First post and questions about Mojo 18w TMB Build

[Bieworm]

Been there too. It were my ⛈JJ power tubes
Said goodbye to my last JJ last week. The PI tube was going bad...

[colossal]

Here’s the last thing. I still get some Intermittent low volume scratchy/staticky/windy sounds from the master volume. These errant sounds will completely go away if I play through the amp for a few minutes. Leave the amp sit turned on without playing and the sounds will come back in a few minutes. Maybe also of note is that when the amp is first turned on, these errant sound will start as soon as the amp warms up and will continue no matter how long I wait to start playing. Start playing and the sounds go away.

The scratching sounds indicate that you have DC voltage on the master volume pot. Check for leakage on your tone stack treble cap. This is fairly common with mica capacitors.

[gwmullins]

Josh,

Please indulge me a little. I assume we’re talking about the filter caps in the B+ rail. I have tapped on them. In this build they are 2 cans @ 32uF x 32uf and one 16uF. No sounds out of the cans, but tapping anything around the 16 uF cap (supplies V1) can be heard in the speaker. Also on the first section of the first can ( supplies the OT) I get 3.4 VAC. I didn’t really expect any significant VAC here. Is this normal. In all of the progressive sections of the B+ rail I get maybe a few mVAC.

FWIW, taking all of these caps out of circuit and checking with a multimeter, all show correct capacitance.

Also, I have replaced all of the caps in the TMB tone stack and the cap between the Master Volume pot and the PI.

Thanks,
Greg

[gwmullins]

Colossal,

I have replaced all of the caps in the TMB tone stack. Multimeter shows 0 VDC on the MV pot.

Thanks,
Greg

[gwmullins]

Bieworm,

I have replaced all of the tubes in this amp with a mix of new TAD, JJ, and Groove Tubes. However, I do have a new, complete set of TAD tubes for this amp. I’m thinking I should just change them out.

Thanks,
Greg

[Bieworm]

Bieworm,

I have replaced all of the tubes in this amp with a mix of new TAD, JJ, and Groove Tubes. However, I do have a new, complete set of TAD tubes for this amp. I’m thinking I should just change them out.

Thanks,
Greg

Just try it. Easy .. if your voltages are ok there is no harm in trying
The symptoms you described are what I experienced with 1 failing power tube.. maybe it's your problem too?

[Daviedawg]

Greg - the noises, the disappearing and coming back and the response to tapping suggest to me that a connection is not right. It could be a valve not seated fully, a bad solder joint, a shorting whisker or a part broken wire all of which can respond to heating and cooling. It sounds like you have isolated the general area as being around the 16µF cap. so another detailed look should bring it to light while moving stuff with your chopstick and with your meter clipped on the HT line as well if possible.

I have suffered the same more than once and spent hours getting worked up tracing the culprit thinking I have it sorted.

Dd

[Bieworm]

Greg - the noises, the disappearing and coming back and the response to tapping suggest to me that a connection is not right. It could be a valve not seated fully, a bad solder joint, a shorting whisker or a part broken wire all of which can respond to heating and cooling. It sounds like you have isolated the general area as being around the 16µF cap. so another detailed look should bring it to light while moving stuff with your chopstick and with your meter clipped on the HT line as well if possible.

I have suffered the same more than once and spent hours getting worked up tracing the culprit thinking I have it sorted.

Dd

Now that you wrote this Dd.. my power tube failed due to a pin that didn't sit properly. It grabbed next to the socket seating. I replaced all sockets with beltons.. new power tubes and problem solved. The tube was a goner

[Daviedawg]

Sorry that happened. In some of the repairs I have done recently the least good component is the valve socket. Commercial amps I mean. Results are damaged pins, bent pins, poor connections.

Lesson for us to buy good ones.

Dd

[gwmullins]

I’ve ordered a few more parts (Belton tube sockets, some better resistors, etc.). I’ll post some new pics and another set of voltages after I get the new parts installed.

Thanks,
Greg

[RxLewis]

apologies if you stated and I missed it- have you tried replace the pot?

[gwmullins]

Hi guys,

My grandkids came to visit for a couple of weeks. Needless to say the 4 grandkids got all of my attention.

So here is the state of this build. It’s now fixed. Everything is working properly, but it took most of your recommendations to get it fixed. I’ll summarize.

1) Complete modification of the preamp section grounding per Josh’s signature link. No ground bus across the pots, moved the PI ground to the Power Amp ground section, etc.

2) Changed all input jack resistors to metal film (from carbon comp).

3) Used shielded wire for input wiring.

4) Moved most of the grid resistors to the tubes.

5) Changed all tube sockets to Belton sockets.

6) Changed all of the tubes. (Replaced the JJs with TAD.)

7) Made some minor but essential changes to the lead dress.

I did change out several resistors and capacitors, but they weren’t really a problem.

The most significant/impactful changes were the grounding, the tube sockets, and the tubes. I have to say that I was taken aback at need to change the tube sockets since the tubes all fit snugly in the Mojo sockets, but going to the Beltons eliminated all of the errant, inconsistent noises. Making all of the other changes made this amp have almost no hiss or hum.

The other thing worth mentioning is this amp is the most sensitive amp I have (and I have 14 amps - been playing guitar a long time) to the quality of the tube in V1.

I put this amp in a combo cab with 2 Celestion Vintage 30s. It sounds fantastic.

Again, I’d like to thank everyone who made suggestions for fixing the problems I had with this amp build. I know you didn’t have to and I want you to know I’m grateful for the help.

Greg

[whboyd]

Anyone using or thinking of using the Mojotone 18 W kit or small parts kit, I should read my post under the thread titled “best kit“. Especially important regarding the locating resistors placed across the “unused pins” on the power tubes.