No sound from 18 watt TMB

[lpresnall]

I just let the last solder joint cool this afternoon on my 18 watt TMB. I used the Ceriatone layout such as it is, and fired it up thru my bulb, got all the good stuff as far as pre and output tubes glowing nicely, no smoking, fuse blowing, popping. So, took it off the bulb, same thing. Then - I only get a test signal at the input jack in use, and pin 7 of V1. This is my first time building this circuit. I have been able to build the ax84 P1Ex and HiOctane but I'm at a loss here. The Ceriatone layout isn't as forthcoming with information as I need and there's no schematic (with which I'd be semi-fluent if I had it!). voltages are:

PRE AMP

PIN 1 3 6 8

V1 160 1.03 280.6 6.01
V2 26.8 60.6 193.1 60.7
V3 282.7 192.3 279.9 280.3


POWER

PIN 3 7 9

V4 1.96 394 1.03
V5 1.96 400 1.05

On pin 2 of V4 and V5 I get 3.08 and 1.51, respectively.

Can anyone help? I don't know what voltages I should be seeing where, or what I may've omitted or done wrong. Thanks!

[Phil_S]

There is a great Excel voltage chart file you can download at the Ceriatone website. I am posting their TMB18W as a PDF here.

We know there is a problem, maybe more than one. To help us get to the bottom of it, answers to a few questions would be very helpful.

In some 18W amps, V2 is designated at the phase inverter and it is physically V2. In others, people have tinkered with the classic design and physically moved the phase inverter to the V3 position. Tell us, which is your PI?

I think the attached voltage chart makes V2 the PI, so adjust your thinking accordingly.

What voltage is your power transformer high voltage secondary supposed to be rated for? Typical is about 290-0-290.

What rectifier are you using? EZ81, or solid state diodes, or something else?

Can you post the schematic you built from? If you can't, can you describe the B+ supply ladder? Typical, after the rectifier is: 2.2K > 8.2K > 2.2K. What are the voltages at pins 1 and 7 of the EZ81, and after each dropping resistor in the B+ supply?

Can you post a few high resolution pictures, including one overview showing the whole chassis (important for orientation to the build) and then maybe 3 closer up so we can see details.

On the PI (V2 or V3, depending) there should be fairly substantial DC voltage on the grids (pins 2 and 7) -- something between 50 and 80 volts depending on your circuit and a few other factors. Post these readings, too, please.

Edit: Sorry, I see you have no schematic, so confirm your B+ ladder and voltages. Did you follow the Ceriatone layout?

[Firestorm]

Looks like multiple problems: On V1, pins 1&6 should be high volts and nearly identical; pins 3&8 low volts and nearly identical. V2 in the Ceriatone layout is the PI, so pins 1&6 should be high and nearly identical; pins 3&8 should be "moderate" voltage and nearly identical. V3 is the CF, so there should be high voltage on pins 1,6,7 and 8; pin 3 should be low. You also have no screen voltage at pin 9 of V4 and V5. These should both connect to the 2W 100R. As it is now, the output tubes are not conducting. You need to go over the layout very carefully and see what you may have miswired. There are schematics for this amp out there, just Google.

[lpresnall]

Phil and Firestorm, thanks for the replies. I'm at work now, but when I get back to town tomorrow evening I'll check stuff and try to answer your questions. Phil, some of your inquiry is over my head, but I'll do my best. I suspect my heater wiring is wanged up. Ceriatone's pictorial layout looked easy to follow at first, by my Heyboer transformer from Hoffman is a little different and I didn't have the schematic from Ceriatone for my exact situation. (Still don't, but close). I'll try to post some pix as well. Thanks again!

[Phil_S]

Here is a common schematic. I don't know if it is true to the Ceriatone layout, but it is a reliable schematic.

The B+ ladder is the high voltage supply taken from the rectifier. Think of it as the main trunk of a tree. The nodes are like branches. It is designed to supply voltage to each stage. Basically, more or less, each preamp tube is a stage, and then the pair of power tubes are the last stage.

Just follow the top of the schematic from the rectifier to the various points. See if you can get a voltage measurement from each resistor to ground. Based on the orientation of the schematic, place the probe to the left of the resistor. Of course, you know that "left" means nothing when you actually build it.

[lpresnall]

Thanks Phil! This looks like something I can follow. I'll check it out tomorrow night or Thursday morning and post findings back here. If I'm not a smoking pile of powder after touching the wrong thing.

[lpresnall]

Ok, following the advice from your replies I went thru the amp again and here's what I have now:


PRE AMP

PIN 1 3 6 8

V1 92.3 .57 81 0
V2 41.8 30.8 116 30.7
V3 96 .42 160 96.1

(someone asked for pin 7 on v3: it's 96.1)


POWER

PIN 3 7 9

V4 30.2 224 210
V5 30.2 231 210

I get volume at the "MV" input plugging a guitar in but the sound decays quickly. I get very low volume at the other 2 inputs. I am getting no voltage at the 100k resistor (lug 3 from the right of board on the Ceriatone layout). And the amp seems hot!

What say ye? I believe I've solved a few issues only to raise others.

[mark enger]

sounds fade, is the pi grounded? did you check all your grounds? mark them off with a marker to ensure everytings grounded might be that 56k is'nt grounded, and it seems v4 and 5 cant be 30v on pin 3, post some pix, mark

[Phil_S]

It would help to have the B+ supply voltage readings:
Rectifier, AC voltage at pins 1 and 7, DC voltage at pin 3.
DC voltage at each filter cap of which there should be 4. The first one is the same as pin 3 on the rectifier.

Your power tube cathodes are at 30V because plate voltage is so low. Once you solve that problem, I suspect everything else will fall into line. On a relative basis the readings seem proportionally in line with expectations. On an absolute basis, they are all too low. I think Mark may be on to something -- the PI tube should have voltage readings that are almost symmetrical and they are not. In addition to checking the tail resistor for proper grounding, check all the other parts, too.

I am guessing here that you have used a resistor of the incorrect value. For example 1K and 100K look almost alike. Check all your resistors carefully. It may require you to unsolder one end to get a proper reading.

[lpresnall]

Ok, more values:

RECTIFIER:

PIN 1: 35.6
PIN 7: 34.4
PIN 3: 275.4

CAPS LEFT TO RIGHT, GROUND BUS NORTH:

31.5 173.4 165 0/.586 (THESE ARE THE TWO TINY 1uF'S AT THE EAST END OF THE BOARD).

I have no idea what or where the tail resistor is, forgive my ig'nance! Thanks guys, for putting up with me.

[mark enger]

Can you post a pix? Your voltage reading are not making a lot of sense.
Are all your tubes lite up? mark

[lpresnall]

hey ark, yep they're all hot! will post pix tomorrow morning.

[lpresnall]

"mark" mm is sticky on my keypad

[Phil_S]

RECTIFIER:
PIN 1: 35.6
PIN 7: 34.4
PIN 3: 275.4

Pins 1 and 7 don't make sense. Are you metering for AC voltage? You must meter for AC. That is what your transformer produces. It is impossible to produce 275DC from 35VAC.

The output (pin 3), I am assuming that is DC volts. If so, I am trying to determine if that voltage is consistent with what the transformer supplies, so please try again on pins 1 and 7.

The rectified DC voltage on pin 3 suggests your PT is producing 460VAC. I think I should have provided instructions on this. To measure this, set your meter for AC volts. Place one probe on pin 1 and the other on pin 7. This is a place where you do not measure relative to ground. We say, "measure ACROSS pins 1 and 7."

After you do this, power off the amp, remove the rectifier tube, power on and take another reading across pins 1 and 7. Post both readings. This information is important.

Insert the rectifier tube and continue...see below.

CAPS LEFT TO RIGHT, GROUND BUS NORTH:
31.5 173.4 165 0/.586 (THESE ARE THE TWO TINY 1uF'S AT THE EAST END OF THE BOARD).

I do not think you are capturing the readings at the locations we need and since we don't have a picture of your board, the orientation information doesn't help, so please post a picture.

I'll try to be more specific about what we are looking for. We need voltage readings at each of the larger filter caps, not the cathode bypass caps. These caps are likely to be 32mf or 16mf, or numbers close to that (anything 10mf to 50mf). Follow the B+ supply. B+ is an antiquated term referencing "battery positive terminal" but it means the rectified positive voltage initially provided at pin 3 of the rectifier tube.

The instructions below follow the schematic I posted and the references are to the very top line, from right to left, starting by the right facing arrow marked HT. HT stands for high tension (Brit/Euro speak) and is what we Americans call high voltage -- same thing. HT=HV, but you are more likely to find HT as the standardized abbreviation, no matter what you call it.

FC1: Follow the circuit from rectifier pin 3. Find the junction of the first 32mf cap and the 2.2K resistor. That is the first filter cap. Find the DC voltage, one probe at that junction, the other at the chassis ground.

FC2: Continue to follow that part of the power supply or B+ ladder. Find the 8.2K resistor and the second 32mf filter cap. What is the voltage at that junction?

FC3: On the other side of the 8.2K resistor, you should find that it joins to a 16mf filter cap, the two 100K plate load resistors for the phase inverter (on pins 1 and 6), and a 2.2K resistor. What is the voltage reading where the 16mf cap joins the 8.2K resistor? If you have trouble finding that, find the two 100K plate load resistors and provide readings on each side of each resistor relative to ground, and indicate "tube side" or "B+ side." Actually check all of this if you can find it. The readings should all be the same.

FC4: Find the other side of the 2.2K resistor attached to FC3. That is connected to the last filter cap in B+ ladder. Get the voltage reading at the junction of the 16u and 2.2K.

Report your findings labeled FC1, FC2, FC3, and FC4. Remember to get the VAC readings to the rectifier, too, and that is a single number across the two pins.

If you find that your parts are different values (particularly resistors) please report what you are using. We need that information to calculate the amount of current flowing. Current flow may be a useful diagnostic.

I have no idea what or where the tail resistor is, forgive...

All is forgiven. At the beginning learning the terminology can be taxing. We all start someplace. You are doing a good job of articulating your questions.

Look again at the schematic. Find the phase inverter (PI). Working from the right, you'll see the pair of E84's stacked vertically. To the immediate left, you'll see a pair of triodes marked V3. That is the PI. At the bottom of the PI, connected to the cathodes, you'll see in a cross pattern, one 8200 resistor (vertical) connected to both cathodes, two 470K resistors (horizontal) one to each grid, and one 56K resistor (vertical) connected to the 8200 and pair of 470K. That 56K is the tail resistor. Make sure it is connected to the ground buss.

Given that you have 275V from the rectifier, it is not surprising that all the voltage readings are low. We are looking for EL84 plates at 324V. When you start with 275, that is about 50V low. That could be an explanation for many of your voltage readings.

If you happen to have a PT that is not putting out the correct voltage for this amp, there are two reasonable options. One is to get a transformer that puts out higher voltage and the other is to live with it and modify the B+ ladder to up the preamp voltages. This is why we need to VAC reading supplied to the rectifier, so we can understand this. It may be that there is something wrong with the rectifier, or some error in the build causing excessive current draw, maybe an unintended ground or an incorrect resistor value. Let's take one thing at a time and not jump to conclusions.

It appears, however, that 42V on V2 pin 1 is not even close. It should be close to the 116V you see on pin 6.

I will be traveling Saturday and Sunday and don't know if I'll see anything you post until Sunday night or Monday. I will probably see if you post anything today or tonight, but I may not return here today. If I don't answer, maybe someone else will or just be patient. Your problem is probably an ordinary one. Pictures and voltage readings will help us sort this out.

[lpresnall]

Phil, you are awesome! I will go up and get these readings and post by tonite in hopes of getting this thing sorted by Monday. I believe i did keep the meter on dc for the rec reading. This time I will not fail!