Further thoughts on 5R4GYS rectifier tube/amplifier compatability

Like others, I bought a pair of 5R4GYS rectifier tubes from Upscale Audio some time ago, in the hopes that they would upgrade performance compared to the GZ34 and GZ37 tubes I had been using in two amplifiers - the VTA ST70 and the Sonic Frontiers/Parts Connection SET-300B.  The latter is a single ended triode design, based on two 300B tubes, which uses two rectifier tubes.  Both amps are deemed compatible with GZ33, GZ34, GZ37, and 5U4G rectifiers by their respective manufacturers.  I tried the 5R4GYS in both amps very briefly, not enough to test for early failure, and did not find a significant audible performance upgrade.

I recently noticed the discussions in this forum concerning early 5R4GYS failure in VTA amps.  This spurred me to verify that I could at least salvage the tubes for use in the SET-300B.  The spirit, as well as some of the personnel, from Sonic Frontiers days lives on at Parts Connexion, so I emailed the site for that company and inquired.  Chris Johnson and another staff member named Glenn informed me that the 5R4GYS will suffer early failure in the SET-300B because "...input capacitance load is too high for this tube - it will burn out prematurely."

So we have a reason for early 5R4GYS failure for an amp not made by VTA.  I do not have the electrical engineering knowledge to determine that the problem identified by the Parts Connexion staff also causes early failure in VTA amps.  In my opinion the limited warning  in the Upscale Audio website, i.e.,  to not use this tube in Bob Latino's amps, is at best incomplete.  I think Upscale Audio should advise prospective 5R4GYS buyers to ask the manufacturer of the prospective buyer's amp about compatibility.  Perhaps Upscale Audio should also research the matter further, so that more detail could be provided on the Upscale website.

Robert

A simple visit to the data sheet for the 5R4GYS should let you know the direct, lumped load capacitance your rectifier can tolerate. The power supply schematic for your amplifier will let you see how much lumped capacitance the design includes on the rectifier side of the choke or current limiting resistor (Mk II, et al).

A GZ-34/5AR4 is specified to have no more that 60µF directly loading its output. My guess is that Bob/Roy/VTA apply less that 60µF directly to pin #8. In my MkIII amplifiers I run 23µF tied directly to the rectifier output and am very happy with the operating life I get from my SOVTEK 5AR4 rectifiers.

For a GZ-37 the load capacitance spec is 4µF. I would not use that in a socket designed for GZ-34.

For a GZ-33 the load capacitance spec is 60µF.

For a 5R4* the load capacitance spec range looks to be 4µF--I saw one datasheet with a 20µF spec. I would not use a 5R4* in a socket designed for GZ-34.

I'm not sure why or what performance improvement you would expect by replacing the intended rectifier with 5R4. Higher B+ voltage? In that case why not use silicon diodes in place of the tube rectifier? Many of us are interested in lowering B+ back to the intended levels in our vintage tube gear. Modern tube gear (e.g. Sonic Frontiers) should not have this issue.

...j

My failure rate with the 5R4GYS rectifiers bought from Kevin Deal was about 20% right out of the box. Hardly worth the postage to send them back The ones that did work were very sluggish on the low end. No comparison with a GZ 33/34/37/5AR4/. For the el-cheapo price, you're better off with the Webers, provided you've got the B+ time-delay circuit.