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arkay 18th December 2001 11:27 AM

Help - Troubleshoot
Hi all,
I have finished construction of my first tube power amp.
I have built it based on a schematic found on the Bona's site. The url to the schematic is EL84 Quad Single-Ended amp.

The only change I have made to the schematic is that I have added a 100K pot as volume control. I have connected the input signal to one end of the pot, the center lug of the pot is connected to the amp, and the other end is grounded.

I am using this amp with a home built tube preamp (schematic again from the Bona's site). The url to the schematic is:
Link to preamp schematic

I am facing the following problems now:

1. When there is no input signal, if I retract the volume pot to the minimum postion, the amp is producing a howling sound in the speaker. If I reduce the volume control when input signal is present, the amp is not howling.

2. The amp is producing a kind of motorboating sound. This is very evident and annoying during soft music passages.

I request all of you to help me in solving this problem. Any help or direction in this regard would be very highly appreciated.
Thanking all of you in advance,

GRollins 18th December 2001 03:41 PM

1) What value did you use for the pot? Some tubes have a maximum resistance they will tolerate in the grid circuit. Above that, they oscillate.
2) Classic problem. Your gain stages (or channels) are coupling through the power supply. You'll need to decouple the various stages and/or channels. Regulated supplies are another option.


arkay 19th December 2001 04:05 AM

Dear Grey,

I have used a 100K dual pot as the volume control.

>2) Classic problem. Your gain stages (or channels) are coupling >through the power supply. You'll need to decouple the various >stages and/or channels. Regulated supplies are another option.

Can you please tell me how to decouple the various stages and /or channels? Any information would be very helpful to me.
Thanks and regards,

GRollins 19th December 2001 04:31 AM

100k sounds good for a volume pot, but I don't understand why you've put one on the amp when the preamp already has one.
The amp shows a 100k grid stopper resistor (the resistor in series with the grid on the first tube). That sounds really excessive to me. It also adds to the grid circuit resistance. People can argue back and forth all day about grid stoppers, but in this case I think it might be worthwhile to see what would happen if you removed it from the circuit entirely, or at least reduced it drastically. When I use a grid stopper, it's usually on the order of 100 to 1k.
If you've built the amp as shown, the first stage rail should already be decoupled from the output rail. The 10k 1W resistor leading into the 100uF cap for the front end decouples that cap from the output rail. Did you use that resistor?
Motorboating results from successive stages drawing current at slightly different times. The small variations in rail voltage modulate the output of the tubes. Setting the rail for each stage up with its own cap, separated from the other parts of the amp, usually cures it. Sometimes you have to juggle values a bit, though.


Just noticed something. That preamp has two outputs. Which one are you using, the low Z or high Z?

arkay 20th December 2001 05:08 AM

Dear Grey,

If I remove the 100K grid stopper, is there any other changes in the circuit that I should make?

Yes, I have used the 10k 1W resistor leading into the 100uF cap.

I have connected the amp to the LowZ output of the preamp.

Yesterday, I turned up the volume pot in the amp to the max volume position before switching it on, and used the volume pot in the preamp to control the volume. The motorboating has reduced a lot, but is still audiable. Now the hum has gone up to a noticeable level. Also, I think the motorboating is getting triggered by any sharp percussion beat in the music. Once the motorboating starts, it persists, till I switch off the amp, and switch it back on after some time.

Please help.
Thanks & regards,

GRollins 20th December 2001 01:37 PM

That grid stopper isn't really related to any other part of the circuit, so you can play with it all you want without having to recalculate anything.


arkay 21st December 2001 04:52 AM

Dear Grey,
Thanks for yout reply.

In the schematic, one end of the 100K resistor is connected to the frid of the first tube, and the other end is connected to one end of a 22pF, 160V capacitor, the other end of which is connected to the anode.

When I remove the 100K resistor connected to the grid, what will happen to the 22pF capacitor? Should it be left as it is, with one end connected to the anode and the other end floating?

How and where do I connect the volume control and the input signal? If you are advising me to remove the volume control altogether, where should I connect the input?

Please bear with me for all these simple doubts. I am a software professional, and my only experience with hardware is in the digital domain.

Please advice.
Thanks and regards,

GRollins 21st December 2001 05:28 AM

Um, hang on a second. There's something wrong here. I just took another look at the schematic so as to be able to visualize what you're describing.
Granted, it's late, and I'm so tired my eyes are crossed, but I don't see a resistor of any sort from that grid to ground, and there should be.
I'm betting that there's an error in that schematic, and that the 100k resistor should go to ground instead of to the grid of the tube. That's a much more normal arrangement. Check it against some other designs and you'll see what I'm talking about.
What voltage are you reading on the grid? Measure it as you change the pot on the amp and see if it changes. (It may or may not change, depends on whether you've got grid current.)
The 22pF cap is a local feedback loop. Leave it alone for the time being.
The 100k pot is presently your only connection to ground, so as you vary the value, you're changing the nature of the feedback through that 22pF cap.
Could be giving you the oscillation (the howling, I still think the motorboating is power supply related).
I'd say to connect that 100k between the grid and ground, run the signal straight into the grid, take out the pot entirely, and leave the 22pF cap where it is.


paulb 21st December 2001 07:07 AM

Gee whiz, Grey, do you ever sleep?

arkay 21st December 2001 07:27 AM

Dear Grey,
Thanks for the response. I am at work now. I will connect the 100K between grid and ground, connect the 22pf to the ground side of the 100K, and feed the signal direct the the grid, once I reach home after work. I will let you know the results, along with the voltage at the grid. Sorry if I delayed your sleep.

Thanks & regards,

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