Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: _Steve on January 21, 2022, 12:07:13 pm
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Hi!
Im thinking of building a parts-bin special, Princeton-esque combo amp with both SE KT66 and PP 6V6S output stages. Both stages will have their own OT and separate speaker mounted together (in phase) in the same cab. If this is just a dumb idea feel free to let me know now!
I'd especially appreciate input on the power supply. I've drawn up what I think might work below - Im not sure about the order of nodes however. Ive never built an SE amp before so im sailing in the dark a little here. My main concerns are around hum, but also having the PP stage sag up-and-down the SE stage. My understanding is that SE amps are usually fairly constant/stiff so im not sure if this will be a big deal in this configuration or not? I did consider maybe regulating the SE stage but I dont have a lot of extra voltage to play with there and also the preamp will be sagging up and down with the PP stage anyway.
Thoughts and suggestions appreciated!
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I never think something like that
From what I read the use of the SE & PP sections will be in parallel (at the same time)
so, as you say, SAG of the PP will "affect" the SE section, are we really sure this will come out as a bad thing ? May be this will give some more chime to your amp (or may not)
Is possible to use a single PT with two separated supply lines, one rated for the SE and one for the PP
To separate the two different lines as the current drawn from a section don't steal current from the other section may be the use of diodes will be adequate but the safest method would be to have two separated PT each one with his PS circuit
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Personally I would first try with a single OT and doubled PS for SE and PP sections, the second option will be to use one OT with two separate different PS using diodes, use two separate OT and two PS, for convenience and weight reasons, it will be my last choice
Franco
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I, like many have tried to get creative.
I wind up ripping it apart and starting over.
You'll be further ahead just building one or the other if part count/supply is an issue.
then when parts/money gets freed up, build the 2nd, jack them together as desired for tonal bliss, or not.
induced hum in SE is an issue, so good PS regulation helps, but lead dress, ground layout, ALL contribute to a quiet hum free SE.
an SE with equivalent PA tubes to PP can't keep up in a volume sense. So if the PP is 6V6, you're better off with an SE using 6L6, '34
or a KT 88
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Did you know you can make a PP amp into an 'SE' amp, by pulling one of the output tubes. (Of course to be 'optimal' you'll want to change the reflected load etc and the PP OT has no air gap etc, but those are all mere details, and it 'can be done')
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Thanks for the comments and suggestions. Any thoughts on whether the order of nodes in the diagram is optimal?
To Franco, thanks for the consideration! I wonder though if going down some of those routes might be a folly as the preamp would still be sagging with the PP stage anyway? Its encouraging that you think having the SE stage sagging might not be a problem, so im inclined to just give it a try to find out! :)
A couple of clarifications:
- Yes the plan is to run both stages simultaneously. Kind of a 'best-of-both-worlds' idea. Obviously could end up worst-of-both :)
- Im hoping to run everything from a single PT
- PP stage is 6V6S and SE stage is KT66. I plan to run the PP stage through a less efficient 10" speaker and the KT66 through a 99dB 12" to try to even up the power imbalance and when all else fails I can hopefully fall back on the individual master volumes
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If you don't want the preamp B+ to sag then put a diode between the PI node and the preamp node.
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I think what im awkwardly saying is I dont know whether sag is a problem or not for an SE output stage and the potential fixes might be worse than just letting it sag :)
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SE suffers from 2 main things;
it's a Class A design in an A/B world. Some SE's will produce great tone when bent, others not so much. There are no easy answers to which version you wind up with.
the 2nd issue is hum, there is no natural cancellation in the PA stage that you get with a PP design. The usual answer is a stiffer PS, reducing the ripple to the PA stage. Design layout, grounding also play a big part.
Anyone can make a happy PP amp, SE takes a special kind of __________ :icon_biggrin:
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I think what im awkwardly saying is I dont know whether sag is a problem or not for an SE output stage and the potential fixes might be worse than just letting it sag :)
BTW there is a way to add something similar to SAG on SE amps
Put a large resistor (as resistance value) in series with standard G2 resistor
in parallel with this large resistor put a switch
When switch is on (closed) you have normal operation
When switch is off (open) you have "SAG" effect also if you are on an SE circuit
Franco
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SE suffers from 2 main things;
it's a Class A design in an A/B world. Some SE's will produce great tone when bent, others not so much. There are no easy answers to which version you wind up with.
the 2nd issue is hum, there is no natural cancellation in the PA stage that you get with a PP design. The usual answer is a stiffer PS, reducing the ripple to the PA stage. Design layout, grounding also play a big part.
Anyone can make a happy PP amp, SE takes a special kind of __________ :icon_biggrin:
So in the crude diagram in my first post I proposed feeding the SE plate after both a CLC AND an RC filter - do you think this will be sufficient for B+ hum in the SE stage?
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Design layout, grounding also play a big part.
As shooter said.
I built an SE AC4 in a repurposed Frontman 15 cabinet. Because of the location of the speaker magnet in such small cabinet I had to put the PT on one end of the chassis and the OT on the other end. As a result, the OT is directly behind the EF86. With the volume all the way down I can hear some hum. At some point I rebuilt the power supply and increased the filter caps (as much as the EZ81 will allow). But, I finally decided that without a larger chassis/cabinet and the ability to move the transformers around, it's just going to hum.
I've never done this, but there are threads on this forum that explain how to hook the PT up to your wall power and then physically move and rotate your disconnected OT and choke around on the chassis until you achieve the lowest amount of hum. Obviously this involves a live PT and is potentially dangerous without the utmost care and attention.
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So in the crude diagram in my first post I proposed feeding the SE plate after both a CLC AND an RC filter - do you think this will be sufficient for B+ hum in the SE stage?
Yes.
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With the volume all the way down I can hear some hum.
Not so unusual on SE amps
Franco