6C33C-B tubes

I have 35w p/p Class A monoblocks which use two 6C33C-B tubes per unit. The manufacturer recommends a bias setting of 150mv. Before biasing (once a month), the tubes' measurements rarely deviate more than 10mv over/under the 150mv. Over the last year, I’m guessing that they’ve been in use for about 1000 hours. I have no idea what their expected lifespan might be. So far, biasing is still relatively easy. The manufacturer is no longer around to answer any questions. So, here’s a few for the Audiogon community.

1) For those of you using 6C33C-B tubes, are you able to guesstimate their lifespan in your amp – in Class A operation?
2) Generally speaking, how necessary is it to bias exactly to the manufacturer’s recommendation – in this case 150mv? Is there an acceptable tolerance +/- %?

Thanks! P.S. I’ve been replacing the sockets once a year.
The pair of 6C33C-B tubes providing 75 wpc in my BAT VK75SE are biased 200ma with 210V plate voltage. They begin to degrade sonically at 1200-1500 hrs. I have not needed to replace a tube socket in 10K hours of service.
Agree with Dave's statements above. I once asked Victor about needing to replace a 6C33C-B tube socket and he said it is quite rare, if ever, in his experience.
why do you need to replace 6C33C socket? in the case of BAT, it's a difficult job so not likely owner is expected to replace socket once a year.
Re: sockets - When a few years old, my amps had great difficulty maintaining proper bias. It wandered all over the place. When I peeked inside, the spring clips were charred and had lost all tension on the tubes' pins. I tried cleaning them up but they were too far gone. Replacing the sockets wasn't so difficult - just patience and a steady soldering iron. Ever since replacing them, the bias has been steady and the amps have been very reliable.

I'm still interested in hearing about tube life expectancy.
A bump.

Besides replacing and comparing the sound of old 6C33C tubes to new ones, is there a way to electronically test them - to see if their values still measure well? I've got a pile of used ones. They still bias okay but have over 1200 hours on them. I don't know whether any of them are still good or to toss them all.
Testing 6C33C at home isn't a trivial task. They are power hungry and could be very unstable due to heat fluctuations. The only market tester capable of is the expensive Amplitrex (but the current will be still limited).

My advice is to get NOS early 60xx Svetlana - there some sources here in Russia that sell them for about $20/piece. Svetlana-made oldies were made using quality materials at St-Petersburg and less prone to 6C33C "unstability".
As for the life expectancy - it depends on stress for 6C33C in amplifier. According to your amp it's similar to my BAT so I would guess it'll be about 2000-2500 hours. I'm pretty sure that $40 for a NOS pair isn't a big issue...

There is also 6C18C - a prototype for 33 tube - but it has more mellow sounding and not recommended although it's older than 6C33C.
You can easily take off about 10% of the manufacturer's recommended bias without a problem. Your tubes will last longer and sound ever so slightly warmer. If you raise the bias, you risk putting too much power through the circuit than it is prepared to handle. The tubes themselves shouldn't mind a higher bias as the bias in the Almarro 318B is 200 with a suggestion in the manual to lower the bias to 180 if you wish to extend the tube life.

BTW, I compared some 6C18C tubes to the 6C33C and they sound very close. I think I may prefer the 6C18C as they seem to have a touch more bass and midrange weight although equally extended. I would not call them mellow sounding compared to the 6C33C. In fact, the 6C33C is not a real warm tube, and a slight increase in warmth is probably a good thing, unless you are trying to avoid that. Again, to keep things in perspective, the 6C33C and 6C18C sound almost identical.
If you do not have access to a tester for 6C33C tubes like most people, one way to test the 6C33C tubes is to put a new pair in your amp and see where the bias is. New tubes will usually not vary much more than 10% or so between each other. As the tubes age, the bias goes up, eventually reaching a point where you cannot bias the tubes down to the manufacturer's recommendations, and they should be tossed. So, you can adjust the bias of the new tubes in your amp, then to test the old, put them in without changing the bias and look at the readings, using a little interpolation to get an idea of how old your tubes are.

Before the tubes go bad, they should start to loose the highs and the tubes may start sounding a little dull or may start distorting before that. I had some older 6C33C tubes which did not require much of a bias change over the last few years, yet now sound a little dull compared to new tubes. No comparison of the sound between old and new tubes is fair until the newer tubes have broken in ideally at least 200 hours or so, although the bulk of break in will occur before that. Then the tubes gradually start to mellow and warm over time as they age. From experience with my system, the tubes sound better at first as they age, then when they get too old, as mentioned, they start to sound a little dull.
Try to find someone, a tube seller etc., in your area with an Amplitrex that will test for a fee. Check tube suppliers, Upscale Audio in LA area offered this service at one time. My experience with 6C33C-B has been they won't hold their bias long once they start degrading.
Going to have to eat my words on this one. Earlier I stated I thought the 6C18C to be warmer than the 6C33C. After pulling out two new pair of tubes and doing a serious A-B, I found the 6C33C to be slightly warmer and the 6C18C tubes to have a splash more treble. Both sound beautiful. The best is where you wish to move the sound of your system. For more treble and a little more upper midrange presence, the 6C18C would be the best, and for a slightly warmer midrange, the 6C33C is better. Both sound similar.
I'm having a batch of 6C33-B tubes tested by tctubes out of Saint Paul, MN. I'll let you guys know how it goes.
Can I ask where you are finding NOS 6C33C tubes? I'm getting an Atmasphere Novacron that uses 8 and would like to buy a few extra back-ups. :-)

(Dealer dislcaimer)
Same here. Where does one find NOS 6C33C tubes from a reliable source? Whenever I've purchased from across the way it was a nightmare. I've had to pay a premium to buy tested tubes from US Vendors.
I have about 8 NOS from the early 80's make me an offer


If you are ordering 6C33s off of ebay, ask if the tubes are in the original boxes. If not, do not buy.
I bought a batch of 8 from a seller here named 'audiomisfit'. They came in what appears to be original boxes. Short answer here is the tubes were fine. I paid $285 for 8 but his prices seem to fluctuate.

Read on for the longer answer…

I decided I wanted them tested and got various responses to that plan. Local dealer who sells amps using those tubes said don't bother testing them. Technician who is repairing an amp that uses that tube said don't bother. But I'm stubborn, new to this, curious, and wanted to control for tube quality so I went ahead with it.

Based on suggestions found in older threads here I looked into Upscale Audio, tubeaudiostore (RAM tubeworks) and TC Tubes. From what I can gather, the 6C33B is somewhat difficult to test so the prices I was quoted were higher than for other tubes.

I ended up testing them with TC Tubes ($10 each).
Upscale Audio said they don't test tubes they don't sell. Fair enough.
Tubeaudiostore (RAM) quoted me $200 to test 10 tubes and $5 each tube thereafter.

Turnaround with TC Tubes was 2 business days after receipt. For any of you who are interested in what kind of data you get out of such a test, I’ve posted all of the results below. Some of this is a bit over my head but I’m thankful for the summary and detail on each tube and I feel like this gave me piece of mind about my purchase and may allow me to place them in the amp more intelligently. Tyler was very responsive to my inquiries and I’d definitely use his services again.

https: //drive.google.com/file/d/0B4HpFMtCMjw7S0NEd0t5UEF2MWs/edit?usp=sharing

Hope the link works!
Hope this works

https: //drive.google.com/file/d/0B4HpFMtCMjw7S0NEd0t5UEF2MWs/edit?usp=sharing

nope !!!
I was able to download all the PDFs from the link Tortilladc provided earlier. If you want the results just shoot me a PM and I'll try and attach the pdfs in my reply.
link is still live and working. now it's time to read up enough to understand the results!
I bought 8 tubes from a guy in Russia in early 2012. They arrived in unopened original boxes. I sent them to Parts Connection for testing. As I recall they had a special setup for testing them since they sell the 6C33C tube.

My Quicksilver 50 Watt 6C33B Triode mono amps are biased at 65ma per Mike Sanders recommendation. The 6C33B tube is no longer made, so all of them are NOS.

Here’s some good information on the tube from Romy the Cat.
Check the New Sensor site and click tube specs/FAQ, there are some great info here. I would recommend buying from a person who gets their matched tubes from them. I have BAT tube amps and get matched 6C33B tubes from New Sensor, I re-match the tubes in the amps before using them, the closer they are matched the better they sound. With age they drift apart, so I normally rematch the pairs every 6-8 months.

Another aspect of tube life could be due to the age and quality of the capacitors, I've noticed that when I upgraded the electrolytic caps in my unit, tube life has increased.
Hi Btselect

How much did New Sensor charge you for the 6C33C-B tubes? A year or so ago I contacted them and they were really pricey per tube. A lot more than other reliable tube sellers like Upscale or even Parts Connexion.

Thanks and advise when you can.
"Another aspect of tube life could be due to the age and quality of the capacitors, I've noticed that when I upgraded the electrolytic caps in my unit, tube life has increased."

Anyone else have similar observations?

I got them from a small local dealer who got them from New Sensor, in the past whenever a group of us were in need of tubes we made a bulk purchase to get a lower price. I've haven't bought these tubes in a while, I have bought hundreds of these tubes in matched pairs, the price of these tubes back then was very cheap. I would check if Jim McShane sells these tubes, I believe he sells tubes from New Sensor, or anybody else who sells tubes from them should be able to help you. Like on ebay, the sellers there are selling a lot of tubes so they can buy tubes from New Sensor in bulk getting a much lower price, and passing the savings to the general public.
I used to have a lot of trouble with the tubes in my VK-75se. Since cleaning the pins and sockets with Deoxit and letting the amp warm up an hour before using it, I haven't had any problems.