15" tone rings

[sniper]

Elsewhere I have written Mojo sucks. Well maybe they don't suck as bad as they used to as they now carry 15" tone rings says I setting here with blueprint plans for the old Fender 15" tone ring cabs.

http://www.mojotone.com/amp-parts/Hardware/Mojo-15-Tone-Ring  

[Pilgrim]

Ignorance here....what does a tone ring do?    Never heard of them.

[sniper]

it was Fenders first attempt at porting a bass speaker cab.





the rings have been very hard to find lately and i was pissed at Mojo for not stocking the 15" versions as they only had the 10" and 12" versions. now they do. i recently had an estimate from Rogers Amps for building a cab. after getting the estimate i got the blueprints so i might have it done locally by a cabinet maker hoping for a supply of rings to open up. i was thinking about using a spun aluminum Wok top to make one.

i simply want a 15" cab to go with my 2x12 Blonde Fender cab as i really like classic (think mid 60's-ish) tones from my bass. now i have another job to make a cab to go with that 15" JBL i have had in a box forever.



i also have had doubts about my little 50 watter being able to push both speaker cabs ... so i bought a trashed Blackface AB165 with no trannies or knobs to rebuild as a 100 watt amp and to use my 100 watt power tranny from a twin 2 reverb and hopefully a new OT if i can find one to have a 100 watt pre CBS Blackface amp with two KT88's. i have a virgin chassis (a Marshall type) to use for this project. i will probably sell the original head cab and chassis to a collector or some one that wants to restore a Black Face amp keeping the board for myself. 100 watts should push both cabs nicely.  

Versteht jetzt?

[Psycho Bass Guy]

so i bought a trashed Blackface AB165 with no trannies or knobs to rebuild as a 100 watt amp and to use my 100 watt power tranny from a twin 2 reverb and hopefully a new OT if i can find one to have a 100 watt pre CBS Blackface amp.

Personally, I'd go for the post-CBS ultralinear output transformer of the later Twin or Bassman 135, two great power platforms that had the misfortune of having a shitty preamp/ bias scheme in them. You could keep the Blackface bias, which is far superior to the later 'balance' control, but still have much more clean current on tap to give your Fender-clone a beefy punch. The only caveat is that you'll have to find VERY good 6L6GC's (probably 7581's or KT66's) and a voltage doubler/reduction network for the higher plate and screen levels out of the PT and possibly a separate filament tranny if you go the KT66/6550/KT88 route or go with a bigger PT entirely.

There used to be guy on Talkbass who made an amp exactly like that, but I'm banned from there and don't remember his name.

[Pilgrim]

Versteht jetzt?

Jah, ganz gut.

[sniper]

the PT out of the Twin 2 Reverb, 82 to 86 model, is 105 watts. i bought it for 39$ which i thought was very reasonable as the guy didn't know what it was out of and is (was) selling them cheaply. i just didn't know what OT to use. the UL OT is a great idea to help prevent saturation.

my concern was using a stripped version of the preamp circuit (one channel ... one input) and over powering the preamp circuit altogether along with trying to fit the new trannies and the 88's in the original head cab. thats why i was planning on using the Marshall chassis i have.

here is the modified preamp circuit as it stands now:

[Psycho Bass Guy]

I'd drop the input cathode resistor, ditch the second 12AX7, and use a 12AU7 or even a 12BH7 for the tone stack. You'll have to recompute the r/c tone curves to the lower plate resistance of the 12AU7 (roughly 1/10th the 12AX7's) if you want the same frequency values, but I think you'll find that it will be a much bass-friendlier preamp just by changing the tubes and little else. It'll still sound like a Fender, but it will be more like a 400PS than a Showman.

 You also won't necessarily overpower the circuit by not having an additional channel; your PT will just run cooler from less current demand, which means you can get a little more out of its other voltages without stressing it.

[sniper]

nice!!! PM sent

[sniper]

on researching your suggestion, unless i missed the point here ... talking about the ultra linear suggestion, it would prolly be a lot easier if i just followed a Sunn 200S or Sorado schematic and used the trashed chassis for parts (or to go a little stealthy and make it look like a "Fender" black face, lol). that would blow some minds huh? *****

*****CAUTION OLD FART ALERT USING OUTDATED 60'S EXPRESSION....CAUTION CAUTION!!!

[sniper]

I have been doing a little research on Fender "ultralinear" taps and this is pretty interesting...from DIYTube. This is from a conversation about replacing a OPT in a Studio Bass with a Hammond OPT that usually will put out about 160 watts RMS non ultralinear.

"The 1650T transformer with screen taps would be a marginal transformer choice for the super twin -- and it would require more circuit modification than indicated to make it perform even at a marginal level.

Many people in the guitar amp world mistakenly refer to the Fender tapped screen circuit as an Ultralinear design. It is not, and the taps were not intended, nor are they capable of producing Ultralinear operation. Typical Ultralinear operation for 6L6 class tubes requires a 43% tap (based on turns ratio), which is about what the Hammond transformer provides at 40%. True Ultralinear operation results in much lower distortion, and much lower power output -- about 35 watts is the absolute maximum RMS power output that a pair of 6L6s can give in an Ultralinear design. Therefore, when using the tapped version of the 1650T in the Super Twin, the amp would not produce much more than about 110 watts continuous RMS output. For this transformer to work to it's best ability, the screen taps would need to be taped off, and then have a standard choke type filter system installed for the screen B+ take off. Under these conditions (strict pentode operation), the amplifier would produce about 165 watts continuous RMS output.

As for the actual design of the Fender tapped screen circuit, the purpose of these taps was a cost saving measure on Fender's part, in that they eliminated the need for the choke to provide adequate filtering for the output stage. That was it, and nothing more. This is not an opinion, but a fact that came out of a conversation I had with the circuit's designer. As such, the taps are appropriately placed at only 12.5% of the winding -- enough to provide a small amount of B+ filtering, but hardly enough to reduce power output and cause Ultralinear operation.

Finally, contrary to what was claimed, the Super Twin is quite capable of putting out 200 watts continuous RMS -- if the P-P impedance is correct to allow for it. In this case, the original Super Twin transformer has a P-P impedance of 1500 ohms, which is proper for six 6L6s in Fender's tapped screen design. This is why the Hammond transformer, as a 1900 ohm unit, will not allow full power output to be produced even without screen tapped operation.

When the impedance mismatch, inappropriate tap positioning, and power rating of the transformer are all considered, you can see why I maintain that this transformer would be a marginal choice at best."

Taking this into consideration, it might be best to investigate as to whether the OPT in a Twin 2 (which uses a choke) would be able to put out 100 watts driven by two KT88's as it shows 465V before the choke and 460V after the choke on B+ on the Twin 2 schemo. It might be a little marginal on the 88's? It would certainly drive them at a lower output and that would give it some reserve on "depth" or reserve current which is what a person would want in a bass rig. I would guess and venture about 75 watts RMS and about 150 peak or program burst. I would prolly gain a bit on the simplified preamp as far as reserve also."

Who needs a Sunn anywho? lol. It turns out that I found a Sunn 200S is a 60 watt amp and not the 100 I thought it was. It follows this same theme by using the 88's instead of 6L6G's with would take away the reserve of the tube which Sunn capitalizes on to sound so loud, like idling a 454 cu in snow blower. For brut force there is no substitute for cubic inches and nitro ... ditto on tubes it seems.

I'am putting the Blackface on the rear burner. Back to the BFA, I'll post a schemo soon. Been working on this one about two years gathering info.

[sniper]

the 12AU7 input:



i have to work on the power amp and output pics then i'll post them. should be good for about 160 watts RMS.

output:



power:



this amp is a design by Don Huebert and is a very noiseless working amp that puts out tremendous power. it has been the house amp in a club since May 07 with one failure ... Don routed a coax too close to a resistor and it shorted. it is used by all types of basses including uprights pushing an 8x10 Ampeg cab. Don says it vibrates the floors at the back of the club.

here is the link: http://www.diytube.com/phpBB2/viewtopic.php?t=1436

[Psycho Bass Guy]

Sniper,
I'm going to address the conversation that you quoted first, then your actual post. I don't disagree with the conclusion in regards to the Super Twin, but there's a whole lot of BS in between about UL in general. Maybe I'm just missing the context, but for the sake of argument:

Many people in the guitar amp world mistakenly refer to the Fender tapped screen circuit as an Ultralinear design. It is not, and the taps were not intended, nor are they capable of producing Ultralinear operation. Typical Ultralinear operation for 6L6 class tubes requires a 43% tap (based on turns ratio), which is about what the Hammond transformer provides at 40%. True Ultralinear operation results in much lower distortion, and much lower power output -- about 35 watts is the absolute maximum RMS power output that a pair of 6L6s can give in an Ultralinear design.

I don't know where this person is getting their info, but UL output results in MORE, not less power; that's the whole purpose of the design; you hold the screen current down while jacking up its voltage, resulting in larger current swings from cathode to plate, which equals more power. The biggest problem with UL is that the high screen voltage can arc in cheap tubes and you need more drive power to push them, which is usually accomplished with a cathode follower drive stage. Strictly speaking for the Super Twin, this is correct because the rest of the circuit, but this is presented as general info, and there are OTHER Fender amps that are UL with 6L6's, and certainly well above 35 watts per pair: ie- the Bassman 135.

Therefore, when using the tapped version of the 1650T in the Super Twin, the amp would not produce much more than about 110 watts continuous RMS output. For this transformer to work to it's best ability, the screen taps would need to be taped off, and then have a standard choke type filter system installed for the screen B+ take off. Under these conditions (strict pentode operation),

Someone is talking our of their ass: 6L6's are beam tetrodes, not pentodes, and they have a different transfer curve, bias point, and higher output power because of it. There is no common audio pentode equivalent to the 6L6GC; the closest is the EL34, a much higher powered tube which is almost impossible to confuse with a 6L6GC.

As for the actual design of the Fender tapped screen circuit, the purpose of these taps was a cost saving measure on Fender's part, in that they eliminated the need for the choke to provide adequate filtering for the output stage. That was it, and nothing more. This is not an opinion, but a fact that came out of a conversation I had with the circuit's designer.

Is this guy seriously trying to say that a transformer choke is cheaper than a cap or that you can't have UL without a choke? There are plenty of UL amps that don't have a choke; I have a few of them.

Finally, contrary to what was claimed, the Super Twin is quite capable of putting out 200 watts continuous RMS -- if the P-P impedance is correct to allow for it. In this case, the original Super Twin transformer has a P-P impedance of 1500 ohms, which is proper for six 6L6s in Fender's tapped screen design. This is why the Hammond transformer, as a 1900 ohm unit, will not allow full power output to be produced even without screen tapped operation

...even though he earlier said that UL tapped screens produce less power???

Taking this into consideration, it might be best to investigate as to whether the OPT in a Twin 2 (which uses a choke) would be able to put out 100 watts driven by two KT88's as it shows 465V before the choke and 460V after the choke on B+ on the Twin 2 schemo. It might be a little marginal on the 88's? It would certainly drive them at a lower output and that would give it some reserve on "depth" or reserve current which is what a person would want in a bass rig. I would guess and venture about 75 watts RMS and about 150 peak or program burst. I would prolly gain a bit on the simplified preamp as far as reserve also."

As I alluded earlier, you could add a cathode follower for the drop in your forward gain, giving you greater current headroom and drive. That's also awful low voltage for KT88's. They run comfortably in the 6-700 volt range, and the higher you can get them, the cleaner they will run. The SVT's drive stage works this way with each side of the inverter feeding a 12BH7 (basically a higher power 12AU7) as a cathode follower, basically a power amp as opposed to a strictly voltage gain stage, to drive the output tubes.

[sniper]

Sniper,
As I alluded earlier, you could add a cathode follower for the drop in your forward gain, giving you greater current headroom and drive. That's also awful low voltage for KT88's. They run comfortably in the 6-700 volt range, and the higher you can get them, the cleaner they will run. The SVT's drive stage works this way with each side of the inverter feeding a 12BH7 (basically a higher power 12AU7) as a cathode follower, basically a power amp as opposed to a strictly voltage gain stage, to drive the output tubes.

Bingo!

I agree with you fully. I couldn't quite get all that he was writing but I get your explanation. I was hoping for some clarity thats why I posted it, thanks. Unless I miss my guess, that is why the Bassman 10 uses no filter and it is not UL. The voltage is way too low for 88's but enough for 4-6L6's (with the Twin 2 PT) and it makes sense looking at the tube data from the 6L6 spec PDF and the schemo* voltage from a Twin 2 Reverb.

* I am still learning to read those things, it is slow to me. It is coming small bits at a time. I did not get the part of the driver circuit becoming a "power amp" driver in effect vs a simple gain and was just looking at the fact of substituting an AU7 for an AX7.

The part that mistakenly made sense and the part I inadvertently left out was about the 43% tap, Hammond uses 40% on some trannies for some reason, is because of harmonics balance. I couldn't quite understand the 12.5% Fender tap but was taking him at his word and trying to make sense of it, then I overlooked the cathode resister not noticing the cathode follower value vs the negative voltage and read the data from NJ Tube about an output drop in UL mode on 6L6's. I was tryng to marry apples and oranges even though the voltages were still fairly close except grid 1. *Note to self ... study the diff in cathodes and anodes with respect to current flow polarity and remember not to juxtapose values for diff tubes, ditto anodes vs cathodes duh!*

How do you guys keep all this info straight? I suppose some self taught engineers vs real engineers don't and thats why tubes don't work, work crappy or smoke/flash dead. "But it worked fantastic for about two millionths of a second!"

Somehow a flash back to a portion of Terry Kath's (RIP) "Free Form Guitar" come to mind:

http://popup.lala.com/popup/360569475234590996

Thanks again PBG.

This is all so far above me I feel like I am trying to reinvent the wheel and not trying to understand how it works.

[sniper]

I'd drop the input cathode resistor, ditch the second 12AX7, and use a 12AU7 or even a 12BH7 for the tone stack. You'll have to recompute the r/c tone curves to the lower plate resistance of the 12AU7 (roughly 1/10th the 12AX7's) if you want the same frequency values, but I think you'll find that it will be a much bass-friendlier preamp just by changing the tubes and little else. It'll still sound like a Fender, but it will be more like a 400PS than a Showman.

 You also won't necessarily overpower the circuit by not having an additional channel; your PT will just run cooler from less current demand, which means you can get a little more out of its other voltages without stressing it.

taking a shot ... this almost duplicates the Fender curve according to Duncan's Tone Stack Calculator, but i am not sure of the resister value on ground from pin 8 as shown. i know the pin numbers have to be changed as this shows them in reverse order, that can happen it's simply late and i have been working on this for two hours:



it bumps the mid boost a small amount but i think that could not hurt knowing the Gibby Mudbucker. i lose about 1 -db across the board (1 db less negative towards the positive) but it looks like a parallel track on the curve.

[sniper]

here is the comparison of the second modification to a sample Fender circuit that was loaded in the calculator:



mine is the top line. JJ (tesla) and EH both making the 12DW7 again.