Newbie here

[AmpGal]

Hi, all. I am new to amp building, having just done two Gretsch 5222 mods successfully. I'm currently in a spot with my 3rd not working properly. I modded it to go back to something I was used to when my first scratch 5E3 Tweed Deluxe build was giving me fits.

So, now I have two different amps that aren't working properly... (I need an emoticon pulling its hair out). After reading through some of the threads here, it looks like I may have found a group who can help me troubleshoot them?

To start, I'd like to get my scratch 5E3 Tweed Deluxe build fixed. If I post all of the voltages I'm getting at various points on the board, can you all help me? Right now, I've checked everything I know to check... resistor values, cap voltages, pins on the 6V6s & preamp tubes. Everything is giving me good readings, but the volume is just pitifully low. I have to volume up to 9 before I get any real sound... a kindergartner could take a nap in front of the cabinet while my husband's playing, no joke. Also, the tone knob does absolutely nothing... either that or its not getting enough volume to hear any difference.

What say you?

Gina Veesaert

[Jazz P Bass]

6V6 cathode voltage

Measure the 6V6 cathode voltage. No signal. Should be around 21 Vdc.
This amp should put out 12 clean watts into a 8 ohm load with the controls at 5 with a 100mv ac input.
Let us know.

[tubeswell]

Measure all the idle (DC) voltages from the B+ to the plates, screens, cathode(s) and grids of the output tubes and to the plates and cathodes and grids of the pre-amp tubes and report them back. Also check the filament (AC) voltages

And welcome to the forum BTW

[loudthud]

Welcome! You'll get all the help you need here.

Check your layout and ohm between any eyelets that are supposed to be connected on the back side of the board. Sometimes these become unsoldered when changes are made to components on the top side of the board. Also ohm out any wires that are soldered on the backside that come out and terminate at a tube, jack, pot or ground.

Check around the speaker jacks for any solder splashes or anything else that could cause a short like a cutoff resistor lead. With the amp off, unplug the speaker and plug a regular guitar cord in. At the other end of the cord you should measure a very low resistance between the tip and barrel (ground). This is normal because the output transformer has a low resistance, less than one ohm. Now, unsolder the wire that goes to the output transformer, usually yellow. Now you should measure an open circuit. If you measure the resistance across the plug connected to the speaker, you should get something around 6 ohms. Everything ok? Re-solder the wire and plug the speaker back in.

[AmpGal]

I checked continuity again for everything that's run under the board. No problems there. Output transformer resistance is at .06 ohms, & 6.5 ohms when it's open.

I plugged it in with tubes, speaker & standby in/on (no guitar) and measured every pin on all 5 tubes.

The 6V6 readings are:

Power Tube A (closest to rectifier)
Pin 3 - 369v (Plate)
Pin 4 - 338v (Screen)
Pin 6 - 339v (What is pin 6 & why am I getting readings on an unused pin?)
Pin 8 - 23v (Cathode)

Power Tube B
Pin 3 - 370v (Plate)
Pin 4 - 338v (Screen)
Pin 6 - 339v (same ??? as above)
Pin 8 - 23v (Cathode)

The 5Y3GT Rectifier readings are:
Pin 2 - 384v (Filament)
Pin 4 - 11.7mv (Plate 2) Should rectifier plates really read in mv, not v?
Pin 6 - 1.34mv (Plate 1)
Pin 8 - 0 (Filament) If pins 2 & 8 are both filaments, shouldn't this have a reading?


Here are preamp readings:

12AX7
Pin 1 - 144.2v (Plate 2)
pin 3 - 1.3v (Cathode)
pin 6 - 118.1v (Plate 1)
pin 7 - 117.1v (Grid)
pin 8 - 116.4v (Cathode)

12AY7
Pin 1 - 112.7v (Plate 2)
pin 3 - 1.9v (Cathode)
pin 6 - 115v (Plate 1)
pin 7 - 0 (Grid)
pin 8 - 1.9v (Cathode)

Should the preamps have similar readings to each other like the power tubes? Pins 7 & 8 are drastically different, but I don't know if that's normal or a problem.

Thanks, all!
Gina

[loudthud]

Pin 8 on the 5Y3 should be the +384VDC. You may have had the meter set to AC Volts. Pins 4 and 6 both should be about 320 VAC. Since there is B+ in the rest of the amp, you can skip readings on the 5Y3 because they are dangerous.

The problem is at pins 6, 7 and 8 of the 12AX7. Pin 7 normally won't give a valid reading because of the circuit. Pin 6 is usually something around +250 VDC and pin 8 around +50 to +100 VDC. Either the tube has gone bad or there is some wiring error. With the power off and the caps discharged, pin 8 should measure 56K ohms to ground and pin 7 should measure 1.06 Megohms to ground. The readings suggest that the tube is shorted pins 6, 7 and 8 together. Something wrong there, got another tube to try?

[AmpGal]

Husband's got tubes out the yin yang. He said he'd tried a few on each, but we'll try again. Maybe I'll take this socket out to get a better look at it.

Will let you know. Thanks!
Gina

[loudthud]

There is a capacitor that connects between pins 1 and 7 of the 12AX7. It could have become leaky or possibly damaged by nicking it with a hot soldering iron or over heating one of the leads. You might try un-soldering one end and testing it with your ohm meter. It should measure infinite ohms. It is normal for these caps to give an initially low ohms reading and then increase the reading to infinite.

[tubeswell]

Power Tube A (closest to rectifier)
Pin 3 - 369v (Plate)
Pin 4 - 338v (Screen)
Pin 6 - 339v (What is pin 6 & why am I getting readings on an unused pin?)
Pin 8 - 23v (Cathode)

Power Tube B
Pin 3 - 370v (Plate)
Pin 4 - 338v (Screen)
Pin 6 - 339v (same ??? as above)
Pin 8 - 23v (Cathode)

Have you got a screen grid resistor between pin 4 and Pin 6 on each socket?. Either that or is there a short between Pin 4 and pin 6 somehow (but 1V difference suggests maybe you have a low value screen grid resistor there that is using pin 6 to anchor the other end to???)

The 5Y3GT Rectifier readings are:
Pin 2 - 384v (Filament)
Pin 4 - 11.7mv (Plate 2) Should rectifier plates really read in mv, not v?
Pin 6 - 1.34mv (Plate 1)
Pin 8 - 0 (Filament) If pins 2 & 8 are both filaments, shouldn't this have a reading?

You probably measured DC (instead of AC) here. Pin 8 will have the B+ (high DC) voltage on it on it because it is connected straight to the reservoir cap (which is how it should be).

Here are preamp readings:

12AX7
Pin 1 - 144.2v (Plate 2)
pin 3 - 1.3v (Cathode)
pin 6 - 118.1v (Plate 1)
pin 7 - 117.1v (Grid)
pin 8 - 116.4v (Cathode)

12AY7
Pin 1 - 112.7v (Plate 2)
pin 3 - 1.9v (Cathode)
pin 6 - 115v (Plate 1)
pin 7 - 0 (Grid)
pin 8 - 1.9v (Cathode)

Should the preamps have similar readings to each other like the power tubes? Pins 7 & 8 are drastically different, but I don't know if that's normal or a problem.

Thanks, all!
Gina

the 12AY7 looks normal.

What sort of PI setup is your 12AX7? If you have a cathodyne PI for 6, 7 and 8, those voltages look wrong. The DC idle voltage on pin 6 should be about 3 x higher than pin 8, and pin 7 should only be a handful of volts lower than pin 8. Have you got a schematic of your build/ and a layout pic?

[AmpGal]

The problem is at pins 6, 7 and 8 of the 12AX7. ...With the power off and the caps discharged, pin 8 should measure 56K ohms to ground and pin 7 should measure 1.06 Megohms to ground. The readings suggest that the tube is shorted pins 6, 7 and 8 together. Something wrong there, got another tube to try?

Okay, I get 55.5k ohms to ground on pin 8, but 79.5k on pin 7, every time. I checked continuity through the socket out to the board & am not finding any shorts; there's tone where there's supposed to be & nowhere else.

Have you got a screen grid resistor between pin 4 and Pin 6 on each socket?. Either that or is there a short between Pin 4 and pin 6 somehow (but 1V difference suggests maybe you have a low value screen grid resistor there that is using pin 6 to anchor the other end to???)

The kit calls for a 1.5k resistor between pins 1 & 5, then a 470 ohm resistor between pins 4 & 6. I've checked their values, too.

You probably measured DC (instead of AC) here. Pin 8 will have the B+ (high DC) voltage on it on it because it is connected straight to the reservoir cap (which is how it should be).

I probably did, since everything else was VDC & I don't remember changing the multimeter to read VAC instead.

the 12AY7 looks normal.

What sort of PI setup is your 12AX7? If you have a cathodyne PI for 6, 7 and 8, those voltages look wrong. The DC idle voltage on pin 6 should be about 3 x higher than pin 8, and pin 7 should only be a handful of volts lower than pin 8. Have you got a schematic of your build/ and a layout pic?

Now we're getting into an area that's sketchy for me. I don't know what this means, or how to tell what kind of PI setup I have. I'm working with a layout that came in paper form with the kit (SDG) & some original schematics I found online. I don't have the layout in electronic form, but will try to take a pic of it as well as the build to help.

There is a capacitor that connects between pins 1 and 7 of the 12AX7. It could have become leaky or possibly damaged by nicking it with a hot soldering iron or over heating one of the leads. You might try un-soldering one end and testing it with your ohm meter. It should measure infinite ohms. It is normal for these caps to give an initially low ohms reading and then increase the reading to infinite.

Again, getting confusing. Pins 1 & 7 both connect to the board at different capacitors, so I'm not sure how to apply this.

Gina

[AmpGal]

Okay, just to make sure, I went back & checked readings on the VAC setting for the filament pins 2 & 8 on the rectifier tube, but I don't understand the readings I got. Pin 2 settled down to 0V. Pin 8 did the same thing, but settled at 4V. Is that right? I'm trying to read online & learn more about how all this works, too. There is just so much information out there, I'm drowning.

[AmpGal]

OH, HAPPY DAY!

My husband swears he changed out tubes, but I just went in & replaced the JJ 12AX7 with a Sovtek and lo and behold... the damn thing works! Woohoo! There seems to be a little bleed in the volume pots... i.e. if you're plugged into the Normal 1 jack & you turn the Bright volume up all the way, it will cut the volume to your Normal side, but other than that it works great. Tone pot works fine, too.

Now, off to my poor little 5222.... Which should be a lot easier to figure out now that you all have taught me how to check things out on a more detailed level. Will let you know!

Thanks, again!
Gina

[tubeswell]

Be careful measuring live voltages Gina. Glad it worked out for you

[AmpGal]

Be careful measuring live voltages Gina. Glad it worked out for you

Thanks, Tubeswell. No worries. I have a healthy respect for electricity. I'm not an EE, but I have experience working on computers, alarm panels & household electrical stuff. With this testing, I'm the "one-hand-behind-my-back-at-all-times" type of careful.

[AmpGal]

Speaking of being careful, I want to learn more about all of this. I can now tell whether the tube I'm looking at is a power, preamp or rectifier, and I've found a site to learn more about them, but will that give me the knowledge to determine if the readings I take are correct or not?

For instance, on the Fender Pro Jr I'm trying to fix, I just took the following measurements:

EL84 -A
Pin 2 (Grid) - 50-55mv
Pin 3 (Cathode) - 330v
Pin 7 (Plate) - .4mv
Pin 9 (Screen) - 47mv

EL84 -B
Pin 2 (Grid) - 50-55mv
Pin 3 (Cathode) - 330v
Pin 7 (Plate) - .7mv
Pin 9 (Screen) - 49mv

Here are preamp readings:

12AX7 -A
Pin 1 (Plate 2) - .4mv
Pin 2 (Grid 2) - 28v
pin 3 (Cathode 2) - 25v
pin 6 (Plate 1) - 0v
pin 7 (Grid 1) - 28.5v
pin 8 (Cathode 1) - 13.12v

12AX7 -B
Pin 1 (Plate 2) - .4mv
Pin 2 (Grid 2) - 1v
pin 3 (Cathode 2) - 0v
pin 6 (Plate 1) - 0v
pin 7 (Grid 1) - 1.73v
pin 8 (Cathode 1) - 0v

Now, you guys can look at that and tell me immediately if something's off. I want to learn to do that.