redplating 1959 w larmar

[Roe]

sorry, I don't know. see randall aikens posts in this tread. you could always try running a 12ohm load or something in order to see whether you'll like the sound of the higher Z

[Kevin]

I use a 12ohm resistive load for my re-amp rig, with the amp set to 8ohms. I like the way it sounds. The low end does seem a little attenuated, which is good.

I am also using the 100V PT primary tap as my "0" and the 230V tap as my "120", which brings the plate voltage down to 465-470. I know this is risky and I probably wouldn't do it with an original PT, but it's a build, so what the heck... The heater voltage drops to around 6V.

The OT is an original C1998, and I was told that if running it into a resistive load, it would be safer to run an upward mismatch by a factor of around 1.25. I went a little higher, just because it was easier to source those resistors. I'm running four 47ohm, 100-watt, aluminum-clad resistors in parallel, and the aluminum load box still gets way too hot to touch.

[Kevin]

Took some measurements yesterday, while swapping around power tubes, if anybody is interested...

My plate voltage drops from 465 at idle (lower than it should be due to my mucked up primaries) to a little under 390 when slammed. I remember it dropping to under 400 even when I had the PT wired correctly (high 480s B+). The screens sag down to around 340. This is with a Marstran T2562.

[Roe]

thanks for the voltage readings kevin!

[justonwo]

Randall Aiken (and others) guided me through a mammoth troubleshooting procedure in 2006 when I completed my Super Lead build. You can follow the troubleshooting here, if interested. I even posted pictures of my oscilloscope during the process.

http://www.thegearpage.net/board/showth ... redplating" onclick="window.open(this.href);return false;

In the end, we agreed that the amp was simply pushing the tubes beyond what they were capable of handling. And, by the way, the worst red-plating does NOT occur at maximum volume. It occurs somewhere around noon on the volume knob for reasons that are best explained on Randall's site. Yes, you had to be in a dark room to see it, and you had to be slamming the tubes pretty hard.

I found that JJs were significantly more robust than Winged Cs when it came to maximum plate voltage, but ultimately I decided to drop my plate voltage to 450 VDC with a variac.

[Roe]

Randall Aiken (and others) guided me through a mammoth troubleshooting procedure in 2006 when I completed my Super Lead build. You can follow the troubleshooting here, if interested. I even posted pictures of my oscilloscope during the process.

http://www.thegearpage.net/board/showth ... redplating" onclick="window.open(this.href);return false;

In the end, we agreed that the amp was simply pushing the tubes beyond what they were capable of handling. And, by the way, the worst red-plating does NOT occur at maximum volume. It occurs somewhere around noon on the volume knob for reasons that are best explained on Randall's site. Yes, you had to be in a dark room to see it, and you had to be slamming the tubes pretty hard.

I found that JJs were significantly more robust than Winged Cs when it came to maximum plate voltage, but ultimately I decided to drop my plate voltage to 450 VDC with a variac.

thanks for the info and link. mine only had problem when running flat out. increasing the tail resistor to 27k get rid of the problem. also low nfb (100k 4ohms) makes the problem worse

a friend's superbass appears to have had the same problem.

[justonwo]

Oh, wow. That's interesting. What was the original value of the tail resistor? 10k? I should try that one of these days . . .

So what resistor are you using for the NFB? Are you running into the 8 ohm tap on the OT?

[Roe]

yes, I changed the tail resistor from 10k to 27k. I had to experiment a little to get the right value. also I experimented with the tail resistors (82k and 100k), ending up with a pair which reads 83k5 and 105k.


For NFB I use a 20k resistor in series with a 80k pot connected to the 4 ohm secondary. this gives everything from 20@4 (=28k@8) to 100k@4, thus everything from the 67 value to the early 70s specs

[justonwo]

Thanks for the information. Hopefully, I can put this issue to rest once and for all . . .

It IS strange that George has never seen problems with his builds. I have gone over all lead dress pitfalls pretty meticulously.

[Roe]

justonwo,

I have lower b+ than you. my amp has 473v or so. if I remember correctly your amp had 490 or something?

[justonwo]

Yep, I believe I had 490 VDC.

[Kevin]

I wonder if the fact that most speakers measure around 12-13 ohms exacerbates this problem. An OT designed to present the tubes with a 1.75k load at 16 ohms would be reflecting something more like a 1.3k load (when they really want something higher like 2.5k)...

Just a thought...

[Roe]

yeah, thats probably true. but the impedance varies a lot on different frequencies

[Roe]

update. I recorded the amp quickly with a new band at rehearsal:

http://snotpope.com/downloads/Superskur ... skurk1.mp3
http://snotpope.com/downloads/Superskur ... skurk2.mp3

you're hearing a korina jr into the 1959 and a SG into a ocd and a superbass

[FourT6and2]

update. I recorded the amp quickly with a new band at rehearsal:

http://snotpope.com/downloads/Superskur ... skurk1.mp3
http://snotpope.com/downloads/Superskur ... skurk2.mp3

you're hearing a korina jr into the 1959 and a SG into a ocd and a superbass

Sounds good!