Tweedle Dee, Take two

[fred.violleau]

Hi guys,

This is my second take on this great build. First one sounded absolutely fantastic, so I needed a new one since the first one found a new home.

I did many layout for this build in order to balance feature set and size. I ended up with a 12x8x2 Hammond chassis.

Mods are as follow:
- diode rectification (my first build was done like this and I loved it)
- single input
- switch to choose between channel one and Channel two
- switch to have channel cascade into channel two
- Channel one voiced differently: 22k, 220k, 2k2, 5uf25v
- 470k grid stopper on V2B
- passive FX loop
- switch for V2A bypass cap (in/out)
- 56k NFB switch (in/out)
- LarMar II Master Volume
- 1k5 power tubes grid stoppers resistors
- 470R 5watts screen resistors (want to try different tubes)
- switch from cathode bias to fixed bias (new experiment for this build... can't help it, I need to try different things)
- footswich with 4 options on tap in order to make this amp gig friendly channel picking, NFB, cathode bypass and cascade option.

I bought an old RCA 400 projector from a fellow member here, so I am re-using the PT iron that seems to be well suited for this project. It was providing current for a rectifier tube, 3x6V6s, 1x6J5, 1x6J7 and a light bulb !

Inputing 12v on the primary side yielded the following results:
- 0.56 v on the rectification filaments (which will provide energy for the four relays this build requires)
- 74.7v for the high voltage filaments
- 0.718v for the heaters filaments

Question : does anyone know if this iron could support EL34 or other tubes than 6V6s ?

References of the iron:
901876-2
138713
Could not find much on the internet about it...

EDIT :
Since the original OT (Ref: 949135-2 450637 ) from the RCA projector seems a bit weak (10w output but not sure about that), I have a hammond 1760E as a backup plan.Again If anyone has more info on this, please chime in!

Chassis drilled, board completed with the eyelets, test fit looks good, now onto the smell of melted solder!

Here are a few pics:

PXL_20210303_144950898.jpg

PXL_20210305_183528125.jpg

PXL_20210305_205704457.jpg

PXL_20210307_123244797.jpg

PXL_20210307_123403570.jpg

Cheers,

Fred.

[martin manning]

...I am re-using the PT iron that seems to be well suited for this project. It was providing current for a rectifier tube, 3x6V6s, 1x6J5, 1x6J7 and a light bulb !

Inputing 12v on the primary side yielded the following results:
- 0.56 v on the rectification filaments (which will provide energy for the four relays this build requires)
- 74.7v for the high voltage filaments
- 0.718v for the heaters filaments

Question : does anyone know if this iron could support EL34 or other tubes than 6V6s ?

So at 120V you should see 5.6V for the rectifier heater winding (good), 7.18 for the other heaters (good), and 747 on the high voltage secondary (Hmm..). That last figure is very high for a capacitor input filter with silicon rectifier. You will get over 500VDC. As for powering other tubes, I doubt that there is enough HT or heater current for EL34 or 6L6.

[fred.violleau]

...I am re-using the PT iron that seems to be well suited for this project. It was providing current for a rectifier tube, 3x6V6s, 1x6J5, 1x6J7 and a light bulb !

Inputing 12v on the primary side yielded the following results:
- 0.56 v on the rectification filaments (which will provide energy for the four relays this build requires)
- 74.7v for the high voltage filaments
- 0.718v for the heaters filaments

Question : does anyone know if this iron could support EL34 or other tubes than 6V6s ?

So at 120V you should see 5.6V for the rectifier heater winding (good), 7.18 for the other heaters (good), and 747 on the high voltage secondary (Hmm..). That last figure is very high for a capacitor input filter with silicon rectifier. You will get over 500VDC. As for powering other tubes, I doubt that there is enough HT or heater current for EL34 or 6L6.

Hey Martin,

Thanks for your input.
Do you know a way to drop the HT after rectification, and before Filter caps?
Could I use a diode or something else in order to get the HT within reasonable range ?

Fred.

[fred.violleau]

Martin,

I could use 2x zener diodes 15V 5W in series on the HT CT to ground in order to get a decent voltage drop on the HT.
I was holding on a mouser order as I knew I would miss a few things... now's thew time

EDIT : will probably need an NTC in-rush Thermistor in series with the zener diode and I was also thinking on adding an inrush current limiting thermistor (Amphenol CL-60A) placed in the white neutral line and a .1uF 275V X Class Capacitor for line noise reduction.

Let me know what you think!

Fred.

[martin manning]

Maybe consider a MOSFET reducer? xtian posted on that not too long ago. See here: https://ampgarage.com/forum/viewtopic.php?f=6&t=27035

[ChopSauce]

The return of the tweedle dee? Great!

About MOSFET reduction, why not a VVR instead of a master volume?

[Synchu]

One more vote for the VVR. Sounds better to me on all my 5x3 types.
Niki

[fred.violleau]

I must say I was impressed with this MV, so I will keep it.

I thought at first I could maybe use the amp with 6L6s so it would have been even more usefull. Maybe not... I'll see if the build can managed it, all wiring and resistors will support it anyway. I even measured and setup the build in case I need to replace the PT, just in case...

A man with a plan

Mosfet looks interesting since it allows flexibility on the B+.
I remember being worried on how hot my first build was running. So I lowered the B+ a bit with the dropping resistors
I am not very familiar with VVR, but from what I get it reduces voltage as well, thanks for the suggestion, will look into it!

Fred.

[fred.violleau]

Ordered the parts for the mosfet B+ dropper, thanks Martin for the suggestion and many thanks for @xtian and @dorissant and TAG community for providing a simple and easy to use solution !
I will need to add this in my already busy build

Fred.

[fred.violleau]

Board is coming along nicely, waiting on the zener diode and final high voltages to adjust the dropping resistors.

PXL_20210312_023732395.jpg

Cheers,

Fred.

[norburybrook]

looking great Fred. How are you powering the relays?

I've been using my teedle dee a lot in the studio recently, it covers so much ground for me , the cascade mode give the ability to have some nice grit when needed.


M

[fred.violleau]

Hey Marcus,

Thanks! I am powering the relays from the rectifier heaters. So plenty of power for the relays and the led!

I tried to add tube rectifier and diode rectification on a switch, but it did not fit in the chassis. I thought switching from cathode bias to fixed bias would add more to this build. I thought I could put on 6L6s hence the fixed bias... We'll see how the PT behaves, if it can handle it.

I just received the Mosfet from mouser and the WIMA safety surge cap.. these are bigger than I thought, so I will have to be creative in order to fit these in the tight space left... Back to the drawing board for this !

Question : can i use a carling SPST mini-switch in order to use two different zener diodes and have two options for the PT current drop? These are usually rated at 6 amps but I want to make sure that I stay on the safe side since current would flow through it as long as the amp is on.

Fred.

[norburybrook]

Hey Marcus,

Thanks! I am powering the relays from the rectifier heaters. So plenty of power for the relays and the led!

I tried to add tube rectifier and diode rectification on a switch, but it did not fit in the chassis. I thought switching from cathode bias to fixed bias would add more to this build. I thought I could put on 6L6s hence the fixed bias... We'll see how the PT behaves, if it can handle it.

I just received the Mosfet from mouser and the WIMA safety surge cap.. these are bigger than I thought, so I will have to be creative in order to fit these in the tight space left... Back to the drawing board for this !

Question : can i use a carling SPST mini-switch in order to use two different zener diodes and have two options for the PT current drop? These are usually rated at 6 amps but I want to make sure that I stay on the safe side since current would flow through it as long as the amp is on.

Fred.

ah of course, didn't spot the lack of tube rectifier... be interesting to see how SS changes the sound. I suppose I could try it myself and see I'm happy with the tube though.

looking forward to this build.

M

[CaseyJones]

Mods are as follow:
- diode rectification (my first build was done like this and I loved it)
- single input
- switch to choose between channel one and Channel two
- switch to have channel cascade into channel two
- Channel one voiced differently: 22k, 220k, 2k2, 5uf25v
- 470k grid stopper on V2B
- passive FX loop
- switch for V2A bypass cap (in/out)
- 56k NFB switch (in/out)
- LarMar II Master Volume
- 1k5 power tubes grid stoppers resistors
- 470R 5watts screen resistors (want to try different tubes)
- switch from cathode bias to fixed bias (new experiment for this build... can't help it, I need to try different things)
- footswich with 4 options on tap in order to make this amp gig friendly channel picking, NFB, cathode bypass and cascade option.

If Uncle Howie relied on information copied from the internet and reiterated ad nauseum we might not have any Dumbles.

Mods are as follow:
- diode rectification (my first build was done like this and I loved it)

A 5AR4 tube is a diode.

If your supply voltage is already high SS diodes are a step in the wrong direction. Besides, the Tweedle is the only Dumble that deliberately uses a tube rectifier. The magic of a 5AR4 rectifier can't be denied although the "stock" 5Y3 tube is more in keeping with prudent supply voltage.

- single input
- switch to choose between channel one and Channel two

Uncle Howie went out of his way to retain the interactive volume controls, one of the many beloved quirks of a stock 5E3. If Dumble retained a feature it is probably worth keeping IMO.

- switch to have channel cascade into channel two
- Channel one voiced differently: 22k, 220k, 2k2, 5uf25v
- 470k grid stopper on V2B
- switch for V2A bypass cap (in/out)
- 56k NFB switch (in/out)

A "stock" Tweedle if there is such a thing is an absolutely transcendental study in minimalism. You're defeating most of the clever Dumble- ness by messing with it.

I bought an old RCA 400 projector from a fellow member here, so I am re-using the PT iron that seems to be well suited for this project. It was providing current for a rectifier tube, 3x6V6s, 1x6J5, 1x6J7 and a light bulb !

Question : does anyone know if this iron could support EL34 or other tubes than 6V6s ?

In general vintage iron is barely adequate for the circuit it was intended to supply. No one cranked old RCA projector amps into screaming overdrive, that is to say they mostly didn't need maximum supply current from the power supply.

Some guys look at the filament current and figure that's the whole story. If that were so we'd have a Universal Plate Transformer with wide availability of different filament transformers according to application. Obviously that isn't so. In general we can use a couple specifications from the tube data sheet to achieve cool operation of our cranked tube amps. We want to see around 50ma B+ per 6V6 tube, fudge upwards for class A while we can get away with a little less for Class AB. EL34s are a lot hornier on both filament and plate current demand. Figure around 100ma per plate and 1.5a per filament for EL34s, in other words lots more. That's double the plate current and triple the filament current, each. That is to say if you insist on learning things the hard way use a PT designed for 6V6s to run EL34s.

If you're building a high powered 5E3 start with a PT designed for a JTM45 or other Marshall shaped object. Bear in mind EL34s function well with supply voltage about 100v higher than 6V6s. Pad the preamp voltage down to specifications found on the Tweedle schematic if you want it to sound "authentic".

Using cathode biased EL34s you may actually achieve the 19 or so watts some math deficient builders claim they're getting from cathode biased 6V6s.

[fred.violleau]

In general vintage iron is barely adequate for the circuit it was intended to supply. No one cranked old RCA projector amps into screaming overdrive, that is to say they mostly didn't need maximum supply current from the power supply.

Some guys look at the filament current and figure that's the whole story. If that were so we'd have a Universal Plate Transformer with wide availability of different filament transformers according to application. Obviously that isn't so. In general we can use a couple specifications from the tube data sheet to achieve cool operation of our cranked tube amps. We want to see around 50ma B+ per 6V6 tube, fudge upwards for class A while we can get away with a little less for Class AB. EL34s are a lot hornier on both filament and plate current demand. Figure around 100ma per plate and 1.5a per filament for EL34s, in other words lots more. That's double the plate current and triple the filament current, each. That is to say if you insist on learning things the hard way use a PT designed for 6V6s to run EL34s.

If you're building a high powered 5E3 start with a PT designed for a JTM45 or other Marshall shaped object. Bear in mind EL34s function well with supply voltage about 100v higher than 6V6s. Pad the preamp voltage down to specifications found on the Tweedle schematic if you want it to sound "authentic".

Using cathode biased EL34s you may actually achieve the 19 or so watts some math deficient builders claim they're getting from cathode biased 6V6s.

Hey Casey Jones, thanks for joining the conversation! At least I will have learned many things today with your valuable contribution.

I thought vintage iron were more robust and built with larger tolerance, you imply the opposite. I guess I will see how the amp behaves with the 6v6s. Thanks for clarifying the hunger of the EL34, it may indeed not be a good candidate.

As for all the comments upon the choices I did for this build, I totally respect your opinion and absolutely respect what Mr Dumble brought, and would not in any way try to compare to an educated and obviously brillant builder

I chose to get away from a well paved way and may do mistakes. I hope these will be valuable lessons indeed as we learn from our mistakes (at least I do, or try to).

You may or may not like what I am trying to do here and that is fine with me. Thanks for sharing your point of view.

Fred.