Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: American_George on March 02, 2013, 08:54:38 am
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Hello everyone.
I'm American_George from Bamberg, Germany and my second build is driving me a little crazy!
Here's what I did, the why: becaus I thought I could: Gutted my Champ 12 because I wasn't using it and didn't like the distortion playing (where I usually am) at home. I did sucessfully build a weber 5f2 Champ and she (they're always she's) is dead quiet and sounds good, day or night! But I was missing reverb and that (very sexy) tremelo. If I was going to build both into an amp I would have just gone to a princeton reverb and there is not a lot of room in the champ 12 chassis. So a Vibro Champ was the logical choice, right?
So I made a eyelet board choosing individual Caps for the power supply. Bought a Weber (not classy but they do work) multitap PT and a new (didn't need it but decided to use it) OT. Orange drops, TAD caps not the whole nine yards but close enough. I don't want to get into Amp Mojo just get her up and running. Any Amp I can buy is probably better than my playing anyway.
Now there is often the problem with too much amp mojo reading and I think I fell into that trap... My power supply started at the 640 VAC center tapped output, I was getting high readings and decided to change over to the 600 VAC tap and got things well in range, with good readings across the mesuring points.
I did manage to kill my 6V6: either with too much V or my 6V6 wasn't healthy to begin with but I'm running a 6L6 now and sounds good, except for the terreble Hum and the tremelo sounds (for no better way to describe) dirty and at minimum speed too fast. Also the tremelo has a loud put-put-put in the back ground.
Due to too much mojo reading I had filtered my PT at 16-16-8-8 the first stage was added as suggested, and was soon changed to 32-32-16-16 (all Caps now added parrallel)
A few wiring mishaps (you can do a bunch of errors and it will still work, not good but will work) Now we're finally getting to my problem!
Hum: V1 and V2 removed 6L6 just bearly making any noise, I could live with it. Info Filament twisted well away from every thing and CT grounded. The OT is probably to close to the PT but no more room. I am using (again room) full-wave rectifier and with a load resistor a LED instead of the Fender light bulb, again no room.
With V1 and, or V2 installed, all knobs at Zero, Hum pretty loud. Turn Volume up, Hum increases to 11 O'clock then it's as quiet as church mouse and at around 1 O'clock begins to be really loud. It makes no difference which imput I use. At maximal Volume the Hum is almost as loud as the guitar signal.
The trebel and bass pots add a little noise but I think that's to be expected and react a little with each other. Maximal signal is all Pots at max. I don't see the Tone stack robbing anything, but at this point what do I know? The Amp is loud and so is the Hum. By the way I'm almost sure it's 100 Herz (50 cycles here in Germany)
The Tremelo sound is just dirty. Probalby a results of the Hum getting into the signal. At minimum Speed, max. Intesity the amp isn't so loud (to be expected) the effect is there but nothing to write home about. With speed almost off, Intensity just on you can hear just a little put-put in the background, I think that is expected, right? Turning speed and Intensity up a little the speed just putters up and the puttering is very loud.
What do I do? Monday I wanted to go and buy some new caps at least 22 UF for stages 2 and 3. But I've added an extra filter stage and it's running 32 UF as well as the first stage. Will just changing stages 2 and 3 to the traditional values have that much impact?
I know I can slow the tremelo down by exchanging one 0,01 UF with a 0,02 Uf which I had originally had and not really noticing an improvement. So.....
Where did I go wrong or am I looking in all the wrong places?
I would be really thankful for any thoughts on the matter.
Thanks in advance from Bamberg, Germany
American_George
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Too much info. You have hum & trem issues. Disconnect the trem & fix the hum first. Is the hum 60 cycle (heaters), or 120 cycle (power supply)? In Europe that may be 50 & 100 cycle.
For more info go to www.geofex.com (http://www.geofex.com). Follow the links in Tube Amp Debug page for hum. Then re-post here as needed.
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Also, if you are able, please post a picture of the interior wiring. We may be able to spot a likely problem from a picture.
Welcome American George!
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Hello Helpers....
First of all thank you for quick replies. Just knowing someone is out there willing to help, is help!
I did some suggested reading and went to work, with some imrovements.....
The Hum is 100 Hz.
I did some remodeling of the imput, and volume grounding points and (after reading) removed the Bus ground on the backs of the Pots. Big improvement!
You can never look at these things often enough....
I discovered that I had connected Neg. Feedback to the Speaker ground instead of speaker +, at the time it wasn't connected but now that is correct.
Tomarrow I'll change the 16 uF at the tremelo (which hasn't gotten better yet) and preamp stage to 22uF and see if things improve.
Pictures will follow soon.
Thanks again
American_George
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Hello DIY Tube Amp Friends,
Thanks for the help!
Hum is almost totally gone, here's what I did: simplified the grounding sceme to Leo's original, that helped a great deal, no Mojo grounding scemes, just simple follow the dots.
Then becaus I was getting wierd readings from filament voltage, I capped the CT from PT and replaced it with 2X 100 Ohm/ 3 watt resistors, wow did things get better (thanks for the reading tips) with no Hum I'm happy.
I didn't add (because the shop didn't have them) any more Caps to the power supply. It's ok as is.
The tremelo is working much better. I changed all the 0.01's to 0.02's and it slowed down to the point where I can use it. With speed, and intensity turned up I still get unpleasant brumm-brumm-brumm in the background but tuned in to something useful it works. If you have any really good ideas to improve this send them along, for the time being I'm going to play it the way it is.
I will change over to a 6V6 when I get one, the 6L6 it works fine though. It'll be a drop in. All Voltage readings (except for filament since I forgot to do it after success - don't change a winning team!) are very close to original.
Thanks for the help so far.
"Guitar beginners buy gear, musicians make music"
"Amp builder beginners buy parts, Amp techs understand parts"
The quotes are from me to you!
greetings from Bamberg, Germany
American_George
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... With speed, and intensity turned up I still get unpleasant brumm-brumm-brumm in the background...
Unfortunately, this is normal for this circuit.
Why? Because the amp is single-ended, it is common for there to be a background pulsating noise.
In the 50's Leo Fender sought to eliminate this problem in the 5E9 Tremolux. He applied the tremolo signal to a cathode resistor shared by 2 halves of the phase inverter. While the guitar signal was push-pull at this point, the tremolo was common-mode to both sides of the output. The tremolo still modulates the guitar signal, but gets eliminated at the output transformer. At the speaker, you get only the guitar signal with tremolo, but if you're not playing there is no background "pumping".
Leo got a patent on that circuit. It does assume everything in the push-pull part of the amp is perfectly balanced (otherwise trem cancellation is incomplete).
The later output-tube bias modulation tremolo was a follow-on development. Even later, it seems Fender accepted the possible background noise with the LDR trem circuit, in exchange for a tremolo strength that's gauranteed no matter how the output stage is biased.
So it seems that a bit of background noise is unavoidable in your amp.
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Hi Amp builder pros
before I sign off on this project I do have one or two questions:
In the original the Intesity pot is 25K's I'm using 50K does this mean maximal Intensity is half way across that Pot?
Can you use Caps (the three in the tremelo that is) that are rated less than 400 V. I don't see high voltage readings in the schematic?
How come some builder just need the 0,01's others 0,02 and still others need o,47 caps to slow the tremelo down? What is interacting here? and if I want to further slow it down which (looking at the layout) should I change first? Or doesn't make a difference?
I'm adding some pictures hopefully you'll be able to look at them in my Picassa Album.
(https://picasaweb.google.com/ggborrelli/TurnedAChamp12IntoAVibroChamp?authkey=Gv1sRgCJLFyrSjz7an3gE#5851894435005736530)
https://picasaweb.google.com/ggborrelli/TurnedAChamp12IntoAVibroChamp?authkey=Gv1sRgCJLFyrSjz7an3gE#5851894435005736530 (https://picasaweb.google.com/ggborrelli/TurnedAChamp12IntoAVibroChamp?authkey=Gv1sRgCJLFyrSjz7an3gE#5851894435005736530)
Once more thank you
American_George
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The caps and resistors that go from the LFOs plate to grid, each pair, have a time constant. So if you make the C or R larger the time constant is longer. The bigger the cap or larger the R the longer it takes for the cap to charge up. So it slows down the LFO speed.
IIRC..... :think1: HBP or Tubeswell posted here that you have to be careful with the R's value, I don't recall the reason. But it's very easy to just up the value of 1 or more of the caps to slow it down. That way you don't have to mess with the R values and throw something off in the LFO circuit.
Different companies used different value C's and R's to achieve the same end result. But the values still had to work out for the LFO to function properly.
Brad :icon_biggrin: