Hammond AO-35 to EL84 plexi or 6V6 plexi?

[Hellawatt]

Gurus, I have a question. I have a Hammond AO-35. This amp had a 5Y3, 2 12AX7s and 2 EL84s. Last year I did a Carmen Ghia conversion on one of these, and I love it. Now I want to try something a little different, like maybe a plexi or jcm800 preamp.

Can I use these transformers to build a silicon rectified amp with 2 EL84s or 6V6s and 3 12AX7s? If I remove the rectifier tube socket to make room for another preamp tube, how do I know whether the power transformer can handle it? Would I add up the heater currents of all the tubes to see if it's more? How much more is too much? What about B+ voltage? Would it be less because of the additional preamp tube? How much? I'm looking at the tube data sheets, but I don't know enough to do the math.

[John]

If you're going from tube rectifier (5Y3) to silicone, you'll have higher B+, maybe as much as 50 volts higher. Maybe not. Adding 1 preamp tube is nothing I personally would worry about. The only real current draw would be the additional heater. IIRC, the 6V6 actually pulls less heater current than the EL84. Don't quote me though, till I go look it up.    But in short, I would not worry about adding the extra 12ax7. The only worry will be the higher B+ without that rectifier tube. And again, that might be no big deal at all in the end.

[shooter]

My notes show 6V6 at .45A, 84s at .6A and AX7 at .3, so if you go 6V6 you save enough to add an AX7, but like John said, you can probably use either.

[dude]

I'd think about the differences in tone between EL84's and 6V6's as the most important decision. Also, that chassis is very small with a vertical bd, you'd have to drill sockets bigger for 6V6's, or at least one if you use a diode rectifier. If you decide on 6V6's making one socket bigger and adding a third 12AX7, make sure you have the room. I put an 18 watt plexi lite with a full tone stack in my AO-35 and that was tight, also a VVR. Very tight and was difficult to build but can be done.


al

[Hellawatt]

I'd think about the differences in tone between EL84's and 6V6's as the most important decision. Also, that chassis is very small with a vertical bd, you'd have to drill sockets bigger for 6V6's, or at least one if you use a diode rectifier. If you decide on 6V6's making one socket bigger and adding a third 12AX7, make sure you have the room. I put an 18 watt plexi lite with a full tone stack in my AO-35 and that was tight, also a VVR. Very tight and was difficult to build but can be done.


al

To be honest, I don't really know what the tone difference is between EL84 and 6v6. Im leaning towards 6v6 though, since I already have one ao-35 conversion with EL84s.

I know it will be very tight. I'm thinking I will put the OT on standoffs to make room for a horizontal turret board, , and maybe do the same to the PT if I have to make more room. I'm not sure where the choke will go. What's a VVR?

[Ed_Chambley]

A vvr is a variable voltage control. A small circiut that usually hs ontrol over the power tube voltages. Like a master volume you can increase preamp with volume control and lower the output tubes power. Easier to do on cathode bias and lower wattage since the current is bled off to a heat sink mosfet. With K bias no bias tracking is required. If fixed bias the vvr must track the bias. Not hard either, just more parts.


It is also called power scaling.


EL84 tubes are easier to distort, especially if you use low voltages. They also tend to sound bright and harsh some tweaking is involved. A lot of 6v6 is easy to work with, carries better bass response and sound similar to a 6L6, but have longer transition into distortion. Easier to play on the edge of breakup.


EL84 also will have a glassier tone when raking strings and also ghost notes ring out a little more. Tweed amps used 6v6, but it is the Fender Blackface Deluxe Reverb i consider to be thee 6v6 defining tone and the Vox ac15 to be the defining tone for EL84.


You can use another couple of preamp tubes if you want. All the old Hammond iron is tough and traditionally nothing in an organ should sag due to lack of current.


Go ahead and build it how you planned. The pt will be fine.

[dude]

I'd recommend getting rid of the AO-35's OT, it's harsh to my ears. Most 18 watt stand-up OTs would work well, especially if you use EL84s. Make sure you mount it perpendicular to the PT. and try to give the greatest distance between the two.


Actually keeping the stand up bd is best, will leave more room for sockets. And the stand-up board's turrets are placed well for an 18 watt 6V6.


Al