Thanks Craig, I'm new to recording. Definitely have a lot to learn.
CraigGa wrote: ↑Fri Jun 18, 2021 8:59 am
I just noticed that you have 0.022uF across the tone switch when all the previous drawings have 250pF, that's a big difference.
Oh, you're right... That's an error, nice catch. I used 250pf as well.
CraigGa wrote: ↑Fri Jun 18, 2021 8:42 am
What difference does the split load resistor make? would a 10k/15k switch be useful?
It determines the amount of pre-amp gain/distortion that makes it through to the power section. Big enough changes have a huge effect on the feel and sound of the amp. I initially thought that resistor may be 1K6 based on that picture, for example, but there was so little gain that a strat could barely overdrive the power section. It's worth experimenting with when you build yours. A 10/15k switch would get you more clean headroom in the 10K setting and shift the overdrive character to more power tube and less pre-amp distortion.
I'm not sure exactly where this version of the amp fits into the big picture. Based on posts Chris Benson made in other forums, it seems like he's changed the design a bit over the years. For all I know this version was a one off with a specific design goal. In my case, I was curious about the signal bleed we noticed with the volume off and I felt like my build was lacking gain compared to demos online. The signal bleed is reduced with this version. It's still there but barely audible. The amp feels and sounds more lively without losing the harmonics or changing the way the low end holds together. I also really like the 0.02uf caps in the PI. It doesn't really take away bass, so much as better balances the high vs low end. I've had it set up like this for a while now, I don't feel the need to change anything, finally.
Mike
Last edited by mikeywoll on Fri Jun 18, 2021 3:43 pm, edited 1 time in total.
Unipotential wrote: ↑Mon Jun 14, 2021 12:22 pm
Just curious, has anyone tried Merlin's floating paraphase PI with this instead of the adjustable 10k pot? Shared Rk, second triode fed from voltage divider of 910k and 1M...
My understanding is that is more if a hi-fi stereo design, and not a very well loved design, based on some other forum discussions. A lot of the character of this amp comes from the PI distortion, I would imagine that all that negative feedback would probably kill the vibe.
Unipotential wrote: ↑Mon Jun 14, 2021 12:22 pm
Just curious, has anyone tried Merlin's floating paraphase PI with this instead of the adjustable 10k pot? Shared Rk, second triode fed from voltage divider of 910k and 1M...
My understanding is that is more if a hi-fi stereo design, and not a very well loved design, based on some other forum discussions. A lot of the character of this amp comes from the PI distortion, I would imagine that all that negative feedback would probably kill the vibe.
Mike
That would be hi-fi stereos of the 50's... Also Vox and Ampeg used the floating paraphase. I tried this using 470R//25uF shared, 910k & 1M, 2x 220k grid leaks. It might be slightly different, but no vibes killed here.
Easy enough to try and rule out oneself...
Reverb transformer is not used in this design. It has a 12DW7 CF stage that feeds an ~800 ohm Z tank.
Most of the reading is related to quirks that needed ironing out per particular build and correct component values. But I think this project is worthy of the read, as many have appeared to have success with this build/design.
Masco wrote: ↑Mon Aug 02, 2021 2:24 pm
Reverb transformer is not used in this design. It has a 12DW7 CF stage that feeds an ~800 ohm Z tank.
Most of the reading is related to quirks that needed ironing out per particular build and correct component values. But I think this project is worthy of the read, as many have appeared to have success with this build/design.
That's really neat, I'm going to get a cup of coffee and really dig into this thread. I've very intrigued.
Just got this bad boy wired up sans reverb circuit. All voltages are well within 10% of Aaron's three node schematic. Most values are on the money w/ the exception of one half of my phase inverter circuit being 10 volts low. I figured that was pretty close though.
I am having a slight issue, there's a hiss that increases as I turn up the volume. Using a jazzmaster in the middle position the amp is dead silent to about 5. It's not ridiculously loud but I'm trying to get it as quiet as possible.
I have the center tap of the heater circuit shorted to the cathode bias resistor/cap and have tapped around w/ a chopstick but have had no luck at getting it to disappear.
I didn't use shielded cable on the input but that was because I could get from the input jack to the grid on the first gain stage w/ my input resistor leads. Was this a poor assumption?
Happy to be a part of this community after so many years perusing threads!
Congrats on your build. I don't think there are any specific gotchas with this amp that would cause hiss. I would investigate the usual stuff... I apologize if you tried this stuff already.
It could still be layout/lead dress... it could be the leads on that input grid resistor. Try shielding the input or try putting the resistor right on the tube so the lead between the resistor and tube pin is as short as possible. It could be that you have another noisy part of the circuit near your input lead/jack and you are amplifying the noise there.
Components can do it... If using carbon comp resistors on the plates of your tubes, they can hiss.
It could be RFI/EMI in general, that often sounds like static or hissing... Try using shielded cable on the leads connecting to both V1 grids. Try like adding a 10K grid resistor to the second (V1b) grid (after moving the input resistor to the tube for V1a). I have had great luck with a low value (50-100pf) capacitor between the plate and cathode on each triod of V1. That shunts high frequency signals, like am/fm radio, to ground without messing with the audible range. I have had less success but usually try a similar sized cap between the input jack sleeve and the chassis. In both cases you want the leads to be as short as possible.
mikeywoll wrote: ↑Thu Aug 05, 2021 7:56 pm
It could be RFI/EMI in general, that often sounds like static or hissing...I have had great luck with a low value (50-100pf) capacitor between the plate and cathode on each triode of V1. That shunts high frequency signals, like am/fm radio, to ground without messing with the audible range.
Mikey,
Did you mean 50-100pF from grid to cathode or actually plate to cathode, as you wrote.
mikeywoll wrote: ↑Thu Aug 05, 2021 7:56 pm
It could be RFI/EMI in general, that often sounds like static or hissing...I have had great luck with a low value (50-100pf) capacitor between the plate and cathode on each triode of V1. That shunts high frequency signals, like am/fm radio, to ground without messing with the audible range.
Mikey,
Did you mean 50-100pF from grid to cathode or actually plate to cathode, as you wrote.
I use a 50pf ceramic cap from plate to cathode. I personally cannot hear any audible change with this setup, except for the lack of RFI. I live on a hill about half a mile from a radio tower serving a large city. My Amps are fm radios without these caps. Values larger than 100pf can reduce audible high frequencies and harmonics. I cannot vouch for the grid to cathode approach, but it makes sense it would work as well. I think you need to use a much smaller value in that case, and maybe in conjunction with a small grid stop resistor?
mikeywoll wrote: ↑Fri Aug 06, 2021 2:01 am
I use a 50pf ceramic cap from plate to cathode. I personally cannot hear any audible change with this setup, except for the lack of RFI. I live on a hill about half a mile from a radio tower serving a large city. My Amps are fm radios without these caps. Values larger than 100pf can reduce audible high frequencies and harmonics. I cannot vouch for the grid to cathode approach, but it makes sense it would work as well. I think you need to use a much smaller value in that case, and maybe in conjunction with a small grid stop resistor?
Ah, ok. Yes, I was wondering if you were using some small added capacitance from grid to cathode as a means to lower your series grid resistance but still get the desired HF roll off. RF can be maddening!
Colossal wrote: ↑Sun Aug 08, 2021 1:12 pm
Ah, ok. Yes, I was wondering if you were using some small added capacitance from grid to cathode as a means to lower your series grid resistance but still get the desired HF roll off. RF can be maddening!
I forget exactly where I picked this trick up... But it's often seen on cheap, mass produced tube amp schematics. If you look at the schematics they have small caps all over the input tubes to stabilize the circuit against oscillation and bleed off RFI.
Most advice I found suggested keeping the RF out of the amp in the first place, which is great advice, but doesn't address really bad situations. RFI can sneak in through pots, switches, reverb tanks, the tubes themselves.
Sorry, I was on vacation and barely got back to troubleshooting. Thanks for the suggestions.
I did the cap from plate to cathode trick. No luck.
Moved from a carbon to metal film resistor, still no luck.
I sort of made the problem worse. Much worse. But maybe that's a good thing.
I think this is a layout issue. Id find that when I'd put my hand near the tone switch there would be a lot of what I'm assuming is RF noise. As soon as I'd cover the switch and touch the chassis right at the switch the noise would disappear almost completely. I put some foil around the control switch and again touched the chassis and the noise would disappear.
I then got some alligator clips and grounded the unused top pins of the switch to the chassis and again the noise would almost disappear completely. I thought this would be great until I realized that I was grounding out the signal when I'd switch from the tone setting I was using to the other.
I thought I could just clip the unused leads of the switch to make things better but there's no joy there.
I found a wire (from the switch to the pot) that I can create noise by putting my hand near it. What I think I need to do is shorten the leads of my resistor & cap, and add shielded cabling to and from the switch/pot. This would be plate to grid cabling as Mikey suggested.
Does any of this make sense?
Heading back down the rabbit hole tomorrow. Thanks for the suggestions. Plus adding a picture as well. Be kind.
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