crap components

[scole]

in building my champ i found that the sovtek 5y3 isnt really a 5y3 at all
http://en.wikipedia.org/wiki/5Y3, now even wikipedia knows. why doesnt someone tell sovtek? im getting tired of all the junk thats out there.

[chipaudette]

In the same vein, I came across the webpage below a few days ago. It talks about how well (or not well) new-build pre-amp tubes (12AX7) meet the specs.

http://www.guitaramplifierblueprinting.com/12ax7.html

This page has tons of text discussing the unpredictableness of the performance of new tubes. TONS OF TEXT. It goes on an on. But I could care less about such blabbering. I care about numbers.

About half-way down the very long page (hit CTRL-F and search for "Preamp tube characteristics") he starts making tables of results that he's measured for BATCH after batch of tubes. Then, over time, he repeats the tests with new BATCHES of tubes. He's done a lot of testing. He's even tested a ton of NOS tubes for comparison.

The conclusions that I take from all his data are:

1) New tubes have lower gain than spec (esp the 2002 tubes). They're running as low as 72% under. That's 3 dB...which is a lot if you're trying to closely manage the overdrive properties of your pre-amp!

2) More importantly, the tolerance (ie, the spread in values in the batch of tubes) is HUGE! In that 2002 set of tests, he shows variation on the order of 25-60% Wow! So even within a brand in the sam eyear, the quality control (or lack thereof) is such that tremendous variation in tube performance from one tube to the next. Amazing!

So, I guess the net result is...you gotta buy and try lots of tubes to find the good ones.

Chip

[Sir Cuitous]

I am a new builder. My first amp was a Champ kit which I built by following instructions. Since then I have studied and studied while saving money and gathering parts for another amp.

I checked the bias on this first amp to find that the 6V6 was drawing too much current. I was able to increase the bias resistor to get it into the correct range, but since then I have wondered why that was necessary.

I just came back from my garage a minute ago where I finally measured my B+ voltages (I know, DUH). 615vac from the tranny, but 370vdc on the first node and 320vdc at the third node!? I was expecting somewhere around 320 at the first. The rectifier is a Sovtek 5Y3.

Should I be surprised by this? Who can recommend a better 5Y3 tube?

[scole]

sir, there's still plenty nos rca 5y3's out there pretty cheap too. try kca tubes.

[stokes]

Thats exactly why I never use any current production tubes.Nothing but NOS in all of my amps.I've got 2 different Ampegs with Bugle boy preamp tubes that have been in the amps since the 1960's and they are still strong.And these amps got a lot of use,back in "72 I put a bass speaker in one of them and my bass player used the amp till 1980.Nowadays my sons bass player still uses this amp,with the same preamp tubes.You will never get that kind of performance from any of the crap made today.

[Sir Cuitous]

So I got an RCA NOS 5Y3GT from work. The PT is a Mercury FTCP-59 (325-0-325). I am now getting 350v at B+1 and 305v at B+3. These voltages are still significantly higher than I would expect. I'm mystified. Does anyone have experience with other 5Y3s? What kind of voltage drops can I expect?

[MWJB]

Sounds ball park to me, for 325-0-325. Not sure why you particularly want less than 350v at B+ 1.

A rule of thumb is that, just as a SS rectifier gives roughly 1.4*AC secondary voltage, a NOS 5Y3 gives 1.1*sec AC, 5U4 = 1.2*sec AC, GZ34 = 1.3*sec AC. 325VAC*1.1 = 357.5vdc.

There will obviously be some variance due to tube brand and the effect of plate current pulling down the B+ supply.

If you want to pull down the B+ a little more try rebiasing the 6V6 to 40mA to 50mA.

[TD_Madden]

that merury mag is probably not wound to compensate for the higher wall-voltage we all have now compared to just 20 years ago.

Some folks (Bruce at Mission comes to mind) use a voltage-adjusted PT to allow for this. In the past (before he had the new xformers), a zener was included to drop the B+ a bit.

[MWJB]

If you look at the tolex Champ schematic, you'll see that they claim to have got 360vdc at B+ #1, with 320-0-320VAC. 5F1 schematic claims 340vdc at B+ #1 (no sec AC given). If period correct Fender voltages are what you are after, I'd say you were right on the money.

[Sir Cuitous]

The 6V6 is drawing 38ma right now over the cathode, but I could increase that. I was going by these schematics which are fairly common on the internet:

http://www.schematicheaven.com/fenderamps/champ_5e1_schem.pdf
http://www.schematicheaven.com/fenderamps/champ_5f1_schem.pdf

320 for the 5E1 and 340 for the 5F1 at the first node. So I guess I'm in the ball park.