Yamaha / Soldano T100C Amp, Clean Channel Low Volume

[TomCarlos]

Mates....

I am working on a Yamaha / Soldano T100C Combo Amp. I just finished replacing the 80uf caps in the power supply. Yeah, I went with IC caps. I didn't want to deal with having to mount bigger caps on the top and bottom of the PC board. Also, the 40uf cap (which was previously replaced with a 47uf) that can be found on page 2, upper right, near the "F" connection point and considered part of the Pre-Amp section.

The amp is quiet and the distortion channel rocks. The problem is with the clean channel - it simply does not have anywhere near the output as the distortion channel.

BTW, the footswitch is working to change the channels.

Someone did a mod this amp. I know there are published mods for the choke. But I could not find any other common mods that would explain what I see. You can see a brown ceramic disc, going through a white shield, connecting to a 220K resistor. That resistor then connects to a .022uf cap. The 220K and Ceramic disc are NOT found in the T50C amp (the smaller brother that I just worked on). These mods appear to be in and around V5 and V7. Oh how I hate mods!!! When someone has done a mod, no one knows where it could end.

So the issue for me is to isolate the signal path that is only used for the clean channel. No sense in worrying about the common path or the output - we know there is plenty of output just by listening to the distortion channel.

Counting the Phase Inverter, there are seven 12ax7 tubes. If V7 is the Phase Inverter and V6 is the reverb tube, that leaves me V1 through V5.

I tested each tube (V1 - V5) on the Sencor tester, replaced each one in the circuit (one at a time), and that did not yield any changes to the clean channel volume.

Can anyone give me some hints as to where I might focus my efforts? In the next few days, I will measure the voltages on the V-1 to V5 tubes but would rather not waste time on anything not related to the clean channel.

And yes, this is one of the amps I am selling on behalf of my friends estate. I just sold the T50C amp for $500. Now I need to figure out if I can fix this one or if I need to discount the amp because of the issue. (It is a time thing that I lack and am nervous about disconnecting parts without knowing the full extent of the mods). But I am hoping to solve the problem and also hope the mod has something to do with the common signal path. And I wish I still had the T50C here so I could see the board and stock components - drats.

Thanks in advance.

yamaha_t50_t100_schematic.pdf

[g1]

V1b and V5a (and clean tonestack) are the only triodes specifically for the clean. The other thing will be all the switching opto's.

[TomCarlos]

Thank you G1. I will take a closer look at those tubes but will also check the others. In addition, I’m gonna check each control for the clean channel once again. The ongoing concern is to try to figure out what this modification might be all about. In any event, I’ll get back to the amp in the next couple days and will report back.

[Leo_Gnardo]

You can see a brown ceramic disc, going through a white shield, connecting to a 220K resistor. That resistor then connects to a .022uf cap. The 220K and Ceramic disc are NOT found in the T50C amp (the smaller brother that I just worked on). These mods appear to be in and around V5 and V7. Oh how I hate mods!!! When someone has done a mod, no one knows where it could end.
- - - -
Can anyone give me some hints as to where I might focus my efforts?

It's not so important what the extra components are, what IS important is what are they doing?

If you could pencil-in on the schematic the extra parts, we'd have a much better idea what's going on and perhaps make suggestions what to do.

Without that information, the best I can do is suggest you very carefully lift one end of each of those mod parts, one at a time, and see what happens. They may be filters removing high frequencies but that's only a WAG. "Very carefully" so that you can put the connection back together in case something definitely unwanted happens, like big hums, whistles, signal totally disappears, etc. If removing the parts gains you a nice clean full sounding amp then you win.

All in all, it's always best to fully understand what you're doing before operating.

Also, good on ya for getting $500 for the 50W Yamaha. Good deal on both sides. Still a "sleeper." The buyer got a good deal on a quality amp, and you got what I expect is as good a price as you could expect.

[vintagekiki]

#1
Yamaha / Soldano T100C Amp, Clean Channel Low Volume

Plug in guitar into jack effects return and try what the sound is like when you switch channels.
If the overdrive and clean channels have a clean sound, it is a sign that the V4, V5, V7 and pwr tubes are working properly.
For now, don’t deal with mods.
Check the clean signal path (red on schematics).
Check resistor 5.6M and 1M (V1), optocoupler 51 and 52 (V2).

1)
https://www.owner-manuals.com/-manuals-1-20a.html?alpha_filter_id=84

[TomCarlos]

Thanks again mates....

Once I double check the controls, solder connections, Tube voltages and parts connected to those tubes, I'll then try to trace out the mods.

Gee, I wonder if I can reach Mike Soldano and get a service manual including a photo of the PC board and components :-)

Tom

[vintagekiki]

About Soldano

https://www.soldano.com/

https://robrobinette.com/images/Guitar/Soldano/

https://www.thegearpage.net/board/index.php?threads/how-to-contact-mike-soldano-about-non-b-a-d-soldano-amps.2164537/

[Enzo]

And Mike would tell you, "I didn't build that amp, That is a Yamaha." And by the way, Yamaha manuals are about the most complete in the industry.

[vintagekiki]

https://i0.wp.com/static-cdn.imageservice.cloud/5295140/soldano-designed-ampyamaha.jpg

https://productselling123.blogspot.com/2021/03/soldano-schematic-soldano-hot-rod-25.html

1)

https://el34world.com/Forum/index.php?topic=18801.0
Help with this preamp schematic (T100)

http://www.huntamps.com/projects/yamaha-t100/
Yamaha T100

https://www.facebook.com/media/set/?set=a.545229788827351.147493.161905970493070
Yamaha T100/SLO conversion

https://www.facebook.com/Lampiaki-serwis-Leszno-607328313041334/photos/pcb.836828010091362/837470970027066

https://www.facebook.com/607328313041334/posts/836828010091362/
Yamaha Soldano T100C

http://www.diyitalia.eu/viewtopic.php?t=4797#p44168
Yamaha T100C - Soldanizzazione

[vintagekiki]

... get a service manual including a photo of the PC board and components ...

MEF has everything, only can't be found.
It should be mined, like as bitcoin.

https://music-electronics-forum.com/forum/amplification/guitar-amps/mods-tweaks/46762-yamaha-soldanop-t100

https://music-electronics-forum.com/filedata/fetch?id=847923
Yamaha T50_100 Service Manual

[TomCarlos]

Dang V-kiki, you are good. I wonder if the mod as mentioned in thread 46762 is what I have. I am going to dig into this tomorrow.

Many thanks !!!

[vintagekiki]

#11

https://music-electronics-forum.com/forum/music-electronics/36648-soldano-avenger-help-needed

https://music-electronics-forum.com/filedata/fetch?id=832552
Soldano SLO 100 Mods

https://www.tube-town.de/ttforum/index.php?topic=7259.0
Mods für meinen T100-C

[Leo_Gnardo]

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.

Another way to go about it: have a look at the cathode circuit of the second triode, 2K2 in parallel with a 0.05 uF cap. That's a brightening combination, tilting the audio spectrum upwards for high frequencies. You could parallel a larger cap and bump up gain across the guitar spectrum. I'd try a 2.2 or 3.3 uF cap. This comes at a price though - you lose the high frequency bump that the 2K2 // .05 combination gave. If you want to add back some Fender-like zing to the high frequencies, parallel that 5M6 resistor with a small disc cap, just a couple of picofarads. Fender uses a 3M3 parallel with 10 pF in many of their classic amps. Around 6 pF would simulate Fender's bright curve, more pF will make it brighter yet.

Hope this helps!

Thanks to VK for posting that preamp schemo! Once I had a good look at it, the solution(s) became clear.

[TomCarlos]

Just an interim update before I take the deep dive on voltage measurements, connections, etc.

Again, my thanks to VK and Leo for helping with the Service Manual and tips. Coincidentally, Yamaha replied to my email last night and sent me a link to the T50C / T100C Service Manual.

https://www.yamaha.com/yamahavgn/Ser...T50_100_SM.pdf

In the Service Manual, it is noted that V6, Pin 8, change the 1.0K to 1.8K. That has been done.

The Service Manual also has a section for a modification titled "Reverb Wash Modification." The description reads as follows:

"When the Overdrive channel (with reverb On) is being played, and a change is made to the Clean channel, a noise is created in the reverb drive circuit. This modification will reduce the "effect return from loading the reverb return", while providing better current gain, through isolation of amplifier stage "V4." The two (2) plates and two (2) cathodes on Amp stage "V4" will be isolated. Five (5) components will be added, and three (3) circuit traces must be opened (cut)."

And that is exactly what I see in this amp. Someone has already made that mod to the T100C. (I did not see this mod when working on the T50C). I can see the components, cut traces on the PC board, etc.

The way I read it, the mod should solve the described problem but should NOT affect the overall gain in the clean channel. I certainly did not experience this when working on the T50C amp.

So I wanted to point this out to anyone who stumbles across this thread in the future.

And now, onto the voltage measurements and especially those around V1 and V5.

More to come...


Yamaha_T50_100_SM.pdf

[vintagekiki]

You can tweak your clean gain upwards very easily by adding a parallel resistor across that 5M6.
Fender uses a 3M3 parallel with 10 pF in many of their classic amps.

Great advice.
Provided if the preamp circuit (V1, V2) and optocoupler is correct (35CL, 52OD), resistor 3M3 can be obtained if with resistor 5M6 in parallel temporarily soldered resistor 8.2M (blue on schematic)

Optocoupler can be tested with ohmmeter, when the amp is at stdby.
With foot switch select OD / Clean, and individually check the resistance of each optocoupler.