Upgrading a Brentwood K14 (with schematic)

[Zaos]

Hey everyone.

I picked up this awesome old tube amp that i finished rebuilding to see if it works.

I replaced all of the 5uf caps with 10uf, and replaced all of the other caps.

I"m a bit at a loss mod wise. I want to add grid stoppers (not sure where exactly in the circuit exactly) and would like to clear up the overdriven sound. It's a fair bit bass heavy, even with the original tone control all the way onto treble.

I'm going to a 1Meg PPIMV which is easy enough, as the circuit is basically a 6v6 push pull.

Thanks all.

[John]

For less bass, replace the 10uF on the first tube with something much smaller, maybe a 2.2 uF or even a .47. I've used as low as .33 on pentodes before, depending on what I was shooting for.


Your grid stoppers, maybe add them to the power tubes. 47k ohm is a "normal" value. You can add in small (22K or so) stoppers on the grids of the 2nd tube, but not absolutely necessary. There should of course, be one on the first tube, your input tube. 33k - 68k is normal there.


For less overdriven tone, lower the screen resistor on the first tube. Tack in another 1Meg in || , see what that gets you. I'm not familiar with that particular tube, but with EF86 and 5789, nice overdriven tone is gotten when the plate voltage is maybe 30% higher than the screen (this is a broad generalization of course) The higher the screen voltage, the more amplification, but less distortion you get, which sounds like what you are going for?


For your MV, it's common to use a dual gang 250 audio taper, and then connect a 2.2M resistor from wiper to ground lug. This gives the grids of the power tubes a path to ground should the wiper lift or lose contact. And you're keeping the grid leak value about the same.


Hope this helps!

[Zaos]

Yes that definitely helps a ton! This is my first endeavor into tube audio at all, as i've mostly worked on solid state gear.

My goal is just to make it less bass heavy, and maybe figure out how to make the treble/bass pot have a wider range? Not sure if that's possible.

If it helps at all the 7F7 is equivalent to a 6SL7 from what I can tell.

With the dual gang pot, I didn't even think of doing that. I connect each pot to one tube, and then have a resistor for each one going from wiper to ground then correct?

Is there anything from glancing you see that you would change? I'll try changing that 10uF pot tomorrow if time permits and update the thread. Thanks again for the help, probably not a common amp to see. Judging by the tubes I think it's late 30's early 40's

[John]

I *think* just reducing that 10uF cathode cap on V1 will help with the bassiness quite a bit. Which will probably then make it appear that your tone pot has more range.


And yes, you got it with the dual gang pot. Hoffman sells them right here. Your 2.2M resistor can go from wiper to grd, or wiper to the ground lug on the pot (which of course then goes to grd), whichever is easiest.


http://www.mif.pg.gda.pl/homepages/frank/sheets/084/7/7C7.pdf
that's the datasheet for your 7C7. It shows max screen voltage as 100, but in guitar amps we don't pay attention to that    But I would say if your plate is running around 110 to 120, and your screen is 80-90, that should be fairly clean and not too noisy. (I like the small pentodes in the V2 position myself, as I use them more for "tone" than for "gain")


I'd check the resistors for tolerance on your PI tube. If they're pretty far out of spec, that could be giving you a lot of distortion that you don't want.

[Twinstarrocket]

Zaos, I have a K-14 in original condition, and I'm about to put it back in order. Some of the most beautiful wiring I've ever seen in an amp, all laced etc. I almost hate to mess with it. Just looking through the schematic, there's nothing to indicate this amp should sound excessively bassy; the coupling caps would indicate quite the opposite. 

[Zaos]

May be just a wee bit late, but I've been in college so didn't have time to work on the amp!

Swapped out the input 10uf for a .47uf and the amp is much less bass heavy! Figured I'd update on the off chance anyone else has this amp.

Now I'm figuring out where/how to put the grid stoppers, and how to get more of that sweet distortion

[trlobdell92]

"Swapped out the input 10uf for a .47uf and the amp is much less bass heavy! Figured I'd update on the off chance anyone else has this amp."

Hi! Interesting choice, but good work! I have two of these things, one was a gift years ago from a friend that he found in a junk pile, the other I found on ebay five years later for 50 bucks. I'm interested to try some of your mods to even out the EQ for more treble, I've also been considering cascading the two preamp pentodes. I'm incredibly grateful someone found a schematic, I've literally been looking for ten years!

Your grid stopper resistors go on pin 6 of the 7C5's, limiting the signal coming in from the .005 caps. That won't really help with giving you more distortion though, it will stop the amp from having any nasty unwanted "farty" distortion. You may be better off tweaking the bias of the tubes, or trying a 5Y3 rectifier tube. Keep us (me ) up to date!

[trlobdell92]

Additionally, here's a link to a video of mine, only one is plugged in, in the video.

https://www.facebook.com/675823232/videos/10157101301363233/

[Zaos]

Is your bass/treble control also more or less useless?

[Zaos]

That's awesome that you have two, and the kind folks over at audiokarma just linked a schematic! I've been having a lot of fun with the amp hooked up to some pedals, I'll definitely keep the post updated with what I try next. I'm gonna try to figure why the tone control sucks, and look into a solid state rectifier setup instead of the 5y4.