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Discussion in 'Amp Input - Normal or Bright' started by GunMonkeyINTL, Dec 19, 2017.
Fantastic build. Those wiring pictures are borderline NSFW...
GAH!!! A rat’s nest.
Because of my chassis design, I decided to install all the leads and components on the chassis-mounted components before the board went in, while I had more room to move around.
Board in, ready for the heart transplant.
@GunMonkeyINTL it is a joy to watch.
Your attention to detail is really top of the line.
Life is good,
GunMonkey, I wish I had a ounce of knowledge to build something like this. Amazing and a beautiful job you are doing. Can't wait to see it when it's finished. Great job my man.
Truthfully, I have almost no idea what I’m doing.
That’s kinda the point, though. I want to learn to repair and modify amps, and it seems to me something that I’ll only be able to wrap my head around in stages: fix-learn, build-learn, fix-more-learn, rinse and repeat.
My first project was to bring an old kid’s amp up to modern safety standards, and I’ve since built a couple pedals from PCBs, but I wanted to actually understand what I was doing- still don’t, but am working on that.
I got tired of reading/listening to people who understand amps, and trying to make sense of the gibberish they were talking, while trying to figure out how to do the little trivial thing I was trying to accomplish. I figure that, if I learn at least a little about how they work, I’ll be able to fix my own stuff, and buy old-used with confidence, without having to ask someone for help.
You could totally build an amp without understanding the first thing about how they work. There are enough options out there to paint one “by numbers” as long as you stick to the directions- without having to spend months sketching things out and reading/asking questions to figure out your own drawings.
....oh, and find a guy like @Robins to help flatten out the learning curve a little.
Got the board in and the rat’s nest tamed a little. Just the effects loop left to go, but had to put it off until tomorrow.
I did plug it in and do some more testing. The b+ appears to be showing up in the right places (and nowhere it’s not supposed to be).
Unloaded, with the SS rectifier, I’m getting 458vdc, and, with the 5y3, 440vdc.
I needed to take a measurement with all the tubes in place so I could find the right power supply resistor for the effects loop, so, I did what any eager young kid would do:
I closed up the cut in the signal path for the effects loop, and plugged a guitar in.
It works in both recitifer schemes, with the tube rectified seeming a little cooler. I was able to get clean and overdriven sounds out of it. I didn’t push it because it got kinda loud quickly, and I was only plugged into a single, crappy 10” 16ohm speaker- but there’s definitely nothing off in the sound, and the controls all act like they should.
So, while I had it in working condition, I decided to take measurements off the power tubes, to see where I’m at before I go any further. Here’s what I got:
Cathode Voltage- 10.2
Cathode Resistor- 120r
Cathode Current- 42.5mA
Plate-Cathode Voltage- 298
Plate Current- 40mA
Plate Dissipation- 12w
Cathode Voltage- 12.3
Cathode Resistor- 120r
Cathode Current- 51.5mA
Plate-Cathode Voltage- 355
Plate Current- 49mA
Plate Dissipation- 17w
@Robins (and others)
This seems kinda hot, no?
It is cathode-biased (which I understand is supposed to allow a little higher “safe” dissipation), and I think it’s supposed to be a pretty “hot” amp, but does 17w, when running as designed, sound right?
17W is the absolute maximum for EL84s - hot. 12W is good.
I bet it sounds killer though and the Taco runs hot but try a bit colder to see how you like it.
Congratulations by the way! you got your baby up and running.
You are the man \m/
Let´s have a cigar xD
All the very best,
Thanks. And it does sound glorious, even when I play it. I wish I had someone around who actually knew how to play guitar!
I think I figured out the hot SS rectifier issue: the one component that I didn’t follow the schematic 100% on was the choke resistor. The schematic calls for a 22r, but the closest 10w I could find for sale was 20r.
At the time, buried in all the drawings and notes, staring at the BOM, it seemed like no issue. It’s normal for a calculation to come out that you need, say, 452.7 ohms, and figuring “470r is close enough”. Being only 2 ohms off seemed like no big deal, but I didn’t think about it being a full 10% low.
I’ll try to do the math tomorrow, but I think that extra 2r would probably take something like 20vdc off my b+. Probably enough to make all the difference, and get me down to like 14 or 15w.
I’ll tell you a secret, though: I kinda like the way the way-hot bias sounded. I played it about an hour in the SS “channel”, and it just sounded.... angry.
I play mostly punk and dirty-blues kinda stuff, and it really worked for me.
I don’t know the right term for it, but I like that “squawk” kinda thing an amp does when you attack your strings hard in high-gain. In the hot bias setting, this amp was giving me the coolest squawk-thing I ever heard. It sounded like shearing plate metal- with a quick feedback yelp. (I’m TERRIBLE with guitar/amp sound terms, but hopefully that describes it a little).
Anyway, I’m sure it was the sound of killing a tube, but I liked it- A LOT!
I checked it with the lights off, and I was getting a very faint red glow on the power tube plates. In tube-mode, I didn’t get any glow, and I’ll gladly play it in that mode until I can get a replacement resistor and get my b+ down a little.
Here it is set up with 2x10s (8ohm Rolas that @montemerrick was selling a couple months back). The in-laws are in town, but they and the wife are going out tomorrow. It gets hooked up to the 4x12 the second I hear the door click behind them.
I don’t have any cigars, but, if you email me your address, I’ll totally go get us a couple and send you one. You can let me know when you’re going to light it, and I’ll smoke mine.
Thanks, again, for all your help, brother.
Oh....and the effects loop works, too. No problems.
When I turn the gain way down (and my pickup volume pot down a little), it does a pretty decent clean. I mounted a reverb pedal in the loop, and it sounds nice...for, like, church and stuff.
Brilliant thread, nice quality workmanship. Well done, the insides would almost put an Hi Watt to shame.
I knew that you would love the Pink Taco and it is the right amp for you. Again, you did an amazing job.
And I totally agree with that barking and angry tone from the Pink Taco - that´s why we love those things \m/
Yeah, that would be a cool thing with the cigar but for now I raise my glass to you and you have my total respect. You brought it to life and you came up with that great working layout and your amp really looks pro.
It is a keeper and you got yourself a great sounding Taco.
Happy Christmas/Happy Hollidays and Happy Taco day
Play it loud brother\m/,
Thanks. There are a couple wires at the end that are bothering me- grounding the output jack, and powering and grounding the effects loop were literally an afterthought. With all the planning, I’m not sure how I missed that, but I did.
Either way, I’ve got some tweaking to do, so will probably have a chance to clean that up.
And, thanks, again, brother.
The Stupid Idea is going to go full circle. I’ve already decided that it needs its own dedicated case and cabs, so the whole idea of buying the cheap Marshall mini stack to build in will end up being silly.
That said, I haven’t hurt anything in the Marshall, so I can put it back together, complete, and either sell it or put it somewhere else.
I’ve got some good ideas for a cab build, but I’ll save that for Vol 4.
I got some additional 10w resistors and was able to get the SS recitifed bias down. I did the math the best I could figure it and came up with 170r as the value that might get me down to 12w (120r is “stock”). Given the holidays and shipping schedules, I ordered from Amazon so I could get them quickly. The only values they had that were usable to me were 50 and 100r, so I figured I could add them series to the 120r in there and get 170r or 220r.
For my figures, I assumed the 5% grid current that the Weber calculator assumes.
The stock (measured) 117r cathode reisitor gave me the following:
Plate - 365v
Cathode - 12.3v
Grid - 360v
P->C - 355v
Plate Diss/Current - 17.7w / 49.9mA
With the (measured) 168r in place, I got the following:
Plate - 385v
Cathode - 14.14
Grid - 378
P->C - 371
Plate Diss/Current - 14.8w / 39.9mA
With 215r in place, I got the following:
Plate - 385v
Cathode - 14.9
Grid - 380
P->C - 370.3
Plate Diss/Current - 12.2w / 32.9mA
The Tube Recitier mode (117r) is the following:
Plate - 305.8v
Cathode - 10
Grid - 298.8
P->C - 293.2
Plate Diss/Current - 11.9w / 40.6mA
I had read that increasing the cathode resistor value would raise the plate voltage. Looking at the schematic, I guess it makes sense- higher resistor raises the cathode voltage, which decreases its current, which increases the plate voltage. Supposedly, raising the cathode resistor has diminishing returns because of this relationship.
The odd thing is that I did see a significant Vp and Vk increase when I added the 50r, but, when I added the 100r, the Vk only went up a little bit, and the Vp stayed about the same.
I’m guessing that is because of some non-linear relationship between the elements of the tube that I just don’t understand yet.
Either way, I did get the bias down to the generally recommended levels, and, even with the higher plate voltages, they look to be within the range that lots of factory PP el84 amps are in.
I do have a feeling that I’m going to end up kicking it back up to the 168r/14.8w bias, and see how it goes for a while- or maybe run it at 12.2w for a while, and then switch to 14.8w and see if I notice the tubes wearing out quickly.
Either way, I have a much better understanding of how power-tube bias works in this amp, and won’t be afraid to tweak and maintain that aspect of it in the future.
Remember there is screen grid dissipation and plate dissipation.
Combined dissipation is the limit and screen grid max. dissipation is about 2w and up to 4w for some JJs.
All the best,
If I’m tracking what you’re saying, my grid dissipation, in SS rectifier mode, right now (assuming 5% of the total tube current) is 0.17mA / 0.65w, and, in tube rectified mode, 0.20mA / 0.65w.
That seems a lot lower than what you’re saying the limit is. Am I mis-figuring something, or missing what you’re saying?
Side note, the one example I’ve been able to find of someone online checking the bias of a stock Pink Taco had the power tubes checked at 16.4, and 17.4w. Another user reports getting about 6 months out of a set of tubes, and I notice that Friedman offers a lifetime warranty on their amps, but only 90 days on the tubes.
Do you think the 17w I got to start with was actually in the range of how Friedman ships the amp?
The screen current increase when the tubes get pushed and being loud and overdriven(!).
Dave squeezed the **** out of those tubes esp. on the amps first run with hand wired turret boards.
In the 2nd run (PCB/turret) he used the resistors like in your build. The tubes get "less" stressed.
Less stress in that context means they hold up a few months longer.
I wrote you in that EMail the EL84 "bark" and really are essential for the tone of the Taco.
I am sure you are in the range of a stock Friedman Pink Taco.
Use those JJs and they are good for a few years. I mean you don´t push them hard every day - it is really one LOUD amp as you know.
All the best,