Prototypers wanted

[n_maher]

I decided to go a bit crazy and try to use the mill to at least some level of its capability at something other than cutting round holes... I'm pretty pleased with the results. It was hard to do as well as I wanted to because it's simply impossible to hold the extrusion rigidly enough while milling. If I ever do this again I'm going to have to work on some sort of custom clamping rig to deal with extrusion flex and vibration. As it stands, perfectly acceptable on DIY gear in my book and certainly passes the look test at distances greater than a couple of feet.

I haven't quite decided what to do with the top panel yet since I know the design intent is for the heat sink to be located above the trafo. I'll probably stick with that and simply do some sort of hole pattern over the Vreg heat sink.

[Nebby]

Wooo, very nicely done Nate!

[luvdunhill]

anyone else tried any other tubes? or is that sorta verboten discussion here (my last post concerning such things sorta flew under the radar)

[dsavitsk]

anyone else tried any other tubes? or is that sorta verboten discussion here (my last post concerning such things sorta flew under the radar)

There are a whole bunch of tube with different names that may or not be different tubes. I know Tomb likes one of the alternatives better, though I can't remember for the life of me which one it is. Some of them list the acceptabe plate dissipation as being much lower than how we are using them. However, I believe this is the case with Tomb's favorite and to date he hasn't had any issues that I know of. Worst case with this amp if you kill a tube is that the tube dies. Nothing else should be damaged as a consequence, with the possible exception that if the tube shorts internally the CCS transistor could overheat. So, please experiment and let us know what you find out.

[luvdunhill]

Cool. So far I like the CV4031. Slightly more Rp (6.8K versus 6K) and if I read the data sheet, slightly less gm. Everything else seems the same to me. Since you mentioned it, the plate dissipation is more than the 6J6 (2 x 1.6W versus 2 x 1.1W). The tubes come in nicely overdone packaging, including pin savers. I paid $12 shipped for the pair from the UK.

To me it seems like there is better bass control. And perhaps more gain, but I'd have to measure or reread your post above in light of the changes. For now, I'm resting and enjoying the amp

Edited August 1, 2010 by luvdunhill

[TomB]

There are a whole bunch of tube with different names that may or not be different tubes. I know Tomb likes one of the alternatives better, though I can't remember for the life of me which one it is. Some of them list the acceptabe plate dissipation as being much lower than how we are using them. However, I believe this is the case with Tomb's favorite and to date he hasn't had any issues that I know of. Worst case with this amp if you kill a tube is that the tube dies. Nothing else should be damaged as a consequence, with the possible exception that if the tube shorts internally the CCS transistor could overheat. So, please experiment and let us know what you find out.

Yep - 5964 is the one that's "over-driven" but sounds quite nice. I haven't tried it again since upping the current in the CCS, though, but I will give it a go tonight. The other one that's in relatively plentiful supply and is still cheap is the 5844. Many of the others listed on TDSL are just slight variations on the 6J6, but those two have noticeably different plate construction.

Here's the basic list that shows up as "close or identical performance" on TDSL:

1216, 5844, 5964, 6030, 6045, 6099, 6101, 6535, 6927, 6CC31, 6J6A, 6MNN3*, CK6101, CV5046, CV8160, CV8231, ECC91, M8081, T2M05

[luvdunhill]

Cool, I found one not on your list (similar to M8081 probably thhough). There also is at least one Russian tube that has the same pinout.

[luvdunhill]

Just noticed some odd with the two original tubes that I got from Tom. Anyone notice anything strange in the pic?

[ATTACH=CONFIG]3415[/ATTACH]

[cetoole]

No getter? Not even an oxidized one, that I can see. You didn't happen to open the one on the right to give a really good cleaning, did you?

[luvdunhill]

heh, didn't open it Here's a "before pic":

[ATTACH=CONFIG]3418[/ATTACH]

[spritzer]

That's not a flaw, it's a feature...

[TomB]

heh, didn't open it Here's a "before pic":

[ATTACH=CONFIG]3418[/ATTACH]

Thank goodness. I didn't think I'd send somebody a tube missing the getter flash. It must've used it up fast! This goes back to what I was referring to with Pars. If the tube sat on the shelf for 40-50 yrs, gas sometimes infiltrates into the tube. I guess if the getter flash is thin enough, it gets burned away leaving clear glass. This is different than when you break one - the getter flash turns white in those cases, but doesn't go away.

OK - anybody else need more tubes while I'm at it?

[Pars]

OK. I got around to grabbing the two RMAA runs I saved last week, as well as running a couple more today.

Last week's runs were done with 75 ohm CCS resistors. I also tried to balance the input levels between channels so these will not show the channel imbalance to the full extent. Bias (PR / PL) was 110Vdc / 130 Vdc. Runs at both 32 ohm setting (33 ohm load) and 300 ohm setting (330 ohm load):

RMAA_test _ l'esp_330_load_up.pdf

RMAA_test _ l'esp_load_33_down.pdf

I paralleled 221 ohm resistors to the existing 75 ohm for 56 ohm CCS. I also matched the BJTs in the CCS as well. When I went to run RMAA, it was complaining that there was too much distortion. I played around with the volume control on the amp, input and output levels to/from the M-Audio box, etc., but could not get rid of it. Bias is now at 148 Vdc (PL) and 129 Vdc (PR)... yes, it got worse.

RMAA_test _ l'esp_330_load_up_56.pdf

RMAA_test _ l'esp_32_load_down_56.pdf

I've never had anything distort so much that RMAA complained about it. THD levels don't look too good. I may pull one end of the 221 ohm and run some and see how it behaves that way. Any other suggestions?

Edited August 3, 2010 by Pars

[TomB]

Just glancing at the results, it looks like dynamic range and noise floor improves, whereas distortion does not. I think one of those tubes definitely looks bad in the final set of results/plots, though. That may be skewing the overall numbers. As far as the channel imbalance - even when well-matched, I think a 1 dB channel variance is pretty good for tubes.

BTW, I got mixed results with the 5964's at the new CCS setting - they may sound a bit worse than before? Hopefully, I'll get a chance to do more listening in the next few days. I would try RMAA, too, but I only have an M-Audio Transit and they never got the drivers fixed beyond Windows XP.

When the Transit worked, I had instances where RMAA refused to give results because distortion was too high on other tube amps/hybrids. JMHO, but I don't think RMAA was created with tubes in mind.

[Pars]

Yes, when I looked at the previous numbers, I thought I would see the 56 ohm numbers much higher based on RMAA bitching at me. The channel imbalnce with equal input levels (the 56 ohm CCS numbers) is ~0.7dB for the high impedance and 1.7dB for the low impedance. BTW, for 120 ohm phones, would the high impedance be the setting most likely to produce good results? Or just listen and see which you prefer?

EDIT: Also, does anyone have dimensions for center on the tube holes in the top? I think I am going to use a 1 1/4" holesaw for these. I may have access to a unibit big enough as well.

Edited August 4, 2010 by Pars

[dsavitsk]

Take it with a little grain of salt. If you look at the 45 data sheet, renowned as the lowest distortion tube around, at full "undistorted" output it's still up around 5%. Now, it's 5% has basically nothing past the 3rd, but the point is that distortion numbers are nt everything. At least the shape of the distortion is more or less appropriate.

My guess is that RMAA is requiring you to run this at a pretty high level, probably higher than you might listen with most common headphones, which results in the 1% THD. If you run it at more like a few mW, the distortion looks a lot better with most upper level stuff buried in the noise. When I switched from using my M-Audio FastTrack Pro as the sound source to instead running it's spdif to my own DAC, a lot of the higher harmonics dropped off quite a bit, so I'd suspect the soundcard is having some influence.

All that said, it is what it is -- the tube is not a super low distortion tube, we are running it at the maximum of what it can do, and the OPTs are not super high quality. If you want better tubes/transformers, it will cost a lot more.

[luvdunhill]

I might slightly prefer the K1000 with 300 ohm switch and WE jumpers. Its a different flavor for sure...

Plan on trying new CCS current this weekend as well...

[Pars]

I think I will socket either one or both (paralleled) CCS resistors to make changing them easier. That way, my brother can decide what he likes the best and go with it.

Doug: yes, I'm aware that RMAA is just a tool. Running it on my Counterpoint tube preamp resulted in much much lower distortion numbers, but it is an entirely different tube architecture. Volume (and gain) were at max for that testing as I used the THD values to bias the tubes in the line stage. Here's a sample.

The M-Audio FW Audiophile is an external box, but yes, functions as a sound card. RMAA has to have a certain level range in order to function. I have control via the control panel of input levels (I presume this is input coming back from the M-Audio, so it is the output of the DUT), and output levels (I presume this is the output of the M-Audio into the DUT). I can of course also vary the volume pot on the amp. Normally, amps are tested turned all the way up (at least to my understanding).

Speaking of the Counterpoint, would you be willing to take a look at it sometime Doug? No hurry, maybe this winter or something if you are up for it. Still having a slight hum problem.

RMAA_test_ SA5.1_32_192k_80407_4.pdf

Edited February 11, 2019 by Pars

[TomB]

<snip> Normally, amps are tested turned all the way up (at least to my understanding).</snip>

AFAIK, this was a practice started by AMB. I'm not sure there's any basis in a standard other than that. AFAIK, Tangent doesn't test his amps that way and I don't think Pete Millett does, either. I wasn't going to go into detail, but Doug hit on a key point. When I tested Millett Hybrids/MAXes/MiniMAXes, I essentially got the same thing when attempting to set the volume to 100% - too much distortion for RMAA to function. As Doug said, tube amps (and hybrids) are not necessarily designed to run at 100% volume. There are certain tubes and bias settings in a Millett Hybrid that will result in the amp's output being "racked" into opposite ends of the signal wave, if that makes any sense (something to do with the voltage swing on the tubes over-taking the voltage swing available for the buffer, I think). In any event, I eventually realized that the amp wasn't able to operate in that regime and it was pointless in attempting to test it at that.

[Pars]

Doug and Tom,

Thanks, I'll play around with it some more and see what I can find. I did already turn the volume down on the amp, but maybe not enough.

[luvdunhill]

AFAIK, this was a practice started by AMB.

<pedant>well, not really, I would assume Stereophile might be the originator of this concept for "amplifiers"</pedant>

[spritzer]

KG tests his amps like that and I believe HFN tests all amps under review flat out. Didn't Marc use this amp flat out so why test it any other way?

[TomB]

KG tests his amps like that and I believe HFN tests all amps under review flat out. Didn't Marc use this amp flat out so why test it any other way?

"Flat out" assumes a lot - that gain is within the controllable testing parameters, for instance. With tubes, you could have gains well into the 20's, 30's, and higher that may have no realistic operating points at the maximum volume position.

[spritzer]

Fair enough but that is clearly not the case here.

[Pars]

The only time in the past that I have touched the volume control on an amp under test is if I can't get the input level low enough for RMAA. It wants to see as close to -1dB IIRC as possible. I don't know what the gain is on this amp, but it doesn't seem particularly high, at least from listening.