triode/pentode switch on self-bias 6l6's

[1blueheron]

The amp is a Webster 88-24 amp, taken from an Allen organ and converted for guitar use.  Some of you probably remember.  This amp is the first amp project I did (Only with a ton of help from Sluckey and the Hoffman amp forum braintrust) and I am returning to it and going to attempt a few tweaks.

The first thing I would like to do is attempt a triode/pentode switch on the power output tubes.  I have googled 'til my eyes are about to drop out, and all the schematics and instructions I find for thsi mod, are a little different and I want to make sure I am doing this properly. 

Here is the Schematic of it as built. 

(edited to correct error as Sluckey noted below.)

[sluckey]

There cannot be a connection between pin 4 and pin 8 on the output tubes. Did I do that?   

[1blueheron]

Sluckey,

Correction posted.  I did update the power supply section to show the cap can revision, pilot light  3 wire plug and power switch.

Here is the way I am thinking the pentode triode switch should go, but not 100% sure...  Does this look reasonable?  Full being Pentode mode and half being triode mode.  I scavenged that from a Mark Huss Plexi mod but it has fixed bias rather than self bias and he had some resistors I don't so it left me some doubts. Not sure if that matters.

[tubenit]

I had done a number of pentode/triode switching amps with 6V6's and 6L6's years past.  I found the pentode/triode switch reasonably useful on the 6V6's in lowering volume and changing the tone.  My experience using the pentode/triode switch on 6L6's did not prove as useful in that it seemed to not change the volume or tone as dramatically as on 6V6's.  However, that was my personal experience and may not be applicable to other amps or situations. 


I don't use pentode/triode switching anymore at all.  I've gotten to where I prefer PPIMV master volumes .......... or in some cases using a VVR.


with respect, Tubenit

[1blueheron]

Tubenit,

Thanks for chiming in.   In the original build thread, that is where we more or less left off, with a variety of MV solutions.   I haven't ruled that out, and will likely end up there, but I thought this looks simple enough to give a shot and I am curious to hear the sound of pentode vs triode mode.

This is step one before I go too crazy.  I'm thinking there is no reason it can't stay, even if I do a PPIMV?

[sluckey]

Your switch will work fine. I would add 470Ω/3W resistors between each pin 4 and the switch. This is a switch that you should only flip when you are not playing, ie, don't start beating out your favorite power chords while someone is flipping the switch so you can get a good A/B comparison. Might get a surprise!  "wink"
 

[HotBluePlates]

... a triode/pentode switch on the power output tubes.  ...

I don't like the difference in plate & screen supply voltages (around 55v).

When the tubes are switched to triode mode, the screen voltage will rise that extra ~55v and cause plate current to rise.  It can work if you select a cathode resistor for the triode mode, then accept a cooler bias when in pentode mode.

The ~55v change probably is not insurmountable.  I ran into the same problem in an amp I made.  However, the voltage-difference in my case was closer to 100v, which made a triode/pentode switch infeasible.

[1blueheron]

So here is the proposed first modification. 

Other than being a DPDT switch, is there any special rating this switch needs? Is the 125V 6A DPDT on-on toggle in the Hoffman Parts section adequate?

[pdf64]

It’s worth noting that the schematic in post #1 seems a bit wonky, in the PT secondaries and rectifier area.
eg the smaller winding labelled 5V is shown connected to the rectifier anodes and with a CT to 0V.
Moving the 5V label to the ‘black-yellow-black’ winding would fix it.

I agree with post #7, in that this amp, with its potential divider feeding the screen grid HT node, seems a poor choice for the proposed mod.

Due to back emf when driving inductive loads (ie speakers), the switch will need to withstand several kV between its poles; no regular switch will be rated for that, though in practice, bog standard switches seem to cope fine in such applications.

[sluckey]

I would not use a mini toggle switch for this task.

[1blueheron]

It’s worth noting that the schematic in post #1 seems a bit wonky, in the PT secondaries and rectifier area.
eg the smaller winding labelled 5V is shown connected to the rectifier anodes and with a CT to 0V.
Moving the 5V label to the ‘black-yellow-black’ winding would fix it.

I agree with post #7, in that this amp, with its potential divider feeding the screen grid HT node, seems a poor choice for the proposed mod.

Due to back emf when driving inductive loads (ie speakers), the switch will need to withstand several kV between its poles; no regular switch will be rated for that, though in practice, bog standard switches seem to cope fine in such applications.

I see that mistake in my PS section.  I will correct/modify.

As for the switch...   Let's say we don't switch while the amp is operational and drawing current.   Would this be a better choice for a switch?  I am not familiar with the term "bog".

[sluckey]

Would this be a better choice for a switch?

Yes. I prefer Hoffman's LAMB ST2 switch.

[1blueheron]

Is listed as a power on/off switch.  Assume I just need to use the additional terminals to make it On-On and it will work as drawn? 

[sluckey]

Is listed as a power on/off switch.  Assume I just need to use the additional terminals to make it On-On and it will work as drawn?

This is not difficult. What else did you see in the listing? Maybe something in BOLD type? So, what do you think?   

[1blueheron]

Long bat handle   

No, seriously, I do see it listed as DPDT  and as such, I would like to think it would work but the function is listed only as On-Off.    I looked up the PDF on the model number and it appears if I want On/On I need to order a "G" series flavor rather than "E" series.  If I am being obtuse, I apologize.  If only On/Off  then why 6 terminals?

[sluckey]

Arrrgh! 

[PRR]

.... Let's say we don't switch while the amp is operational and drawing current. ...

When the amplifier is working and playing loud, there will be 600V peak between adjacent terminals. More when clipping at some frequencies. That's a lot for anything "mini".

[1blueheron]

Seems like I am swimming against the "current" here with this idea.    Since the general tide and collective wisdom seems to be in favor of a MV solution rather than a triode pentode switch on the power tubes, I think I will give up the idea and move straight to a MV instead.  Thanks to those who voiced their thoughts and suggestions.  I will start a new thread with appropriate title.

[tubeswell]

DPDT… why 6 terminals?

Double-Pole/Double-Throw. Goes On on one side when Off on the other side. And vice-versa when you throw the switch the other way. (Hence ‘On/On’) And can do that for 2 separate circuits at the same time.


The Center terminal would go to the screen. One of the outside terminals would go to the plate. The opposite  outside terminal goes to the screen High Tension supply node. Same for each set of poles.

[pdf64]

…
When the amplifier is working and playing loud, there will be 600V peak between adjacent terminals. More when clipping at some frequencies. That's a lot for anything "mini".

Perhaps a lot more 
When using a 100:1 attenuating probe, my finding is that when overdriven into an inductive load, V peak on typical output valve anodes can be well over 2kV, so maybe 5kVp-p.

[1blueheron]

DPDT… why 6 terminals?

Double-Pole/Double-Throw. Goes On on one side when Off on the other side. And vice-versa when you throw the switch the other way. (Hence ‘On/On’) And can do that for 2 separate circuits at the same time.


The Center terminal would go to the screen. One of the outside terminals would go to the plate. The opposite  outside terminal goes to the screen High Tension supply node. Same for each set of poles.

So isn't that  an "On-Off-On" DPDT (three positions) switch rather than just "On-Off"?  I understand the 6 terminals for On-Off-On but it seemed redundant if it is only an On-Off as described on the website.  I was thinking in terms of the below picture and would expect it to be like the middle switch for the purpose of the triode/pentode switch.  Also in the sense that if I was buying a power switch for simple On-Off function, I might not be thrilled to receive a 3 position switch.  Hopefully I am not just digging a deeper hole here.   

Your explanation is perfectly clear however and I appreciate the detail.

[HotBluePlates]

So isn't that  an "On-Off-On" DPDT (three positions) switch rather than just "On-Off"?  ...

Two switch positions (not three)

Flipping switch one way connects middle lug to one outer contact.  ON.

Flipping switch the other way connects middle lug to the other outer contact.  ON.

So the switch is ON-ON.

"Double Pole" means there are 2 circuits that are switched.

Yes, you could use a DPST switch (which would have 2 circuits, 4 lugs, and be ON-OFF).

[1blueheron]

 

So, If I am understanding correctly, the switch that Hoffman is selling and is listed as DPDT "On-Off" switch, is actually an On-On switch which can be used as On-Off, by using the center contact as line and one side of the outer contacts as load or if you want to switch both line and neutral, using the center contacts and the same pair of outer contacts and leave the outer contacts on the other side as unused.   

For my triode pentode switch application, I would use all 6 contacts as an "On-On".

[1blueheron]

I have given up the idea of switching between pentode and triode mode.  However, I am still curious as to what the amp would sound like in triode mode.  Rather than install a switch, what would the best practice be for just using the 6L6 tubes as triode only?   Can i just remove the B node  power connection and run a 470ohm 3W resistor between pins 3&4?   Strap pins 3&4 with no 470Ω resistor?

[mresistor]

I would do it like the drawing with the resistors.

[HotBluePlates]

... I am still curious as to what the amp would sound like in triode mode.  ...

Cleaner, darker.  For my preferences, it sounds "boring."