18w TMB - First Build !!!HELP!!!!

[Gerardg]

Hello Everybody,

I recently began my first amp build ever - actually my first time building anything electronic or using a soldering iron.

The build has been completed, and has some major problems. So, I'm presenting my build to you for any advice.

Mojotone provides mostly carbon comp resistors - I have read on the site I should replace all, except the resistors in the phase inverter portion of the circuit. I suppose that means any resistor not directly connected to the Phase Inverter tube socket.

When I power up the amp, there is a super loud hum (probably the 60hz cycle from power) indicating a ground problem. I removed the ground bus from the pots, and drilled two posts into chasis and put the ground bus there. I made some changes per to the input jacks per recommended layouts on this site as opposed to mojotone layout.

The Volume pots are not engaged on either channel - I can't figure out how this is happening. So, guitar signal comes in, but it is at a constant level and not affected by volume pots.

Most importantly, as soon as the standby switch is turned on, there is a 2 or 3 second moment before a SUPER LOUD, high pitched squeal blasts out of the speaker. This happened after added a missing lead to pin 3 of the middle pre-amp tubes. It's so loud I can't even keep the amp on, and like I said, volume pots not acting on signal. The lead attached to pin 3 lowers the voltage on pin 1 of first 12AX7 to about 155v (which is my target voltage). The pitch can be changed by the volume & master knob once only, until it the signal hits the feedback loop. If I turn gain all the way up, it becomes inaudible.

Any help/advice is appreciated. Please note - I am a total novice, a lot of the lingo I see on the site passes me by, I'm not up to speed on grounding.

Thanks in advance everybody!

I have included a shot of the build, and the original layout from mojotone - keep in mind the change to ground bus and jacks.

TMB Build.pdf

Mojo TMB Layout.pdf

Normal Jack.pdf

Hi Gain Jack.pdf

Tube Chart.xlsx

Tube_Chart.pdf

[JMPGuitars]

Welcome to the site!

First, read the Modern Ground Schemes & Soldering Technique threads linked in my signature.

Second, your ground bus is a big no-no. The bus wire should be connected to the grounded turrets, not to the chassis. There should only be one connection to ground in the preamp - read the thread for more info.

Third, After fixing your ground bus, do a highlighter test of your build. Verify every connection, and every component value before highlighting each part and connection.

Yes, carbon comps are not ideal, but they are not likely the cause of your woes.

Also keep in mind that Mojotone has one of the worst layouts/designs we've ever seen for these amps. They're poorly laid out, and frustrating to work with. If you search this site for Mojotone, you'll see what I mean. I love their cabinets, but their 18 watt circuit design is lacking.

Thanks,
Josh

[crgfrench]

It might be worthwhile to read this other similar thread, some good info there too
viewtopic.php?t=25356

[colossal]

.
Most importantly, as soon as the standby switch is turned on, there is a 2 or 3 second moment before a SUPER LOUD, high pitched squeal blasts out of the speaker. This happened after added a missing lead to pin 3 of the middle pre-amp tubes. It's so loud I can't even keep the amp on, and like I said, volume pots not acting on signal. The lead attached to pin 3 lowers the voltage on pin 1 of first 12AX7 to about 155v (which is my target voltage). The pitch can be changed by the volume & master knob once only, until it the signal hits the feedback loop. If I turn gain all the way up, it becomes inaudible.

Any help/advice is appreciated. Please note - I am a total novice, a lot of the lingo I see on the site passes me by, I'm not up to speed on grounding.

First of all, great job on your first effort, overall. However, as Josh mentioned, those Mojo amps have become the bane of our existence. The layout is TERRIBLE (I'm sorry, but every time I see a new member show up with one of these, I cringe). There is an art and a science to laying out an amplifier.

Before jumping into the subtle details of this or that, regarding your horrendous squealing issue, it's possible that the two leads on the output transformer are reversed. You have a 50/50 shot at getting it right the first time, unless the supplier has been very specific about the winding orientation of the transformer primary, the lead color code, and the layout drawing. Wired one way, the amp will be silent except for a little hiss at idle, politely waiting for note to be played, or, in the opposite, a terrifying, howling mess that is totally unusable.

That said, in reviewing your tube chart voltages, there are some things that jump out in your TMB channel and phase inverter, so that is likely the source of your problem. Focus your efforts there. Fortunately, the voltages in your power supply and power section look good and reasonably within spec.

There is a step-wise troubleshooting process that must be observed. Changing things willy-nilly will most often compound a problem. The main issue with solder-by-numbers amp building is that if the builder has no other skill than to assemble the amp, if there is a mistake(s), then you don't have any point of reference to correct it.

and review the links provided. There have been several recent issues discussed at length with these Mojo TMB amps.

[Gerardg]

Thanks for the tips gents!!!!

I changed up the ground bus, made some adjustments from the layout provided by Josh, and the thread from crgfrench.

This thing is now super quiet, until I turn it up and play and it's screaming - the way it should! It's got a ton of sizzle, which I might have to tweak - but it is definitely in the ballpark, on all channels, all jacks work etc....

Going away for a bit, but when I get back, I will be going through it as everybody mentioned and look at some of what colossal said.
I will re-test my voltages and do some chopsticking - then move on to some tweaks.

This is working out and I'm super psyched - Thanks again to all!!!!

[Daviedawg]

Well done getting it sorted in a calm manner and following advice. It is the way to learn. Enjoy your amp to the full.

Dd

[colossal]

That's great news, good to hear.

[Gerardg]

Hello,

Posting the Voltage chart as I do see a few off-target voltage measurements, particularly on V3 - which is the TMB pre-amp.
(It is V2 on the Mojotone build).
The regular Hi/Lo gain channels sound great.
The TMB channel is a bit tempermental - there is a serious sizzle at high gain levels, and if the gain is cranked to 8+ - the squeal is present unless I mute strings or play notes etcetera.

I poked around with a chopstick and did not hear any crackling.
Some points did have high pitched squealing noise when voltage meter was on them.

I'm going to focus on the voltage to the TMB pre-amp tube first.

Tube_Chart.pdf

[Bieworm]

The high gain sizzle is remedied by the paul ruby mod. No doubt. Easy and cheap. 4x UF4007 diodes.. that's all
The voltages ... they are off indeed. Also your powertube voltages are pretty odd.
You should also post the other voltages from the B+ chain , heaters and rectifier...

Josh will chime in and point you in the right direction..

[JMPGuitars]

The high gain sizzle is remedied by the paul ruby mod. No doubt. Easy and cheap. 4x UF4007 diodes.. that's all
The voltages ... they are off indeed. Also your powertube voltages are pretty odd.
You should also post the other voltages from the B+ chain , heaters and rectifier...

Josh will chime in and point you in the right direction..

4x UF4007 is a SS rectifier. The Paul Ruby Mod is 2 UF4007 and 2 Zener diodes such as the 1N4744 as per my EF86 Xtra schematic: files/JMPGuitars_18_Watt_EF86_Xtra_Schematic.pdf

Thanks,
Josh

[JMPGuitars]

Hello,

Posting the Voltage chart as I do see a few off-target voltage measurements, particularly on V3 - which is the TMB pre-amp.
(It is V2 on the Mojotone build).
The regular Hi/Lo gain channels sound great.
The TMB channel is a bit tempermental - there is a serious sizzle at high gain levels, and if the gain is cranked to 8+ - the squeal is present unless I mute strings or play notes etcetera.

I poked around with a chopstick and did not hear any crackling.
Some points did have high pitched squealing noise when voltage meter was on them.

I'm going to focus on the voltage to the TMB pre-amp tube first.

Tube_Chart.pdf

It seems to me that you need to highlighter test the TMB wiring and component values. It also sounds like you need more chopsticking. Chopsticking isn't just tapping stuff, it's also to move wires around to adjust your lead dress and see if that's causing problems. You might have a parasitic oscillation- posting sound samples might be helpful.

The Paul Ruby mod is probably a good idea.

Thanks,
Josh

[Bieworm]

The high gain sizzle is remedied by the paul ruby mod. No doubt. Easy and cheap. 4x UF4007 diodes.. that's all
The voltages ... they are off indeed. Also your powertube voltages are pretty odd.
You should also post the other voltages from the B+ chain , heaters and rectifier...

Josh will chime in and point you in the right direction..

4x UF4007 is a SS rectifier. The Paul Ruby Mod is 2 UF4007 and 2 Zener diodes such as the 1N4744 as per my EF86 Xtra schematic: files/JMPGuitars_18_Watt_EF86_Xtra_Schematic.pdf

Thanks,
Josh

Ah shoot!! Thanx for correcting that. 2x UF4007 and 2 zenerdiodes in the 15V region... already forgot about that..