Tremolo Help

[C Moore]

http://i111.photobucket.com/albums/n143 ... oto008.jpg

http://i111.photobucket.com/albums/n143 ... oto007.jpg

OK so I built this head quite a while back.
Finally got around to ordering a cab for it...and having a few guys play it (I DO NOT play guitar)...they say it sounds good...but the Trem is Real Weak.
The tubes are good...I tried several that I know are good.
Have I got a problem with the values in my Trem Circuit.?
The amp is basically a BF Fender with ...6K6...power tubes.
I used a Trem Circuit from an Ampeg, becasue they had one with a 6SL7.

Quite a while back, Martin Manning told me..."The resistor with a ? value will have to be adjusted for the LFO signal that you have. I'd find a value that makes the p-p voltage swing about the same magnitude as Vbias."
I have dual bias. So 2 different bias voltages...they are -40.0 and -45.0.

So what is he telling me to do exactly.?
Do I need to put a Scope on/across that LFO resistor, and get a p-p voltage reading.?

It was his post...about #12...in this post... https://tubeamparchive.com/viewtopic.ph ... highlight=

Any help in getting my Trem to work would be great.
I have used this trem circuit in an Ampeg...but not in this dual bias Fender scenario.
Thank You
sorry for the long post

[tubeswell]

Maybe try a 5G9 trem circuit instead. The idea of a plate-driven 'follower' on the LFO is interesting but I wonder about the output impedance of that method compared to if you use a CF as a buffer for driving the output tube grids into cutoff.

[C Moore]

Here is the Ampeg I stole it from.
My intensity pot is 2x250k...theres is 1M.
There LFO resistor is 470k.
Mine is 130K.
There amp is Cat Biased. Mine is fixed bias at -40.0VDC +/-

[tubeswell]

Same difference. The 270k grid load resistors for the 6V6s in the Ampeg present a lower input impedance than your typical pre-amp stage would, and coupled to the plate driven bias wiggle, which has a relatively high output impedance compared to that of a CF buffer, means that there is more room for the wiggle to be lost in translation, especially as the intensity is attenuated by having a 470k in series with the 1M intensity pot. The 6K6 also typically requires more bias voltage than a 6V6, so that affects how much LFO wiggle voltage is needed to drive the output tube grids into cutoff. You could try is bypassing or reducing the (470k) series resistor between the coupling cap and the intensity pot.

[C Moore]

Yeah...that 470k on my amp is actually 130k.
Maybe I need a different trem circuit...as you suggested.
Thanks

[tubeswell]

Try taking the 130k out first - that would be easier than rewiring.

As an afterthought, given the reliability issues of most modern-production tubes these days in terms of their h-k insulation, I can see the potential attractiveness of using a plate drive follower (although you could overcome this potential problem with heater elevation, or using level shifting drivng a direct coupled CF with lower load resistor, or using a 12DW7 tube with an AU7 CF stage and different load etc). However I think a CF buffer would generally deliver better results intensity-wise. But try taking out the 130k first just in case.

[C Moore]

10-4
Talk to you guys tomorrow.
Thanks Again TAG

[C Moore]

OK...I jumped that 130k resistor, and that "helps".
Have to have the Intensity dimed, and it seems like if I lightly strum a chord...there is no trem at all. But if I rake the chord pretty hard, that sets off the trem, but it is not a nice sounding effect.

So...dual...fixed...adjustable bias.
Bias voltage is about -45.0VDC on each power tube.
Would really like to keep the single 6SL7.
Would you still recommend the Fender 5G9 circuit.?
Thank You

[martin manning]

Can you measure the voltage swing on the bias with the tremolo full-on? Maybe you just don't have enough. WIth bias at -45V, I'd look for 45V p-p on a scope.

[C Moore]

Hey Martin
Yeah...I can do that.
Where are my leads going exactly.?
Thank You

[martin manning]

Ground and the grid pins, AC coupled.

[C Moore]

OK.
Let me take a shot at this.....
Is this an Idle/Quiescent measurement.?
Or do I need to inject a sine wave into the front of the amp.?

[martin manning]

No signal needed. Just looking to see how much bias swing you're getting from the trem.

[C Moore]

I do not know what I am doing wrong.
With the Vol Treb Bass half way.
The Trem all the way up.
I am only seeing several hundred milivolts, and a very "unconventional" looking sine wave.
The scope is on AC...tried both Auto and Normal.
I just cannot get any kind of real reading on either power tube grid.
Tried 3 different chassis ground positions.

[martin manning]

What do you see at the output side of the coupling caps on the 6SL7? Do you still have that 0.1 cap from the Ampeg circuit there? As I suggested above you can eliminate that. The two 0.1's feeding the intensity pot will block the DC.