Tremolo for Cathode Biased Push Pull Amp

[BrainDeadAmps]

Hello everyone,

I'm trying to get a better grip on tremolo circuits.

I've implemented a vibrochamp tremolo circuit into a Filmo 385 conversion. Because the amp has an empty octal socket after the conversion I used a 6sl7 inplace of the 12ax7. Basically identical to the schematic except for replacing the .01 oscillator caps for .02 to slow the speed and increased the cathode bypass cap to 47uf/25v in place of the original 25uf which made the oscillation a little deeper. I have it connected to the 3rd gain stage, immediatley following the Tone Stack. I experimented will all preamp stage cathodes, even the phase inverter with no real changes worth mentioning.

B+ going into the plate of the buffer, 265vDC straight from the phase inverter filter cap.
470k from the buffer plate to the oscillator plate giving a B+ of 110-120vDC

I've noticed that most tremolo circuits have their b+ coming from the power tube Filter cap.
On most tube data sheets the 6sl7 is rated for 250-300vDC, so in fear of stressing the tube I opted for the adjacent power node for a lower B+. I have 320vDC on the Power Tube Filter Cap, Is it really that risky to bump the B+ up to 320 on the 6sl7

In the end, in does work with a very minimal thump after battling lead dress but I know it could be improved.

I recently stumbled upon the Matchless Hurricane Schematic and it's one of the only tremolos i've come across on a cathode bias amp that alters the power tube bias.

It appears to be worth taking a shot at replacing the pre-amp tremolo with the power-tube tremolo for a more intense modulation.


Before desoldering and swapping anything out I figuired I'd throw the idea out on here.

Any insight will be greatly appreciated.

[sluckey]

There are plenty of tremolo circuits that wiggle the grid of cathode biased output tubes. Since you are using a 6SL7 you may be interested in this...

     http://sluckeyamps.com/RCA/Ampeg_J12B.pdf

The LED provides a very strong tremolo.

[PRR]

6SL7 is rated 300V design center. In any practical use the plate sits at half or 3/4 of the supply voltage. (Which is why the Typical gives 250V.) 400V of B+ is perfectly rational. Anyway it won't blow-up like a cherry-bomb.

[BrainDeadAmps]

Sluckey that circuit looks promising, I'm intrigued to try it out.

PRR thanks for clarifying the voltage rating!

I will be sure to report back once I get it swapped out.

[BrainDeadAmps]

There are plenty of tremolo circuits that wiggle the grid of cathode biased output tubes. Since you are using a 6SL7 you may be interested in this...

     http://sluckeyamps.com/RCA/Ampeg_J12B.pdf

The LED provides a very strong tremolo.

Sluckey, I have already implemented a TW Type 2/Lar Mar Master Volume. I was wondering if the tremolo would interfere and found an old topic where you mentioned it should work.

So essentially I would disconnect the ground on my MV pot and connect the ground leg of the intensity pot in it's place?

[sluckey]

So essentially I would disconnect the ground on my MV pot and connect the ground leg of the intensity pot in it's place?

Not quite right. Connect the intensity pot ***WIPER*** to the MV ground lugs.

[BrainDeadAmps]

So essentially I would disconnect the ground on my MV pot and connect the ground leg of the intensity pot in it's place?

Not quite right. Connect the intensity pot ***WIPER*** to the MV ground lugs.

I implemented the tremolo circuit with great results. Max intensity gives a nice helicopter throb.

I did run into a grounding issue.

I have attached a simplified drawing of my grounding scheme.

Something is wrong with how I'm grounding the input and v1. There is a what I believe to be a ground loop. When I adjust my tone pot on my tele I even get a sort of scratchiness like there is dc on the pot. When I jumper from the input jack to the input of v2, the Amp is perfectly silent.

Any ideas on where I went wrong with my grounding scheme?

[PRR]

...When I adjust my tone pot on my tele I even get a sort of scratchiness like there is dc on the pot. ...

Did you check for DC on the input?

(That seems more likely than DC coming from a ground.)

[BrainDeadAmps]

...When I adjust my tone pot on my tele I even get a sort of scratchiness like there is dc on the pot. ...

Did you check for DC on the input?

(That seems more likely than DC coming from a ground.)

Checked for dc on the input, meter read Zero.

[pdf64]

Checked for dc on the input, meter read Zero.

What was the scenario at the input when the measurement was made? eg with nothing plugged in and shorting jacks, the resistance between grid and 0V common may only be 34k, at the actual input it’ll be 0.

[BrainDeadAmps]

Amp plugged into the wall
 Guitar was plugged into the input jack.
 Turned the amp on with my black probe alligator clipped to the amp chassis and I checked the tip of the cable with the red probe.

[pdf64]

So what were the guitar’s controls set to?
I suggest to monitor the mVDC at the input stage grid, whilst adjusting the controls so as the induce the noise mentioned.

[BrainDeadAmps]

So what were the guitar’s controls set to?
I suggest to monitor the mVDC at the input stage grid, whilst adjusting the controls so as the induce the noise mentioned.

I decided to revise the ground wiring of the v1 5879 circuit, I attached a drawing below.
Got most of it done last night, will hopefully get a chance to try it out tonight.

PDF64, I will definitely try your troubleshooting method if the problem remains.

I have a question about the best grounding design. Like I said earlier when jumpering past v1 the rest of the amp was very quiet. All of those grounds are connected to the power filter caps which are bolted to the PT on the opposite end of the input.

The Ground tab that is riveted underneath the output transformer is on the input side of the chassis, so I figured I could tie in both V2 and V1 Filter caps with their respected circuits.

But my input jack is not isolated from the chassis.

Would it be best practice to _____?

A.) Run V2 to the Pre-amp ground tab and V1 to the Input jack Ground Tab.

B.) Connect both V1 and V2 to the ground tab and leave the input jack isolated with only the contact from the chasis ground.

[BrainDeadAmps]

So what were the guitar’s controls set to?
I suggest to monitor the mVDC at the input stage grid, whilst adjusting the controls so as the induce the noise mentioned.

Noise persists after grounding v1 to input jack tab.

Tried monitoring the input grid like pdf64 suggested.

Still read 0mVDC while rolling both guitar controls up and down.

[BrainDeadAmps]

At last, my new ground scheme works!

Turns out I tied the cathodes together of v3 than ran a ground wire to the (-) filter cap.

(Revision) Gave each triode its own ground wire to the filter cap and noise issue ceased.

Thank you all for the wealth of knowledge found here.

[Willabe]

Glad you found it. 

For your next build/re-build, on grounding;

http://www.valvewizard.co.uk/Grounding.html

[BrainDeadAmps]

Glad you found it. 

For your next build/re-build, on grounding;

http://www.valvewizard.co.uk/Grounding.html

That is actually what got me through it. Wonderful little write up.