Bias Question on 1971 50 Watt 1987 lead small box

[Jude]

Hi all,
I recently picked up a 1971 50Watt small box and am getting what looks like some high bias readings and am having some difficulty getting a set of tubes to match in the desirable range - the amp has a bias adjuster but i am unable to get the reading below the following - I have tried various tubes as follows: the amp came with some old tesla E34L and the readings on pin 8 of V4 & V5 were 60mv and 70mv approx - i then tried some new JJ E34L tubes which i picked up and were supposed to me a matched pair and got readings of 57mv and 40mv - I then tried a set used of Marshall winged =C= and am getting a good match but again in the 70 mv range. I am waiting on a new set of factory matched mullard el34 and hope that these will bias at the correct idle current. I have also tested the current winged =c= for mA on pin 8 and am getting a reading of around 21mA which i thought was curious. The other readings i am getting through the amp are as follows:

Amp is running on 240V AC supply in Europe

With No Tubes installed:
V1 pin1 469V
V2 pin1 473V
V3 pin1 479V

With Tubes Installed:
AC = 3.36

V1 pin1 135V Pin3 .96
V2 pin1 160V Pin3 1
V3 pin1 205V Pin3 41
V4 pin2 3.36 AC Pin3 449 pin4 443 pin5 -35 pin6 449
V5 pin2 3.36 AC pin3 449 pin4 443 pin5 -53 pin6 449

-36.5 neg bias volts

Rectifier pin1 451 pin2 346AC

The amp sounds great - just dont want to run it to hot if that will damage anything - Any of your comments, opinions and help is much appreciated!

Thanks,

Jude.

[neikeel]

Hi Jude

I am a little confused!
First, you say it is a 1971 Model 1987 - that amp does not normally have a rectifer valve - they are solid state, that era is typically a cream circuit board with a square diode block mounted on the board between the bias supply (220k resistor and diode) on one side and a pair of snubber caps (often missing) the bias feed should be taken off the hot side of the standby switch.
So, is this really a '71 Model 1987 converted to valve rectifier or an earlier JTM50/JTM45 or a rare valve rectified '67 JMP50?
Pictures might help...........

Next, I presume that you have high stability, close tolerance 1R (1 ohm) resistors mounted between the earth lug and tied pin1 and pin 8 of the output valves? And you are using a decent multimeter? If not both of those things need correcting. Sorry to be pedantic but if we do not sort that stuff out we are p&*^ing in the wind .


-if all the above is correct you can go ahead.
-that bias voltage is a smidge too high (or too low if you do not flip for the -ve sign). Normal starting point for overall bias voltage is around -40v or so before you start fine adjusting each tube with your bias pot.
-depending on what has been done to the amp (mods, 6550 vs EL34s and back?) you will need to change the value of the bias resistor. It is usually a 47k in series with a 25k pot (range 47-72k) but you can see 56k and 68k there depnding on what mods/valves people have used.
Remember your plate voltage will vary depending on how much current your valves are drawing even at idle.
Typical pin 3 voltages on a ss '71 JMP50 are 420-440vdc.
This equates to a bias current (measures as mV across your resistor extrapolates as 39-41mA).

Post a couple of really clear well lit pics (or links to) of your board and amp and you can get it dialled in

[Jude]

Hi Neil,
Thanks for your reply, The amp is definitely a 1971 50 watt small box - its a rosemary and mostly original from what i can tell - i tried to post some pics - but the site is saying they are invalid - i posted some on the marshall forum at the following address :

http://www.marshallforum.com/lets-talk- ... stion.html" onclick="window.open(this.href);return false;

It is has a solid state rectifier ( sorry for mentioning pin when detailing the readings there) - i took the readings a few weeks ago when i pretty much got a reading from all points of relevance and have no reason not to trust my Volt Meter - the amp has definitely had some work done on it as it has diodes across the output tubes, the speaker ohm selector has been replaced and it looks like the screen grid resistors have been changed along with one of the tube sockets and the power cord - other than that the amp seems pretty stock... transformers and caps etc....

Thanks,

Jude

[toner]

Have you measured the 1 ohm resistors to see if they are accurate? If they are, you have a weird batch of mismatched tubes there!

Based on most of the measurements listed, you need more negative bias voltage to bring the idle current down. I would reduce the first "dropper" resistor before the diode. It looks like a 220k. Try 180k or 150k.

[neikeel]

Sorry can't see the pics (members only) I only hang here, Plexipalace and Marstran, life too short for too many fora.

As we both suggested check your 1ohm resistors, their location and quality of contacts. Measure them too.

There are different ways of skinning that bias voltage cat! I never change the feed resistor or the 15k and tend to vary the 'bias' resistor itself.

Try to post a single pic of the bias section of the board and the output sockets here before changing stuff!

[Jude]

Hi Toner and Neil,
Thanks for your help - Yes the resistor before the diode on my amp is a 220k but looks like it is original to the amp - i saw on the metro build schematic George recommends a 150k ????- I will try to test the 1 ohm resistor and let you know.

Tyr this link to see the amp - this is the link to the add from where i purchased it! pics still up !!!:)

http://www.adverts.ie/guitar-amps/1971- ... ge/1078602" onclick="window.open(this.href);return false;



Thanks,

Jude

[neikeel]

http://www.adverts.ie/guitar-amps/1971- ... ge/1078602

Yes typical 71 lead amp, probably a real snorter? Pretty stock including bias circuit.

Those 1ohm resistors are Welwyn 5% and not good enough for setting your bias.

1ohm 1% or better are needed.

Diode block and snubbers (and lamp) replaced. Personally I would do those resistors first then play around

[Jude]

....Yes the amp is really sweet - i am using it as the front end daisy chained with a JTM45 which I put together with the help of MetroAmp.

I notice that George said to use a 1 ohm 1 watt resistor in the 5o watt kit and a 1 ohm 2 watt in the JTM 45 kit - does it make a big difference what wattage resistor to use and do you have a recommendation as to what would be the very best brand/type of resistors to put in?

Thanks again for all your help!

Jude.

[neikeel]

....Yes the amp is really sweet - i am using it as the front end daisy chained with a JTM45 which I put together with the help of MetroAmp.

I notice that George said to use a 1 ohm 1 watt resistor in the 5o watt kit and a 1 ohm 2 watt in the JTM 45 kit - does it make a big difference what wattage resistor to use and do you have a recommendation as to what would be the very best brand/type of resistors to put in?

Thanks again for all your help!

Jude.

Does not really matter. There is very little current there and only mA so 0.5 to 1w is fine. Just get the ones that are close tolerance <1% is optimum.

In a 50w you can use transformer shunt method if you have a decent meter (that is what I use on vintage two pot amps) and you are careful

[Jude]

... i know it been a very long time but i only got to get this solved now! - as Toner suggested the "Dropper Resistor" was the culprit - it was way out of spec - I replaced it with a 70's NOS PIHER 180K and the amp is dialed in and sound great! - Thanks Very Much for all your help guys!!!