Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: McAbe on December 21, 2022, 08:33:50 pm
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Hi All! Got my 6v6 plexi running, so now I am into the tweaking phase. I am using a lower voltage winding on my PT, so I only have a B+ of 345. I adjusted the power dropping resistor between the screen and phase inverter to compensate. My voltages on the plates of the phase inverter and preamp tubes are 250/210/200. Which got me thinking about the ideal preamp voltages for a plexi circuit. The sluckey/hoffman plexi 6v6 has voltages in the preamp around 270/230/215, but it looks like real plexis can be as high as 320/275/260.
Has anyone here experimented with preamp voltages in a plexi? What do you all like? Also how small of a resistor can I use between the screens and the phase inverter before it has a negative effect on the filtering? Right now I have a 15K resistor in there, but to get the higher voltages I would have to drop that to around 4K.
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A choke can provide better filtering in the B+ line with less resistance than a resistor, thus less voltage drop ie keeping voltages higher.
If I am reading your post correctly though, it seems like you could do some very simple hands on research though by switching to the higher voltage winding on your PT.
I’d imagine though in a plexi that I’d enjoy slight higher preamp voltages but without raising power tube voltage too high for volume sake. Maybe that’s where you’re headed too. Always a dance.
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The high voltage winding was giving me a B+ of 460 which was too much. I tried dropping voltage with zeners and series resistors, but it was too much. In the end, it seemed easier to go with the low voltage winding.
What I am looking at is changing the value of the voltage dropping resistor between the filter caps to change the preamp voltage. I am fine with the lower B+. Had to change the bias resistor, but I got it biased to 10 watts per 6v6 tube.
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Are you using a tube rectifier?
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I am using a full bridge rectifier which gets me a B+ of 345 with tubes installed and biased. I already have a choke between the first and second filter cap, so the screen voltage is about 340 or so.
How high of a voltage can I realistically aim for on the phase inverter and preamp? What do you all prefer in a plexi?
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What value HT cap are you using at each supply node?
From that, it’s probably a good rule of thumb for the minimum resistor value between them to form LPFs that have a significantly lower corner frequency than the high pass filters in the signal path.
Ideally, aim for at least a (frequency) decade between them.
The HT voltage at the V3 LTP supply node just needs to be sufficient to ensure that the max clean output from the LTP can drive the output valves to Vg1=0.
However, in regard of bias shift (leading to blocking distortion), it’s not good to have significantly more clean output voltage than is necessary.
So the dropper between the screen grid node and V3 LTP node should be sized with that in mind.
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How high of a voltage can I realistically aim for on the phase inverter and preamp? What do you all prefer in a plexi?
Marshall did not put voltages on their schematics (at least the ones I have). But Fender AB763 Twin Reverb shows 450V for PI node and 410V for the preamp node.
I was not locked in on any particular voltage as long as it was typical for 6V6 operation. Anything between 350 and 425 would have been OK. The old Hammond PT I used put my B+ at 400V (loaded) at the first node and that made me very happy. I didn't even bother with the other node voltages. I just copied the original Marshall dropping resistor values (20K, 10K, 10K) and never looked back and didn't experiment with different voltage values.
I think you may be a bit too focused on proper voltage. Marshall didn't provide any information and there really is no standard established for a Plexi 6V6. If you like the way the amp sounds I would say you're done. If you're not satisfied, then experiment (including different PT) until you are happy. If still no joy, maybe the Plexi 6V6 isn't the right amp for you.
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Serviced a few Marshall 1959 Super Lead 100 Plexis (including the Handwired vintage reissue ones that came out around 2008. The plate voltage on the EL34 quads in those is typically between 500 to 510V. That would kill a 6V6GT, and many EL34s besides). But a JJ6V6S can handle 440 or so
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Yes, I think the particular preamp anode voltages aren’t that critical. Bear in mind that bogey valves are few and far between, in the real world different valves draw different currents, so anode voltages will differ, hence the +/-20% note on voltages with Fender schematics.
And these Marshalls are just a variant of a tweed Bassman.
(https://el34world.com/charts/Schematics/files/Marshall/Marshall_ampchart.gif)
I think that the above is a 3rd party compilation of voltage charts from Marshall service info sometime in the 70s.
eg
(https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45tr.gif)
and
https://el34world.com/charts/Schematics/files/Marshall/Marshall_1959mkii_volt_chrt.pdf
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Thanks all. Sounds like I should just experiment a little and report back. Based on nothing, my assumption was that the voltage on the preamp would have a larger effect on tone than the voltage on the power tubes (assuming they are biased to the same level).
Not sure I understand all the talk about the relationship between the filter caps and resistor values and the filters they form. Any reading you all can suggest?
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Thanks all. Sounds like I should just experiment a little and report back. Based on nothing, my assumption was that the voltage on the preamp would have a larger effect on tone than the voltage on the power tubes (assuming they are biased to the same level).
Not sure I understand all the talk about the relationship between the filter caps and resistor values and the filters they form. Any reading you all can suggest?
https://www.valvewizard.co.uk/smoothing.html
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Here are my 6V6 Plexi voltages. I'm totally happy with how mine sounds.
You could increase the B+ by 10% which would give you around ~418V on the plates, and still be totally fine. But, I like the way lower voltages sound.
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Thanks for that. Your preamp voltages are basically exactly what hoffman and sluckey show on their schematics. My voltages are a bit lower, and from what I have seen "real" plexis can have much higher voltages in the preamp.
My amp is open on the bench, so easy enough to clip in parallel resistors to increase the preamp voltage and see how it sounds. Amp sounds great, but I am still interested in experimenting before I close it up.
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FWIW, in my amp with a B+ around 340, I ended up replacing the 22k power resistor with a 5.6k resistor. It was not a major change, but I preferred the higher voltages on the preamp. My voltages ended up a bit higher than the sluckey/ Hoffman design, but lower than what some of the real plexis show. YMMV but it is really easy to experiment with preamp voltages by clipping in parallel resistors, so I recommend experimenting.
This amp really shines with the master volume cranked up to 10 though, so the preamp distortion is not all that important.
On a related note, my next project should probably be to build an attenuator of some kind. Any designs/schematics you can recommend?
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Here’s a layout similar to Dr Z’s airbrake attenuatir, designed by Ken Fisher of trainwreck.
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My last build was 1K resistors across the board, each node, from plate to V1 and sounds great.
Plate 360V
V1 285V
I personally try to be in that 280V on V1, but have amps with wildly varying voltages.
One of my favorite amps currently has 339V on V1... And 450V on 6v6s
Voltages are just one part of the tone, but an important part.
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Voltages are just one part of the tone, but an important part.
I tend to like higher voltages on the preamp as well, but lower voltages can give a warmer sound with earlier breakup. Definitely, worth experimenting with.
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Here’s a layout similar to Dr Z’s airbrake attenuator, designed by Ken Fisher of trainwreck.
This looks really interesting. Has anyone compared an airbrake to an attenuator with a reactive load. There is an interesting project on the marshallforum: https://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/
Anyone have experience with these?