yamaha T100 low output on the gain channel only

[soulreaper_msd1]

i have a Yamaha T100 amp head i never really used the gain channel hardly ever since i bought it new, the amp has very little volume on the gain channel but works fine on clean (which is the channel i used a lot) i just changed all the tubes(pre-amp & power tubes, which were the factory ones untie now- a lot of years) anyway i was hoping this would fix it cause i know at least 1 power tube was bad cause the glass was loose from the tube base. what should i check next? thanks...

ok I just did a bias check on the power tubes, there a 4 of them, from left to right tube 1=31.3 and fluctuated , tube 2=26.9 to 27.0 and fluctuated , tubes 3=29.6 to 30.0 , tube 4 was 00.1 no fluctuation ,, I hear 66l6gc tubes should read 30.0 milliamps to 27.0 milliamps..

[Richard]

Sometimes tube bases have a little slop in them. The power tube should affect both channels equally. Has the amp always had low power on the gain channel? Is the sound good except for limited volume and are we talking about not as loud as a 100w amp should be or is it not as loud as any amp should be. Do the rest of the controls on the gain channel work normally? Have you tried swapping all the preamp tubes through the clean channel to verify you haven't replaced a bad tube with another loser. After verifying that I am working with good tubes I would start twiddling knobs and see if any reacted unusually, that may narrow down the search.

http://music-electronics-forum.com/a...a_original.jpg

There appear to be several switches in the circuit, any one of them could be making poor contact. One of the professionals will chime in soon with a much more thorough guide to troubleshooting.

[soulreaper_msd1]

thanks Richard..

[g1]

Here's another schematic. V2 is for the gain channel.
Then on the power amp page, there are separate tone stacks and switching for the channels.

[soulreaper_msd1]

thank you ..

[soulreaper_msd1]

1 thing I forgot to add , on the gain channel only when I turn the reverb dial it pops and kinda tries to get a little louder... it did this before and after the new tubes..

[soulreaper_msd1]

not as loud as any amp should be..

[Enzo]

Your whole dirt channel is just V2.

The clean channel works, right? So that means V1 is working as is V3 and everything after.

Look at the schematic. The input feeds V1 grid, pin 2. That stage amplifies and the result is on the plate, pin 1, Note that that signal goes to both the clean and the overdrive volume controls. The clean control feeds the other grid of V1, and then the path is over to V3 and so on. The dirt channel control feeds the grid of V2 pin 2. Then the plate of that stage feeds the other side of the tube. Notice that the input signal is always amplified by both channel. The only reason you don't hear overdrive all the time is the two optical switches - S1, S2. S2 is turned on to pass the overdrive signal to V3. S1 turns on to ground off the signal of the OD channel when the clean is selected.

SO check the signal at the plate of V2, pin 1, it ought to have a similar level as the plate of V1 pin 6. More or less. Whatever is on pin 1 of V2, there ought to be a much more driven signal at pin 6.. If there is, then maybe S2 is not being turned on or it is defective. Likewise, S1 kills the OD for clean, but if it isn't turning off, it squashes the OD, or maybe it is defective.

The schematic shows a dotted line around the volume controls, indicating they are not on the circuit board. SO I assume there are wires or connectors to them. Make sure those connections are working.

Having said all that, there are a couple other optical switches involved. There are a pair that select either the clean or the OD master volume controls. The same circuits are controlled, there is simply two controls. A bad or uncontrolled opto on the OD master would kill the signal. Likewise, there is only one reverb circuit, but two level controls for it, and one is selected by the optical switches.

Plug the guitar into the FX return. That bypasses the preamp, but not the master volumes. Switch channels back and forth. If the OD channel still drops way down, then suspect the opto there. If however, both channels sound OK through the FX return, then the master volume selector is OK.

[g1]

Enzo, it splits up again between V5 and V7 for separate tone stacks, on for clean ch., one for gain channel. (pg.3 of pdf).

Edit: Sorry Enzo, I see you did touch on the separate masters. Sometimes I only see parts of fresh posts, then the rest shows up later. It's strange as I would think the whole post should show up at the same time?

[soulreaper_msd1]

I hooked up to the effects return still the same deal as before.. if I turn the volume up all the way and the gain when I strum the strings I get a loud popping kinda noise for a split second every time I hit the strings, clean channel works as it always has..

Thank everyone . for there help by the way..

[g1]

Then I think you can ignore the area around V2 and concentrate on the area between V5 & V7.
That would be the OD tone stack, master, and switch 36 OD. You could try jumpering across the switch to test.

[soulreaper_msd1]

1st I want to thank you and everyone else for their help.

whats the best way to test these controls?

I tested the current of the tube pods with an adapter made to test (6k6 6v6 6l6/5881 el34/6ca7 6550/kt88 7355& 7591 tube pots with the tube in top of the adapter with a multi-meter..

thanks.

[soulreaper_msd1]

thanks G1 , I been at it for a while now and my brain seems to be - not to good at reading wiring diagrams lol, but ive found no loose connections so far and iam not sure which leads to jumper to bypass the OD switch, but if I turn the gain all the way up and the volume on the OD channel and pull and push the gain knob it pops realm loud and acts like it could output more for a half a second then back to the low output,, iam good with wiring on car but really not to good ac current or this kind of wiring,, thanks.

[rosssurf]

Older thread I know, But I have having similat issues with my T100c. Did you find and fix the problem? Did you figure where to jump the opto switch

[soulreaper_msd1]

haven't orderd parts yet

Older thread I know, But I have having similat issues with my T100c. Did you find and fix the problem? Did you figure where to jump the opto switch

I haven't ordered parts yet but from what I've seen these guys no what they are talking about......

no I haven't figured out which to jump to check. (forgot to add that earlier)