Amp build for a first time builder?

[bcmatt]

Maybe Yoyo has never actually seen a transformer or his experience with them is so "light" that 3.5mm could be plausible measurement in his mind? We who have repeated exposure to the physical stuff know right away, it's got to be 3.5" x 3.75". Yet, we typically don't express inch measurements in decimal fractions.

Haha! 3.5mm would not be plausible, but non-americans would be thinking in cm, which is not so ridiculous especially if you don't have a specific idea of the size of these things.
I must admit that my first experience of tube transformers was from my 70s Traynor. The first time I saw some supposed fender build of someone's I could not believe how tiny the OT was and assumed it must not be more than a 5 watt amp... because my perception was skewed by my experience.

So, if you said the OT mounting holes were 4cmx4cm, I probably would be thinking ah, typical Fender Time Lord technology. (It must be bigger on the inside like the TARDIS).

[Phil_S]

Maybe Yoyo has never actually seen a transformer or his experience with them is so "light" that 3.5mm could be plausible measurement in his mind? We who have repeated exposure to the physical stuff know right away, it's got to be 3.5" x 3.75". Yet, we typically don't express inch measurements in decimal fractions.

Haha! 3.5mm would not be plausible....

But I think you got the drift.

[martin manning]

I'm hoping this thread can be kept alive, warts and all, until a Bassman comes out at the end. These are the same issues and descisions anyone new to amp building will encounter, so I think it will be a valuable archive.

I've tinkered a bit more with the modernized layout. If I were building a 5F6-A, this is how I'd do it. It now has a 50k bias trimmer for a larger bias range, and the trimmer has been relocated to keep more distance between the B+ and bias eyelets. There is also an elevated filament CT now to go easy on the CF's Vh-k, and it also doubles as a HT bleeder. The 10uF across the 47k in that network is probably unecessary since it branches off from the screen node, but I've left it in anyway. The required modification to the board for the adjustable bias and elevated CT is limited to adding four new eyelets, which are shown in blue.

Edit: Layout and BOM updated, build guide updated Dec 2014 (v. 3c)

[M Fowler]

Well I'm with Dan on this one, I don't think anyone should be building an amp with 450 volts involved when they ask the kind of questions that have been asked on this thread, sorry but that is my opinion. I'm not just talking about the unit of measurement either.

Go ahead and spend the money on parts not knowing if they fit I have and learn each time from my mistakes that's why I was passing along information on chassis vs transformers.

Mark

[martin manning]

Go ahead and spend the money on parts not knowing if they fit I have and learn each time from my mistakes that's why I was passing along information on chassis vs transformers.

Mark, I don't think anyone is suggesting that Ryan should do that. The possibility of an issue with the fit-up has been raised a couple of times, and it has to be resolved before ordering parts.

[skyboltone]

That scheme looks really good Martin. I'll have to build one now....after my DR. I've got all the parts gathered for it now; including more than $500 worth of NOS tubes. I'm going to use a JBL 123 initially.

I've just been so tangled up with living over these last couple of years that I really haven't gotten anything done.

Anyway Ryan; if you buy the iron you want and the mojo chassis (a good one I think) you can patch the existing power transformer cutout and make a new one, or modify the existing cutout. As Mark mentioned it isn't ideal from a cosmetic standpoint but it is certainly usable.

Most of us sell these things when we've finished horsing around with them so we try to make things as custom looking as possible. But that really isn't your goal with your first build.

Keep at it.

Dan

[tubeswell]

If you buy the weber 5F6A kit sans transformers and tubes, you can put whatever 5F6A-spec PT and OT and choke you want on the back of it.

https://taweber.powweb.com/store/kits_50a.htm#5F6A

If it is 5F6A spec it should fit, and you can ask CJ Sutton at Weber about what ones will fit the chassis if you are in doubt. ( And even if anything you buy doesn't fit, its usually nothing that a hand-drill and bit of elbow-grease with a metal file won't fix).

There are other brands of trannies on the Weber site for 5F6A amps e.g.:


OTs

Heyboer HY022855 ($68 )

Mercury FO50BM-2 (OFBF-50) ($136)

(But even the Weber one will work fine, and it is included in the overall kit price of $680)


PTs

Heyboer HY022798 ($78 )

Mercury over the top range depending on voltages etc that you might want. I'd shoot for something that gives a B+ of ~430VDC when plugged into your particular mains wall voltage.

(But even the weber brand one will be fine if its US voltages, and its included in the kit price)


Chokes

Mercury C FC-TBAND ($38 )

Even the weber brand one will work fine (and its included in the kit price)


The Weber speakers are awesome, and you can put whatever combination you want in (if you don't go for the stock sig10s included in the kit price).

The other parts, pots, resistors caps etc and rats and mice cost and you can easily ditch whatever you want.

Regarding your question about cap markings earlier, see the attached blurb on E-series cap markings. For the purpose of guitar amps, caps are typically marked for a maximum DC voltage rating (e.g. 400 or 600 etc) and their capacitance value either in microfarad (uF), nanofarad (nF), or picofarad (pF) values. (1farad = 1,000,000 microfarads, or 1,000,000,000 nanofarads, or 1,000,000,000,000 picofarads). Hence .1uF = 100nF and .022uF = 22nF and .001uF = 1nF = 1,000pF etc.

Resistors are usually colour-coded, but the colour-coding varies in different countries, so you should have a multimeter handy to check them before you solder them in anyway. (And you'll need to tool-up with a range of basic tools including one or more multimeters, if you're going to build your own amps). Theres a lot of mojo talk around about which resistors are better for what. If you don't like hiss, then use metal film (MF) types for plate and grid-load and cathode load resistors, otherwise the higher the power rating of the resistor, the less likely it will be to hiss anyhow. You need to be aware that carbon comps (CCs) are the most prone to heat damage by over-soldering (causing drifting in resistance values) if you're planning on using them.

Tubes you can trying to your heart's content once you get the amp built and functioning normally.

[martin manning]

Sorry, had to update the modernized layout above- forgot to make the schematic match the new location of the bias trim pot, and there were a couple of other small typo's.

[jbefumo]

Hi All,

Have built a few amps over the years, always simply following a stock schematic, and not really knowing what I was doing. This time I actually studied up, read some books, and took a shot at designing my own.

So far, it's pretty promising, and I'm optimistic that it will ultimately sound pretty decent. Having a problem that has me a bit baffled, illustrated in the attached schematic.

Basically, when I engage the cathode bypass capacitor in the second preamp stage, a get a high-pitched squeal - not overwhelmingly loud, but loud enough. Happens regardless of the value of capacitor I use.

This only happens when I'm running through the power stage. If I remove the power tube and tap the signal as shown, and run it to an external amp, everything sounds fine, so I'm guessing maybe some kind of feedback issue from the power amp?

Any tips would be much appreciated. Really just shooting in the dark at this point.

Thanks.

[martin manning]

Hi All,

How about starting a new thread for this?

[Structo]

Yes jbefumo, please start another thread about your amp.

Tell us about your prototype and I'm sure someone can help you out.

Welcome to the Amp Garage.

[yoyohomieg5432]

ok, so I just called mojo and asked for the dimensions for the 5F6 chassis. here's what i got:

PT cutout: 3.25 x 2.5
OT: 3.25x2
choke: 2.75

if i'm reading things correctly, this probably won't work out. the PT should be 3 7/16 x 2.75. am I looking at the right dimensions? (from here http://www.classictone.net/40-18029.pdf). the dimensions are reasonably close and it sounds like he was quickly measuring things while he was on the phone with me, so I'm not sure how much error might be involved with that.

the OT seems to be 3.5x2 (from here http://www.classictone.net/40-18000.pdf). again, it's pretty close and I'm not sure how much error might be involved from his end.


the choke is 2 13/16 (from here http://www.classictone.net/40-18003.pdf). this essentially 2.75, so this one seems like it should work out.

any opinions? not really sure what to do.

[martin manning]

I agree it doesn't look like they are compatible. Here's another idea: Hoffman Amps http://www.hoffmanamps.com/ has a 5F6-A chassis which I believe is Mojo's for $80, and the cap pan for $22. He has transformers including an OT with 2/4/8 secondaries for $75, a PT for $90, and the choke for $18. If he will confirm that the transformers he sells will fit in the chassis he sells, you would be in business. Its kind of funny but there are people selling Mojo stuff for both a bit more and a bit less than what Mojo charges for it.

[ToneMerc]

ok, so I just called mojo and asked for the dimensions for the 5F6 chassis. here's what i got:

PT cutout: 3.25 x 2.5
OT: 3.25x2
choke: 2.75

if i'm reading things correctly, this probably won't work out. the PT should be 3 7/16 x 2.75. am I looking at the right dimensions? (from here http://www.classictone.net/40-18029.pdf). the dimensions are reasonably close and it sounds like he was quickly measuring things while he was on the phone with me, so I'm not sure how much error might be involved with that.

the OT seems to be 3.5x2 (from here http://www.classictone.net/40-18000.pdf). again, it's pretty close and I'm not sure how much error might be involved from his end.


the choke is 2 13/16 (from here http://www.classictone.net/40-18003.pdf). this essentially 2.75, so this one seems like it should work out.

any opinions? not really sure what to do.

I think that PT (18029) will work, it has the same mounting centers as the PT that should be used. It appears you were given the cutout verses mounting centers. IIC, 3.5*2.5 should be about the cutout for that PT. I would have them verify the mounting centers.

TM

[martin manning]

Here's the link to the cut-out drawing: http://www.classictone.net/OOA-85285_Ch ... _C-400.pdf The required cut-out is 3.46 x 3.13 so maybe it is ok, but the mounting centers are still in question. FWIW the Bassman PT Mojo sells has 3.44 x 2.75 mounting centers same as the MC.

The page with the various options is here: http://www.classictone.net/40-18073.html From my earlier post, 120V 40-18073 PT at $81.11, 40-18000 OT w/ 2/4/8 secondaries at $64.63, and 40-18003 at $17.60 for a total of $163.34

Another way to go about this is buy the chassis so you have it in hand, then measure for yourself and find transformers that fit. It's really kind of sad that it is so hard to get this information out of these guys.