MegaPlex - Plexi meets ODS?

[groovtubin]

Since you now have an extra triode section parallel them, halve the plate and cathode resistors and double the cathode cap value. Same gain, half the noise, a little more warmth....just sayin'

Couple more options to consider.

EDIT: Schematic needs a ground on the DPDT clean/OD switch

With respect, 10thtx

on #3 i`d try dumping the fat, hardwire in instead, and add a MIX control after the couplers, w/wiper out feeding the next stage, just a sugg, lookin good!

jim

[groovtubin]

Since you now have an extra triode section parallel them, halve the plate and cathode resistors and double the cathode cap value. Same gain, half the noise, a little more warmth....just sayin'

Couple more options to consider.

EDIT: Schematic needs a ground on the DPDT clean/OD switch

With respect, 10thtx

oh yeah, and use 1959 plexi values, .022, .0022, i` BET .001 on BOTH would sound good

jim

[renshen1957]

AFAIK.. The tone stack is a filter network that is designed to ideally be driven from a zero source impedance to achieve it's proper frequency response....
The cathode follower provides a very low source impedance that allows the tone stack to work as designed. If the tone stack is driven from too large a source impedance, not only will there be a loss of gain, but there will be a different frequency response to the network, typically quite a few dB loss of the highs... The cathode follower prevents this loss, allowing the tone stack to retain more of it's theoretical frequency response...

So with this being said CF driven stacks are IMHO more dynamic..

A good example of this would be the Bluesmaster TS (very close to the Marshall stack) but without the follower.. As you will see in this post

https://tubeamparchive.com/viewtopic.php?t=13323

Most complain of the loss of high end and or very bass-y sounding.. This IMO is because the Bluesmaster is plate driven and the Marshall style is CF driven..BIG Difference!!..
The Downside
IMO Cathode follower driven tone stacks with preceding gain stages don't sound very good to me bright and grainy (2nd order harmonics).. Definitely not in the Dumble vein..

Tony

Hi Tony,

Cathode Followers voltage gains are between 0.7 and 0.9, less than unity, and the due to the nature of tubes the output impedance is quite low but not zero.

Variable loading placed on the gain stage (bypassing the Cathode Follower and driving the EQ from the plate of the preceding stage) allows the EQ to be more effective and allows a wider variation as the EQ is manipulated. A more dynamic sound is produced.

The Cathode follower in the 5F6-A (from which the Marshalls were cloned) was a left over from the 5E6 feedback controlled stage to drive the EQ and the loop provided a virtual-earth mixing function, and the impedance of the stage was reduced and drove the EQ more easily; a low impedance makes the loop more accurate, a cathode following is the simplest way to do this in a tube circuit.

The preceding feedback loop stage was eliminated in the 5F6-A ( a different tone stack design from the preceding 5E6 bassman), so the Cathode Follower was a vestige of a discarded circuit.

Its no secret that Ken Bran and Jim Marshall cloned the Tweed Bassman (it happens, Peavey Butcher was a Marshall 800 clone, Mesa cloned Soldanos to make the dual rectifiers, Behringer and Bugera continue the "tradition" by cloning everything they can get their hands on) for the first Marshall head and later tweeked circuit values to make the amp brighter. The subsequent amps that followed by Marshall (and were copied by others trying to cash in on Marshalls bright sound) copied the circuit as it was perceived to be part of the sound.

Many amps do not have Cathode Followers to drive tone stacks (Tweed Deluxes, Black Face Fender Twins, NON-top boost AC-30, Hi Watt Bulldogs, Kelley FACS, the lead channel of the ENGL Blackmore signature mode, Mesa Mk 1 & MK 2, Komets, and Dumbles to name a few) with excellent tonal results. These amps with either early or late plate driven EQs do not suffer from loss of highs.

Bluesmaster being a Blues amp wouldn't be my idea of a best comparison with a bright rock amp like a Marshall; a Marshall 2205 has a clean and gain channel neither of which have Cathode Followers driving the tone stacks. Still sounds like a Marshall, too.

Any loss of gain from early EQs is usually compensated by the following gain stages and rarely do you find a cathode follower pushing the tone stacks to compensate in these amps. For what it is worth, a Cathode Follower driven tone stack can sound hard or cold in this arrangement

A late EQ tone stack is preceded by a gain stage and the next stage is the power amp (I include the PI in the power amp as it is in the PA feedback loop) what losses which occur from the EQ stack driven by a plate aren't missed.

If you like the sound, that's cool, but a plate driven EQ can sound good, too.

Best Regards, and a Happy New Year.

Steve

[talbany]

Steve
An interesting topic for sure and thanks for the wonderful explanation on your take..I'll take 1 more stab at it and go a bit deeper and hopefully this will make a bit better sense of where I am coming from when I mean dynamic FWIW>>

Variable loading placed on the gain stage (bypassing the Cathode Follower and driving the EQ from the plate of the preceding stage) allows the EQ to be more effective and allows a wider variation as the EQ is manipulated. A more dynamic sound is produced.

The problem is that people associate "gain stages" with, well...gain. The cathode follower has a maximum theoretical gain of unity, and typically a gain of around 0.5 to 0.7 or so just like you mention, but to those engineering types, a gain of less than unity is still called a gain (that is, unless it's called a loss or an attenuation, in which case the attenuation is the reciprocal of the gain), so a stage with a gain of unity still has a gain of 1 is how I look at it..
The cathode follower is an amplifier stage, but not a voltage amplifier in the typical sense. It is used as a "buffer" amplifier, which means it has a high input impedance and a low output impedance. This means it does not appreciably load the previous stage it is connected to..For example, if the previous stage had an output impedance of 100K, and you tried to connect a stage with a 10K input impedance to it, you would only get 9% of your original signal when you connected the second stage, because of the voltage divider formed by the 100K output impedance and the 10K input impedance. If you insert a cathode follower with a 1K output impedance and a 1Meg input impedance, the 10K stage can be driven with 90% of the original signal, because there is now effectively a 1K:10K voltage divider instead of a 100K:10K voltage divider therefore less losses in the stack..

If you like the sound, that's cool, but a plate driven EQ can sound good, too.

Also... I never said plate driven stacks didn't sound good... Most of the amps I own design and build at VVT are mostly plate driven although I still am a BIG Plexi fan..LOVE em ..

Bluesmaster being a Blues amp wouldn't be my idea of a best comparison with a bright rock amp like a Marshall; a Marshall 2205 has a clean and gain channel neither of which have Cathode Followers driving the tone stacks. Still sounds like a Marshall, too.

Maybe the 2205 might sound like a Marshall to you but not to me..
Also the tone stack cap, pot, bypass caps used in the 2205 totally different than the BM different topology..2205 and BM is IMO a not so good comparison..

Have a Great New Years Back..

[10thTx]

There have been enough differing ideas and suggestions regarding this Plexi meets OD concept that I decided to post an editable schematic and layout.

IF you use ExpressSCH, you can find an editable schematic and layout here:

http://www.el34world.com/Forum/index.ph ... 7#msg99077

There is also a version that uses a 5879 pentode in the second OD gain stage and is cathode biased.

With respect, 10thtx