New build 2nd gen hybrid reverb with high plates on v1

[Guy77]

This looks great Erwin! Your attention to detail is A+.
I noticed on your main Preamp PCB board it looks pretty much like a regular eyelet board except I see a fairly long trace running along the board that connects 3 eyelets. What is this used for?
Cheers

Guy

[Aaron]

I see a fairly long trace running along the board that connects 3 eyelets. What is this used for?
Cheers

Guy

Looks like an earth for V1, V2 & trimmer.

Aaron

[talbany]

10k VS 5k. I agree 10k has a wider useable sweet spot: my thoughts werd merely on the differences between the 12ax7 pi 100-110k (10% difference) and the 12at7 pi 47k-51k (almost 10% difference). Putting a balance pot on top of this with the 10% difference. Anyway the 12at7 has pi resistor values that are closer together, hence my thoughts on the 5k pot.

I gotcha. My thoughts are the AT will pull more current (than AX will)as well as a smaller voltage drop across the lower value plate resistors so everything should be lower vs an AX w/110/100k plate values. Ultimately what your looking for is swing so the higher value pot will give you this regardless of what tube or plate resistor values your using. My notes say for a 12AX7 and 10k pot I have .25W should be sufficient. Still most of these larger trimmers (like the one you have I believe)are rated around 1 watt and the smaller ones .5W
Overall the only downside to the higher value trimmer is you will lose a few more volts in the process but shouldn't really be enough to affect the performance of the PI . You will probably see more of a voltage drop just switching from AT to AT.
I find the 10k generally gives me between 7-10 volts of swing vs a 5K which gives me 4-6 volts swing which more than likely lets me open my drawer full of old stock tubes and be able to find one that works vs calling Mojo or someone and have them send me one that is supposed to be matched only to find it wasn't matched or noisy or sounds awful send it back and have them send me another a week later Your mileage and patience may of course very

I have 10k's on all my abused builds and have never had any issues with them at all going on 10 years and at least 500 shows with my 2nd gen

Tony

[erwin_ve]

This looks great Erwin! Your attention to detail is A+.
I noticed on your main Preamp PCB board it looks pretty much like a regular eyelet board except I see a fairly long trace running along the board that connects 3 eyelets. What is this used for?
Cheers

Guy

Thanks, still seeing things in my builds that are subject to change. Im nowhere compared to the big boys here. And my math skills suck

As Aaron said above: grounding points for cathodes v1, v2 and trimmer to ground. They all go by "good old fashioned wire" to the grounding point near the input of the amp.

[Guy77]

Thanks Aaron and thanks Erwin! These boards look like they could be an interesting way to save assembly time.

Cheers

Guy

[martin manning]

10k VS 5k. I agree 10k has a wider useable sweet spot: my thoughts werd merely on the differences between the 12ax7 pi 100-110k (10% difference) and the 12at7 pi 47k-51k (almost 10% difference). Putting a balance pot on top of this with the 10% difference. Anyway the 12at7 has pi resistor values that are closer together, hence my thoughts on the 5k pot.

I gotcha. My thoughts are the AT will pull more current (than AX will)as well as a smaller voltage drop across the lower value plate resistors so everything should be lower vs an AX w/110/100k plate values. Ultimately what your looking for is swing so the higher value pot will give you this regardless of what tube or plate resistor values your using. My notes say for a 12AX7 and 10k pot I have .25W should be sufficient. Still most of these larger trimmers (like the one you have I believe)are rated around 1 watt and the smaller ones .5W
Overall the only downside to the higher value trimmer is you will lose a few more volts in the process but shouldn't really be enough to affect the performance of the PI . You will probably see more of a voltage drop just switching from AT to AT.
I find the 10k generally gives me between 7-10 volts of swing vs a 5K which gives me 4-6 volts swing which more than likely lets me open my drawer full of old stock tubes and be able to find one that works vs calling Mojo or someone and have them send me one that is supposed to be matched only to find it wasn't matched or noisy or sounds awful send it back and have them send me another a week later Your mileage and patience may of course very

I have 10k's on all my abused builds and have never had any issues with them at all going on 10 years and at least 500 shows with my 2nd gen

Tony

Wider sweet spot with 10k? Seems like a 5k would have 2x the rotation angle for a given shift of the load resistance side-to-side. For adjustment range, I think 5k is fine considering the lower value plate loads (edit: 47k/51k). A 5k trimmer should provide approximately the same percentage change as a 10k with ~100k Ra's. The 15mm Piher trimmers (and most others) are 1/4W, which should be fine for a 12AT7. Some Cermet trimmers are 1/2W.

[erwin_ve]

Wider sweet spot with 10k? Seems like a 5k would have 2x the rotation angle for a given shift of the load resistance side-to-side.

In the context of discussion with Tony I ment : wider sweet spot base to use different 12AT7. That
wasnt clear, sorry.

[martin manning]

Ah I see, no worries. 5k should provide about 7VDC swing plate 1 to plate 2, and after some more study, 47k and 51k looks like it will come close to balancing the AC at the output with the trimmer centered (assuming perfectly matched triodes, and 1 kHz, 22V RMS signal at the PI output).

[stelligan]

Very nice looking boards.

[erwin_ve]

Little progress: All boards populated and also made the filter and transistor driver/recover for the reverb.

Still puzzling were to put the reverb pot on the chassis. Any suggestions welcome. The faceplate and drilling the hole for the reverb pot is done after that decision.
On front is nice but no room for ampname etc
On back between fuse and iec inlet is possible but noise issues might be a problem there: Anyone experiences on that part?

[pompeiisneaks]

Looks grand!

Sadly I've never had much success with reverb, I've added it on one amp so far and it was massively noisy . I ended up gutting that amp later and building the Dual Marshall in the same chassis.

~Phil

[erwin_ve]

Looks grand!

Sadly I've never had much success with reverb, I've added it on one amp so far and it was massively noisy . I ended up gutting that amp later and building the Dual Marshall in the same chassis.

~Phil

What kind of reverb driver and recovery circuit did you use Phil? Did you find out what caused the noise?

[pompeiisneaks]

I did a one tube reverb on a Vox AC100/2. The reverb was always weak and added noise and I had a hard time positioning the tank so it didn't have massive hum, and before I could figure it out, the amp itself died. I never got around to figuring out why, and it was my second build, so I gutted it and rebuilt the marshall dual in it. long story . I now know the major problem with the amp was the PT having an intermittent short to heaters. I replaced that damn piece of crap.

~Phil

[erwin_ve]

Thanks Phil, your experiences on the reverb are exactly what I fear.
The circuit I use is borrowed from a peavey deuce. It was fairly quite before gutting it.
My plan is to take the signal after the cathode follower of the built in dlator and use a 10k mix resistor.
I will post the schematic soon.

[amplifiednation]

Put the jacks and transformer under the preamp board on the chassis belly.