Synchu wrote: ↑Tue Jul 21, 2020 12:31 pm
Should be close to half the input voltage which is your case and I'd go with 9.4
Just finished a 2nd gen hybrid and the FET boost sounds absolutely phenomenal with this one.
I highly recommend going with the adjustable FET option (i.e. mounting a 10k pot in place of the FET jack, i.e. lifting the trimmer of the board).
Niki
yes, I agree 100% I was never a FET fan until I did this on a build and since then I've done it every build. It really is a great clean boost and having it on a switch is perfect.
Synchu wrote: ↑Tue Jul 21, 2020 12:31 pm
Should be close to half the input voltage which is your case and I'd go with 9.4
Just finished a 2nd gen hybrid and the FET boost sounds absolutely phenomenal with this one.
I highly recommend going with the adjustable FET option (i.e. mounting a 10k pot in place of the FET jack, i.e. lifting the trimmer of the board).
Niki
9.4 it is. Thank you for the advice. I've already got a push/pull pot ready to go and it'll definitely be on a footswitch too.
I've had an abundance of free time this week as I got a bit ahead in my work schedule and I was able to get most of the amp wired up. Working with PTFE cables is no joke and I'm incredibly impressed with all of you who can do such an amazing job at keeping your wiring tidy. I made a few planning errors and didn't wire the layers in the most logical order...I found the area around the relays to be especially challenging but it's also incredibly relaxing to do. All I have to do now to finish the wiring is to connect the relay transformer and the power transformer but I have questions to which I haven't found specific answers or have seen conflicting information.
The easy one first. If I'm using the full wave bridge relay supply and the transformer that Martin recommended (Hammond 166F12C) does the center tap go to ground or get capped?
I may have just made a mistake in purchasing a ClassicTone 40-18004. It is 320-0-320V @450mA. Can I use this or should I try to swap this in the order ASAP for something with a higher voltage?
On the relay transformer, you have a 12V CT secondary. Using the 12V (CT not connected to anything) the full-wave bridge will produce a little less than 12 * 1.414 = 17VDC. The regulator will take that down to 12V for the relays.
40-18004 is a Fender Twin/Showman PT, which will be fine. Classic Tone's data sheet does not say, but I believe they list the loaded voltage. Quoting amp voltages, most people are referring to voltage measured at idle (no signal), which is really about half-loaded. Measure the primary and unloaded secondary voltage when you get it so we can find out.
Last edited by martin manning on Sun Jul 26, 2020 6:04 pm, edited 1 time in total.
While I'm waiting for the transformer, I thought I'd post some photos of where I am right now. I need to triple check the wiring I've done so far, and go over the power tube connections again (I don't think I've quite finished that up) but so many of your have an eagle eye for mistakes so any input is welcome. Thank you all for all the content on this board. I'm still far from understanding all of this but have certainly learned a lot on this first build.
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Looks like some careful work there, nothing obvious jumps out. Look into start-up procedures, and build yourself a bulb limiter (see sticky post in Tech Discussion area). You can do some quick resistance to ground checks at all power supply nodes to identify a real goof, but it's often hard to spot less obvious wiring errors. Start with a very low wattage bulb and no tubes installed to see if you get reasonable voltages (proportional to the reduced voltage on the PT primary) in all the right places. I can't see your HV rectifier board, but it should be set up for a full-wave rectifier, not the FWB shown in the 102 layout.
Pay no attention to the PT. That one will end up in the 002 I'm building next, provided I can get this one working.
Thank you for looking over it. I've already built the bulb limiter and will be reading up on startup procedures over the next few days while I wait for the other PT.
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After finishing the wiring and getting the new PT in, I let it sit for about a week while I read as much as I could about the startup procedure. I'm still not 100% confident that I'm going about this correctly but I pulled up my pants last night and started it up (in standby only) with a 25W bulb in the limiter.
Heaters - 4.95V
Secondaries - 478
Bias Tap - 43.4
B+ - 333VDC
I'm assuming that the limiter did what it's supposed to do and is what's preventing proper voltages from being reached. It glowed bright for an instant at power up and then settled down to about what I expected to see. Nothing has smoked yet, there were no sparks.
One question...I put the meter on all the B+ points (only in standby) and read 2.1mVDC, 2, 1.9, 1.5, 1.5 going from B+1 through B+5. With the standby switch in the off position, I expected to read zero. Is this screwy?
I think you’re ok. I don’t think the very small voltages are anything to worry about. I would keep the bulb limiter in and take it off standby. You should see another brief increase in brightness and then a return to approximately where it was. Check the power supply nodes down the line and expect to see the voltage drop only a small amount as you go down to the last preamp filter. If the voltages are all present, I would turn the bias pot so you have the most negative voltage, power down, put a speaker load on the output, and put the power tubes in. Power up again with the limiter, and this time you should see more brightness initially due to the heater surge current, and it won’t dim as much as before. If you can take it off standby, you are done with the limiter. BTW 25W is pretty low for a 100W amp, I would use a 60W.
If you made it to power tubes in, and standby set to play, with no limiter, you can bring the bias up. Something like 35 mA would be fine to start with. Then put the PI tube in. If it howls, the OT primary leads are probably reversed. If it stays quiet, put the preamp tubes in and see if you get sound from the speaker.
martin manning wrote: ↑Sat Aug 08, 2020 2:23 pm
No problem, happy to help. So, what happened?
If you made it to power tubes in, and standby set to play, with no limiter, you can bring the bias up. Something like 35 mA would be fine to start with. Then put the PI tube in. If it howls, the OT primary leads are probably reversed. If it stays quiet, put the preamp tubes in and see if you get sound from the speaker.
I got the power tubes in. Nothing blew up yet. I have not put any tubes in the PI or preamp at this point
Heaters: 3.35
Bias: 61
Secondary: 334
Bias pot (min). -60.5V @ -0.15mA (read at the wiper)
Bias Pot (max): -50.2V @ -.0.12mA (read at the wiper)
B+: 453VDC with it dropping slightly through the nodes to B5 at 386VDC
Screen: 452
Plate:452
I have a 47K resistor off the bias pot to ground. With the bias readings where they are, do I need to swap this out?
edit: the reading on the bias test points are: 34.5mV, 34.3, 35.3, 34.4 (I just also realized that this is with the pot at max. I'm going to turn it all the way down and try again)
~35 mA is ok for now if you want to keep going, but you would like to have the bias pot at about mid sweep so you have some adjustability. Reducing the value of the 47k to ground is the way to do that. I'd try something like 43k. The readings at the bias test points are what you want to pay attention to. The mV readings are equal to mA of current since you are measuring across 1Ω resistors. Turning the pot down (making the bias voltage more negative) will reduce the idle current.
martin manning wrote: ↑Sat Aug 08, 2020 9:39 pm
~35 mA is ok for now if you want to keep going, but you would like to have the bias pot at about mid sweep so you have some adjustability. Reducing the value of the 47k to ground is the way to do that. I'd try something like 43k. The readings at the bias test points are what you want to pay attention to. The mV readings are equal to mA of current since you are measuring across 1Ω resistors. Turning the pot down (making the bias voltage more negative) will reduce the idle current.
Right after my last post I turned the pot down so I'm reading ~30mV at the test points. I'll move on to the PI now.
