High B+ using SS rectifier

[expat]

This is my own Franken-amp design so there's no precise 'original' to compare things to. It's a 6at6-12ax7-6v6 lineup.

I dig that the SS rectifier is going to put out a lot more voltage than a tube rectifier.

Without the tubes in, I'm seeing 550v on the 6v6 plate, which is rated for a max of 500v.

I've read that they can handle 550v AND that once the tubes are in, the overall voltage would drop back down into normal operating range.

Trying to get some expert advice before I put any tubes in!

Schematic is attached.

 

[rock soderstrom]

Without the tubes in, I'm seeing 550v on the 6v6 plate, which is rated for a max of 500v.

That's an paradox, how can you measure the anode voltage at the plate without a tube?

Take care of your filter capacitors if they can really only handle 450V.

With the tubes in, the voltage will go down a little, how much also depends on the transformer. I would give it a try, the filter capacitors, if they really are 450V types, worry me more...

Edit: You should also keep an eye on the maximum screen grid voltage...

Edit2: You are running quite high overall with your voltages, I would go a little off the throttle and maybe increase R17 and/or install a sag resistor in front of it.

 

[volker]

No, you must certainly won't drop 50V. Even if you were to run the 6V6 at full power (you shouldn't), the overall current draw of the amp might be around 30mA. There are three resistances you need to take into account: DCR of the power transformer, that 100R and the DCR of the output transformer.

You also need to make sure your capacitors can withstand that no load voltage, since the tubes at switch-on don't draw any current at first. Plus you're probably gonna blow up V2B before the 6V6. That 500V rating is only for JJ 6V6 by the way.

To increase the voltage for this kind of circuit this much is rather nonsensical, nothing of worth to be gained.

 

[abbey road d enfer]

Without the tubes in, I'm seeing 550v on the 6v6 plate, which is rated for a max of 500v.

That's in contradiction with the 275V rating of the transformer. 550V DC computes at 390VAC.

 

[rock soderstrom]

That's in contradiction with the 275V rating of the transformer. 550V DC computes at 390VAC.

according to schematic it is a transformer with 375VAC secondary, but this P-TF22772 is rated different, see attachment.

 

[dmp]

The PT in your schematic (P-TF22772) is 325V, not 375, 275, ect...
Unless the part number is wrong too.

Without the tubes in, the B+ rail will be higher than it will be with tubes in. It is pretty meaningless to test a tube circuit like this without the tubes in. Not only do you have no voltage drop across the resistors in the PI filters (R17 etc) but the filter caps charge up to the V_peak of the AC where they will sit closer to the V_rms under load.

You might want to bring it up on a variac.

 

[expat]

Thanks a lot everyone. Let me clarify a few things -

I'm seeing 550v at pin 3 of the 6v6 (I misspoke saying it was on the plate, its on the pin that goes to the plate).

The diodes I'm using are U8100E. My schematic program doesn't have those, so I substituted the 1n4007.

The T1 voltage was from a previous version of the schematic, my bad. It is indeed P-TF22772 (325-0-325) that I have installed.

I've tried to correct these on the attached revised schematic. Also, a shot of the guts for fun.

I just moved and can't find my darn variac, so I'm using a bulb limiter for now.

Also - Rock Soderstrom - I've got some 5 and 10ohm thermistors here that are high-voltage, was thinking to pop one in (after the fuse).

 

[rock soderstrom]

Thanks a lot everyone. Let me clarify a few things -

I'm seeing 550v at pin 3 of the 6v6 (I misspoke saying it was on the plate, its on the pin that goes to the plate).

The diodes I'm using are U8100E. My schematic program doesn't have those, so I substituted the 1n4007.

The T1 voltage was from a previous version of the schematic, my bad. It is indeed P-TF22772 (325-0-325) that I have installed.

I've tried to correct these on the attached revised schematic. Also, a shot of the guts for fun.

I just moved and can't find my darn variac, so I'm using a bulb limiter for now.

Also - Rock Soderstrom - I've got some 5 and 10ohm thermistors here that are high-voltage, was thinking to pop one in (after the fuse).

As it is at the moment, I would not switch it on, your electrolytic capacitors (as marked) are all latently at risk to explode, as they get plenty of over-voltage when switched on (and partially also during operation). This will not work, also because the 6V6GT (depending on the manufacturer) is operated in the red range.

You have to get rid of some voltage by using more series resistors (sag+ripple filter) or better, install a tube rectifier, which converts the excess voltage at its internal resistance into heat.

Another mains transformer would also be an alternative.

PS. A thermoresistor only helps to a limited extent...

 

[dmp]

The peak voltage from an AC signal is V_rms*sqrt(2)
The V_rms at the wall may be somewhat higher or lower than 120V, but if it were 120v exactly, the V_rms of the secondary would be 325v. The V_peak then would be 460v. That should be the highest voltage you could see in your circuit (and you'd want all your HV caps to be a safety factor above that). I'm not sure how you are seeing 550v in this situation.

 

[expat]

Thanks Rock S. i figured i'd try something different with the SS rectifier, but I'm really starting to lean towards a tube. This PT has a 5v tap and I've got some 5Y3GTs and 5U4's around here.

I'll put a thermistor in there for sure, primarily to slow down the voltage on startup.

dmp: Thanks man, I don't know what's up - I can recheck it if its safe - as soon as I saw that voltage on the DVM I shut it down. Missing my variac big time.

 

[rock soderstrom]

Thanks Rock S. i figured i'd try something different with the SS rectifier, but I'm really starting to lean towards a tube. This PT has a 5v tap and I've got some 5Y3GTs and 5U4's around here.

I'll put a thermistor in there for sure, primarily to slow down the voltage on startup.

That's what I would do! If you use a tube rectifier, you don't need a thermistor, as the tube ensures a soft start-up. Nice side effect

 

[expat]

Ah, ok, rockin'. I thought a thermistor was a good plan either way.

I guess its back to the drawing board with this PS - and back to drillin' the chassis! That's what prototypes (read: baking pans) are for!

 

[rock soderstrom]

That's what prototypes (read: baking pans) are for!

I did the same thing a few years ago. My first 6G15 type tube spring reverb was also mounted in a baking pan. They are not particularly easy to drill, but that was just what was quickly and cheaply available to me at the time.

 

[expat]

I've promised myself to buy a real chassis for it if it sounds cool
So, my old version of Circuit Maker doesn't have tube rectifiers, so i went full janky and Photoshop'd it!
I just pulled in the circuit from the Fender 5F2A Princeton that i built.
Will something like this work?

 

[rock soderstrom]

Will something like this work?

Yep, I think this could work but you do not want the "death cap" C15. Here is a schematic from the Vibro Champ with a very similiar mains transformer. It shows some voltages, you will have slightly less voltages due to the prefiltering with R17.

 

[expat]

Allrighty, thanks alot for the input.
Couple questions: doesn't this vibro champ have the 'death cap', too?
What's the difference, why this one instead of the 5F2A? Does it have to do with where it taps into the filtering caps? I do see a difference there.

 

[rock soderstrom]

Couple questions: doesn't this vibro champ have the 'death cap', too?

That's true and it's just as redundant (or even dangerous?) nowadays as in the Tweed Princeton or other amps from my point of view. I admit, I'm not that familiar with the US power grid, but as far as I know, it's not done that way anymore.

What's the difference, why this one instead of the 5F2A?

I only chose the Vibro Champ as an example because the mains transformer you use for your amp is sold as a drop in replacement transformer for that amp and the wiring diagram indicates voltages.

The differences are not that great compared to the 5F2A. The Vibro Champ got more filter capacity for less hum, the 5F2A got bigger serial resistors for the same reason.

 

[expat]

Got you, cool. Thanks a lot for pointing that out. I'm sort of glad to rip the SS out of my TUBE AMP!!! I'm not a purist, I just tend to do better when its one or the other.

 

[rock soderstrom]

I think it just fits better this way, your transformer is built for tube rectification. Personally, I like the sound of tube rectification better in exactly this application, but that's a matter of taste.

 

[expat]

All of my amps so far use tube rectification, so I figured i'd try something different. But not a huge thing. And tube sag is fine by me.

Now, I'm off to see my cousin Oliver Anthony play at the old "Grand Old Opry" in Nashville! Gotta step away from the soldering iron once in a while and hear some actual music being played.