rectifier questions

[ALBATROS1234]

i recently acquired a kodak av-154 16mm sound projector. i serviced the amp and it sounds pretty darn good yet one thing confused me. it is a push pull 6L6/gb that came with a 5Y3 rectifier. it is rated at 15 watts , but it is a hell of loud 15 watts if you ask me. the first question is why would someone do this? if you wanted to have 15watt output why not pair 6V6s with the 5Y3 like the rest of the world.  this amp has a ton of sag , which you can witness visually when you strum a hard chord the blue glow noticeably dims for a second. i know these 6L6s must be starving for current. i have other rectifiers like some 5u4 types but those draw 3a filament rather than 2a of the 5Y3. i have a couple 5R4GYB types which seems more apporpriate for the circuit. 1st they draw 2a filament just like the 5Y3 but can theoretically supply twice the current and have the same pinout while having a similar volatage drop. is there any reason i should be leary of putting one of these 5R4GYB recti tubes in place of the 5Y3 it was given by kodak?

[DummyLoad]

6L6GB loafing at 15W w/ 325V on anodes will last a VERY long time. maker wanted projector to be reliable / low maintenance.


--pete

[ALBATROS1234]

Yes that makes sense I suppose .  And from what I can tell I don't see any reason I cannot use the 5r4 but I just wanted to check to see if there was something I had not considered. Although I am not sure what difference it would make without also adjusting the bias . Perhaps it would give less power sag with the 5r4. I have never seen it where a tubes blue glow dims visibly when a chord is struck. Let me say that I had to rebuild the input because with that array of mess on the orig circuit inputs the volume was so low it was  sounding like a 2 watt. A standard guitar input with a 56k grid stopper with a 1 meg grid leak to ground worked wonders. Oddly when I clipped in to the grid to experiment with a guutar it was so sensitive with no grid stop I could hear the guitar vol scratching when rotated

[bmccowan]

I'm curious about a couple of things. Have you considered using 6V6s and adjusting the cathode resistor to suit? A lot of old non-guitar amps seem to have under-taxed 6L6s in them. They were avoiding the crunch that we seek.
And besides changing the input - have you stuck with that stock schematic.
The scratching of your guitar parts is likely some DC getting back to them, eh?
Cool project! I wonder how similar this is to one of the B&H Filmosound amps?

[ALBATROS1234]

I have. At least I put 6v6s in it and they sounded decent, a little more breakup and bite but I prwfwred the 6L6s they had more dynamic range maybe because of the bias or ot impedance.

[ALBATROS1234]

I just figured it would be simpler to give the 6L6s more current by plugging in a 5r4 which is filament compatible but I was worried what if the 6L6s overheat the h't winding because the tranny doesnt supply enough high voltage current and burns up

[pdf64]

Yes, that is the risk you take by raising the HT above what the designer intended, ie higher HT will result in the circuit drawing more HT current and increasing the power output.
So the PT, OT, caps and HT dropper to the screen grids need to be able to accommodate that increased voltage /  current / power.
But as it's already plenty loud, why bother?

Bear in mind that this amp was probably intended to provide reliable reproduction of speech and music, the designer wouldn't expect it to be pushed into heavy overdrive, especially of a prolonged nature.
But the lowish HT, cathode bias and 3k HT screen grid dropper will help to keep it reliable even with such 'abuse'.

...Oddly when I clipped in to the grid to experiment with a guutar it was so sensitive with no grid stop I could hear the guitar vol scratching when rotated

There looks to be a Vdc supply from the power tube cathodes to at least some inputs, so that could have made things 'scratchy'.

[Willabe]

They were avoiding the crunch that we seek.

6L6GB loafing at 15W w/ 325V on anodes will last a VERY long time. maker wanted projector to be reliable / low maintenance.

And not everybody 'seeks crunch'.

Plenty of guys want clean or slight break up and if they want some distortion, they want it smooth, no crunch.

[PRR]

The "glow" always changes when a tube passes big signal, due to voltage swings steering electrons to stray more/less into the glass.

It is a nicely conservative design. Not "starved".

It is never going to be a Stadium Blaster. Bumping from 15W to 19W won't make you a star.

I say leave it as is.

[ALBATROS1234]

fair enuff i will leave it as is, it sounds pretty good after i rebuilt the input and changed electrolytics and all of the waxy caps. i just thought maybe with a beefier rectifier which i have on hand it would improve it somehow. i have never seen a 5y3 on a schematic with pp 6L6s. i knew using one of my 5u4 types wouldnt have worked with the 3a fil draw but it dawnwd on me i had these 2 5R4GYB types and a bit a reading seemed to imply it was workable. thats why i ask the pros questions when i am unsure. thanks fellas

[ALBATROS1234]

ok, ok, ok, so im hard headed. i had to try the 5r4 to see what difference if any. actually there was a difference. not so much in the tone of the amp although there was a slight change more in the way it felt playing through it, the 5y3 was kinda spongy and squishy sounding whereas the 5r4 was punchier and boxier sounding, weird, didnt think it would be such a noticeable difference. i actually prefer the 5y3 tone and feel so in it stays. it late so i could not really turn up to crunch level though.

[DummyLoad]

i have never seen a 5y3 on a schematic with pp 6L6s.

supro S-1624 - a very cool amp! the paraphase inverter and the sag of the 5Y3GT are part of the mojo.

--pete

[ALBATROS1234]

It's definitely a unique tone with the 5y3 .hard to describe a feel but spongy rubbery with a squishy feel like walking in a marshy area where your feet sink in a half inch then you have to put forth effort to lift your foot out of the sticky sponge. Lol.  And the original Sylvania 6L6gb sound phenomenal. All if the tubes were original Sylvania with identical date codes except the 5y3 was GE so I guess they burnt UP one or more 5y3  tubes . From what I remember it would not be hard to change a few components to make this thing a 5f4 tweed super. It was built in that same time frame.

[bmccowan]

I think you would need one additional preamp tube for a 5F4 Super. There are many good paths to follow without attacking the chassis.
And, I think the 1624 Supro has 6973 power tubes, with a few early ones having 6V6s.
But I applaud the hardheadedness, and seeking the tone that suits you. Be safe/careful and go for it!

[ALBATROS1234]

youre right i need another 12ax7 for a 5f4 , i would have to change values and redign the tone stack, too much trouble, it already sounds very much fender tweed, more like a channel of a 5e3 with an extra gain stage a 12ax7 instead of 12ay7 in v1 but with a bass control added and 6L6s though. few component changes though, i think it might be too hot with 5e3 components on v1 though. it has fairly low headroom as is and 4 on the vol starts getting crunchy and gets somewhat marshally when cranked to 8. its a cool ,loud unique 50s/60s fender tone machine for sure.sound nice with the stock 8" 1950s rola but even better with my 1950s 12" alnico magnavox which is quite jensen like.