Need any advice re: Switchable diode & tube rectificatio

[Noel Grassy]

I'm just not getting the big plate voltages on a SE EL-84 amp. It has a 5Y3 in it currently and I thought I could get a bit more voltage with a full wave diode array. I still like it's qualities using the tube recto but I don't want to buy another PT. I'm thinking Mesa's treatment would work here although I've never seen his schematic diagram of the switching circuit.

I'll keep looking through some of my fat EE tomes for this info in the meantime. None of which is newer than 1950. LOL.

BTW, the xfmrs I'm using are the stock iron from an old Harmony amp (Model # escapes me) that has 1 X 5Y3, 1 X EL-84, 1 X 12AX7 and a seven pin Tremelo tube that I disconected in order to increase VDC originally.
The little guy only develops approx. 225V at the power tube plate and a whopping 110V on the 12AX7. It's circuit is curiously very much like the Epi amp that's so popular to goose currently. I can't justify buying the MM xfmr party pack for an amp that I paid $20 bux for. The original 8" Jensen AlNiCo was shot when I got it so I made a 5/16" baffle that holds 2 X 8" 4 Ohm speakers now. The bottom end defies believe ability when just looking at the tiny cab. I now want the ferocity of a goosed power/pre-amp tube. Thank you in advance for any brilliant work-arounds,

Noel Grassy.

[Wayne]

Your PT will no doubt NOT handle a voltage doubler.

You'll gain some voltage (40 - 50V? Dunno. Never played with tube rectifiers) by going with solid state rectification. You could gain a bit more by wiring the now unused 5V windings in series with the primary. This will put your heater voltages up as well, but maybe they'll still be within spec. If they're not, a small (less than 1R) power resistor in each heater leg would fix that.

If you've got a separate heater transformer & room to add it, you could do the same thing with the 6.3V windings.

Or, if you've got a lower voltage tranny kicking around, you could connect it's primary to the wall and use it in series with the HV windngs & gain whatever the secondary voltage of the add-on transformer is.

As I discover more about the expense & hassle of ordering parts from away, waiting for them, and paying for them, I'm getting better at making the most of what I've got on the junkpile

W

[solderstain]

You'll gain some voltage (40 - 50V? Dunno. Never played with tube rectifiers) by going with solid state rectification.

I use the Mesa scheme to switch between SS and Tube rectifiers in my "Bassman Combo" (formerly a Pro Reverb). I don't have a dedicated rectifier selector switch; I use a DPDT center-off switch for my Standby switch. Up = solid state rectifier, Down = tube rectifier, Center = Standby.

Wayne is right - you'll gain voltage with the SS rectifier. How much of an increase you see depends on the tube rectifier being used. In my case, I use a GZ34 rectifier tube, and the difference in B+ is roughly 22vdc at the plate of the output tubes (with the higher voltage being the SS rectifier). That's as close as you'll get. Different rectifier tubes will give you a larger voltage difference. When I tried a 5U4 rectifier tube, the difference was greater. Meaning the tube rectifier voltage was lower.

If you really want to get the BIG VOLTS, there may be no substitute for replacing the PT.

Or just live with the little volts and experiment with the output tube bias resistor/capacitor to get the bias better with the volts you've got.

[Noel Grassy]

I use the Mesa scheme to switch between SS and Tube rectifiers in my "Bassman Combo" (formerly a Pro Reverb). I don't have a dedicated rectifier selector switch; I use a DPDT center-off switch for my Standby switch. Up = solid state rectifier, Down = tube rectifier, Center = Standby.
So, do you have a transferable copy of this method in the form of a schematic? I haven't seen how it's employed.


Wayne is right - you'll gain voltage with the SS rectifier. How much of an increase you see depends on the tube rectifier being used. In my case, I use a GZ34 rectifier tube, and the difference in B+ is roughly 22vdc at the plate of the output tubes (with the higher voltage being the SS rectifier). That's as close as you'll get. Different rectifier tubes will give you a larger voltage difference. When I tried a 5U4 rectifier tube, the difference was greater. Meaning the tube rectifier voltage was lower.
I indicated a 5Y3 as the rectumfrier and while in my library I found the factors of different forms of rectification. The SS diodes will yield secondary voltage times 1.4. Your right Wayne, almost fiddy volts. That's probably "just right" as Goldilox once said.

If you really want to get the BIG VOLTS, there may be no substitute for replacing the PT.

I also found an Xfmr in my box that formerly drove a small stereo amp with an EL84 pair. It was still mounted to this dagnasty chassis with a pair of OTs that look to be the same size as the Harmony's. So I might be building 2 of these pups for a cool scaled down stereo rig since I really like running my signal through the Lexicon nowadays.

Or just live with the little volts and experiment with the output tube bias resistor/capacitor to get the bias better with the volts you've got.

I have the cathode resistor about 60 Ohms below the recommended value and brought the tail out to the front panel on a 10 turn 100K 5 Watt pot. Sick minds think alike.
Thanks for your helpful observations,

Noel Grassy.

[Noel Grassy]

I screwed that $hit up!
I tried to "itemize" my responses to each quote and ended up becoming
part the quotes.
Vicious learning curve on this Interweb bidnez. You kids with your Hula Hoops and 8 track tapes...whyeyeoughta...

[solderstain]

After rereading my previous post, I can see that I worded it poorly.

The switching between SS and tube rectifiers is pretty brainless. I did it differently than Mesa does. Mesa switches the AC on PT secondary between the two rectifiers. I did the opposite - I have the AC going to both rectifiers all the time, and then I choose which B+ I want with the Standby switch.

The part of the Mesa scheme that I DID use on my Bassman Combo is how they accommodate the difference in bias current required, since the Mesa amps and my Bassman build both use fixed bias.

Since the SS rectifier and the tube rectifier produce different B+ (again, by ~22vdc in my case), one bias setting isn't really suitable for both rectifier options. If you set the bias at a nominal 70% for the tube rectifier option, it makes the SS position quite hot. And similarly, if you set the bias at a nominal 70% for the SS option, the tube rectifier position is on the cold side. The answer is to have a second resistor in the bias adjustment part of the circuit that gets switched in and out depending on which rectifier you've selected.

Mesa does that, and I adopted it for my Bassman build. I tried living with the bias setting being 70% for the SS rectifier and the attending cold-ness of the tube rectifier option, but it just left me cold (sorry for the bad pun). Since I adopted the second switchable resistor, both rectifier options are now biased up where I like them.

Your build doesn't use fixed bias, so I didn't elaborate on the "Mesa scheme" since it isn't really applicable to your build.

If you want to see it anyway, I can upload the Mesa Dual-Rec power supply drawing. But I don't think it will help your build.

[Firestorm]

Mesa's scheme was deliberately complex in an attempt to create something patentable. Just string your SS diodes in parallel with the tube, insert an SPST switch with the cold side connected to the first filter (same place as the output of the tube rectifier). When the diodes are engaged (switch closed), they'll essentially bypass the tube since they offer virtually no resistance. Depending on the rectifier tube, that should get you another 25+ volts of B+. I'm not sure how much higher you want to go since the EL84 tops out at 300 plate volts (and less than that in Class A, which is what you've got).

If you really want to go higher, you need to get a 7189A -- 440 plate volts -- but way expensive now. IMO, you could try a voltage doubler -- it shouldn't stress the PT since it "pays for" the doubled voltage by cutting the current in half.

But before I'd go to such lengths, I'd take a look at the dropping resistor that feeds the preamp. If it's anything like the schematic, it could be WAY smaller to get the preamp plate voltage up to reasonable levels. Then adjust the preamp plate and cathode resistors to maximize gain. I think you'll hear that much more than higher plate voltage on the output tube.

[Colossal]

I use a DPDT center-off switch for my Standby switch. Up = solid state rectifier, Down = tube rectifier, Center = Standby.

FWIW, I do the same thing on my 6V6 Plexi; works great. Each rectifier goes to its own filter cap and then onto the switch. On a Plexi, you get a more aggressive tone with SS rectification and higher plate voltage (think Van Halen) than with a tube rectifier (which feels/sounds more Zeppeliny/Old School).