Weird voltages

Nestori · Post by **Nestori** » Thu 07/10/08 3:22 am

I finally got my 36Watt complete, built it by Richies REV5 and measured the voltages. First I checked the rectifier and the PT and they were OK (secondary output is a bit lower than should be, it's 275-0-275 = 550V). Then I installed the powertubes and the OT plus speaker and something went wrong.
There was a weird hum coming out of the speakers. Suddenly the hum started to change from hum to fizz and I switched the standby on. I encouraged myself and hit the switch. The 9th and 7th pins were ok, about 320-340V all of them. But I got 33 Volts from the third pin! I checked the poweramp section and everything was ok. Should I get bigger cathode resistors? The current are 150ohms like they should be. The OT is a H*mmond 1615 Hi-Fi Push-Pull, the wiring schem is on the end. Tubes are JJ EL84 and some chinese GZ34.
And I'm wondering also, what are those Sg:s on the OT? Screen grounds? What should I do with them?

Forgot to say that the amp is a normal Richies 36W TMB, with 2 inputs less and with added FX-Loop and channel switching. All the parts are (quite) quality (M*llory150's, Or*nge Dr*ps, F+T, A*pha etc.) and I used teflon-plated silver wires. Only thing that annoys me is that I used a crappy soldering iron, it's a home-made iron (maybe in the 70's?) and the soldering quality is reeaally poor. But next time will be different

Nestori · Post by **Nestori** » Fri 07/11/08 3:36 pm

Sorry to add! I measrued the voltages again after a couple of days break. The final results were these:

EL84 x 4
Pins 7 & 9 about 220-250V
Pins 3 about 33V

My knowledge ends here.

sub · Post by **sub** » Fri 07/11/08 4:16 pm

Nestori wrote:Sorry to add! I measrued the voltages again after a couple of days break. The final results were these:

EL84 x 4
Pins 7 & 9 about 220-250V
Pins 3 about 33V

My knowledge ends here.

Pins 7 & 9 is too low
Normal pin 7 voltage is about 320-340V
pin 9 is about 300-310V
pin 3 is very high... should be about 10-12V

morcey2 · Post by **morcey2** » Fri 07/11/08 5:35 pm

Check the voltage on pin 2 and make sure that the grid leak resistors are properly grounded. Also, make sure your cathode bypass cap is installed the right direction.

What you described sounds like the classic floating grid/leaky coupling cap scenario. If you're dropping 33V over a 150ohm resistor, you're pulling 220mA. The resistor would be dissipating almost 8 watts. Are you using a single 150Ohm or 2 separate resistors? If they're separate, are you getting 33V on all 4 cathodes? (pin 3).

From pin-2 on each of the EL84s, measure resistance to the chassis. It should be about the same as each of the grid leaks. From the description of the soldering iron you used, a bad ground is definitely a possibility. If those measure ok, lift one end of each of the power stage coupling caps and power the amp back up. If it starts to behave normally, one of the coupling caps is bad.

If none of that helps, start reflowing solder joints in the power-amp area. I would bet it's a bad ground connection for the grid leaks. Once that's fixed, your high-voltage should jump back up to normal since you aren't pulling 300+ mA through the power supply.

Also, pictures would help.

Matt.

Nestori · Post by **Nestori** » Sat 07/12/08 2:22 am

morcey2 wrote:Check the voltage on pin 2 and make sure that the grid leak resistors are properly grounded. Also, make sure your cathode bypass cap is installed the right direction.

What you described sounds like the classic floating grid/leaky coupling cap scenario. If you're dropping 33V over a 150ohm resistor, you're pulling 220mA. The resistor would be dissipating almost 8 watts. Are you using a single 150Ohm or 2 separate resistors? If they're separate, are you getting 33V on all 4 cathodes? (pin 3).

From pin-2 on each of the EL84s, measure resistance to the chassis. It should be about the same as each of the grid leaks. From the description of the soldering iron you used, a bad ground is definitely a possibility. If those measure ok, lift one end of each of the power stage coupling caps and power the amp back up. If it starts to behave normally, one of the coupling caps is bad.

If none of that helps, start reflowing solder joints in the power-amp area. I would bet it's a bad ground connection for the grid leaks. Once that's fixed, your high-voltage should jump back up to normal since you aren't pulling 300+ mA through the power supply.

Also, pictures would help.

Matt.

I used two separate 150Ohm resistors and yes, all of the cathodes are screaming 33 Volts. I was thinking that there might be a bad soldering in the groundings, and I actually found yesterday one misgrounding (hah, is that a word

) but it was in the pre-amp section. But I'll check the grid resistors etc. when I have more time!

In the other hand it doesn't matter if the amp won't work at the first, or the second try. This build was sort of a prototype-like, built-in-a-hurry amp. If this baby lights my fire up, then I'm sure this amp won't be the last one. I'm just curious if it will be worth the money and work! And I also like challenges!

Thanks!

I hope these advices will help me!

Nestori · Post by **Nestori** » Sun 07/13/08 2:11 pm

It worked! Thanks morcey2! And the cause was an ungrounded grid, how stupid man can be. So now the voltages are fine: about 340V on every 7th, about 320V on every 9th and 12V on every 3rd.

But...

...I encountered a new problem! No sound comes out yet! I installed the pre-amp tubes and measured the voltages. Everything was fine, except on the third pre-amp tube V3. When I tried to measure from pins 2 and 7, a horrible crackle came out from the speaker. My amp hums a little and everytime I touch those pins: "KRAGOOM!" and the hum is gone, and when I stop, the hum is back! The voltagemeter shows about 55 Volts on both pins. Also the NFB-switch is a pain in the ass. When I lift the switch it starts to squeal, and a lot! The feedback is incredible.

Waiting for the answer!

morcey2 · Post by **morcey2** » Sun 07/13/08 3:05 pm

For pins 2&7 on V3, don't bother measuring them. Unless you have a VTVM with a really high impedance, they won't show up correctly anyway. For a better explanation than I can make, look here:

http://www.ax84.com/faq.html#grid_dmm2

If you're getting squealing when the NFB switch is engaged (when its actually connecting the OT secondary to the tail resistor), you need to swap the OT primary leads, usually blue and brown. Just switch which set of EL84's they're connected to. What's happening is you're getting positive feedback instead of negative feedback, which causes the squeal. Swapping the leads changes the phase of the signal in the OT and turns it back into negative feedback.

If you're not getting sound through, but hearing a hum, odds are that they're something wrong with the input jacks. Do any of the controls alter the volume or tone of the hum?

Matt

Nestori · Post by **Nestori** » Sun 07/13/08 5:14 pm

morcey2 wrote:For pins 2&7 on V3, don't bother measuring them. Unless you have a VTVM with a really high impedance, they won't show up correctly anyway. For a better explanation than I can make, look here:

http://www.ax84.com/faq.html#grid_dmm2

If you're getting squealing when the NFB switch is engaged (when its actually connecting the OT secondary to the tail resistor), you need to swap the OT primary leads, usually blue and brown. Just switch which set of EL84's they're connected to. What's happening is you're getting positive feedback instead of negative feedback, which causes the squeal. Swapping the leads changes the phase of the signal in the OT and turns it back into negative feedback.

If you're not getting sound through, but hearing a hum, odds are that they're something wrong with the input jacks. Do any of the controls alter the volume or tone of the hum?

Matt

Good to know! Thanks again!

Yes, when I turned master volume, the hum (of course) got louder and with normal volume the hum got more crunchier, like adding more gain, naturally. And when I touched and turned the plug around in the input jacks I heard a crackling sound, although very small. So as far as I know, there might be something wrong, maybe in the groundings. But I'll check everything again and make sure I'm not mistaken again!

Nestori · Post by **Nestori** » Mon 07/14/08 9:52 am

Hooray, at least I can now hear my guitar through the amp! So, the hum is still there and the tone is really thin and volume is very poor. But all the knobs works fine and everything but there's still something wrong. I can hear my guitar but the sound is SO thin and poor and humming and almost no gain at all! And when I turn the master volume to half, the volume level is about normal when I play at home. Could it be the tubes?

morcey2 · Post by **morcey2** » Wed 07/16/08 7:44 pm

Glad it's working more better.

Can you measure the voltages on all the tube pins for each of the tubes and post them? Also, do you have any gut-shots? Those would help immensely.

Matt

Nestori · Post by **Nestori** » Thu 07/17/08 10:06 am

So here's the fuzz:

morcey2 · Post by **morcey2** » Thu 07/17/08 10:28 am

Something is very wrong with the fx loop (V4). If those are the voltages you're measuring, you're dropping a grundle of voltage somewhere. Have you tried taking the fx loop completely out of the circuit and seeing what you get? Connect the output from the tone stack directly to the MV and see what you get soundwise.

The rest of the the voltages look good.

Again, is there any way you can post gut shots?

Matt

morcey2 · Post by **morcey2** » Thu 07/17/08 10:39 am

Your cathode follower is wrong. If it's hooked up that way, the grid is sitting at 0V. You need a DC-blocking cap before the grid leak (1M) and the grid leak needs to connect to the bottom of the CF bias resistor.

Look here. Merlin (merlinb here) explains it better than I can:

http://www.freewebs.com/valvewizard/accf.html

Basically, between the cathode of the CF triode and the top of the 100k resistor, you need a bias resistor. Something between 800 and 1.5k is usually good. The 1M resistor then connects to the junction of the bias resistor and the 100k resistor. The signal can still be taken off the 10k.

You'll probably want a cap on the return before the return level pot. That'll keep any rogue effects from messing with the bias of the recovery stage.

Matt

Nestori · Post by **Nestori** » Thu 07/17/08 4:10 pm

Ah, that explains. I'll have to test when I have time again! Thanks again Matt!

Nestori · Post by **Nestori** » Thu 07/17/08 4:28 pm

A quick information!

The Amp is working! HUH?! I should've done it slower and with more thinking, because I forgot the B+ from the FX-Loop! Ah what was I thinking. I'm hopeless

But now the amp is good and oh yes, the sound! Brilliant! Only thing that matters is the hum that is always there, but not when the amp is in standby, I have to check all of my ground points. And the amp makes fizzy noises sometimes and the normal-channel hums even more than the TMB-channel? There's always a little fizz on the backgoround and the fizz kinda cuts through the sound of guitar. When the fizz gets louder it's all hell loose... It starts when I turn the amp louder or turn more tone. And the Normal-channel is useless because of the hum is so loud. And after playing for some time, you can't hear the guitar almost at all on the normal chan. Bad soldering I think. And and and.... well.... back to the workshop, eh?

But should I still wire the fxloop cathode follower correctly? Now the voltages are normal in the V4 but how would it differ from the present situation?

And morcey2! I don't have a camera at the moment! Darn...

Also "get-rid-of-hum"- tips and hints are welcome!

lasvegasira · Post by **lasvegasira** » Thu 07/17/08 6:31 pm

Would you be willing to repost the schematic for the reverb with the changes on it? I'm pretty sure I'm getting what you said needed to be corrected, but I want to be sure so I don't blow anything up.

Thanks!
Dave

Nestori · Post by **Nestori** » Fri 07/18/08 9:55 am

So here's a short clip. There's a small click in the beginning, it's the standby switch, and then hummmmmmmm... You can hear how I turn the gain, volume tone etc. and then when I turn more volume the annoying oscillating begins.

Nestori · Post by **Nestori** » Sun 07/20/08 1:38 pm

Yup, I've done some research and stuff.

I love the amp. It sounds just perfect for me, but the hum is so irritating and I can't turn the master volume louder than 8 o'clock and the squealing begins. I tried to poke the wires with a chopstick and checked some other things too. So I think that:

1. I have too many wires crossing other wires and the tube sockets. When I moved couple of the wires closer to the PI-tube socket the nasty oscillating began. Also there were areas where the hum wasn't so loud. What was the trick to change the PI grid wires?

2. My wires are too long, so I have to chop chop chop around.

3. I have to check my channel-switching unit, because when I tapped the board with the stick, the hum was nearly away.

4. I think I'll remove the whole FX-loop, the delay and other effects sounds great in front of the amp!

5. I also have to check my solderings. This time with more concentration and time.

6. Plus when I run the amp for over half an hour, the PT gets very hot. I can touch it but it's hot, so should I be worried?

7. AND The hum goes COMPLETELY away when I remove the PI tube. I tried to remove the other tubes but the PI was only one with the anti-hum reaction!

Tips and hints are welcome!

Nestori · Post by **Nestori** » Mon 07/21/08 2:15 pm

And oh yes! The hum... what hum?

Finally, the amp shows no signs of hum! I removed the channel switch-unit and the fx loop and twisted some wires. I also did the cascade-mod without the switch (think I'll add one more input for the normal channel and the cascade switch) and now I have a roaring monster on top of my speaker cabinet! The tone is awesome, I can play anything from jazz to heavy riffs with this baby! It cleans up so smoothly when you roll down your guitars volume and when on full steam, the power is unstandable! With a tele you get lovely harmonics and a very dynamic tone! This won't be the last amp I'll build! I'll have to clean up the mess a little, rewire some parts and give the amp a finishing touch! The top cabinet is empty and waiting for a companion...

And thanks to morcey2 and all of you for your patience and help! I learned a lot while building my frankenstein!

Thanks!

-Nestori

Donations

Links

Welcome

Weird voltages

Weird voltages

Voltages.