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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Brook 12A Mono blocks

unique1

Member
2014-11-13 3:58 pm
A very long gestation period for these as I started accumulating tubes almost 10 years ago-good thing as they're much more dear now. As for the particulars, outputs are 6A5G's, subbing for the 2A3's in the original. The input is 6F8G, an obscure and great-sounding 6SN7. Power transformer is custom wound by Heyboer, OPT's are Electra Print. I had to add a choke in the power supply as hum was unbearable without it in spite of increased capacitance. They sound wonderful, like listening to all my music for the first time. Many thanks to some of you who answered my queries concerning this build and especially to Briney who has built the same, although more true to the original.
http://www.diyaudio.com/forums/attachment.php?attachmentid=547561&stc=1&d=1462561142
http://www.diyaudio.com/forums/attachment.php?attachmentid=547562&stc=1&d=1462561142
 

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45

Member
2008-12-18 2:29 am
UK
I think you meant 6AV5 and 6F8.

The 6A5G is the heater-cathode version of the 6A3. It was made by Sylvania only in my knowledge. The filament works at 6.3V/1.25A and has a center tap internally connected to the cathode pin so that the tube is almost hum free.
The other specs are identical to 6A3 and 2A3.

The 6F8G is a ST glass double triode with top cap with the same specs of a 6SN7. The top cap is for one of the two anodes. This creates a slight asymmetry concerning its inter-electrode capacitances that differ by about 10% on average for the two halves.
 
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unique1

Member
2014-11-13 3:58 pm
Thanks for that 45. A slight correction if I may, the top cap is grid #2, or at least #2 in my application. I have Sylvania and Philco branded 6A5G's. Some are square plate and some rounded. The Philco's are probably Sylvania's as well. I haven't used the rounded ones yet so I'm not sure of the sonic differences, if any.
 
The 6A5G is the heater-cathode version of the 6A3. It was made by Sylvania only in my knowledge. The filament works at 6.3V/1.25A and has a center tap internally connected to the cathode pin so that the tube is almost hum free.
The other specs are identical to 6A3 and 2A3.

Ok, that makes much more sense than an ST envelope beam tetrode (6AV5).

Where can we find a schematic, unique1?
 

unique1

Member
2014-11-13 3:58 pm
Here's the schematic:

http://www.ampslab.com/SCHEMATICS/Brook12A.gif

Please note I built a C- supply for the bias and used a 5AR4 rectifier which resulted in a 300-0-300 PT with the low impedance caps. Plate voltages are within 4v of schematic values. The triode side of the VT-99 with the grid cap was used for the splitter. It would have been rather weird to have mounted an RCA jack on top of the tube if that was to have been V1. No, I can't take credit for the chassis, Hammond. The PT also has two 6.3v filament supplies which I specified as the 6A5G's have their own centre tap and I elevated the centre tap for the preamp tubes.
 
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When the DoD gave Sylvania a contract for 6B4 spares, they fulfilled it by internally triode wiring 6AV5s. The 6B4 is Octal based, but electrically equivalent to the UX4 based 6A3. Other than filament voltage, the 6A3 is identical to the 2A3.

Wait. Where does the 6A5G come in?

I built a 6AV5 amp recently, actually. Very nice sound. 7F7 > 6SN7 driver stage (I think you turned me on to the 7F7, as a matter of fact).
 
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yeah. I think 2a3 is just as nice if you implement it well.

Otherwise if you go for 6B4G then a nice high impedance regulated DC supply is the way to go.

I personally would try out the cap plate triode for input differential amplifier on the 6F8G... and make sure there is a decent sized grid stopper on the phase splitter as well. ;)
 
Sodacose, the 6AG5 is simply a rare version of the 6B4G.
Sorry the specs sheet is French, but you can see how the cathode is connected to the mid-point of the heater. Its still kind of a DHT...
It is a directly heated cathode with the mid point connected to a pin. People have taken liberties and incorrectly drawn the picture with both a cathode and a filament symbol. I have all those tubes (6A5G, 6B4G, 2A3, 6A3) and looked.
 

unique1

Member
2014-11-13 3:58 pm
Mr. Briney, they sound spectacular, in my estimation. I've never heard things in my music that I'm hearing now. Super solid and extended bass, nuance, fluidity, detail, sweet top end. They're not overly loud at 10w or so but drive my Duevels nicely. I'm going to get the Cornwalls out for an audition as well.
Many thanks again for your help although I deviated quite a bit, especially with the power supply and the 5AR4 rectifier.