Tweedle-Dee Master Volume

[martin manning]

A few people have been adding mods to the Tweedle-Dee, including a master volume. Here, IMO, is the correct way to do it. Also shown are an improvement to the balance control, a tweak to the plate load, and a combined cathode load resistor.

[ChopSauce]

Thanks!

I put the schematic suggested by sluckey https://ampgarage.com/forum/viewtopic.p ... 15#p385476 here, for comparison



since you both seem to agree about the proper way of doing it.

I am looking forward to reading - possibly - about your plate load "trick", though, which may look like one step backwards, with respect to Mr D. mods...

[fred.violleau]

So Martin you advise to :
- add the 470k grid stopper
- lower the plate voltage a bit
- add resistance on the PI tail
Besides adding the 1Meg pot and a 22nf cap.

So I understand that adding resistance on the tail with less voltage on the plate is to balance back towards the initial scenario ?
But why not keep the original scenario ?

Fred.

[fred.violleau]

I will try to mod my build towards your proposal and report as soon as it's done !
Thanks Martin!

Fred.

[martin manning]

So Martin you advise to :
- add the 470k grid stopper

Yes.

- lower the plate voltage a bit

Let the voltage come out where it will. I reduced the 110k plate load to 100k because it helped to balance the outputs, assuming a perfect 12AX7.

- add resistance on the PI tail

I found a value that should balance the outputs with the trim pot centered (again assuming a perfect tube), and with lugs 2 and 3 on the trimmer connected. The balance control should work a bit better that way.

Besides adding the 1Meg pot and a 22nf cap.

Yes, the PI operating point will be affected unless the volume pot is isolated from the grid with the second cap.

I'll be interested to hear how it turns out!

[norburybrook]

I will try to mod my build towards your proposal and report as soon as it's done !
Thanks Martin!

Fred.

I think I will do the same Fred, after adding the master volume etc I noticed something had changed in the amp I didn't like as much as before. I've been mulling over in my mind what that was is. I was going back to removing it in my mind this morning. I think perhaps the variable grid stopper method might be more useful, i.e replacing the 470k grid stopper with a pot. The amp isn't THAT loud anyway so a master volume wouldn't be that useful unless you need bedroom volumes for heavy OD. I also prefer the sound of the FX in a wet dry set up coming from a line out rather than the FX loop. that does mean 2 amps though




@MArtin, thanks for this.


M

[martin manning]

Marcus, the grid stopper will only affect the extreme high frequency end of the response, so I don't think a pot would be very useful. Having the 470k there will reduce the possibility of HF oscillations, though, so I'd recommend keeping it. Regarding the MV, of course one could also do the post-PI Lar-Mar type with a dual pot.

[norburybrook]

Marcus, the grid stopper will only affect the extreme high frequency end of the response, so I don't think a pot would be very useful. Having the 470k there will reduce the possibility of HF oscillations, though, so I'd recommend keeping it. Regarding the MV, of course one could also do the post-PI Lar-Mar type with a dual pot.

yes, you're right i was getting confused with the 680k which is the resistor that cut's the gain down when cascading channels.I'm still going to remove the Master though and see if it has affected the tone. It's funny when i first did the cascade something wasn't quite right but it sounded good. I've changed to many things to work out what's the cause of it loosing something.



M

[fred.violleau]

So I did Martin's mods.

Here is what I hear:
- amp sounds a bit stiffer,
- a little less output volume (around 15% to 20%),
- less brightness on the bright channel
- Channel 1 breaks up later ( around 5ish...)
- double distortion happening earlier
- overall, distortion is less "creamy"

I still have to check PI voltages to see if I messed up something.
I did not adjust the PI tail to 86, mine is still 99k (56k + 33k in series)

Is it possible that adding the 0.22uf coupling caps made the amp stiffer, and a bit less bright, would it be responsible for the drop of output volume as well ?
EDIT : forgot the mod on pin 2 and 3 on the trimmer... would that make a difference ? WIll do it later tonight.

Fred.

[martin manning]

Hi Fred, if you have changed the plate load from 110k to 100k, and still have the original cathode resistance and trimmer connection, then you are probably unbalanced. I'd expect some improvement afterward. I don't think the extra cap would cause those effects you mentioned. The gain and frequency response should be essentially the same.

[fred.violleau]

I added the connection between pin 2 and pin 3 on the trim Pot... and yes I am now using a 100k resistor for the PI

And it sounded a bit better, like a bit more volume , maybe a better balance.. but.. still sounded sterile to my compared to what I had.
EDIT : the amp had a lot more headroom, but was lifeless, and when I cranked it up, I had tons of double distortion, signal being squashed and compressed... really not what I had before

So I walked back slowly and tested the amp each time I removed something.
First I removed the 22nf cap on the output of the MV pot => still sterile, no change on the sound
Then I removed the 470 grid stopper => stil sterile, no change on the sound
Then I rewired my pot the way I did in the first place.... and it's not wired as a master volume, nor a sweet pot.
I connect pin2 of my MV pot after V2A .022coupling cap and connect Pin3 of the MV at the junction of the 1M and the 3.3M resistors, while Pin1 is grounding at the control bus bar. (Maybe not the best grounding place as it adds some hum, I will play with the grounding and see if I can find a better place)
So I am just adding a 1M pot between the output of the 2nd triode and the input of the PI.
Holly moses, that makes the amp sing !!! like fabulous !! I mean It really shines!!

So I know I am derailing from the original topic... but this is really worth trying, because it turns the amp into a different beast.
Now my question to Martin : could it be harmfull ? Since I am not using a grid stopper anymore, the pot is turning into some sort of a grid stopper ?

Fred.

[martin manning]

Your 1, 2, and 3 are CCW lug, wiper, and CW lug?

If that's correct, I think you are getting clipping on opposite peaks of the two outputs (positive swing on one and negative swing on the other), which turns into a strong second order harmonic at the output. With the CCW lug of the pot grounded you have placed a 1M resistance between the grid and ground, and so the stage is not functioning like a typical cathodyne inverter.

Also as you turn the control down the resistance following the coupling cap is reduced, which will increasingly cut the low frequencies.

I don't think you will damage anything running it that way, you just have an new type of tone control and overdrive going on.

[fred.violleau]

Thanks Martin for the detailed explanation. The 1M pot would produce those big current swings which could explain also why the amp is much louder?

I will try to bypass the pot today to hear how the amp sounds.

Test, fail, learn, test, fail, learn... Love it!

Thanks Martin for your time and valuable knowledge!

Fred.

[norburybrook]

Thanks Martin for the detailed explanation. The 1M pot would produce those big current swings which could explain also why the amp is much louder?

I will try to bypass the pot today to hear how the amp sounds.

Test, fail, learn, test, fail, learn... Love it!

Thanks Martin for your time and valuable knowledge!

Fred.

after some experimentation today I've removed the master volume. When in cascade mode the second volume control almost acts like a master anyway so for the sake of tone I think it's better without one.



M