Problem with excessive current draw

[joe1968]

Hi,

Some days ago a friend brought his old bass & organ amp over for an overhaul. The amp is a head from the 70's (brand: Top Sound - nothing really valuable) which is basicly an AC30 circuit with 4 EL84 and a 2 channel input section.

The amp was working to a certain extent but the sound was poor and had really bad overall noise level (hum).

He want to use it for guitar and I convinced him to change the circuit closer to an AC30 TB. It looked like an relatively easy job, but it turned out to be a nightmare.

Here is what I've done so far.
1. Re-built the power section (new caps, power resistors, tube sockets, new rectifier diodes, new tubes, etc.). I've built the power section pretty much to specs from the Hoffman AC30 layout. The original was just using a cathode resister 84 Ohm instead of the 50 Ohm. One side of the filament was connected to ground. I changes this for 2 x 100 Ohms to ground (but also changed it back later during trouble shooting).
2. Rebuilt the pre-amp section to AC30 specs too (no tremolo).

So far so good.

When I turned it on first time w/o tubes everthing looked quite within the ballrange.
B+ around 310 VDC (filter caps higher without load of course), Heaters 6.9 VAC, etc. current limiter installed.

After I installed the power tubes (the old ones which have been working before - didn't want to risk the new glass) my current limiter was shining quite bright and the I had a serious voltage drop on B+. The Voltage drop over the cathode resistor was way over 15 Volts DC.

Double checking all connections and grounds didn't tell me anything new.

Then I tried a rather stupid thing by switching it on again without the current limiter. B+ was around 300 VDC, but I had a voltage reading at my cathode resistor of 50V (only for a couple of seconds!) Before I was able to switch it off, the cathode resistor was going up in smoke together with one of the 100 Ohm R's. My Variac unfortunately was at line voltage. Don't know what I was thinking.

Connecting the currernt limiter again. As a next step a swaped the cathode resistor for a 150 Ohm 10 Watt. But no difference. Still excessive current draw (of course not the same with the current limiter. The resistor was getting hot but didn't burn up). B+ dropping to 170 VDC. Bulb bright on when bringing the voltage up to 230 VAC.

I tried already with OT connected and w/o. Same result. The amp by the way is humming with the light bulb installed and AC voltage at line level!

Without the light bulb and AC line voltage reduced to 120 VAC. B+ is around 160 VDC, voltage drop over cathode resistor is about 15 VDC (with the 150 Ohm). Heater was around 3 VAC. When I tried to go up with the AC line voltage the thing is drawing way way too much current.

I douple checked the power section now over and over and couldn't see what I have done wrong. And I have done something wrong, because the amp was working with the same components (values) and circuit before.

I know it's something stupid I am missing here. But maybe there is someone out there who has some more ideas for me where to look at.

Any ideas are appreciated.

Best, Joe

[Billy R aka DynaFreak]

First things first. I would disconnect all secondary leads from the circuit, isolate them from each other and try your current test again. if it pulls current then you know you have a PT problem. If the pT is not the problem then you connect the power supply circuit (caps, etc) but not what it supplies (tubes). then test again. Isolating sections of the amp is a good troubleshooting technique, especially for a homebrew.

Another technique that I use on amps that I am repairing is to connect a 6VAC wall wart pwr supply to the rectifier circuit and let it power the amp at very low voltage (I have one set up with gator clips). That way I can safely get in there with a DVM and start checking voltages at different points to look for trouble. I usually start at the power supply and work downstream. of course the low voltage will not power up the tubes but that is not objective. This method lets you get in there and do some trouble shooting without blowing a bunch of fuses and the aggravation that causes.

[Chuck H]

The power supply is almost certainly wired wrong. I would suspect is the filament circuit because you tried to wire it differently and then put it back to stock in the course of trouble shooting. Re check to see that it's wired correctly. OTOH you may want to wire it correctly with the 100 ohm resistors as a CT. The nasty hum is likely partly due to the fact that your filaments are not wired to cancel out heater circuit hum impressing on the circuit.

I might suspect the HV wind because it's drawing excessive current independantly. But that could also be a bias issue. Are you sure all the el84 grids have a proper 0V reference?

Are all your new caps in correct polarity? All your diodes?

Are you familiar with power supplies or did you re wire it rote from some outside information, schem, personal notes, etc?

Things are almost certainly miswired. That's actually good news because you know that your looking for mistakes in the power supply (for now). Since it's not a really involved circuit it shouldn't take too long to look it over. If you don't find the error/s it shouldn't take too long to look it over again Keep looking.

Chuck

[Alex R]

Might be worth trying it without the power tubes.

[joe1968]

Thanks, I tried a couple of things today.

1. I rewired the filament back to original design. One side of the filament is connected from pin 4 of the output tube to the ground of the cathode resistor now. I have now AC heater voltage reading to ground (6.5 VAC with tubes installed).
2. Disconnected all secondary wiring from the PT and made some testing. All voltages seem ok and no shorts.
3. Disconnected the choke for testing. Resistance 147 Ohm.
4. Put everthing back in. Still too much current drawn.
5. Changed for a 300 Ohm Cathode resistor. Problem is still there.
6. Tried with new tubes. No change.

What do you mean exactly with "does all the grids do have proper 0V reference"? Sorry my English isn't so good. What do I have to measure to check?

All caps and diodes are in correct polarity!

I have built a couple of amps so far and restored some vintage amps. I am not very experienced but know about the basics (obviously not). I took a couple of pictures before I started to work on this amp. So I have quite good information what I've changed in the process. As I mentioned before, I have built the power supply according to the AC30 Hoffmann Layout. The original design was more or less the same except the solid state rectification and that the power section and pre-amt section are located in 2 different parts of the cassis.

I will once more go through my wiring and layout. You are right, it must be miswired somewhere. The layout isn't that complicated.

Joe

[Chuck H]

You should stop trying to correct the over current problem by increasing the cathode resistor. That is not the problem and may only serve to confuse the repair. You should put the correct value back in the amp.

The 0V reference on the power tube grids are a critical part of the bias supply for the amp. Every power tube grid should have a resistor (probably 100k to 470k) that references ground with NO CAPACITOR between that resistor and the grid. There may be a series resistor of some lower value between the grounded resistor and the grid. That is OK and normal. Even if you do see the grounded resistors in place you should use your meter to test the value of these resistors and that the end that is supposed to be grounded actually is grounded. It sounds like this might be a problem based on your symptoms.

Chuck

PS: You metioned that you replaced the power tube sockets. Carefully check the pinout for the tubes compared to your wiring. Check for miswired pins. If there are any unused pins being used as mounting posts for components check to see that there are no mistakes there.

[joe1968]

Thanks.

I checked the 0v reference on the power tube grids. You have been spot on. I missed a ground connection from the 220 k resistors.

No excessive current anymore. Installed the original cathode resistor (84 Ohm). Now I am a bit worried about the voltage readings.

1. 292 VDC after rectifier.
2. 1st main supply filter cap: 398 VDC (there have been two 47mfd 450V installed - I changed them for 500 V)
2. After choke: 396 VDC
2. pin 3: 12.7 VDC
3. pin 7 384 VDC
4. pin 9 394 VDC

I didn't take the voltages when the got here. Should have done it.

Could this be normal or do I still have a problem?

I didn't put in pre-amp tubes so far. But the power section seems to work. There is a hum coming off the speaker. Not that bad. Maybe I have to take care of it later.

Joe

[Alex R]

Hum from this power section could be due to an imbalance in the push-pull. You can generally minimise it by swapping tubes around if it's that.

[Chuck H]

The hum is there because you don't have the filaments wired as a humbucking circuit. You started to wire them that way and then undid it during troubleshooting. It can still be done and probably should.

We don't know what the HV was before the work so I can't say if your HV is normal for that amp. Many amps did run very high volts on el84 tubes. Often, for bass amps and PA type amps (like the one you have) the actual power tube is supposed to be the 7189a which is basically an el84 all beefed up. Higher maximum specs. Modern versions of that tube do exist but with different numbers. There is the Sovtek el84m (m for "military version") and the russian 6p14p-xx (ep, eb) tubes. I don't know much about them except that they're durable and can take higher volts.

IME it's OK to run regular el84's at or near 400Vp but the sound will be very wrong for an AC30 type amp. Too hard, bright and stiff. To avoid rebuilding the entire preamp you may want to add one or two top end bleeders to calm any brittle treble frequencies. But test the amp first as it is of course and decide for yourself.

Two things about your voltages that stand out are the after rectifier reading being lower than the main cap reading. This doesn't make any sense to me because in my head I see those as the same point on the schematic.

The other thing is your cathode voltage of 12.7V. We can use this number and your cathode resistance to figure the current being drawn by the tubes at idle (bias)

12.7/84=.151, this is the current being drawn by all four tubes. So, in a perfect world, each tube is drawing 1/4th of that. So... .151/4=.0378, You can multiply the current by the voltage to figure the watts, so .0378*396=14.968 watts. Since those tubes have a max dissapation of 12 watts I'd say that's too high. This isn't the whole story because we haven't subtracted cathode volts or figured for screen current. But 15 watts at 400Vp is just too much for a normal el84 tube. The Vox AC30 is a class A push/pull design and runs the tubes quite hot with that resistor and only 330Vp. Class A simply isn't going to be possible at your plate voltage, the tubes won't take it. And the stock cathode resistor for the Vox is simply much too low. With your plate volts you should aim for about 9 or 10 watts toltal dissapation per tube. Try a 100R cathode resistor for starters. You may need to go higher than that.

So, being at higher plate volts and biased in class AB1 instead of class A that amp won't really be much like an AC30. Sure it's using 4 el84 tubes like an AC30 but my truck and my car both use four 15" wheels and they're very different vehicles. It's OK though. Once it's up and running you can revoice if needed and maximize the amp for what it is.

Your other option would be to reduce the voltages with a big honkin zener diode. You could use a diode AND a resistor to simulate the "sag" of a rectifier tube also and get a more authentic Vox tone. If your interested in reducing volts there are a bunch of posts here. Just put "zener" and "voltage" in the search box and you'll be reading for a couple of hours.

Either way one thing I would definitely do is rewire the filaments to be humbucking. It's really not hard to do. Two 100R resistors are grounded together at one end. The two filament leads go one each to the ungrounded ends of those resistors and then to filaments of the tubes. Keep track of which lead is which and wire all the tubes the same. Niether end of the filament wind is grounded anymore with this wiring. I can draw a schem of the principals if you like.

Chuck

[joe1968]

I rewired the filament with 100 ohm to ground. Did the hookup of the pre-amp section today. No more miswiring there ;-).

The amp sound great but a bit stiff due to the overall voltage level in the amp. The overall noise level of the amp is quite good already. Still have to work on the lead dress a bit. I will try a higher cathode resistor to get the dissipation down to 12 Watts.

I want to get the amp in a "working condition" before I take care of the plate voltage issue. I have to speak to my friend (it's not my amp) if he would like to invest in some EL84M or maybe in a filament transformer and a rectivier tube. What would be the voltage drop with 5V4 roughly?

Joe

[Chuck H]

A 5v4 will only drop about 25 volts in that amp. That will get you into the safe zone but not into the class A volts zone. I think a 5ar4 will get you down about 35 volts and a 5u4 will get you down about 45 or even 50. With four el84's I think that's pushing it for a 5v4 anyway, but I'd have to look that up so don't quote that.

FWIW I think if you want some sag that the "add a filament transformer and rectifier tube" plan is a good one. If you have room on the chassis and $$$ in the budget. Otherwise a BAR (big ass resistor) can give you the same thing.

The amp may continue to sound a little stiffer than an actual AC30 (which may not be good considering that they are bright amps anyway). The Vox has a Vp of 330 I think. With a 5u4 you'll still have 360+ Vp. I think a zener string on the CT of the B+ wind is the easy way to go. When you get the Vp down the amp will soften a lot without as much need for 'sag' to soften the attack.

Or you could just revoice it to maximize what it is like I suggested earlier. You could probably do all the revoicing with the coupling cap from V1A and the tone stack component values. To me trying to make it into a Vox is like trying to make a truck into a car. It's a lot of work and everyone will still be able to tell it was a truck. But soup up the truck as it is and you've got something.

Chuck

[joe1968]

Changed the cathode resistor to 120 Ohm. Plate voltage is now at 394 VDC and Cathode is 13,9 VDC. If my calculation is right the tubes running at appr. 11,5 watts (or a bit less if I do the complete calculation). Maybe I try to get a 100 Ohm.

I am still a bit confused about the voltage measures I took.

1. Each side of the HV winding is showing 320 VAC. (between HV CT and one side of the HV winding).
2. After the rectifier I measure 288 VDC to ground. Standby switch off. 0 V at the first filter cap.
3. When I switch on the standby, I get 401 at the first cap and the rectifier. Which makes sense as the 1st cap is connected to the rectifier.

Maybe I am missing something. But should the first filter cap just load up to the DC coming off the rectifier?

Joe

[loudthud]

The 288 VDC is the "Average" you should get after full wave rectifying 320VAC when you don't have a filter cap connected.

RMS * 1.414 = Peak
Peak * .707 = RMS
Peak * .636 = Average
RMS * .900 = Average

[Chuck H]

Still learning. That confused me too.

Try your 150 ohm resistor on the cathodes. Using a 100 ohm is going to heat things back up a little.

[joe1968]

Hi I am back. It took while, but finally I was able to speak to my buddy and he decided to invest in the rectifier tube solution.

I bought a Hammond transformer (266M5) and a 5Y3GT. Drilled a whole for the tube socket and some more for the transformer put everything together and fired up the amp.

The plate voltage is no at 330, which I guess it quite acceptable. The amp sounds nice. The amp had a very good noise level before. Now I hear an annoying hum coming off the speakers that hasn't been there before.

I guess it has something to do with the rectifier tube or transformer. Any idea? I hooked up the transformer at the primary side to the line AC voltage and the secondary in series (5V) to the pins for the heater. I do not need any grounding, do I?

Thanks for your advise.

Joe