What do you guys think?

[Jeb-D.]

The topic of Single-Ended OTL's has come up a lot lately, so I decided to give it a go. It still needs work, but all and all what do you think? Should put out about 20Wpk, dissipation per tube is aprox 40W. Wanted to do it open loop, but the Zout would be about 6ohms, which is too high. The output tubes are 6c33c-b's I forgot to add that text to the schematic.

[Giaime]

I see some problems in the design, frankly.

First of all you shouldn't return feedback directly to the input: in that case the nfb would be influenced by the output impedance of the previous audio device in the signal chain.

Also 10k grid stopper resistors on the 6S33 tubes seem a bit high.

20mA at that voltage in a 6DJ8 also is wrong, go for 10mA. Use two resistors in series for the cathode: an unbypassed one in series with a bypassed one, the sum of the two being the cathode resistor value needed for your biasing. Inject feedback across the unbypassed resistor and not the input.

Watch out for the 30W you'll dissipate in the transistor below the output tubes!

[rcavictim]

I dunno. Looks like a massive cathode follower with some sand across the speaker. If I were trying such a thing I'd be using an iron and copper device there replacing holes with Henries.

You will need a LOT of drive voltage swing. Can that 6DJ8 do it with a sand CCS as a plate load? I'd be using a choke there too, doubles the available theoretical swing.

Best way to know is build it and let us know how it works.

[Jeb-D.]

First of all you shouldn't return feedback directly to the input

I First had it going to the cathode resistor with it unbypassed, but couldn't get it to simulate that way. I figured since Rf would be much larger the Zo of the amp. Zout's effect on the feedback would be ignorable.

20mA at that voltage in a 6DJ8 also is wrong, go for 10mA

Thanks for catching that, The curves I was using did not have the Pmax line.

Watch out for the 30W you'll dissipate in the transistor below the output tubes!

The power transistor I'm using is rated at 120W so I'm not too worried. It will probably be darlington paired with a smaller transistor just to shrink the size of the base resistor.

I dunno. Looks like a massive cathode follower with some sand across the speaker. If I were trying such a thing I'd be using an iron and copper device there replacing holes with Henries.

Theres nothing wrong with using sand for a current source. I couldn't find a piece of iron with an even comparable bandwidth to the SS CCS.

You will need a LOT of drive voltage swing. Can that 6DJ8 do it with a sand CCS as a plate load? I'd be using a choke there too, doubles the available theoretical swing.

Well, 1.6A peak into an 8 ohm load would be 12.8V peak on the load. The 6DJ8 can supply a 34V peak signal to the grid of the output tubes. With the active load at that B+. As long as the gain of the output is above .37 then it should be capable of driving it

[Giaime]

I would consider, however, using a cathode follower for driving the output tubes. I'm concerned with the massive input capacitance, and the tendency the 6S33 has to have grid current even with negative Vgk.

[Jeb-D.]

I would consider, however, using a cathode follower for driving the output tubes. I'm concerned with the massive input capacitance, and the tendency the 6S33 has to have grid current even with negative Vgk.

That would be a concern, But I've built a 6c33c SET cathode follower amp. The driver stage was a 6su7gty with active load idling at a mere 2mA. And it drives the 6c33c past 30khz before cutoff. The pull down resitor of the 6c33c was quite a bit larger then the one in this design though. I figure the 6DJ8 idling at 15mA will be adaquate to drive it and keep the Pd at an exceptable level. That's also why I went with larger value grid resistors, since miller effect doesn't plauge cathode followers.

[Giaime]

30kHz bandwidht without "iron" in the middle is pretty poor...

[tubetvr]

I would consider, however, using a cathode follower for driving the output tubes. I'm concerned with the massive input capacitance, and the tendency the 6S33 has to have grid current even with negative Vgk.

I don't think that a cathode follower is necessary, the input capacitance of a cathode follower stage as is used here is Cgp + (1-G)*Cgk which with a 6C33C will be something like 31 + 10 = 41pF and for 4 tubes in parallell ~120pF is that so high?

Where you have got the information about grid current in 6C33C I don't know, it is however not true, grid current is negligible up to practically 0V and I for instance use 68k grid leaks with very good results.

I would however reduce the series grid resistors to 1k or something similar, anything higher is not necessary.

Regards Hans

[Giaime]

Originally posted by tubetvr

Where you have got the information about grid current in 6C33C I don't know, it is however not true, grid current is negligible up to practically 0V and I for instance use 68k grid leaks with very good results.

I tested with my hands Hans, and probably got a defective tube. I had 3-4mA flowing at -1V Vgk.

[tubetvr]

Hi Giaime,

Interesting and unfortunately it seems not an unusual experience. It seems that defective 6C33C tubes are still available in Europe and US. I had hoped that the supply of defective tubes or rejects was exhausted by now but it seems that it is not so, I heard recently from someone who bought 4 6C33C from an European vendor and all 4 of them where defective with either low emission or grid to cathode short.

I can really recommend to buy directly from Russia, quality, (speaking from my own experience) is excellent as is customer service and prices are the lowest in the market, the only problem is that there is a minimium order sum but if you have some friends who need tubes at the same time it is not a problem.

I have bought 6C33C from 3 sources, San-Ei in Japan, (they where the first to import 6C33C then relabeled EC33C to the western world), tubes.ru and GS tubes, I had one faulty tube of almost 100 and that one was a grid to cathode short. Viktor Khomenko at BAT say that you should expect about 1% failure not more.

In my OTL amp I use a simple split load inverter to directly drive the output tubes with very good results and I can easily see that there is practically no grid current even close to 0V. You should expect to see a little bit of grid current close to 0V with a but it should be in the uA range and not more. The original russian datasheet says that the grid leak should be 200kohm or less so it is not different from any other power tube,

Regards Hans