Based on the graaf 20 schematic as beforementioned, I breadboarded the amp and got it working. The tricky part was to get the right amount of feedback, which according to GRAAF is about 6 dB. I found ~12 dB better, also less hummm.
The feedback is set by altering the anode resistors of the input phase splitter. Took me a while to figure that one out
Tubes are ECC88-2xEF184-2x6c33cb per channel.
After some hours burn in time, the bias remained stable. I set it at 200mA (B+=145 V)
Gives around 20 Watts @ 8 Ohms
I like the sound as it is, fluent and a sense of 'being there'.
I find myself not listening to highs, mids and lows anymore, but a whole picture.
It is still on the breadboard, but it is by far the best sounding amp i made...
The feedback is set by altering the anode resistors of the input phase splitter. Took me a while to figure that one out
Tubes are ECC88-2xEF184-2x6c33cb per channel.
After some hours burn in time, the bias remained stable. I set it at 200mA (B+=145 V)
Gives around 20 Watts @ 8 Ohms
I like the sound as it is, fluent and a sense of 'being there'.
I find myself not listening to highs, mids and lows anymore, but a whole picture.
It is still on the breadboard, but it is by far the best sounding amp i made...
Just took a quick mugshot of the board and inhabitants
The board is 95cm in length, most space is taken up by the supply
The 4 chokes in the supply will probably change into a crcrc setup,
wich seems to work just as well according to Duncans psu- designer.
I put CSS under the phasesplitter and driver stages; no difference to the sound in my ears, but makes it easy to set the correct current for each stage and also better for se-input.
Used regulators LM317 & TL783 as ccs for an initial setup, but haven't found anything better yet.
Still looking to reduce hummm, when i use lower feedback, maybe humbucking the filaments?
They are series connected, 25V, each channel a separate winding from the Xformer
That's it so far. I hope to make some progress in the coming long nights......
The board is 95cm in length, most space is taken up by the supply
The 4 chokes in the supply will probably change into a crcrc setup,
wich seems to work just as well according to Duncans psu- designer.
I put CSS under the phasesplitter and driver stages; no difference to the sound in my ears, but makes it easy to set the correct current for each stage and also better for se-input.
Used regulators LM317 & TL783 as ccs for an initial setup, but haven't found anything better yet.
Still looking to reduce hummm, when i use lower feedback, maybe humbucking the filaments?
They are series connected, 25V, each channel a separate winding from the Xformer
That's it so far. I hope to make some progress in the coming long nights......
Attachments
How about 845 driving 6c33cs?
I just returned from this Japanese website
http://www10.big.or.jp/~dh/work/9403.html
Well, his works are well-known among Asians. What do you guys find in his design?
Man. This is an amp I dare not to try....
The most interesting one is the next project: 845 drives 845...
I just returned from this Japanese website
http://www10.big.or.jp/~dh/work/9403.html
Well, his works are well-known among Asians. What do you guys find in his design?
Man. This is an amp I dare not to try....
The most interesting one is the next project: 845 drives 845...
Cool didn’t realize this thread was still going. Mark, your prototype otl amp sounds really interesting!! keep us all update with the build. I'm still saving up for the transformers for my 6c33 monoblocs, Audio Note quoted me £481.75 for the mains and output transformers, £587.50 for more over rated versions. I’m guess custom wound transformers are always gonna be pricy so I should just deal with it. I’ve had all the other bits needed for months, just cant seem to save up enough money 
Hesky
Hesky
I remember the first time that I heard an OTL amplifier, on a show.
It was a CROFT with ECC81 input and concertina splitter, ECC82 driver and 2x 6AS7 per channel. Not much power, but boy was I impressed by the live feeling i got. The best illusion so far! That's 10 years ago....
I tried my hand at some tube amps with EL84 se and also pp, but never got that special sound back again, not even with a EL84-300B se amp I had on lone....
Now I feel that I found it back with this amp
Anyway this is were I've got so far....
I've just got my notes and put them in the original schematic.
I left out some parts, that are not essential for it to work and made things easier for me to do an initial setup.
Since my electronics skill is limited i have some problem:
In the original schematic there is something I can't figure out:
there is a resistor parallelled with the coupling capacitors. I've seen that before, but don't know it's function and how to come at the proper value. Any help on this would be great.
In the original schematic it also has a function in the bias setting for the power tubes, that i can figure out with Ohms law.
The values shown are experimental and also because that was what I had in the parts bin.
Comments are welcome anyway!
It was a CROFT with ECC81 input and concertina splitter, ECC82 driver and 2x 6AS7 per channel. Not much power, but boy was I impressed by the live feeling i got. The best illusion so far! That's 10 years ago....
I tried my hand at some tube amps with EL84 se and also pp, but never got that special sound back again, not even with a EL84-300B se amp I had on lone....
Now I feel that I found it back with this amp
Anyway this is were I've got so far....
I've just got my notes and put them in the original schematic.
I left out some parts, that are not essential for it to work and made things easier for me to do an initial setup.
Since my electronics skill is limited i have some problem:
In the original schematic there is something I can't figure out:
there is a resistor parallelled with the coupling capacitors. I've seen that before, but don't know it's function and how to come at the proper value. Any help on this would be great.
In the original schematic it also has a function in the bias setting for the power tubes, that i can figure out with Ohms law.
The values shown are experimental and also because that was what I had in the parts bin.
Comments are welcome anyway!
Attachments
gm20 clone
Here is the link to the original schematic again
http://www.audiodesignguide.com/otl/graaf20.gif
Here is the link to the original schematic again
http://www.audiodesignguide.com/otl/graaf20.gif
Well, in case that there is a need for output transformers. We take look at Lamm ML1.1
http://www.lammindustries.com/PICS/ML1.1/modified/ML11_inside.jpg
I just ordered two plitron 6c33c output transformers and pair of chassis for the amp. I can't change my plan for GraafGM20.
So, here is the circuit I tried to build which I don't where it comes from.
See the attachment.
Hi,
He,he...Glad you liked what you heard.
I'm the proud owner of a pair of CROFT OTL Series II. The exact same pair that was used for the Penta Show way back in 1987.
About 110W/8Ohm of muscle per channel using 10 6080s per side so no complaints there.
Have been an OTL afficionado ever since.
Nowadays, anno 2004, if I'd be asked to design an OTL I'd almost certainly go for the 6C33-C valves as I feel it's much easier to control a smaller number of output tubes.
The only OPT coupled amps that ever came close to mimicking the openness of the OTLs IME were the Megaherz designs.
Not surprisingly the HF response of their OPTs goes way up in the hundreds of KHz as well.
BTW, congratulations on your Graaf "clone".
Franco,
I'll try to find some time to look into it ASAP.
Unfortunately it may take a little while before I have a little spare time.
Since you want to use the PCBs Ernst designed I'd suggest you build the amp "as shown".
I'll base my suggestions for modifications respecting the PCB layouts so no major surgery will be needed.
Cheers,
He,he...Glad you liked what you heard.
I'm the proud owner of a pair of CROFT OTL Series II. The exact same pair that was used for the Penta Show way back in 1987.
About 110W/8Ohm of muscle per channel using 10 6080s per side so no complaints there.
Have been an OTL afficionado ever since.
Nowadays, anno 2004, if I'd be asked to design an OTL I'd almost certainly go for the 6C33-C valves as I feel it's much easier to control a smaller number of output tubes.
The only OPT coupled amps that ever came close to mimicking the openness of the OTLs IME were the Megaherz designs.
Not surprisingly the HF response of their OPTs goes way up in the hundreds of KHz as well.
BTW, congratulations on your Graaf "clone".
Franco,
I'll try to find some time to look into it ASAP.
Unfortunately it may take a little while before I have a little spare time.
Since you want to use the PCBs Ernst designed I'd suggest you build the amp "as shown".
I'll base my suggestions for modifications respecting the PCB layouts so no major surgery will be needed.
Cheers,
I got them from Ebay. Usually, you need to order directly from Plitron. The transformer is made like rock! of highest quality I have ever seen. They have posted the test data on their website. The bandwidth is amazing. Take a look.
The advantage of useing two 6c33cs' are obvious. Lesser matching problem and lesser transformer demand.
I am still looking proper toroidal power transformer for my project. I don't want to use two power transformers at the same time. I will use VTL's input and driver stage probably. David Manley use 12at7+12bh7 to push everything. I think I can do that too.
Lamm ML1.1 use 12bh7a for the driver stage. What you guys think? A little more power is better?
Hi,
VTL has used about anything even remotely close to a 12BH7A since the war in former Yugoslavia_Ei being the only current producer of 12BH7As_ 6350s, 6463s etc.
If I read the schematic correctly it says 12B4A which is not the same as a 12BH7A at all.
The 12B4A is an exceptionally good sounding tube but its use depends on what the circuit requires.
It's certainly capable of kicking a pair of 6C33-Cs around, no worries there.
Cheers,
VTL has used about anything even remotely close to a 12BH7A since the war in former Yugoslavia_Ei being the only current producer of 12BH7As_ 6350s, 6463s etc.
If I read the schematic correctly it says 12B4A which is not the same as a 12BH7A at all.
The 12B4A is an exceptionally good sounding tube but its use depends on what the circuit requires.
It's certainly capable of kicking a pair of 6C33-Cs around, no worries there.
Cheers,
6c33c project change
hi,
I almost finish my 6c33c project, but the thing is that my design is poor. The power transformer is right under the output transformer and the power tubes are too close to power tanny too. I am thinking about aborting monoblock design and puting all power transformers inside one chassis. I think power-signal-seperation design is much better in this case.
Also, 6c33c runs too hot. I need to take care of safty.
hi,
I almost finish my 6c33c project, but the thing is that my design is poor. The power transformer is right under the output transformer and the power tubes are too close to power tanny too. I am thinking about aborting monoblock design and puting all power transformers inside one chassis. I think power-signal-seperation design is much better in this case.
Also, 6c33c runs too hot. I need to take care of safty.
Attachments
Hi
Here you can find the description of VERY simple 6S33S SE amplifier (only two tubes per channel). Text is in polish but schematic diagrams and photos say everything:
6S33S SE amplifier
Here you can find the description of VERY simple 6S33S SE amplifier (only two tubes per channel). Text is in polish but schematic diagrams and photos say everything:
6S33S SE amplifier
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