My first build - AB763 Deluxe Reverb clone - opinions please

[Greg_L]

I would ground the 12AT7 reverb driver cathode, grid leak and RT secondary at the same filter cap that you are taking the reverb transformer primary from. The other ground returns for teh reverb recovery etc can be with the filter cap node that you are taking the reverb recovery stage from. YMMV

If I'm understanding Chuck H correctly, you're saying the same thing, and I think I'm going to try that. It would be less intrusive and less labor intensive to move one wire on the filter cap board than it would to reroute all of those other grounds.

[Greg_L]

Well friends...made some changes.

Grid stopper on V3Bb - this definitely smoothed the top end a bit with the reverb. Thanks pdf64.
Grounded reverb tranny at RCA jacks - this improved the kink in my waveform to being almost not even there. It's so slight now you have to really zoom way in on it. So great tip there, thank you Chuck.
Grounded filter nodes B, C, and D together at buss bar to input jacks pretty much as Fender spec'd it out - this yielded no significant change except the amp is actually even quieter...which I didn't think could be possible, so that's great.
Did some frequency sweeping while scoping the output. It was all over the map so I don't know what I learned from that, but it was neat to do. The waveform never got too odd anywhere, so I guess that's a good thing.

Tone test...haven't mic'd it up yet, but just playing it in the room sounds great. Very loud, very punchy, reverb and tremolo work beautifully, sounds like a typical Blackface to me...so all that's good.

I think I might just chill for a little while and play the snot out of it and check again later.

[tubeswell]

Can't remember what you said you'd done about the filter cap voltage ratings. Did you actually put 500V caps in after all?

[Greg_L]

Can't remember what you said you'd done about the filter cap voltage ratings. Did you actually put 500V caps in after all?

Yes. The A node got 22uf/500v caps.

[The Dude]

Firstly, I know we're talking about 2 different amps here and my apologies for butting into your thread. I'm only posting as it may help you figure out your distortion issue as i had the same symptom. Keep in mind that even though symptoms are similar, solutions may be quite different. All that said, I'm kicking the amp I'm working on out the door and here's what I found.

Scope told me the distortion on this amp started at the PI. The '68 reissue has a 1.5K NFB resistor- not sure why they changed it from the original 820. I installed an 820 and things got better, but still a tiny bit of distortion. This also took care of a motor boating issue. I lowered the bias to about 18mV at roughly 400V. That's where the amp sounded best and the ugly distortion was even a bit less, but still there. Stubborn thing. I then stuck a slightly lower gain 12AT7 in the PI position and distortion gone.

[Greg_L]

Firstly, I know we're talking about 2 different amps here and my apologies for butting into your thread. I'm only posting as it may help you figure out your distortion issue as i had the same symptom. Keep in mind that even though symptoms are similar, solutions may be quite different. All that said, I'm kicking the amp I'm working on out the door and here's what I found.

Scope told me the distortion on this amp started at the PI. The '68 reissue has a 1.5K NFB resistor- not sure why they changed it from the original 820. I installed an 820 and things got better, but still a tiny bit of distortion. This also took care of a motor boating issue. I lowered the bias to about 18mV at roughly 400V. That's where the amp sounded best and the ugly distortion was even a bit less, but still there. Stubborn thing. I then stuck a slightly lower gain 12AT7 in the PI position and distortion gone.

Thanks for the update.

I moved my negative feedback wire to the 8 ohm tap on my OT and it seems to be better in the room, but I have yet to try it mic'd up - which is where the issue revealed itself. I had to go out of of town, but I'm back now and will test it tomorrow.

[The Dude]

I'm surprised you can't hear it, but the mic can. Don't discount the idea that the mic or the mic preamp might be adding the clipping sound and it has nothing to do with the amp.

[Greg_L]

I'm surprised you can't hear it, but the mic can. Don't discount the idea that the mic or the mic preamp might be adding the clipping sound and it has nothing to do with the amp.

Yeah, I've already verified that. Different mics, channels, preamps, speakers, cabs...all of it. The sound is the same.

The sound is very very subtle. You can't hear it in the room, and it hides itself in a full mix. I didn't notice it until I solo'd a recorded track during a mix and I was "wtf is that"? Ever since then I've been a on a mission. Lol.

I'm really hopeful that getting the negative feedback right has fixed it.

[Greg_L]

Well the negative feedback tweak didn't fix my brittle sound. Fuuuuuuudge. It's still there.

What now? I'm thinking maybe upping the power tube grid resistors?

I'm using stock value 1.5k right now. Would bumping them up to 10k...50k...help maybe?


The brittle sound goes away if I roll the vol back on the guitar, or turn the amp down. When I dig in or hit it hard, that's when the brittle sound happens. It's not the guitar - it does it with any guitar. Any speaker, any cab. I really think it's something power tube/phase inverter related. When the amp is really clean, it doesn't happen. It only happens when it's breaking up or pushed hard - either channel.

[tubeswell]

Did you fit 220pF grid-to-cathode on each 6V6 socket?

[tubeswell]

Another idea is to scope the LTP duty-cycle when the amp is under high-signal conditions. LTP’s like to be centre-biased if you’re going to run the amp real hard all the time.

[Greg_L]

Did you fit 220pF grid-to-cathode on each 6V6 socket?

No. I can try that. Do I just span a cap from pin 5 to pin 8?

Another idea is to scope the LTP duty-cycle when the amp is under high-signal conditions. LTP’s like to be centre-biased if you’re going to run the amp real hard all the time.

No idea what this means.

[tubeswell]

No. I can try that. Do I just span a cap from pin 5 to pin 8?(

Yep.

Re: the LTP, I’m in the field on my iPhone now, and I’ll give a full explanation when I get back to my work-station.

[Greg_L]

Yep.

Re: the LTP, I’m in the field on my iPhone now, and I’ll give a full explanation when I get back to my work-station.

That would be awesome, thanks.

One thing I have not done is scoped it while playing a guitar.

The more I read about "blocking distortion" the more I think that's what I have going on. What I've read describes symptoms similar to mine.

On the plus side it seems a simple and common fix is to just up the value of the grid stoppers, and I'm currently using damn near the minimum.

[Justin Thomas]

The '68 reissue has a 1.5K NFB resistor...

Slight derailing here... the "Custom 68" series are NOT reissues. Hence all the changes. These are not intended to be as spot- on as possible Like the 65 series are. Sure, they look the part, but they're not supposed to behave like a real 68 Fender. Otherwise, they'd have that "mixed bias" that supposedly everyone hates, etc.

I know it's a small point, but I feel it's worth making...

Carry on.

Justin