Stiffening a power supply

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Re: Stiffening a power supply

Post by RectifiedAmps »

Lowering the sag resistors will not have a huge effect on preamp voltages at idle. The reduction in voltage is proportional to how much current the power amp is sucking through them - at lower volumes it’ll be small but once you start to overdrive the power amp it’ll have a more noticeable effect. This is why sag is often described as a type of compression.

I’d be careful with changing a lot of different components, which could get you even further from where you want to be and introduce other undesirable issues. If reducing the sag resistors (or tone caps) has cut the bass and given it too much clean headroom, then increase them a bit until you find the sweet spot. 100r will probably be a good balance I bet. You could probably get away with 5w ones since theyll be dissipating less than the stock values.

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Re: Stiffening a power supply

Post by Reg18 »

RectifiedAmps wrote: Sun Jun 04, 2023 11:39 am Lowering the sag resistors will not have a huge effect on preamp voltages at idle. The reduction in voltage is proportional to how much current the power amp is sucking through them - at lower volumes it’ll be small but once you start to overdrive the power amp it’ll have a more noticeable effect. This is why sag is often described as a type of compression.

I’d be careful with changing a lot of different components, which could get you even further from where you want to be and introduce other undesirable issues. If reducing the sag resistors (or tone caps) has cut the bass and given it too much clean headroom, then increase them a bit until you find the sweet spot. 100r will probably be a good balance I bet. You could probably get away with 5w ones since theyll be dissipating less than the stock values.
I think bass isn’t actually much different, it just has more headroom as you say so it’s louder at the point it starts to saturate and fill out. Any other advice on a MV installed across R25? Is this a single pot version, as I’ve seen ones that require a dual gang pot.

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Re: Stiffening a power supply

Post by RectifiedAmps »

You can’t use R25 as a MV - at least not easily. It has voltage across it originating from the top of the 56k cathode resistor and acts to establish the grid biasing. It would only have one gain stage between it and the previous volume control, so it probably wouldn’t get you where you wanted to be (and it’s not a PPIMV).

A good PPIMV option here might be a Vox-style Cut control but with the cap as a 1uf. Instead of acting as a tone control, itd cancel all frequencies out so effectively act as a master vol and it shouldn’t interact with the tremolo.

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Re: Stiffening a power supply

Post by Reg18 »

RectifiedAmps wrote: Sun Jun 04, 2023 5:33 pm You can’t use R25 as a MV - at least not easily. It has voltage across it originating from the top of the 56k cathode resistor and acts to establish the grid biasing. It would only have one gain stage between it and the previous volume control, so it probably wouldn’t get you where you wanted to be (and it’s not a PPIMV).

A good PPIMV option here might be a Vox-style Cut control but with the cap as a 1uf. Instead of acting as a tone control, itd cancel all frequencies out so effectively act as a master vol and it shouldn’t interact with the tremolo.
That actually looks virtually like the Trainwreak type 3 MV?
Based on a quick google on this site and looking at the schematic, are the bits where I’ve circled the phase inverter outputs?

https://robrobinette.com/Voicing_an_Amp ... ut_Control
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Re: Stiffening a power supply

Post by sizzlingbadger »

Sorry I meant from the top of R25 to ground and feeding the grid from the wiper, it will only limit the signal going into the PI, may be enough for your needs, depends how much the PI is overdriving. Then again you could just try a pot across the PI output (after the coupling caps) and see if it effects the tremolo or not. Vox and Marshall do just that with reasonable results.
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Re: Stiffening a power supply

Post by RectifiedAmps »

Reg18 wrote: Sun Jun 04, 2023 6:54 pm That actually looks virtually like the Trainwreak type 3 MV?
Based on a quick google on this site and looking at the schematic, are the bits where I’ve circled the phase inverter outputs?
Yep, actually that would work quite well. I had something else in mind but the trainwreck option is simpler.

And yes, then circled spots would be suitable parts of the PI output for this.

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Re: Stiffening a power supply

Post by sizzlingbadger »

You could try a Lar-Mar using R32/R33, would probably work fine with the tremolo too.
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Re: Stiffening a power supply

Post by Reg18 »

Ok so looks like I might need some more advice so I don’t kill my self by accident.
I attached a 1m pot, lug 1 to R32 and lug 2 to R33 with Lug 3 not connected.
Just tacked on to test at this point with the pot loose not mounted to the chassis.
When I grabbed the pot it gave me a shock with the amp on, is that because it needs to be attached to the chassis to earth it or have I wired it wrong?
Circled is the end of the resistors I attached to.
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Re: Stiffening a power supply

Post by RectifiedAmps »

Sounds like you’re connecting to the wrong side of C1 and C3! Do you have a multimeter? Always a good idea to test for voltage before you do anything.

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Re: Stiffening a power supply

Post by Reg18 »

Ok so I sort of figured it out, the pot was resting on another “live” component on the pcb which gave me a decent shock.
And I didn’t have the pot quite wired correctly either, it need pins 1 and 2 going to R32/R33 and pins 2/3 connected together to work,
The 1m pot reduces most of the volume in the last 10% of the rotation so I might try a 250k audio pot.

Update: it works better with a 250k pot in terms of rotation but I’d say the sound still doesn’t “fill out” untill the volume is up a bit.
Wondering if I should try the Dual gang pot Type 2 version?
Or would that basically achieve the same thing at low volumes?

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Re: Stiffening a power supply

Post by jeremyb »

Step power supply what are you doing??
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Re: Stiffening a power supply

Post by Reg18 »

I ended up trying the Vox tone cut while I was there as it just required adding a cap on the type 3 MV setup.
I tried there suggested 4.7nf and tried a couple of other ones as well, I actually loved the sound with a 100nf cap with the tone cut on full so I’ve hardwired it in and not going to bother with a MV. It’s filled out the lower mid range and made the whole amp more to my liking.
I guess it’s changing the ratty, fizzy saggy quirks of the original amp which is some of the appeal, but it sounds like it’s not messing around anymore! Tight and robust with some more low mids. I’ll try and do another demo if I get time this week.

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Re: Stiffening a power supply

Post by Reg18 »

Used the new and improved amp to demo my latest FET based boost pedal. And if someone can point me in the direction of how to use the YouTube link via a ph I’d love to know, I’m sure I’m doing it exactly how I used to but it doesn’t seem to work anymore.
Last edited by Reg18 on Sun Jun 11, 2023 8:29 pm, edited 2 times in total.

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Re: Stiffening a power supply

Post by Bg »

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With =

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Without =
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