B+ reduction

[lowell]

hey all,
I want to reduce the b+ in my amp. It's a 5E3X2. I have the parts from RGs mosfet follies page on geofex, however he says to put this in series w/ CT of POWER transformer. Can I install this to only lower the B+ and not ALL plate voltages in the entire amp? And if so, how?

[KingCameron]

B+ will affect the entire circuit

If you attenuate the transformer to lower the B+, it will subsequently lower all voltages in the amp. You can adjust each plate voltage acording ny changing the resistors to that plate, or by adjusting the resistors to GND on that side of the triode

[lowell]

Can I put this assembly in series w/ the CT of the OT?

[KingCameron]

not sure I understand...

If you are going to provide a path for current from the CT to ground you are bleeding power off of the transformer, before it ever gets to the tubes, therefore lowering the B+. Since the pre-amp tubes set on a power supply that comes after the Power tube stage, you will be reducing the voltage that is going to them as well.

You can...
Adjust the resistors in your power supply to change how much it steps down the voltage to the pre-amp stage

Adjust the voltages on a tube by tube basis by looking at their plate voltages and what is going to GND

[lowell]

Ok I'm strictly referring to putting this amplified zener in series w/ the B+/CT of output transformer. I understand that the subsequent power supplies are after this point in the power supply chain. If I put this in series w/ the OT center tap it will affect the C+ too?

[lowell]

KC,
Still not sure if we understood each other. I THINK your talking about putting this on the center tap of the power transformer... per RGs article.

Anywho if you're interested or anyone else is interested I put the amplified zener in series w/ the OT center tap and it worked like a charm. The plate voltage is at 300v on the 4 6V6 and the idle current of tubes are at 32ma each. They are quite close to the max dissipation so I'm gonna keep an eye on 'em for a while. This amp cranked now sounds EXACTLY like Neil Young on the classic recording of Cinnamon girl where his amp sounds like it's barely hangin' on. When I really dig in it farts and compresses like crazy... sounds like a square wave at times... almost synth'ish.... very cool.

[lowell]

If I put a switch in to switch b/t this mod and stock mode does that mean the switch needs to withstand the 30watts that this amp put out? Would a DPDT power switch for a tube amp be sufficient for this?

[Bruce / Mission Amps]

KC,
Still not sure if we understood each other. I THINK your talking about putting this on the center tap of the power transformer... per RGs article.

Anywho if you're interested or anyone else is interested I put the amplified zener in series w/ the OT center tap and it worked like a charm. The plate voltage is at 300v on the 4 6V6 and the idle current of tubes are at 32ma each. They are quite close to the max dissipation so I'm gonna keep an eye on 'em for a while. This amp cranked now sounds EXACTLY like Neil Young on the classic recording of Cinnamon girl where his amp sounds like it's barely hangin' on. When I really dig in it farts and compresses like crazy... sounds like a square wave at times... almost synth'ish.... very cool.

With respect to your power transformer and it's center tap.... if you follow the OT center tap backwards, you'll see it is already in series with a diode, probably two of them if your power supply uses a full wave rectifier! ha ha... so yes, I see no reason another one (a high wattage Zener with the cathode facing the power transformer) shouldn't work too. Added noise?... Maybe a .22uF to .47uF 500v/630v cap to ground at the anode end would help.

[KingCameron]

yup. I was thinking PT, not OT...cool stuff

KC,
Still not sure if we understood each other. I THINK your talking about putting this on the center tap of the power transformer... per RGs article.

Anywho if you're interested or anyone else is interested I put the amplified zener in series w/ the OT center tap and it worked like a charm. The plate voltage is at 300v on the 4 6V6 and the idle current of tubes are at 32ma each. They are quite close to the max dissipation so I'm gonna keep an eye on 'em for a while. This amp cranked now sounds EXACTLY like Neil Young on the classic recording of Cinnamon girl where his amp sounds like it's barely hangin' on. When I really dig in it farts and compresses like crazy... sounds like a square wave at times... almost synth'ish.... very cool.

Sorry bud, I was thinking PT, not OT...interesting stuff though...

[lowell]

no prob... sometimes it'd be nice to chat on here.

I would like to see if this amp is now operating in true class a. How can I do this? Can I use my scope to see it or is there an equation I can use to mathematically see the operating class?

[Bruce / Mission Amps]

no prob... sometimes it'd be nice to chat on here.

I would like to see if this amp is now operating in true class a. How can I do this? Can I use my scope to see it or is there an equation I can use to mathematically see the operating class?

The 5E3 or 5E3X2 amps are not a Class A amps.
There are so few real Class A guitar amps out there that is getting almost impossible to find one unless it is a single ended amp, which, for the most part, has to be Class A in order to stay linear for audio.

Very roughly speaking, Class A just means that with respect to it's power tube's DC bias voltage, one or both of the power tubes are not driven by a previous stage so far negative as to cause one or both tubes to be shut off during any 360 degree portion of the signal and the power tubes are in fact both swinging signal for the entire 360 degrees of any sine wave.
Biasing the tube to a very hot/high current actually means that the bias voltage is closer to zero and the drive signal appearing on the power tube's grids can push negatively far enough to over ride the smaller grid bias voltage so far as to effect the bias state of the tube so it goes into cutoff.
The 5E3 (as an amp circuit) can easily do this with the relatively low voltage seen in the power tube's cathode biasing and the two stages of gain prior to the cathodyne driver.
That means the driver and PA of the 5E3/5E3X2 are merely working in high idle current power tube, Class AB1.

[lowell]

Hi Bruce,
Thank you for the info. I know the amp is Class AB when in stock wiring. Not sure if you read all the threads, but I modded the plate voltage to be a low 300v and the current is now quite high on all tubes. I would now like to CONFIRM if the amp is in Class A or still in some form of Class AB. I would like to know how to CONFIRM this whether it be with a scope or an equation.

[Bruce / Mission Amps]

Hi Bruce,
Thank you for the info. I know the amp is Class AB when in stock wiring. Not sure if you read all the threads, but I modded the plate voltage to be a low 300v and the current is now quite high on all tubes. I would now like to CONFIRM if the amp is in Class A or still in some form of Class AB. I would like to know how to CONFIRM this whether it be with a scope or an equation.

If you did not modify gain, the preamp and phase inverter, then I'd say you still have a class AB amp because there is enough drive to do what I mentioned in the previous thread.

[lowell]

Oh Ok... so if you were doing this what gain would you set the cathodyne PI at? Would you adjust the plate resistors or something in the cathode resistor circuit of the PI?

[lowell]

Another question regarding the amplified zener. I used a 75v zener and 88watt power mosfet. I am only getting a 30v change when the amplified zener is engaged... not the 75v I would think the zener would drop. Is there something I'm missing that is causing these unexpected results?