Add tube rectifier?

[Axtman]

Hello. I have a Fender Bassman Ten 50 watt version amplifier. I would like to add a tube rectifier to the amp. The amp has a hole in the chassis and a plate covering it. So I unscrewed the cover plate and screwed in an appropriate size tube socket. I am going to follow the schematic for the Bantam Bass and wire up the tube. Oh, the power transformer has the appropriate extra wires for a rectifier tube.

The problem is that I would like to add a switch to switch from tube to solid state. How can I do that? My thought was to use the "Ground" switch since I have already disconnected the infamous death cap. OR should I put the switch on the standby position with up being tube, middle being off and down being solid state.

Your thoughts please! Thanks!

[deci belle]

Hi Axtman, I've done this to a bf Showman. I got a 50w PT with a 5v 4A tap that opened up two sockets for rectifier tubes.

I got a burly DPDT ON-off-On switch from Weber PN#WSW-DPDT-F-15-CO, and it fits perfectly in the GND SW's 1/2" keyed hole.

I used short, bare 20ga. jumpers to connect each tandem pole just to make the beefy switch beefier.

The wire connected to the output of the SS rectifier is moved to one of the center tabs of the switch.

The output of the SS rectifier goes to one of the outside tabs of the switch and the output of the rectifier tube (pin 8), goes to the remaining outer tab.

The *Off* position works as a built-in safety, but you ought to turn the amp completely off before switching from SS to tube rectification.

[hewo]

PT_HT_AC_Tappable_Windings_Switchable_Via_DPTT(center_of f)_Using_Both_HT_AC_Leads

can the DPTT (center_off) be wired in circuit utilizing BOTH POLES, for BOTH HT_AC_winding_leads_(CT_earthed)?
My HT_AC_winding has taps for either 330VAC (full_winding) or 300VAC (tapped_partial_winding) so I wired in another DPTT (center_off), utilizing BOTH POLES at each switch position, to hard wire BOTH LEADS (pair_330 and pair_300).
the HT_AC_Windings_Tap_DPTT must be upstrm of the RECTIFICATION_METHOD_DPTT.
do you recommend AGAINST my scheme of utilizing BOTH_LEADS (in lieu of only one lead)?

[lowell]

I would add a switch. The easiest way to wire this is to put the rectifier diodes in.parallel with the tube rect. Your new switch is gonna switch them in and out of circuit. All you really need is a spst switch. You can just have the cathodes of the diodes connected together and to one switch pole, the anodes on the tube rect socket AC input pins, and the remaining switch pole goes to the DC side (pin8?) of the tube rectifier. Hope that makes sense I'm writing this on my phone. The idea is that when the diodes are switched in they swamp out the tube rectifier.

IMO, I'd just switch a series resistor in and out of the B+, instead of going to all the work with the tube rectifier. It's not THAT much work, but tubes ain't cheap and you can quite successfully simulate a tube rect with a B+ resistor or possibly more effective is using an added shared series screen resistor which is not in series with the B+. It goes from the PS screen node to the screen resistors (470ohm). And a rectifier tube is more susceptible to failure than silicon diodes...

[Steve Conner]

I'd recommend putting the diodes in series with the tube rectifier, not parallel. That way, if the tube rectifier shorts out, the silicon diodes are right there as a backup, the amp will keep going instead of blowing its fuse and dying. You would switch to "silicon" mode by shorting the tube rectifier's anodes to its cathode with a DPST switch.

But yeah, what Lowell said, I've converted a few amps from tube to SS rectification, but never the other way round.

[lowell]

Good call on series vs parallel Steve.

[hewo]

Thankyou Lowell and Steve, responses

can the DPTT (center_off) be wired in circuit utilizing BOTH POLES, for BOTH HT_AC_winding_leads_(CT_earthed)?
My HT_AC_winding has taps for either 330VAC (full_winding) or 300VAC (tapped_partial_winding) so I wired in another DPTT (center_off), utilizing BOTH POLES at each switch position, to hard wire BOTH LEADS (pair_330 and pair_300).
the HT_AC_Windings_Tap_DPTT must be upstrm of the RECTIFICATION_METHOD_DPTT.
do you recommend AGAINST my scheme of utilizing BOTH_LEADS (in lieu of only one lead)?

My question whether the use of DPTT gating of BOTH AC high tension (either 330 or 300 both leads as a pair into the DPTT) is prudent or not was not addressed. Response claimed dealing with only one lead suffices for gating desired selectable high tension AC into the 5Y3GT.

Further examination of this scheme (dual leads in lieu of single lead gating concept) raises concern whether the 5Y3GT can be idled hot with 5VAC heaters energized while htac is disengaged altogether by the DPTT switch positioned to NONE (center off). Comment requested. Is a htac bleed resistor (short to ground, both legs), say, 150K-220K ohms, needed, to pacify the PT from experiencing zero transfer when htac is DPTT selected NONE ?

[Enzo]

There really is no danger to the tubes or transformer. You can leave the 5vac on at all times, the tube doesn;t care. You can leave high voltage AC on its plates at all times, the tube doesn;t care. You can have one or the other or both of those things, the tube won't care. The power transformer windings may be left unconnected and no harm will come to it. Many amps create a standby condition by opening the AC leads to the rectifier (Marshalls for example).

[hewo]

Thankyou Enzo for your responses

There really is no danger to the tubes or transformer. You can leave the 5vac on at all times, the tube doesn;t care. You can leave high voltage AC on its plates at all times, the tube doesn;t care. You can have one or the other or both of those things, the tube won't care. The power transformer windings may be left unconnected and no harm will come to it. Many amps create a standby condition by opening the AC leads to the rectifier (Marshalls for example).

Okay,
1. Idling the 5Y3GT with heaters alive and no htac is permissible (irregardless legacy schemes do not employ 5Y3GT idling this way).
2. the PT htac does not require bleed resistors, of each leg, to earth (htac CT earthed), to keep the PT preoccupied, psuedo idling, with scant energy transfer.
3. there will be no, or trivial, reactive currents by the PT when the DPTT instantaneously engages htac feed to the already heated 5Y3GT.
4. similarly upon disengaging instantaneously htac, reactive currents by the PT will be none or trivial.

Looks like elsewhere in this forum others have commented (directed) backup contingency diodes should be inline installed downstream of the 5Y3GT not upstrm but I disagree with the circuitry protectionism concept because that desireable "spongy" signature of the 5Y3GT would be compromised this way. the scare is 5Y3GT catastrophic failure and consequences. is the 5Y3GT, in current shuguang factory production, of untrusting reliability? if I revise circuitry to permit selectable rectification modes, will the diode mode actually be musically desireable as compared to the legacy mode 5Y3GT ? in other words u can have all these selectable options, but really, there's going to be a preferable config u end up always using.

[Enzo]

No, the solid state diodes in series with the tube rectifier will not compromise its tubiness at all. If you have a diode in series with the rectifier tube plate, it will only allow positive going pulses to get to the plate. What the tube does with them is of no concern to the diode. The tube only lets positive going pulses through anyway, so not having to block negatives is irrelevant to the tube. The tube will still have its own internal resistance and so will still add all the sag it ever did alone.

Adding diodes after the tube rectifier will not protect the tube in any way, or the amp. Putting diodes before the tube rectifier allows a tube to fail and the amp will continue to function. SUch diodes protect the transformer and the rest of the circuit from a failed rectifier tube.

[hewo]

5Y3GT_Diode_Safeguarding

No, the solid state diodes in series with the tube rectifier will not compromise its tubiness at all. If you have a diode in series with the rectifier tube plate, it will only allow positive going pulses to get to the plate. What the tube does with them is of no concern to the diode. The tube only lets positive going pulses through anyway, so not having to block negatives is irrelevant to the tube. The tube will still have its own internal resistance and so will still add all the sag it ever did alone.

Adding diodes after the tube rectifier will not protect the tube in any way, or the amp. Putting diodes before the tube rectifier allows a tube to fail and the amp will continue to function. SUch diodes protect the transformer and the rest of the circuit from a failed rectifier tube.

Thankyou Enzo for your responses:
1. Okay will solder inline diode into both HT_AC_Leads, (CT earthed), dwnstrm of htac selectable voltage DPTT (center-off), and before hitting the 5Y3GT. Polarity of diode for either htac lead fed to 5Y3GT shall be FORWARD STROKE PERMISSIVITY ONLY.
2. This will give me an opportunity to utilize some great britain obtained ultrafast diodes (fat like tipparillo's) that wouldn't feed thru the pcb's of slm vintage club series amps.