Authentic JTM 45/100 build diary. Clips added.

[neikeel]

Thanks Ted! I got him wrong. Then the 16ohm tap it is for this amp.

Yes that is correct. Marshall moved to 27k on 8ohm and then 47k on 8ohm followed by some randomness (speakers/OT taps) and later 100k on 4ohm. I have played with it and for early amps I like 27k on 16

[VintageCharlie]

Amp is ready, will pick it up later this evening. The tech said it is working fine, he wants to test it on a proper cab before handing it over, to see if all is going well. Can't wait! Will report my first impressions as son as i can turn it up loud.

[Spodeeodee]

Looking forward to your impressions.

Plenty of pics too, right???

[Xplorer]

Very exciting ! Can't wait to hear your clips.

[VintageCharlie]

Hi guys, i just tested the 45/100. The basic tone is there, but i think the bias has to be set up properly. I hadn't bought the 1ohm resistors and the tech didn't have them at hand either, so we just went without them - hence bias is a tricky thing now. Will probably put the 1ohms in there in the future to be able to bias it more easily. I am not sure what the following symptoms mean - if the bias is too hot (too high idle current) or cold (too low idle current):

- volumes set at 0 i still get sound from the amp (but i think this is normal for these amps?).
- very low headroom - at vol. 1 on bright channel and an LP with buckers i can already hear some slight distortion with heavy chords, at 3-4 i'm getting loads of audible distortion and it is approaching ac/dc territory (cleanish aussie crunch thing). From what i've read these amps are pretty much clean even at 4-5 with humbuckers, without any noticeable distortion?
- bit more fizziness to the distortion than these models should produce.
- from about 5 upwards i get audible ghosting pretty much all over the fretboard. (from what i've read jtm 45/100's aren't usually very prone to ghosting? not to speak at such "low" volumes)

At the moment there is a 56k bias range resistor. The tech adjusted it by ear and trimmed the bias pot to the coldest (lowest idle current) setting it allows. He has no direct experience with this specific model, hence adjusting by ear is a tricky thing if you don't know what to expect.

What would you guys say - does all this indicate hot or cold bias? Will the 56k bias range resistor have to be changed to 47k? (also, lowering the bias range resistor allows for colder bias (lower idle current)?

Otherwise the amp is quiet with almost no background noise to speak of.

I'm running it through a 4x12 cab that has 2x g12h 75hz speakers and i run it with -8db on the hot plate (-4db gave the same results, cant try it without the attenuator, but it works fine with my other amp and these issues shouldn't come from the attenuator).

[VintageCharlie]

The ghost notes come out on volume 3 already (with humbuckers), together with quite audible distortion (very slightly audible and get stronger when increasing volume). The low E string sounds a bit farty/fizzy when the amp is set to 5 onwards. The higher strings are quite balanced with the bass strings in volume. The tech said that cold bias is among other stuff indicated by silent higher strings that get louder when the lower strings start to drive the tubes properly.
From what i have read on the net, it seems like the bias now should be quite a bit too hot and a the bias pot is maxed out with the 56k resistor on the lowest idle current setting. This would mean it would have to be changed to the 47k to get it even lower? Does this sound correct?

[Xplorer]

Hello ! You can probably compare with the billy batz layout, having your amp this way and later mod it to your specs again ? And you could verify the voltage grid and measure the bias. I think that it's what i'm gonna do, it worked well with the metroamp build, made to the stock specs, close enough to the voltage grid. I could hear a difference when it was wired on 220 or 240 volts.. I prefered 240, so avout 230 volts came in and the amp is drived less hard. More cleans. Now, i'll mod it little by little... But i have a standard base to refer to.
A bit more patience, you're almost there

[VintageCharlie]

Yes, i think i will add the 1ohm resistors, would have done it right away, but i had forgot to order them.

[Ted B]

What would you guys say - does all this indicate hot or cold bias?

What you're describing sounds like you may be hearing crossover distortion (cold bias). Reserve judgment until you can verify the bias.

Be sure the 1ohm resistors you're getting are low tolerance (e.g. 1%).

[VintageCharlie]

Yes, i can order 1% 1ohm resistors in either .6w or 1w - which would be best for this application?

Thanks!

[Ted B]

The voltage drop across those resistors is so small that they don't need to be large. The 0.6w should be ok, and the 1w plenty.

[Roe]

0.6w acts as a fuse when the tube shorts

[neikeel]

Ted is probably right about crossover but I find that only really kicks in with the amp cooking. My 45/100 is very clean up to 5 or 6 on the dial.

Is that a 4k7 in the middle of the PI? Most common issue with early break up on kits is substitution of 470k instead of the correct 470R. I looked at your pics and certainly not 470k but maybe a 4k7?

If you do not want to add the resistors (or they have not arrived yet you can use the transformer shunt method )

[crane]

Hello everyone!
Hope I'm allowed to participate in this conversation. I'm actually the one who built this device.
Some of you mentioned that the issue could be cold bias - this seems strange to me because the amp sounds fine when played silently. Wouldn't cold bias result as unpleasant uneven harmonics when playing really silently? I actually have biased some amps by turning the bias to the coldest and than slowly turn it back while plucking really silently and check if I haven't reached max current allowed.
My idea of solving this is to measure idle current first, second - put a volume pot after preamp and drive the signal to a sound card - just to locate the problem - if it's in preamp or poweramp.
Right now the biggest problem is the distance between me an VintageCharlie, my tight work schedule and him being the only one who knows the "correct" tone

[VintageCharlie]

Hi guys!

Hi Crane, glad to see you here Thanks for joining in!

Thanks for the tip, Neil, that is a 470r resistor for sure - i measured it together with all other resistors - the code is yellow, purple, brown.

Knowing the "correct" tone is a tricky part, as i haven't heard an original live - just listened to lots of clips and recordings and read many descriptions from owners.
What i have gathered from all this is the following (guys, please correct me, if something is wrong here) basic characteristics of a stock 45/100 (mine has the Larry Grounding and runs on the 490v taps, not 560v though):

- the amp stays really clean up to about 5 on the dial (and it gets a REALLY loud clean before breaking up).
- if ghosting appears, it happens higher up on the deal, when approaching crunch territory at 6-7 on the dial.
- from clips the distortion, especially on the heavier strings doesn't show much fizziness/fartyness

The last two characteristics might be very subjective, but the first one is quite reliable. At the moment the amp shows some noticeable breakup at 1.5 and at 3 it starts to get into slight crunch and ghosting becomes clearly audible. We got the 1ohm resistors and when Crane will be able to free up some time, we will open up the amp, install them, check bias and probably also check relevant voltages (i have a voltage chart from a build with a 490v mains tap from George and i think another one from a forum member to compare) - after the measurements we should have a good starting point.
I'm sure it's a minor problem, as the amp is silent and the basic tone is there apart from what was mentioned above.

My schedule is a mess now too, so it will probably take quite a bit till we can deal with this - so don't hold your breath just yet guys!

Here's a link to the finished amp (high res pic): http://www.failiem.lv/down.php?i=cywpvh ... 0.jpg&view" onclick="window.open(this.href);return false;