs are tough; if it was made in the 60s, failing resistors and capacitors are far more likely.
In addition to the scope, have you got a good meter, signal generator and a dummy load for the amp?

Tested qith another matched EL84 quad,
Changed pre-amp tubes to check,
I have a fluke, signal generator and dummy load. I can post some pics for the waves.

This low headroom is a problem in all channels, with a telecaster the amp distort on 3, with les paul is even worst, I already had a couple of Ac30, itīs the first time I see sth like that


I will give you the voltage on the PI plates later.

Thanks for the help

[Mick Bailey]

The main OT failure that will produce a distorted output is a shorted turn. With this you get very low volume and ugly distortion. You say that the amp is loud - this suggests to me that the OT is probably OK.

If you have a scope and signal generator you should be able to inject a guitar-level sine wave at the input and trace this through each stage to find out where the distortion is happening. If you first look at the output (ideally into a dummy load) you can see if one half of the waveform is clipping excessively. Check that the PI outputs are the same. I usually work back from the output.

[Swamp]

The main OT failure that will produce a distorted output is a shorted turn. With this you get very low volume and ugly distortion. You say that the amp is loud - this suggests to me that the OT is probably OK.

If you have a scope and signal generator you should be able to inject a guitar-level sine wave at the input and trace this through each stage to find out where the distortion is happening. If you first look at the output (ideally into a dummy load) you can see if one half of the waveform is clipping excessively. Check that the PI outputs are the same. I usually work back from the output.

Thanks Mick

I will check the output waves and on the PI, than I will see from where is coming this distortion (Preamp/power amp).

[Swamp]

Removed those "tube amp doctor" caps 8uf with new F&T caps.

new 1k2 at PI, one of the 100K plate resistor was reading 88K.

Itīs seems more loud now..

[Tom Phillips]

Swamp,
You indicate that you have the test equipment to measure the amp's output power. What is the maximum clean power that the amp will produce into the dummy load just before it starts to clip? That measurement will give a good indication of the overall health of the amp.
Tom

[Swamp]

Swamp,
You indicate that you have the test equipment to measure the amp's output power. What is the maximum clean power that the amp will produce into the dummy load just before it starts to clip? That measurement will give a good indication of the overall health of the amp.
Tom

Exactly.
I will connect it today and post pics and values.

I set bias of amplifier by calculations, never used another method. I searched a lot about this, and canīt find some good info about it. There is any way to see how much power my amp is dissipating, with the maximum voltage value given in a dummy load?

I always connect to the scope to check is the wave is symmetrical, I see amps going from 45V to 90V or even more sometimes, (also to warn people to not fuck with the speaker cables) but never really used those values for anything. Just trust the mA of idle current on tubes, mark the 60% and the 70% and set the bias between that where I find the sound better.

[Jazz P Bass]

FYI, the formula for the output power of an amplifier is V x V / R.

Where:
V= the output voltage (usually stated in RMS, not critical if simply looking for a ballpark figure)
R= the load resistance.

[Mick Bailey]

.Just trust the mA of idle current on tubes, mark the 60% and the 70% and set the bias between that where I find the sound better.

The AC30 is cathode biased - no adjustment and often 100% dissipation (sometimes more) giving a short but glorious tube life.

[Dave H]

There is any way to see how much power my amp is dissipating, with the maximum voltage value given in a dummy load?

No, the voltage across the dummy load will give the power the amp can drive into a load not the plate dissipation of the tubes. You measure plate dissipation by measuring plate current and multiplying that by the plate to cathode voltage. Check it, It should be as Mick said for an AC30.

To measure the power output use a sig gen and dummy load, turn the signal up until there is a small flat on the peaks of the sinewave at the dummy load. Note the p-p voltage, divide it by 2.828 (to get rms), square the result then divide by the dummy load resistance (or measure the voltage with a DVM to get rms directly). If the output is 30W or more (AC30 is about 33W) then there isn't a headroom problem. There's just too much gain making it give full output at low vol settings.

[Swamp]

No, the voltage across the dummy load will give the power the amp can drive into a load not the plate dissipation of the tubes. You measure plate dissipation by measuring plate current and multiplying that by the plate to cathode voltage. Check it, It should be as Mick said for an AC30.

To measure the power output use a sig gen and dummy load, turn the signal up until there is a small flat on the peaks of the sinewave at the dummy load. Note the p-p voltage, divide it by 2.828 (to get rms), square the result then divide by the dummy load resistance (or measure the voltage with a DVM to get rms directly). If the output is 30W or more (AC30 is about 33W) then there isn't a headroom problem. There's just too much gain making it give full output at low vol settings.

Hi Dave!

I am a reader, I read a lot before asking something.

For long time I try to make this calc!



Voltage on 16Ohm dummy load = 65.7V

65.7 / 2.828 = 23.2319

(23.2319)^2 = 539,72

539,72 / 16 = 33,73 WATTS

And we are good now

After the changes of yesterday I canīt put the volume of the amp more than 3 at home. Before I was at 6 or even more.


THANK YOU!!!

Portuguese wine for the boys!!

[TimmyP1955]

I would think that such a nice scope would allow the voltage readout to be changed to VRMS instead of P-P - you might have a look.

[Swamp]

I would think that such a nice scope would allow the voltage readout to be changed to VRMS instead of P-P - you might have a look.

I have no Idea!

The DMM is a Fluke 115, and its true RMS. No Idea if the scope is. Have to search about it

The thing I love about this Scope is the AUTOSET

http://www.testequipmentdepot.com/ha...507-3_manu.pdf

Thanks for the tip