Quote from: Joe on June 29, 2015, 03:13:07 PM
My motivation for doing this is simply to make the volume control a little more usable. Right now it goes from "zero" to "loud" with a 1MM turn of the knob. It's loud or nothing basically.

That is rather strange and makes me think there is a problem with the volume control, as in the track is broken on one end.
A volume control is a logarithmic pot which allows a gradual change of the volume level. If the track is broken at one end either by way of a dry joint or the carbon track is broken, you will get a leap in volume for a small movement of the volume control. Volume pots usually form a voltage divider with one end to the out of the preamp, the wiper out to the power amp, and the other end of the pot going to ground. If the ground side has lifted you will get this effect.

 I don't think so, unless I have missed something here.

Quote from: ilyaa on June 23, 2015, 09:32:45 PM
with rectifier tube removed:
transformer end of the main filter resistor shows short to ground - other end shows 50 ohms to ground....

If the output transformer was blown, that would not account for there being no heater Voltage.

If the Power transformer secondary had melted then there would be a short to ground via either of the 2 centre taps.
Best to start with the Cap anyhow just unhook transformer side and test for short on the cap and the winding, although caps usually show physical signs of a problem, so I doubt it is the cap. Hopefully I am wrong though as it is cheaper.

 I have never seen the need to replace any cap. It all depends on what caps you are talking about, but I have never seen a cap blow an amp up by itself, it is usually a symptom of the fault, rather than the cause.

I am surprised you want to do this, other than you want to put the head on a smaller cab.

I do agree that 25W is no where near good enough, given that you need to have something which will protect your amp.

Probably the easiest way would be to use a switched resistor in line with the volume control, but this takes a little bit of a calculation and use of a signal generator and a scope and a dvm. A scope will show you if the signal is clean or clipping.

Output impedance should always be matched as J M Fahey has stated and the combination of series and parallel resistors is of course a must, don't forget that the speaker forms part of this.

Resistor Wattages should really be all the same not only for the ease of purchasing but because you are strapping a resistor across a speaker whose impedance does change with frequency.  If a speakers impedance increases, you will find that the load resistor across the speaker has to dissipate more Wattage. You can use wire wound Aluminium Can or Ceramic resistors, whichever has the values you need.

Speakers are reactive in nature so are Not, strictly speaking, resistive elements. They are really just motors.
As far as the capacitors you have seen are concerned, they could form part of a reactive filter circuit in the output.

Use your nose. Transformers tend to whiff if the insulation melts between the windings. this can be a cause of the short.
Best way to check is to lift the ground off the centre tap of the secondary on the mains transformer, the only other way it can short is a melt down between the laminations and coil.

Don't forget to check the caps as these can also short.

Output power is determined by the characteristic curves, which I don't have access to. However it also depends a lot on the transformer used.

As far as the transformers are concerned, if you got to Hammond or Sowter, or indeed any company that produces quality transformers for audio, they will be able to supply whatever you want.

I did wonder about the validity of the current figure shown, so did a search which may help. The book is tattered and sello-taped together.

http://www.diyaudio.com/forums/tubes-valves/255577-12al8-head-amp-utc-output-transformer.html

The circuit shown claims 40mW into the 800 ohm Load.

Data Sheet http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCUQFjAA&url=http%3A%2F%2Ffrank.pocnet.net%2Fsheets%2F127%2F1%2F12AL8.pdf&ei=Nl2RVcK3HMHC7gaRjLrwBg&usg=AFQjCNHaXi67E9EqKxXg4n2-cG8semS7Ow&bvm=bv.96783405,d.ZGU

The caps may have been replaced with better ones. 100uF @ 25V sound like smoothing caps on the preamp section, the Voltage is no where near high enough for the power amp sections.

Can you post the schematic and I can tell you for sure.

Quotethere is still a bit of hum on the effects section of the pre amp

How have you determined this ?

Is it a low frequency hum ?

If it is low frequency and it only happens when you increase the volume, then it is more than likely mains hum.

Any amp with a PSU that is inadequately smoothed will have hum. There is also the fact that poor earthing can also be a cause, so make sure that you have not, inadvertently, pulled off any wires.

As an aside the resistors you are calling cement are ceramic heat dissipating resistors, with a wattage rating that can go from 1W up to 100W or more. Browning of the ceramic is quite normal as these are used in the output section of the power amp.

To answer that question.

Quote from: difly on April 14, 2015, 06:33:17 PM

I would like to make his preamp to insert into another poweramp,

Could anyone help me ? To build the preamp I do like the image ?

Thanks in advance!

To answer the jfet question of why it is used at the front end. You have to look at the characteristics of Valves, Transistors and FET's, which will make it become clearer.
Both Valves and JFETS are Voltage amplifying devices, whereas a transistor is a current amplifying device.

What difference does this make I here some saying. Well when a transistor starts to distort, it is the second harmonics which effect it the most, which gives, to me, the horrible course sound. Now compare that with a Valve, which has its distortion effected more by the third harmonic, which gives the characteristic waveform. This tends to not clip as badly as the transistor, and sounds a hell of a lot better.

Another thing with FET's is that their impedance at the gate is very high, so is less likely to effect the input signal, it also makes input impedance matching easier. Read up on small signal analysis with a transistor taking particular attention to how all the resistors at the input effect the input impedance.

Standard practice for most people, is to rip out all power transistors and check them, then forget about the protection circuit, then wonder why nothing works.
Many times you will find a protection circuit fails, with the result that there is no output, despite everything else looking OK.

Power amps are a pain, take nothing for granted as it is all DC coupled and a blown driver can cause you to loose a new set of output transistors. Check everything, all it cost is a little time.

Anode current is around 75mA for a 12AL8 from my tattered reference book.

Quote from: Ace-of-guitars on June 24, 2015, 09:26:19 AM
Ok. So I have a weird problem. I was just going to play when I noticed no sound coming from my amp. I tried playing around with the volume knob and magic! The PRESET changed!

Please help!

Did you try plugging it in ?

Whenever you report problems to anyone, be that on line or over the phone, be as detailed as possible, and always check what you would consider silly little things.
Someone trips over a lead and pulls a wire out of a mains plug, or dislodges a plug is not unheard of, check everything and report everything. You would not believe how many times you can get "my x doesn't work". "Well is the little light on", "erm no".

Quote from: difly on April 14, 2015, 06:33:17 PM
Hello Friend!

I wonder what it means M9 M8 M10 M16 M13 on the first page of the schematics that are connected in 100n capacitors. Could you tell me?

I would like to make his preamp to insert into another poweramp, but I did not understand where they link these capacitors 100n in components denoted with the letter M.

Could anyone help me ? To build the preamp I do like the image ?

Thanks in advance!

In each case given that they are next to a VR? which is not used, I would presume decoupling either across or to the wiper of the unused VR to suppress noise. No idea as I have not seen the M? symbol used before. Easy enough to check if you have the machine.
If you look at the diagram, all IC decoupling is shown relative to the IC's, as on the first sheet which shows IC 9,10,11,14 with decoupling caps.
I am a bit surprised that full decoupling has not been done on the digital IC's as in 10uF Tant with 100n decoupler, but guess it is cheap.
I have no idea about the earlier comment about this

QuoteEDIT: now I understand why: it´s an absolute ripoff of a Sansamp !!!!!

as there is nothing about that circuit that can be attributed to anyone in particular.
As far as circuits coming from somewhere is concerned, there are only so many ways you can design an amp circuit, some people can design some really good amps, but goof up in small areas, so why not improve upon something.

QuoteThe JFET input is actually a circuit idea conceived by Behringer

No it isn't, I designed an amp late 1970's with FET front ends, so no that is not original either.

Every design you can think of has similarities to other amps, it matters little where it came from as long as long as there is no patent.

I just wonder why you want to use a Behringer circuit for the front end to a power amp ?

What you really need to do is write down your criteria for a pre-amp as in what is its usage going to be and do you want something that is make do, or do you want something a bit special.

As far as power amps are concerned, when you design a system, you match your pre-amp to the power amp. So it is not just a question of banging an output into some power amp, and expecting quality.