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In Reply to: Tanscendent T8 with 12 ooutput tubes posted by ackcheng on November 23, 2004 at 08:04:02:
I have built a T8 with 10 output tubes and later I built another OTL similar to T8 (other driver stage) with 12 output tubes.
Pictures of these two amps:
http://hififorum.knaak.dk/Bernt/2xOTL.JPG
http://hififorum.knaak.dk/Bernt/Nyotl41.jpg
http://hififorum.knaak.dk/PAC/20040228@CH/OTLandThiel.JPG
About building the 12 tube OTL, many pages. Text is in swedish but lots of pictures: http://www.hififorum.nu/forum/topic.asp?TOPIC_ID=17321
Follow Ups:
Thanks!Any tips I need particular attention? I am planning for a bigger transformer already.
One thing, the fuses need to be changed for the higher current capabilities.
As info: I arranged my output tubes in banks of three tubes, each bank with its own bias arrangement and fuse. Unfortunately that doubles the number of coupling caps too. Itīs more flexible that way. If I lose one bank I get 0,5V DC on the output, which is not much. Normally DC offset is within +/- 10 mV and drifts slowly. The T8 output stage is very stable, I have had no trouble at all except for one tube that went bad. I just swapped the tube with glowing red anode, readjusted the bias and that was it. No trouble with harming the speakers.
I use PL519, 40V, 0,3A heater. 6 in series can be connected to 230V AC from the wall. No hum.
Thanks very much for your tips.
Not much, a bigger transformer will do the trick. Make sure to use the best possible coupling caps. And besides that, I prefer carbon resistors. I got a bit "edgy" sound with metal film resistors.
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