1940's GE YGA-4 Audio Oscillator, My Frankenstein Preamp

[millzners]

I see some definite possibilities with the 6SN7 common plate mixer that's already wired in.  As a line mixer for use in parallel compression apps or FX blending.  It should only take one more OT (Edcor 10 or 15K:600) and a few resistors and pots to do it.  I could draw something up if you're interested.

Thank you for the offer.  I'm still unsure of whether or not I'll be able to get 2 channels out of this guy, and a mixer like that would be fantastic way to blend compression in there if I go the 1 channel route.  I'll think about it while the parts are shipping...  Do you think there's value in having 2nd stage after the NYD style one bottle that uses the 6J5?  I'm guessing without NFB it will be tricky.

 

[lassoharp]

Thank you for the offer.  I'm still unsure of whether or not I'll be able to get 2 channels out of this guy, and a mixer like that would be fantastic way to blend compression in there if I go the 1 channel route.  I'll think about it while the parts are shipping...  Do you think there's value in having 2nd stage after the NYD style one bottle that uses the 6J5?  I'm guessing without NFB it will be tricky.

There looks to be plenty of supply & sockets to do 2 channels of preamp only.  Assuming you're going to use the stock OT for the 1st channel, you will have a different OT for channel two.  I've no idea what the original OT may sound like response wise.  You may have two channels that sound different if that's a concern.

If by 2nd stage you mean adding another stage to the NYDave amp, yes, that would involve a redesign and could get tricky.  If you meant just using the 6J5 for another small amp, that would work.  The 6SN7 mixer amp would use a 6J5 for it's output driver and would function as a small 'stand alone' line amp.  So, you could run out of the preamp into the mixer/line amp just for some extra gain if you weren't using it for mixing apps.  Off the top of my head I'd say you'd get maybe another 15-20db net gain in addition to the NYDave pre which I think is around 40db.

You may be able to do 2 preamps and one mixer amp.  The concern here is the current rating on the choke and PT plus the extra 300ma filament current another SN7 would pull (or reduced to extra 50ma if you remove the pilot lamp).  Is the current rating by chance printed on the choke case?  You'd probably need to take some stock voltage and current measurements before attempting to do all 3.

Another option would be to use a 1M pot in place of the 1M grid resistor for the Hi-Z instrument input on the one-bottle.  This would allow you to run the preamps one into the other with decent gain control for situations where you wanted gobs of gain.  This would be much easier than trying to add a 3rd stage to the one bottle and net you more gain and tube/transformer sound.  The use of a 1M pot for Hi-Z input is actually a pretty decent general feature to have as it allows the preamp to function as a utility line amp with critical gain control.  So maybe you really like the sound of one of your SS preamps but wish it had a little more oomph - now you could run it direct into the one bottle without using heavy U pads in front of the input transformer.

 

[millzners]

Yeah I'm excited to see what the OT sounds like.  I don't mind having a different OT for Channel 2, actually.  If I'm mic'ing stereo I exclusively use MS anyway, so no need for matching L & R.  Assuming things go well with the first channel, I'm definitely leaning toward 2 channels.

The real question is how to use the existing 2 6SN7's and 2 6J5's effectively in this design.  I'm not really interested in adding any other tubes, so maybe this ends up as a 3 channel preamp?  Variety is nice.  I'm excited to begin experimenting with everything once I get the NYD cookie cutter circuit in place.  Looks like my parts are shipping today or tomorrow so I'll have to wait unfortunately.

 

[lassoharp]

The real question is how to use the existing 2 6SN7's and 2 6J5's effectively in this design.

2 SN7s = 2 one bottles

2 6J5s could as you said be used as a 3rd one bottle or utility line amp - with pot on input and an OT only.

Lots of possibilities.  I think you'll have some good sounding tube pres either way.

 

[millzners]

Parts arrived this afternoon, which is extremely fortunate b/c it's a great weekend for DIY.  Pouring rain.  I ripped out (litterally) the old C11 power supply filter cap can, and replaced the original 10uF's with a set of 3 10uF and Lassoharp's suggested 22uF for C11C.  Plugged it in using the new 3 lead power cable I added, turned it on, and poked around the outside with the DMM to make sure I wasn't going to get shocked.  No smoke, no stray voltages, so I plugged in the output. The big ugly 120Hz noise is gone, which was the only problem with the unit to begin with.

Since I had already tapped into the original circuit, I plugged in my bass and gave 'er a whirl.  A nice big sparkly sound emerged, with just a little bit of 60Hz buzz.  Since it's late, I used headphones, so nothing difinitive about the OT's frequency response, but it certainly didn't seem to be a problem.  I'm officially excited about this thing.

So here's the plan, with Lassoharp's input very much considered.  I'm going to make 3 NYD-ish Simple One Bottle pre's.  2 from the 6SN7's and 1 from the two 6J5's.  I want each channel to be a little different:

CH1 Will be pretty much be the audio amp from the schematic plus an input xfo.  I'm leaving the original components as they are including the original OT.  I want to preserve the 40's mojo, and I especially want to feel that I've got something totally unique.  This channel will also have the eye tube intact, assuming it's not causing a problem.

CH2 Will be the other 6SN7, this with new components and NYD's values.

CH3 will be the 2 6J5's and some version of NYD's design with a few things thrown in that I'll borrow from what Kingston is doing with his latest design.  This will be the "fun" channel.

No idea about the 5 XFo's I'll be adding, but I'm partial to Cinemags.  Time to brush up on my lead dress. 

 

[leadbreath]

hehehe another frankenstein about to come alive, i love it!!

 

[leadbreath]

damn!! now i want one too!!! ;D

 

[millzners]

On the topic of lead dress...

I have the following sequence going from right to left, is a star ground or bus ground a better idea?

OT, 6SN7, 6SN7, 6J5, IT, 6J5, IT, Rectifier Tube, Power Xfo.

Obviously there's an IT and 2 OT's missing from that list, I don't have a place for them yet...  Open to suggestions.

If I use a star ground it seems like I'll have one for the preamp tubes and xfo's, and one for the power supply, while the earth ground from the wall is totally separate from all those.  Does that sound accurate?

Thanks!

 

[lassoharp]

If I use a star ground it seems like I'll have one for the preamp tubes and xfo's, and one for the power supply, while the earth ground from the wall is totally separate from all those.  Does that sound accurate?

The earth (safety ground) is not really separate.  They all reference the chassis.  If not using a bus bar, arranging the grounds heirarchically makes good sense and should give good results.  Just a matter of tying the grounds of each stage - input to output(low current to high current) together so that each stage references the previous.  So each is tied together but is only tied to the chassis at a single point - usually the ground of the 1st filter cap in the PS.  You can see that part of the rationale is based on wire lengths and the minute resistance of longer wire vs shorter wire because technically the ground is electrically the same point.

Bus bars work great if you can make them fit.

I wouldn't be surprised if the original unit tied grounds locally at the tube socket lugs.  Commonly done and can work just fine.

 

[millzners]

If I use a star ground it seems like I'll have one for the preamp tubes and xfo's, and one for the power supply, while the earth ground from the wall is totally separate from all those.  Does that sound accurate?

The earth (safety ground) is not really separate.  They all reference the chassis.  If not using a bus bar, arranging the grounds heirarchically makes good sense and should give good results.  Just a matter of tying the grounds of each stage - input to output(low current to high current) together so that each stage references the previous.  So each is tied together but is only tied to the chassis at a single point - usually the ground of the 1st filter cap in the PS.  You can see that part of the rationale is based on wire lengths and the minute resistance of longer wire vs shorter wire because technically the ground is electrically the same point.

Bus bars work great if you can make them fit.

I wouldn't be surprised if the original unit tied grounds locally at the tube socket lugs.  Commonly done and can work just fine.

I took out the eye tube and I think there's room for a bus ground bar that will run all the way to the PS.  I think this will give me a cleaner layout than a star ground.  My only question is about the earth ground coming in from the wall.  I read this should just be terminated to the chassis, but that the bus or star ground for the rest of the design should not be tied directly to this lug.  I sort of doubt the impact that will have considering the PS caps and earth ground lug are about 1" apart.  I'll try a few things either way.

 

[lassoharp]

I did a bus bar on a mixer project and really liked how the ground wiring worked out.  I do believe it made for a much less cluttered ground tie.

I usually tie the mains earth to one of the PT mounting screws which is, as you point out often only an inch or so away from the main circuit star point.  Much easier to remove too if you ever have to replace the AC cable.

 

[millzners]

Waiting to see about another piece of gear I'm after so I haven't ordered the transformers from Cinemag yet.  In the mean time I implemented a star ground and put a 47uF cap in C11C where I had a 22uF before.  I re-routed some high voltage wires away from the signal wires.  The noise floor is now -66dB which is a good start, and it unit itself sounds fantastic on electric bass and guitar.  If I stopped now, I'd have one hell of a DI, and all I've done is tap into the original audio amp. 

 

[millzners]

This project is back in action...  The Cinemag transformers finally arrived after 2 months, I've got the 2nd channel half finished, and I'll be finished with the DI portion tonight.  I'm going to work on one channel at a time, starting with the DI and get the best possible noise floor, then move on to the next channel until all the output transformers and DI's are set.  Then I'll drop in the input transformers one by one and make sure they're not coupling to anything... 

Time to start thinking about the 3rd channel, which is going to be the "fun" channel.  I'd like to try a thing or two from Kingston's Overkill Drive-1 project.  Just an extra switch or knob that will give me something a little different...  It'll still be an NYD one bottle w/out feedback except it'll be a 2 6J5's.

 

[millzners]

Nice.  I wired up the 2nd channel from DI to output, and with the jacks and pot dangling from wires hanging out the front the noise floor is -70dB.  The sound is nice, a little more high end, a little less 2nd order harmonics when compared quickly with the first channel using electric bass.  No major hiccups so far, time to get the 3rd channel wired up...

 

[millzners]

Here's my thoughts for Channel 3, borrowed almost completely from Kingston's Drive-1 design -- with a few things changed to fit the components I actually have on hand like using 2 6J5's instead of 1 6SN7.  I have a variable cap from the original circuit I may throw on the bottom of V1 just to see what happens...  I'll start with this and work my way out from there. 

 

[lassoharp]

I have 5K rheostats as cathode resistors for the output tubes on the pair of RCA BA2 clones I built.  I installed them to keep tabs on the amount of unbalanced DC on the OT pri.  Out of curiosity I played with the adjustment - from 5K to nearly short.  To the ear,  I couldn't tell any great hearable difference until approaching really low values - probably around a 100r, which really only sounded like more output drive ability due to higher idle current (plus low end response drop from being direct coupled to OT). 

Adding or removing bypass caps on either tube will make an audible difference in gain and distortion.

A 10K vol pot will likely give more apparent high freq response than say a 100K in the same position.

I'm not sure if the use of 10K pot was to purposely work V1 into a lower load to create distortion.  Normally you would go 2 to 5 times higher than the V1 plate resistor.  Triodes are apparently somewhat tolerant of breaching this rule of thumb.  Check PRR's comments:

http://www.groupdiy.com/index.php?topic=32319.60



Edit:  Also, I think the 22uf decoupling cap for V1 needs to go between the 36K and the 27K plate resistor.

 

[millzners]

I have 5K rheostats as cathode resistors for the output tubes on the pair of RCA BA2 clones I built.  I installed them to keep tabs on the amount of unbalanced DC on the OT pri.  Out of curiosity I played with the adjustment - from 5K to nearly short.  To the ear,  I couldn't tell any great hearable difference until approaching really low values - probably around a 100r, which really only sounded like more output drive ability due to higher idle current (plus low end response drop from being direct coupled to OT). 

Adding or removing bypass caps on either tube will make an audible difference in gain and distortion.

A 10K vol pot will likely give more apparent high freq response than say a 100K in the same position.

I'm not sure if the use of 10K pot was to purposely work V1 into a lower load to create distortion.  Normally you would go 2 to 5 times higher than the V1 plate resistor.  Triodes are apparently somewhat tolerant of breaching this rule of thumb.  Check PRR's comments:

http://www.groupdiy.com/index.php?topic=32319.60



Edit:  Also, I think the 22uf decoupling cap for V1 needs to go between the 36K and the 27K plate resistor.

Thank you for taking a look at that for me.  The 10k pot should have been a 100k pot, and I agree about the filter cap even though I believe that's where it's shown on Kingston's schematic.

Great advice on the cathode resistor vs. cap for the switch -- I've redrawn the schematic so that I'll have 2 switches for 3 options of cathode bypass cap on each stage.  I hope that yields more diversity in gain/distortion.

I have these options for cathode bypass caps: 10, 100, 220, 470, and 1000 uF, I'll have 2 caps for each stage plus the off position will have no cathode bypass cap...  I was wondering if 1000uF cathode bypass cap on the first stage would cause a problem -- it would be switchable so if it's crazy but safe that's a good thing, I just don't fully understand the implications.

Except for the switchable cathode bypass caps, the 3rd channel is wired up and working from DI to output.  I tried to use more of a laid back approach to grounding whereas the first two channels are strictly star ground and sure enough I've got a 120Hz buzz on channel 3... 

No time to play around but each channel is distinct so far which was my intention.

 

[millzners]

I've updated the schematic above with how I plan to wire up the cathode bypass cap switches.

One other really interesting thing.  I wired a 15k in series with the B+ feeding into V1 6J5 -- just like the schematic above.  But at the time I didn't realize that the way I wired it, the 15k is now also in series with the B+ that feeds the 2nd channel's 6SN7 -- before the 4.7k in NYD's schematic. 

The result is that 2nd channel now has a mellow fuzzy distortion to it, sort of like overdrive.  It's actually really cool sounding -- if you wanted it.  Is this starved plate?  Is it harmful to the tube?  Can I put a switch in to get this effect when I want it, or is that going to create a giant pop and blow something up?  Maybe a 15k 2W pot?  If making that kind of effect was that easy everyone would be doing it...

 

[leadbreath]

man, this is turning into an interesting thread, ive actually also been looking around for some type of tube oscillator to frankenstein.

 

[lassoharp]

I have these options for cathode bypass caps: 10, 100, 220, 470, and 1000 uF, I'll have 2 caps for each stage plus the off position will have no cathode bypass cap...  I was wondering if 1000uF cathode bypass cap on the first stage would cause a problem -- it would be switchable so if it's crazy but safe that's a good thing, I just don't fully understand the implications.

If it were me I would try the two cap plan you have on V1 (10uf & 1000uf - except maybe just 470uf).
The large bypass caps lower the low freq roll off point.  It's been a while since I did the math but 470uf puts it at a  very low freq and 1000uf may shift it just slightly lower.  Using 1000uf shouldn't hurt anything - it just may not be necessary. 


The 10uf is interesting.  From another thread here I recall it being mentioned that this particular value would cause a slight rise in the lower mids.  Full-bass/flat, mid boost, unbypassed sounds like it would be useful.

http://www.groupdiy.com/index.php?topic=6711.60

One other really interesting thing.  I wired a 15k in series with the B+ feeding into V1 6J5 -- just like the schematic above.  But at the time I didn't realize that the way I wired it, the 15k is now also in series with the B+ that feeds the 2nd channel's 6SN7 -- before the 4.7k in NYD's schematic. 

I'm not quite clear how the wiring has been done.  You may want to post a schematic that includes the entire PS showing the feeds to all 3 sections.

Lowering the B+ will tend to "brown up" the sound.  It's not exactly starved plate (usually meant in reference to hybrid circuits using chips to do the clean work). 


For the 15K pot - If just for adjusting branch current/voltage on one channel - possibly.  It depends on current draw.  2W sounds like it's cutting it close on dissipation.  Dropping from say 250V B+ to 180V or close to it should get some of the brown effect.  So maybe a 5K or 10K pot/rheostat might work better.  Best to see full schematic with at least a pair of voltage measurements - one at rectifier output, another at the main B+ feed (post the filtering resistors).