Cool old tubes for tone gurus

[J M Fahey]

almost all the getter flash has turned clear/white.

Proof enough: junk it.
Sorry.

[Richard]

I was looking at the SVT schematic and wondering why the 12DW7's are repeatedly used as a high gain stage into a cathode follower. Could someone explain the purpose of this?

[Leo_Gnardo]

I was looking at the SVT schematic and wondering why the 12DW7's are repeatedly used as a high gain stage into a cathode follower. Could someone explain the purpose of this?

The lower-gain, hi current triode in those 12DW7's is supposed to be able to drive what the engineers saw as a low-impedance load, the EQ stacks, and potentially an effects loop. Early SVT's had two "line out" jacks driven by a DW's c.f., later ones had line out - power amp in jacks. The earlier ones could be easily converted to loops, I've done a few. The engineers wanted to conserve a wide bandwidth while driving both EQ stacks, and at the preamp output possibly long cable runs and/or fairly low input impedances to boost amps and/or transformer DI boxes. For a while 12DW7's were rare-to-nonobtainium, lucky we now have some pretty good ones made by JJ. In the interim there were suggested circuit mods so you could sub 12AX7's. They worked OK but as you must expect they did change the tone/feel of the amp so it's good to have the DW7's available once again.

Also granted, if there's no low impedance device to run from the line out/fx send, the power amp is awfully sensitive, can be driven to clipping with only a 250 millivolt signal.

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And thank you Juan, on the 12BH7 question. I was thinking maybe nsubulysses's tube was really OK, just has a bubble in it or something:

[Mick Bailey]

The Blackstar HT5 runs its output tube at 465v. I got 5.4W before any sign of clipping on the clean channel. Not bad for a single output tube, though it uses a SS PI.