bkbirge

175b Drip rev1 help
« on: December 24, 2018, 10:55:17 AM »
re: 175b vari-mu compressor builds...

Hey all, been a long time since I've had time to DIY and I'm just now getting to some projects that I started many years ago. One of them is the Drip 175b version 1 pcb's. I'm nearing the finish line but when I look at the transformers I bought for this they don't match the build docs. Since the drip forum has been closed for years I'm trying to figure out if I bought the wrong stuff or I bought a set that is supposed to "work better" or something like that. I vaguely remember there being a few builds that changed things up for better performance and I suspect I was going to do those but I really can't remember for sure. I'm hoping some of you more experienced folks have better memories or can point me in the right direction...

What I've got:
input: Cinemag CMLI 15/15b
output: Sowter 9185s (these are actually what Sowter says to use for the Gates Sta Level)
interstage: Sowter 1133e

What the build docs say:
input: Sowter 1009e, or UTC ha-100x
output: Sowter 8650 or 9900
interstage: Sowter 1133e

Thanks in advance for any help!


« Last Edit: February 01, 2019, 08:48:31 PM by bkbirge »


alexc

Re: 175b i/o transformer help
« Reply #1 on: January 08, 2019, 01:35:35 AM »
Should be fine  ....  it's not all that particular, generally speaking. Some of the recommendations for the v1 of drip 175 can be a little daft. Like the psu traffo specified back then  ...  big enough to power a battle ship, not a little 100mA limiter.  Also good for welding.

Using something 'scaled' a little more appropriately, can have good results in terms of 'radiated noise' into signal traffos - the main bugbear imho of all these 'amp head' type builds.

I ended up using my Hammond 373 (from memory) to instead power a parallel 6550 SE amp head. And I still don't use anywhere near it's capacity. It's a monster.

I bought a smaller (though still impressively sized) Edcor for the psu traffo.

Those signal traffos you mention are top shelf. I used the same interstage traffo from Sowter .. very good performance  (but do check the orientation and wire code ....  as I recall there was a 'reverse' in the drip build guide.

The other thing is to try a few of the gain reduction cell tube  ....  it's quite important to get a decent match from the two triode stages in the envelope. 

If I was to do mine again, I'd 'adjust' it for some proper 6386 remote cutoff dual-triode tubes - I used the 6bc8, and I think they can be a little 'light-weight'  ...  and they can vary about the place quite a bit.  It remains my only '6BC8' limiter of this type.

If I had access to a 'conversion pcb' for the gain reduction tube, I'd use something good yet inexpensive  ..  like a pair of properly selected remote  cutoff 'frame grid' pentode-in-triode-mode tubes. But that requires the 'duo 9 pin pentode to 6BC8' pcb  :)

Good luck with it - it's hard to get sounding bad the drip v1 175b  limiter  ..  generally speaking.

« Last Edit: January 08, 2019, 01:49:30 AM by alexc »
I ping therefore I am

bkbirge

Re: 175b Drip rev1 help
« Reply #2 on: February 01, 2019, 09:05:55 PM »
Thanks alexc, good info. I did end up building both with the recommended transformers. One works great, no prob calibrating once I put a GE nos 6bc8 in. I love what it does though I would like a mod for more attack range.

The other has much lower gain on input and has a nasty ground hum that boosts with output level and it won't calibrate at all, the test switch acts like an rf amp everytime my hand goes near it. I checked test point voltages and they all test ok if a touch low but that's the same as the build that works. Except on pin 5 of the 6al5 I've got 3.6 v ac roughly that is unaffected by attenuators. It is only on that one side of the tube, pin 7 seems good no ac. Did a full tube swap with working unit and problem didn't move. Choke ohms test fine. Checked for solder bridges and component vals around the diode circuitry those all  seem ok. Any ideas on how to track this down?

scott2000

Re: 175b Drip rev1 help
« Reply #3 on: February 03, 2019, 08:19:12 AM »
Except on pin 5 of the 6al5 I've got 3.6 v ac roughly that is unaffected by attenuators. It is only on that one side of the tube, pin 7 seems good no ac.

Pin 1 is the other cathode? and that checks out fine?
What do you mean by " that one side" ???  The cathode side???

Maybe double check grounds.....
« Last Edit: February 03, 2019, 08:51:34 AM by scott2000 »

bkbirge

Re: 175b Drip rev1 help
« Reply #4 on: February 03, 2019, 09:55:48 AM »
Pin 1 is the other cathode? and that checks out fine?
What do you mean by " that one side" ???  The cathode side???

Maybe double check grounds.....

You are right, for some reason I was looking at pin 7 when I should have been looking at pin 1. Pin 1 and Pin 5 both have the unexpected AC voltages. For DC they both test at approx. 38 VDC which is consistent with the one that is working.

Grounds seem to be ok near as I can tell, continuity checks with no noticeable drop in ac or dc across.

bkbirge

Re: 175b Drip rev1 help
« Reply #5 on: February 07, 2019, 11:22:41 AM »
Still haven't figured this out, if anyone has insight I'd appreciate it. Also I've got several drip Fairchild time constant boards I'm going to build, curious if anyone has interfaced these with the 175b and how they did that.

mjrippe

Re: 175b Drip rev1 help
« Reply #6 on: February 07, 2019, 04:18:37 PM »
The AC signal you are seeing could be the signal used for balancing  the 6BC8.  Check wiring of the test switch and surrounding circuitry - you may be stuck in calibration mode.


 

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