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Quirk in Mullard 5-20 circuit

Hi there.

I'm finishing off a Mullard 5-20 amplifier, someone else's abandoned project. (Having just paid for all the iron I now see why someone would give up! Sheesh, that was expensive!)

My circuit is very slightly different to the other variants I've found on the net. I don't have the means to add a diagram so I will try to describe.

Immediately after the phase splitter, B+ is supplied to each leg via 2 resistors of equal value - typically 180k. In my circuit there is an additional 5.1k resistor in the supply to the "pull" leg. Can anyone tell me what this is for?

I'm curious, but tempted to take it out & revert to standard circuit.

Apologies for the non-technical description, but it's the best I can do. Help much appreciated!
Thanks
Eduardo
 
alot of times the plate resistor in a LTP on the "pull" side (the triode that does NOT have the input signal fed to its grid) is larger than on the directly driven input of the LTP. This is because the LTP is not perfectly balanced and that second output will be a little bit lower. In some amps there is a trimmer pot in series with the resistor (sometimes a slightly smaller resistor. maybe one fixed 100K resistor on one side, and a 91K resistor in series with a 15k or 25K trimmer). I'd just leave that small resistor in there, it probabally does some good, and shouldn't cause any significant harm if it doesn't
 
1st monoblock reached first test over the weekend.

Rather disappointingly I was greeted by motorboating as soon as B+ came on voltage, and after a few seconds the rectifier tube glowed brightly, the main fuse went pop. Very sensibly I didn't have a spare to hand...

I suspect my main filter capacitor is the culprit. I got caught up with the "authentic" thing & bought NOS PIO caps. Never used old caps before. These were hermetically sealed in foil bags, so I felt safe in regarding them as good-as-new. All other caps ARE new, modern electrolytics (in the power supply).

Before I rip out the suspect & bin it's expensive ***:

(1) what are other people's experiences with NOS caps? Am I right to be suspicious? Should I try re-forming?

(2) Are there any other ****-ups leading to motorboating not related to power supply?

Thanks
Eduardo
 
Paper anything has a bad habit of finding moisture when there's no way for it to get in. Wax paper is notorious for leakage and redplate. I don't care if it's hermetically sealed, and I don't care what audiophools say about it, heck if anything I'll do the exact opposite of what they say! At least I'll be following logic then! :D

Glowing rectifier can only mean shorted B+ rail. Gah, why'd you even leave it on long enough to turn red? Plates don't glow red!!! Use a multimeter!

Tim
 
eduardio said:

Before I rip out the suspect & bin it's expensive ***:

(1) what are other people's experiences with NOS caps? Am I right to be suspicious? Should I try re-forming?

(2) Are there any other ****-ups leading to motorboating not related to power supply?


I'd troubleshoot it a little more. Not so sure you can blame an old cap for motorboating all the time as you can do other things to improve the situation.

Is the amp stable without a signal applied? Or does it start up on its own?

Open up the signal path by removing a coupling cap from say the gain stage to the phase inverter stage and see if it still does it. Motorboating is more of a ripple effect down through the amp. Like when a fast transient signal, from a kick drum for example, puts too much current demand on the power supply and causes the B+ to sag, which ripples down through the other stages and the amp swings back and forth. There are various ways to work around this.

Not sure if reforming will help so much as you've already have been applying high voltage. But what you might want to do is to insert a small resistor, like 1 or 10 ohms, in series with that cap and then monitor how much leakage current you have for that cap. It should settle down to next to nothing in short order. If it creeps up high enough to blow the fuse eventually, then scrap it.
 
Hi Ed,

Firstly, check everything again, by means of a multi-meter as well as visually. Secondly, reform electrolytics before putting into use. An easy way of doing this (after confirming that your circuitry is correct) is by wiring up a normal household 100W globe in series with the AC mains to the transformer (assuming you do not have a variac).

The globe is a good means of testing, as the circuit stabilises, the glow will dim slightly. You can measure voltages in the circuit and allow for the lower voltage readings accordingly.

After a couple of hours, if all is well, remove the globe and connect directly to the 240v AC mains, whilst monitoring the rectified DC output of the rectifier on the first smoothing cap with a multimeter. Take care, the voltage is high. If in any doubt over the HT, wire either a 250mA slow-blow fuse or a small value resistor in series with the HT between the rectifier cathode and filter cap/choke.

You *are* running a GZ34 off its own 5v filament winding and not a 6v winding, I presume?

Keep us informed...

-Eric
 
Hi. thanks for the replies.

Power was only applied for about 25 seconds before the fuse blew, & I haven't had chance to get it stable long enough to check any voltages.

I've checked the circuit relentlessly for weeks - it's on a commercially produced PCB & I'm pretty confident there are no mistakes. This was someone else's abandoned project, I had to verify everything I inherited very carefully.

There was no signal applied during the test. There is a 500mA fuse in the HT line between the rectifier and the main filter cap (which remained intact). Rectifier has it's own 5v supply (CT taped off, not grounded). Heater voltages were measured without the rectifier in place & were a little high (5.9v from the 5v tap, but under no load I don't think that's a concern).

A few things I'm a little dubious about apart from that old cap.

The cathode bias resistors - which are off the board attached by flying leads, bolted to the case for cooling (is the length of the leads an issue? Maybe 6 inches away.)

Connection of the secondaries on the output transformer. It has 4 separate 4 ohm windings, not common & taps, so the feedback could have been positive rather than negative (but I think that would have resulted in a squeal rather than a buzz.) There is no connection for the ground side of the feedback loop on the PCB, so it's connected to the ground plane near the phase splitter.

Unlikely I'll get a chance to investigate further until the weekend, & I'll stock up on the fuses this time.

Anyway, your ideas would be much appreciated. Thanks for the support.

Cheers
Ed
 
eduardio said:


Power was only applied for about 25 seconds before the fuse blew, & I haven't had chance to get it stable long enough to check any voltages.

There was no signal applied during the test. There is a 500mA fuse in the HT line between the rectifier and the main filter cap (which remained intact). Rectifier has it's own 5v supply (CT taped off, not grounded). Heater voltages were measured without the rectifier in place & were a little high (5.9v from the 5v tap, but under no load I don't think that's a concern).

The cathode bias resistors - which are off the board attached by flying leads, bolted to the case for cooling (is the length of the leads an issue? Maybe 6 inches away.)

Connection of the secondaries on the output transformer. It has 4 separate 4 ohm windings, not common & taps, so the feedback could have been positive rather than negative (but I think that would have resulted in a squeal rather than a buzz.) There is no connection for the ground side of the feedback loop on the PCB, so it's connected to the ground plane near the phase splitter.


Sounds like your drawing too much current off B+ as the supply comes up and as the rectifier warms up. The buzz is caused by no filtering action since your drawing too much current and have way too much ripple voltage and/or the buzz is coming from the power transformer under too much strain.

The *relative* size of your fuses is probably off. That is, your AC input fuse is too small relative to the B+ 500ma fuse so the AC fuse blows way before the B+ fuse.

Did you pull the power tubes and try it? Beware of over-voltaging your supply caps as B+ can go very high without the power tubes to load it.

Monitor the voltage across the cathode bias resistors individually as the B+ comes up and quickly shut it off before the fuse blows. You should be able to catch it easy enough. But, using ohms law, figure out the voltage needed for your bias point. 6L6's? No more than about 40ma each. If one or all of the bias resistor voltages shoot past the bias voltage needed for say 50mA maximum, then you've got biasing problems. What is the size of your cathode bias resistors?

The length of the wires on your cathode resistors shouldn't really be a problem for audio stuff.
 
I believe you have "hit the nail on the head" in your post #12. I 'd bet money you do have positive feed back. This is really fairly common when your building a tube push-pull amp with negative feedback. Unless you are building a complete kit where the kit designer has worked out all of the connections to the output transformer, you have a 50-50 chance of getting the primary connected with the right relationship to the secondary connections. Get it right and the amp usually works great; get it wrong and, congratulations, you just made a power oscillator! And to answer you doubt about it being low frequency instead of a high frequency squeal, in my experience it has always been low frequency. Yes, I've had it happen to me a few times;javascript:smilie(':bawling:')
bawling
luckily I've never toasted any speakers - yet.

I think if you just swap the output transformer primary connections, you will fix your problem. If this is a real copy of the 5-20 Mullard shows in their book "Tube Circuits for Audio Amplifiers", it uses "distributed loading" (ultralinear) connections for the screen grids of the EL34s. You will have to swap those also.

If you disconnect the negative feedback like Tim says and the problem goes away, you'll know you have to swap the connections. By the way, when you do disconnect the feedback, you will probably get a lot of hiss and maybe some hum. If this is the true 5-20 circuit, Mullard specs it as 220 mV input for 20 Watts output WITH the 30 dB of negative feedback in place. Without the feedback, you have 30 dB more gain so, in this case, microphone amp might be a better description. You might want to short the input to ground before you turn it on.

Good luck and let us know how it turns out. I've looked at this circuit several times and wondered how it would sound.:bigeyes:
 
Hi, thanks for all your posts.

Not had much time to make any further progress yet - having to balance work, amplifer, & demanding wife and kids with care!

However I did find 10 mins to power up without the EL34's in place. After 20 seconds or so there was what looked like arcing in the rectifier. I switched off fast enough to save the fuse, but there was no time to investigate any further.

Ed
 
Hi Ed,

This sounds like the GZ34 is dying on you, internal short circuit. It can happen.

As others have said, you would be advised to at least short the input of the amplifier to earth before switching on. Better still, use a 1M pot as a volume/level control.

Should your amplifier howl upon switch on, simply remove the earth & neg feedback wired from the SECONDARY of the output transformer and swap them around.

Good luck!

-Eric