Fender Red Knob Twin w/GZ34

[Dagan8]

Hello all. This is my first post.

I just recently finished building my 1st amplifier. A JTM45 type pre on a EL84 Marshall 18watt power. I loved the experience of building this amp. The reason I built the amp was because I wasn't too happy with my Fender Red Knob Twin amp. I find that this Fender is just lacking "feeling". Don't know how else to explain it.

I started to play around with the Fender today to see if I can make some improvements to the tone. I Started by removing the 2 inner power tubes effectively cutting the power in half. I also swapped the 12AT7 PI with a 12AX7. I am definitely heading in the right direction, but It is still missing something.

Before I resort to gutting the amp I would really like to try one last thing. I would really like to squeeze a GZ34 rectifier into the amp and remove the diode rectifier. Can this be done??

I have 2 extra 8 pin sockets since I removed 2 power tubes. I will also have to add a 5v Transformer for the GZ34's heater. I quickly cut up the schematic and added the GZ34 but I'm not sure if it will work with this amps power scheme. Here is what I have:

MODIFIED:


ORIGINAL


I appreciate any advice.

BR
Seth

[Chuck H]

No, you cant simply add the tube rectifier like that. The original diode rectifier is a "bridge" type and generates a lot more voltage than a "full wave" type that employs a tube rectifier. Your voltage would drop almost in half. To use a tube rectifier in that amp you would need either two rectifier tubes to make a bridge or a different PT that would generate the correct voltage with a full wave type rectifier. FWIW there are other elements to your design that would actually make it dangerous to operate if you wired it that way.

JMHO but the attack envelope and power supply sag of an amp is not at all a make or break tone quality for me. Many things add to the feel of an amp. I don't think increasing sag, and that's all a rectifier tube does, could change an amp from a loser to a winner.

Chuck

[Bruce / Mission Amps]

Not too tough...

Here's a simple mod if you have the tube, socket, 5v@3a tranny and a little room...
It would be even easier to add a 25-50 watt, 150 ohm resistor into the B+ rail for sag.

[Chuck H]

Bonus Bruce... You know, I thought about it after posting too but I'm just now getting back here.

Just to be sure you understand, Seth, each half of the rectifier tube would replace one of the foreward two diodes in the existing bridge arrangement. The way your original idea is drawn up I don't think you know enough about power supplies to experiment with circuits in this dangerously high voltage area of the amp. So I'm glad you asked here before trying it.

Chuck

[Dagan8]

Thanks Chuck & Bruce. This is exactly why I asked.

I was a little leery because I am used to seeing the type of power supply that has the CT of the high voltage winding grounded. This one looks different due to low/high power switching. I'm sure I would have asked either way. I can see my big mistake which makes this dangerous. I connected the HV winding directly to ground at point "D". Oops

So one GZ34 only takes the place of a 2 diode arrangement. I see how Bruce is showing a hybrid tube/ss power supply.

I think I might experiment with the large resistor to ground. This would be an easy way to see if i like the added sag.

I can see now that 2 GZ34's would be needed to go to full tube rectification, but I 'm guessing that 2 GZ34's would be fairly stiff anyway. If I had a higher high voltage winding on my PT I guess it wouldn't be to difficult just to make a new power supply section using the scheme I'm used to seeing with the CT grounded and the single GZ34. Kind of like a 5F6A supply. I would have to omit the high/low power switch, but I never use it anyway.

With the proper dropping resistors and caps would my B+ be way too low if I totally rebuilt the power supply in this fashion using the current PT??

[Dagan8]

OK. SO in reality the are 3 ways I can do this.

1. Add the 150ohm 30w to ground.
2. Hybrid SS/Tube
3. Use 2 Tubes

I quickly mocked up the mods in paint. I didn't include the 2 caps in the hybrid design. Not sure if these are necessary or if you where just using it to represent the rest of the circuit.

The dual rectifier design is probably flawed. Seems way too easy. I will do a lot more research if I decide to go this route. I do have 2 spare sockets available so it is an option. Would be cool.

SAG RESISTOR:



SS/HS Hybrid:


Dual GZ34:

[Steve Conner]

Nuh-uh, you can't make a tube bridge rectifier out of two GZ34s.

The reason is that the GZ34 (and all the other popular rectifier tubes) have two diodes with a common cathode. That does fine for the positive half of the bridge, but for the other half, you need a common anode and two separate cathodes.

So, you need to use two separate rectifier tubes for the negative half, TV damper diodes or similar, each with its own isolated filament winding. Much easier to just use two silicon diodes as Bruce suggested.

Easier still to use the sag resistor, I challenge anyone to hear the difference in a properly conducted test.

[Chuck H]

Nuh-uh, you can't make a tube bridge rectifier out of two GZ34s.

The reason is that the GZ34 (and all the other popular rectifier tubes) have two diodes with a common cathode. That does fine for the positive half of the bridge, but for the other half, you need a common anode and two separate cathodes.

Hey... your right. I have been dupped by the Duncan PSUD2 that allows for a two 5AR4 bridge rectifier. My bad.

Easier still to use the sag resistor, I challenge anyone to hear the difference in a properly conducted test.

+1 I never design with rectifier tubes anymore because "I" can't hear a difference. I prefer the easier to implement, less expensive and more permanent "sag resistor".

Chuck

[Chuck H]

Steve, does this make any sense?

[Dagan8]

Thanks for all your help guys. Ya. I was sure the 2 tubes as a full wave rectifier was just too easy to be true.

I will use the resistor to create the sag. I found a 30W 150omh with a heatsink. Like you said Chuck I don't think the effort of the hybrid SS/Tube set up will be worth the difference if any.

I will be ordering new caps for a cap job and I will add the resistor when I open the amp up for that. I might even make the sag resistor switchable.

Can you please verify the circuit that shows the resistor in my previous post.

Thanks again

[Steve Conner]

Chuck H: Neither of your two right-hand sketches will work, because the filament connections will short out the HV supply. There's no way (I know of) to do it without three isolated filament windings. That's one reason why tube bridge rectifiers were unpopular.

I guess theoretically it could be done using diodes with separate filament and cathode, but I've never seen any whose heater-cathode insulation could stand the voltages in a 50 watt amp.

Dagan8: Your circuit with the sag resistor looks OK, as far as I can see.

[Chuck H]

Ok. It looked funny to me too. But since it was on the Duncan program, you know, I figured there must be something I was missing. That's what I get for learning without knowing. Thanks

Seth, your sag resistor can be placed on the grounded end of a bridge rectifier (where you have it), on the CTR of a full wave type or inall cases in line after the rectifier in the B+ rail. For simplicity with your circuit where you have it is best.

Chuck

[Dagan8]

Thanks again guys. Sag resistor it is!