On the typical marshall style amp with a choke between the 2 caps in the first cap can, where exactly is the difference in sound between a choke and a resistor heard? In other words, we all know that using a choke instead of a resistor give the amp a different feel if not a bit different tone. But my question is WHERE that difference the choke makes is happening. does it influence the amp's tone/feel in the preamp or PI at all, or only the PA? I ask because i want to impliment VVR in my amp, but i only want it to scale the PA. So in order that i don't affect the sound the preamp now has, i intend to use the same filtering that the preamp now sees without VVR. When i impliment VVR i will have to break the B+ connection after the screens and tap the B+ from the standby switch to go to the PI node. (yes, i'll use diodes to isolate) This way the PI and preamp are seeing the same B+ cap/R lineup as before EXCEPT for one thing....no choke. I will be using a resistor instead. I am not about to put a second choke in for more reasons than i care to explain.
So thats why i want to know. My amp now sounds so awesome i am afraid to do anything that may change it. Heres my schematic that I just sloppily edited to reflect how i intend to do it. Notice that the preamp should see the same thing as before except for the R instead of choke. I'd also like to hear any comments on whether you think there will be any loss of interaction between the pre and power amps now that they don't share the same components even when i bypass the VVR by connecting the board's B+ in and out wires. In other words, do you think it will sound different than the amp does now with the VVR board bypassed?
Well, no replies but i'll update the sitch anyways and hopefully find out why VVR isn't working. I did it just as on the shematic, tho first i seperated the PA from the pre as in the schematic with a extra can and choke but without the VVR just to make sure that PSU setup doesn't change the tone or feel. Seemed the same, so i went ahead with the VVR. It doesn't work and i have gone over and over this many times and it is just as on a schematic i posted that dana said was right. I made sure the diodes were correctly orientated, checked the mosfet to be sure it wasn't shorting to the chassis thru the insulator, and checked every connection many times. Made sure i had the drain source and gate right as per the drawing on the package. it all looks right. I thought about maybe having ruined the mosfet, but i kept my wrist grounded and didn't even move the static foam till i was ready to solder after it was mounted. Even used gator clips on the mosfet leads when soldering s heatsink. kept all leads as short as possible.
Bottom line is that when i turn the pot down it begins a loud buzz that continues all the way down, and the volume of the amp hardly drops at all. This is really puzzling unless i damaged the mosfet. But as careful as i was i would thing that if it's damaged that easily few people would ever get this thing in w/o the same damage.
EDIT: forgot to mention a slightly important thing....voltages at the plates and screens do show it's working. But even at the lowest voltages which i think was something like 30v, the volume seemed normal !
iknowjohnny wrote:Hmmm.if i scale the entire amp i still get the buzz, but the volume then goes down as it should. But the buzz is so loud it drowns out the guitar.
Everything you have proposed looks good to me. BTW I tried to reply over at the other forum and they have locked the thread . Anyway, Can you give me some voltages? You say when you scale the entire amp the volume goes up and down but have you measured the voltages? When you scale only the power tubes are your voltages varying. What is the voltage difference between the plates and screens of your power tubes without the VVR installed?
The loud buzz you are experiencing can be a few things. If you have long wire runs from the mosfet to the rest of the circuit they can pick up noise. I have also found that some mosfets will buzz like crazy and some don't. I have actually replaced a 2sk3675 that was buzzing like a mother (even though the vol was going up and down) with the NTE2973, I supply in my kits, and the buzz vanished. What part number Mosfet are you using? Are you planning on using PPIMV?
The longest run from the mosfet is the length of the mosfet's leg plus about 1/8". The voltages do go up and down at both the PA and pre when scaling the whole amp, and at the PA when scaling just that. lowest PA voltage is 65v at full counter clockwise on the pot. The normal plate to v is 399v and screens usually measure about 10-15 less if i recall.
The part unfortunatly is the 2377. Maybe thats the cause of the buzzing. As for the volume, i just realized it DOES go down when i scale the PA only, but not till the master is pretty far up. At say 9:00 where the amp is much too loud for home practice there seems to be no attenuation. But at 11:00 there is, tho not as much as i'd hoped. still too loud for home. So this device just may not be for me after all. It would work for gigs, but i don't need attenuation there because i get my gain from the preamp with this amp.
So it probably IS the 2377. But i think this circuit isn't for me, so i think i'll remove it and see what the best attenuator is and build one. I think the airbrake is supposed to be good.
iknowjohnny wrote:The longest run from the mosfet is the length of the mosfet's leg plus about 1/8". The voltages do go up and down at both the PA and pre when scaling the whole amp, and at the PA when scaling just that. lowest PA voltage is 65v at full counter clockwise on the pot. The normal plate to v is 399v and screens usually measure about 10-15 less if i recall.
The part unfortunatly is the 2377. Maybe thats the cause of the buzzing. As for the volume, i just realized it DOES go down when i scale the PA only, but not till the master is pretty far up. At say 9:00 where the amp is much too loud for home practice there seems to be no attenuation. But at 11:00 there is, tho not as much as i'd hoped. still too loud for home. So this device just may not be for me after all. It would work for gigs, but i don't need attenuation there because i get my gain from the preamp with this amp.
So it probably IS the 2377. But i think this circuit isn't for me, so i think i'll remove it and see what the best attenuator is and build one. I think the airbrake is supposed to be good.
I was just looking at your schematic again. Since you are using the normal style master vol before the PI you might want to include the PI in the part of the amp that is scaled along with the PA. I think what you are seeing / hearing is PI running at normal voltages overdriving the PA. Most people who scale the PA only use a Post PI master vol just for this reason. Since your MV is before the PI and the PI is contributing some gain this might be why you aren't getting the results you want. Different amps present different challenges when retrofitting the VVR into the circuit and some require a lot more effort and experimentation to get the results you want. If you want more help I'm here and if you want to give up then thats fine too. It's all good
I don't think it's OD'ing the PA because with the master down i can turn the VVR anywhere and the volume not only doesn't change, but the tone doesn't either. Not in any way including amount of distortion. Then at around 11:00 it works and the volume goes down, but still no change in amount of OD or anything. On the other hand it seems to work completely normal with the entire amp scaled which speaks to your theory somewhat. But the buzz is still there so i think i'll pass because at $12 a pop i really don't want to end up buying another FET only to find thats not it, or even buying a third or whatever it takes. i think an attenuator will do me ok if i can just figure out what to build. But i'll leave the VVR there to experiment for another day or so till i feel i've done all i could. Thanks for your kind help tho !
Post a pic of your install if you can. I've used the NTE2377 in 3 different amps with no issues after I sorted out the lead dress. One worked great from the start, 2 of them buzzed and hummed like mad at first. Both were fixed by merely moving wires - and not the ones to the FET.
FWIW, I have never handled the FET with any kind of special care. Maybe I'm just lucky but they seem to be pretty durable.
I can tell you w/o a pic. There are no wires. the mosfet is mounted under the pot which is turned with the lugs pointing down. So for example the 100k R that comes off the wiper goes right to the gate with only a tiny bit of lead left on it because the distance to the gate from the center lug is about the same length of a 1/2 watt resistor body. The longest thing would be the B+ source from the standby switch where i left the full length of lead on it and it goes to the pot then the drain. So about a 1/2" of diode lead from the diode body to the drain. But thats just B+ and should have no effect on noise till after the mosfet. the rest is the same. all components just directly soldered with most of thier leads cut off leaving just enough to reach.
Now LOCATION may be a different story, I dunno. But the pot is next to the standby switch at the end of the turret board with no components within a few inches of it. The presence however is next to the VVR pot. But i really don't want to drill more pot holes only to find thats not it.
It works !!! I did an archive search at 18watt last nite and someone who had the same buzzing said that if he loosened the mosfet's mounting screw the buzzing stopped, so i tried it an amazingly it stopped !
There are still issues tho. One of them makes sense, tho i have never heard of anyone complaining about it. That would be that when you turn the VVR level down on a amp with a high gain preamp, the high gain preamp progressively becomes cleaner and cleaner. At low levels it's almost totally clean. that leads me to the second problem...
which is that when i scale only the PA, it doesn't have any affect till the master is turned up to well past the level i was hoping to achieve for playing at home where i have to be pretty low to avoid bothering people upstairs. At about 9:00 on the master the VVR knob does nothing. However, say you're at 9:00 and you have the vvr knob at 0 and the volume is as tho the was no VVR. Now you start turning the master up......from 9:00 to full up the volume barely gets any louder. So it DOES work, but it just doesn't change the volume at all between off and 9:00 on the master.
But that leads me to the 3rd issue....i obviously can't be using it like this for more than a few minutes because the mosfet isn't tightened down. I never saw a "i fixed the problem" reply from the guy i mention who had posted the same thing at 18watt. And theres no PM feature there.
But all that said, i don't think this thing will really be worth it to me. I can hear a difference between the vvr tone and the non vvr tone when both are played at the same level. I never seem to hear anyone saying they hear the difference, but to me the tone suffers a tad. It sounds sloppier to me. Not that it's bad, just not quite as good. But i think it's a useful circuit for some people/amps. Probably not for me tho. Having just got it working tho i cannot say for sure. But theres a good chance i will remove it because switching it to non VVR will be a lot more complicated than it would be if scaling the whole amp. In fact, now that i think about it, it may not be the VVR that changes the tone, bu8t the fact that to scale the PA only i had to take the choke out of the B+ for the preamp. In fact, that makes total sense because chokes keep things from getting sloppy ! Anyways, a bit more playing is required to seee what i want to do.
I don't think it is the choke issue that is causing the sloppiness.
First, I haven't used one of these VVR's so this is just thinking on my part:
What is happening to the bias voltage as you turn down? Since you're biasing at 100%, which you have gotten figured out for your plate voltage now, what is happening when, say, the plate voltage is 300, or 200, or 100?
At 300 volts from plate to cathode (your plate voltage) then you should be drawing .083 amps on each tube. At 200 volts it would be .125 amps. At 100 volts it would be .250 amps per tube! 25 watts / volts = amps. Right?
I am wondering if the sloppiness you describe is from the tubes getting biased cold and trying to bias into class AB or beyond with cathode bias.
Like I said, I haven't messed with a VVR before, just thinking out loud here.
Jana wrote:I don't think it is the choke issue that is causing the sloppiness.
First, I haven't used one of these VVR's so this is just thinking on my part:
What is happening to the bias voltage as you turn down? Since you're biasing at 100%, which you have gotten figured out for your plate voltage now, what is happening when, say, the plate voltage is 300, or 200, or 100?
At 300 volts from plate to cathode (your plate voltage) then you should be drawing .083 amps on each tube. At 200 volts it would be .125 amps. At 100 volts it would be .250 amps per tube! 25 watts / volts = amps. Right?
I am wondering if the sloppiness you describe is from the tubes getting biased cold and trying to bias into class AB or beyond with cathode bias.
Like I said, I haven't messed with a VVR before, just thinking out loud here.
We are talking about a cathode biased or self biased amp here. The bias is directly proportional to the current flowing through the output tube. As you turn down the voltage the bias is automatically adjusted because you have less current flow and less bias. The other option is a fixed bias circuit which has extra components to change the bias so that it "Tracks" along with the change in B+.
618]http://i24.photobucket.com/albums/c39/dazco/vvrmod.jpg[/img]
But even at the lowest voltages which i think was something like 30v, the volume seemed normal !
. Anyway, Can you give me some voltages? You say when you scale the entire amp the volume goes up and down but have you measured the voltages? When you scale only the power tubes are your voltages varying. What is the voltage difference between the plates and screens of your power tubes without the VVR installed?
