1X6S33S=6X6S19P which one gives less impedance?

[kleist]

Hello, i am new member in this site. I want to build an OTL amplifier. I will use 8ohms speaker with it. I want t know, which one is gives me less impedance and more power? 1X6C33C tube or parallel connected 6X6S19P tubes? Which variant is gives better sound quality?
Thank you!
Andrey

[Alastair E]

Both the 6C33C-B and the 6C19 types have been used for OTL.

Unfortunately, the 6C33C-B has aquired a reputation for bias-drift. This is due in part to incorrect use, (Not understanding the valve's high perveyance and other perculiarities) and to no break-in done by the user, or running too much dissipation in the standing-state, often done in OTL and SE work to lower the O/P impedance, But there are Much better ways than running heavy currents through the O/P stage to reduce the O/P impedance/damping-factor in an OTL!.......

I prefer the 6C33C-B and providing that you keep dissipation to below 40W quiescent bias, and the current through it at or below 200mA, they are very stable providing that they are 'broken-in' properly.

TubeTVR (Hans) has an excellent site which details his OTL work, and also gives 'break-in' details for reliable No Drift 6C33C-B valves.. Follow those instructions, and you should have drift-free 6C33's and happy OTL's, no matter what design you follow....

As you are looking to drive an 8 ohm load, you'll need loads of 6C19 for any reasonable power, but its possible to get 56W from a single pair of 6C33C-B--My current OTL uses just a single pair per channel and achieves 56W power O/P into 8 ohm load The 'Bias' conditions for my valves are 190V, and 75-100mA. The dissipation at this standing-bias is just 19W, so for general listening, nothing really gets that hot...For an OTL that is!
To my ear, it has an excellent sound.

My bread-board of this OTL used the same pair of O/P valves for well over a year, No Bias-Drift, and no failures yet. That pair is currently in the 'half-completed' proper build under extended test..

There are various 'formats' of OTL, Have you decided on what type you'll make, ie, Futterman, SRPP, Circlotron, or something new perhaps...
I went the Circlotron route as this has a more 'symetrical' signal path than some of the other designs, but has the drawback of two independant floating-supplies for the O/P stages and extra PSU complications..

If you decide to go for the big Russian bottle, The best place Ive found is fleabay for good deals. Some of the 'tube-dealers' charge a small fortune in comparison, and I have my reservations as to whether they are any better than those I have had off fleabay!

[tubetvr]

In addition to all the excellent advice that Alastair gave, (it seems that the only thing we don't agree about is the relative merits of the Circlotron compared with other OTL circuit) I can offer one more advice or opinion.

If it where up me I would always select a design using a single tube over one using multiple smaller ones. The reason is not due to a possible difference in sound or amplifier performance but more to the inevitable problems that you will encounter when tubes need to be connected in parallell.

Even if the tubes are closely matched, both static and dynamic which is quite a problem for as many as 6 tubes, there are problems with individual drift that will upset the matching as time goes. The 2nd problem, (and one that I believe is responsible for the bad reliability of many older OTL amps) is that parallell connected tube need to be derated, i.e. the max current for each tube need to be reduced, this is mandatory even if you use individual cathode or anode resistiors in order to equalise currents.

Typical derating can be, (taken from a datasheet): 70% for 2 tubes, 60% for 3 tubes, 56% for 4 tubes, 53% for 5 tubes and 51% for 6 tubes for matched tubes without cathode or anode resistors, the figures with cathode resistors dropping 25% of the anode voltage is 88, 86, 85, 84 and 83%.

In some OTL amps this has been completely ignored and tubes has even been running much over max value of dissipation, no wonder that OTL amps has had a reputation of low reliability.

In summary there are a lot of advantages of using as few large tubes as possible in an OTL amp, 2 6C33C tubes give as Alastair write up to 50-60 W in 8 ohm, I only take about 25W in my amp but I have had excellent relaibilty using the same tubes now for more than 7 years.

I have had very good experience buying 6C33C tubes from GS tubes, no failures and very good price compared to other sources.

Regards Hans

[Alastair E]

Hi Hans,....I thought you might show up! Nice to 'see' you...

One of the reasons I chose the Circlotron over other designs, was I found a design on the 'Web quite some years ago. It looked to be the most well thought-out and had an excellent description of operation with it. At that time there wasnt too much data on the other designs around and not very good details of their operation. The design my amp is roughly based on, was the 'Sweep-Tube' monoblock from Allan Kimmel. It seems this diagram and detailed description have now gone from the 'web, which is a shame...

To be honest, the Circlotron is a very complicated device in comparison to most other types, whether its any better or not who knows. If I build another it probably wont be a circlotron, Ill go for something with more 'conventional' set up....

[kleist]

thank you very much!