Mainline version history question

[Rossputin]

You talked me into it, Paul...My VCap ODAMs arrive today. I went with 18uf because I found them MUCH cheaper than the 22uf, and there's no 20uf. Hope that's gonna be fine.

Do you know if the 6C15P tube is a drop-in replacement for the 6C45PI? I like using tubes from the year I was born and I don't think the 6C45 was made in the 1960s. I'm having a hell of a hard time finding out if there's a nearly identical replacement that was made in the 60s. This 6C15P seems like the only possibility but I just can't find out much about it.

[Paul Birkeland]

When this last came up, I don't believe I was able to find much in the way of a datasheet, but I believe they will work.

[Rossputin]

how much risk do I have if I put one in and it's not actually a perfect substitute? I have a feeling it's more risk than I'd want to take.

[Paul Birkeland]

The Mainline is current regulated, so catastrophic damage is generally unlikely.  If the pinout is the same but the tube isn't electrically the same, the most likely negative result would be bad sound.

[Rossputin]

Paul, this is how you know it's gone too far:

Last night I had a dream that you said (in a posting on this board) that the transformer in the Mainline could/should be upgraded so I woke up this morning thinking I needed to go research potential replacements. And I was kinda excited about the prospect.

What does that say about me?

And why did you say to upgrade the transformer? 

[Rossputin]

Sorry for the dumb question but if I'm replacing the Dayton 10uf caps with the recommended VCap ODAMs, does it matter which end of the new caps go to which solder terminal? I suspect it doesn't matter because I don't see anything like a "+" indicator on the old or new caps but I don't want to make a mistake!

[Rossputin]

OK, as long as I'm burying you in questions today, Paul: Is there any reasonable way to change the volume control to something more "traditional" without harming audio quality? Like one high-quality pot that moves smoothly instead of the steps? Not at all sure this is a good idea or something I'd do but I wonder what's possible for someone with only a modest level of experience building these things.

[Paul Birkeland]

Sure, you can upgrade the output transformer.  The Magnequest TL-404 will work in the Mainline circuit but you'll lose the balanced output. The Sowter 8983 would also function similarly but also without balanced output.

The V-caps are directional in a certain sense, but not at the signal levels seen in the Mainline.  The short leads on those caps should go to the plate of the 6C45 and the long leads to the output transformers. 

Yes, you can put a single stereo pot into the Mainline.  If you do that, you should take off all the hardware on the front panel and pull the labeled overlay panel off and throw it away since it's no longer correct for the kit.  The stereo pot will definitely not perform to the level that the stepped attenuators do, and you will nuke the resale value of your Mainline by doing this.

[Rossputin]

Thanks for all that...I hadn't even realized that there were short and long leads on the caps but indeed there are!

No way I'd want to nuke the value, nor reduce performance, so I won't change the volume pots.

Also don't want to lose balanced connection so won't change the transformers. Wasn't really looking to do that anyway. Just shows a certain psychological problem that I had a dream about that. ;-)

I'm trying to do research on the 6C15 vs 6C45 tube question and will let you know if I learn anything.

[Rossputin]

Hey Paul (and anybody else interested in this question), check this out:

6S45P datasheet: https://rudatasheet.ru/tubes/6s45p/

Older tube, 6S15P datasheet: https://rudatasheet.ru/tubes/6s15p/

Got this from long-time tube guy: "Yes they are very similar and should be interchangeable. The 6C45 is supposed to be a "better" version with lower noise."

[Rossputin]

Attached are pictures of my new VCap ODAM capacitors installed. I went with 18uf because I found them MUCH cheaper than 22uf and I figured (probably wrongly) that either is a huge improvement when going from the original 10uf Dayton caps (not that those are bad).

Thanks for all your guidance, Paul!

[Rossputin]

Hey all,

I swapped 6C15P (6S15P) for the two 6C45P (6S45P) -- which can have -E as an identifier as well for a better-built tube -- into the mainline and it works great and sounds great. Not yet to the point of being able to tell if it sounds different. But if it does it's not obvious.

For those asking why I'd do such a thing since the 45 is almost certainly a better quality tube than the 15, the answer is very idiosyncratic: I try to always use tubes made in the year I was born and the 45 only began being made well after I was born. This was one of the most challenging experiences I've had finding a tube from the year I was born. Ended up getting them from sellers in Kyiv, Ukraine.

Best,
Ross in Denver