My Latest Build

[JazzGuitarGimp]

10thTx,

Yup, DaGeezer's amp sounds great! I hope mine sound that good. Do you know if was playing a single coil or humbucker guitar?

I am hoping I can manage the levels betwwen Clean and OD with just two triodes in the Clean preamp. Until I actually hear it, I don't know if the Y-Mode sound will be usefull. But if it is, I need to keep the two channels in-phase - unless I add two active loops, and then the two preamps could go into opposite inputs of the PI, which would keep the channels in phase. But I have a Zinky Tonemaster (for many years) and I can tell you from experience two effects loops is not a plus....

I have placed a Level control at the output of each preamp, and I have relabeled the EFX Receive control to Master Volume, so I should be covered..... Hopefully.

Cheers,
Lou

[jcsifu]

Jcsifu, i have a friend who has a two rock modified black face bandmaster head. It has been dumblized, and I agree - it's a total compromise between getting a great clean tone, and a great OD tone. Keep n mind, and I should have made this clear when I posted the schematic, that this is an unproven design at this point. But as I massage the amp into maturity, I will update the schematic.

Ok, noted and thanks. Looking forward to the finished schem and pics....and hopefully clips

[tictac]

Just curious, is this supposed to be a Dumble type amp? I ask because that 42K tail resistor and a 50kish feedback resistor is going to take you into Vox-tone territory...

Maybe that was a typo... 42k vs 24k.....


my .02

TT

[JazzGuitarGimp]

Hi TT,

I actually am not sure where this came from. I've been tweaking the schematic for a few years. It's possible it came from the original weber DRx2 schematic. I know that these values will likely need to be adjusted, since I am running 6V6's. I expect the bias to be in the -30-ish volt range, and I suspect the gain of the PI will need to be adjusted. Once I get Phase I built, I will check the gain of the preamp, the preamp's maximum p-p swing, the gain of the PI, the PI's max p-p swing, and compare these to the bias on the 6V6's (or 5881's, if I decide to stay with these). I will also keep in mind that I will be trying AB2 for the output stage, and make sure I have enough swing from the PI to drive the output grids to say, +25-ish volts. I will adjust the PI resistor values as needed.

This is definitely a work in progress...

Cheers,
Lou

[jcsifu]

Hi TT,
Once I get Phase I built, I will check the gain of the preamp, the preamp's maximum p-p swing, the gain of the PI, the PI's max p-p swing, and compare these to the bias on the 6V6's (or 5881's, if I decide to stay with these).

This is the approach I almost always think is the surest way to go when combining irregular stages, get it working as drawn and then swap items out one by one.

[JazzGuitarGimp]

Some of the parts came today, including the resistors I needed for the B+ string. So I got that wired today:

[JazzGuitarGimp]

Starting to get the board populated.

[xtian]

Starting to get the board populated.

My favorite part.

[JazzGuitarGimp]

Hi All,

A little more progress on the board. Initially, my plan was to build out the board with just the PI and Clean Preamp circuits (previous photo) - do some testing and some playing, then pull the board and add-in the Loop circuit, then pull it one more time to complete the board with the OD Preamp. I bought some resistors to load the B+ rail on the missing OD and Loop nodes so that the voltages would be correct in their absence.

Then..... I started adding up the number of wires to the board, and when I realized I'd have to unsolder 12 wires (!) just at the PI, I decided I should probably build-out the rest of the board before it goes into the chassis.

This photo has the Clean Pre, the EXF Loop, and the PI circuits loaded and ready for solder. I'll get the OD Preamp circuit loaded onto the board today (hopefully).

After having made several purchases for parts, I am finding I have still overlooked a few things here and there. Originally, I was going to build out the whole amp with 1% metal film resistors. I seem to have a thing for being precise. After cracking open Merlin's Preamp book again, I've decided 5% carbon film resistors is probably the better choice in terms of tone. But I am missing a few values, and after having made six purchases in the last week, I'm done with buying parts for a while. So I will use metal film for some of the resistors and hope for the best.

My initial thought on the board is, after populating all the components, I am going to solder every eyelet that has component leads. There will be no connections made from the bottom of the board. So when I get it mounted into the chassis, I will strip and tin the ends of the wires that connect to the board, then reheat each eyelet that gets a wire and push the wire in, keeping heat on it so that the wire is heated sufficiently to make the connection. Anyone want to talk me out of this approach?

Cheers,
Lou

[JazzGuitarGimp]

Hi All,

So I've had a change of heart - in keeping with the old adage "it's not worth doing if it's not worth doing right", I went ahead and placed another order with Mouser for big quantities of Xicon 1/2W 5%, 1W 5%, and assorted brands of 2W 5% carbon film resistors. There won't be a metal film resistor in sight (on the board) when the build is done. Project is on hold until the order comes in - should be mid-next week.

In the meantime, if anyone wants to advise against soldering all the eyelets before the board goes in, I'm all ears

Thanks for watching.

Cheers,
Lou

[JazzGuitarGimp]

Board is populated and the PI / power amp is working. I'm getting 30 clean watts from the quad of Chinese 6V6's that came with the Weber kit. They aren't well matched. When I biased the output stage, the tube I chose to measure while adjusting just happened to be the strongest one. I biased it at 21mA, then I went back and measured the rest of the quartet. The others came in at 15mA, 17mA, and 18mA.

My B+ is about 14V lower than expected at 372V vs 386V - well within Fender's +/-20% tolerance. I currently have a JJ GZ34 of unknown condition in the amp, so maybe it is contributing to the low B+. But for 6V6's, I think it's fine where it is.

Thinking about tictac's post, I decided to change the PI resistors. After looking at several schematics, I chose values that are close to the 6G12-A to start with. I have 1K at the cathodes, followed by 24K tail, followed by a 5KB presence pot.

I had a heck of a time getting sound out of it last night. I could hear the OT singing. I finally discovered that I had mangled the impedance switch - it's such a tight build, that I had to bend all of its terminals at a right angle, so that they point straight back from the switch and in doing so, the common terminal loosened to the point where it was no longer making contact with the rotor. So the common wire is hardwired to the 8R tap until I can replace the switch.

Next up, I am going to wire the clean preamp and hardwire it into the PI for the next phase of building / testing.

[JazzGuitarGimp]

I do have a question, if someone can help me out. It's my understanding that when a CF is used, it is customary to elevate the heaters to about +80V. I will have a built-in d-style effects loop - it has not been wired yet. But I just had a look at the stand-alone loop layout in the files section (top post, here: https://tubeamparchive.com/viewtopic.php?t=12876), and it references the heater winding to ground. Can I get away with leaving my heater string CT at ground, or should I elevate?

Many thanks,
Lou

[JazzGuitarGimp]

I may have figured it out. I am thinking that since the CF is not DC-coupled to the plate of the preceding stage, Vgk (correction; Vhk - thanks, Martin) is not stressed to the point of needing elevation.

And yes, I am the guy who had the recent meltdown and flew off the handle. But I assure you, my bark is much worse than my bite...

[martin manning]

Most 12AX7's are spec'd at 100V max Vh-k. The CF in a D'lator only runs the cathode at about 30V, so it's fine. It's those Fender and Marshall CF's with the 100k Rk running 180V Vk or more that really need it.

[JazzGuitarGimp]

Thanks, Martin!