building Single Ended amps

[akimball442]

Hi, I am new to this forum, although I've been looking at it for a while now. It looks like one of the better, if not the best places on the internet to gain knowledge.

I first fell in love with tubes as a guitar player, in fact, the only solid state amplifier I own anymore is the one my dad bought me as a kid, and only because I can't part with it for that reason.


I later figured out they made audio equipment out of tubes, and got my first Eico ST70 and rebuilt it, and I still use it to this day.

Since then, I've collected a garage full of goodies, amps from old consoles, record players, you name it, if it had tubes in it, I bought it. I've rebuilt lots of magnavox and zenith amplifiers with good success, some of them sounded good, others sounded not so good.

Right now, I have 2 amps I would like to make an attempt at turning into REALLY good amps. I have lots actually, but would like to start getting ideas for two.

One is an RCA 6v6 (with black plate tubes in it) unit out of a suitcase style stereo record player which had 3 speakers. The wiring for the speakers was a joke, and this unit had two dissimilar output transformers. I don't even know what they were trying to do there. I simplified it and put in a pair of zenith OPTs from some SE el84 amp I had laying around that had a burned up power tranny. I also eliminated the common cathode on the 12ax7, and added a little feedback. The amp sounds ok. Much better than it did, but I would like to find a schematic for a REALLY good SE 6v6 amplifier that I can build on a new chassis using the iron I have here.

The second project was a really interesting disaster from the past, an el84 SE stereo amp with 3 OPTs.

Somehow they powered up three speakers by making a hybrid-- combining both sides into one larger P/P OPT and supplying the sound to the single bass speaker that way. Each channel has a tiny SE OPT that powered the left and right HF speakers. These speakers were apparently (on purpose) wired out of phase. Perhaps the different phase is how they powered up that P/P OPT for the bass channel. I followed a schematic I found online and converted it to a regular SE stereo amp. I added a small amount of feedback, this amp sounded HARSH (read: horrible). The feedback made it sound good. This one sounds good, except for some bass distortion, and a general poor low end. I'm certain that's because the output transformers are so tiny.

I know there are a million single ended EL84/12ax7 amps out there. Which one is the best?

Any schematics/links would be appreciated, as well as personal experience... ie.. has anyone on here actually built a 6v6 SE amp?

Furthermore... I keep reading here and elsewhere that people are against negative feedback for whatever reason.... But in the cases of both these amps that didnt have any they sounded horrible until I added it. Why is that?

I know this is a long winded post, and future ones should be shorter.

I'm interested in anything anyone has to say.. or suggestions.

Thanks...
Aaron Kimball

[Miles Prower]

One is an RCA 6v6 (with black plate tubes in it) unit out of a suitcase style stereo record player which had 3 speakers. The wiring for the speakers was a joke, and this unit had two dissimilar output transformers. I don't even know what they were trying to do there.

Sounds like what was called a "matrix amp": it's a way to do stereo on the cheap. The results can be pretty good, if the design is done right. More often than not, the results weren't so good.

This one sounds good, except for some bass distortion, and a general poor low end. I'm certain that's because the output transformers are so tiny.

That would be correct.

Any schematics/links would be appreciated, as well as personal experience... ie.. has anyone on here actually built a 6v6 SE amp?

Try this: Power amp / integrated 6V6SE (DIY) The schematics - a design process walk-through

Furthermore... I keep reading here and elsewhere that people are against negative feedback for whatever reason.... But in the cases of both these amps that didnt have any they sounded horrible until I added it. Why is that?

There's a whole bunch of audiophool nonsense out there, and this is but one example. NFB is not "evil". It may have gotten such a reputation due to the tendency of the "Big Box" designers using it to compensate for fundamentally poor designs. Used properly, NFB will improve any design.

As for your situation, pentodes almost always produce more of the nasty sounding third and higher order harmonics. SE amps that use low-u triode finals can sound pretty good with no NFB applied at all since triodes tend to produce mainly second harmonics, which aren't so objectionable as third and higher harmonics. Also, pentodes have higher r(p)'s, and so worse speaker damping. These require NFB if they are to sound good. However, even adding a few db's of NFB to a triode SE design can improve its sonics. (Of course, there is always the possibility that some folks like the effect of the added second harmonics that SE triode amps produce.)

[poynton]

Originally posted by akimball442




These speakers were apparently (on purpose) wired out of phase. Perhaps the different phase is how they powered up that P/P OPT for the bass channel.

HI.

Usually, it is to give the impression of a wider soundstage and is quite common on 'BoomBoxes'. [Mono centre and anti-phase L/R]

Andy

[Eli Duttman]

Triode wired 6V6s sound VERY good. The O/P trafos you already have are quite suitable.

Simple circuitry along the lines of the DECWARE SE84 using a 5965 for voltage gain and triode wired 6V6s will work very well. High efficiency speakers are essential, as power O/P will be about 2 WPC.