Bringing an amp to life (tube content)

okcrum · #1 06-13-2011, 04:52 PM

Awhile back, TBer spankdaplank gave me a really sweet deal on a homebrew amp built from the plans in Chapter 14 of Ultimate Tone Vol. 3, which in essence is an updated Traynor Custom Special.

Spank hadn't built the amp. He'd bought it from someone intending to get it working, but didn't have the time. Hence my buying the amp.

Part of the deal between spankdaplank and I was that, in return for the great price, I'd get the amp working once and for all.

That brings us to this thread. For the purposes of discussion, I've assumed that spank didn't get a chance to do any work on the amp. He reported to me that the amp was wired as a power amp only, and that there was something wrong with the preamp. So, the amp has never completely worked.

Above is the amp in my shop under test. It came with 6550s (and many spares) and I've gotten some 6CA7s for it, since those were in the original design and are a lot less expensive.

The power amp works fine. The preamp looks like it has just too much wire in it to be stable (see below), though it seems to be wired correctly. The aluminum foil is actually effective, though I'll replace it with copper braid or screen eventually.

Above is the amp roughly as received. I lost one 22 uf/450v on powerup; that's the odd one in the pic. I've started to get little bits of time here & there to work on the amp, so I'll chronicle getting the amp fully alive (for the first time) and gig tested.

One big problem is that there are relatively large transformers in the original design, and the amp was built on a 20 gauge aluminum chassis - instead of steel. The result is that each time the amp's been shipped, the chassis has deformed a little more. At this point, I'd say it might not survive another shipment, but it won't need to. I plan to mount it in a rack case, with a 4U rack panel bolted to the front of the amp to help stiffen the chassis - the transformers are near the front of the amp.

So, I'll update this as I get things fixed in the amp. The preamp (with too much wire) is in the lower left of the 2nd pic. Note the unattached yellow wire near the upper left of the large eyelet board. That's the B+ for the preamp. I also have some optimization to do for crossover distortion, and some concerns about the screen voltage, which I'll address later.

And thanks to spankdaplank, without whom this thread would not exist.

JimmyM · #2 06-13-2011, 05:11 PM

Interesting. This is what? 200w? Can you actually replace the chassis so you don't get stuck having to use the rack all the time?

okcrum · #3 06-13-2011, 05:35 PM

Quote:

Originally Posted by JimmyM

Interesting. This is what? 200w? Can you actually replace the chassis so you don't get stuck having to use the rack all the time?

It's certainly capable of 200W, though the output transformer currently limits it to about 160 (continuous RMS, measured). Hardly audible. That's with a pretty beat up quartet of Sovtek 6550s I'll use for testing. I have some JJ 6CA7s I'm interested in trying in this, plus several different sets of 6550s that came with the amp.

Most likely is that I'd build any redesigned versions on steel chassis in real cabinets, and keep this as a giggable prototype. The rack mount is a simple and effective packaging solution for the moment. Plus, I have all the parts already.

One nice thing about this design is it already has individual bias settings, so I don't need to use matched tubes.

JimmyM · #4 06-13-2011, 06:10 PM

Well I envy guys like you who can work on amps. Hell, I envy anyone who can solder anything more difficult than two wires. Should be interesting...keep it updated.

spankdaplank · #5 06-14-2011, 06:48 PM

Hey - great to hear the amp is being re-animated. I'm kind of ashamed I let it sit in a box for so long. However my life took an unexpected and surprising right turn.

Sorry for this hijack, but here I am with the next chapter of my life in the Philippines. We're building a house with a spectacular view of Leyte Gulf. My wife is letting me put my music studio in front with large windows for the view.
Life is good. Sorry again for the hijack, here is the view from where our house goes:

JGR · #6 06-14-2011, 07:06 PM

Cool, keep us posted. If you are using the 278 and 1650T, the PT will be the limiting factor, not the OT.

okcrum · #7 06-15-2011, 02:32 PM

Quote:

Originally Posted by JGR

Cool, keep us posted. If you are using the 278 and 1650T, the PT will be the limiting factor, not the OT.

Will do. Yeah, 278CX & 1650T, exactly as in the book. I'm not expecting to see the power supply run out of gas till about 200W out of the amp, but there may be other factors at work here. Hammond rates the 1650T for 120W, but there's enough iron in it to get to around 180-200W, IME.

okcrum · #8 06-15-2011, 02:36 PM

Quote:

Originally Posted by spankdaplank

Hey - great to hear the amp is being re-animated. I'm kind of ashamed I let it sit in a box for so long. However my life took an unexpected and surprising right turn.

Sorry for this hijack, but here I am with the next chapter of my life in the Philippines. We're building a house with a spectacular view of Leyte Gulf. My wife is letting me put my music studio in front with large windows for the view.
Life is good. Sorry again for the hijack, here is the view from where our house goes:

NP at all - looks like you ended up in a beautiful place!

Sneakypete · #9 06-15-2011, 02:54 PM

Great project but I bet it has the same problem as doing an oil painting - how will you know when it's finished? Will you be tidying up the wiring - all neatly tied bundles with crisp 90 degree corners etc etc (actually could be quite a lot of etc's!)

okcrum · #10 06-16-2011, 03:16 AM

Well, the goal of this thread is to get the amp working and giggable, using the TUT-3 circuit. There's no telling what happens after that.

okcrum · #11 06-20-2011, 06:35 PM

Quick update since I didn't get much free time last week:

1) PA or driver has asymmetric clipping - this is the output power limit right now, not the OT
2) tone controls and buffer amp + DC cathode follower are back in circuit and appear to work OK

Spankdaplank filled me in that the pre seemed to have way too much gain. As he said, it might just be a wrong resistor value in there. Or it could be something else again, like a stage oscillating.

What's remaining is to move the input back to the original preamp, so the whole circuit is in play, get that working, work out the wrinkles in the power amp, add some protection diodes to the PA circuit, and package the amp up. I'm hoping to wrap it up this coming weekend, plus or minus gigs and rehearsals.

JGR · #12 06-20-2011, 06:58 PM

That OT is rated 120 watts with a flat 30Hz-30Khz - you can indeed push 200+ through it, no problem, and you won't lose much low end response. It is as big if not bigger than the OT in a lot of 200+ watt amps.

The PT B+ and VA rating will limit overall power output, but you should be able to crank into the 200+ range with some clipping. I think it is a great combination of iron. Good to see you are making progress.

okcrum · #13 06-26-2011, 05:26 AM

OK, I'm about ready to test the amp with a cab. I just have to heatshrink a few unused wires. The amp had a ton of gain - 20 mv p-p at the input would drive it to full output. That was with all 12AX7s. I got the gain down to manageable and still get grit with 12AU7s in the DC cathode follower and PI/driver. I'll need to clean up the layout some; there were stability issues with all 12AX7s.

Rough measurement is about 180 watts continuous RMS just below clipping, which is now symmetric, save for the compression in the preamp.

okcrum · #14 06-26-2011, 05:29 AM

Oh, and the 4U case is out, the amp is too tall. So I'll rig something in the gigantic head cab that spankdaplank included. Seriously, thing's bigger than an SVT head cab.

JGR · #15 06-26-2011, 07:59 AM

Awesome! What is the line voltage in your house- is it closer to 125?

okcrum · #16 06-26-2011, 05:40 PM

Quote:

Originally Posted by JGR

Awesome! What is the line voltage in your house- is it closer to 125?

Oh yeah, 122-124.

The amp sounds quite nice through a 1x15. Loud and ballsy. The tone stack needs work. It does things, but not nearly enough.

okcrum · #17 06-26-2011, 06:06 PM

...and another pic, SVT-CL at lower left gives an impression of size:

JGR · #18 06-26-2011, 08:38 PM

Cool, I figured you would get a bit more juice since the PT is spec'd at 115. I am getting ready to do an all tube version of my preamp with that same iron and was figuring I should be able to squeeze around 180 clean. How much more noticeable headroom does the CL have? Might be hard to tell until you get the preamp sorted, but figured I'd ask...

okcrum · #19 06-27-2011, 02:15 PM

The tone stack is a passive 4 terminal network between the preamp and the power amp, so it'll be easy to sort out.

I'm measuring continuous RMS so I can see what the worst case PS sag is going to be. The amp is certainly capable of 200W RMS tone bursts.

I'll need to gig this to see how it compares to the CL. I'm guessing for small gigs it'll be roughly a tie, since this amp has 4/8 ohm taps, and if I drive an 8 ohm cab with the SVT I'm getting around 200 watts due to the mismatch.

okcrum · #20 07-01-2011, 03:03 PM

Quick update: looks like the tone stack was rewired, so there's the problem. This network is a little different than most to start with.

I'll trace it out over this weekend (to see what was being tried) and return it to the original setup; then it should be time to mount and drunk-proof the head to get it out on a gig.