squealing out of control when powered on

[dscottguitars]

Hey Chuck, obviously you've been there, that's exactly what was going on.

[dscottguitars]

The finished product...

It's finished and ready for testing by anyone willing to try it out. As I said before it's 40-50 watt 6L6 tube amp (original solid state rectified). I put a blend pot to choose between two preamp triodes, one with a clean sound and one that distorts. There is also a boost switch that adds quite a bit of gain and a bit of crunch. I played the guitar shown, my own hand carved D-Scott guitar, and that cabinet using only one speaker in it and the tweeter off. It cranked!! It's really quiet. It has tone!! Using all four pickup configurations on my guitar I went from muddy crunch to bright piercing sustain to low mellow clean. And with the boost switch there are many sounds you can dial in. I then plugged in my acoustic guitar, a Taylor 30th anniversary beauty with a Schatten pickup, and was amazed at the clarity of the sound. But only with the dial on clean and the boost off. I had quite a bit of volume w/o breakup, I didn't expect it to be so nice for acoustic. This would work as a PA amplifier if you use a good mixer. I then used a Boss pedal eq and that allowed me to dial in a perfect representation of the sound of my guitar. My neighbors loved it...

It was made with all vintage Allen Bradley resistors. I used as much original parts as I could. All the caps are new and good quality. Those big copper caps really put the E in the low E on the guitar. It has one input, one 8 ohm speaker output and a line out for PA or another amp. There is a hum balance pot on the back to easily adjust the hum for different venues. On/off, standby switch and pilot light also. It's painted a 'metallic' midnight blue and an dark blue cage with light purple overtones. This was originally a McMartin, but I gutted it and put in my own schematic design. My arm is hurting from patting myself on the back so much, it's my first amp.

This amp will be for sale soon-I want to make sure it works for awhile. Price?? Hoping to get $500. FB friends will get a discount.

[dscottguitars]

Hi All...

I didn't notice at first but when turned up all the way there is a persistent buzz. It gets louder w/ each pot, even the tone. I think I've traced it to the input line, it is very sensitive and when touched with a probe it squeals. When touching the probe to the plate I get a radio station-very strong. I've tried rewiring the schematic w/ many different configurations but end up with the same problem. I separated the input ground-directly to the chassis and no change. I use shielded wire, connected at one end, and the rest of the lead dress seems to be in order according to what I read.

Is there a way to eliminate this, or cut it down? How much is normal? -a relative, subjective question. It's not so bad that it affects the sound or you can hear it while playing. It is only there, and I would like to sell this and I'm not sure if any buzz is acceptable. Unfortunately I don't have another to compare it to. I did have a Motorola, stereo tube amp I used for a bit and I don't remember that having any noise when turned up.

It is still there with or without the guitar plugged in. The guitar position can make it louder, but after a certain distance it' still there. Turning down the guitar pots does very little. Only a small background hum goes away, not the buzz. It is definitely worse when I throw the coil cut switch to a single coil, but if I move far enough away it gets back to the original buzzing.

I've also tried different preamp tubes, no change. The above picture is close to the way it is now. I added a master volume, and a 220 resistor between the coupling caps of the blended tube.

This is not hum, I can adjust for that and it is a different sounding noise. I'm thinking it's a parasitic oscillation but I don't know what else to do.

Any suggestions??

Would shielding the input area somehow help? Or maybe a bottom cover plate? I'm tired of the trial and error method, so far it's all error. Is the gain too high?

Here is what I have:



Thanks, Daniel

[Chuck H]

Any effect on the buzz from guitar pickup configuration can be ignored. Guitar pickups "hear" a lot of buzz, it's normal. Hopefully you have an input jack that grounds when the guitar cord is unplugged from the amp. If so then unplug the cord and determine if THAT amount of buzz is not acceptible. Most amps "buzz" very little. Most amps DO hiss when gain is increased. Hiss is a byproduct of power through resistance and a little (or a bit on high gainers) is unavoidable. All you should hear as the volume is increased (with no guitar cord plugged in) is some hiss (relative to gain) and maybe a very small amount of hum and the amp should be stable (no squeals or wierd noises) well into gain levels that allow the guitar input to get the amp into some clipping.

[dscottguitars]

I don't have a grounding jack, but if I short it out the noise goes away. Also, if nothing is plugged into the jack it is loud with hum, but that seems normal to me.
thanks...

[dscottguitars]

Hi again,

Changed the preamp again. i couldn't get what i was looking for and still had buzz so this is what i did. i put a pot as the cathode resistor to adjust plate voltage that way and dial in preamp distortion by making the resistor smaller. it works great. Then, i added another pot to the feedback circuit to adjust the amount of feedback to produce output tube distortion or not. This works really well too. in fact these two mods are the best i've done so far-wrt sound, response and breakup. A schematic is include with the voltages. Can anyone see anything 'odd' that i may not know about?

i still have buzz, not as bad, but it's still there when cranked. i put in a shorting jack, no help. i regrounded everything to two star ground; all the preamp grounds go to the chassis by the input jack and all the power supply, phase inverter, and output grounds are on the chassis by the power transformer center tap. This did help some, as i have hiss now with less buzz, but at full volumes, no NFB, and a low resistor on the first stage there isstill buzz. Not to be confused with hum, that is very quiet and can be adjusted with the pot.

i'm thinking two possibilities: One is too much gain, it really is quite a bit and playing at good levels that are quite loud, there is very little noise. it's only at the extremes that the buzz is loud, no FB and or low value cathode resistor on the adjustable. The other possibility is the extremes may not be able to be that quiet. in order to play it at the extremes you need to be at least 15 feet away from the speaker, and there the noise is less. You can't hear it while playing.

One thing that takes all the noise away completely is when i check the voltage on pin #2 of the phase inverter. When i touch the probe to it, the buzz is gone. When i check the other input, pin #7, a radio station can be heard. This is also true if i touch the probe, w/o the ground, to the plate of the first tube into the tone and volume controls.

My multimeter specs say there is a 10M impedence using the voltage meter. if i were to connect a 10M resistor to that grid to ground, would that be the same as using my meter? is there any capacitance i could/should include? There is nothing mentioned in the specs.

Another question: plate voltages for a tube wired in parallel; A single 12A-7 can have 300 volts on the plate. is it the same in parallel, 300v max? Or do they add to 600v?

There is a chart on the schematic for the Rc and NFB showing the position of the pot and its corresponding values.



Thanks for your input,

Daniel

(the capital i on my computer isn't working for some reason...)

[dscottguitars]

Unless I'm not understanding something it must be the high gain causing the buzz. I tried a 10M resistor to ground from pin #2 of the PI and no change. I find out that when I'm touching the probe to that pin, it cuts everything out completely, signal too. I had my guitar plugged in when I tried it and it cut that out as well.

So unless there's something I haven't heard about, it must be the high gain. I've tried grid resistors, small RF caps at different places and changed tubes but no change. I added a 220k resistor from the output of the tone pot to cut some gain there and that helped. I may try a higher value to cut more. There is plenty of gain still on my bridge pickup w/ the coil cut-the lowest of volumes on my guitar. With the neck humbucker, whoa... BTW, the tone pot is where I got the most buzz cut so that's why I'm cutting the gain there.

Any ideas, comments??

[Ran Randle]

Unless I'm not understanding something it must be the high gain causing the buzz. I tried a 10M resistor to ground from pin #2 of the PI and no change. I find out that when I'm touching the probe to that pin, it cuts everything out completely, signal too. I had my guitar plugged in when I tried it and it cut that out as well.

So unless there's something I haven't heard about, it must be the high gain. I've tried grid resistors, small RF caps at different places and changed tubes but no change. I added a 220k resistor from the output of the tone pot to cut some gain there and that helped. I may try a higher value to cut more. There is plenty of gain still on my bridge pickup w/ the coil cut-the lowest of volumes on my guitar. With the neck humbucker, whoa... BTW, the tone pot is where I got the most buzz cut so that's why I'm cutting the gain there.

Any ideas, comments??

The chassis is small so the buzz may be unavoidable due to high gain to some extent and the lead dress may contribute to noise, buzzing etc.

I have a very similar amp I'm converting to a tweed style 6L6 amp and had trouble with the power supply and high b+ voltage. The power tubes are cooking and not sure of the value cathode resistor needed to get it in the range needed. I tried a parallel 250ohm and 270ohm 5w resistors with a 50uf 100v bias cap. I'm thinking a 300ohm 10w cathode resistor and the series 10k5w resistors you used in your amp?

It appears my amp has the same rectifier diode and parallel 560k resistor layout as your amp. I'm still learning and revamping power supplies is tricky. Don't mean to high jack the thread but this is almost exactly the same set up as my amp.

Thanks for any help.

[dscottguitars]

Many changes and no buzz

Hi, since my last post I've made quite a bit of changes. Added a footswitch, found the Marshall crunch and more...

The buzz problem was with the hum balance pot. I changed the configuration using the 5K pot and 2-100 ohm resistors. The other way I found would not let me get 6.3v exactly and made it buzz. When I added the other resistor-see schematic- I could get 6.3v and the buzz also went away.

I used values on the preamp tubes to be close to Fender and then Marshall w/ the footswitch. I also found my NFB idea was similar to Orange, but I have more range to go from really clean to really mean. Also, Orange uses it as a tone control combined w/ NFB, I only use it for NFB.

It also works great with my acoustic guitar, no preamp-using a Schatten pickup. With a preamp and a Boss pedal EQ, it sings.

So, it's a screaming electric guitar amp and a pristine acoustic/PA amp all in one.

This amp sounds so good to me I am going to reproduce it and try to sell them on ebay...

Here is the latest:

[Ran Randle]

Hi, since my last post I've made quite a bit of changes. Added a footswitch, found the Marshall crunch and more...

The buzz problem was with the hum balance pot. I changed the configuration using the 5K pot and 2-100 ohm resistors. The other way I found would not let me get 6.3v exactly and made it buzz. When I added the other resistor-see schematic- I could get 6.3v and the buzz also went away.

I used values on the preamp tubes to be close to Fender and then Marshall w/ the footswitch. I also found my NFB idea was similar to Orange, but I have more range to go from really clean to really mean. Also, Orange uses it as a tone control combined w/ NFB, I only use it for NFB.

It also works great with my acoustic guitar, no preamp-using a Schatten pickup. With a preamp and a Boss pedal EQ, it sings.

So, it's a screaming electric guitar amp and a pristine acoustic/PA amp all in one.

This amp sounds so good to me I am going to reproduce it and try to sell them on ebay...

Here is the latest:[ATTACH=CONFIG]16388[/ATTACH][ATTACH=CONFIG]16387[/ATTACH][ATTACH=CONFIG]16386[/ATTACH]

Great work, sounds like it’s a really versatile amp. I rebuild organ amps and have made a Marshall plexi 18 watt and a matchless lightning clone from the Hammond L100 chassis.

I will try to use your schematic to get power supply voltages where they need to be for the 6L6 PA amp I'm converting, thanks for posting the schematic and photos. I'd like to hear your amp. Do you have any sound clips posted?

There are pics of more conversions in my album if you want to check them out. I have some clips posted here of the Marshall clone I did: organdonor amplification

Cheers.

[woodyc]

I think it might be wise if you got an agent from whoever provides your general liability to come over and look at that thing and hear your sales plans. And I would make sure to show them your layout and solder joints and tell them about the 400V and the old parts you are using. Just a suggestion.

[dscottguitars]

Thanks, but I don't have anything like that. I just make things out of my garage. And when I do list an amp for sale, I'll always disclose any used parts and warnings about high voltage. And I'll guarantee the amps for one year-labor only. The amp pictured is mine and any new ones I make will have a much nicer layout. This is my first complete rebuild, I've made one from scratch and modded a Fender. But, I spent a year redoing each amp as I discovered new ways to make it sound better. Hey, if it takes off on ebay, maybe I'll be able to get an agent and all. That would be awesome...

Here are the other ones: