Super Twin reverb questions

[Aldo]

Hello,
I'm working in my Super Twin to get the best out of it but it seems some work was done on it previously.

If you have a look at the tone stack board (3rd pic) there are two red Wima 1.0uf caps in parallel with resistors. They don't seem stock... Do you know what are they doing? Looking at photos on the web (2nd pic) I see an unknown grey big cap on the left side (no resitor?) and at the bottom of the picture only one resistor, no cap. What is that big cap? I also see a small ceramic cap on the upper left corner not present in my amp.

Should I leave my amp like that, I mean with the two red caps in parallel with resistors?

I would like to make this amp sound as close as possible to a standard Twin. A DRAMATIC improvement was changing the speakers. I'm testing with what I have at hand, a Texas Heat and a stock Blues Jr. Fender-Emi speaker. Much more clarity now. I also installed a pot to lift the NBF and changed the bright cap to a 250pf mica. The original is 330pf, but in this case there was a 470pf cap that I don't know what kind is it (1st pic, grey unsoldered cap). Then I changed the blue .1uf bass cap for a Mallory and will change the mid cap as well.

Thanks

[DRH1958]

From the looks of pic #3, the 820 ohm resistor on the bottom of it looks like they used the cap to bypass the cathode on V1b. See if this connection goes to pin 3. On the original, it is unbypassed. And maybe they changed the value of the bypass cap for V1a with the other one? This is the 1.5k resistor on the left. See if this goes to pin 8. Here's the schematic.
http://el34world.com/charts/Schemati...180w_schem.pdf

[Aldo]

Thanks! But... what is the effect of bypassing the cathode on V1b? What do you achieve with that?
I don't have acccess to the amp right now, but yes, on the schematic the 1.5k resistor goes to v8, and they changed the stock cap (should be 750). Maybe this bigger image gives more info... what a mess of cables!

[Jazz P Bass]

Typically, when a cathode resistor is bypassed, that stage provides more gain.

[Justin Thomas]

Bypassing the caps basically "makes more gain." I put it in quotes because it's not technically accurate, but that's the apparent effect. As in, I worked on a Lectrolab once with an unbypassed cathode resistor. Adding the cap about doubled the grit and boosted the volume. The value of the cap determines what frequencies get boosted. That huge 750mF cap boosts every frequency down to about 1Hz. The cap was probably made smaller to lessen the amount of bass, adding a bit more apparrent "slice" without the famous Fender "fart." It's part of why the Marshalls sound different from a Fender -the Marshall uses a .68mF bypass cap, keeps it tighter in the low end, as opposed to Fender's normal 25mF.

The following is my opinion: Regarding your desire to make the Super Twin sound like a "standard" Twin, good luck. Not to discourage you, but the Super Twin Reverb & Twin Reverb are radically different beasts, which you are probably finding out. I think Super Twins can sound great, and some people love them. It's definitely worth making it sound as good as it possibly can, but, after some work, if you REALLY want a Twin Reverb, sell the Super Twin & get a Silverface Twin Reverb. Late 70s is fine; but the Super Twin would require basically a total rebuild to sound like a Twin Reverb - new power supply, new tubes, new circuit form the ground up, etc. Super Twins should just be taken on their own terms. It's a great sound for what it is, but it shouldn't be asked to do something it just wasn't designed for. I'd never try to turn a 5E3 into a Deluxe Reverb II...

Good luck with it, and I hope you can get it into shape for yourself, because I think they are cool amps. I owned one for a while, and though it didn't quite work for me, it did go to a good home where it is getting gigged.

Justin

[DRH1958]

Jazz is of course, correct but to add to his response, since the cap is 1uF, it only adds gain above 194 Hz, which is equivalent to the open G string on the guitar. This is referred to as partially bypassed. To provide gain for all frequencies, the cap will need to be around 10uF or higher. The formula for the cutoff frequency is 1 divided by the quantity 6.28 X R X C where R is resistance in ohms and C is capacitance in farads. So in the formula, you would have 1/ 6.28 X 820 X .000001=194 Hz. So this cap and resistor combination will add gain to frequencies above 194 Hz. If you use a 10uF cap, it changes to 19.4 Hz. This is essentially fully bypassed since open low E is 82Hz. This is for V1b.
By changing the cap on the 1500 ohm resistor for V1a, they essentially took away a fully bypassed stage and made it partially bypassed. It is now at 106Hz. This is equal to the G# on the low E string. So it took away low bass boost on the input. I usually like the input to be fully bypassed and then do tone shaping later in the amp. So these lower frequencies are kind of lost right off the bat. Others might do it differently.

[Aldo]

Thank you both for your time and the in-depht explanation about bypassing, it's being very helpful.
I thought the Super Twin was like a hot rodded Twin and small changes could bring it closer to the original, I'm definitely not going to rebuild the amp nor I have the expertise to do it! Actually the amp has not the bass response or scooped feel I expected but has a prominent mid bump I can't get rid of (although changing speakers helped) and it makes sense part of it may be due to the bypassing of these resitors.

I want to try it as stock and see if I like it better. Or leave the cap in V1a at stock value and change the value of the added V1b cap to 10uf as DRH1958 suggests.

One last question, what is the stock value of the bypass cap at V1a, 750uf? I may be getting this wrong because I'm only finding radial electrolytics or very expensive axial caps at that value...

[Tone Meister]

Hi Aldo. I appreciate what you want to do with your amp, and no doubt you can make some noticeable changes that you can hear. But that amp with its six 6L6GC power tubes @ 180 watts RMS and ultralinear output can never sound like a blackface or even an early silverface Twin. It will always be more in your face, brash, and strident as compared to an earlier model.

The preamp tweaks you're doing and a speaker swap are about all you can do to improve the sound of that particular amp, but it cannot and will not be an 85 or 100 watt Twin, it just isn't ever gonna be either of those. A pair of Eminence Swamp Thangs or one Swamp Thang and your Texas Heat is a good match for that amp, speaker wise.

In 1978 at age 17, with money I had earned all summer truck farming, I bought a brand new Super Twin in an attempt to sound like Ted Nugent. All I ever managed to do with that amp was piss off my dad and kill small rodents within a 50 yard radius of my practice building. Buying the Super Twin ended up being one of my worst choices in music gear. However, five years later I was able to trade the ST for a 70's JMP Marshall head and cabinet, which ended up being one of my best decisions.

Good luck with your tone quest, Aldo.

[DRH1958]

Glad to help Aldo. It seems like a good idea to put a larger cap in V1a because if you lose frequencies right away at the input, you really can't get them back again. If you are short on bass, then you will have them back again. You can always dial out some bass later in the tone stack or EQ. You don't have to go crazy like Fender did and put a 750 there. Just a 10uF should help.
One other little bit of advice, the amp has a couple of tubes that aren't made any more. The 6C10 is normally not used in guitar amps and only other amp company to use this tube I know of is Ampeg. It is actually three tubes in one. It's easy to spot because it's the shortest one. It isn't manufactured any more, so if you are going to keep the amp, look for a spare on ebay or elsewhere. They are expensive, so look carefully and look for good deals. Also, the 6CX8 is a triode and pentode in one tube, also not made any more. Again, look on ebay or old TV repair guy's sell-offs.

[Jazz P Bass]

I want to try it as stock and see if I like it better. Or leave the cap in V1a at stock value and change the value of the added V1b cap to 10uf as DRH1958 suggests.

One last question, what is the stock value of the bypass cap at V1a, 750uf? I may be getting this wrong because I'm only finding radial electrolytics or very expensive axial caps at that value...

Here is a cool bypass cap calculator.

Cathode Bypass Capacitor Calculator

It provides a visual interpretation of how the bypass cap value affects the frequency response of the stage.

As an aside, I feel that bypassing V1B is counterproductive as that is the tone control circuit.

[DRH1958]

On second thought, I can see why Jazz would not bypass this stage(V1b). But I would also remove the 1uF cap as this increases upper frequencies, so has the effect of having less bass. Unbypassed will give you an even frequency response. I guess I had bypassing stuck in my brain and thought don't partially bypass it, fully bypass it.
I would still fully bypass the input(V1a). Use 10-25uF. You don't need the stock 750uF.
Explore that site Jazz linked you to. Lots of good and useful info there.

[Aldo]

Thanks all! And Tonemeister for your help with speakers, again! For now i'll give a go at the Super because i need a lot of headroom, some day i'll aim for a Twin. I could only find electrolytics at these values, except a huge 250v poly cap for 7€. I got a non polarized 22uf 50v. Hopefully i'll try it later and let you know

[Aldo]

I did make the changes, the 22uf cap brought back the bass as predicted here. I removed the added bypass cap at V1b, and I can see why they put it there, it gave more bite, gain at the input but also more noise, it was easier to overdrive the signal. However I found you can get the same effect with the tone controls EQ controls, master and volume.

I'm happy with the results! I see this amp doesn't have a good reputation but I don't think it sounds bad, at least with the NBF you get more sparkle). I tried a different cab with celestion gt75 speakers and liked the results, they are more scooped than the Eminences in my cab. It's amazing the effect speakers have on tone. I'm waiting to test a Jensen Tornado
Thanks everybody again

[DRH1958]

As long as YOU are satisfied with the amp, it's reputation makes no difference! Glad to hear that you liked the change with adding the cap. Also, the G12T-75s are known for less midrange, so if you are trying for that sound, good choice. Speakers probably have the largest effect on tone as any single part of the signal chain, so if you are looking for different sounds and can afford or have access to different speakers, good for you. Give them a try until you get your sound just right. You can also mix different speakers, as long as they have the same impedance. For instance, use one that has more mids along with one that has more lows and highs for a more complete sound. Like a G12T-75 with a Vintage 30.

[Tone Meister]

As long as YOU are satisfied with the amp, it's reputation makes no difference! ...

You bet. ^^^

My comments were in response to Aldo wanting "to make this amp sound as close as possible to a standard Twin" which can never be the case. If headroom is the goal then Aldo certainly has the right amp. I just know from experience that my Super Twin didn't work all that well for me in 150 seat lounges in 1978-79.

Congrats on a headroom machine Aldo!