Anderton '64 reissue JTM145 - any info?

[Smokebreak]

Ok I found the thread you're talking about. good read : http://www.ax84.com/bbs/dm.php?thread=281497
While the plate to cath cap may be towards the same effort,
I'm talking about a small cap across the 100K feeding the plate.
My CF knowledge is limited as well. In the past I've fiddled with the cap value. You should be able to tell one way or the other if it sounds like you're getting more of a blanket over the amp effect, which may be good to tame the highs, but may choke the life out of an amp.

edit : you could also increase the 100p at the input

[Littlewyan]

The .1uF Caps on the HT line are for filtering as far as I know. If the 33uF Caps are doing their job properly and the amp is laid out well then the .1uF Caps shouldn't be needed. I wouldn't increase the 100pF Cap on the input as this is just for shunting RF Frequencies to ground at the input. Also caps across plate resistors are just band aids and are quite an extreme measure. Lets start simple

Have you used Pots for the Tone Controls or fixed values? If you've used pots have you tried turning down the treble and then increasing the mids?

Edit: Just read most of that thread, very interesting. I can see how the .1uF Caps would affect the sound, however they wouldn't affect the low end as they aren't large enough to affect the low end. If anything they'd lower the brightness.

[arjepsen]

Yeah, I put in the tmb pots.
I can dial down the treble some, but I'd prefer an amp that didn't have the treble all the way down constantly...

I don't know enough to give a technical explanation of what the .1's actually do... but to my ears the bass sounded more flabby when I disconnected one of them.

[arjepsen]

any other suggestions as to where I could tame some of the treble?

[Littlewyan]

I know what you mean, its nice to keep all the dials at 12 and have a nice. This is very odd though as normally with a JTM45 you'd turn the treble up to 8 and I believe its the same with this circuit. When I watch youtube vids of this amp the treble is always up. You could remove or lower the value of the 470pF Cap that is bypassing the 470K Resistor after the first stage. I think there is something amiss here though.

Have you double checked your values against the schematic? What speakers and what guitar are you using? Have you tried swapping valves?

[Matec]

Hello arjepsen

I agree with the Littlewyan said.

The amplifier has a bright sound, but also has good bass.

There may be some mistake in your mounting?

The idea of changing the capacitors of the source might help, but it sure does not solve these problems.

The output transformer has the correct specifications? Correctly connected?

The problem may be in the path the signal travels.

Maybe with photos and sound samples we can help you.

Good luck

[arjepsen]

Yeah, I'm fairly certain the values fit with the schematic.
I'm playing it through a 12gh30 in a cabinet I built from the plans at ax84.com. Right now I'm playing my own home made warmoth strat through it. (lindy fralin pickups). It's a fairly bright guitar.
But I also built a sort of "DC-15" or 2-channel lightning style amp, which is inherently bright - but this little thing is brighter than that.
There are other posts in this thread that suggests that I'm not the only one finding the stock schematic too bright.

Hope you understand - I'm not saying anything is wrong with the stock schematic - I'm just trying to find ways to tame that brightness.

[Matec]

I understand.

Following this scheme:

[img609]http://s26.postimg.org/8ujnrkm15/HTM1_R ... 4_2014.png[/img]

Disconnect C201, C20 and C24.

The sound must have been "normal"

If you need more bass frequencies, make a connection eliminating C21.

If still, the sound is too bright, your amp really has a problem.

[Littlewyan]

Also double check the Tone Stack connections, although if this wasn't hooked up properly NORMALLY you'd have a hell of a lot more bass and mid.

[Smokebreak]

Pictures, transformer specs, voltages could go a long way here.

I think we're also assuming you've tried every suggestion, and none of them have worked for you?

To clear up, I believe the only mention of the amp being too bright, according to the second schematic posted, was by me. I've stated that I built the amp up without the bright cap, and the clips represent that build. When I later put the bright cap in, yes, the amp was too bright for my ears, and Matec confirmed that he couldn't read the value clearly. Suggestions have been made to keep it around 100p, or remove it completely, or put values on a switch, to taste

Clipping the 470p peaker in parallel with the 470K WILL kill some treble, but also kills that Marshall sizzle, to my ears. I go the other direction and add a larger cap to the peaker, usually 2n2. It fattens things up, and gives the illusion of less highs. You can clip it in parallel to the existing 470p/470K to see if that's headed in the right direction.

[arjepsen]

Well, I haven't tried every suggestion yet.
I tried without the bright cap, and it helped some.
I also changed c202 to 220pf instead. (Any estimate on how big a voltage swing is going across that one? I put in a 160V cap, and it seems fine - though I like to be sure
These changes have brought it down to a bearable level.
I've tried putting in a bright cap, since I felt there was too big a difference when turing up the gain. But so far, it actually even get a bit too shrill with 47pf in there.
I also have a little bit of fizzyness going on when fully cranked.
I'm wondering about the filter across the plates at the output stage.
Any suggestions for value changes for that filter?

And sorry for not saying it before: Thankyou guys so much for your help so far!

[arjepsen]

Ok, here's a few pics. It's a bit of a quite cramped ratsnest....
There is a few changes to how it looks now - but mostly I tried the cap across the first 100k plate resistor, but removed it again while trying to deal with some weird sounds that turned out to be from connecting the heater ground to the cathode of the 12au7.
The OT is a heyboer with 23k across the plates.
The PT is a toroidal I bought from tubetown in germany. I've connected it to the 240V taps, to lower the volt a bit.
b+1:261
b+2: 248
b+3: 242
12au7: Plates: 257, Cathode: 9,18
PI: P: 217, C: 32
V3: P:166, C:1,2
V2: P: 156, C1,2
V1: P: 181, C: 1,6

[arjepsen]

doh - forgot pictures.

[Littlewyan]

Ok looks like we have a few possibilities here.

1. The fizzyness is due to the Negative Feedback actually being positive. Try swapping the 12AU7 Grid wires or anode wires around. Either one will do, both will have the same effect.

2. What do you mean by 9,18 on the Cathodes of the 12AU7? Do you mean 9V on one side and 18V on the other?

3. How can you have 181V on V1 when you have 156V on V2? V1 should be lower than V2.

4. The underside may look messy but looks pretty smart on top. Nice and compact.

Must admit I can't see much in the photo, not very good with tracing that way.

[arjepsen]

1. The fizzyness is due to the Negative Feedback actually being positive. Try swapping the 12AU7 Grid wires or anode wires around. Either one will do, both will have the same effect.

2. What do you mean by 9,18 on the Cathodes of the 12AU7? Do you mean 9V on one side and 18V on the other?

3. How can you have 181V on V1 when you have 156V on V2? V1 should be lower than V2.

1: I believe you mean changing the blue and brown wire to the 12au7 plates right? Otherwise I think I need more explanation.

2: The cathodes of both triodes of the 12au7 are tied together - measuring 9,18 volts to ground.

3: Yeah, I found that curious too. Can it be related to the cathode components on that first stage? The next two stages only have 1k5.?