Cornet2 tube and voltage questions

WGH · « **on:** 26 May 2007, 11:55 pm »

I successfully completed a Cornet2 a week ago and tube rolling is interesting - I have some holographic imaging going on over here. The unit is completely quiet and seems to work perfectly, but as Alexander Pope said in 1709, "A little learning is a dangerous thing". I had to go check voltages and this is what I found:
B+ R107 350v
H+ R223 7.21v and 5.56 on the other side
R219 145.6v, 145.7v
R212 144.4v, 144.3v
R211 320v, 219v
R204 142.5, 143.4
Heater 5.57v measured at center tube hole
Line voltage is 115v - 117v AC.

The tubes (except the 5V4GA) are from Jim McShane so they should be good.I started a with an RCA 5Y3GT, but voltages were even lower. A GZ34/5AR4 can be used to bring up voltages, didn't have one of those but did notice a person used a 5V4GA and didn't blow it up, so tried one and it seems to work, voltages are as stated above. As a bonus the highs and lows are much better defined too.

Tubes:
RCA 5V4GA - tight bass, extended highs
Raytheon USA 12AU7 square getter 90-84 - very nice and precise
Sovtek 12AX7LPS

The capacitors are Sonicap Gen1's (200, 202, 204, 206), Sonicap Platinum (C203), and Mundorf SIO (C208). (These caps are why I got the FryKleaner and iRIAA)

So voltages seem OK (sort of) except the heater. Should the voltage dropping resistor be changed? What would be a good starting value to try out? And what do you think about the 5V4GA?

Thanks again.

hagtech · « **Reply #1 on:** 27 May 2007, 03:16 am »

Sure, you can try adjusting the heater resistor. Just start adding stuff in parallel. Like a 10 ohm 1W. Voltages are for 120V nominal.

jh

WGH · « **Reply #2 on:** 27 May 2007, 09:54 pm »

Thanks Jim, 10 ohm resistors brought the heater voltage to 6.1v, though it did take 4, which changed the filter resistor value to 1.0465 using a parallel resistor calculator.

The construction of the Cornet2 went very smoothly, just follow the silkscreen and you can't go wrong. But I did have one observation, the positive lead of electrolytic capacitors goes in the square hole, I thought diodes have a positive (anode) and negative (cathode) leads, but the cathode lead (with the stripe) goes into the square hole. The silkscreen and construction photo make the orientation very clear, but it is slightly counter intuitive.

Thanks to ScottRT for discovering the Radio Shack switch #275-625 fits perfectly, Amandarae and samplesj for the tweaks and the photos to use as a guide, and Dan for the free lens.

Some photos just for fun:

Wayne

amandarae · « **Reply #3 on:** 1 Jun 2007, 05:03 am »

Very nice!!!

Glad you are enjoying your preamp. I built mine many many moons ago. No complaints, works like a charm. Heater voltage on mine was 6.12V . One of these days, when I have some free time, I will try to bump it up to 6.3V.

regards,

Abe

amandarae · « **Reply #4 on:** 2 Jun 2007, 12:09 am »

I open up my Cornet today. Measured the heater voltage and confirmed 6.12V. From there, I calculated the current across the ceramic resistor and come up with 1.5 ohm resistor to give me approximately 6.3V for the heaters.

I use a 10W 1.5 ohm (Eagle Brand) resistor. After installation, the voltage measurements were as follows:

One side of the resistor(between R223 and C209) with respect to ground = 6.29V
Heater pins (4,5) for the 12AX's and 12AU = 6.28 Volts (6.27V after 20 minutes)

Results: In my opinion base on my system, I think there is a substantial improvement. Maybe because the bump up heater voltage (from 6.12V to 6.27V) at 150 mV difference made the tubes operate better. I do not know why but if people reading this who owns the Cornet2 have some time to kill, check your heater voltages and verify it is close to 6.3V.

I had no complaints with my unit prior to this step. In fact, it sounds very good to me! After many many months of using it, I never imagined that it will still improve. I am very happy with the results. I am not sure if this will work for others, just posting my experience about it.

Many thanks to WGH for initiating this thread and Jim for making the suggestion how to experiment on what to do. Simple to check, simple tweaking, simple solution. But in my system, it brought a very very positive result.

regards,

Abe

WGH · « **Reply #5 on:** 2 Jun 2007, 02:05 am »

These Cornet's sure are fun to poke around in, and they respond so positively to a little tickling. I am still learning, the last time I had electrical courses was back in 1969, what Jim does looks like magic to me.

From reading the old posts, I think a lot of people are using their Cornets with low heater voltages. One thing to keep in mind is the in-coming line voltage. Since summer is coming (101° F in Tucson tomorrow) setting the H+ voltage on a low voltage afternoon will result in too high heater voltages late at night. I recommend keeping a log of your house voltage to see how close it stays to 120V and adjust accordingly. Heater voltages over 6.3 is not a good thing.

I came across an interesting little note about the 5Y3 while researching tubes:
http://en.wikipedia.org/wiki/5Y3
"Currently, a plug-in replacement is being manufactured by Sovtek in Russia, which is has very similar specifications, but is indirectly-heated, and can support currents up to 144 mA, versus the 120 mA of the original. However the Sovtek 5Y3 is not a true 5Y3. It drops less voltage than vintage 5Y3's, and may cause voltages to run too high."
I believe JH designed the Cornet2 using a Sovtek 5Y3, so plugging in a NOS 5Y3 will result in slightly lower voltages. A GZ34/5AR4, GZ32, or 5V4GA will bring them up. Another reason why tube rolling can fine tune the Cornet to fit your listening preferences.

Wayne H.

robertwstephens · « **Reply #6 on:** 2 Jun 2007, 04:36 am »

Dang it Abe! I am in the middle of a high efficiency speaker building project and now you have me wanting to waste an afternoon playing with my Cornet. I am still waiting for your report on the Piccolo with tweaks before I order up. Thanks for keeping this fun! Peace. Robert

Relaxin' - Miles Davis Quintet (200g reissue)

amandarae · « **Reply #7 on:** 2 Jun 2007, 07:14 am »

Quote

One thing to keep in mind is the in-coming line voltage. Since summer is coming (101° F in Tucson tomorrow) setting the H+ voltage on a low voltage afternoon will result in too high heater voltages late at night. I recommend keeping a log of your house voltage to see how close it stays to 120V and adjust accordingly. Heater voltages over 6.3 is not a good thing.

Hello Wayne,

Very good point! After reading your post, I did a little observation experiment. I waited until its close too midnight here and see the results when the AC is at its peak where I am at. I am a little surprise of the results being lower compared to the previous measurements I had. But I think that I will go with this data instead.

Here's the resistor pic

Here's the measured AC at the wall

Here's the voltage reading just before the resistor

After the resistor...

the voltage at the last 12AX7 (front)

In my opinion, it will take a considerable swing in the AC supply to produce greater than 6.3 Vdc in my unit. Looking at the very first pic, you will notice that the resistor is 1.5 ohm 5% (non inductive type). But it really measures at ~1.7 ohms. The original value was 1.8 ohms 5% (the ceramic one).

Quote

Dang it Abe! I am in the middle of a high efficiency speaker building project and now you have me wanting to waste an afternoon playing with my Cornet. I am still waiting for your report on the Piccolo with tweaks before I order up. Thanks for keeping this fun! Peace. Robert

Hello Robert,

Unfortunately, I do not think I will have my Piccolo ready this month. I have not started building it. I asked the question on the other post because I am reading the manual in preparation. Like you, I have some projects now in the making.

I am convinced that I will build my unit stock, with only one exception. The exception is pictured below....

I will have Jim's Bugle power supply side by side (with substantial space) to the Piccolo powering it. I figure that this config will serve me better in the long run.

Regards and goodluck on your ongoing project.

Abe

Eric H · « **Reply #8 on:** 2 Jun 2007, 11:15 pm »

OK, I couldn't stand the curiousity any more so I opened up my Cornet2 to check my heater voltages. With 119.9 VAC and a GZ34 I was at 5.97V. I happened to have a single 10 ohm 5W resistor so I put it in parallel and that got me up to 6.2V. Can't say if I can hear a difference in sound yet - just started listening.

I also checked my Clarinet w/GZ34 and it was already at 6.2V so I didn't mess with it...

robertwstephens · « **Reply #9 on:** 8 Jun 2007, 11:46 pm »

Hello! I checked my Cornet 2 and my heater voltage was 5.3 volts. I bumped it up to 6.3 volts by changing resistors and adding some. If I change rectifiers, do I need to check this again? I have a NOS RCA I got from McShane. I am not sure if the rectifier affects the voltage for the heaters or not. The sound did change a little I think. I rolled a couple of tubes to get things where I like it. Thanks for keeping this fun! Peace. Robert

machine · « **Reply #10 on:** 9 Jun 2007, 01:51 am »

robertwstephens:
My voltages are around what you had - what did you exactly change?

Anyone:
Just to verify - you have to ADD resistance in parallel to the ceramic resistor to increase the voltages?

And what is the calculation to find out where you should be?

Thanks

robertwstephens · « **Reply #11 on:** 9 Jun 2007, 03:24 am »

Hello! I took Abe's lead and put in a 1.5 ohm resistor. That wasn't even close. I had to add 4 10 ohm resistors to get to 6.3v. If I even had half of a clue as to what I was doing I could have calculated exactly how to do it with 1 resistor maybe. This is all fun and games for me! Peace. Robert

Eric H · « **Reply #12 on:** 9 Jun 2007, 06:09 am »

A week after getting my heater voltage up from 5.97V to 6.2V I'm sure it made a significant improvent. Here's an outline of the "math":

1) With your Cornet2 on (careful!) and tubes in measure the voltage on both sides of R223, the 1.8 Ohm heater voltage dropping resistor.

2) Use Ohm's law (I=V/R) to calculate the current through R223, where V is the difference between your voltages in 1) and R is 1.8Ohms. Should be about 0.9A. For me it was 1.62V/1.8Ohms= 0.9A.

3) Use Ohm's law again (R=V/I) to calculate the desired resistance for R223. Here I is the current you measured in 2) and V is the difference between the higher voltage side of R223 that you measured in 1) and the desired voltage (6.3V). This is your desired resistance for R223.

At this point you can just sub in a 5W resistor of the value you got in 3)... which should be something less than 1.8Ohms. For me, with a heater voltage of 5.97V, it was 1.43Ohms. It's hard to source this resistor, though, so it's probably easier to piggyback another resistor in parallel with the 1.8Ohm R223, as Jim suggests. The formula for this resistor value is

1/R = 1/x + 1/1.8

where R is the desired resistance from 3) and x is the value for the additional resistor. This can probably be a 1W resistor as Jim suggests.

Hope that helps... somebody should probably proofread it...

amandarae · « **Reply #13 on:** 9 Jun 2007, 10:09 am »

Quote

A week after getting my heater voltage up from 5.97V to 6.2V I'm sure it made a significant improvent.

Yep, me too! I was surprised at the results. The ~110 mV increase (I had 6.12 V before) really made a difference to me.

To add to the excellent post Eric had above, here is another method I will calculate the value of the resistor(X) paralleled to the 1.8ohms ceramic resistor to produce 6.3 volts(you can choose 6.25 or 6.27 to be safe, its up to you)

(V1-V2)/1.8 = I

R= (V1-6.3)/I

X = 1.8R/(1.8-R)

Where....

V1-V2 is the voltage difference across the 1.8 ohms resistor; V1 as the greater value between the two

I is the current from the ceramic 1.8 ohm resistor

R is the resistance value to adjust the heater to the desired voltage (6.3V in this case).

X is the resistor value in parallel with 1.8 ohms to give the desired value of R

Note:

A.)This assumes that the ceramic resistor is 1.8 ohms absolute value to make the calculation easier.
B.) In my case, where V1=7.7 and V2 chosen at 6.25V, R=1.7 Ohms approximate. Since I found a resistor close in value to this, I just pulled the ceramic and replaced it so as not to perform the parallel calculations.

cheers...

WGH · « **Reply #14 on:** 9 Jun 2007, 04:15 pm »

I used a parallel resistor calculator to find out what values of resistors would work best to reach the adjusted R, there are many to choose from, here is one of the easiest to use:
http://www.1728.com/resistrs.htm

Many resistors measure slightly different than their values, I have a few 1 ohm that measure 1.3 ohms, so getting exactly 6.3 V will take a bit of luck too.

Wayne

machine · « **Reply #15 on:** 9 Jun 2007, 05:53 pm »

WGH - Awesome!

Braden · « **Reply #16 on:** 19 Sep 2007, 12:08 am »

Ok, I'm sort of puzzled on this one. I decided to check my resistor voltages (R21 on the original Cornet) and they read just over and just under 11Volts. So I started poking around and found that almost everything read around 11Volts. The chassis itself reads around 11Volts. Any ideas why this might be? Also, my R21 is a Mills 2.5Ohm. It's been 4 years since I've built this thing so I have no idea why I used that sized resistor instead of the listed 1.8Ohm.

[insert puzzled emoticons here]

Braden

samplesj · « **Reply #17 on:** 19 Sep 2007, 12:44 am »

Quote from: Braden on 19 Sep 2007, 12:08 am

The chassis itself reads around 11Volts.

When you touch both red and black leads to the chassis you get 11 volts DC?

Maybe you just not getting a solid chassis ground for the black lead?

Braden · « **Reply #18 on:** 19 Sep 2007, 12:59 am »

Quote from: samplesj on 19 Sep 2007, 12:44 am

Quote from: Braden on 19 Sep 2007, 12:08 am
The chassis itself reads around 11Volts.
When you touch both red and black leads to the chassis you get 11 volts DC?

Maybe you just not getting a solid chassis ground for the black lead?

Did I also mention that I'm running on a few hours sleep. Woops! That should have been a total giveway, getting 11 volts AC like that. I didn't have a proper ground for the board. Now when I check it I'm getting 8.4 volts on one side of the resistor, and 6.2 on the other.

ecir38 · « **Reply #19 on:** 30 Oct 2007, 04:56 pm »

I installed these terminal blocks
http://www.mouser.com/search/ProductDetail.aspx?R=571-214376679
where R223 (cornet2) and R313 (clarinet)go to make the resistors easy to replace when tube rolling the rectifier.

Note that the t-blocks are 4 position and that I cut them up to make two single position t-blocks. A three position one wound have worked also. The only other thing I had to do was to trim a little off the pin that penetrates the pcb being it was a litlle bit bigger than it needed to be.