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In the Forum: Melquiades Amplifier
In the Thread: Valve Preamplifier for Macondo/DSET
Post Subject: DHT's testedPosted by anthony on: 8/16/2020
Have put all my DHT's on the curve tracer to see if pairs can be matched for preamp duties.  

My stash of 10y/VT25/801a/VT62 are generally all very new or NOS specs with mu and plate resistance varying a little, but generally close to nominal specifications.  The VT25's in particular were very even and I have put a pair of them in the breadboarded preamp with pretty much identical specs at the operating point.  The VT62's all had higher plate resistances and lower plate current than the VT25's.

The YO186 were all over the shop...which you could guess by the margins offered in the spec sheet I suppose.  The glass bottle shape and size varies with the year.  Dates range from 1938 to 1956.  A pair of April 1956 YO186 test almost identically and likewise a pair of 1938/1956 that I am running in the DSETs.  Mu varied from 3.1 to 4.1, which is quite substantial, the low mu sourcing loads of current, up to 65mA where the spec sheet says 38ma, and the high mu source about 24mA at that same point.  In between the mu3.7 tubes have the correct current and plate resistance.  So, two closely matched pairs that sit right on the specification sheet parameters (one pair in the DSET's, one pair to try in the preamp), a pair of low mu/high current which I am not sure are particularly useful, and two pair of high mu/low current which may also be good in the preamp...they may actually be preferable there.

The CX371a pair tested 85% and NOS, which is nice.  One 45 ST was more or less dead after the abuse it suffered breathing life into two DSET amplifiers while its mate tests perfectly.  The four UX245 globes that tested NOS on a TV7 and AVo MK3 show up as about 70% emmission in the eTracer.  Shame...they are really beautiful tubes.

After all that, I have identified three matched pairs of YO186 to trial in the preamp and gained a better understanding of the voltage gain they are able to offer.  The low current sourcing pairs are more or less the same gain as a 45 but with a lower plate resistance, not that the plate resistance matters when using a mu-follower but it does if using an output transfomer.  Both the 45 and YO186 are therefore borderline candidates for the preamp because they offer 12dB of gain which would barely be enough with the mu-follower, and they are unsuitable in my situation for use with an output transformer because gain would be lost.

I will try them with a hybrid mu-follower though.

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