Bad driver stage voltages

[pedro]

Hi Fellas.... long time since I wrote here.........

I have a problem with my 50w ODS build

been OK for ages and Ive just installed a new PI circuit, which was sounding great.

But now my PI is really badly balanced - Ive tried loads of different tubes but have the following issue.

One side ( input side ) has plate of 330v. Other side has very low plate of 240v

Ive replaced the plate resistors and the trim pot. Tried taking the trimpot out completely, and tried various values of plate resistors ( 100k/82K, 100k/91K, 120K/100K ) Tried re-soldering everything. Always the same issue - the "presence " side of my PI is at MUCH lower plate voltage. I only checked it because I wanted to get it fairly balanced.

Any ideas on what would commonly cause such a problem ?

Rgds
Pete C

[Funkalicousgroove]

what value tail resistor do you have on your presence circuit?

[pedro]

Hi Funk ,

presence circuit has 4.7K tail with .1 cap

Pete

[pedro]

https://tubeamparchive.com/viewtopic.ph ... ght=driver

Trawling back in the files I had the same problem before a year and a half ago. Something strange about my circuit !

I think I'm going to remove everything around the PI, feedback and presence and re-do it .

Still, any thoughts on usual causes of such imbalance ?? - feels like the two sides of the driver are biasing very differently.

Rgds
Pete

[odourboy]

Probably a silly question, but have to tried a different tube? I had two Sovtek 12AX7WAs from the same batch that each had one side completely out of whack. The plate voltage on the 'B' side of each measured so low that I thought I had a circuit problem. Frustrated, I changed out hte tubes for a Tung Sol and the probem vanished.

[pedro]

OB,
thanks for suggestion - tried 4 different 12ax7s so far - Tung Sol, Sovtek and EH - all have similar, but not exactly the same, voltage issue.

have noticed when hooking up meter that the voltage rises quickly to around 301v then drops suddenly soon afterwards to 260v or even lower. I have tried 2w 100K resistor on the plate assuming that maybe the resistor was breaking down under B+ ( its at 442v BTW where the plate loads split and are fed from PS )

Damn frustrating - even tried wiring up a spare socket alongside and exactly same problem - so its not the socket !!

maybe I just have some duff 12ax7s - I have a couple of Mullard somewhere - I will try them but dont want to fry one side of the twin triode.

Any more suggestions ?

Rgds
Pete

[dogears]

This style PI will yield about 40v less than the traditional one. Seeing around 260v is normal. Seeing 300v on one side is not normal. PM me your exact resistors on you PI. Not the plates.

[glasman]

Sounds like the tube is not drawing any current on the 330v side.

Check the following

1. Grid connection at both the tube and the junction above the tail resistor.

2. Check your cathode connection on that side of the tube. Theses are about all there is that could create the problem.

The voltage on both cathodes should be the same. You can measure the voltage at the grid, but you will get some funny results due to the loading of the volt meter.

Gary

[pedro]

Thanks all,

I'm rebuilding the whole PI to cover for any problems.

Also thinking about repalceing the original phenolic 4mm circuit board as dont think it fully meets HV specifications after all ( despit ebeing bomb-proof ! ) and this might be the culprit somehow.


One last try before rebuilding the board !

Rgds
Pete

[pedro]

Oh well,
afer many frustrating tube swaps and a complete PI rebuild I'm back to where I started with 100v difference between the two plates of the PI.
Never seen anything like it before, and Ive run out of ideas and energy for now.
I give up - there must be something fundamentally wrong with the board.

For now I'm going to leave it and build a whole new board.

Rgds
Pete

[llemtt]

Hi Pete

Have you checked for ultrasonic oscillation? If so any DC reading would be misleading.

cheers
Teo

[pedro]

Teo,

no I havent checked that. Ive tried everything else so maybe youve found the cause.

I do have a scope ( loaned by my neighbour ) and an sine / square wave oscillator to use so could you tell me how to go about checking for the ultrasonics ?

Thanks for helping

Rgds
Pete

[dogears]

Pete,

Did you place the 47pf between the PI plate resistors? It's supposed to be there.

Teo,

no I havent checked that. Ive tried everything else so maybe youve found the cause.

I do have a scope ( loaned by my neighbour ) and an sine / square wave oscillator to use so could you tell me how to go about checking for the ultrasonics ?

Thanks for helping

Rgds
Pete

[pedro]

Hiya dogears !

I did try it both ways before I gutted it and rebuilt the PI.

I must admit though that i didnt add it again after I did the rebuilt PI again this weekend.
I will try first thing tomorrow when I'm back at home - I'm sitting in Heathrow airport catching up on posts from the last year or so .... wishing I was near a guitar.......or a soldering iron.

The new phenolic board turns up tomorrow so I might just get the main board rebuilt anyway before I travel to Florida next week. No soldering then for a week or two.

I'll let you know how i go with the 47pf plate snubber back in..

I think Teo might be right about the oscillation problem though - I guess it could cause the weird symptoms Ive been getting. Especially as this amp is built into a donor rather than from scratch. I couldnt bear to give up on the Partridge TX's in this thing though and use another chassis - they are wonderful things.
Other than knowing that the PI isnt right somehow the amp still sounds bloody wonderful - so when its sorted its going to be even better.

Anyway, I'm rambling now......

speak soon
Pete

[pedro]

Update......., all my problems seem to be solved !

i found that the phenolic board I'd used for my turret board wasnt fully spec'd for high voltages ( despite being 4mm Tufnol board ) , so I gutted the amp and rebuilt the main board and the PS board using eyeholes rather than turrets on 2mm HV tufnol board. ( i prefer eyeholes because i find i need less heat from the iron to get a god solder joint )

I decided to incorporate the bluesmaster PI values as well.

The great thing is my amp is cured - the wierd PI voltages have now gone - I now have 263 and 255v either side on the bluesmaster style PI, and around 203v on the V1 plates.

The relay popping problem I had when channel switching when I first built the amp about 18 months ago is also cured.

It just goes to show sometimes its not the layout , the wiring, the routing of wires or bad solder joints - it can be something more fundamental and obscure.

Anyway - amp sounding great now - updated with all latest tweaks and circuit values based on ODS HRM and the bluesmaster PI values.

Now I need to build another.

Thanks for all the help along the way over the last 2 years !

Pete C