5C2 question

[armillary]

I thought I'd start by trying to preserve the original schematic of each preamp tube for comparison. The 6SJ7 may suffer (or benefit) from the tone control. But I hadn't thought about putting them both on grid leak or cathode bias. Good idea.

[armillary]

Progress report on the Princeton/Champ. I wired it up this weekend. I wish the layout was prettier, but I can correct that later. In the picture the 6SJ7 is at the left end with the 6SC7 next door.

It worked first try, but I've been getting a bit of rattle/ripped speaker sound at high volume. My other plain Princeton clone has the same problem. I've swapped speakers and it's not the speaker. I increased the preamp's plate resistors to 330K to lower the plate voltage which helps reduce the rattle/rip and definitely increases the bass response, but it didn't completely eliminate the problem.

So I decided to try a grid stopper resistor on the 6V6. I used a 100K pot as a variable grid stopper resistor. It doesn't reduce the volume one bit, but it has eliminated the rattle problem and changes the tone in many interesting ways. Especially the bass. I haven't measured, but I think the sweet spot is at about at 10K ohms. So I'm sold on grid stopper resistors now!

Still more work to do, but there's progress.

[rp]

Rip and rattle? It's so simple I can't imagine you built it wrong or it's the layout. Rip if fuzzy distortion sounds like the circuit but if it's more a rough paper tearing sound with a component of rattle then sounds mechanical to me, maybe a socket pin, reflow the solder joints on each socket one at a time, or if it's on both just the rec and 6V6. I chased a raspy rattle (through the speaker) in a champ clone for 3 days - was an untensioned pin on the power tube socket. I had bought the cheapest sockets from AES and had pulled the valve like 30 times. Adding the resistor might just have changed the rattle frequency or the gain or something and but it's actually mechanical.

Which valve you like better?

Try hitting that grid leak side with a high gain boost pedal if you want a unique sound.

[armillary]

I think the problem might be my 380VDC CT PT being a bit too hot for the tubes. The 6V6 plate voltage is 374. The 6SC7 plates are 182/192.

I don't want to judge the preamp tubes until I have the sound worked out. It's like you said, they're very different.

[rp]

I think the problem might be my 380VDC CT PT being a bit too hot for the tubes. The 6V6 plate voltage is 374. The 6SC7 plates are 182/192.

That can't be it. I don't think Champs sound as good after 370V but they work just fine and certainly don't rip and rattle. I've seen SFs with 430V on them.

[armillary]

What's really odd about the grid stopper pot is that it eliminates the rip/rattle even when it's set CCW to zero ohms resistance. So whatever the rip rattle issue was it didn't take much of a grid stopper to get rid of it.

I've had misgivings about those stupid curly pink wires I used for the spring loaded socket. Maybe I'll dress them better and see if that solves the problem without needing a grid stopper.

[armillary]

An update on the Princeton/Champ and the Princeton 5C2 clone. A 1K grid stopper resistor on the 6V6 eliminated the rip/rattle at high volume on both projects. I didn't redress the wiring but will when they find cabinets to live in.

Both projects sound good now. Each preamp tube in the Princeton/Champ sounds different. I like the 6SC7 because it doesn't require a pedal to get that overdriven sound. I need the right overdrive pedal to give the 6SJ7 a fair test though. Suggestions?

rp you are so right about the muddy bass TF103 OT on the 5C2. That thing has got to go. The bass from the Hammond 1750C in the Princeton/Champ is crisp and Fendery. So I ordered a Hammond 1760C from Mouser. I'd have ordered the Allen, but Mouser takes credit card orders late on Friday night when I do my most inspired shopping.

Neither project is using a choke at the moment. I moved the 5H choke before the first filter cap on the 5C2 (a choke input filter?) and the B+ voltage dropped by 80. The 6V6 plate went from 360 to 270. Surprisingly the result was a louder amp! The choke wasn't hot, but I wonder if it was designed for that much current. Both amps are quiet without chokes.

So still tweaking, but no more technical problems. Thanks again for your help.

[pops]

The 5H choke should be fine if it can handle the current of the whole amp, power tubes included. This is the best way to lower voltage in an amp. I have used this many times.

[armillary]

Thanks pops. The technical tidbit I learned from this is that a choke placed after the first filter capacitor reduces rectifier ripple with a surprisingly low drop in B+ voltage, and a choke placed before the first filter capacitor reduces ripple with a surprisingly high drop in B+ voltage. I'll remember this.