Building tube amp in wooden box(es)

[Saurav]

Does fire hazard mean anything to you at all??

If you ask my wife, you'll get a vehement "Yes!" to that

Yes, of course, that is a concern. I also realized that my PS design which had resistors feeding the output tube's B+ would require some very high powered resistors. I wondered by most PSU designs I've seen fed the output tubes straight from the choke, now I think I know why.

Maybe I'll just spend the money and buy the punches/hole saws I'll need.

 

[Morse]

Hi Frank;

>>>...Does fire hazard mean anything to you at all??...<<<

Good point. Yes that's a valid concern with any DIY electronics no matter of chassis type. For my own personal 'risk/reward' ratio, I'm willing to take the tiny added risk of a wooden chassis with metal mountings for combustables but others may want to delete all wood from their projects (though it breaks the heart of this long time amateur woodworker to type it). That's one reason that I advocate the use of the metal mounting plate for the tube sockets themselves.

Anyway, some other fire hazards to consider are the use of carbon comp resistors in spots where they can fail catastrophically (and burn!) and worst of all the wax coated cotton insulated wire that was used commonly in the good 'ole days. Covering insulation wire in cotton's bad enough - but to soak it in wax just makes your wiring into a candle waiting for a match.....

For all novice woodworkers, another BIG fire hazard is the infamous 'oil soaked rag' - particularly if you're using something like linseed oil. That stuff is a notorious spontaneous combustion hazard!

As far as moisture absorbtion in wood is concerned, I've never been able to measure anything other than "infinity" for resistance on any of my amp chassis. Still, it's best to ensure that no 'live wires' can contact the chassis directly as with any other chassis type.

A good post Frank. Thanks for injecting a little sobriety into the discussion.

All the best,
Morse

 

[Bas Horneman]

Does fire hazard mean anything to you at all??

Offcourse it means nothing to an American. They build their houses of wood..

 

[Saurav]

I just ordered all the parts for my first amp. I decided to play it safe and went with a standard Hammond aluminum chassis, and bought the metal punch kit from Triode Electronics. So my first amp won't be unique and exotic and wooden, but hopefully it'll be a little safer

FWIW, I'll be building the Angela/JE Labs 2A3 SET, the 6SL7 SRPP version.

 

[Morse]

Hi Saurav;

Nothing wrong with playing it safe. If you want to dress up a plain jane chassis, you could consider some wood side panels. Just go to the local 'big box' home improvement store and pick out a nice stick of the hardwood of your choice.....

By the way, what brand valves you going with? I'm using SovTek 2A3's, Reflektor (Russian NOS, same as SovTek) 6SL7's, and an RCA 5U4GB in my JE Labs 2A3 (I built the 2001 ed rather than the SRPP).

Please let us all know how the SRPP sounds by the way! I've considered trying mine out in SRPP since the rewiring is not really that big a change.

Good luck and all the best on your project!!
Morse

 

[arnoldc]

hey saurav, great to hear that. let us know how it progress.

morse, i also built the SRPP 2A3 and one thing that impresses me is that it sounds good from classical to heavy metal i also use Sovtek 2A3, red base RCA 5691 and RCA 5U4G.

one benefit of the JE Labs SRPP 2A3 is that a change in cathode resistor to 1K5 and addition of 1K from coupling to grid and it can now accommodate a 45 it's on Esmillia's site as well.

am still looking for a furniture maker to make my wooden base for my other projects... i might give up though and just use acrylic

 

[Saurav]

I just bought the tubes that Triode Electronics sells, so I think I have Sovtek everything.

Morse, I've thought about wooden side panels, that should be easy enough to do and look pretty good. I'll probably leave the bottom open, or put something perforated there, so I didn't buy that piece.

How does one calculate gain and output impedance for an SRPP stage? I looked through my Morgan Jones book this morning, and the closest thing I could find that's been analysed is a mu follower. Maybe I should read the Tubecad SRPP article for a third time and see if I understand any more from there.

For those of you who've built the SRPP version, do you have a rough idea of the input sensitivity? My linestage is passive, so I'm a little worried that it won't drive this amp to full output.

Thanks,
Saurav

 

[Morse]

Hi Saurav;

The SRPP Deconstructed article is less of a number crunching exercise than it is a treatment of variations on the SRPP theme.

There's actually more than one TubeCad article on SRPP, so the best answer I can give on SRPP design parameters like gain, etc. is for you to look them up at http://www.tubecad.com/may2000/page5.html and following.

Frankly I would not trust myself to type those formulae in by hand, so I just gave the link to the page with 'em! Please note that there's a correction to the formula for finding GM of the SRPP given in the next issue of TubeCad so you'll need to find that in the index of past issues.

You'll do well ordering from Ned; I've bought a LOT of valves, etc from him and never had a problem yet (knock on wood for luck after typing something like that!! ). The SovTek 5U4G rectifier is a beauty in a lovely ST type coke bottle - you'll be very pleased with the cosmetics on that one!!

If you should happen to need to brace the underside of the chassis to better bear the weight of the trafos, you can buy aluminium bar stock cheaply at most any hardware store.

Good luck and all the best,
Morse

 

[Saurav]

Good luck and all the best

Thanks. I finally "took a deep breath and jumped in", so to speak. Let's see how this goes.

Thanks for the link, I'll go browse the Tubecad site for the other articles.

 

[Morse]

Hi arnoldc;

Thanks for the tip on the mod to use the 45 in that circuit.

Hmmmm, acrylic for a base material?

Will you use a metal top plate then? It's certainly got potential...you might want to draw out some designs and try for an art-deco look with something like that. Have you ever seen the Shanling 2000 CD player?

Maybe you could use a rounded rather than square shaped base? Or perhaps a coloured acrylic in conjunction with an anodized alluminium top plate? You could even combine some mirror polished wood components for a really unique look.

Only real hassle I can think of with all this is that the thermal expansion/contraction cycle might be problematic - in other words, you'll have to allow for the more "mobile" parts to expand freely without stressing out the rest of the chassis. You could take a look at how furniture makers allow for the expansion and contraction in real wood via moisture absorbtion.

Interesting idea, though. Please keep us all posted - I bet it could be a really nice looking amp!

Good luck with it!!
Morse

 

[arnoldc]

hi saurav, i use an ALPS 100K pot for the volume control and does not use a preamp on my SRPP 2A3. i've tested it with good results on "non-efficient" speakers such as my Infinity Kappa 200 @ 89dB, JBL Ti2K @ 88dB, Red Rose Spirit @ 87dB as well as Axiom M3ti @ 92dB, Coincident Triumph Signature @ 94dB, Voigt TQWL pipes, and Bob Brines (licensed) Fostex TL speakers. I like it best with the Coincident Tri Sig though. I hope to be able to test with an Omega TS2 and Super 3 soon.

hi morse, my CX-345 arrived and will get it soon. i'd be able to give you some feedback when used on this SRPP circuit, but I don't have a 5K OPT other than the 125ESE (my 2A3 is currently using MagneQuest DS-025). i'd be making another amp specifically for the CX-345 with Tamura F-475 and this is where is will use the acrylic, cost permitting. yes, i've seen the shanlings as some friends have them, but i don't have the urge yet to splurge on a solid state player that expensive as i'm pretty happy with my Oracle turntable.

 

[Morse]

Hi ArnoldC;

>>>...i've seen the shanlings as some friends have them, but i don't have the urge yet to splurge on a solid state player that expensive as i'm pretty happy with my Oracle turntable...<<<

Sorry, I didn't write that very clearly. I wasn't trying to imply that you should go from analog to digital (*shudder*) but just pointing out how much fun the avant-garde styling of something like that can be. Personally I loathe the current generation of soul-less black boxes that make up mainstream hifi, though at least their cosmetics correctly advertise the contents - as warm and individual in character as a tray of ice cubes.

By the way, what did you think of the 125ESE's? I'd like to put something serious but affordable on my next SET project for OPT's and the 125ESE is at the top of the current list of candidates.

>>>...i've tested it with good results on "non-efficient" speakers...<<<

Yep, total agreement with you there. You really don't need high SPL speakers with SET so long as the speaker's crossovers, if any, are 6dB/octave slope. Right now I'm doing the listen/fine tuning on a pair of 89dB SPL RS40-1354's and they've got way better dynamics and more bass response than I thought they would with only 4 watts from my 2A3. It's almost a shame to use 'em with an amp like the 6BM8 Gnat that's built for moderate listening levels with limited bass response. Just got to get rid of a little upper midrange "shoutiness" and they'll be perfect for a midsize little stereo like the 6BG6 SET that i've got on the drawing board.....

One other question if you don't mind - what did you think of the Axiom M3Ti's? I've got a buddy here who's looking for budget speakers for his midfi system and he's sheepish about DIY'ing it. The Axiom looks like a good candidate for him, along with a subwoofer.

All the best,
Morse

 

[Saurav]

hi saurav, i use an ALPS 100K pot for the volume control and does not use a preamp on my SRPP 2A3.

Well, it should be fine with a passive linestage then. The webpage says .7V input sensitivity for the cathode follower version, and I'm hoping that the SRPP version doesn't reduce the gain too much.

 

[arnoldc]

Sorry, I didn't write that very clearly. I wasn't trying to imply that you should go from analog to digital (*shudder*) but just

hi morse, no i'm sorry you wrote it perfectly but my writing does not end with a period... ha ha ha. it was altogether another thought

By the way, what did you think of the 125ESE's? I'd like to put something serious but affordable on my next SET project for OPT's and the 125ESE is at the top of the current list of candidates.

this 125ESE continues to amaze our group. IME, i've found it no slouch, as it sounds really good on different circuits.

1. on my SRPP 2A3, everybody was amazed at how good it did 1812 Overture (the real thing, not the wimpy digital cannons). no distortion on the cannons and crystal clear on the bells. it's driving a modified Voigt.

2. on my 76-6SN7-300B amp, it also sounds very good. and reacts well with 300B tube rolling such as Valve Art, Electro Harmonix, TJ Mesh and WE300B.

3. a friend who has a Monica (Bob Danielak's direct coupled darling, DC Darling) has been using this for years. yesterday, he swapped in a Tamura F-475 and found out that the F-475 is more "refined" but that's it. i told him the F-475 is overkill for Monica

IMO, this OPT is a great workhorse, you won't be disappointed. and for the price you pay, it kick ***. he he he

One other question if you don't mind - what did you think of the Axiom M3Ti's? I've got a buddy here who's looking for budget speakers for his midfi system and he's sheepish about DIY'ing it. The Axiom looks like a good candidate for him, along with a subwoofer.

i've auditioned them in my room, using my equipment and here's what i found using the 2A3.

pros:

- no crossover
- it did not cause the amp to clip
- it has clarity and resolution
- it conveys that magical midrange of SET
- it has smooth highs
- excellent soundstaging ang imaging

the bass, for an enclosure that small, is surprisingly good. it delivers punch and dynamics that are hard to ignore.

cons:

- it may not be 92dB as claimed by the manufacturer

it's worth the $275 list price.

 

[Morse]

Thanks ArnoldC!

That's just the info I needed before dropping the $$ on the 125ESE's.

>>>...1812 Overture (the real thing, not the wimpy digital cannons)...<<<

That wouldn't be the old Telarc recording, would it? I used to have that one on LP and used it to show off the biamped dual 15" subs I'd built back in the early '80's. That recording rocks!!! I had a heck of a time getting a turntable/cartridge combo that would track it though - IIRC I ended up with an NAD 5120 and a Radio Shack Shure knock off, and that did work.

By the way, you're into LP's, aren't you - where's a good place to buy LP's online these days? I've thought about getting back into that but my LP collection is long gone and would have to be built back from scratch. Still have the 5120 and a couple of pretty decent phono cartidge/styli combos, but I need to piece together a good RIAA preamp.....

I'll pass on that recommendation on the Axioms to my friend for his Yamaha sand amp (assuming I can't talk him into DIYing the speakers).

Thanks again and all the best!
Morse

 

[arnoldc]

hi morse,

yes, it's the old Telarc

on vinyls... i have a big disadvantage being in the Philippines. we have our own sources here, but some titles i'm looking for are kinda expensive or not available. from time to time, some people go to the US and haul a thousand or so vinyls and sell it here.

my friend buys at reasonable cost from ameoba records, but not online. i buy from other sources such as audiogon, then have it shipped to my friend in CA, then she ships it to the Philippines.

so which phono pre-amp have you been looking at? been using Musical Fidelity X-LP and i think it's time to do some tubes too. been looking at Hagerman Tech phono stage or JE Labs, he he.

 

[Morse]

Hi ArnoldC;

Thanks for the heads-up on Audiogon - I'd heard of 'em but never knew anyone who'd bought from there. I may have to give that a try after I visit the local second hand shop and see what their selection and price's are like.

Hmmm, which phonostage?

I was thinking of taking a circuit out of RC 30 with a 12AX7 and building a 'make do' power supply consisting of a pair of 12.6V 3A trafos back to back (and drawing the .3A heater current off the junction of the two). I'll take a look at the JE Labs circuit and see if it's anything that's do-able on my budget these days. OTOH I do have some oddball valves around here - 6AK6's, 6021, nuvistors and such that might have potential.

In the longer term, once I find a circuit I really like, I might integrate it into my JE Labs 2A3 - my PS has almost 100% overdesign on the secondary winding so there's current to spare in there, and an oversize chassis to work with. Then I'd have to add another pair of RCA jacks and a switch but that's no big.

What do you think about all this? I don't know much about valve based phonostages.

Thanks again for the info!
All the best,
Morse

 

[arnoldc]

hi morse, unfortunately, i have no experience myself with tube phono stages... we have to rely on someone else