New build : micro bassman troubleshooting no indicator light but tubes glowing

Pat_rocks92

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Hello i finished my micro bassman from rob robinette however i tried it and got no indicator light but all tubes are lighting. No sound coming from the speaker, no hum, no buzz.

Edit : it now works after replacing the pilot light but i got more issues :

- I replaced my pilot light with another one and i got light.

- but i didn't get sound then i removed the 3.5 jack wire linked to the speaker jack and i finally got sound. however it is weak even for a 1w amp


- here are my current issues :

- Weak volumes however when i jump my send return fx loop together it seems better.

- Master Volume doesn't seem to work unless the raw switch is on.

- When both volumes are at 0 i still get sound.

- my treble, middle and bass doesn't seem to affect anything

- When i use my raw switch the master volume works in reverse which is weird and when it is fully turned clock wise it cuts completely the sound. It makes a scratch noise like it lacks oil before cutting sound.

- It has like no gain.

perhaps do i have a dead pot and my switchcraft are not working as intended ?

or is it because of the tubes ? I use a tung sol 12au7w and 2 electro harmonix 7025 could it be that those EH tubes be the issue ?

IMG_4080 (1).jpg

IMG_4079.jpg


Second suspect : wrong wiring Perhaps on the pilot light ? tube heaters ? Artificial center tap ?

For the tube heaters here are the pictures :






IMG_4061.jpg





IMG_4065.jpg

IMG_4074.jpg

IMG_4075.jpg


here is the layout :


Bassman_Micro_Eyelet_Layout_With_Rectifier_Tube (3).png



Things we might consider ( i don't know if it is right) :

- i didn't add the red wire going from B+ (first filter cap 47uf) to the SSR rectifier i thought it was unecessary as i use a tube as a rectifier.
- Power amp ground from 3 watt resistor is grounded to the chassis.
- My 250 uf capacitor is perhaps too big (250v) ?
- My 2 100ohm resistors on v3 might have a wrong wattage ?
- Soldering is a mess but components don't seem to touch each other.
 

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Last edited:

dan40

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Silly question but did you make sure to install a bulb in the pilot light? If so, make sure that the two solder lugs on the light do not have any stray pieces of wire or solder causing a short across them. Also check that solder didn't drip down and make a connection with the metal frame of the light.

I also noticed that you have many cold solder joints on your turrets. The joints on top don't look to bad but the wire connections on the side of the turrets look cold and could use reflowing.
 

dan40

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Place your meter leads on both pilot light wire connections at the same time and tell us what AC voltage you are seeing there.
 

Len058

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Ground the amp before you electrocute yourself. Remember, the fairy dust of the magical 120V can't travel unless you use a proper ground prong.
 

King Fan

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i didn't add the red wire going from B+ (first filter cap) to the SSR rectifier i thought it was unecessary.
I'm not sure which red wire you're talking about. Can you clarify? Most red wires do something....

Also, good close-ups, but at some point we need backed-out straight overhead shots of the whole chassis, and then the left end through the filter caps, and the right end.

If you don't have a multimeter, and your amp isn't grounded, it may be a sign you should take it to a tech. I say this with respect -- and concern.
 

Pat_rocks92

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okay guys
Silly question but did you make sure to install a bulb in the pilot light? If so, make sure that the two solder lugs on the light do not have any stray pieces of wire or solder causing a short across them. Also check that solder didn't drip down and make a connection with the metal frame of the light.

I also noticed that you have many cold solder joints on your turrets. The joints on top don't look to bad but the wire connections on the side of the turrets look cold and could use reflowing.
- I replaced my pilot light with another one and i got light.

- but i didn't get sound then i removed the 3.5 jack wire linked to the speaker jack and i finally got sound. however it is weak even for a 1w amp

- here are my current issues :

- Weak volumes however when i jump my send return fx loop together it seems better.

- When both volumes are at 0 i still get sound.

- my treble, middle and bass doesn't seem to affect anything

- When i use my raw switch the master volume works in reverse which is weird and when it is fully turned clock wise it cuts completely the sound. It makes a scratch noise like it lacks oil before cutting sound.

- It has like no gain.

perhaps do i have a dead pot and my switchcraft are not working as intended ?

or is it because of the tubes ? I use a tung sol 12au7w and 2 electro harmonix 7025 could it be that those EH tubes be the issue ?
 
Last edited:

Pat_rocks92

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Wait a minute x) i think i perhaps found one issue, shouldn't the ground buss wire be grounded to the chassis ?

well if it is the case it doesn't seem to be on robs layout. Perhaps it is meant to be grounded with the jack.
 
Last edited:

Pat_rocks92

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UPDATE : tube heating seems relatively weak. My rectifier tube heats and glows a lot however my other tube don't glow as much and are colder even if they produce heat.

i might perhaps build new tube sockets and rewire the heaters.
 

Pat_rocks92

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Good job pressing on. You may just need that DMM about now… :)
I just have one issue in switzerland, prices are extremely high, it is 120 dollars for inspection + then they charge you a big amount to replace faulty parts. For this price tag i could almost rebuild the entire amp... as my transformers seem to be ok.

the 3 prong power cord is there. I will only use one hand when inspecting in the chassis plus i will discharge the filter caps.

For the price of a revision, I better rebuild it or get a multimeter however mine has no more battery i shall buy one soon.
 

Pat_rocks92

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Is the terminal circled in green attached to the ground bus?

View attachment 988064
You're my Hero !

Thank you it works almost perfectly now !!!!

every issue disappeared !

man if i did take more time to check this layout but this part of the drawing is somewhat not clear.

I just have one issue left

jumping the FX Loop greatly increases the volume why ? i don't know. I had a switchcraft which the tip shunt touched the tip so i bended it for them to not touch thinking it was an issue. Should i bend it back ?

here is what rob says about this :

Dirty FX loop jacks. Insert a jumper or guitar cable from FX Out to FX In to see if that cures the problem. If it does fix your problem try cleaning all the jack contacts, especially the switch contacts. If that doesn't fix it replace the jacks.

especially the switch contacts what does he mean ? the switch on the contact or the toggle switches ?
 
Last edited:

Lowerleftcoast

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switch contacts what does he mean ?
The jack *switch* has a contact point to the tip part of the jack. New jacks may have poor quality control or maybe have been damaged in transport. The contact point may not make good contact. Old jacks... the contact point will oxidize making contact iffy. Adjusting and/or cleaning that contact point is what Rob is talking about.
 

dan40

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had a switchcraft which the tip shunt touched the tip so i bended it for them to not touch thinking it was an issue. Should i bend it back ?


Yes. The return jack should have a shunt switch which makes contact with the tip and allows the signal to flow through to the master volume when you are not using the loop. When you need to use the loop, plugging in a patch cable opens the shunt switch and allows the signal to pass through the loop. You need to bend the switch back into place so that it makes good contact with the tip when no cable is inserted.
 

Pat_rocks92

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The fx jacks are hidden from view...
Need new pix. hah.
well according to the schematic, they are wired correctly. I suppose that there is one defective jack. I will order new ones.

This micro bassman is amazing i used a p10r from jensen and it sounds great. It just lacks some gain despite having a 12ax7 (i use eh 7025) perhaps should i go for jj tubes ?

It is very loud for a small amp i will build the ltp version perhaps should i go for a 8 inch speaker ?
 




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