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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 4th May 2016, 04:01 PM   #21
w5jag is offline w5jag  United States
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Location: Arkansas
I've fiddled with quite this a bit over the last few weeks, mostly with 6V6, since that is what I originally built it for.

I knew from playing around with another board and external supplies that I needed to get my B+ down around 250 volts or so for reasonable SMPE IMD numbers, which, unfortunately, is my only means of analysis at present. When all you have is a hammer, everything looks like a nail, so I'm stuck with this for awhile longer.

The only way to get there with my existing PT is a choke input supply. I pulled the first cap, kept the 5H choke, and left the existing cap which was a 400 volt 120 or 150 uF device. On 96dB speakers, I could hear a bit of hum within a few inches of the speakers, so I pulled the last cap and substituted a 1200 uF, 250 volt cap, in it's place. It is totally silent - you can't even tell the amp is on.

This got the B+ acceptably low, but presented another significant problem - there is a voltage floor, below which the front end just won't work without alteration. That floor is right around 245 volts, and, at these very low voltages, 12AT7's have to be hand selected. Most won't work correctly and show horrific distortion numbers - it takes a large pool to draw from to find an example that will show low distortion. At 255-260 volts, a few more will work correctly, and as voltage increases, more and more will work. Obviously, this is tedious and wildly impractical if one does not have a large quantity of tubes on hand.

There is a further problem - at these low B+ voltages, the 6AX5 voltage drop compounds the problem: I could not find a 12AT7 in my stock that would perform acceptably well in this configuration. A 6X5 has a lot less voltage drop, but, in theory, can't supply the needed current even with the current boost allowed by an LC supply.

With a 6X5 rectifier, and some gnarly old GE 6V6 that I hand selected, here are some numbers. This is in Ultra Linear with cathode feedback, which doesn't seem to make much difference one way or the other at these very low power levels:

Plate voltage: 249
Screen voltage: 252
cathode voltage: 10
cathode current 56 ma
Power dissipation: 13 watta
Rk 180 ohms

IMD at 1 watt was 30%; at 0.5 watt 10%; at 0.10 watt 3%; and 50 milliwatts was 2.2 %.

SE Amp cad predicted 3.3% 2nd harmonic and 1.1% third harmonic at this point, as a triode amplifier.

The 6X5 seems to have a bad reputation and running one at twice the permitted current would look to be an invitation to disaster, but, it didn't give me any trouble. I used a National 6X5 of all things, that they had thoughtfully mis marked as a 6AX5, that had a big bulb and heavy ceramic base, similar to a 5852, and it shrugged off this insult and worked fine for several weeks, with long hours of daily use. Just looking at it, you can't even tell it's been used.

Win W5JAG
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Old 4th May 2016, 04:35 PM   #22
w5jag is offline w5jag  United States
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After blowing some diodes to smithereens, smoking a meter, and melting a test lead making a voltage quadrupler from my free 5 volt secondary for a fixed bias supply, I dismissed that idea temporarily ( maybe permanently ) and decided to use it for it's intended purpose: a rectifier supply.

For those interested in converting the SSE socket to 6X5/6AX5 it's easy:

jumper pins 3 and 4;
jumper pins 5 and 6;
6.3 volt filament power goes to pins 2 and 7.

Conversion back to traditional 5 volt rectifers, gave me a bit more flexibility in establishing a range of B+ voltages. Along the way, I also found, maybe, a solution to the 12AT7 problem at these very low B+ voltages. That solution may be the 12AU7. For whatever reason, 12AU7, with no need to hand select, seem to work reliably at voltages down to 225 volts, with no significant distortion penalty.

As an example, here are some numbers from a combination of a 5W4 rectifier, some RCA 6V6GT, and an awesome looking triple mica, two support rod, black plate, RCA 12AU7, still in UL with CFB:

Plate voltage: 225
Screen voltage: 228
cathode voltage: 9
cathode current 50 ma
Power dissipation: 10.75 watts
Rk 180 ohms

IMD at 1 watt was 38%; at 0.5 watt 13%; at 0.10 watt 3%; and 50 milliwatts was 2.5 %.

The 12AU7 presents its own unique problems. The gain is considerably less. This could be a problem in some applications. It is not in mine. There may be other distortion issues with it - I plan on expanding to a pc type of analysis but that is presently thwarted by a dead pc.

However, it sounds quite good. The amp is eerily quiet - signals just jump out from a black background of silence. More improvement may be available, but it clearly sounds better than it's original build. I'm weighing replacing the OPT's. I think with a higher load, I could have done a lot better at the higher voltages. I'm leaning to the Hammond 125CSE since I've been well satisfied with the 125ESE, but that is still up in the air, and not going to happen soon, if at all.

Along the way, I also changed the Auricap coupling caps to Orange Drops, for no particular reason other than to try them. The distortion readngs in this and the previous post were made with the orange drops.
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Old 4th May 2016, 05:02 PM   #23
w5jag is offline w5jag  United States
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Join Date: Feb 2007
Location: Arkansas
Here are a few pics of the latest iteration, dressed out in righteous cold war tube glory.

Interestingly, the 5992 seem to have higher distortion than other 6V6 types. I remember when I was playing with 6889, they had a very apparent audible distortion drop after a half hour or so. I haven't really had the time to let the 5992's just sit on the meter for an hour to see if they have the same characteristic.

Also interestingly, while the 6106 and a 5Y3WGTB/6087 rectifier put out voltage within a few volts of each other, the 6106 actually being a few volts less, 12AT7 tubes that would not show an acceptable distortion characteristic with the 6087, would show acceptable distortion characteristics with the 6106. So, maybe there is an audible difference among rectifier tubes apart from their change in operating points, and people that say they can hear differences with different rectifier tubes may in fact be able to do so. I didn't try, and doubt that I could.

Also are some pics of the extensive packaging that the RCA 12AU7 came in. I have quite a lot of these tubes. You may not be able to see from the pic, but the pins were snaggle toothed, and I am sure it had never been out of the packaging - removing one destroys it.

Win W5AJG
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Old 4th May 2016, 05:40 PM   #24
spendorite is offline spendorite  Canada
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Quote:
Originally Posted by w5jag View Post

This got the B+ acceptably low, but presented another significant problem - there is a voltage floor, below which the front end just won't work without alteration. That floor is right around 245 volts, and, at these very low voltages, 12AT7's have to be hand selected. Most won't work correctly and show horrific distortion numbers - it takes a large pool to draw from to find an example that will show low distortion.

Win W5JAG
Hi W5JAG, Did you try a 12AU7 ?

For a while last year using my low voltage SSE for 6V6 and other tubes, I swapped out the power transformer
and used another which gives suitable B+ voltages for 6Y6, 6W6 and some others. Depending on the rectifier used,
solid state, 5Y3, 6087, 5Z4 , 5W4 , 5V4 I could get B+ voltages ranging from around 170V to around 265V.
I found that with the 12AT7, of which I have several different brands, the amplifier sounded terrible below about
250V regardless of which one I tried.

I decided to try a 12AU7 of which I also have several brands. Using a 6087 rectifier and 6Y6 power tube I got a B+
of 195V if memory serves me right. The SSE sounded quite good with the 12AU7, not as good as when using the 12AT7
at higher voltages but still very acceptable.

I don't have the equipment to measure distortion so I can't offer any numbers.
I know the SSE wasn't designed for use with the 12AU7 but it worked okay at these lower voltages.
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Old 4th May 2016, 06:00 PM   #25
w5jag is offline w5jag  United States
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Quote:
Originally Posted by spendorite View Post
Hi W5JAG, Did you try a 12AU7 ?

For a while last year using my low voltage SSE for 6V6 and other tubes, I swapped out the power transformer
and used another which gives suitable B+ voltages for 6Y6, 6W6 and some others. Depending on the rectifier used,
solid state, 5Y3, 6087, 5Z4 , 5W4 , 5V4 I could get B+ voltages ranging from around 170V to around 265V.
I found that with the 12AT7, of which I have several different brands, the amplifier sounded terrible below about
250V regardless of which one I tried.

I decided to try a 12AU7 of which I also have several brands. Using a 6087 rectifier and 6Y6 power tube I got a B+
of 195V if memory serves me right. The SSE sounded quite good with the 12AU7, not as good as when using the 12AT7
at higher voltages but still very acceptable.

I don't have the equipment to measure distortion so I can't offer any numbers.
I know the SSE wasn't designed for use with the 12AU7 but it worked okay at these lower voltages.
Yes, see post #22 above - 5814 is for our purposes, a 12AU7.

Your report is exactly what I would expect based on my tinkering.

In the near future, I will take another look at 6Y6, 6W6, and probably 6U6 and get some numbers on those.

Do you have any 6K6? They work well at these low voltages, and sound different being a true pentode. With a B+ of 225, and a 5814A tube, I got 5% at 1/10 watt, and 3.9% at 1/20th.

Win W5JAG

Last edited by w5jag; 4th May 2016 at 06:06 PM. Reason: clarified numbers were with 12AU7 front end
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Old 4th May 2016, 07:02 PM   #26
spendorite is offline spendorite  Canada
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Quote:
Originally Posted by w5jag View Post
Yes, see post #22 above - 5814 is for our purposes, a 12AU7.
Oh yes, that post must have come up while I was getting ready to do my post..


Quote:
Originally Posted by w5jag View Post
Do you have any 6K6? They work well at these low voltages, and sound different being a true pentode. With a B+ of 225, and a 5814A tube, I got 5% at 1/10 watt, and 3.9% at 1/20th.

Win W5JAG
I have 6K6G tubes but I use them mostly with the SSE set up for 6V6 tubes.
Using a 6087 rectifier and 680ohm cathode bias resistor, plate is 284V and screen is 286V which
is right at the maximum for this tube but they do sound very nice though their lifespan may be shorter.

In future I'll rebuild my chassis to accommodate both transformers and switch between the two when using the lower voltage tubes.
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Old 10th May 2016, 03:36 PM   #27
w5jag is offline w5jag  United States
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Spendorite, I played with the 6Y6 and 6W6 last night, but left my notebook at the office, so I didn't get anything written down.

I had them around 190 volts and at about 80 mils ( 12AU7 front end ), so they were over their dissipation rating, but they performed well. They matched 6V6 at lower powers and significantly bested 6V6 at one half watt and beyond, never going above 9% up to one watt. That was in UL, with CFB.

I'm just not seeing the type of output wattages I expected with some of these smaller tubes, even when I had them at the higher voltages, and regardless of the type of front end tube in use. It's not a big deal as they have no trouble driving my speakers, but I'm just not sure what's up with this.

Looking back, I have to say that the 6146B in UL looks to be the real deal here, but it was hardly a low power amp, which was the whole point to start with. Maybe different output transformers will help.

Win W5JAG
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Old 11th May 2016, 12:18 PM   #28
spendorite is offline spendorite  Canada
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Quote:
Originally Posted by w5jag View Post
Spendorite, I played with the 6Y6 and 6W6 last night, but left my notebook at the office, so I didn't get anything written down.

I had them around 190 volts and at about 80 mils ( 12AU7 front end ), so they were over their dissipation rating, but they performed well. They matched 6V6 at lower powers and significantly bested 6V6 at one half watt and beyond, never going above 9% up to one watt. That was in UL, with CFB.
Thanks for the info W5JAG


Quote:
Originally Posted by w5jag View Post
I'm just not seeing the type of output wattages I expected with some of these smaller tubes, even when I had them at the higher voltages, and regardless of the type of front end tube in use. It's not a big deal as they have no trouble driving my speakers, but I'm just not sure what's up with this.
Yes, I find the same thing and I'm surprised it is the case with the 6Y6 tube given
its rated maximum dissapation of 12.5 watts.
In addition my speakers are only 88db efficient but presents a very tube friendly load.
I don't listen at very loud levels so for small jazz combos, solo voice, smale scale
classical and some other music it gets loud enough for me.

For my lower voltage SSE for 6V6 and other tubes I use an Edcor XPWR009 - 550V(275-0-275)@175mA
and for 6Y6 and others I use another Edcor XPWR186 - 370V(185-0-185)@200mA, the opts are
Edcor GXSE15-8-5K.

Using the 6Y6 and 12AU7 with a 5Z4G rectifier and 440 ohm cathode bias I get
plate = 185V, screen = 188V, Vk = 20V, Ik = 46Ma. giving a dissapation of 7.6W which is
on the safe side.
Sometimes I run them hot. I switch over to SS rectification and using 360 ohm cathode bias
I get plate = 208V, screen = 212V, Vk = 21.6V, Ik = 60Ma giving a dissapation of 11.2W.
I both cases they play very well with my speakers at the levels I listen particularly when I
run them hot.
I use UL mode with CFB.
I like running the 6Y6, it is one of my favourite lower voltage tube along with 6V6, 6K6 and 6EZ5.

The 6W6 because of the much lower rated screen voltage I don't run very often, when I do
I ususlly go for a plate voltage around 155V and screen 157V which is still higher than rated.




Quote:
Originally Posted by w5jag View Post
Looking back, I have to say that the 6146B in UL looks to be the real deal here, but it was hardly a low power amp, which was the whole point to start with. Maybe different output transformers will help.

Win W5JAG
I don't have any 6146 tubes tubes and with prices relatively high on EBay or even at online suppliers
plus required mods on the board I doubt I'll go for these.
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Old 15th June 2016, 11:52 PM   #29
w5jag is offline w5jag  United States
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12BH7 looks to be another viable alternative for low power SSE's running below 250 volts.

Tube CAD shows ( like 12AU7 ) that some component values could be changed for better optimization.

Regardless, just dropping it in the socket, and running it through my IMD analyzer without any changes, it shows significantly less distortion than 12AU7. I haven't had the opportunity to do much testing at all, but my gut is that the 12BH7 ( like 12AU7 ) is a very satisfactory replacement for the 12AT7 in these low voltage SSE's, provided that the lower overall gain is not problematic.

The unexpected arrival of another R-174/URR on the bench has had me playing radio, rather than audio, the last few weeks.

Win W5JAG
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Old 18th June 2016, 11:38 PM   #30
phase is offline phase  United States
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My Dad managed to keep both myself and the Scott tube amp intact long enough so I could enjoy it later when he gave it to me to use. This was accomplished by having the gear up high, where it wasn't accessable to the shorter ones.
It looks like even with the perforated case that it would still be possible to spill liquids down into the amp inside. Don't mean to be a downer, but it's a safety thing you know...
Congrats on the recent family members!
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