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Running a valve preamp without a full complement of valves

James

James

Lord of the Erg\o/s
I'm running a Copland CTA-301 preamp, which I really enjoy. It has a pair of 12AU7 and a quad of 12AX7 valves for the line and phono stages respectively. The phono stage is MM only, and so I don't use it. Is it safe to remove the 12AX7s from the circuit? @RustyB suggested that his Conrad-Johnson preamp sounded better when the phono valves were taken out. I just want to be sure doing so does not upset the electrical apple cart.
 
If the filament supply is unregulated, then removing the tubes could cause the filament supply voltage to rise due to the lower load. This could cause problems for the remaining tubes, shorting their life.

TBH, same goes for HT, depending on how it's fed, e.g. if both are fed from a common supply and not regulated, though I'd expect this to be less of an issue.

You'd need a schematic to be sure.

You might be better off just finding some worn out tubes to put in there to save burning up good ones.
 
I would be very weary of removing any valves in your preamplifier. The power supplies will be designed for running the full complement of valves all running at their optimum operating points, or at least they should be. If you remove valves from the circuit you will alter the current draw from the HT and filaments supplies (LT) which can increase the HT and LT voltages, unless of course they have regulated supplies. Raising the voltage could put the existing valves under more stress or running them outside of their optimum operating envelope. This means they might not work as well, or it might make them fail more quickly, or the audio fidelity might not be as good.
 
Tony L said:
Maybe consider a step-up transformer? It may be better than your current phono stage!
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I'd like to, but the good ones seem rather expensive. Been looking out for an original Ortofon T-30. I'd even settle for a T-20.
 
James said:
I'd like to, but the good ones seem rather expensive. Been looking out for an original Ortofon T-30. I'd even settle for a T-20.
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I’d be inclined to figure out the optimal parameters for your chosen cart and make one using vintage Partridge, EMI or whatever transformers. I don’t think you’d find the Ortofons hard to beat, they were fairly budget devices that seem to go for over value today IMHO. Plenty of new options too.
 
I've had the same question about my Audio Note OTO, since I don't do vinyl, but it was recommended to not try it for the same reasons given above. Of course, it's circuit dependent, but I'd be cautious. The use of old worn out tubes was also suggested.
 
Different amp' - Croft - but I asked Glenn and he said that removing the 2 off 12AX7 in the MM stage wasn't a problem.
I then goy an SUT, out of curiosity, and that beat any phono stage I'd tried (a few of the common ones to a few £100's S/H) into the line level.
I now use a head amp' into the MM stage.
 
Get a SUT. They are not that crazy. I had a Noteworthy Audio SUT and it was £200 new. The Rothwell which I now use is £485 new (unless prices have risen) and I have used it with a Koetsu Black, Ortofon Jubilee and Benz Micro Wood SL and Ref S. The sound is vibrant and alive.
 
Tony L said:
I’d be inclined to figure out the optimal parameters for your chosen cart and make one using vintage Partridge, EMI or whatever transformers. I don’t think you’d find the Ortofons hard to beat, they were fairly budget devices that seem to go for over value today IMHO. Plenty of new options too.
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Thanks, and the SUT idea would work for my other preamp (Pioneer C-21) which has a fully adjustable (capacitance and impedance) MM stage. Does anyone use a Hana ML with an SUT?
 
Vinny said:
I then goy an SUT, out of curiosity, and that beat any phono stage I'd tried (a few of the common ones to a few £100's S/H) into the line level.
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You still need a phono stage with an SUT. You can't just put it between a turntable and a line stage. A SUT only provides some voltage gain (10dB to about 30dB depending on the ratio). You still need more gain (around 40-50dB) but more importantly you need the phono stage to provide the RIAA equalisation.
 
Perhaps a dummy load would be a good idea. It should be possible to figure out the filament resistance and disapation. Worst case you need a few resistors.
 
G T Audio said:
You still need a phono stage with an SUT. You can't just put it between a turntable and a line stage. A SUT only provides some voltage gain (10dB to about 30dB depending on the ratio). You still need more gain (around 40-50dB) but more importantly you need the phono stage to provide the RIAA equalisation.
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AFAIK @Vinny is using a head amp into the Croft MM stage.

I've been toying with trying a SUT or headamp into the MM stage in my Copland @James same MC as yourself, Rothwell makes some pretty affordable units, I quite like the idea of the Headamp for its adjustability.
 
Hooch500 said:
I think he'd still have to run it through the MM stage as there would not be enough gain to connect through pre amp direct.
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Yes, I was suggesting using a SUT with the Copland valve pre’s MM input rather than whatever stand-alone MC phono stage he is currently using.
 
I'm using a DV-P75mk4 into a line input for LP duties, but could easily use the MM stage with a suitable SUT.
 
Darren L said:
I've been toying with trying a SUT or headamp into the MM stage in my Copland @James same MC as yourself, Rothwell makes some pretty affordable units, I quite like the idea of the Headamp for its adjustability.
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What headlamp did you have in mind?
 
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