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Old 22nd June 2010, 08:31 AM   #1
neazoi is offline neazoi  Greece
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Default 6C33C OTL DC schematic needed

Hello,
I have found some russian 6C33C in good price so I would like to builg an OTL amplifier.

1. I am interested in Class-A only designs please, not class-AB.

2. I would greatly like them to be direct coupled through the whole amplification stages (difficul eh?)

3. Two tubes per channel, as I have only 4 of these 6C33C


any suggested schematics?
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Old 22nd June 2010, 09:25 AM   #2
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1. Output tubes have to be replaced frequently.
2. 6C33C runaway makes it really poor choice for the first tube project.
3. There are some really nice sounding tubes not much more expensive like EL84 and the amp will have output transformer to protect valuable hi eff speakers. 6C33C is a voltage regulator tube not audio tube.


Ciuffoli 6C33C OTL

I would suggest google “6C33C Survival Guide” as well.
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Old 22nd June 2010, 01:22 PM   #3
banat is offline banat  Serbia
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Quote:
Originally Posted by neazoi View Post
Hello,
I have found some russian 6C33C in good price so I would like to builg an OTL amplifier.

1. I am interested in Class-A only designs please, not class-AB.

2. I would greatly like them to be direct coupled through the whole amplification stages (difficul eh?)

3. Two tubes per channel, as I have only 4 of these 6C33C


any suggested schematics?
See the post:`What tubes for a tube amp` 7.3.
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Old 22nd June 2010, 02:46 PM   #4
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Quote:
Originally Posted by neazoi View Post
2. I would greatly like them to be direct coupled through the whole amplification stages (difficul eh?)

3. Two tubes per channel, as I have only 4 of these 6C33C


any suggested schematics?



2. Not difficult at all. How many speaker pairs have you got?

3. 4 at a good price? I bet these are leftovers after someone matched the rest. Chances those four will work together without being matched after burn-in: nil


If you are lucky and they are indeed matched why don't you build a clone of a Graaf 20? Circuit has been posted here and some members have either the original or a clone, so you'll probably get some help.
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Old 22nd June 2010, 04:24 PM   #5
kevinkr is offline kevinkr  United States
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Class A operation into reasonable speaker loads is unrealistic with a single pair per channel. If you want more than just a few watts you WILL need to operate them in class A/B.

The simplest amplifier conceptually to get running would be a variant of the Futterman although a Circlotron might be a better choice for other reasons.

You do NOT really want to DC couple these to your speakers, I learned this the hard way. (Unless you know exactly what you are doing. Big zener clamps can provide a measure of protection)

B+ should be delayed until the tube cathodes have had a chance to fully warm up, this will prevent cathode stripping and reduce the likelihood of a speaker frying arc.

You will need to burn these tubes in before use for at least 100hrs, starting with just filament voltage applied for maybe 24hrs and then run them at about 2/3 of their rated dissipation for the balance of the time. This will stabilize their parameters and reduce their propensity to arc during warm up.

Bargain tubes will turn out to be anything but. You need a good number of samples in order to match tubes section to section and to each other. A dozen is not unreasonable.

Done right this tube sounds great with or without feedback. Low output impedance will require a lot in parallel or gobs of NFB. A single pair is good for roughly 15W - 20W into 8 ohms in class AB, significantly less than 5W in class A. Increasing VC impedance will result in increased output power until you reach the voltage compliance limits of your design.

If you are not an old hand at building and debugging tube amps, run don't walk to the nearest exit - an OTL amp using this tube is not a beginners project.
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Old 23rd June 2010, 02:52 AM   #6
djn is offline djn  United States
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Here is the write up for the amp I am building. I have most everything laying around so there is not a lot of risk. Your speakers do need protection though.

Information • DIY Audio Projects Forum
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Old 23rd June 2010, 03:53 AM   #7
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Quote:
Originally Posted by neazoi View Post
Hello,
I have found some russian 6C33C in good price so I would like to builg an OTL amplifier.

1. I am interested in Class-A only designs please, not class-AB.

2. I would greatly like them to be direct coupled through the whole amplification stages (difficul eh?)

3. Two tubes per channel, as I have only 4 of these 6C33C


any suggested schematics?
Sure. I've used this with 6C33s in the past, running class A although with only 2 tubes per channel you will not be able to do that unless you speaker is 32 ohms or so.

You will have to adjust some things for the 6C33; the bias voltage will not be as negative. What is nice about this circuit is its less prone to runaway.

This circuit is quite proven, is very stable and relatively fail-safe.

You will need a manual bias control, about 10K wired as a reostat to replace the 22.1K and 110K resistors in the bias network. You will also need a spring-loaded switch wired to disconnect one of the power transformer secondaries from one of the rectifier packs for the output section. When you depress the switch, that supply will then shut down, allowing to to read the bias current with a DC Ammeter across the speaker terminals as shown.

The power supply is not shown in this schematic but can be found at http://www.diyaudio.com/forums/attac...-power-117.gif

I would set the bias at 200ma after a 5 minute warmup and not let it exceed 250ma after one hour of operation. The circuit shows a small amount of negative feedback but you will not need it if your speaker is at least 16 ohms.

If you use more than 2 power tubes I would set the cathode resistors of the power tubes to 2 ohms 10 watts, and I would install a fuse in series with the plates of the tubes in any event. Wire the fuseholders so that the B+ line is on the inner contact of the fuseholder. The fuse value should be 1.5amps.

With only a single pair you do not need to do a lot of matching, although you will be rewarded with better sound if you do.

The 6C33 sockets need a hole in the chassis larger than you would expect, as it is expected that the socket is mounted with a standoff of at least 1/8" to allow air past the socket. There should be a gap between the tube residing in the socket and the chassis that is at least 1/4" all the way around the tube. I would also install two rings of cooling holes around the socket and make sure that the bottom cover has cooling slots or holes as well.

I would wire the filaments for 12V as the wiring is a bit easier to manage. The American sockets (EF Johnson) hold up a lot better than the Chinese or Russian sockets, which have a service life of about 5000-7000 hours. I recommend wiring the socket with silver or silver-plated wire, insulated with Teflon and use SN96 or other 800 degree (F) solder, as it will hold up a lot better. Regular solder can come right off the socket in time. The filament connection on the socket is overloaded at all times and so tends to run the temperature of the socket up high, and the tube itself runs so hot that even without B+ on it burns are a serious risk!

If the socket is installed properly, it will be a shock hazard if the tube is not in the socket due to exposed contacts. Extreme caution is advised!

As always, have fun and don't zap yourself.
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Last edited by atmasphere; 23rd June 2010 at 03:59 AM.
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Old 23rd June 2010, 01:08 PM   #8
neazoi is offline neazoi  Greece
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Thank you all for the schematics,
I have build 4 transformer based amps before and two solid states, one being the le monstre which is direct coupled, but as far as I go to the OTL it seems that it needs more experience, especially if you have two expensive cones...

Now, I am not an expert butas far as I can understand these schematics are all push-pull or topologies where the upper tube works on 180degrees and the lower one the rest 180degrees. If they are not class-A designs they should suffer from intermodulation distortion (even class-A push-pull does).
On the other hand I have not seen any OTL SE topology schematic enywhere!
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Old 23rd June 2010, 03:28 PM   #9
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Quote:
Originally Posted by neazoi View Post
Now, I am not an expert butas far as I can understand these schematics are all push-pull or topologies where the upper tube works on 180degrees and the lower one the rest 180degrees. If they are not class-A designs they should suffer from intermodulation distortion (even class-A push-pull does).
On the other hand I have not seen any OTL SE topology schematic enywhere!
Apparently! In the schematic that I provided, there is no 'top' power tube as it is a Circlotron.

IM distortion does not arise from push-pull operation. Usually it comes from intermodulations elsewhere in the circuit. For example in the circuit I provided if the plate of V4 is not sufficiently bypassed, you will increase IM distortion. Another technique to reduce IM is to use seperate power supplies for the driver and output sections. A third method is to avoid direct-coupling from input to output, as a fully direct-coupled amplifier can easily modulate its power supplies. This is assuming that the actual circuits in the amplifier are in fact linear, if they are not of course IM will arise there as well.

If you build this amplifier correctly, IM is well below 0.02% at full output.
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Old 23rd June 2010, 03:53 PM   #10
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Quote:
Originally Posted by neazoi View Post
Thank you all for the schematics,
On the other hand I have not seen any OTL SE topology schematic enywhere!
Here's one: http://www.audiodesignguide.com/otl/otlse.jpg

Jim
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