I'm building a tube amp, HELP!

Posted by Todd on Sat, 01/05/02 - 04:10:21.

I am trying to build a Fender Champ amp, the simplest of all tube amps. I've built it three times and still can't get it working. Anyone know about this who can help me out? One specific thing I was wondering is the function and if it is neccessary to have the .047mF 600V capacitor from the AC line to ground. My electronics instructor said I dont need it. But my amp doesn't work, what does that cap do?! HELP!



Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Re: I'm building a tube amp, HELP!

Your instructor is correct. The amp will work just fine without it. All it does is filter some mains-borne hash to ground when the amp is on and it also helps to prevent the "on/off" switch contacts being chewed up too soon from arcing, which happens whenever a highly-inductive load like the power transformer is switched. OK, let's start from the obvious:

1)... are all the filaments glowing?
2)... are you able to measure voltages? If so, do you have B+? This could be around 300VDC so be careful. Make sure it is present at the output transformer, not just at the rectifier cathode. This is because the series dropping resistor in the power supply could be open-circuit, so you need to measure it down-stream of this resistor.
3)... Are you sure the output transformer windings are intact? An open circuit on either will of course result in no output.
4)... Have you tried a different 0.02uF/600V coupling capacitor? If this cap is open-circuit, you get no output.
5)... Have you checked the volume and tone pots for continuity? Used pots can be particularly bad.
6)... Are you sure all resistors are the correct value? Don't assume color codes are correct. Measure every resistor before soldering it in. I once had a 10Meg resistor clearly marked as 1K ohm. You can imagine what that did to my signal level. Likewise, if the normal 1 meg grid-leak resitor on the 12AX7 grid was actually something like 100 ohms, you'd have a pretty quiet amp.
7)... Are you 100% sure the tubes are OK?. No emission=no output. What do you measure on the tube cathodes? The 12AX7 cathodes should have around 1 volt (very roughly speaking)each and the output tube should have something more like 5 - 8 volts. Huge departures from this would suggest a dud cathode resistor, and/or plate resistor, or a dud tube.
8)... Are you certain connecting wires are not broken? It does happen. Never assume a wire has continuity. Test it first.

Do you get any sound at all, or is it dead quiet? A dead amp is easier to fix than a sick one, as the cause is usually simple. Something in the signal path is open circuit - whether it be a resistor, capacitor, potentiometer, tube, or transformer winding. If everything checks out OK, redo all your solder joints and check continuity of wires. Let me know if this helps to fix the problem. The Champ is a very simple and reliable amp. It should run the first time it's switched on, with no problems.

Re: I'm building a tube amp, HELP!

Dear Frank S.,
Thank you for taking the time to respond to my post about helping me build a Fender Champ amp. You seem very knowledgeable as well as polite and useful. I have no idea what could be wrong with it.
I am getting a signal out, but it is even less than the input. I used an oscilloscope to discover this, and also signal traced from the input through the stages. The first stage of the 12AX7 was amplified and everything appeared fine, including voltage measurements. However, the second stage had no signal and voltage measurements were not correct. It was apparent that the tube was not conducting for some reason. The filaments are all lit and, yes, as it wasn't originally done, both parts of the 12AX7 filament are lit.
I have measured the power supply voltages and they seem to be measuring correctly. I have tried a different coupling capacitor, especially since this was the section where something was wrong, I thought for sure the cap was leaking voltage bc the grid is reading a much higher voltage than expected.
I havn't made voltage measurements recently and unfortunately I did not write them down, so this is based on memory of the problem. What I believed was happening was that the grid had a very large voltage on it and the plate had very small (close to no) voltage on it. I don't remember the cathode voltage, it may have been zero? I know the operation of the tube has electrons boiling off the heated cathode, which are attracted to the oppositely biased grid, and then flow through the grid to the greater attraction of the plate, where they are collected. If my grid is reading high voltage (which I am positive was happening) then wouldn't it be logical for the plate to collect electrons and produce a voltage? It makes sense that if the cathode was not receiving much voltage, it would not have electrons to boil off. So, that could be my problem I think? Question 1 is: what would make the cathode read with no voltage? And Question 2 is: What would make the grid read with extremely high voltage? I don't know the answers, maybe you might have a clue. It is possible that the tubes are bad (though I have two sets that I have tried) or the transformer has a problem (I only have one. Is there a way I can test it?). I don't think there's anything wrong with the wiring, considering that I have build this thing 3 times!
The Champ amp that I have constructed and that we are talking about here a model AA764. Since the discovery of a rare 6SJ7 tube, I am going to begin construction of the even simpler (yes, the absolute simplest amp possible!) model 5C1.
This is a link to the schematic for the AA764..... http://www.kbapps.com/schematics/tubeamps/fender/champaa764.html
The only problem with building the 5C1 is that some of the parts are unavailable to me. Do you know where I could get 8mfd 450V, .02mfd 600V, .05mfd 600V caps? If so, please let me know. One question I have about this is: can I use the more common 10mfd 450V filter cap instead of the 8mfd?
Thank you very much for all your initial help, and if you've even bothered to read this far in my extremely long response, thank you again for that. If you can provide any help or information, please post another follow-up (that goes for everyone else too!). If not, wish me luck and thank you again.

-Todd

Re: I'm building a tube amp, HELP!

No problems, Todd. That's what forums like this are for. There's a couple of possibilities here ... first check out every capacitor in the tone stack. Apart from tailoring a tone contour, these caps also block DC from the first stage plate from appearing on the next stage grid. They must be rated the same as any coupling cap - i.e., at least 400VDC. Before you do this check, lift off the volume pot center lead. If the high grid voltage disappears, you're getting it through the tone stack. If it is still there, the second possibility is likely ... a short in the 12AX7, or even the tube base. Have you tried other 12AX7's? With regard to your question about low voltage on the cathode - this is not necessarily a problem. A lot of cathodes are actually tied directly to ground and you don't get a lower (positive) voltage than this. The cathode resistor just serves to bias the grid negatively so the tube works more into its linear region. But your amp should have a 1500 ohm cathode resistor, biasing the grid to around one volt. If you have a very low voltage and there are no tube shorts, check the 25uF bypass capacitor. If this is shorted, it will pull the cathode voltage right down. In any case, zero or very low voltage would be expected if there is no current flow through the tube, as the cathode resistor would put the cathode at ground potential. To test the output transformer, first check winding resistances. The primary could be anywhere from 200 - 500 ohms. The secondary more like 0.5 to 1.5 ohms. You can test an output transformer very easily and safely, plus identify its impedance ratio by connecting a wall wart to the primary. I use a 24VAC 0.25 amp unit. Measure the actual AC voltage on the primary and note it down. Now measure the secondary voltage. Expect to see something between 0.8 and 1.5 VAC. Divide the measured primary voltage by the measured secondary voltage. This of course is your voltage ratio, which is the same as the turns ratio. Now square this number. This gives your impedance ratio. If you multiply this number by your speaker impedance, it will give you the primary impedance seen by the tube. For example if the voltage on the primary were 24.0 volts and the voltage on the secondary is 0.96 volts, the voltage ratio is 25:1. Squaring this gives you 625:1. So for an 8 ohm load, reflected primary impedance is 625 x 8 = 5000 ohms. Don't be too concerned if you expect a certain figure and the sums are not 100%. Transformer action, meter accuracy and actual winding characteristics all have an affect. But what you should NOT see is massive differences. This would suggest a wrong transformer, or a faulty one. Keep plugging ... you'll get there!

Re: I'm building a tube amp, HELP!

No problems, Todd. That's what forums like this are for. There's a couple of possibilities here ... first check out every capacitor in the tone stack. Apart from tailoring a tone contour, these caps also block DC from the first stage plate from appearing on the next stage grid. They must be rated the same as any coupling cap - i.e., at least 400VDC. Before you do this check, lift off the volume pot center lead. If the high grid voltage disappears, you're getting it through the tone stack. If it is still there, the second possibility is likely ... a short in the 12AX7, or even the tube base. Have you tried other 12AX7's? With regard to your question about low voltage on the cathode - this is not necessarily a problem. A lot of cathodes are actually tied directly to ground and you don't get a lower (positive) voltage than this. The cathode resistor just serves to bias the grid negatively so the tube works more into its linear region. But your amp should have a 1500 ohm cathode resistor, biasing the grid to around one volt. If you have a very low voltage and there are no tube shorts, check the 25uF bypass capacitor. If this is shorted, it will pull the cathode voltage right down. In any case, zero or very low voltage would be expected if there is no current flow through the tube, as the cathode resistor would put the cathode at ground potential. To test the output transformer, first check winding resistances. The primary could be anywhere from 200 - 500 ohms. The secondary more like 0.5 to 1.5 ohms. You can test an output transformer very easily and safely, plus identify its impedance ratio by connecting a wall wart to the primary. I use a 24VAC 0.25 amp unit. Measure the actual AC voltage on the primary and note it down. Now measure the secondary voltage. Expect to see something between 0.8 and 1.5 VAC. Divide the measured primary voltage by the measured secondary voltage. This of course is your voltage ratio, which is the same as the turns ratio. Now square this number. This gives your impedance ratio. If you multiply this number by your speaker impedance, it will give you the primary impedance seen by the tube. For example if the voltage on the primary were 24.0 volts and the voltage on the secondary is 0.96 volts, the voltage ratio is 25:1. Squaring this gives you 625:1. So for an 8 ohm load, reflected primary impedance is 625 x 8 = 5000 ohms. Don't be too concerned if you expect a certain figure and the sums are not 100%. Transformer action, meter accuracy and actual winding characteristics all have an affect. But what you should NOT see is massive differences. This would suggest a wrong transformer, or a faulty one. Keep plugging ... you'll get there!

Re: I'm building a tube amp, HELP!

: No problems, Todd. That's what forums like this are for. There's a couple of possibilities here ... first check out every capacitor in the tone stack. Apart from tailoring a tone contour, these caps also block DC from the first stage plate from appearing on the next stage grid. They must be rated the same as any coupling cap - i.e., at least 400VDC. Before you do this check, lift off the volume pot center lead. If the high grid voltage disappears, you're getting it through the tone stack. If it is still there, the second possibility is likely ... a short in the 12AX7, or even the tube base. Have you tried other 12AX7's? With regard to your question about low voltage on the cathode - this is not necessarily a problem. A lot of cathodes are actually tied directly to ground and you don't get a lower (positive) voltage than this. The cathode resistor just serves to bias the grid negatively so the tube works more into its linear region. But your amp should have a 1500 ohm cathode resistor, biasing the grid to around one volt. If you have a very low voltage and there are no tube shorts, check the 25uF bypass capacitor. If this is shorted, it will pull the cathode voltage right down. In any case, zero or very low voltage would be expected if there is no current flow through the tube, as the cathode resistor would put the cathode at ground potential. To test the output transformer, first check winding resistances. The primary could be anywhere from 200 - 500 ohms. The secondary more like 0.5 to 1.5 ohms. You can test an output transformer very easily and safely, plus identify its impedance ratio by connecting a wall wart to the primary. I use a 24VAC 0.25 amp unit. Measure the actual AC voltage on the primary and note it down. Now measure the secondary voltage. Expect to see something between 0.8 and 1.5 VAC. Divide the measured primary voltage by the measured secondary voltage. This of course is your voltage ratio, which is the same as the turns ratio. Now square this number. This gives your impedance ratio. If you multiply this number by your speaker impedance, it will give you the primary impedance seen by the tube. For example if the voltage on the primary were 24.0 volts and the voltage on the secondary is 0.96 volts, the voltage ratio is 25:1. Squaring this gives you 625:1. So for an 8 ohm load, reflected primary impedance is 625 x 8 = 5000 ohms. Don't be too concerned if you expect a certain figure and the sums are not 100%. Transformer action, meter accuracy and actual winding characteristics all have an affect. But what you should NOT see is massive differences. This would suggest a wrong transformer, or a faulty one. Keep plugging ... you'll get there!

Sorry about the multiple postings ... it didn't seem to work (nt)

.

SAFETY! And, oh yeah, what was wrong with the amp.

Ok, I think I got it down about that .047mf cap placed between the AC line and ground. When Fender made this amp, it was during a time when there was a.) no ground prong on AC cords and b.) no specified "hot" or "return" prongs. In other words, the AC plug was two prongs exactly the same and you could plug in the amp with the sides reversed. You may be familiar with a ground switch on certain amps, and this is also the reason they were used. Well, what I have recently learned is that not only is this cap unneccisary but it is actually very DANGEROUS! In fact, I remember once shocking myself because of this. With this cap, the entire chassis is at a dangerous potential difference in respect to my body. That's why it hurt like a bitch. I also tried adding this cap back in later during my process but did not get shocked bc this time I had the ground from the three prong cord going to chassis ground, so that dangerous potential was sent to earth ground and away from me! The other thing I learned was that it is neccissary to wire the hot side of the AC line (the right slot when you're looking at a wall outlet) directly to the fuse first. I believe this is to prevent damage to the transformer or the rest of the amp. My theory isn't the greatest, but I know this is what you have to do.
As for what was wrong with my Champ amp, which I have working now by the way and am incredibly proud and happy, was simply the wiring. This will remain a mystery until I have a chance to compare the schematic to the chassis layout, but I just know that both me and my electronics instuctor compared my wiring to the schematic multiple times and found no problem. However, the other night I was tracing all the parts on the layout and found some complications. I fixed the wiring and plugged it in. It works, though doesn't sound that great. Today I learned a lot of information about techniques to reduce the hum (its a rat's nest of wires in mine)and other modifications to make to the Champ.
For anyone building a Champ model AA764, change the cathode bias resistor of the 6V6 from a 470ohm 1W to a 560ohm 5W. The higher resistance will help to protect the tube and the higher wattage will stop the resistor from dissipating too much heat. Also, add a 1K ohm 5W resistor from the + of the second filter cap to the screen grid of the 6V6. This is also done for protection of the tube. Make sure the 1M ohm at the input is connected directly to the 1500 ohm cathode bias resistor of the first stage of the 12AX7. Yes, they are both grounded, but make sure they are TIED TOGETHER. Also, a tip to reduce hum is to only use ONE spot for chassis ground. Do not screw a bunch of places on the chassis and use them for ground. Also, which is even worse, do not do what I do and tie all the grounds together with wire, because the resistances of that wire and add you get currents running through there. These are called ground loops and are undesirable. Make sure to remember to twist filament windings and you may want to create and "artificial center-tap" for the 6.3V section. You can do this by placing a 100 ohm resistor to one wire and another 100 ohm resistor to the other wire and then connecting the ends of those resistors to ground. I think that's about all I can tell you. Kinda funny how I am giving advice about building a tube amp that I originally posted as "HELP! I am building a tube amp!" Well, that's progress! Everyone tell me if I am on the right track or not. Please respond with umm... anything you want. -Todd

Re: About those modifications ....

I'm glad you got it going! It had to be something simple, as you found. With regard to your suggested modifications - the tips to avoid hum are well documented and make sense. But the proposed changes to the 6V6 cathode and screen resistors are something that warrants discussion. It all depends on what you want from your amp. The classic singing tone with sweet overdrive that the early Champs were famous for, has a lot to do with running the amp very "hot". Fender in particular are known for this and the results are the stuff that legends are made of. Sure, if you want long life from your output tube, reduce the cathode current by increasing the resistor. Likewise, for the screen. But then you can expect to lose some of the classic tone. Much depends on the output transformer impedance and your power supply voltage. The early champs ran from a 320 - 0 - 320VAC transformer which gave a B+ from the 5Y3GT rectifier/filter caps of around 360VDC. Actual anode and screen voltages were around 350 VDC. This is right on the 6V6's maximum anode voltage and is 35V above the maximum screen voltage. So with these potentials, the tube is being pushed to the limit. You would want at least 7K output transformer primary load impedance - preferably 8K ohms. If the supply voltage is lower (like 250 VDC), you can use a 5K ohm load with a 300 ohm cathode resistor. The trade-off to great screaming overdrive tone is short tube life. If you're prepared to change the 6V6 every 100 hours or so, you will get tone that can't be beat.

So what's next?

Thanks for the info, it all makes sense and as you said, is a matter of preference. I think I'm going to completely rebuild the amp with a more "friendly" layout so I can easy access certain components and test the effects of different values. So I may see what that "screaming tone" really sounds like, and then change back to conserve tube life.
So I pretty much know what I have to do with the Champ amp. My question now is, what amp should I build for my next project (in the approaching future)? I build the Champ bc I know a lot of people build these simple little amps and it is a good place to begin. What is the next step up? Post if you know. Thanks. -Todd

Re: So what's next?

That all depends on what you want - it's the reason so many different amps can exist on the market. You started with a basic all-tube amp with no frills. Where do you want to go from hee? More power? Pub/club gigs? Large auditoriums? Lots of knobs to twiddle? Reverb? Tremolo? Only you can answer these questions. No one amp will give you everything. As a second DIY project, I would go for for some useful gigging power - say 50 watts, using established output tubes (EL34's, 6L6 etc) in a push-pull class AB1 configuration. Maybe a compact all-in-one combo with a single 12" speaker, or a pair of 10" ... the Fender blue alnico model 036457 is cheap and sounds great. It's rated at 30 watts so a pair being pumped by 50-watts is a nice match. Think of re-sale value when you tackle a project like this. You are more likely to get a good price when you move on to your next project, if you follow long established guidelines and construction techniques. So my suggestion is to pick a well-known commercial circuit and build a clone.

problems! HELP!!!!

I have my Champ amp built and working, but something's wrong with it. It sounds great but when I hit the strings really hard the amp not only cuts out but starts flipping out. A lot of static is created and it sounds similar to the annoying sound of a bad input jack. I'm thinking that the tubes may be driven to cutoff and saturation. I have tried different tubes and different speakers. Can anyone help me?

Re: I'm building a tube amp, HELP!

I started building a amp about 8 months ago using a replacement power transformer for a vibroverb and a output transformer from the new custom vibrolux reverb
The power section is a copy of a old vibroverb and it is this section that I have problems with, I seem to get this blown speaker sound fron the output section and when I add a 2000 pf cap to the tube side of the 1500 resistor on each power tube to ground like a super-reverb aa270 the problem goes away but the amp goes from sounding super to super bad!
What could be the problem?

Post new comment

The content of this field is kept private and will not be shown publicly.
CAPTCHA
This question is for testing whether you are a human visitor and to prevent automated spam submissions. Do not include any spaces in your answer.
Image CAPTCHA
Copy the characters (respecting upper/lower case) from the image.

Contact | Contents | Privacy Policy | Forum

This site is published by Hitsquad Pty Ltd. Copyright © 1999 - 2022 , All Rights Reserved.

Affiliate Notice: As an Amazon Associate we earn from qualifying purchases.