Tube amp advice needed

halo71

Anyone familiar with old Newcomb tube amps? I came across a pair of mono blocks that use one 6X4 rectifier tube, one 6AV6 preamp tube, and one 6V6 power tube. Rated at, I think 5 watts each. They look to be in excellent shape.

I was really wanting a cheap tube preamp. But for $150 for these mono's....are they worth checking out?

mhardy6647

I'd opine that is a little steep for a pair (is that for a pair?) of SE 6V6 amplifiers. These Radio Shack PP 6V6 mono amps (Japanese made Grommes clones) were $110 the pair, restored (and their restorer knows the value of the equipment he restores and owns).

halo71

Yes that is for the pair.

Here is a pic of one of them.

Joe08867

Little pricey in my opinion. Offer $90 and see where it goes.

dcmeigs

I would encourage you to think about their application.

The amps you are looking at are single ended 6V6 (rather than push pull). The 6V6 is a 12 watt tube and can be biased close to that in class A, much less in AB - like you say perhaps as little as 5 amps. Do you think that will be enough power to drive your load?

What kind of speakers are you driving? What is their sensitivity and how reactive are they? I have a single ended EL84 stereo amp which is essentially the same as the pair of these and it sounds very nice driving my efficient Zigmahornets but my reactive Signifers would destroy that amp in no time.

halo71

Doesn't matter now, dude flaked out. Decided he didn't want to ship. :rolleyes:

Thanks for the info though.

dcmeigs

halo71 wrote: »

Doesn't matter now, dude flaked out. Decided he didn't want to ship. :rolleyes:

Thanks for the info though.

I think that's a good thing.

You can get that sort of thing (and sometimes considerably better) out of an old maggie console off of craigslist for next to nothing. If you want to fool around with tubes on the cheap, that's a great way to start. If you don't want a DIY project, why not post an ad on the wanted board here at the Polk Forum and buy from someone you trust?

BTW, I think you will find that most of those output transformers are wound for 4 ohms and that can be an issue if you are driving 8 ohm speakers. Look for output transformers with 2 or 3 wires on high voltage side (SE and PP resp.) and four on the speaker side. The single tap transfos will only have two on the speaker side. 4 ohm taps are not a deal breaker, but the impedance mismatch will cost you about 3db.

dcmeigs

Here is a nice console find:



It's a Stromberg Carlson EL84 push pull with almost new condition vintage RCA EL84s. The output transfos are the pair on either side of the power transfo in the middle. Note the taps for 4,8 and 16 ohm speakers. Recaped and equipped with separate bias pots, it's sounds fabulous.

halo71

dcmeigs wrote: »

If you don't want a DIY project, why not post an ad on the wanted board here at the Polk Forum and buy from someone you trust?

I am fine with a project. You have anything to offer up?

dcmeigs

Oh yeah, I always have projects around. How knowledgeable are you about high voltage? I don't want to kill anyone! Do you have a scope and signal generator?

halo71

I am no expert with tube amps by any means. With nothing more than a voltage meter.

But I have modded a few guitar tube amps before. Epi Valve Jr. and a Valve Standard and an old Soldano. I know how to ground out the caps if that's your main concern.

A tube preamp is really what I am wanting though.

mhardy6647

Alanis Morrisette was kind enough to write a song to help us all remember how to measure high voltages with a DMM or VTVM...

One Hand in My Pocket

dcmeigs

mhardy6647 wrote: »

Alanis Morrisette was kind enough to write a song to help us all remember how to measure high voltages with a DMM or VTVM...

One Hand in My Pocket

I like that.

Over at DIY Audio there is a sticky on safety that is excellent reading and it is written so that newbies and grey beards alike can appreciate it. No matter how bad assed you are, there is always a guy who has worked on something more deadly than you, and his comments can usually discourage the rest of us from tearing into that _____ fill in the blank (like that 1.8kv scope that needs some work).

Here is a Link

No preamps. If you are interested I have a 7408 (high voltage 6V6) push pull Harman Kardon Receiver that sits on the self. I'm not sure that it's that much of a project. The tuner variable capacitor is hard to turn and I just leave it on the only station I listen to (can't tell you - mods don't want us talking politics) and I have modded out the bias circuit once or twice. It has great iron and a good tube set. I'd sell it for salvage value. For parts or repair as they say on Feebay. I think the shipping costs would make the sale prohibitive. It's heavy.

halo71

Thanks, but I may just hold out for a preamp.

Unless you have some small power amp project(s).

dcmeigs

Check out the Aikido preamp threads on DIY Audio. It comes as a DIY kit, meaning you can buy the circuit cards and then go to Mouser and buy the components and then stuff the boards. Then you need to build a power supply and put it all in an enclosure with jacks and switches. It's a reasonably involved endeavor but you will have the support of the DIY community and Brosky, who sells the cards and designed the circuit, has written enough about it that you will fully understand the circuit. There are plenty of photos to see how to do a proper build.

mhardy6647

It is (unfortunately) easier to build a good sounding, well behaved vacuum tube power amp from scratch than it is to build a preamp (even just a line-level preamp, to say nothing of a phono preamp). Sad but true.

A kit is a good place to start for a preamp.

www.bottlehead.com
www.transcendentsound.com

are but a couple of good vendors - I have long been tempted to build Bruce Rozenblit's grounded grid preamp kit for a few years...

dcmeigs

mhardy6647 wrote: »

It is (unfortunately) easier to build a good sounding, well behaved vacuum tube power amp from scratch than it is to build a preamp (even just a line-level preamp, to say nothing of a phono preamp). Sad but true.

Absolutely right. Starting with a phono or preamp is diving into the deep end.

If you can get hold of some good iron, a piece of plywood and some really ugly carpet, a power amp is easy in comparison. I wish I had taken a better photo.



It sounded superb by the way.

tonyb

Man, I'd be afraid the house would catch fire if I walked away from that puppy.

mhardy6647

tonyb wrote: »

Man, I'd be afraid the house would catch fire if I walked away from that puppy.

That's less of an issue than being very careful about house pets and small children :-P

Ideally, one doesn't walk away from such a breadboard amp unless it's in a steady state (i.e., been operating for 1/2 hour or so with no sign of the magic smoke* escaping from any components) - putting it on a table is probably a good idea too - although it's true that wool rugs are fire retardant! :-)

The breadboard approach is a venerable one. Clip leads and octal relay sockets are wonderful things. This is especially good for folks like me who cannot make a straight cut or drill a clean hole even if life itself depended upon it.

EDIT: A Heathkit, EICO (heh, or Lambda!) HV power supply makes the task of breadboarding an amplifier much, much easier. These are plentiful and inexpensive - heck, even a big Lambda isn't crazy 'spensive, and it will run forever. I only have a little low-ish voltage (250V) Lambda power supply. You know it's little because one person can actually carry it without effort. The Heathkit HV supply I have has some sentimental value - it's the one we used in grad school to do protein electrophoresis; my thesis adviser gave it to me a few years back (it was long since retired from active duty in the lab)!

dcmeigs

tonyb wrote: »

Man, I'd be afraid the house would catch fire if I walked away from that puppy.

Yeah, cat fires can be a problem. I'm a big fan of fuses on the secondaries (B+). They will all but eliminate cat fires and save your power tubes when the bias fails.

That photo was taken at a trade show, which accounts for the stylish carpet (Hilton no less). I'm was a little surprised that the fun police didn't object but it was an adult crowd and back in the corner behind some tables. Nobody died.



Mhardy, check this out. I picked up this Lambda 500 vdc, 500ma supply off craigslist a couple years ago for under a hundred. I blew the dirt out with an air hose and no further work was needed. It has about eight 6L6's, IIRC. One person can lift it but two is much better. It's a very handy thing to have. Notice that the tag says GE Computer Department. I would like to see some photos of the computers they used this thing with.

mhardy6647

They use oil-filled caps that have essentially infinite lifespans.

500V at 500 mA. The mind boggles.

For the OP's benefit, another aphorism worth remembering: It's the volts that jolt, but the mils [milliamps] that kill :-O

dcmeigs

I thread jacked this post something awful. My bad.

Mhardy, l want to discuss with you an interesting ChiFi amp with a paraphase circuit. I'll start a new thread later today. Please look for it.

mhardy6647

ahem I think we thread-jacked it; sorry!

halo71

No need for apologies. I will continue my search for a tubed pre or "project". :biggrin: