July 25, 2013 at 2:51 am #5158
I put a pentode/triode switch in my two stroke, but I never use it because the triode mode results in a significant idle hum…and in pentode mode I hear virtually no hum at all.
I never understood why until I came across this from the aikenamps website:
Q: Some time ago I purchased an amp from a well-known boutique amplifier maker. This expensive amp has “new/old” switch on the back. The manual explains that the “new” mode corresponds to the pentode mode and the “old” to the triode one. The trouble is that in the “old” triode mode the amp produces ghost- notes that are out of pitch! Hearing out-of-pitch after-sounds really confuses my playing. The manufacturer says “that’s normal” and points to the Matchless brand that “does the same”… I’d be curious to hear Randall’s opinion on this subject.
A: In general, triode mode is much more sensitive to insufficiently-filtered power supplies (which is what causes most “ghost-noting”) than pentode mode. This is because in pentode mode, the screen grid supply is usually taken off from a point after a filter choke/filter capacitor. The plate supply is usually taken off from the first filter cap, which has a lot more ripple voltage on it. This is okay in pentode mode, because, in a pentode, the screen grid voltage has far more control over the plate current than the plate element does, so the ripple/hum/noise on the plate supply doesn’t get into the output. Also, there is some inherent power supply rejection due to the push-pull arrangement.
When you switch to triode, mode, what you are doing is moving the screen grid connection from the nice, quiet, filtered point and connecting it to the plate of the output tube. Now, there is no relatively stabilized screen supply to keep the plate current quiet, and the triode-mode tube will respond to the under-filtered supply.
The solution to your triode mode problem is more filtering in the power supply.
Ok so I am thinking about adding more filtering. My amp has the 40/16/8 filter caps per the standard schematic.
1) Is this a reasonable idea or will adding more filtering mess with a good thing?
2) If yes, where and how much? Could I simply throw another cap across one of the existing caps in parallel? Swap one or more out for a bigger value? It would seem to me that I’d need to increase the 40uF cap since that is where the plate supply is coming from? And that tweaking the downstream caps wouldn’t do anything further?
Thanks for any thoughts.July 25, 2013 at 4:11 am #5912
Can anyone tell what this is doing? ie, what are resistors, a mosfet, etc doing to help filter the power supply?July 25, 2013 at 5:32 pm #5913RobinParticipant
Near the back of Merlin Blencowe ‘s ” designing power supplies for tube amplifiers” there is an in depth explanation of the mosfet circuit voltage control….London calls it power scaling, 65 amps calls it Master Voltage, etc. An easier method of output power reduction would be to use a simple attenuator. 6db attenuation is about half power, while 12db attenuation would be half of that. The advantage with voltage control or an attenuator over the pentode/ triode switch is that you don’t have the noise and tone shift, just less output. Gerald Weber has a simple DIY 6/12db attenuator chapter in his book “All about vacuum tube guitar amplifiers”. Also 6db & 6/12db versions of his designs were published in Vintage Guitar Magazine a while ago (like maybe 2006-07, I’m guessing).
You could add more filtering as you’ve suggested by putting another cap in parallel with the 40uf cap and see if it does anything for you. 40uf is already plenty of filtering though, many TwoStrokes are using 30uf or even 20uf in that location. I’ve never heard a “triode” mode that sounded very musical to me.July 25, 2013 at 8:59 pm #5914
Thank you Robin.
The london power link was not to their power scaling circuit but rather to a power supply filtering circuit that includes (From what I can see in the photo) a mosfet, some other transistor, 3 resistors, and 2 (22uF 450V)capacitors.
I’m curious what this circuit is doing that a filter cap would not…they claim that this product will make a single ended amp as quiet as a PP and also quiet triode mode hum.
I already have a resistive 3/6db attenuator on my two stroke that I pretty much have engaged all the time. Even with all that and only an 8″ speaker my two stroke is quite loud (did I mention also with only a 6k6 power tube?!) when cranked up and while the triode mode is not as great as pentode mode I do not personally find it “unmusical,” rather just a little damped in the highs but much of that can be adjusted with the tone knob.
I’m just interested in any simple things I can do to make that triode switch useful since I put it there – the hum is bothersome enough that I don’t use it but would gladly do so to reduce volume if it didn’t hum so much.
I am not gigging with this amp as you can probably tell, and I think the amp sounds great no matter what mode/attenuation I put on it.July 26, 2013 at 7:01 pm #5915RobinParticipant
I’ve not built a Two Stroke with the Pentode/Triode switch but here are some ideas.
Assuming that you’ve optimized the 6.3v filament circuit with the techniques outlined here: http://www.valvewizard.co.uk/heater.html
then check out this page:http://diyaudioprojects.com/mirror/members.aol.com/sbench/humbal.html.
Note that if your PT has a center tap for the 6.3v, you could not add an artificial center tap. If you do have an artificial center tap, using resistors that are matched to the same value will maximize the humbucking benefit (or use a balance pot instead).
Although SE amps have a reputation of being noisy, the Two Stoke can be a VERY quiet circuit. If your amp has any hum in pentode mode (other than a very slight hum from the PT), you should resolve it and see if it reduces the triode mode noise issue.
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