Micro Bassman LTP issues

Camdenharr

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Hi everyone,

This is my first time posting here, but I’ve been a lurker on TDPRI for years. Admittedly, I’m green at this and it’s only my second build, but I do generally understand wiring at a basic level.

My current build is a Micro Bassman LTP from Rob Robinette’s site. Everything is “stock” and the only changes I’ve made are 1) no effects loop, and 2) no SS rectifier, aside from some troubleshooting steps I’ve taken, which are listed below.

I had an issue when I first put it together where guitar audio wasn’t making it out of the speaker. I’ve spent a bit of time troubleshooting since then and have made improvements — guitar audio now comes out of the speaker — however it sounds quite awful. Oscillation and phasing sounds.

I am going to attach pictures but *fair warning* the amp is sloppy right now. In my troubleshooting I’ve been taking it apart and putting it back together, not worrying *too* much about lead dress. I plan to rework the ugly bits once I make some progress on the troubleshooting. You will see some connections lifted up off the circuit board: those are like that because I was working to diagnose any issues with the underboard wiring, so I made temporary connections on the top of the board. Also for troubleshooting purposes I wired the leads right to the speaker for now as well.

No guitar audio would come through the speaker at all unless I jumped the cathode of the LTP to the ground bus, so you will see that temporarily soldered in place. I believe this effectively “deletes” the NFB but not 100% sure on that. Also when using the amp, if the presence control is at anything other than maximum it oscillates/motor boats like crazy.

And finally, steps already taken:

1) reversed OT leads in case OT was in opposite phase of stock from the factory - no impact

2) confirmed filter caps are working as they should, via testing them directly and also jumping in another known good filter cap in parallel

3) lifted connections off the main board where I was suspect of them shorting out somewhere

I look forward to any input you all may have!

Thanks again,

Camden
 

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Camdenharr

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I’m on my phone so can’t help with the nice pics. Another thing helpful in these threads is a complete voltage table. Do you feel ok doing one?
So I don’t think I’ve made one before as a “complete” table - do you just mean current voltages of the tube socket pins and nodes?
 

King Fan

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Yes, exactly, the voltages of the DC socket pins, the B+ nodes, and the 6.3V heaters pin-to-pin, at least on the lamp.
So I don’t think I’ve made one before as a “complete” table - do you just mean current voltages of the tube socket pins and nodes?
 

D'tar

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Grounding that cathode is obviously bypassing the entire cathode network intended. Lets power down and ohm our way through to determine whats happening.

remove the temp ground and ohm the
1.ltp cathode to chassis/ground=
2.ltp cathode to each grid of the ltp=
3. each ltp grid to ground

we can get a volt chart once you verify proper component/wiring
 
Last edited:

Camdenharr

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Thanks guys - right now I’m able to play it with pseudo normal sounding guitar tone coming out of the speaker, BUT — I have the cathode of the LTP (V3 12ax7) grounded and also the cathode of the 12au7 power tube disconnected from the 820ohm cathode on the far left of the board and grounded on the ground bus.

Again I’m sure this isn’t “right” but it does at least allow a somewhat normal tone with less “out of phase” sound than I had before.

I’ll get to work on the voltages now.

Thanks again!
 

King Fan

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I’ll get to work on the voltages now.

Thanks again!
remove the temp ground and ohm the
1.ltp cathode to chassis/ground=
2.ltp cathode to each grid of the ltp=
3. each ltp grid to ground

we can get a volt chart once you verify proper component/wiring

How about voltages *after* undoing the temporary ground and after measuring and reporting those spots with an ohmmeter...
 

Camdenharr

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DISREGARD THE B+ AND PIN READOUTS HERE AS THE 820ohm CATHODE WAS DISCONNECTED FOR READINGS. PROPER READINGS ARE IN POST #15

Hi everyone - I just took the measurements. To confirm: any test connections have been removed and the amp is "stock." The only thing I am not sure of is the 820Ohm Cathode Resistor 3watt on the far left side of the board -- my PT doesn't have a 6.3v center tap, so I used the artificial center tap shown in the layout/schematic consisting of two 100Ohm resistors tied in to V4. As such, I do NOT have the jumper wire installed from the 820Ohm Cathode Resistor to the 8th pin of V4. Originally I did have this connected but I realized it wasn't required due to there not being a 6.3v CT. Anyway, onto the readings:

ALL READINGS ARE IN vDC UNLESS SPECIFIED AS vAC/mvDC/etc

Between the Heaters:
All at 6.4vAC, from the lamp to each tube

B+ readings:
B+1: 333.0
B+2: 330.1
B+3: 273.7

Pin readings:
V1 (12AY7)V2 (12AX7)V3 (12AX7)V4 (12AU7)V5 (EZ81)
1143.3152.3222.9332.8120.9
2.2mv // 0?-1.1mv21.8608mv
32.3361.0229.729.8332.9
429.2329.629.729.829.7
529.2329.629.729.829.7
6141.8273.3210.3333.78mv
7.2mv // 0?152.322.029.9121.4
82.34153.329.829.92mv
929.4329.629.829.92mv

Notes on Pin Outs: V3, the LTP valve, would motorboat/oscillate when grids pins 2 and 7 were probed. Note sure if that helps us ID any issues, but thought I'd mention it.

Resistances:
LTP Cathodes to Ground (both same): 11.8K Ohms // 11,800 Ohms
LTP Grids to Ground (both same): 1.008M Ohms // 1,008,000 Ohms
LTP Cathode to LTP Grids (both same): .998M Ohms // 998,000 Ohms

I look forward to any thoughts you all may have!

Thanks again,

Camden
 
Last edited:

Camdenharr

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Hi everyone - I just took the measurements. To confirm: any test connections have been removed and the amp is "stock." The only thing I am not sure of is the 820Ohm Cathode Resistor 3watt on the far left side of the board -- my PT doesn't have a 6.3v center tap, so I used the artificial center tap shown in the layout/schematic consisting of two 100Ohm resistors tied in to V4. As such, I do NOT have the jumper wire installed from the 820Ohm Cathode Resistor to the 8th pin of V4. Originally I did have this connected but I realized it wasn't required due to there not being a 6.3v CT. Anyway, onto the readings:

ALL READINGS ARE IN vDC UNLESS SPECIFIED AS vAC/mvDC/etc

Between the Heaters:
All at 6.4vAC, from the lamp to each tube

B+ readings:
B+1: 333.0
B+2: 330.1
B+3: 273.7

Pin readings:
V1 (12AY7)V2 (12AX7)V3 (12AX7)V4 (12AU7)V5 (EZ81)
1143.3152.3222.9332.8120.9
2.2mv // 0?-1.1mv21.8608mv
32.3361.0229.729.8332.9
429.2329.629.729.829.7
529.2329.629.729.829.7
6141.8273.3210.3333.78mv
7.2mv // 0?152.322.029.9121.4
82.34153.329.829.92mv
929.4329.629.829.92mv

Notes on Pin Outs: V3, the LTP valve, would motorboat/oscillate when grids pins 2 and 7 were probed. Note sure if that helps us ID any issues, but thought I'd mention it.

Resistances:
LTP Cathodes to Ground (both same): 11.8K Ohms // 11,800 Ohms
LTP Grids to Ground (both same): 1.008M Ohms // 1,008,000 Ohms
LTP Cathode to LTP Grids (both same): .998M Ohms // 998,000 Ohms

I look forward to any thoughts you all may have!

Thanks again,

Camden
Grounding that cathode is obviously bypassing the entire cathode network intended. Lets power down and ohm our way through to determine whats happening.

remove the temp ground and ohm the
1.ltp cathode to chassis/ground=
2.ltp cathode to each grid of the ltp=
3. each ltp grid to ground

we can get a volt chart once you verify proper component/wiring
Just posted my readouts!
 

D'tar

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my PT doesn't have a 6.3v center tap, so I used the artificial center tap shown in the layout/schematic consisting of two 100Ohm resistors tied in to V4. As such, I do NOT have the jumper wire installed from the 820Ohm Cathode Resistor to the 8th pin of V4. Originally I did have this connected but I realized it wasn't required due to there not being a 6.3v CT.

Cam, V4 pin 8 needs to be connected to the 820 ohm resistor. This is the cathode resistoer and sets the bias for the output tube. Your 2x 100 ohm resistors are your heater artificial center tap so the only connection you will not have compared to the layout is the green/yellow wire from the PT to the 820 ohm cathode resistor.

Your ohm readings on the ltp tail seem reasonable. reconnect your v4 cathode and retest.
 

Camdenharr

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my PT doesn't have a 6.3v center tap, so I used the artificial center tap shown in the layout/schematic consisting of two 100Ohm resistors tied in to V4. As such, I do NOT have the jumper wire installed from the 820Ohm Cathode Resistor to the 8th pin of V4. Originally I did have this connected but I realized it wasn't required due to there not being a 6.3v CT.

Cam, V4 pin 8 needs to be connected to the 820 ohm resistor. This is the cathode resistoer and sets the bias for the output tube. Your 2x 100 ohm resistors are your heater artificial center tap so the only connection you will not have compared to the layout is the green/yellow wire from the PT to the 820 ohm cathode resistor.

Your ohm readings on the ltp tail seem reasonable. reconnect your v4 cathode and retest.
Great input thank you - so I’ll reconnect that wire and remeasure just V4 again correct? Funnily enough that’s the way I did it first, I just thought it was an error so I undid it.
 

D'tar

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That may change everything downstream once the power tube is conducting properly so a whole new chart may be necessary.
 

Camdenharr

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Alrighty - new measurements with the cathode resistor reattached...

B+1: 318.8v
B+2: 316.4v
B+3: 262.7v

V1V2V3V4V5
1151.8149.5260.9318.1119.5
2.7mv-1.0mv3.8530.1mv7.4mv
32.525.97 (volts, not mv)11.5411.71318.8
410.1111.311.5411.7111.84
510.1111.311.611.7111.84
6150.4263.4262.9317.87.7mv
7.3mv146.74.3130.1mv120.6
82.51149.111.6511.772.6mv
910.7211.4611.6511.772.6mv
 

D'tar

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Any guitar signal passing through here?
Your LTP voltages look odd to me.
can you see heater glow on v3 same as v1?
Do you have another 12ax to try in v3?
 

Camdenharr

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Any guitar signal passing through here?
Your LTP voltages look odd to me.
can you see heater glow on v3 same as v1?
Do you have another 12ax to try in v3?
So guitar signal is NOT passing through all I can hear is motorboating/oscillation.

All heaters glow the same, yes.

And finally I do have another 12ax - I’ll pop it in now.
 

Camdenharr

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Just put in the new 12ax7 and got the following for V3

1) 268.4
2) 1.51
3) 10.58
4) 10.71
5) 10.71
6) 268.5
7) 3.86
8) 11.02
9) 11.02
 




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