The Newport Stereo - Series 444; adjustment process? Balance and Hum

endervox

endervox

Active Member
Good afternoon! I recently picked up a The Newport Stereo - Series 444 tube integrated amplifier on trade. Inside someone had previously partially recapped it. This obviously needs further restoration, but what I'm looking for is how to adjust the amplifier. There are left and right balance pots and a hum adjustment.

When I got the amp it was my understanding one of the 6BQ5's on the right side is red plating, so I haven't done anything with it. I have a couple replacement tubes I can throw in but would would like to understand an adjustment procedure first.

I've done a lot with SS but less with tubes, but know all the risks and dangers.

Any help?
 

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What's in the box?
The little foil box?
The balance is used to balance the phase inverter.
Don't mess with it unless you have a scope.
Theres no bias adjustments.
a redplate tube indicates a bad tube, bad connection, or failed coupling cap.
start by verifying that the tube does in fact red plate then move the tube.
if that tube still red plates then its safe to assume the tube is shot.
sifti the new tube in that socket red plates then you have a connection issue or a failed coupling cap.personally I'd change all of the coupling caps out, clean the sockets and try again.

The big aluminum cans should be replaced. Best bet for those who want a clean install and quick turn around is to order them from hayseed hamfest.
i dont recommend scabbing in individual caps under the chassis for a variety of reasons. Heat being one and ground current routing being the other.
You could also restuff them but its time consuning and not a lot cheaper...

Knobs look like they belong to a C8.
Don't know anything else about it.
Looks to be cathode biased and would probably benefit from some recapping and an EFB conversion if you can live with a little sand in your amp...
 
Last edited:
Thanks for the quick reply! The foil box appears to be the input, which I assume is shielding? Since posting I got my tube tester out and 3 are marginal, the 4th very low (the one that apparently was red plating). I don't have a matched quad but do have 2 different matched pairs I can test with.

I do have a scope but wouldn't know exactly what would be required to balance the phase inverter. Sounds like the first test is bringing it up to see what's red plating.
 
The balance adjustment in that case gets adjusted once the power supply is recapped the coupling caps are verified to verify good and any drifted resistors have found their way into the waste basket.
The last step before you put it into your system is to balance the phase inverter to drive the power tubes evenly.
We'll guide you through it once you're there...


The weak tube could be weak due to being subject to dc leakage from a failed coupling cap.
Financially, it makes sense to change them because you can change all 4x for the price of a good 6bq5. (The tube that gets destroyed when one fails)
Modern film caps don't fail, so this is a once and done type fix...
 
Looks like mostly original parts in there. I see what appears to be 2 newer caps but thats it.

Agree its pretty likely one or more of those wax paper caps or the red one thats likely paper in a cute shell are bad and causing the problem. the can caps likely aren't much good either. Looks like typical old tube amp type stuff, old parts that have failed from age. Not a big deal and this doesn't look intimidating to work on.

can't sat I've ever heard of this model though.
 
I'm going to inventory the parts and put together a list for Digikey. I might have some in my stockpile but most are lower voltage for SS equipment. Thanks for your help thus far! Stand by!
 
Any recommendations on budget cap types for the lower values? Here are my values.

Multisections to bypass
1st can - 40/450v x2
2nd can - 30/450v, 50/400v, 40/25v

Other individual caps
100/450v
220/25v
25/3v x2

Small value caps
0.01/600v
0.047/400v x4
0.25/400v x2
0.1/200v x2
 
for the small value, any polypropylene film cap will be fine. I use a lot of Illinois MPW but I'm not overly particular about it. The Vishay 716P are another good choice. There is a C-D cap that I can't think of too, I seem to recall its white but otherwise similar to the Illinois.

Probably going to have to make the 600v cap a 630, and the 200v one 250 just for availability reasons. You can make them all 630 if you want, it won't bother anything.

If there is one from line to chassis, replace that with an X1Y1 250vac safety cap. I see a green cap towards the back that looks like it runs from where the power cord attaches to chassis. Thats the one that should be a safety cap. Those fail open circuit rather than shorted.

The 3 volt electrolytic probably isn't going to be found, a 10 or 16v part at 22uf will be fine. I might bump the 25v one to 35 or 50v.

30 will become 33
50 will become 47

might want to check the resistors in circuit. Fair guess many will be out of tolerance. Most will measure in circuit with no fuss, replace as needed. Carbon film or metal film are fine here. The small ones should be 1/2 watt, the big ones I'm guessing are 2 or 3 watt.
 
Thanks for the response. I've recapped/repaired/restored dozens of SS pieces so I'm very familiar with the values. I'll probably get some caps ordered later this week.

I'll look into the safety cap.
 
gadget73 said:
If there is one from line to chassis, replace that with an X1Y1 250vac safety cap. I see a green cap towards the back that looks like it runs from where the power cord attaches to chassis. Thats the one that should be a safety cap. Those fail open circuit rather than shorted.
Click to expand...
As it goes to ground, I'm assuming it's a Y1 safety cap, as it's line to ground, would that be correct?
 
trainbuffton said:
What's in the box?
The little foil box?
The balance is used to balance the phase inverter.
Don't mess with it unless you have a scope.
Theres no bias adjustments.
a redplate tube indicates a bad tube, bad connection, or failed coupling cap.
start by verifying that the tube does in fact red plate then move the tube.
if that tube still red plates then its safe to assume the tube is shot.
sifti the new tube in that socket red plates then you have a connection issue or a failed coupling cap.personally I'd change all of the coupling caps out, clean the sockets and try again.

The big aluminum cans should be replaced. Best bet for those who want a clean install and quick turn around is to order them from hayseed hamfest.
i dont recommend scabbing in individual caps under the chassis for a variety of reasons. Heat being one and ground current routing being the other.
You could also restuff them but its time consuning and not a lot cheaper...

Knobs look like they belong to a C8.
Don't know anything else about it.
Looks to be cathode biased and would probably benefit from some recapping and an EFB conversion if you can live with a little sand in your amp...
Click to expand...

A baked potato.

Lol
 
Hello! I've finally received the parts and am starting to replace them. I have colour coded the 'balance' adjustment relative to the EL84's I'm curious what steps I should take to 'balance' them. The trim pots seem to be connected to pins 2 (Control Grid) and 3 (Cathode/Suppressor Grid). I still have to finish replacing caps and checking drifted resistors but I'd like to get an understanding of what I'm measuring and from where. Thoughts?

Circuit.jpg
 
If its tied into the grid circuit, its likely tickling some positive voltage from the cathode into the grid to set the overall current. Usually with these, you adjust them by setting for minimum hum in each channel. Thats where the current will be equal in both tubes.


looks like a 330 ohm per-output tube connected to the same ear as the grid resistor for that tube. The wiper should be going to ground. Where its positioned along the way will move one grid more positive as it moves the other less positive until the current ends up balanced.

you'll want t make sure all of those resistors are fairly on-value or all of that ends up screwy.
 
Thanks for the response. Interestingly enough, there's also a hum pot which I'm also not sure how to adjust.

The 'balance' pot I had assumed would be used to balance current to be equal between the two tubes in push pull configuration? That said, I wasn't sure if (a) that's correct (b) where I would measure between and (c) if I was to be measuring voltage or amperage.

Writing this, I sound pretty useless...
 
The hum pot is probably wired across the heater circuit. Normally you'd null the balance pots out with the volume all the way down, put it on phono, turn the volume up, and tweak the hum pot for minimum.


You can measure across the 330 ohm resistors if you want to know what the current actually is, but the audible null will get you to the same place without any test gear.

It would be volts / 330 (or actual measured value for better accuracy) = amps. Or just make sure the 330 ohm resistors are the same and just target the same voltage.


this is all pretty non-scientific old school crap. Its along the lines of "hit it with a hammer until it acts right"
 
gadget73 said:
The hum pot is probably wired across the heater circuit. Normally you'd null the balance pots out with the volume all the way down, put it on phono, turn the volume up, and tweak the hum pot for minimum.


You can measure across the 330 ohm resistors if you want to know what the current actually is, but the audible null will get you to the same place without any test gear.

It would be volts / 330 (or actual measured value for better accuracy) = amps. Or just make sure the 330 ohm resistors are the same and just target the same voltage.


this is all pretty non-scientific old school crap. Its along the lines of "hit it with a hammer until it acts right"
Click to expand...
Fair enough! Thanks for the response. I'm not sure I'll have time tonight to tinker but I'll measure a few things.

Just to confirm, if I want to use my DMM, I can set it to Amps, and measure across the 330ohm resistors and just adjust the trim pot until each tube are equal? Or, just do it by ear. Perhaps I'll do both to learn.

As for the hum pot, it can be seen in the first pictures I posted at the top of the screen but I can't see from the photos what wires go where. I can take a better photo later.
 
Set it to volts. The volts divided by the resistance will calculate to amps. if you use the meter in amp mode it will shunt the resistor and completely screw up the bias on the tube.
 
endervox said:
Good afternoon! I recently picked up a The Newport Stereo - Series 444 tube integrated amplifier on trade. Inside someone had previously partially recapped it. This obviously needs further restoration, but what I'm looking for is how to adjust the amplifier. There are left and right balance pots and a hum adjustment.

When I got the amp it was my understanding one of the 6BQ5's on the right side is red plating, so I haven't done anything with it. I have a couple replacement tubes I can throw in but would would like to understand an adjustment procedure first.

I've done a lot with SS but less with tubes, but know all the risks and dangers.

Any help?
Click to expand...
What is in that foil covered rectangular box?
 
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