$300 tube amp budget. Which Amp??

[Runco990]

Hi guys,

I have been reading till my eyes bleed, and bought a few books on tube theory and modification, etc.

So I got hooked. I bought and built the K-12 amp kit and it's "ok"....
Sounds good, but NO bass at all.

Then I snagged an Eico ST40 amp without tubes for a great price. Re-tubed with JJ Tesla and nursed it up on my variac over a few hours.
It sounds pretty darn good with my Visonic David 70 speakers.
Bass is much better, and vocals are just sooo sweet. At this point there is no going back. I seem to be hooked. BTW, sources include my modest Thorens TD-160 and Sony CDP-101 in perfect running condition.

I am a component level tech by trade and am un-afraid

I have been eyeing the Dynaco ST-70, and the HH Scott 299-C. Also MAYBE an Eico ST70.

Problem is: I really don't have much more than $300 to spend on an amp that still needs to be re-capped, etc. I am also looking for a bit better bass than the Eico ST40. Not groundshaking, but good.

So time to draw on others experience...... What would you suggest, and why? I'd prefer NOT to use an integrated amp, BUT if the sound/price ratio is good, I'll go with an integrated. I can always bypass the pre-amps if the Iron and output stage would make a better amp.

So lets hear it.... can I do OK with a roughly $300 initial budget, or am I dreaming?

Thanks.

Marc

[Eli Duttman]

Marc,

An "El Cheapo" built with DynaClone Z565 O/P trafos should meet the need. Eliminate the volume control. Approx. 89 nF., instead of 56 nF., is appropriate for the I/P high pass cap. Wire the O/P tubes in ultra-linear mode by connecting the screen grid resistor to the taps on the O/P trafos, instead of tieing them to the plates.

If you use a line stage with gain, switching to 6J6s as splitter/drivers is indicated.

[Runco990]

Interesting, but I'm not ready to build from scratch just yet. I did copy the schematic, though.

Also, I'm looking for 20-30 wpc range to properly drive my speakers.

To begin with, I'd like to restore and maybe upgrade an existing amp to listen to. Homebuilt will be a later adventure. So far I have learned a lot just reading here, and drooled at the pictures of many DIY amps built by members. Some of you guys have raw talent!

Back to my original question: Since I have the Eico st40 (and it sounds darn good to me) would the ST70 or one of the others I'm looking at be a better choice to get just a BIT more grunt? The Scott 299-C looks to have very large OPT's, which by my reading should do better?

Or...... can I get a hair more out of the Eico st40? My eventual plan is to gut it and re-assemble just the "AMP" in a new chassis to give it a nice clean look and allow use of a seperate pre-amp.

Any thoughts?

Marc

[Eli Duttman]

Marc,

The Eico ST40 uses 7591 O/P tubes. To get really noticeable extra power, you will have to move into KT88/6550 territory and your budget stops that "show" dead in its tracks.

The 7591 is an EASY tube to drive. The splitter/driver in "El Cheapo" can do the job for you. You have an excellent trafo set in the ST40. You can squeeze a little extra power from a pair of 7591s by switching to combination bias and moving the operating point closer to Class "B". Construction of a C- bias supply is not at all difficult.

No matter what you do, a good regulator is in order for the 7591 screen grid supply.

[phn]

Considering that the Dynaco ST-70 is (in)famous for its bass deficiency, I don't think it's the right way to go.

Having that said, there's virtually no difference between Dynaco, Eico, McIntosh, Marantz, Leak, etc. It's McDonald's vs. Burger King, Adidas vs. Nike, Hulk vs. Superman. It's all about brand nuthugging. It has very little to do with audio.

[rcavictim]

Forget vintage. Get a MingDa MC34B and the job`s done. P-P 6L6`s, can accept 6CA7`s. Ive got vintage Mullard 2nd generation 6CA7`s in mine with some NOS classic USA small TV tubes (6BK7, 6CG7) in the other holes and this amp delivers 28 watts/ch and sounds awesome.

I cannot say enough in the praise department about this amp, period! The MC34B only costs about $350 US in China new so you might find a used one on US soil in yor price range.

[5u4]

I'd be looking for a Heath AA-40. Very nice sounding EL-34 vintage amp!

[DougL]

From my research, and reading Designing Valve amplifiers, the low frequency limit on a tube amplifier is determined by 2 things:

1: The primary impedance of the output transformer interacting with the RP of the Tube.

2: The RC time constants of the inter-stage coupling cap, grid leak resistor pair.

When you are scratch building a tube amp, you have control of The primary Inductance and RP. Its generally uneconomical to change for modification.

The other factor, the RC time constant was probably set high to prevent low frequency stability issues. Modifying the power supply can allow you to increase the coupling capacitors.

Some of the ways I am aware of to change the power supply to allow this are regulate the Front End B+, or create a separate B+ supply.

Hope this helps;

Doug

[Runco990]

Thanks for the constructive replies.

So the Eico ST70 really won't do more than the st40.... same output tubes and transformers by the looks of it, so that makes sense.

How much "budget" should I have? Realistically, for what I want?

Also, I have been looking at the Chinese amps, and I could go that route since it's NEW. (No money for rebuild needed)
However, without hearing much feedback from actual users of these amps, I have been rather sceptical as to their quality.....

Though I'm willing to go this route if I see enough positive responses.

Oh dear.... what HAVE I gotten into?

I used to have tube amps and receivers when I was in Jr High school. I was always fascinated by tubes, but I didn't have the appreciation for it then. Wish I had kept all that stuff. (Already kicked myself many times over)

Thanks for the responses so far. I apreciate all the suggestions, it really helps me narrow down where to drop my money.

BTW, any thoughts as to the Dynakit ST70? It is a candidate since, if nothing else, I can get ALL new parts to build one. Pros, cons???

Marc

[Runco990]

Doug, yes this helps. I have plans...........

I am currently reading Priciples of power, and tu-be or not tu-be, modification manual for tube electronics.

My brain hurts. But I'm a very fast learner.

I repair broadcast video gear to component level, so I'll get the hang of this as well.

Marc