tightening up the trainwreck lowend

Express, Liverpool, Rocket, Dirty Little Monster, etc.

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martin manning
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Re: tightening up the trainwreck lowend

Post by martin manning »

From the above, I now think what you have is two resistors with a switch that parallels a second resistor across the standard 1k, and the resulting value with the second resistor switched in is 180 ohms, correct? That would make the second resistor 220 ohms.

The resistor there is part of an R-C filter, so it is not functioning as a choke in the way that an iron core inductor of 4H or more does. Power supply chokes are used in L-C filters to reduce 120 Hz ripple. The inductance of a wire wound resistor is only a couple of mH, so its inductive reactance at 120 Hz is only an ohm or two and therefore insignificant. While both parts have inductance and resistance, to function as a choke the inductance would have to be much more significant than the resistance at the frequency of interest.

Re the dynamic effect of screen current, I am referring to the voltage sag as the screens begin to draw more current through the resistor.
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rooster
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Re: tightening up the trainwreck lowend

Post by rooster »

Yes on the 220.

In regards to the 1K inductor vs. a trad steel core inductor in the Express circuit I certainly understand the the function of a pi filter, but seriously wonder how much improvement I would realize if I made the swap. Well, provided someone made a 1K steel core inductor large enough for the Express. And I say 'large enough' because I think core and wire size would be big factor as far as emulating the particular speed of energy transfer that Ken had in mind for the circuit. Eh, but maybe I overestimate it's construction paremeters.

However, even if such a piece were created, again, I have to say I doubt I would realize any real sonic benefit that would produce a better live play or recording experience, 60/120/240htz ripple considered. Ken had ears, least we forget.

I understand the sag description and can experience it in realtime as I play the amp at volume. But this is true using either inductor I select. When you think about it, considering the TUBE screen resistors (inductors?), using the stock 1K resistor, each tube is seeing 1.5K ohms resistance before the screen supply voltage. Using the 180 ohm resistance this number will change to 1090 ohms (a 37%(?) reduction of total screen resistance to the individual tube). I point this out because some might miss the obvious - changing the screen supply inductor does not leave the tube screen unprotected or without an opportunity for screen sag. (BTW, Marshall used 2.2K TUBE screen resistors in their 900 series amps for a similar result.)
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martin manning
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Re: tightening up the trainwreck lowend

Post by martin manning »

I guess we went around on this 6 months ago: http://ampgarage.com/forum/viewtopic.ph ... sc&start=0 I don't know why you want to go on calling resistors inductors and describing inductors by their DC resistance. You wouldn't describe the value of a capacitor by its ESR, would you?
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rooster
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Re: tightening up the trainwreck lowend

Post by rooster »

Ha, yes we did.

And of course by definition a coil of wire is an inductor because it exhibits induction behavior, even in the form of a wire wound resistor. This is why I call it an inductor. Carbon comp, metal film, or carbon film resistors do not have inductance, of course. But I am not saying that a wirewound resistor performs exactly like a typical steel core choke in a typical guitar amplifier power supply, only that it does exhibit inductance in this application - small though it might be.

I suggest - my point - that Ken made some choices when he designed his Express. He could have used a 100 ohm trad choke and a 1.5K resoistor on either power tube, right? By your description the amp would exhibit less 120hz ripple. But he didn't do this. Is the stock Express overwhelmed with 120hz ripple? Why not?

Eh, I'm not trying to be difficult, Martin. Remember it wasn't me who designed and built the Express - that guy was the 'difficult' one. 8)
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martin manning
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Re: tightening up the trainwreck lowend

Post by martin manning »

rooster wrote:Carbon comp, metal film, or carbon film resistors do not have inductance, of course.
Ah but they do! All R's, L's, and C's have some of the other two properties along for the ride. In fact the conductive path in film resistors is in the form of a coil.
rooster wrote:...He could have used a 100 ohm trad choke and a 1.5K resoistor on either power tube, right?
A 100 ohm choke? This isn't telling me much. In large inductors (several henries) the DC resistance is not the reason for being. It's almost insignificant.
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Littlewyan
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Re: tightening up the trainwreck lowend

Post by Littlewyan »

Ken could have used a choke but he wanted screen sag from using a 1K resistor and probably thought it provided good enough filtering. In the Komet Amps however he did use a choke, probably because he didn't want the same sag in those amps but knew he wouldn't get as good filtering with a 100Ohm resistor. So he used a Choke that had 100Ohm of DC resistance. Thats what I think anyway.

A 1K resistor doesn't provide anywhere near as good filtering as say a 3H Choke but its much better than a 100Ohm resistor. Plus the inductance of a resistor is negligible at the frequencies in the power supply.

From Aiken

Q: One intriguing thing about some amps is the 1K resistor used for the screen supply filter instead of an inductor. Does it really work like a small choke? How much inductance would such a resistor really have? Would this inductance be enough to significantly reduce power supply ripple (compared to e.g. a metal film resistor of the same resistance)?
A: The inductance of the resistor is negligible at the frequencies of interest in the power supply. For example, using a typical inductance of 0.04mH for a 1K wirewound resistor, you would get an inductive reactance of 0.03 ohms at 120hz. This, in conjunction with the usual 50uF filter cap, which has a reactance of 26.53 ohms at 120Hz, would give an attenuation of around -0.01dB at 120Hz (the power supply ripple is twice the 60Hz mains frequency because it is full-wave rectified).

By contrast, the 1K resistor itself, which has a reactance of 1K at 120Hz, gives an attenuation of around -32dB at 120Hz in conjunction with the 50uF capacitor, which would reduce the residual 120Hz hum by a factor of around 39 times.

Okay, you might ask, why use a choke? Well, a 6H choke would have a typical resistance of of only 100 ohms or so, so it would drop less voltage than the 1K resistor, for DC. However, for the 120Hz AC ripple riding on the DC voltage, the inductive reactance would make it look like a whopping 4.5k resistor. This 4.5K, along with the 50uF cap, would result in an attenuation of -44dB at 120Hz (ignoring the effect of the 100 ohm choke resistance, which would be negligible compared to the 4.5K reactance of the inductor, but it does help to dampen the overshoot at the corner frequency of 9.2Hz). So, by using a 6H choke, you get 12dB more attenuation of hum (a factor of 4 times less ripple) along with less voltage drop to the screens (more power output) and less preamp hum.

The only thing a large resistor buys you is screen supply sag, which you may or may not like. You can always do both - use a choke and add a largish series resistor for better ripple rejection and screen supply sag.


Also, you will get a louder amp from having less screen sag. I found out recently that Marshall's power control in a few of their amps only reduces the screen voltage and not the anode voltage. Cool eh!
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rooster
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Re: tightening up the trainwreck lowend

Post by rooster »

Littlewyan, :lol: .
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Littlewyan
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Re: tightening up the trainwreck lowend

Post by Littlewyan »

Did I completely miss your point?
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rooster
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Re: tightening up the trainwreck lowend

Post by rooster »

No, not at all. The Aiken info was most timely. :D
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Roe
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Re: tightening up the trainwreck lowend

Post by Roe »

The Express sounds bad with a standard 110ohms choke in my experience. For tightening of bass I prefer a .68uf bypass cap on the second gainstage
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Re: tightening up the trainwreck lowend

Post by RJ Guitars »

Roe wrote:The Express sounds bad with a standard 110ohms choke in my experience. For tightening of bass I prefer a .68uf bypass cap on the second gainstage
Thank you for posting this note. Seems like a good candidate for a mini switch option to make the effect audible in real time. It's worth trying since it is simple enough to do especially in comparison with adding a choke.
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Guy77
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Re: tightening up the trainwreck lowend

Post by Guy77 »

Roe wrote:The Express sounds bad with a standard 110ohms choke in my experience. For tightening of bass I prefer a .68uf bypass cap on the second gainstage
I just did this change on my Liverpool amp. I used a 1uf cap since I did not have a .68uf cap. Everything else is more or less a stock Liverpool layout (except that my first power filter is a 50uf as opposed to an 80uf and I put in a Master volume).

All I can say is wow!! Changing my second stage cap from 22uf to 1 uf tightened up my overdrive tone and the overall tone of my amp !
With and without my Master volume engaged the tone is nice and tight but not too brittle either when playing at lower volumes. I love this mod!!!

Cheers
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RJ Guitars
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Re: tightening up the trainwreck lowend

Post by RJ Guitars »

Guy77 wrote:
Roe wrote:The Express sounds bad with a standard 110ohms choke in my experience. For tightening of bass I prefer a .68uf bypass cap on the second gainstage
...I just did this change on my Liverpool amp. I used a 1uf cap since I did not have a .68uf cap... All I can say is wow!!... I love this mod!!!...
Guy
Pardon me for editing this down to the pieces that I like but I really think you are on to something.

1 - Would it be worth it to A/B it with a mini switch or is the original flavor altogether unusable?

2 - Can you tell if you had any significant loss of gain?

3 - Can you tell if there was any loss in the bottom end response?
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67plexi
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Re: tightening up the trainwreck lowend

Post by 67plexi »

I have not built an Express amp in many years looking at my notes.
B+1 417 volts
B+2 402 volts
B+3 314 volts
B+4 295 volts
B+5 280 volts
I used 9k1 on the dropping string not 10k
I like ROE .68 Idea I did that with my 150 watt SSS amp.
With the 4 Express I built tuning them with the proper 12AX7 was key
Power tubes were RFT branded
Mallory capacitors and Rubycon
Resistors Stackpole preamp board.

That said my best sounding EL34 amp has a B+1 500 volts.
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Guy77
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Re: tightening up the trainwreck lowend

Post by Guy77 »

RJ Guitars wrote:
Guy77 wrote:
Roe wrote:The Express sounds bad with a standard 110ohms choke in my experience. For tightening of bass I prefer a .68uf bypass cap on the second gainstage
...I just did this change on my Liverpool amp. I used a 1uf cap since I did not have a .68uf cap... All I can say is wow!!... I love this mod!!!...
Guy
Pardon me for editing this down to the pieces that I like but I really think you are on to something.

1 - Would it be worth it to A/B it with a mini switch or is the original flavor altogether unusable?

2 - Can you tell if you had any significant loss of gain?

3 - Can you tell if there was any loss in the bottom end response?


Hi RJ. Regarding your questions..

1. Yes I plan to setup a toggle switch this weekend . I like the idea of changing back and forth but honestly the 1 uf is more my flavour. With the 22uf and my strat it is a nice warm woody tone at lower volumes that I play at but still fairly loud. With the 1 uf it is more gnarly sounding (36 watt marshall).

2. In regards to loss of gain (volume) I will know better when I have the A/B switch setup this weekend. I will post the results.

3. I found the bottom end response was better with the 1 uf cap especially when I engaged the Master vol, the overdrive tones were much tighter!

Just to let you know I am running a Heyboer TW-35 output tranny from mojotone and my B+ is 345 on the plates.

Cheers!

Guy
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