Plexi 50 bright cap confusion

[bnwitt]

Fellows, I'm a little confused regarding the use of the bright cap on the lead channel of the typical 50 watt Marshall plexi amp.  The Unicord schematic shows the cap bypassing treble from the V1b plate coupling cap across the Lead channel volume pot to the V2a grid.  But most of the layouts I find out there delete this capacitor.  The layout by Mark Huss uses it switched so you can take it in and out.  I'm not a Marshall guy as some of you know so I'm asking, in, out or switched.  I'm leaning towards switched for the optional aspect but I have no way of knowing if the addition of this cap is a good thing to Marshall ears or not.

[RicharD]

It's all a matter of taste but ultimately has a lot to do with the speaker.  I would lean towards switched so you have a choice.  The bright cap often sounds shrill with an open back cabinet but is almost necessary if you have a low brow 4 x 12" closed back cabinet.  (when I say low brow, I mean a cheap cabinet with cheap murky speaker).

-Richard

[bnwitt]

Thanks for the response Buttery.  This head is coupled with a Marshall 1960AX with 4@ Vintage 30 12 inch speakers.  Do you see any value to the boost switch on the 0.68uf bypass cap as shown on the Huss Schem?  I have seen some layouts that omit that cap altogether.

[RicharD]

If you play rock, the boost switch is a good thang to have.  It will slap the cathode follower that's driving the tone stack considerably harder.  Personally, I don't stay married to that .68uF value.  That cuts a lot of low end.  Too much for my taste so I usually run with a 1uF or a 2.2uF.  I seldom run up to the 22uF like Fender used because that doesn't attenuate enough low frequencies.  If it were hifi, that's be a different story, 47 or 100uF.  It's all a matter of taste.  A lot of times, I'll jack with cathode bypass caps right at the end of a project, kind of a final tweak... voicing if you will. (for lack of a better term)

[bnwitt]

Thanks again buttery.  I went ahead and added a 2.2uf bypass cap on the board with a separate turret to allow wiring to and from the normal channel volume control switch for pull boost.  I'll put the bright cap on the Bright channel volume control switch.  Both like Mr. Huss did on his amp. 

[RicharD]

You're most welcome.  Like I say, it's all subjective to personal taste.  Mark Huss is quite the gentleman.  He talked me through a Hiwatt repair that was driving me insane.  Ultimately he told me to replace the output tube sockets.  I did and he was spot on.  That guy knows his Hiwatts.

[Fresh_Start]

For whatever it's worth, Kevin O'Connor's take on the Plexi completely eliminates that bright cap on the lead channel and he uses 1k // .68uf on the second stage's cathode.

BTW can somebody explain to me the point of bypassing an 820 cathode resistor with a 330uf cap in a guitar amp?  Using a 47uf cap gets you below the typical Fender 1.5k // 25uf frequency response. (IOW it's below 10Hz so why do you even want to amplify it?)

Don't know if this interests you at all, but I've been playing around with a Plexi-like preamp for a while and recently came up with an idea for switching the cathode of the second gain stage.  When the cathode resistor is not bypassed, it's 822 ohms.  When it is bypassed by a cap, the resistance is 1.5K.  Also, 1.5k bypassed by .68uf is roughly the same as using 1.2uf bypassing 820 ohms in terms of frequency response.   I did something similar on the second gain stage of my Super Reverb build and liked the result.  You get the frequency shaping and gain goes up when the resistor is bypassed, but it doesn't boost the gain all the way.

Haven't built anywhere near as many amps as you all, but Buttery's approach to tweak cathode bypass caps at the end of a build has worked well for me too.  I've taken to using separate turrets for the cathode caps & resistors which are likely candidates for tweaking so it's easier to do.

Have fun with the Marshall build and keep us posted!

Chip

[RicharD]

>can somebody explain to me the point of bypassing an 820 cathode resistor with a 330uf cap in a guitar amp?

I was wondering the same thang.  That's a f-3dB of 0.5 Hz.  I can only speculate 3 things that come to mind:  they wanted the 2 channels to be quite different, to use with a bass guitar, tape player, or drum machine, or so they could say that channel has full frequency response.

Those final tweaks are game winners when you bring a customer in and treat them like they're at the eye doctor, "is this better or worse."  Everybody loves a personal touch.

[HotBluePlates]

BTW can somebody explain to me the point of bypassing an 820 cathode resistor with a 330uf cap in a guitar amp?

Copying Fender's high-valued cap on the input stage of the 5F6-A, but with locally available parts.

If you have a fast internet connection, look around for any tube manuals, especially short-form data, that show the 12AY7 along with other special-series tubes from RCA or G.E. around the late 50's. The 12AY7 was designed and marketed to be low-noise and low-hum, and specially suited to input stage use.

Now think about the original intent of the Bassman. It's a bass amp, and should have decent response down to 60Hz or so. So any hum contributed due to heater-to-cathode leakage is added in to the desired signal, and is a problem.

If you have a tube that has hum caused by heater-to-cathode leakage, how do you fix the problem? You either select from a large lot of tubes for low leakage samples, or or you find a way to swamp the leakage. Fender picked a special low-hum type (12AY7), but obviously went an extra step to zap the leakage, since they probably knew that the user would expect any 12AY7 to have good performance.

The heater is electrically insulated from the cathode, and that insulation forms a capacitance. Say you have leakage that allows hum to appear on the cathode; the cathode as an input is low impedance and high gain. The capacitance to the cathode would be a small value, maybe a few pF, and amplified by Miller effect. So you have an effective 100pF cap from heater to cathode when leakage is present (maybe more or less).

If you apply an a.c. signal to 2 caps in series, they divide the voltage between them. The ratio of division follows the reactance of the caps, so it appears opposite that with resistors; i.e., the smaller cap has more voltage across it due to its higher reactance at a given frequency.

If the heater-to-cathode is a small-valued cap, you swamp hum by putting a very large-valued cap from cathode to ground. If the input capacitance represented by the leakage capacitance is really 100pF, it looks like 26M at 60Hz, while a 330uF cap looks like 8 ohms. That's a lot of voltage division, and kills any hum due to leakage if it is present in a given tube.

It's not a tone thing in my opinion, it's all about hum.

For what it's worth, it has happened in the past that people have reduced a 25uF bypass cap to 1uF or 0.68uF (or even removed it), and wound up with hum they didn't notice before. And whether the hum appears is haphazard. Not all tubes leak, and not all that do leak do so by the same amount.

[RicharD]

>Fender picked a special low-hum type (12AY7), but obviously went an extra step to zap the leakage, since they probably knew that the user would expect any 12AY7 to have good performance.

Very interesting.  A while back, I bought several gallons (yes gallons) of used tubes.  fter sorting and doing a "Champ" style listening test, I noticed the 12AY7s had the highest reject ratio as compared to the rest of the 12A*7 family of tubes based upon microphonics, noise, & hum.

[bnwitt]

Ok, so what would be a more useful bypass cap value on that input stage wit the 2k7 resistor?  10uf?  Obviously we don't have the hum problem with the 12AX7 in this amp.

[7string]

1.5K and 25uf is a good one. The 10uf might be perfect though. Gotta clip them in and crank it up.

Another place to mess around is at the output coupling caps. .022 / .047 / .1

[Fresh_Start]

Ok, so what would be a more useful bypass cap value on that input stage wit the 2k7 resistor?  10uf?  Obviously we don't have the hum problem with the 12AX7 in this amp.

Since I was annoying enough to bring this up, my take on it is 1.5k || 25uf more than covers all guitar frequencies, right?  Cutoff frequency = [1 / 2*pi*R*C] where C is capacitance in Farads.  Instead of driving myself crazy trying to remember how many zeros to put after the decimal for a microfarad, I figure that if you double the resistance, you can halve the capacitance and get the same answer.  Does 47uf float your boat?

(1500/820) = 1.83
1.83 * 25uf = 45.7uf, then round up

Or you can go to our handy dandy Frequency Reactance Chart.  Pick 80Hz on the bottom scale (low-E), go up to the intersection with 800 ohms on the Y-scale, then diagonally to approximately 2.7uf.  Yes, I cheated and have an excel sheet which calculates the cutoff frequency.  But common sense tells me that's way too low a cap value in this spot based on any other circuit I've seen.

Our friend HBP knows a hell of a lot more about the theory and math here than I ever will.  With only 820 ohms on the cathode, that 12AX7 is biased pretty hot and producing a LOT of gain.  More gain means more noise IIRC.  If a 100uf or 300uf cap reduces the noise level at the most sensitive stage in the amp, there's plenty of room to cut out those frequencies below 80Hz later in the circuit, starting with the .022uf coupling cap following the plate of that triode.

Sorry for sidetracking your post!

Chip

[bnwitt]

I usually use the Bruce Collins rk-ck table but of course it stops at 47uf so the 330uf isn't shown.  With an 820 ohm rk it seems a 1uf or 2.2uf would give the closest -3db point to the treble channel's 2k7/.68 setup.  However, I'm currently at 2.2uf/2k7 on the treble channel which is 72 hz.  So, to keep frequencies the same for both channels and gains the same as stock (keeping the stock rk values on each channel) it looks like a 5uf/820R would be the closest to the 2.2uf/2k7.

[HotBluePlates]

Since I was annoying enough to bring this up, my take on it is 1.5k || 25uf more than covers all guitar frequencies, right?

A clip-in substitution box and your ear will be faster than any calculation or chart.

But if we really wanted to be obsessed with calculation (I've been guilty in the past), the cathode resistor isn't the only resistance to consider. The cathode of the tube presents an impedance of rp(mu + 1) in parallel with the cathode resistor. The problem is rp changes depending on the operating point, but since it is high anyway with a 12AX7, and much higher than the cathode resistor, all you really need to remember is the effective resistance used in the forumla is slightly lower than the value of the resistor.

I am seriously thinking of going back into my amps and swapping bypass caps for 1-2uF caps, pending any leaking tubes (maybe I'll have a huge bypass cap on a switch to zap hum when needed). I noticed that in a great vintage amp that I once owned, and measure parts values, the bypass caps had largely dried up and dropped in value to 1-8uF. I would like a little bit clearer bass in my amps.

A while back, I bought several gallons (yes gallons) of used tubes.  fter sorting and doing a "Champ" style listening test, I noticed the 12AY7s had the highest reject ratio as compared to the rest of the 12A*7 family of tubes based upon microphonics, noise, & hum.

I'm not saying they are better tubes, just that they were marketed for low hum, low noise. As for microphonics, the highly valued long-plate blackplate preamps tubes of the 50's should have higher microphonics than the later short-plate tubes. The pplates were shortened over time to reduce microphonics.

I did not have the cash on hand back in the day when I could buy tubes by the moving-box full at a hardware store in Nashville. But if I did, a certain large number would probably be tubes pulled fro equipment because they started acting up. Doesn't mean that it wouldn't work in a guitar amp, but also that there could be issues.