how to trouble shoot a tremlo problem- Help

[jamme61]

just finished a build of tube depot 18 watt marshall

schematic-

https://d1sjrnpi226dnf.cloudfront.net/s ... 1382030148

layout-

https://d1sjrnpi226dnf.cloudfront.net/s ... 1382030168

every thing works except the tremlo? The channels both work, normal and trem but, when I turn on the trem it dosen't go on, just plays without trem? I checked all wiring and was wondering if there's any tips on how to trouble shoot this? thanks for any help

[xtian]

Looks like a plug must be inserted into the vibrato footswitch jack and it must not be shorted, for the trem to work.

If that's not it, what voltages do you have at pins 6 and 7 of V3?

[Richie]

Well that's not your typical marshall 18 watt trem circuit. Compare this layout and values of your circuit to this. For one, using all .022 trem caps may make it hard for the trem to start up,if at all.
Also the trem in 18 waters is very quirky to lead dress,and depends on what 12AX7 you use in the trem circuit. Some 12AX7 tubes, the trem will just not work.

Try making the trem more like what you see in the ceriatone layout. and it should work.

http://ceriatone.com/images/layoutPic/m ... iatone.jpg

[jamme61]

Looks like a plug must be inserted into the vibrato footswitch jack and it must not be shorted, for the trem to work.

If that's not it, what voltages do you have at pins 6 and 7 of V3?

I put the plug in the jack open not shorted- didn't work then I measured the voltages pin 6 is 263 volts and pin 7 is 43 volts does this sound right?

[jamme61]

Well that's not your typical marshall 18 watt trem circuit. Compare this layout and values of your circuit to this. For one, using all .022 trem caps may make it hard for the trem to start up,if at all.
Also the trem in 18 waters is very quirky to lead dress,and depends on what 12AX7 you use in the trem circuit. Some 12AX7 tubes, the trem will just not work.

Try making the trem more like what you see in the ceriatone layout. and it should work.

http://ceriatone.com/images/layoutPic/m ... iatone.jpg

I agree and if I can't get this one to work i'll change it over to ceriatone. I used the ceriatone before and it worked but did click sometimes- weird sometimes it worked fine other times noise.

[R.G.]

All tremolo problems eventually devolve down into one of two classes of problems. These are:
1) Is the oscillator oscillating?
2) Is the modulator modulating?

There are many refinements and sub-issues, but these two are the places to start.

Which one do you have? Or both?

[jamme61]

I have neither- its like the trem is turned off? I checked that its not grounded off, on pin 7 v3 with trem poy in on position. when I switch the pot to off, the ground comes on for pin 7 which is right. I checked the wiring over and over again with my DVM, everything is right, that I can see. So wether the trem is in the on or off position the trem channel plays fine except no trem? can there be to much oscillation on the trem so its on and oscillating and I cant hear it? but still non trem sound on the channel? I know this is painful for you guys but I do appreciate it- I have a call into tube depot and am searching the web- thanks for trying to help.

[R.G.]

I have neither- its like the trem is turned off? I checked that its not grounded off, on pin 7 v3 with trem poy in on position. when I switch the pot to off, the ground comes on for pin 7 which is right. I checked the wiring over and over again with my DVM, everything is right, that I can see.

So - could you see an oscillating LFO if it were there? If you use your voltmeter setting and probe the cathodes of V3, do they wobble up and down, whether there is signal or not, when the tremolo is turned on? If not, the oscillator is not oscillating.

This is an odd version of a tremolo, using the main gain tube to be the LFO oscillator and the modulator as well as a gain stage, and tapping off some of the plate signal to provide feedback to the second section and cause low frequency oscillation.

But the same rules apply - if there is not enough gain through the feedback loop at the LFO frequency, it will not oscillate, and will not change the cathode bias of the two sections, causing no tremolo effect.

So wether the trem is in the on or off position the trem channel plays fine except no trem?

As noted, the signal gain section is the same as the oscillator gain section, so yes, if this section has too little gain for the LFO to oscillate, it will play fine except no trem.

can there be to much oscillation on the trem so its on and oscillating and I cant hear it? but still non trem sound on the channel?.

Yes. However, that would probably mean ultrasonic oscillation, and you can't see that with only a meter.

Do you have an oscilloscope?

[jamme61]

dont have an oscilloscope. I put my DVM set to dc volts measured from the cathode on V3 and got 3.2 volts steady with the trem on and 2.9 volts with the trem off. so no up and down. Is that what you meant to check it? thanks again for the time.

[jamme61]

Well that's not your typical marshall 18 watt trem circuit. Compare this layout and values of your circuit to this. For one, using all .022 trem caps may make it hard for the trem to start up,if at all.
Also the trem in 18 waters is very quirky to lead dress,and depends on what 12AX7 you use in the trem circuit. Some 12AX7 tubes, the trem will just not work.

Try making the trem more like what you see in the ceriatone layout. and it should work.

http://ceriatone.com/images/layoutPic/m ... iatone.jpg

yea after trying everything with no luck- I took your advice went with ceriatone version trem and it works fine - thanks guys

[tubeswell]

(What R.G. means is that with your VDC meter on the trem tube's cathode, you should see the voltage going up and down on the slowest setting when the trem is switched on.)

Do you have the correct kind of switching jack installed at J5?

Apart from what the other's have said about the unusual design for a LFO stage, you could try simplifying it by removing the .047uF cap (C10) between the plate and the cathode, and changing the values of the RC network between the plate and the grid. Compare the two schematics at the bottom of this page http://www.valvewizard.co.uk/trem1.html - the one using .022uF caps and 470k resistors is designed for a 12AT7 triode. Whereas the 12AX7 is using .01uF caps and 1M resistors.

(Edit: Oops I see you replied reporting your fix as I was typing my post - never mind.)