More clean headroom for a homemade Tweed Super...

[Ammanford Fats]

I've got a chance to buy a homemade Tweed Super (Weber Kit) which I tried out at a gig the other night. Fell in love with the amp, but had a problem with feedback when using my Epiphone ZBD - which I don't get when using my usual amp, a Blues Deluxe.

Would putting a low gain valve or two in there give me more clean headroom while still keeping the overall volume of the amp?
Cheers,
AF.

[loudthud]

A low gain valve will change where you set the volume control but won't make a substantial change in headroom. Upgrading the speaker(s) will do much more to increase headroom.

[Tom Phillips]

I've got a chance to buy a homemade Tweed Super (Weber Kit) which I tried out at a gig the other night. Fell in love with the amp, but had a problem with feedback when using my Epiphone ZBD - which I don't get when using my usual amp, a Blues Deluxe.

Would putting a low gain valve or two in there give me more clean headroom while still keeping the overall volume of the amp?
Cheers,
AF.

The original 5F4 Super used 12AY7 per-amp tubes which are lower gain. I'll bet that the homemade one has 12AX7s installed. However, Supers in good condition are usually well behaved with the 12AX7s installed and most people do use the 12AX7s because of their better availability. Also people prefer the sound with the 12AX7s which is a mild form of hot roding the old Fender circuit.

Are you saying that the feedback is through the guitar? If you can play at higher volume with the same guitar through another amp then it seems that there is something suspicious about the homemade Super. It would be best to track down the actual problem by trying out the Super again. Switch guitars, etc and see if the problem follows the amp.

Cheers,
Tom

[Chuck H]

Your hollow guitar is probably feeding back with the Super because the combination of the Super and your particular guitar share a resonant frequency. If you EQ'd the Super to sound like your Blues Deluxe the problem would likely go away. But then the Super wouldn't sound like it did that night either.

Chuck

[Ammanford Fats]

Thanks for the input guys.
I think part of the problem is that the P90's on my hollowbody are quite high output and it's driving the amp at too low a volume. With the Blues Deluxe, I can tame it by adjusting the master and volume controls - unfortunately I've now been bitten by the Tweed bug and I think the search may be on for a Bassman!!!

AF.

[macpgh]

headroom

Ok, first off I don't know which super plans were used as in which schematic was the basis of the build. So to drop some gain out of the amp you could swap out the pre-amp tubes with lower gain type. !2ax7's have the highest gain amplification factor & will be the dirty most responding tube but not all brands of 12ax7 will sound or respond the same. Going to a 12at7 will lessen the gain factor & clean it up, as well as a 12au7. A mid point tube would be a 5751 tube which is like 3/4's of a 12ax7 gain factor & is a nice tube to play with......I like them in the reverb driver on fenders with reverb as the 12at7 used in this position is kind of over clean & a 12ax7 is too dirty there.

In the alternative and probably a better approach would be something simple but requires using a switch. So you'd either have to drill a hole for the switch or use a push/pull pot as a volume pot.....the pot pulls out as its a switch. Basically you would look at the first preamp tube & identify the cathodes..pin 3 & 8, there is a resistor there which is in parallel with an electrolytic cap and they both connect to ground on one end. This cathode situation is what bias's the first gain stage. The resistor determines the bias point & gain and the cap determines the frequency of sound going thru. Typical guitar caps are 25uf & a bass would be like 250uf or higher. Some manufacturers of amps use this cap as a gain booster by making it switchable, meaning they switch it in & out of that part of the circuit. SO you would want to lift it out of the circuit by putting the switch at the pin 3 & 8 junction point. (I am assuming that the pins 3 & 8 are connected on the pre-amp tube) The resistor is still connected to pins 3 & 8 & grounded on the opposite end with the ground of that cap.

Now if lifting the cap out of the circuit doesn't make for enough clean room, then you can also increase the value of the resistor there. If it happens to be 820 ohms, then you could go to a 1.5K ohm or higher to further dial in some clean. Then be able to switch the cap back into the circuit to get dirty.

Take notice that I am saying if the Pins 3 & 8 are connected together. If they are not connected & are separate, you'd have to do each side with a switch to lift the cap out. ANd then if need be, as mentioned....change the resistor to a higher value.

One a different basis, there may be a microphonic tube in the amp. To check you would have to tap on each tube glass with a pencil to see if it sets it off into a squeal or if it makes a noise. Sometimes when they squeal it will continue to squeal as if its a sustained note being played.
Opposite this you could also have a microphonic pickup on your guitar. It doesn't mean that the pickup is bad, it means that you would have to have the pickups potted. To pot a pickup is to have it dipped in hot wax as the wax soaks into the wires of the coil to keep them from vibrating. You should use surfer's wax for surf boards, heat enough to melt it & dip in for several minutes but as not to cook the pickup. Its not like cooking pasta. You have to give a few minutes for the pickup to heat some and let the wax infiltrate the coil wire. Some pickups are potted & some are not......you'd see wax residue if you remove the covers & look under the tape thats usually wrapped around the coils.

Are you sure you didn't get feedback from a stage mic that was close by?
Feed back can be a good thing if you can master it....think jimi hendrix

[hasserl]

Going to a 12at7 will lessen the gain factor & clean it up

While the 12AX7 and the 12AT7 share the same pin out design, and can physically be substituted, it's not generaly a good idea to do so. There is more difference between the two tubes than the amplification factor. The internal plate resistance is far greater in the AX7, which means the circuit which you plug it into really should be set up differently with different plate load's than it would be for an AT7. I've never been happy with the sound of a 12AT7 plugged into a spot designed for a 12AX7.

Also, this will not increase the headroom of the amp, it will only increase the point on the Volume control where breakup occurs. In order to increase headroom you need to increase the volume at which breakup occurs, you cannot do that by using a lower output tube, which will produce less voltage gain.

Now if lifting the cap out of the circuit doesn't make for enough clean room, then you can also increase the value of the resistor there. If it happens to be 820 ohms, then you could go to a 1.5K ohm or higher to further dial in some clean. Then be able to switch the cap back into the circuit to get dirty.

Careful here. Changing the value of the bias resistor will change the operating point of the tube. If the tube is biased to operate in the center of the linear range than changing the bias resistor will result in either increased saturation or increased cutoff, neither one is cleaner, and neither one will increase headroom of the amp. And if you change the value of the bias resistor, than you also need to change the value of the bypass cap to maintain the same roll off point.

[ThermionicScott]

Try moving the OT centertap to the rectifier side of the choke. Switching to a 5AR4 or SS rectifier and rebiasing cooler would help, too.

[macpgh]

super is as super does

Well we do not know specifically which super this kit was modeled after. If it has a long tail pair phase inverter or the old style. Much less if its cathode bias'd or fixed. I think that this might be a cathode bias'd amp which would make it rather a raw dirty gnarly amp......so how would you clean it up? You wouldn't want to go the route of upping the plate voltages to get it to sound brittle via a SS or a different rectifier that whats in it now....but we don't know what it has for a rectifier as its not stated. Some of the 50's tweeds had dual rectifiers in order to get rid of sag but yet keep the plate voltages down where they are in the "brown" tone zone...making for a warm amp rather than a shrill brittle brite amp like the late silver faced type amps.

The give away for the guy is a statement in the original post..."clean" headroom.
Subbing out a phase inverter thats using a 12ax7 would help to clean up the amp to a degree....by going to a lesser tube like a 12au7 or 12at7. If you run the first pre-amp under the same pretext, it further cleans it up. But then it depends on the phase inverter design with the long tail pair being the clean of all designs but yet then there is the bias situation.
So biasing colder might have a small effect but its the pre-amp thats going to be giving you the buzz saw type distortion of the signal....which some might call dirty or overdrive. This is like a lesson in vocabulary.
The output tube "distortion" isn't of that buzzy variety....its where the saturation begins the creamy sustain & rich harmonic content. So if you subtract the pre-amp buzz & get the other end to maintain the creamy sustain in a clearer fashion than being all buzzed out.......we are not talking the sound of a FUZZ pedal like a big muff.

Loudness associated with headroom would be a situation of exponential factors. !0 watts would be twice as loud as 1 watt. In the realm of music & listening to music via a stereo, most listening is in the 1 watt range. To get the effect of full concert volume, you need some rather big amps. Going back to guitar.......100 watts is twice as loud as a 10 watt. But where is the most versatile rig in wattage in a practicle form.......its something thats either 2 power tubes on the 40/50 watt range or in junior tubes like 2 6V6's or 4 6V6's. The sweet spots are not always at the end of the scale on plate voltage.

I would agree that I don't care for the cleanness of a 12at7 for a first stage pre-amp as I prefer a dirty sound. But its an easy thing to do in swapping out a tube for a tube rather than doing surgery. One thing to do is to get rid of the scope & engineering books & think with your ears.
I once serviced a soldano head, really nice high end piece, but thats a part of a marketing scheme in a business plan. But with the implied quality, it was a piece of junk that looked like it was designed by somebody not knowing how to design for tubes & high voltages. The engineers had the circuit crossing over itself & the guy wondered why it had a mysterious oscilating problem. The thing looks nice with the chrome & the power tubes there in front giving the nice light show as you play & how the glow pulses & shifts for a visual display. When I further examined some soldano schematics, there is some interesting designs for pre-amp circuits but there are also factors of a design flaw in creating phase problems leading to phase cancelation......which makes for another engineering critique

But anyway.......

Lets hear from the man in question.....what is it that you seek. What has the siren of the super done to catch your ears?

[macpgh]

tubes

BTW, in my first post when I refered to tubes & brands not sounding the same or responding the same, I was trying to put across, based on a 12ax7 tube, that a 12ax7wa sovtek brand tube does not in anyway sound like a new old stock MULLARD 12ax7 tube (and thats not the new manufactured mullards by sovtek/electro-harmonix). The nos mullards have a rich harmonic content with the ultimate dirty for a guitar amp if you want to pay $100 for a tube or more. so you need a small collection of samples from various tube manufacturers to get a full picture on the sound content quality of a tube. For a current manufactured tube with good dirty to its response is a JJ/tesla 12ax7.
Amps having several pre-amp tubes to the circuit, I prefer using particular brand 12ax7 tubes to insert at specific points in the circuit......and that wouldn't be all of the same brand

[ThermionicScott]

Well we do not know specifically which super this kit was modeled after. If it has a long tail pair phase inverter or the old style. Much less if its cathode bias'd or fixed. I think that this might be a cathode bias'd amp which would make it rather a raw dirty gnarly amp......so how would you clean it up? You wouldn't want to go the route of upping the plate voltages to get it to sound brittle via a SS or a different rectifier that whats in it now....but we don't know what it has for a rectifier as its not stated. Some of the 50's tweeds had dual rectifiers in order to get rid of sag but yet keep the plate voltages down where they are in the "brown" tone zone...making for a warm amp rather than a shrill brittle brite amp like the late silver faced type amps.

The only "tweed Super" kit Weber sells is for the 5F4 circuit: fixed bias, cathodyne phase inverter, output tube plates downstream from the choke, 5U4 rectifier, etc.

- Scott

[loudthud]

If the lack of headroom is caused by really hot guitar pickups, then changing the first preamp tube will clean up the signal. You can also try using the #2 input which will give you 6dB more headroom. I agree that a 12AT7 just doesn't sound good in circuits intended for 12AX7s but it will work. The 12AY7 has even lower gain but to really gain any headroom, an increase in the cathode resistor is needed.

[macpgh]

kits

Ok, took a quick glance at the 5F4.
What we do not know is if the builder went with the plans or deviated, so it would have to be checked out.
If he did stay with the original circuit the whole way thru, then the phase inverter type is going to make the thing always sound gnarly as its not going to present a balanced wave form to the power tubes. It will function presenting opposite out of phase signals but not exact mirror images.
The long tail pair phase inverter presents the wave more better & matched.
But its the mismatch that gives the guitar signal the dirt that makes for a guitar amp. Back in the day, there was not a big guarantee that the power tubes were going to be matched & balanced and its was that variance that contributed to the magic. When you try to balance the whole way thru, from a design aspect, its when it become more of a stereo application...HiFi where you are exploiting for accuracy in signal & reproduction.

But back to the super, The only quick fix would be substituting the pre-amp tubes to tame it for his guitar. But if its his only guitar & its an all purpose guitar & not quite knowing what style of music is being played.....what more could you do. I have several guitars, single coil strats & humbuckered strats. tele's & les pauls.....and obviously each one drives an amp differently. I have no hollow bodies or p-90's yet. But certain configurations of guitars & their pickups combined with particular amp circuits combined with particular speaker types.....all make for that certain magic.

From a business perspective, how do you arrive at the magic in a cost effective manner. Do you buy that amp & then put money in it for surgery? Do you buy the kit & build it but deviate? Do you buy the kit that will present you exactly what your needs are?

switching the phase inverter to a long tail pair would make the circuit more bassman-ish....which is mentioned in the original posting. So just short of also changing the tone stack on the cathode follower would complete a bassman conversion.
Personally I prefer head units over combo's as I can always change the speaker configuration.
But if he is pursuant to a bassman type clone & wanted a kit. I'd say go to a metropolis JMP marshall clone. It has the classic circuit & parts, 4/8/16 ohm Output transformer & has a solidstate rectifier. (the purpose of fender using 2 rectifier tubes in the late tweed circuits was to rid sag out of the amps performance). Go with 6L6's or 5881's & play with the preamp tubes too match the guitar & modify for a master volume Then figure what speaker type & cabinet type you prefer too hear.

I recently purchased a fender super reverb with 4-10 JBL speakers off an older guy that plays a gretch chet atkins & country music in the classical vein. He prefers a peavey solid state amp with reverb & trem but with a 15 inch speaker. Imagine that.

[Bruce / Mission Amps]

Try moving the OT centertap to the rectifier side of the choke. Switching to a 5AR4 or SS rectifier and rebiasing cooler would help, too.

I have not tried this but I'm curious what you think moving the OT center tap to the other side of the choke would do to increase the headroom.

[ThermionicScott]

I have not tried this but I'm curious what you think moving the OT center tap to the other side of the choke would do to increase the headroom.

Well, I'm sure you already know most of this, but I'll run through my thought process... with the 5F4/5E5A/5E6/5E7/5E8 power supply, all of the current is being drawn through that choke, so there is a considerable voltage drop (my 14864 choke measures 125 ohms), especially at full power. When the OT CT is moved upstream of that choke, the plate voltage goes up, and since less current is being drawn through the choke, the voltage on the other side goes up as well. I have to assume that's why any of the Fenders after those I mentioned above have the plate supply ahead of the choke.

Having a CLC filter before any of the signal tubes makes good sense from a hi-fi perspective, but doesn't help when you're trying to squeeze every last watt from your guitar amp...

- Scott