Yamaha / Soldano T100C Amp, Clean Channel Low Volume

[TomCarlos]

Ok.... some updates....

One Part 3 of 3 of the Schematic, the Power Amplifier Section, I see an error in the Tube/Voltage table. For V3, Pin 8, it shows 3.3 volts. On Page 10 of the Service Manual, V3, Pin 8, it shows 174 volts. I measure 165 volts.

Next, the Service Manual calls for replacing the 1K ohm resistor on V6b Pin 8 with a 1.8K ohm. That mod was done on this amp.

I am using the instructions and table found on page 10 of the Service Manual. It calls for the following:

V1, Pin 1, using 1khz signal, adjust input to .4vrms – got that.

At V1, Pin 6, I should expect 5 vrms. I am only getting 3.25 vrms.

The goal is to see 11vrms at V3, Pin 11.

So with V1 having less ac on Pin 6 ss expected, everything down the line will be less than expected. I have tried 3 different tubes here and checked the dc voltages. So it must be something around V1 that is contributing to the low volume issue. I’ll lift each of the resistors and caps to check them.

Regarding Leo's comment...

Tom, I just had a look at the preamp schematic VK provided in post #5, with red highlighting the clean channel path. Zounds! There's a 5M6 resistor following the second triode, with a 330K to ground, the junction of those goes to the next triode's grid, V3. That combination is a huge voltage divider knocking the signal level down to about 1/18 of what's coming off that second triode's plate. You can tweak your clean gain upwards very easily by adding a parallel resistor across that 5M6. Poke through your megohm resistors & see if you have say a 3M3. I'll bet that would bump your clean gain up noticeably. If not enough try 2M2, 1M5.
=.

I guess you could do that but I do not believe that was done on the T50C. So for now, I will leave that one alone as I seem to have an issue before the 5M6.

[Helmholtz]

V3 is a DC coupled cathode follower. Pin 8 voltage should be about the same as pin 1 voltage (within 1V or so).

[Helmholtz]

If V1 has less gain than expected, check the cathode bypass caps.

[g1]

At V1, Pin 6, I should expect 5 vrms. I am only getting 3.25 vrms.

Do you have the 150mV RMS at pin 7 ?

[Leo_Gnardo]

I'll lift each of the resistors and caps to check them.

Lucky components & lands are top side of the board, makes checking & tweaking so much easier. Thank you Yamaha!

V1 pin 6, 3.25V instead of 5V isn't that much of a deficiency. But who knows, maybe one or both of V1's cathode bypass caps are old & tired, in need of freshening up.

And a quick check of the opto's certainly makes sense.

If after all that, clean channel is still lagging, you can alter the clean signal level as I mentioned. The point is to sell the amp, right? If a potential buyer is impressed with the way it sounds, they're not gonna care much that you swapped a resistor or cap. Forge ahead, I think success is right around the corner. Especially when you collect $$$ the day it gets sold to a happy buyer.

[TomCarlos]

Negative G1 .... I am measuring just over 100mv.

[g1]

Is clean vol. control 500K, and turned up full? Also, they do not specify whether 'bright' switch should be engaged, does it make any difference?
Agree with Leo that it may not be that far off (AC voltage at pin 6). Maybe turn up the signal so you have the 5VAC at pin6 and carry on with the AC voltage readings to see if there are any other gross discrepancies.

[TomCarlos]

Making some progress...

G1 - the "G" is for GENIUS... Yup, you pull out the Bright switch and voila, you get 5 vrms on pin 6. (Make a note of that future visitors to this thread)... I am also measuring 11 vrms on V3 pins 1 and 8.

I checked the bypass caps on V1. They measured good but I replaced them anyway. I also replaced the .022uf off pin 6 just because.

VK asked that I go into the Effects return and test the amp. I did that... both channels are clean and have the same volume output.

VK also asked me to check a two of the Optocouplers. With the amp in Standby mode and measuring the resistance across the Cell end of the Optos, 52CL is at 8.2K in the Red mode (distortion) and 3 meg in the Green mode (clean). For 35CL, I get 222K in Red mode, 5.5K in Green mode.

I also tacked an 8.2 meg resistor across the 5.6 meg. I didn't hear much difference.

So I am still left with the problem of the clean channel not producing the same level of output as the distortion channel. With all other settings equal, when the Distortion Preamp level is at 2, the Clean Preamp needs to be at 6 in order to hear the same type of output level - and that is with both Master Volume levels turned down.

tomorrow is another day...

[Leo_Gnardo]

I also tacked an 8.2 meg resistor across the 5.6 meg. I didn't hear much difference.

So I am still left with the problem of the clean channel not producing the same level of output as the distortion channel. With all other settings equal, when the Distortion Preamp level is at 2, the Clean Preamp needs to be at 6 in order to hear the same type of output level - and that is with both Master Volume levels turned down.

So far, so good. Now try tacking some lower values across that 5.6 meg. Or . . . accept the gain of the clean channel "as the factory made it" and dial the clean volume control way up as you did. If 6 is the happy spot, so be it. What happens past 6? Should get huge!

FWIW lots of folks who are used to the general run of Fender amps especially the 60's-70's classics have the idea that they should hear lots of clean volume with the control turned up to 3, maybe 4. Then ... one day they run across a Bandmaster or Showman. "Hey what's WRONG? I gotta turn the amp up to 6 or even 7 to get a big clean volume." Nothing wrong, those amps don't have as much gain as the rest. So what, dial it up to 6's & 7's, smile & play.

If you think you're going to encounter a "3 should be enough" character when auditioning the amp for sale, sure slap in a parallel resistor to that 5M6. Say 2M2 or even 1 meg, that oughta get you some obvious boost for cheap. If you think your buyer is in the frame of mind "it MUST be as the factory made it, or my money stays in my pocket" then leave it be, you've done as much as you can and that's the way it is. No further problems? Time to bolt the chassis back in & post this gem for sale. When the cash $$$ is in your hand, mission accomplished. NEXT!

[g1]

If you get the proper 11V at pin8 of V3, and there is no difference between channels when going in the FX return, then the output level must be normal for this model.
If there were a problem after V3 pin8, you would notice a difference between channels when going into FX return.

[vintagekiki]

Many new generation amplifiers require different settings compared to the amplifiers we grew up with.
Therefore in their instructions, should be find the recommended factory sample sound settings.

https://ru.yamaha.com/files/download/other_assets/0/313730/T50_T50C_T100_T100C_en1.pdf

[TomCarlos]

Thanks again for your time and help on this thread.

The buyer has agreed to take the amp as-is and have his tech look into the volume difference and potentially undo the Yamaha mod. So I will button it up and ship it.

I will follow up with this thread on these issues.

1. Does the T50C and his other Yamaha/Soldano show similar behavior when comparing the clean channel to the distortion channel on the T100C (knowing his other amps do not have the Yamaha mod)?

2. Follow up with the tech on his findings after he does an analysis and any work to the amp.

3. Post a final update to this thread once I get a report from the tech.

[g1]

If it's still handy, just set it up like one of the examples from the owners manual VK posted (pg.22 or 26). Toggling between channels with recommended settings I would expect some lead boost, but not night and day volume difference.