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In Reply to: RE: Croft Series X info posted by bcarlos on July 18, 2023 at 07:43:33
As a newbie, it is IMPORTANT to note that tube amps operate at high voltage that can fry you if you do not know what you are doing, and are not careful (even if you do know what you are doing).
If it is currently wired for 220v and you are using it with 120, it is effectively running at 1/2 power, which MIGHT explain the low volume. If it has 120 taps on the transformer, you might want to change the wiring from the 220 taps to the 120 taps.
The amp looks to be fairly clean, and may have been upgraded at some point in time. Likely the output is fairly low, on the order of 10W (see Croft Troubadour, which looks quite similar).
It seems that Croft used a capacitor to isolate each output side from the speaker, so blowing up the speakers is not so likely. You would have to trace the signal from the power tubes to the output lugs - the blue capacitors behind the power tubes might be isolating capacitors.
Not sure how to address the tube issue if you think they are not correct. You could look at other Croft OTL amps and see what tubes he used. Apparently Croft did not release schematics. The amp is wired point to point, so you could make a schematic yourself by mapping out the wiring.
Perhaps the amp will come up at 120V when wired for 120V, and all will be well. If not, then a deeper dive will be required. If you speakers will operate well with 10 watts, then it could be a fun project.
regards -- Roger
Follow Ups:
You are right Roger, both blue capacitors are in between tubes and output lugs.
Maybe I was not clear, sorry, I live in Argentina and the voltage here is 220V. Apparently taking power cord in consideration, originally the amp was for 120V, but then somebody change the transformer tap to 220V. I see a cut cable isolated with tape.
Maybe can return the amp to 120V and did a new test.
I was finding others 6080 OTL schematics, and they use 12AX7 and 12AU7 too and maybe thatīs all and the power should be no more than 10W.
On the Troubador, Glenn Croft used 2xECC83 and 1xECC81 driver/phase splitter. My amp came with 2xECC83 and 2xECC82, thatīs my questioning and doubt about the output power, ECC82 is the lowest gain of its.
Both amps look very similar in design.
It would appear that the driver tube is the 12AT7 (ECC81) and the input voltage amplifier tubes are the 12AX7s.
This amp likely uses negative feedback. So it would need the gain of the 12AX7s to be successful at that.
Iīm thinking the same Ralph, 12AT7 in place of 12AU7.
Certainly you should be using the correct power taps on the transformer for your local power (220V).
If you look at the data sheets for the two ECC tubes, you should be able to determine if they are interchangeable. At an initial glance, it looks like the operating requirements and pinouts are the same. I am no expert on this. You might want to give them a close look to confirm, and then give it a try - could be an easy solution. The ECC82 may have been used to effectively decrease the sensitivity of the amp, for whatever reason.
https://frank.pocnet.net/sheets/084/e/ECC81.pdf
https://frank.pocnet.net/sheets/084/e/ECC82.pdf
Iīm very confused, saw a Croft Troubadour sold on Audiogon, saying "4 x 12AX7 ; 4 x 6AS7 tubes".
You can do a search on Croft Troubadour, or Croft OTL, here, and between current and ended items there should be a number of different amps listed. You can take a look at the tube configuration, and if most are the same, then that is likely the original tube configuration. People swap tubes for various reasons, and with various effects.
https://www.hifishark.com/search?q=croft+troubadour
A good strategy might be to set the power configuration of the transformer to correspond to your local power. If there is still a volume problem, you might want to try the alternate ECC tube with more gain. Once you get the amp up and running, it is pretty easy to do tube substitutions to get where you want to go based on the audio result of the swap.
It may be challenging to get specific information on the Croft amp, but you will have no difficulty getting replies from the board on various tube substitution possibilities, and often all the info you need from a web search.
Sounds like you are trying to figure a precise end-game on the tubes, which I think is going to be hard without hearing what the results are.
You are right Roger, but Iīm only trying to find some deep information before to start with tests.
The Troubadour I saw have 12AT7 and 12AX7 in driver position, never see a 12AU7 on this amp.
I promise after all it, to publish the schematic of the Series X, in case another one need helps.
No problem - some times it is too easy to circle the problem rather than going right at it. Something I do on occasion, like a Futterman H-3 that I need to finish.
Looking forward to progress reports. That is a very neat amp, and should provide some good listening.
-
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