5879 SE redo

[RWood]

Hello,

This has been a very interesting thread, and I would like to build one for m'self. I have a donor, an RCA tape recorder, that I've gutted and am about to build back up. It already has a shocked-mounted, shielded 5879, a 12AX7 and a 6AQ5 power tube. Rectifier is a 6X4. I love the idea of the triode >pentode > CF preamp as described by Tubenit and built by Firemedic

My concern is that the RCA's PT, at 200-0-200, won't get me quite enough voltage for the preamp tubes. Using the tube recto, the schem shows 210vdc on the power tube screen and 200 on the plate, loaded. I am okay with lower voltages on the power tube.

Any ideas on how I should approach the B+ for the two preamp tubes?

Thanks

RWood

[tubenit]

Have you thought about a solid state rectifier instead of the 6X4?   That should give you around 200v x 1.4 = 280v on the plates.  The Gibson Scout using 6AQ5 has 305 volts on the plates.  That would allow more room to work with on the preamp tubes.

With respect, Tubenit

[firemedic]

Funny you guys should exhume this thread: I found the source of my noise problem. It was a bad pickup selector switch on my guitar. I guess the noise was canceling out or something in my P-P amps, making it harder to diagnose.
 

[RWood]

Thanks; I will try ss rectifier. I assume I would just use low value dropping resistors in the B+ in order to retain as much voltage for the preamp tubes.

Btw, in the RCA there was a 470Ω after the first cap, before the tap to the OT, so that's why the voltage was lower than expected even for a 6X4 tube. I like using a 'poor man's choke' there, too but usually a much lower value like 47Ω, just to get that extra stage of filtering in a SE amp.

I will reply back with my progress.

Firemedic - glad you found your noise.....and great threads deserve to be brought back to life!

RWood

[llama]

Hi,

Sorry for the reply on this old Post, But I'm hugely interested in building this as my first 5879 amp (second amp overall) 

The Chassis I want to use, as well as the PT/OT are all for an el84 amp, I cannot fit an octal socket into this (its an old Roberts R2R mono block chassis).

1. as the schem shows, how is having the 5870 as the second stage tube different in tone/sound/ vs stage 1?

2.  Would using an EL 84 power section (ala ax84 P1/HO) make this build less desirable?

I've been through the ax84 forum, where I was guided into using the HOSO Pre with the HO power, but this schem seems simpler for my experiment.  Any thoughts.  Thanks, I'm new to this forum, but have read through many of your posts and am very impressed with what many of you have accomplished, and level of expertise  .

Thank you.

[firemedic]

1. I changed the 5879 to the 2nd stage because according to Merlin, who seems to know what he's talking about, the very tiny grid voltage you get from a guitar pickup makes an input pentode sound like a triode. Or words to that effect. So I switched the order of the gain stages to take tonal advantage of having a pentode in the first place.

2. EL84's and 6V6s are electrically equivalent tubes for the most part.

3. You should know I scrapped the whole thing & made a slightly modified Vibrochamp anyway.

I rebuilt the amp several times because I was convinced the 5879 was sensitive & noisy. When I finally figured out where the crappy noise was coming from, it turned out to be the cheap tone cap in my guitar! Thus endeth the lesson.

llama, you are now the 5879 guy, congratulations! Please post w/ results, I'm sure you will build something cool. You're discovering that there are a lot of ways to make these circuits and they're mostly all good. I strongly suggest getting Designing Tube Preamplifiers for Guitar and Bass by Merlin Blencowe, or at least checking out his website (the name escapes me at the moment).

[kagliostro]

Can we hope in some samples ?

Thanks

K

[llama]

Thanks FireMedic,  And also for your reply to my message.

I've been digesting Merlin's Web site (valvewizard.co.uk)  Much of it is above my head, but I'm growing taller.

I will build the Redo with the el84 and gladly post sound samples , now that the "studio" is back up in my basement.

It is a slow process since my life is owned by my family  , but I will avail!  and will gladly take over the reigns of 5879 guy - as long as it pays well....

More to come.  is there a quality layout for this one anywhere?