Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Papa Jim on November 28, 2016, 04:43:45 pm
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I did the attached schematic before I do this modification. Not sure if this is the correct way to wire in this Master Volume or not. The biggest question is whether the amps existing 1M Grid leakage resistors get removed or not. The original installation schematic did not make this very clear. This schematic I did is an install for a Fender 65 DRRI
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That's all wrong. Never seen a MV on the input to the PI. If you want a pre PI master look at this...
http://sluckeyamps.com/tdr/tdr.pdf (http://sluckeyamps.com/tdr/tdr.pdf)
Or, if you want a post PI master, search for Lar Mar master volume. It uses a dual pot that is connected to the grids of the output tubes. This is a popular MV.
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I've used something similar (minus R49 and R52) on several builds and had no issues. I'm not sure if the effect is the same as the one Steve posted however.
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Yeah, one of my friends that helps me out came to that conclusion that R49 and R52 have to go and relayed that info to me after posting here. The original install schematic from the manufacturer who sells this MV was very badly drawn and kinda deceiving about the resistors.
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Well, that's a new one on me. Learned something new today. Which manufacturer sells this? Got a link?
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http://www.londonpower.com/all-kits/london-power-master-volume (http://www.londonpower.com/all-kits/london-power-master-volume)
Steve, I don't know if all is true until I install it probably by Wednesday. I'll reply again on it then. I really do hope it does all it say's. Who knows. At least I won't be out much other than a little time to install.
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Attached is the drawing with the resistors removed.
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I've used something similar (minus R49 and R52) on several builds and had no issues.
How similar?
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That circuit is explained in TUT-4. Supposedly it's very transparent, no change in tone like some of the more common MVs. Got me curious to try. What value pots are you using? Please let us know how you like it.
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1 Meg
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I am the good friend of rolloman. lol He is a very kind and humble fellow. I like him very much. He is my brother in Christ.
The pot is also dual gang (1M audio dual gang).
I believe the 1M Phase Inverter grid leak resistors (R49 and R52 in the 65 DRRI) must be removed so the dual gang pot can become the new grid leaks. This keeps the bass cutoff frequency the same as the stock cutoff frequency, if you leave the stock .001uF C25 and .01uF C26 (65 DRRI) cap values. With the pot on 10 the pot shouldn't affect the tone at all. Except for the affect of the 10K grid stoppers. And their affect should be beneficial.
I haven't tried this MV yet, but I am very interested in doing so because I have to reduce the volume in my amp (tinnitus is terrible). This is also the MV that comes in the London Power SV1 power scaling kit. I will be installing the full power scaling eventually.
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I tried this a few years back and i don't remember much about it other than i never kept it and went back to per Pre PIMV. So i know it didn't work well for me or i'd have kept using it.
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What the idea behind the bottom pot? At first I thought, the idea was to attenuate the NFB input via C26 AS primary signal input is attenuated with the top pot (which would be a nuance at best), but the NFB signal also "lives" between R50 and R51 at the same amplitude and phase that it does at the input to C26, so it would seem to me that the lower pot doesn't have a virtual ground as far as the NFB AC signal is concerned, so it doesn't seem like the lower pot would do anything significant.
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If you have KOC's TUT-4 turn to page 5-7 for his explanation of this circuit. It's his baby. Not sure if I really believe it is superior, but I do want to hear it for myself.
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What the idea behind the bottom pot? At first I thought, the idea was to attenuate the NFB input via C26 AS primary signal input is attenuated with the top pot (which would be a nuance at best), but the NFB signal also "lives" between R50 and R51 at the same amplitude and phase that it does at the input to C26, so it would seem to me that the lower pot doesn't have a virtual ground as far as the NFB AC signal is concerned, so it doesn't seem like the lower pot would do anything significant.
That's interesting. If I didn't have 3 projects on the go I'd do a Q&D to check it out.
How similar?
As above. I found the implementation to be very transparent, not changing the sound of the amp at all, or very little.
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Well, the install is done. Works perfectly, and transparent as advertised. Now I can dime the volume instead of being stuck in 2nd gear like before. Got me exactly what I wanted for an indoor amp now.
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Thanks for the report.
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Well, the install is done. Works perfectly, and transparent as advertised. Now I can dime the volume instead of being stuck in 2nd gear like before. Got me exactly what I wanted for an indoor amp now.
What is it exactly you find about it that's better? Not sure i understand what you mean.
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Well, the install is done. Works perfectly, and transparent as advertised. Now I can dime the volume instead of being stuck in 2nd gear like before. Got me exactly what I wanted for an indoor amp now.
What is it exactly you find about it that's better? Not sure i understand what you mean.
Well before there was no way I could stand the volume of the amp on anything above 2 on the regular volume knob. At that setting the amp was only a very clean sounding. To get any overdrive or distortion at that same volume level it required me to use a pedal which added in their own individual tone depending on the pedal. Now I am able to get rid of the pedals, crank the volume knob on the amp up all the way to 10 if I want, then turn the LP Master Volume to get back to the volume I could stand before when the volume pot was on 2. The advantage is now I can get the raw overdriven sound of the amp naturally at lower volumes without having to use any pedals that can influence the tone. Until today I have never heard my amp sound so good tone wise to my ears at that low volume.
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Ahh, ok, i was thinking you were comparing it to a different MV you had b4 this one, not a non MV scenario.
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Hi All,
I've implemented this MV in my Dual Marshall build. It sounds indeed very transparent. I've put a dual 1 M audio pot in parallel with the 1 M resistors, thus resulting in 500K to lower the resitor hiss and added the 10K grid resistors.
The thing is that the MV becomes very loud very quickly. Is there a way to get more of a gradual sweep out of the MV?
With regards,
Auke
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Hi All,
I've implemented this MV in my Dual Marshall build. It sounds indeed very transparent. I've put a dual 1 M audio pot in parallel with the 1 M resistors, thus resulting in 500K to lower the resitor hiss and added the 10K grid resistors.
The thing is that the MV becomes very loud very quickly. Is there a way to get more of a gradual sweep out of the MV?
With regards,
Auke
Do as I did, and remove the 1M resistors. I believe this was the intended way London Power intended it to be wired..., although the drawing they send is deceiving, and the gap where the shielded cable passes over the 1M resistors is meant to be a cut in the wiring. With the resistors in place you have less total resistance so the pots sweep is closer to 500K.
Maybe sluckey can verify the circuit which he found in London Powers TUT-4 Book.
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The circuit in the book is the same as the one you drew without the original grid leak resistors. However, KOC says "In a modification, one could leave the existing grid leaks in place. In a new build, one could go either way, adding the grid leaks or leaving them out. In either case, it is beneficial to add identical grid stops to both grids; 10K is a good starting point."
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I replaced the dual 1M audio pot with a linear 1 M dual pot. This change gives me more control over the mv. I'm very pleased with the result.
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I replaced the dual 1M audio pot with a linear 1 M dual pot. This change gives me more control over the mv. I'm very pleased with the result.
I'm very surprised at that.
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For a control to give mostly 10%-100%, linear may be best.
If you want few% to 100%, Audio taper is needed to get down to the very small levels.
On say a 50W amp, 10% voltage is 0.5W output. So the 10%-100% range of linear may cover all needs.
2% voltage on a 50W amp calculates to 0.02 Watts. It does seem unlikely you would want that much downside, or the compromise with the 5W-50W zone being scrunched-up into 8-10 on the dial.
If it comes up VERY fast in the 8-10 dial zone, you just got a crap "audio" pot. Sadly good ones are really rare.
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One thing to consider, if you leave the stock 1M grid leaks in parallel with the 1M pot to achieve 500K grid leaks, is the bass cutoff frequency. If the amp does not have NFB you may increase the bass cutoff over guitar frequencies by putting the pot in parallel with the grid leaks. This is only a concern if the stock caps are small like the DRRI caps. The DRRI has a lot of NFB, so this is not a problem. Unless you disconnect or reduce the NFB. Then it would be a problem.
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Resurrecting this thread... I am very interested in this master volume schematic. KOC claims that this is neither a pre-phase inverter or PPIMV master volume. I am curious about the effect of this master on negative feedback. I know George Metropolous has gone away from PPIMVs because of the resulting loss of NFB at lower volumes. Would this master have the same effect, or would NFB stay the same throughout the sweep of the pot?
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NFB will be reduced, just like with a PPIMV.
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http://sluckeyamps.com/tdr/tdr.pdf (http://sluckeyamps.com/tdr/tdr.pdf) I keep trying to figure out why R23 is in this schematic (220K) is needed, of course it's lowering the signal to PI but why?
Well, no reply's, seems that it's needed to reduce the signal to the PI to prevent over loading the PI at full volume. The resistor was there before the MV was installed to reduce the signal for the same reason it's there with the MV volume installed. Only reason I can think of, did I answer my own question...
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http://sluckeyamps.com/tdr/tdr.pdf (http://sluckeyamps.com/tdr/tdr.pdf) I keep trying to figure out why R23 is in this schematic (220K) is needed, of course it's lowering the signal to PI but why?
Well, no reply's, seems that it's needed to reduce the signal to the PI to prevent over loading the PI at full volume. The resistor was there before the MV was installed to reduce the signal for the same reason it's there with the MV volume installed. Only reason I can think of, did I answer my own question...
R23 does not reduce the signal into the PI significantly (it's not part of a voltage divider). The intention of R23 is to keep the tone consistent at lower master volume settings (avoiding hi/low frequency rolloff). It's a circuit described by Kevon O'Connor in the book The Ultimate Tone (TUT). Not sure about the theory behind it though.
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> why R23 is in this schematic (220K) is needed
Not everything on a plan is "needed".
220k is a popular mix-resistor. Does this derive from a two-channel plan? Maybe the other channel was just cut-off, and the now-excess 220k not noted for removal.
It makes hardly any difference. The PI has a 1Meg grid load, but this is bootstrapped to several times that value. Say 2Meg. Then an added 220k in front is only a 10%/1dB "loss", not much at all.
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220k is a popular mix-resistor. Does this derive from a two-channel plan? Maybe the other channel was just cut-off, and the now-excess 220k not noted for removal.
Bingo! :wink:
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Thanks for the reply's, some circuits have it some don't, "was" just wondering if it's needed in the 6V6 Plexi MV I added. I don't think I put it in the first one I built, works fine. Didn't see it in Steve's Plexi schematic drawn (with added MV) from Mark Huss's plexi, either. But I "think" Steve had the 220K in his single channel Deluxe Lite. So, bottom line: doesn't make much difference if it's there or not.
al