18 Watt TMB Build issues

[Richie]

the buzzing, from the pics, it looks like you didn't use the 6.3 CT of the heaters. did you install 2x100ohm resistors in place of the CT?
If not add one or the other.. this should make the buzz more quiet.
I think the preamp or ceriatone labels the rectifier as the 6.3v and the preamp is labeled Fil 1 and Fil 2 ,3.15-0-3.15

[Spunky99]

Richie,

Thank you,
I was a little confused on which 6.3 to use. There is a 1A, 2A and 3A tap on the PT.
I used the 3A tap for the heaters and the 2A tap for the rectifier.
It is a GDS PT.

I don't know what a CT is.

[rock_mumbles]

CT is center tap

[Richie]

The CT on the GDS is Center tap, add a wire from CT lug to your main ground.

[Spunky99]

Thanks again Richie,

That last suggestion corrected the buzz. Now the amp is nice and quiet.
Yayyyyyyyyyyyyy.....

1 issue left to fix.
When the MV is turned up to almost 3/4 or above I get a horrible squeal. I tried this on all volume settings and get squeal starting just over 2/3 and just before 3/4 volume. I don't have a faceplate so I can't give you the volume level in 1 - 10. Possibly the PI?

There are some transient noises I hear but I think the MV issue maybe to blame.

The normal channel has no issues at all except the HI input is not working at all. I'll look at what I did wrong there. The LOW is fine. No squeal or anything.

[Littlewyan]

Is this without a guitar plugged in? And where is your master volume in the photos you've posted? I can't see the back of the pots or the wiring very well.

[RockinRocket]

The normal channel has no issues at all except the HI input is not working at all. I'll look at what I did wrong there. The LOW is fine. No squeal or anything.

You know I built a 18 watt that had the normal channel and also the lite 2bs bright channel and the normal channel was dead quiet but the bright channel squealed through out the entire vol range. Changed grounds, pots caps resistors. Never figured it out..

[Spunky99]

Wow....encouraging, RockinRocket.LOL..

"You know I built a 18 watt that had the normal channel and also the lite 2bs bright channel and the normal channel was dead quiet but the bright channel squealed through out the entire vol range. Changed grounds, pots caps resistors. Never figured it out.."

Sometimes it is really hard to find the problem. I spent 5 years chasing a nagging squeal on a Valve Junior and then a light bulb came on in my head. Yes, it was dim as usual but the light bulb came on.....The amp works great now (for a VJr anyway). They say you can polish a turd but it will always be a turd.... Moral of the story is never give up..keep boring at the rock until you drill through like a gimlet....

Now for the latest in amp adventures. I spent about 5 hours today redressing and checking solder joints especially at the tube sockets. I found one connection that was a cold solder joint. After resoldering I noticed nothing at all....nothing different...maybe less hum, hard to tell.

I rebuilt the normal channel inputs 3 times using new parts each time and gave up for the moment. Now the HI works and the LO doesn't. I need to go buy 68K resistors anyway so I let that sit for now. I did hook up the power light and it works so I am going forward.

Littlewyan asked me if I got the squeal with the guitar plugged in or unplugged. I get it either way, plugged in or unplugged. I get it at just under 3/4 full on the MV no matter where the VOL is set at.
I took very careful voltage measurements again and attached the chart.

The amp sounds really good now and the wierd sounds are gone. I think it was the cold solder joint reacting to the vibrations of the amp when playing. The sound is solid now and very nice at all volumes and MV volume until you get to that 3/4 point and then SQUEAL.....

[Spunky99]

board, tube socket and pot pics

[Spunky99]

5 at a time.

[Spunky99]

I changed out R20 (Shared cathode bias) from a 130 ohm to a 150 ohm and measured the voltages again.

I still have the squeal as before but the cathode voltage on the EL84s dropped from 14.4 to 11.4.
The voltage as measured across pins 3 and 7 of the EL84s are 345 and 348 for V4 and V5.

I have grounds to the rail on all the pots. Could this be a problem as it isn't shown that way in the layout.

Also I see my V3 pin 6 and 8 are way too high. That seems like where the problem is. Not sure what to look at.
Any suggestions?

[sluckey]

It appears that you have a 10K resistor (brn-blk-blk-red) on the V3 socket pin 1 to pin 6. That should be 100K. Would you confirm with your ohm meter? You can measure it in circuit with power off.

[Littlewyan]

Assuming V3 is your phase inverter pins 3 and 8 are the cathodes and should be connected together so should measure the same voltage. Thats probably why the voltage on pin 6 is so high.

[sluckey]

V2 is the PI. V3 contains the cathode follower tone stack driver. Pin 6 ties to B+ so it will be naturally high.

[Spunky99]

sluckey
V3 socket pin 1 to pin 6 is verified as 100K