Add Pentode/Triode switch to two stroke

[Paul]

I’d like to add a pentode/triode switch to the two stroke – this is on my other amp and i like the option alot.

Seems I simply need a SPDT switch to do this, and tie the center lug to pin 4 (screen grid?) and one lug to the 16uF cap (where it’s currently connected) and the other lug to the plate (pin 3).

While I’m at it does a screen grid resistor make any sense – the other amp I have is a PP 6V6 amp and it has a 1k/5w screen grid resistor in there.

Also – since I have a heavy duty DPDT on off on switch handy, any reason not to use it to add standby to the amp?

ie – switch up, pentode; switch middle, standby; switch down, triode.

Would a standby be implemented by simply using the other pole of the switch to open/close the connection from pin 8 of the rectifier to the 1st filter cap?

Thanks!

[Andy]

What is your reasoning behind the Pentode/Triode switch? Typically this is done to cut the power in about 1/2. It does change the tone a bit, maybe thats what you are after?

If you did as you were describing, you probably want to put a couple resistors in. I would have a 1.5K 1W resistor on pin 4, then a 1M resistor, in parallel with the switch lug (see crappy drawing). Then the other lug like you described.

You would be be fine to put standby here, thats what I do on our Renovo amps. Very similar to what you described. We use an On-OFF-ON NKK switch so standby breaks the connection to the Output Transformer and the Power line has to go through the 1M resistor.

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[Paul]

thanks! i’ve been rigging up various resistors between the plate and grid to see what happens but i’ll try this arrangement. How high a wattage rating on the 1M resistor? for what it’s worth my commercial amp (which uses exactly the same types of output tubes in cathode bias) uses a 1.5k resistor as you do but uses a 5w (and does not have the 1M).

Basically, even with a 6k6 (and only into 1 8″ speakers) this amp is alot louder than I was hoping for – and on my other amp (a commercial one, but not a mile away from a PP version of the two stroke) there is a pentode/triode/standby switch and I really like the triode mode because of the volume drop and earlier breakup. I find it a little darker but this can always be compensated with the tone knob.

Also, i like having things to tweak on the amp.

thank you i’ll try it once I can get some resistors and report back.

[Paul]

How high a wattage rating on that 1M resistor? 1W also?

[Andy]

1W should be fine. In my production amps, I use 2-3W that I get from Mouser.com which is a great source BTW.

[Paul]

works great! a nice volume drop and the standby is a handy feature

I used the 1.5k and 1w resistor but no 1m.

thanks!

[Rohbiwan]

This sounds like an interesting mod to me, if you don’t mind, could you give me a better idea of the effect on tone? And what is your estimate of volume drop?

Thanks,

Bill

[Paul]

It is an interesting mod and worth it if you’re the type who wants to have as many reasonable options as possible with this amp.

It’s just like many other amps out there nowadays with such a switch – the volume drop is no 50%, but more like ~30% ish. Breakup comes earlier. Tone is a little bit darker, you can compensate with turning up the tone. It is not a perfect “attenuation” per se, the tone is a bit more compressed and not as punchy but I find it a great way to lower the volume for practical reasons.

You probably wouldn’t gig or record with this amp in that setting. But I’m not a gigging or recording guitarist and appreciate the volume drop and means of changing up the sound sometimes.

Interestingly, i get more hum with the amp in the triode position.

My other amp, a VHT special 12/20 (a really nice amp by the way) has such a switch and I get less hum with that amp in triode than pentode. but’s it’s a push/pull and I don’t know enough about amps to know why that would be.

stay tuned for alot more posts from me about modifications i’ve done to this amp…

[Author]

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