any switchable tube/solid state rectified rockets done?

[wsaraceni]

anyone have a layout or thoughts on doing an 18 watt ac-30/rocket style amp with a switchable tube rectified/solid state diode switch. I'm wondering if i could do it with an on/off/on standby switch. can someone help me out with the wiring. i'm thinking it should be as simple as the transformer going to a turret, one wire to the tube socket, one wire to a pair of diodes in series.

the other side of the tube/diode goes to the standby switch.

just not sure how to wire the switch.

[Firestorm]

The simplest way to do it is to wire the tube rectifier tube normally and parallel the SS rectifier. Put your SS diodes on the tube rectifier plates and the other ends to the hot side of an SPST switch. The cold side of the switch just goes to the same point as the B+ off the tube rectifier cathode. When you close the switch, 99% of the current will go through the solid state diodes because of their very low resistance compared to the tube.

[wsaraceni]

essentially this

http://i266.photobucket.com/albums/ii24 ... cat_30.jpg

without the 100uF and 5m1 resistor?

[hans-jörg]

Hi, I guess with.
Different rectifiing -different capacitors.
A tube requires around 50µF, the diodes around 100µF.

Best

Hans-Jörg

[Firestorm]

That's what I was thinking. I suspect the 5M1 is there to quiet the switching transient when connecting the 100uF. You shouldn't have that issue since the two ends of the switch will see only the difference between the tube B+ and the SS B+; 20-40 V or so. You could put a small cap across the switch if need be.

[wsaraceni]

im wondering how it would be if i put another 40uf cap in there off of the solid state rectified side as I am planning on less filtering then the rocket. 40-40-8-8-8. it may make it a more noticeable difference between the two. what do you think?

still hoping to do this off of a standby switch. tube switch up, standby in middle, solid state down. this all i need for that?

http://www.mojotone.com/amp-parts/ampli ... witch-DPDT

right side:
B+ from tube into bottom lug,
B+ from diodes into top lug,
B+ out in middle lug

left side unused

[Firestorm]

That should work fine. You may as well parallel both sides of the switch (rather than leaving one side unused), nicer for the switch. Also be sure the "extra" 40uF cap is rated for the full unloaded B+ voltage.

[John_P_WI]

Many years ago -15- I built a bassman with both a tube and 1N4007 ss rectifiers. After switching it "hot" a number of times it developed a terrible buzz. When I would switch the SS out of the circuit, they were "hanging in air" no circuit path connected. Years later I was reading one of KOC TUT's talking about this and the explanation was that if the SS diode is switched hot in the middle of a cycle there can be a large transient that will take out the diode in the conduction cycle.

The simple solution is to place a power resistor to ground after the diodes and before the switch allowing the "transients" a path to ground. I don't recall the exact value of resistor, but something like a 220k @ 450 v would only "steal" 2 ma of current and offer cheap protection for the diodes. I think Egnater did something similar IIRC.

Of course the best solution is to switch between the rectifiers when the amp is powered off.

[diagrammatiks]

if you are running el84s you are going to want to check your voltages.

The hot cat gets away with it since it runs el34s with a fairly low plate supply voltage in tube rec mode and kicks it up to normal levels for ss rec. Something like 380 to 410.

with the 500-506 secondaries you'd get maybe 340-350 on the plates. That might require a bias adjustment to keep things safe.

[wsaraceni]

will be running a pair of el84s:

transformer is 290V @ 120mA secondaries CT

3.14V @ 3a heaters

and a 1.4A recifier tap for an ez81.

anyone want to share the math to figure out the voltages?

[greekie]

Or do it like Mesa Boogie:

[img1081]http://music-electronics-forum.com/atta ... ier-ps.gif[/img]

Nicer for the switch too...

[wsaraceni]

anything to consider if doing this to a fixed biased amp? i decided to abandon it for this project, but the express transformers I'm using for a future project have a 5v feed so i could do this to that. it'll be pretty close to an express power section with a ef86 front end.