oscillating plate voltage

[munkeyboy]

So i'm taking a break from some tougher projects and thought I'd try something simple.  Well it wasn't as simple as I was hoping.  Found another tape recorder to convert... it is the columbia 690-TR (you can find the specs here http://sportsbil.com/other/Columbia%20C690-TR.pdf).  I thought it would be simple to convert to something cool, so I found the "Micro champ". https://robrobinette.com/Champ_Micro.htm 
Seemed to fit the bill, but the transformer is quite underpowered vs what Rob recommends and it is different (no center tap).   So I did a different bridge and can only get it to 285v vs the recommended 390v.  But I think this will still work, just need to dial in the cathode resistor as it is currently cold.   All I am using are the transformers and the old cabinet.  In order to fit a new chassis in there, I used the harmond 6x4x2 chassis (as no 7x4x2 was available).   Things are tight and sure I'll have a tough time getting rid of hum.

My question is this.   When turning the volume up all the way and the master volume up past half way (or volume up half way and MV up all the way), I start to get a odd tremolo type effect.  Reading the voltage on the pre-amp plate with both volumes at 50% or lower, i get around 165v.  But turning up either volume past that and the plate voltage starts to oscillate between 165v and 200v+.  Where should I be looking for the problem?

[jjasilli]

With no schematic of what you actually did, the ability to help is impaired.  Sounds maybe like motorboating.  Maybe inadequate B+ filtering -- too many, like more than 2, gain stages served by the same filter node???

[munkeyboy]

I followed the previously linked schematic, but with the attached changes.  You could be correct on the filtering.   The proposed schematic has 3 filter caps, but I only have 2 since i'm not using a tube rectification.   But looking at my changes, maybe I need a bigger dropping resistor.  If so, I wouldn't understand why that would cause this issue.  There is only one preamp tube, so shouldn't be a problem with shared gain stages.

[PRR]

Do it like Rob drew it. He's not so dumb.

[munkeyboy]

Do it like Rob drew it. He's not so dumb.

I agree.  To be fair that 3rd filter cap was a recent change to address hum.  Previously it was only 2 caps.  But my dropping resistor is off.   I am cramped for room and was hoping i can get away with two, but if I had excessive hum, I'd add the 3rd.

[John R]

My limited understanding and experimenting has shown that keeping to the plan is better for time saving and blown parts, but the mistakes also give insight. 
   My hunch is you have to have that 3rd 'reservoir' cap to maintain an even voltage as the input signal voltage swings or you get 'bad sounds'. The cap will boost to cover the drain smoothing the output signal..I think. But the EE's here will explain if they have time, but most of this knowledge is book learnin' per internet, and our own investment in pondering and doing what your doing now. Trial and error, but be sure to be safe - that fuse ought to be covered of course, but you know how your using this.
   'Reservoir' is a good visualization for what is happening, ie a plumbing system. But it also is a scaled proportional system with relativities involved-Ithink . Like some tweed designs running 2 or more preamp tubes off 1 node with a small value cap with no problem. This I assume works because the PT has enough umph to push for demand all the way at the other end. Because copper is expensive now it seems MAYBE modern designs get by with smaller TX using now cheap caps at higher reservoir capacity to supply the umph needed...again I'm guessing.
   You mention this PT being small, so this lends to not having enough voltage 'pressure' down stream...I think/guess.
  I'm not an expert for sure and have no over shoulder mentor. Have tinkered with many builds now, and PRR's council is learned so either tag on another cap and R over the 2 already or get a more potent PT...?   Anyway is a interesting hobby that I have found to deepen my insights into other areas understanding modern auto electronic diagnostics, it is fun now to learn that where I assumed before I could never do it.
   Oh another thought The TX's are awefully close maybe that could be doing something...or not.  Good luck, The little 2-au7 has always been intriqueing.

[sluckey]

You don't have enough decoupling between your filter caps. Take that 470Ω up to about 5K or 10K and see what happens.

[munkeyboy]

Thanks PRR, John and Slucky.  I'll give those changes a go and report back.

[munkeyboy]

Trial and error, but be sure to be safe - that fuse ought to be covered of course, but you know how your using this.

Yeah, but the exposed lugs for the pt and ot are just as problematic.  It will be going inside an enclosure.  But trust me, it gets my respect.

   Oh another thought The TX's are awefully close maybe that could be doing something...or not.  Good luck, The little 2-au7 has always been intriqueing.

The original on the tape recorder was just as close, but the OT was turned 45 degrees instead of 90.  I should have done the same, IDK why I didn't.

You don't have enough decoupling between your filter caps. Take that 470Ω up to about 5K or 10K and see what happens.

That was it!  10k works.  Sounds good except for a bit of fizz at high gain.  Guessing that could be the bias i still need to dial in.   There is a little bit of hum, but not bad. If i can fit a 3rd cap in there I will.  Just need to figure out how.

Second question.  Am I reading the PT and/or original schematic wrong?  I assumed it is a 250-0, not 250-0-250.   I also assumed from the schematic that it has no center tap.  But looking at the actual PT, there is a ground lug at the top with the primary leads.  However, it was wired to the bottom lug where the 6.3v taps are and I assumed it was just for the 6.3v center tap.     (see attached)

[sluckey]

It is a 0-250 winding. It's always a good idea to use your ohm meter to verify which lugs (or wires) are grouped together.

If you want to reduce hum, do as Rob did. Add a third filter cap connected directly to the rectifier. Then a 470Ω resistor from the rectifier to the next filter cap (the one you will connect to the OT.

[munkeyboy]

What is that ground tho?  It is a center tap for the 6.3v?  Not sure how center taps work truly.

[sluckey]

That ground coming out the bottom of the PT on the schematic is just a core shield. It's not connected to any of the windings.

[munkeyboy]

Ok, so then they didn't have a center tap on the original, artificial or not. 

[munkeyboy]

BTW thanks all for the help.  Like John R said, I wish I had an local mentor, but you guys are next best thing.  I appreciate the help and time you put into this forum.

I'm to a point where hum is minimal and really only in the pre-amp (volume adjusts hum).  I added a artificial center tap too, no real change.  Still going to add a 3rd filter cap, but might have to make a new board and compress the turret spacing.   There is one more separate issue, but I'll try to tackle that on my own before I start a new thread.

[d95err]

Add a third filter stage and use the second node (B+1) on your schematic to supply the second tube.

Unlike push-pull poweramps, single ended amps have no natural hum rejection, so you need as much filtering as you can get.

[kagliostro]

if it was my amp I'll swap the single rectifier diode with winding connected to ground

for a FWB rectifier as to improve B+ quality







Franco

[sluckey]

if it was my amp I'll swap the single rectifier diode with winding connected to ground for a FWB rectifier as to improve B+ quality

He did.

[kagliostro]

 

I had missed it




 

Franco