Quirk in Mullard 5-20 circuit

[eduardio]

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

[alexistheo]

the different plate load on the second tube in the LTP is there to help AC balance. mullard didn't specify this (because 1% resistors were unheard of back then?), but i believe a ~3% higher plate load on the 2nd half is now good practice.

[eduardio]

Thanks, much happier now. I'm nearly ready to apply some power & see what happens. One last check over first maybe...

Much appreciated
Eduardo

[Sch3mat1c]

Can also use a CCS, either an additional pentode (6AU6, etc.) or a transistor or MOSFET (400V+ Vce/Vds, >20mA Ice/Ids), for the tail. Then the signal balances perfectly by math alone (whatever the tube splitting it!).

Tim

[kopite]

Not sure if you have seen this site. This is the Mullard 5/20 with lots of information.
http://www.lundahl.se/claus_b.html
Hope it helps.
Phil.

[Dr. Photon]

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

[eduardio]

Thanks a lot for all the advice. The resistor stays.
Should be finishing & testing over the weekend - I'll let you know how it goes.

Cheers
Ed

[eduardio]

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

[Sch3mat1c]

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!

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

[Gilbert]

Originally posted by eduardio

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.