F5Turbo Illustrated Build Guide

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Hi guys, I’m building the F5T-V2 with 25v primaries and I believe 32 volt rails. I’m going to cascode the FE boards. I’m confused what the resistor values for R25, R26, R27, & R28 should be. The BOM says 4.74k-10k and could change depending on the rail voltage. I looked at the formula as described but I’m just not sure. Can someone please assist? Thank you in advance.
 
Building two - one for me and one for a friend. Have a kind of an issue with testing - I built a light bulb tester (incandescent, 75W and 100W tried) - light bulb stays on on both amps and I get 0.6VDC rail voltage. DMM showed 11VAC at the fuse and after testing the bulbs they`re 52ohms where the 75W ones were 64. I suspect this high resistance (which should go up when lit) does not allow the thermistor to heat up and cannot deliver voltage to the rails, but decided to post here. I read your bulbs are 10 ohms or so, could this be the culprit?

When I built two gainclones and a hybrid LM3886 one, it was the same - 100W bulb would remain bright but amps worked fine.
 
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Did you do the dim bulb test with the amp fully built, or were you just testing your PSU?

Kudos for measuring the cold resistance, but it's the hot resistance that matters most for this application. They're going to be in the ~450 ohm (100W) and ~600 ohm (75W) range give or take when they run at full current. Assuming you're on 220VAC.

You may enjoy this video and this article. The video is nice. It even shows (a bit) some measurements of the limited current / voltage to the working amp when still on the dim bulb tester. Article shows some visuals re: bulb choices and how they'll react to the same load.

What is a dim bulb tester - YouTube

Powering Your Radio Safely with a Dim-bulb Tester

Overall, if you're just testing your PSU, you may have an issue. If you're testing a fully built amp, then I'll leave the math to you re: whether your bulb should be "bright" or not with a fully built F5T. Either way, the bulb should still flash brightly and dim some with a fully built amp as the PSU caps charge and then the amp draws steady current. If it immediately goes "fully bright", just like flipping on the bulb normally, then I suspect an issue.

Enjoy the amps. Your friend has a very good friend. :)

PS - Are you certain your dim bulb tester works properly? Asking just to be sure. Your results for your gainclones and LM3886 (fully bright with a 100W bulb) are interesting. However, I don't know the power draw or the PSUs you used.
 
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Fully built with P1 and P2 set to 0 ohms, the PSU is a CRC type and has 162 000uF per channel, 800VA 2 x 24-0-24 transformer. Not even coming close to that chap`s video...(thanks for the links by the way :) ). I didnt use bleeder resistors in the PSU and have not tested it alone yet as discharging post-testing would be slow...
 
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Did you test your PSU separately before attaching the amp boards?

Is your bulb immediately going fully bright just like a normal bulb or is it flashing bright and then dimming and then going brighter, but not fully bright?

If it's immediately going fully bright, you likely have a short somewhere.
 
Speaking of testing the PSU separately, I'm real curious what the output DC voltage is going to end up being. Is this where I can use this variac? And start dialing up the voltage until I hit my wall voltage, 120v? Then measure the outputs on the PSU board, ST_V+ and ST_V-?
 
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You should be in the neighborhood of 35VDC unloaded with 25VAC secondaries (again depending on your PSU and 115 / 120 mains voltage etc.)

I like to use a dim bulb with a 25W bulb on an unloaded supply first to check for dead shorts and other goofy things I may have done.

But to answer your question, yes. The rails will ramp up as you increase the voltage from the variac.

Edited to add, and yes, those are good test points. V+ to GND and V- to GND.
 
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Awesome thank you. Also: this is the F5T thread, so I assume our amps are all relatively close in similarities? How many amps (heh heh) will this puppy be drawing? The IEC inlet I got has space (and requires) two fuses. They're supposed to be about 1.1 - 1.5 x's of what it should be.

Ah also, Mouser screwed up my order of the CL-60 thermistor and mains +/- 3.3 uF safety capacitor. Can I move forward without them and not burn anything down? (Knowing I might throw a breaker here and there during power up)
 
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Any time. You're welcome.

There are 3 primary variants to the F5T that I know of (not including the balanced or X versions). Then, there are variants within those variants based on output devices, bias, rails, choices for source resistors, cascoding etc. Not trying to sound like I know anything at all, but the idea is that it's super versatile.

TL;DR - Yes, there are many overall similarities, but they're each individual to some extent.

For just the purposes of testing your PSU maybe once or twice, particularly when using a variac to ramp up the voltage, I personally wouldn't worry too much about having the inrush limiters in place.

Stable operating current will depend on which version you've built and what bias you're running.

If you throw a breaker, then something went pretty poorly. Tough to tell if you're tongue in cheek or actually worried. If you're worried about any of that, perhaps post a pic or two for people to scan.
 
Awesome thank you. Also: this is the F5T thread, so I assume our amps are all relatively close in similarities? How many amps (heh heh) will this puppy be drawing? The IEC inlet I got has space (and requires) two fuses. They're supposed to be about 1.1 - 1.5 x's of what it should be.

Ah also, Mouser screwed up my order of the CL-60 thermistor and mains +/- 3.3 uF safety capacitor. Can I move forward without them and not burn anything down? (Knowing I might throw a breaker here and there during power up)

Bummer, first time I've heard of Mouser messing something up. I'm just up in Bellingham and have some spare CL-60s, I could mail you some if that would be helpful. I think I saw you on the "premiere" of NocturneVid's part 5 last night?
 
@ItsAllInMyHead yes, sarcasm is often lost in translation. You'd think I would have learned that by now! But nope: not sarcastic. I have tripped breakers from time to time. Always operator error. I'd hope that this amp won't have that sort of immediate draw without the CL-60s (capacitors 8x 22,000µF =176,000 µF) that would trip a breaker. I know the CL-60's serve a purpose, to "limit the inrush of current," but I don't actually know what that looks like yet.

@verticalmammal thanks for the offer! Mouser did upgrade it to overnight, and they're showing up tomorrow(Tuesday 2/23/21) - just in time to miss me by one day on my way back to work for weeks. NocturneVid's? Can't say I recall being in a movie yesterday... <shrug> But I did get into the Laphroaig.... stranger things have happened.
 
Awesome thank you. Also: this is the F5T thread, so I assume our amps are all relatively close in similarities? How many amps (heh heh) will this puppy be drawing? The IEC inlet I got has space (and requires) two fuses. They're supposed to be about 1.1 - 1.5 x's of what it should be.

Ah also, Mouser screwed up my order of the CL-60 thermistor and mains +/- 3.3 uF safety capacitor. Can I move forward without them and not burn anything down? (Knowing I might throw a breaker here and there during power up)

@ItsAllInMyHead yes, sarcasm is often lost in translation. You'd think I would have learned that by now! But nope: not sarcastic. I have tripped breakers from time to time. Always operator error. I'd hope that this amp won't have that sort of immediate draw without the CL-60s (capacitors 8x 22,000µF =176,000 µF) that would trip a breaker. I know the CL-60's serve a purpose, to "limit the inrush of current," but I don't actually know what that looks like yet.

@verticalmammal thanks for the offer! Mouser did upgrade it to overnight, and they're showing up tomorrow(Tuesday 2/23/21) - just in time to miss me by one day on my way back to work for weeks. NocturneVid's? Can't say I recall being in a movie yesterday... <shrug> But I did get into the Laphroaig.... stranger things have happened.

Oh yeah, Texas got hit pretty hard by that cold snap last week. Mouser shut down for a few days, I believe. NocturneVid66 is a guy on YouTube from somewhere near Everett I believe. He is doing a series on his F5T build. I looked and it was not you in the chat for his new video last night.
 
Did you test your PSU separately before attaching the amp boards?

Its what I always do but this time decided to not use bleeder resistors and rely on the bulb so did not test the PSUs alone. Will have to build a small resistor discharge device and disconnect and test the PSUs.

I measured the boards today with a DMM and couldn`t identify any shorts anywhere...I guess will have to test the PSUs alone and if all fine, to swallow a bravery pill and try. I don`t have a variac, unfortunately...
 
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I finally finished my F5T v3 Balanced mono blocks. I have 48v rails, cascode for the JFET's with a cascode voltage of 15.3vdc. I bought the stores transistor kit and PCB's, ordered all the parts off the BOM (making sure that R26 was 10K).
Assembled it using 6L6's build guide.

Using my dim bulb tester, everything is fine, no smoke from the PSU or when the amplifier boards are connected.
While adjusting bias, I can only get 210mV, then I hear both P1 and P2 click at the end of their range.
What do I have to change to increase the bias?
 
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I assume you have 1K at R5 and R6? If you cannot get the bias current up to
the desired level, you need to increase the value of those two resistors.

Do you have anything in the 2k range? Exact value doesn't matter.

You'll need to adjust P1 and P2 back down after the change and before you power up and
start the bias procedure again. (You can verify this with power off by checking that the resistance
values across R5 and R6 are low.)
 
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