Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: smackoj on March 20, 2022, 09:26:53 am

Title: 1966 Kay model 720: Mod it a little or leave stock?
Post by: smackoj on March 20, 2022, 09:26:53 am

Ola amigos;  I have been restoring this 1966 Kay Model 720. She ain't pretty but her framework is solid. I am asking for opinions from you electro tech pros. Should I add a smoothing choke and some negative feedback? I realize this amp did not come from the factory with either of these additions, but possibly the choke was not used because of cost and maybe negative fb was just an untested concept at that time? I have already modified it to cut down on the crazy amount of shielded wire they used to wire the input jack and volume/tone controls (pre and power amp together in one chassis at the bottom of the 1x15 cabinet) from the faceplate on top. Or perhaps you see other improvements that could be made?

If you have opinions and/or suggestions, throw them my way please. Thanks

Title: Re: 1966 Kay model 720: Mod it a little or leave stock?
Post by: sluckey on March 20, 2022, 10:11:30 am

Experiment with cathode bypass caps, 50µF on 6L6s, and 5µF on small tubes. Connect guitar to one input and notice volume level. Now take two unwired phone plugs and insert into the unused inputs. Should get a very noticeable volume increase.

Title: Re: 1966 Kay model 720: Mod it a little or leave stock?
Post by: PRR on March 20, 2022, 12:07:49 pm

Stock.

Title: Re: 1966 Kay model 720: Mod it a little or leave stock?
Post by: bmccowan on March 20, 2022, 12:24:54 pm

There are many ways to set up a 5879 preamp tube and this link has several of them. https://el34world.com/Forum/index.php?topic=21831.msg231846#msg231846 (https://el34world.com/Forum/index.php?topic=21831.msg231846#msg231846) I imagine you will try it stock first before trying mods.

Title: Re: 1966 Kay model 720: Mod it a little or leave stock?
Post by: smackoj on March 20, 2022, 01:08:31 pm

Good suggestions / opinions all. Thanks amigos

I am not familiar with a 5879 vac tube. It only took one short search to find out they bring a premium price! Anyone care to share some history on this tube and why they may have chosen it ?

p.s. The forum link you sent was very helpful.

Title: Re: 1966 Kay model 720: Mod it a little or leave stock?
Post by: bmccowan on March 20, 2022, 01:20:20 pm

Well PRR is the master of history. But this is what I know (or think I know)
5879 is a pentode. A lot of early amps used octal pentode preamp tubes - mostly the 6J7 and 6SJ7 tubes. 5879s, EF86s and a few others are the 9 pin versions of those octal tubes. They sound different (not better/not worse) than dual triodes. Merlin (Valve Wizard) has a good chapter on "small signal pentodes" and its posted on the Web - simple search will find it. I have built and rebuilt several amps with 5879 and 6SJ7 preamp tubes. The most famous amp with a 5879 preamp tube is likely the Gibson GA-40 Les Paul - the early versions had 6SJ7 tubes.
That's about it for my knowledge.

Title: Re: 1966 Kay model 720: Mod it a little or leave stock?
Post by: smackoj on March 20, 2022, 01:36:33 pm

Thanks for the info. I am more familiar with the Octal tubes, 6SJ7 etc used in the preamp section. I experimented with a couple early Valco amps with those preamps. Not a big fan of the ones I played but I will experiment with a few of the bias schemos in the drawings and see how it strikes me.