Tweed Deluxe Mods -- HELP!!!

[m.edwards]

Hey all, I'm new to this forum and as a tech who is still wet behind the ears, I want to reach out and ask for help! I built a 5E3 Tweed Deluxe kit from mojotone, and hand built the cab.


Unfortunately, it's not quite the sound I'm looking for, so I am going to attempt a few mods. First, I do a lot of fast fingerpicking on an archtop I built with a Kent Armstrong Mini humbucker, and the Deluxe seems to have an overwhelming amount of bass, and a tone that is not clear or immediate enough for my liking. I also play a Les Paul Jr. copy i made a lot with Lindy Fralin P90's, and I LOVE the tones I get out of it, but I would like to be able to play cleanly at louder volumes (the signal distorts around 6 or 7 on the volume knob). SO, Here are the mods I was thinking:

--Switchable SS rectifier, to create more headroom and a versatility between my folky archtop/acoustic playing and the bluesy, distorted feel on my electric.

--Adding a pot for adjustable gain, allowing me to choose the amount of distortion to add.

My first question is, are these mods appropriate to acheive the sounds I want out of the amp?

Second, it's my understanding that the cathode biasing resistors are the main contributing factor to gain control. It stands to reason (in my inexperienced brain) that replacing one of these resistors with a potentiometer would give me gain control, since potentiometers are effectively variable resistors. Is this correct? If so, which tube should I wire this pot to? Also, what value pot should I use? Are there any special precautions I should take?

Third, I'm not sure if the switchable rectifier mod would be a better solution to the heavy bass tones than changing the coupling caps to a lower value, or if I should do both.

Are there any other mods that you guys would suggest or are particularly fond of?

I'm slowly but surely becoming more acclimated to tube circuitry, but I'm still very much a novice, so I appreciate any help you can offer. For the sake of ease, here is the schematic I used:

[Leo_Gnardo]

<the Deluxe seems to have an overwhelming amount of bass, and a tone that is not clear or immediate enough for my liking>

First try removing excess bass response by swapping out those 25 uF cathode bypass caps for smaller values. Both 1st & 2nd stage. Somewhere in the 1 to 5 uF range should do. ALSO smaller coupling caps, try 0.022 or 0.01 instead of those 0.1 uF. When your Deluxe isn't trying to amplify those blimpy low tones, the mids & highs will seem to gain more power.

<--Switchable SS rectifier, to create more headroom and a versatility between my folky archtop/acoustic playing and the bluesy, distorted feel on my electric.>

Not a bad idea at all, BUT make sure your hi voltage filter caps can take the extra voltage a SS rectifier will deliver. You might also want to increase the value of the output tubes' cathode resistor, as the extra hi voltage in SS mode will force more current through and send you into red-plate territory. You should increase the voltage value of the output tubes' bypass cap as it will almost certainly exceed 25V in SS mode.

Lower gain preamp tubes can help here too. 12AY7, 5751 and 12AT7 can work to your advantage.

Try these simple cheap mods before commiting to anything more complex. You might find that's all that needs to be done.

Let us know you it works for you!

[printer2]

Usually better to use a volume control to bring down the signal rather than using a pot to change the gain of a stage. Here is a take on cleaning up the 5E3. Lower value coupling caps and also switching out the second stage bypass capacitor (reduces gain 6dB) and the addition of NFB (See the Harvard schematic).



Also if you are going in one channel try adjusting the other volume control. The pots are hooked up to each other and turning up the unused channel loads down the other reducing gain.

[stokes]

IMHO the Tweed Deluxe is the wrong amp for you.It is known for its early break-up.The mods laid out here will improve things some,but I dont think it will get you where you want to be.You arent going to get much more real clean volume.The changes will be subtle.I would not recommend using a pot as a cathode resistor replacement hoping to make it a gain control.This resistor sets your idle point,too cold and you will get nasty crossover distortion,too hot and your tubes will explode.You can choose a different value bias resistor to make it somewhat colder,but it isnt going to get louder,it may not break up as much,but again,it isnt going to be drastic.And this resistor should be 10 watts,most pots are 1/2 watts,a ten watt pot is going to be quite large.I have a Tweed I built and experimented with for years.I like the .02 coupling caps,I made a stiffer power supply,the biggest improvement in headroom came with doing what is called the "Paul C mod",its a mod to the PI stage.The stock PI breaks up quite early,with the mod it breaks up a bit later,but overall,the amp is still what it is,it cuts through a bit better in a band situation,but it still has a lot of break-up,a bit tighter,but still a nice tube distortion machine.If you are looking for clean,the Tweed is not it.

[hack]

Changing from Cathode Bias to Fixed (adjustable with a pot) bias will make it cleaner and faster. See Fender Harvard for fixed bias 5E3 (albeit a single channel instead of two channels).

[barfoden]

IMHO the Tweed Deluxe is the wrong amp for you.It is known for its early break-up.The mods laid out here will improve things some,but I dont think it will get you where you want to be.You arent going to get much more real clean volume.The changes will be subtle.I would not recommend using a pot as a cathode resistor replacement hoping to make it a gain control.This resistor sets your idle point,too cold and you will get nasty crossover distortion,too hot and your tubes will explode.You can choose a different value bias resistor to make it somewhat colder,but it isnt going to get louder,it may not break up as much,but again,it isnt going to be drastic.And this resistor should be 10 watts,most pots are 1/2 watts,a ten watt pot is going to be quite large.I have a Tweed I built and experimented with for years.I like the .02 coupling caps,I made a stiffer power supply,the biggest improvement in headroom came with doing what is called the "Paul C mod",its a mod to the PI stage.The stock PI breaks up quite early,with the mod it breaks up a bit later,but overall,the amp is still what it is,it cuts through a bit better in a band situation,but it still has a lot of break-up,a bit tighter,but still a nice tube distortion machine.If you are looking for clean,the Tweed is not it.

I am not so sure that the cathodyne PI is the major culprit in the 5e3. A modern amp like the peavey classic with 4 x EL84 uses a 12ax7 cathodyne with 68k input resistors (225V), and 1.5k cathode resistor. Very similar to the 5e3 cathodyne.
The peavey has no grid resistor in front of the cathodyne such as a stock 5e3, but it has very large grid resistors (47K) on those EL84 tubes compared to Fenders 1.5k on the 6V6 tubes. Maybe the EL84 are more sensitive to blocking distortion than beam power tubes. ??

Ok the peavey has negative feedback going into the gain stage before the cathodyne instead of the 5e3's 25µf bypass capacitor. This could of cause make a difference.

An amp I keep comparing to the 5e3 is the Dearmond R15t or Martin 112t. This amp is very similar, though has a 12ax7 in V1, so more gain in the first stage but then it omits the bypass cap on the second gain stage before the cathodyne that is identical to a 5e3. It has a huge OT, and the 6v6 are cathode biased with the same 25µf/250ohm setup. But the main difference is that the plate voltage is 330V and the screen dropper is smaller at 2.7K that gives the amp a little less compression than the 5e3 with its 5k dropper and Dearmond's main filter cap is very large at 40µf (though 30µf would be more healthy with the 5y3 recitifer). The Dearmond also has less bass with 0.047µf coupling capacitors in both preamp and power stage. The tremolo channel actually has a small 0.01µf on the preamp stage.

I believe that most modern 5e3 are biased to hot with plate voltages often found from 380-400V for the 250 ohm bias resistor. With such a hot biased the amp will overdrive with just a small signal. You could increase the bias resistor to 330 ohm but any higher will often make a cathode biased amp sound worse when overdriven. Trying to bias the power tubes over a large value resistor is not a good solution for sweet overdriven sound (Bruce collins from mission amps talks about this a lot).

My take on a 5e3 with a little more headroom and later breakup would be..

Use a 12ay7 in V1 (if you want to use a 12ax7 in V1 then remove the 25µf bypass cap on the second gain stage).

Lower bass (decrease coupling caps slightly to 0.047µf in both preamp stage and power stage as a Fender 5c3 and 5d3)

Increase main filter to 30µf (only the first cap, you want the bouncy feel from the 16µf caps on the screen and preamp stage)

Keep 6v6 bias within specs (calculate bias, increase the resistor to 330 ohm if needed, if that is not enough, zener down the +B)

Add grid resistor (in front of the cathodyne (100k -470k) and maybe increase the value of the 6v6's 1.5K grid resistors sligtly).


Lastly two mods that are up for discussion:

Bypass caps:
Preamp stage bypass caps can be lowered to limit bass but the clean fat fender sound comes from using large full range bypass caps (10µf -25µf). Hasserl recommends 4.7µf on both stages as he feels it keeps a full signal but takes out the lowest bass. Try different approaches and see what you like.. For the bypass cap on the 6v6 it is a bit more tricky. Increasing the value to 100µf-220µf will stabilize the cathode bias a little but it will let all frequencies be amplified whereas the 25µf with the 250 ohm resistor combo will limit some bass. So with a large bypass cap on the 6v6 one should probably decrease the value of the 0.1 µf power stage coupling caps.

Decrease resistance in the +B line:
The 5e3 has a large 5k screen dropping resistor and a 22k PI/preamp resistor. Some people have tested a screen supply choke in the 5e3. This has very little resistance (~150 ohm) and the screen voltage do not sag and are similar to plate voltage. A major problem in a 5e3 with a high plate voltage and a 250 ohm bias resistor. Now you really have to rebias the amp and either use a larger bias resistor or zener down the +B, you also need to add screen resistor to the 6v6 tubes.
This mod (if bias is handled well) should give you more headroom as the voltage do not sag on the screen/PI/Preamp section when current is needed and maybe a slightly chimier sound as the preamp voltage is higher. You could achieve the same effect with a smaller resistor but then you have less ripple filtering in +B line. The 22K resistor can be dropped a little to experiment with higher preamp voltages.

Modern amps like my cathode biased Laney LC/VC 30 uses a small 470 ohm resistor in the screen dropping position which has very little ripple filtering effect, but the main power supply is from a large torodial transformer with diode rectification and a hefty main filtering at 2x50µf = 100µf. But Laney kept the screen/PI and preamp filter caps to a nice vintage value of 15µf. Nice and bouncy. You can kill vintage amp dynamics with to much filtering after the main filter cap.

Stiffer rectifier:
If you are using a PT with a high secondary voltage, use a vintage 5y3 rectifier. It is the proper way to get the correct +B voltage. And a 5y3 rectifier should be strong enough for two vintage 6V6GT tubes rated at 12 watt each. When building a 5e3 clone with larger power tubes (5881 or 6L6GC) and larger transformers a stronger rectifier is needed to supply enough current.