OT impedance and break up relationship

[Justin Thomas]

I hear what you're saying, but don't discount the rest of the circuit, too. The Super Twin & Super Twin Reverb ( and 400PS & 300PS) were not exactly "normal" Fenders. I do agree that someone probably got them.cheap, but there are ways to mitigate the huge value later on in the signal chain.

Nearly every tube book I've read says to use as wide a band as possible on the first stage and then block them out later... well, that's about as wide as it gets!

Justin

[J M Fahey]

pedal steelers and keyboardists? they tend to love amps like the Super Twin Reverb that has that big "wooly" sounding 750uF cap in the first stage.

Not even.
25uF + 1k5 is already ***4 Hz*** ... nothing to be gained by going to 0.4Hz or lower.

[Chuck H]

I find most of those GW suggestions dubious to some degree. But then "improve low end" doesn't have to mean the same thing as INCREASE low end. Perhaps a large value cathode bypass cap increases LF linearity due to it's comparably larger charge reservoir.?. Indeed there are plenty of discussions here about what "fully bypassed" means relative to the caps knee frequency @ impedance, but other performance characteristics relative to that cap value almost never come up. Though I'll wager that we all suspect it's a very real phenomenon. Would any of us build a 1959 without the 250uf first stage cathode bypass cap value?

[bob p]

1/(2PiRC) doesn't give the complete answer

Historical perspective matters. Those who remember the progression of HiFi amplifier technology in the 70s will remember what a big deal it was to be able to publish spec sheets in your ad copy claiming that your amp was linear all the way down to DC. Designers of SS amps were proudly listing frequency response specs as DC-to-supersonic, marketing their products as superior to the old bandwidth-limited tube technology which was still their direct competitor.

If you put on the f(half boost) = 1/(2PiRC) blinders then you miss the other half of the story.

It's been mentioned several times that the STR was never designed as a guitarist's amp. If you insist on viewing the amp through guitar-goggles where nothing below 80 Hz matters then you'll never understand it's design.

The exact same amplifier was concomitantly being marketed to bass players as the 200W Fender Studio Bass combo. In that embodiment the reverb circuit was left out and the combo carried an EVM15L. For a miniature bass amp it was a wolf in sheep's clothing. It was the first bass amp from Fender to give the small/medium Ampegs and Acoustics a run for their money. 40 years later the guys at TalkBass are still oogling over it.

Back to the STR -- it's never been popular as a guitar amp because it included a number of design changes that were not made with guitarists in mind. It was primarily envisioned as a solution for keyboardists and pedal steelers.

Why is the first stage Ck so big? Not because CBS was trying to unrealistically expand the sub-bass frequency response below 5 Hz, but because the cathode bypass cap is not smart enough to read the number that's stamped on it and behave the way it's told. In the real world capacitors are non-linear devices that produce low frequency distortion (the "wooliness" I mentioned earlier). Designers will often choose a capacitor that's 10x larger than it "should be" because larger caps prevent increases in LF THD. (Reference: non-linearity in the input's DC blocking capacitor is the the 8th mechanism for distortion discussed in Doug Self's Audio Power Amplifier Design.)

You have to bear in mind that this amp was developed late in the game for Fender -- by the late 1970s the CBS amp designers were becoming hip to the problems that existed in the SF amps and were addressing them in a battle to stay competitive as solid state gear was taking over the world. Things like capacitor-induced LF distortion were definitely on their radar screen as this amp was being sold to people packing a Rhodes 73/88.

The Rhodes had always been sold as a 2-piece "suitcase" outfit comprised of the piano itself, and a separate cabinet that was just as big that housed the amp and the speakers. The keyboard was latched on top of the huge speaker cabinet. Keyboardists hated hauling around the 2-piece setup because it was a Royal PITA. You had to have a van or a pickup truck to haul a Rhodes setup.

By the mid 1970s many keyboardists were avoiding the Rhodes suitcase system in favor of a Rhodes stage piano and a Twin Reverb because this combination was easier to haul. but keyboardists weren't completely happy using the Rhodes with Twin Reverbs because the Twin Reverb lacked sufficient power and LF response compared to some of the SS competition. To address these complaints CBS responded by giving them an amp with even more power, the Super Twin Reverb, whose design included measures to lessen the LF distortion problem. From 1977 to 1980 if you went into a music store that carried both Fender and Rhodes and you tried to buy a Rhodes suitcase piano chances are that the salesman was going to try to upsell you a stage piano and a Super Twin Reverb.

Look at the STR schematic. It uses an "inappropriately large" 750uF/3V Ck in the first stage, with an appropriately low polarizing voltage rating of 3 VDC, rather than the 25uF/25VDC value always seen in BF/SF Fenders. Why? Is this a chance occurrence or was CBS directly attempting to directly address both aspects of the LF distortion problem by choosing high capacitance and low voltage? By this time CBS' designers knew that it was common for Fender inputs to be driven beyond the recommended 100mV rating, so they designed an amp that could tolerate large input signals without generating as many LF distortion artifacts.

Though we have a lot of guitarists on this site it's worth considering that the world isn't centered around electric guitar.

[Malcolm Irving]

Another factor, which hasn’t been mentioned so far in this thread, is that a very large bypass cap can reduce hum or buzz caused by unwanted current from the heater to the cathode. Either due to poor insulation or due to heater-to-cathode capacitance there is the potential for a significant hum voltage on the cathode. This is grounded out (for ac) by the large bypass capacitor. The filtering in this case is pure low-cut (with no shelving effect). So the larger the capacitance, the more that hum will be reduced.
I read somewhere (on the internet no doubt) that when Marshall ditched their very large bypass cap in favour of a more ‘reasonable’ value, they found unexpectedly that they had to be more selective about choice of V1 samples to minimize hum.

[bob p]

Geez ... with all that I typed I managed not to mention heater noise. Malcolm, you're absolutely right -- another reason that big bypass caps were put on the first stage was to assure that the amp had the best heater hum rejection possible, thereby lowering the amp's noise floor. Many people today will diode-bias the inputs for this same reason.

If the truth be told I've been thinking about converting the 750uF/3V Ck on my STR's input to diode biasing just so I don't have to deal with the cap again in my lifetime.

[bob p]

I find most of those GW suggestions dubious to some degree. But then "improve low end" doesn't have to mean the same thing as INCREASE low end. Perhaps a large value cathode bypass cap increases LF linearity due to it's comparably larger charge reservoir.?. Indeed there are plenty of discussions here about what "fully bypassed" means relative to the caps knee frequency @ impedance, but other performance characteristics relative to that cap value almost never come up. Though I'll wager that we all suspect it's a very real phenomenon. Would any of us build a 1959 without the 250uf first stage cathode bypass cap value?

Chuck, I think that you're right -- "increase" and "improve" don't necessarily mean the same thing.

On the subject of fully bypassed Marshalls -- there were some 50W Marshalls that used 330uF Ck. Those design considerations were made before the time when people started overdriving the inputs with huge (> 1 volt) input signals. The combination of overdriven inputs and flat frequency response invariably caused some blocking distortion in the following stages that needed to be addressed.

When high gain guitar amp designs became the norm we began to see Ck values being selected more for their frequency shaping effects and less for their noise and distortion elimination. On rock guitar amps we started seeing partial cathode bypass instead of complete bypass, along with the reduction in the size of coupling caps. This works because a guitar amp doesn't have very much in common with a HiFi amp, a keyboard amp, or a pedal steel amp, which are intended to be more linear devices. Guitar amps are intentionally bandwidth-limited devices and they are specifically designed to provide frequency-dependent "musical" distortion.

We have to keep that in mind when we read schematics because not every amp is intended as a specialist device for guitarists. The Super Twin Reverb, for example, will make your ears bleed with 180 watts of clean guitar sound. Rock guitarists hate that, but the pedal steelers love it and the amp is very popular with them.

[Enzo]

I was just thinking last night about my Crown amp. It actually does have response down to DC. It is DC coupled throughout, and if you apply DC to the input, the output will indeed move to a DC voltage and sit.

[Mick Bailey]

And if they did matching speakers I bet there are folk who would sit there and 'listen' to DC as an extreme art form. Sadly, I actually know someone who that idea would appeal to.

BTW, if anyone's interested I have a few amps that output DC. Not by design, but they do it just fine.

[Enzo]

Crown amps started as lab gear, so DC had legitimate uses.

[Leo_Gnardo]

I was just thinking last night about my Crown amp. It actually does have response down to DC. It is DC coupled throughout, and if you apply DC to the input, the output will indeed move to a DC voltage and sit.

Also known as a "programmable power supply." An old hi school buddy picked up a trio of scrap but working Kepco PPS's from the dumpster at his college science building. They had DC to 30 KHz response and he used 'em for his stereo rig. Only hassle was inputs and outputs were all screw heads on a barrier strip, but that's not too much of a hassle compared to the advantage of getting 60 watt industrial strength power amps for free.

[Gnobuddy]

With all due respect to all the posters involved, some of the tales about the virtues of 750 uF cathode bypass capacitors remind me very much of the Baron Munchausen stories I loved to read as a child.

The full book (long out of copyright) is available, in various digital formats including PDF, for your reading pleasure here: https://archive.org/details/munchausen00unknuoft

-Gnobuddy

[Leo_Gnardo]

With all due respect to all the posters involved, some of the tales about the virtues of 750 uF cathode bypass capacitors remind me very much of the Baron Munchausen stories I loved to read as a child.

Taken to the extreme (I'm sure the Baron would approve!) wouldn't the cathode be at a virtual ground with a very large bypass cap. So, let's leave out the Ck and Rk, grounded-cathode triode preamp does offer some virtue: a built-in compression effect. Some old guitar amps have this, notably very old Gibsons & similar from the 40's & early 50's. Also used in hi-tech (for the time) preamps such as Crown's tube mic preamps intended for use with their tape recorders. Input signal passes thru a film or disc cap, and the value of that cap can be used to limit the low-frequency extension of the amplification pass band. Forget about trying to pass DC, I <LIKE> the compression effect of those grounded cathode circuits.

There was a Munchausen movie too. Lots of fun for all ages.

[tubeswell]

No need to get drastic and change the OT just yet. Have you re-capped and re-tubed the amp? I had a 67 Super Reverb in for service a couple of months ago where the customer was complaining about hummy-ness and lack of clean headroom. I checked the voltages and the B+ was 385 (with -33V bias voltage - see attached pic).

I replaced the old electrolytics and the hum stopped. I rebiased to a cleaner setting and the B+ went up to 420 (Vg1= -45.6V) Ik = 34mA - see next pic. Some improvement in tone and headroom.

Then I replaced the old output tubes and rectifier and immediately the B+ leapt from about 420 to about 470 and the headroom and bandwidth came clean to about 7 on 8 on the dial.

[bob p]

+1. To find a problem you have to be willing to look for it.