Troubleshooting Amp

[SpareRibs]

Hello,
The input transformer of a tube amplifier turns AC into DC correct ? So anything past that will be measured as DC right ?

[Justin Thomas]

Hey Spare,

I'm pretty sure no transformer changes AC into DC. I think they're either step-up, step-down, or isolation. I'm thinking they step a small signal up to a larger one, before a first gain stage. I can imagine a time when such small trannies were cheaper and more space-efficient than adding a tube gain stage... but a transformer can't work on DC. One of the reasons for an (IT is most common example) transformer is to separate out - eliminate- any DC from the following stages.

Justin

[SpareRibs]

Hello,
OK but the voltage going through the tubes is DC right

[Robert Technology]

Transformers are AC devices, so the input and output voltage/current must be AC.

If you need DC voltage/current after a transformer (like we do in a guitar amp) you must RECTIFY it in some way. In an amplifier this is either done with a rectifier tube, or an arrangement of diodes.

You are correct that the voltage seen at a tubes plate from the power supply is DC, but there are AC voltages on the tubes as well...

[The Dude]

Voltage doesn't "go through the tubes". It is a potential causing current to flow through the tubes. Can you be more specific about what you mean or provide a diagram so we know more precisely what you are asking? What is this input transformer you speak of and where in the circuit is it?

[SpareRibs]

Hello,
This is the amp I am trying to check the voltages.

[olddawg]

That's simply an impedance matching transformer on the input. With that jumper in it isn't even in the circuit and you plug a standard unbalanced TS 1/4 plug with 2 conductors into it for a high impedance mic or an instrument. With the jumper out you would use a TRS type plug for a balance 3 conductor low impedance mic or whatever. The signal from the source is AC even if it is complex. This transformer has nothing to do with DC. It's an input matching device. You could take it completely out and installs a standard TS jack if you are using it for guitar.

[SpareRibs]

Hello,
So checking the values between the tubes and such will have DC current correct ?

[eschertron]

Hello,
So checking the values between the tubes and such will have DC current correct ?

With the amp at idle (no signal present) any currents that flow would be DC currents. [caveat: a broken amp that self-oscillates is not considered here] the DC currents that flow are the idle currents from the HV rail to ground through the tubes and whatever other circuits are present. In the case of a transformer (using the output transformer as an example) the DC current flows through the transformer, through the tubes, and to ground. Since the transformer does not "pass" DC, the speaker gets no signal (no induced voltage) from that DC current.

When there is a signal, an AC current is superimposed ONTO the DC current. For a tube this means that the average, or net, current is always DC, but what is of interest to the user is that there is an AC current that can be used for amplification. Using a preamp tube for example, the voltage dropped across the anode resistor changes as the current through it changes, according to Ohm's Law. Higher current -> more voltage dropped across anode resistor -> anode voltage drops. Lower current -> less drop across resistor -> anode voltage rises. This is the way we use tubes as transconductance devices. But the net current through the tubes is always 'one way', or DC, because any reverse current forces the tube into cutoff.

Most techs will measure voltage at any point in the amp (quicker, easier, safer) and calculate what the current might be. Ohm's Law, again.

[52 Bill]

Are we sure of the correctness of the question here? Spare Ribs, what is it that you are wanting to measure? Are you asking how to set your meter? Are you really looking to measure currents or do you mean voltages?

Tell us directly what it is that you are trying to do and it will get you a more direct answer.

[SpareRibs]

Hello,
Here goes, I tried modifying the Newcomb amp using this LT Spice diagram . When it did not work I decided to put it back to stock.
I put it all back together, now it works but the volume is very low. From how it was before to now on a scale from 1-10 the volume would be about 2-4.
That is why I think there is less voltage getting through. The 6v6 tubes do not seem to glow as bright as I recall them glowing before.

[eschertron]

The cathode-biased preamp on the 'modernized' drawing will probably bring more joy. Which circuit do have before you? The original, or a modified?
Also, I see what looks like test, or actual, readings in parentheses on the spice drawing. Did you take those readings on your amp? Trying to find out what is known here.

And I'll echo 52 Bill to say I'm concerned that you say current when I'm hoping you'll say voltage. My personal rule is never set my meter into current reading mode, unless I'm doing some rocket-science-level stuff. And then I must be careful to set the meter back to voltage mode. Unless of course I want a shower of sparks and a trip to the store to buy a new meter

[eschertron]

That is why I think there is less voltage getting through. The 6v6 tubes do not seem to glow as bright as I recall them glowing before.

Good observation! So one thing to definitely check is the AC filament voltage for the heaters. you should have 6.3vac across pins 2 and 7 for both 6V6s, and the same voltage for the heater pins on the other tubes. Note that the pins for the 6AU6 tube is different than those for a 12AX7, there's a potential for an issue with that wiring. Also, the way the schem depicts the heaters on each half of the dual triode is funky, be sure that pins 4 and 5 of the 12AX7 are tied together to one filament wire and that pin 9 has the other filament wire.
Then measure the actual DC voltages that you get at idle (no signal) and write them on the schem (or simply post them here) so we can see what it's doing.

[Amp Kat]

Hello,
Here goes, I tried modifying the Newcomb amp using this LT Spice diagram . When it did not work I decided to put it back to stock.
I put it all back together, now it works but the volume is very low. From how it was before to now on a scale from 1-10 the volume would be about 2-4.
That is why I think there is less voltage getting through. The 6v6 tubes do not seem to glow as bright as I recall them glowing before.

[ATTACH=CONFIG]37970[/ATTACH] [ATTACH=CONFIG]37969[/ATTACH]

Usually when that happens it's either a wiring issue or something you put back in the wrong place. Make sure you have it wired up correctly.

Also the input transformer is a mic input transformer that is used to raise the amplitude of the tiny microphone signal to a higher level so the tube can further amplify it. Usually around 15dB of gain is achieved by these gems and there is no DC involved.

If you measure across the cathode resistor of the output tube and divide it by the resistance should give you the current and if you multiply that by the plate voltage should give you the wattage that the tubes (2) are outputting.

[SpareRibs]

Hello,
I have the amplifier working now, I used the mike input for the guitar. Would reducing the value of the resistor ( from 330 to 270)on the wiper of the volume control, in addition to reducing the value of the resistor (from 560 to 270) going to pin to pin#1 of the 12AX7, increase the volume any at all ?