Basic tube amp question(s)

phatt · Post by **phatt** » 06 Mar 2012, 13:40

Rolbista wrote:@ phatt - I'll try that, thanks, but what's bugging me is why is the OT CT (output transformer center tap?) connected to the 470 ohm resistor near the power supply? shouldn't it go to ground? i mean center tap primary, like in the assembly notes i posted. Also, just got a call from the maker of the PT and he claims to have measured it and says the voltages are fine... He mentioned something about a voltage doubler in my amp, i don't know what to think. Let's assume that my DMM measured incorrectly and the voltages were alright. why were the tubes redplating than? and the amp made no sound...? weird

If the Ct of primary went to ground then the power tubes would have Zero effect.

The Ct is where the power tubes get their DCVoltage to drive the output section.

This is what happens when you try to build amps without a reference to the schematic.

Sure layouts can help but with little knowledge you are stuck when the wheels fall off.

Primary voltage issue,
Look just use the Cal sheet to work it out for yourself?
290 x 290 Volts AC = 580VAC x .71 = 411.8 Volts DC

You are already over a safe margin for the amp you are trying to build. Without big redesigning the power stage you will kill the tubes in hours not weeks.

My best guess is the 290x290 Secondary is meant for a EZ81 Rectifier where the diode drop is big and brings the VDC back down to a useable level. (maybe around 350VDC)

As to the Heaters well we are almost certain the valves where glowing very bright and your meter is questionable so tiss hard to make comment.
My advice is back off and read up on stuff you are unclear about.

Other Advice;;;;

The best thing I ever did was to learn to read schematics,,, you use **NODE numbers** to establish points on the schematic then find them on the layout as you build.

Ground is node 1, then HT1/HT2 etc. Read the schematic and count how many components connect to that point then go find them on the layout.
If they don't connect right then you recheck until it all adds up. (make nodes in colour might help to see more)
Don't despair you are doing well. These hiccups you will look back on one day as the most useful learning tool you ever had.

Phil.

deltafred · Post by **deltafred** » 06 Mar 2012, 20:04

Rolbista wrote:... Also, just got a call from the maker of the PT and he claims to have measured it and says the voltages are fine...

Can you measure your mains voltage and make sure your meter is indicating around 230VAC and not some stupid high value. There is a possibility that your meter is over-reading. (I have a digital multimeter that does that when the battery is dying. It frightened the shit out of me the first time it happened.)

Rolbista · Post by **Rolbista** » 06 Mar 2012, 21:02

yes, that seem to be exactly what happened, low battery in the DMM, put in a fresh one and everything reads fine (apart from PT and amp since it's away at the moment)! So the PT is probably correct, but it still doesn't tell me why there is no sound coming out, i'm comparing my wiring with the schematic as i'm writing this. As for dropping the voltage, what about a zener like kleuck mentioned? 50V down from 410 should be fine already right?

deltafred · Post by **deltafred** » 06 Mar 2012, 22:28

Rolbista wrote:... but it still doesn't tell me why there is no sound coming out, i'm comparing my wiring with the schematic as i'm writing this. ...

Ok.
Check :-
Your layout - correct component placing.
All component values correspond to the layout. (Most resistors can be measured "in circuit" because valves have very high resistance when not powered, but check the bands first.
All links are present and in the correct places.
All electrolytic capacitors are the correct way round.
For dry (cold) solder joints and shorts between joints and also ground.

... As for dropping the voltage, what about a zener like kleuck mentioned? 50V down from 410 should be fine already right?

I would get it working first then worry about voltages (just as long as they are not stupidly high). If you do want to drop some volts then power resistors are probably a better option due to power dissipation.

Rolbista · Post by **Rolbista** » 07 Mar 2012, 16:42

hmmm, not sure on this one, but what should I do with the OT secondary black lead? On the schematic it goes to chassis ground and so does the ground pin of the speaker jack (i made only one instead of two). I wired OTs black lead to ground pin on the socket and to common ground from there. Is that correct? The layout on the other hand doesn't show the secondary black lead at all. What bugs me is that i checked the speaker jack for continuity and signal and ground pins are shorted either with a plug inside or without it, maybe that's the point where the signal stops and reason for no sound? Gonna hardwire the 8 ohm tap to the jack and wire grounds separately, but no sooner than when i get my PT back.

deltafred · Post by **deltafred** » 07 Mar 2012, 19:08

The resistance between the speaker jack tip and body contacts will be quite low because of the OP transformer secondary winding wire is thick and there are a lot fewer turns than the primary.

I'm not sure what it should read because the 4, 8 and 16 ohms refer to impedance (AC resistance) and your meter measures DC Resistance, not the same thing. Impedance takes account of the frequency (and is the basis of (analogue) filters).

It would be worth your effort to Google the subject and get familiar with it as a lot of people have difficulty separating resistance and impedance (probably because they are both measured in ohms).

Have a look at Sinner's Vox AC30 layout here https://www.freestompboxes.org/viewtopic ... 90#p182247 the OT common (black wire) is earthed next to the speaker connector terminal block. There is no switch on the AC30 as it is a combo with no extension speaker socket, the OT transformer impedance tapping is selected by wiring the speakers to different positions on a terminal block.

The grounds in an amp should be "star" wired wherever possible. That is all grounds go back to a central point rather than "daisy chaining" from one place to another. This will help keep hum to a minimum and assist with stability (reduce the tendency of the amp to oscillate).

Rolbista · Post by **Rolbista** » 07 Mar 2012, 19:57

i didn't measure resistance, i selected the continuity tester that beeps when a connection is made (just to be clear what i mean). shouldn't the signal and ground on the OT always be separated?

deltafred · Post by **deltafred** » 08 Mar 2012, 00:14

Rolbista wrote:i didn't measure resistance, i selected the continuity tester that beeps when a connection is made (just to be clear what i mean). shouldn't the signal and ground on the OT always be separated?

Continuity testers beep below a certain resistance, exactly what that is I have no idea because I usually use a continuity tester for checking fuses, switches, anything that is either shorted or open circuit and not much else for that very reason. If I want to know resistance I use the resistance range.

Measure the secondary resistance, I don't have a valve amp on my bench at the moment so have nothing to measure to give you a typical figure but I would guess that it is few ohms (which will make most continuity testers beep).

The output will be connected to ground by the transformer secondary, the value you have just measured. If they were completely separated then you would get no output.

Do you have a schematic for this amp because it would make more sense if you see what was in the OT instead of viewing it as a "black box" with just inputs and outputs.

Rolbista · Post by **Rolbista** » 08 Mar 2012, 08:51

yes i have two schematics, one drawn by some guy from the recent reissue, and one is a factory schematic from marshall. ok, i understand why two leads were connected now, i supposed the answer would be like this but i was hoping i found a simple mistake and my marshall was just a few solder joints away :'(

deltafred · Post by **deltafred** » 08 Mar 2012, 09:25

Rolbista wrote:yes i have two schematics, one drawn by some guy from the recent reissue, and one is a factory schematic from marshall. ok, i understand why two leads were connected now, i supposed the answer would be like this but i was hoping i found a simple mistake and my marshall was just a few solder joints away :'(

Can you post the one that matches your amp please, that will make it easier for people who can read them to help you fault find.

Rolbista · Post by **Rolbista** » 08 Mar 2012, 20:38

the schematic seems to match, i don't have one from Ceriatone however, there is only a layout on their site.

phatt · Post by **phatt** » 09 Mar 2012, 07:51

400 Plus VDC is just plain Effing stupid for a pair of EL84.
Expect to change valves OFTEN.

Don't assume the drawings on the net are the real thing.

Phil.

Rolbista · Post by **Rolbista** » 10 Mar 2012, 19:33

Got my PT back and took some measurements, heaters are normal, here are voltages from tubes:
EL84:
pins:
2. 0V
3. 14V
7. 375V
9. 372V

EL84:
2. 0V
3. 13V
7. 373V
9. 368V

12ax7 gain stage:
1. 294V
2. 0V
3. 4V
6. 282V
7. 0V
8. 4V

12ax7 PI:
1. 201V
2. 0V
3. 2V
6. 207V
7. 0V
8. 0V

diagrammatiks · Post by **diagrammatiks** » 10 Mar 2012, 19:53

There you go that's right.

juanro · Post by **juanro** » 10 Mar 2012, 20:14

There's something wrong around V2 (the PI)
Both cathodes are tied together (acording to the schematics posted) so pins 3 & 8 should be the same voltage, and you reported

12ax7 PI:
..
3. 2V
..
8. 0V

Can you double check that?

Juanro

Rolbista · Post by **Rolbista** » 10 Mar 2012, 23:11

yeah, 3. and 8 are the same of course, 2V, mistake written down. I'm thinking I'll get a pair of 12ax7s and try another pair, maybe they're bad? The signal doesn't seem to go through the PI at all, touching pins 2 and seven has no effect, no crackles, noises, nothing, a little hum is present however when i touch 2.'s on the EL84s

phatt · Post by **phatt** » 12 Mar 2012, 14:21

Hey Rolbista,
It's late but I just noticed that your PI input is DC coupled. Yikes!

I'll leave better qualified folks to peruse that point.

In my understanding that LTP PI setup needs to be lifted from ground to work,, but I could be wrong.
Can't think now,, bed time for my brain.
Phil.

diagrammatiks · Post by **diagrammatiks** » 12 Mar 2012, 16:37

hmm that can't be right...there should be another cap at the pi input.

It's not set up properly for a dc coupled ltpi.

as it's drawn the other side of the pi has no bias.

juanro · Post by **juanro** » 12 Mar 2012, 16:51

Mh, I don't see where the PI is DC coupled. Input is _after_ the decoupling cap of the previous stages, and it's ground referenced through the vol pot. (A common practice)
Also, it's not really a LTPI, (it lacks the "long tail" for that), it's just a commond cathode ("self-splitting") phase inverter, or differential stage, etc, it has many names. One triode get's the input through the grid, the other has it's grid grounded, and both share a cathode resistor.
Unless I'm seeing a different schematic? I'm looking at the two PDF's posted earlier.

Regards
Juanro

diagrammatiks · Post by **diagrammatiks** » 12 Mar 2012, 17:12

juanro wrote:Mh, I don't see where the PI is DC coupled. Input is _after_ the decoupling cap of the previous stages, and it's ground referenced through the vol pot. (A common practice)
Also, it's not really a LTPI, (it lacks the "long tail" for that), it's just a commond cathode ("self-splitting") phase inverter, or differential stage, etc, it has many names. One triode get's the input through the grid, the other has it's grid grounded, and both share a cathode resistor.
Unless I'm seeing a different schematic? I'm looking at the two PDF's posted earlier.

Regards
Juanro

you might be right. I don't remember seeing that topology before. Lemme do some research.