1W JTM build

[Bieworm]

I thought ground loops identify as buzz.. not ghosting?
BTW. Preamp ground go to the buss.. buss goes to the ground lug where the input is grounded

[TriodeLuvr]

That's a beautiful amp, BTW. I really like the black faceplate. Very cool!

Jack

[Bieworm]

That's a beautiful amp, BTW. I really like the black faceplate. Very cool!

Jack

Thx. I with it sounded healthy...dang!!!

[TriodeLuvr]

I thought ground loops identify as buzz.. not ghosting?
BTW. Preamp ground go to the buss.. buss goes to the ground lug where the input is grounded

Hum and buzz is usually the symptom. Why was that last clip so different from the first? Was it only a difference in playing level, or did you change the circuit?

Jack

[Bieworm]

I thought ground loops identify as buzz.. not ghosting?
BTW. Preamp ground go to the buss.. buss goes to the ground lug where the input is grounded

Hum and buzz is usually the symptom. Why was that last clip so different from the first? Was it only a difference in playing level, or did you change the circuit?

Jack

No, just playing level..

[TriodeLuvr]

I thought ground loops identify as buzz.. not ghosting?
BTW. Preamp ground go to the buss.. buss goes to the ground lug where the input is grounded

Hum and buzz is usually the symptom. Why was that last clip so different from the first? Was it only a difference in playing level, or did you change the circuit?

Jack

No, just playing level..

Well, it might be due to blocking. That's where overdrive causes bias level(s) somewhere to shift in a way that obscures half the waveform. The remaining energy is half-frequency. This probably needs a scope to troubleshoot, but OTOH, there is such a thing as simply having too much signal. Maybe the gain control needs a governor.

Jack

[Bieworm]

I don't think the grounding is the problem in this amp.
I scoped yesterday and I can get perfect sine waves all through the amp with low volume and gain settings. I'm not sure that counts for the output...so maybe the problem is the OT or that totally unuseable .1W switching. I'm going to try connecting the OT secondaries directly to the output jacks and evaluate that.
Jack may be onto something too with the too much gain thing. But this being a 1W and my hearing I'm not sure the amp dialed to clean and audibility is even possible...
One other thing that may be important.. when I connected the preamp from the jtm145 to the output of the tremolo TMB the sound was really thin and sharp.. that would indicate the fatness in the jtm145 is created in it's power section? No?
Well.. that would mean the power section there is something overcompensating things?

[Bieworm]

Hum and buzz is usually the symptom. Why was that last clip so different from the first? Was it only a difference in playing level, or did you change the circuit?

Jack

No, just playing level..

Well, it might be due to blocking. That's where overdrive causes bias level(s) somewhere to shift in a way that obscures half the waveform. The remaining energy is half-frequency. This probably needs a scope to troubleshoot, but OTOH, there is such a thing as simply having too much signal. Maybe the gain control needs a governor.

Jack

Jack, if I understand this correctly.. too much signal comes from the preamp and hits the power stage. That might be the problem.. or at least part of it. I noticed when I injected the preamp output signal from the tremolo TMB into the power stage of the jtm145 that it was much more quiet than with the jtm145 preamp. Does that explain things to some degree?

[TriodeLuvr]

when I connected the preamp from the jtm145 to the output of the tremolo TMB the sound was really thin and sharp.. that would indicate the fatness in the jtm145 is created in it's power section?

I noticed when I injected the preamp output signal from the tremolo TMB into the power stage of the jtm145 that it was much more quiet than with the jtm145 preamp. Does that explain things to some degree?

The second test is somewhat inconclusive because the level from the tremolo TMB preamp isn't known. The first test seems to indicate that it's the power stage that is being overloaded. And honestly, I would expect a 1W output stage to overload easily. You could confirm it by pushing the amp like you did in the second sound clip and scoping it under those conditions.

Do you have access to a Marshall 1W that you could test with your guitar? It would be interesting to know how it sounds in comparison to your amp at high gain settings. Also, is there any reason to think the 12AU7 might be a little tired? Just thinking out loud here. That could also contribute to early overload.

Incidentally, I really like the tone of that first clip. Be sure to document your changes as you work through this. Don't want to lose that combination!

Jack

[Bieworm]

when I connected the preamp from the jtm145 to the output of the tremolo TMB the sound was really thin and sharp.. that would indicate the fatness in the jtm145 is created in it's power section?

I noticed when I injected the preamp output signal from the tremolo TMB into the power stage of the jtm145 that it was much more quiet than with the jtm145 preamp. Does that explain things to some degree?

The second test is somewhat inconclusive because the level from the tremolo TMB preamp isn't known. The first test seems to indicate that it's the power stage that is being overloaded. And honestly, I would expect a 1W output stage to overload easily. You could confirm it by pushing the amp like you did in the second sound clip and scoping it under those conditions.

Do you have access to a Marshall 1W that you could test with your guitar? It would be interesting to know how it sounds in comparison to your amp at high gain settings. Also, is there any reason to think the 12AU7 might be a little tired? Just thinking out loud here. That could also contribute to early overload.

Incidentally, I really like the tone of that first clip. Be sure to document your changes as you work through this. Don't want to lose that combination!

Jack

I don't have access to another 1W... I never came across one either. So that's a no-go
The 12AU7 is a NOS GE JAN 6189W... would be a shame that one is a goner. I will have to buy and try another. Tried a 12at7 but that's uncontrollably OD

[JMPGuitars]

I don't have access to another 1W... I never came across one either. So that's a no-go
The 12AU7 is a NOS GE JAN 6189W... would be a shame that one is a goner. I will have to buy and try another. Tried a 12at7 but that's uncontrollably OD

You shouldn't build new stuff with NOS tubes unless they're cheap and fully tested. Otherwise, you're asking for trouble.

[Bieworm]

I don't have access to another 1W... I never came across one either. So that's a no-go
The 12AU7 is a NOS GE JAN 6189W... would be a shame that one is a goner. I will have to buy and try another. Tried a 12at7 but that's uncontrollably OD

You shouldn't build new stuff with NOS tubes unless they're cheap and fully tested. Otherwise, you're asking for trouble.

It was a bargain at 10€, cheaper than a new production tungsol or mullard
But I'm ordering a mullard anyway, and also a hammond 125A for that matter. I accidentally connected CT to ground when I built this amp. And started it up with the bulb.. the OT still works, but Who knows it's compromised?
If it's still ok I can always use the hammond for reverb TX of a next build

[JMPGuitars]

I don't have access to another 1W... I never came across one either. So that's a no-go
The 12AU7 is a NOS GE JAN 6189W... would be a shame that one is a goner. I will have to buy and try another. Tried a 12at7 but that's uncontrollably OD

You shouldn't build new stuff with NOS tubes unless they're cheap and fully tested. Otherwise, you're asking for trouble.

It was a bargain at 10€, cheaper than a new production tungsol or mullard
But I'm ordering a mullard anyway, and also a hammond 125A for that matter. I accidentally connected CT to ground when I built this amp. And started it up with the bulb.. the OT still works, but Who knows it's compromised?
If it's still ok I can always use the hammond for reverb TX of a next build

Sovtek or EH are both usually cheaper, and okay tubes. At least good enough for testing, and some of them actually sound great. Good NOS 12AU7 tubes can easily run 10 times or more than what you paid, so that might be a clue right there too.

[Bieworm]

You shouldn't build new stuff with NOS tubes unless they're cheap and fully tested. Otherwise, you're asking for trouble.

It was a bargain at 10€, cheaper than a new production tungsol or mullard
But I'm ordering a mullard anyway, and also a hammond 125A for that matter. I accidentally connected CT to ground when I built this amp. And started it up with the bulb.. the OT still works, but Who knows it's compromised?
If it's still ok I can always use the hammond for reverb TX of a next build

Sovtek or EH are both usually cheaper, and okay tubes. At least good enough for testing, and some of them actually sound great. Good NOS 12AU7 tubes can easily run 10 times or more than what you paid, so that might be a clue right there too.

Ordered the mullard and 125A . I'll see from there..

[Bieworm]

I managed to lower the sub notes somewhat.

* lowered B+ to 229V
* V1a 1k5 cathode resistor to 2k2. No bypass cap
* added 47uf to the 33uf on the preamp filter cap
* (tail) resistor of the PI from 56k to 28k ( paralleled 2 56k's)
* 1k 5W cathode resistor. Got 9.4V on the cathode and 229V 9n the plate

Now waiting for the new 12AU7 to see if that takes out the subtones some more

[JMPGuitars]

I managed to lower the sub notes somewhat.

* lowered B+ to 229V
* V1a 1k5 cathode resistor to 2k2. No bypass cap
* added 47uf to the 33uf on the preamp filter cap
* (tail) resistor of the PI from 56k to 28k ( paralleled 2 56k's)
* 1k 5W cathode resistor. Got 9.4V on the cathode and 229V 9n the plate

Now waiting for the new 12AU7 to see if that takes out the subtones some more

I'm gonna go out on a limb and say that 80µF is too high for your preamp filter. Use one or the other, not both. You can also try strapping a film cap in parallel with the preamp filter cap. Try the 47µF filter cap with a 0.047µF film cap in parallel. Or the 33µF with a 0.033µF.

I don't think lowering the B+ will help with muddiness? If you're feeding the power amp too much, you need to reduce the level going to it from the preamp, not from the B+ line.

Post an accurate schematic of what you're currently working with.

Thanks,
Josh

[TriodeLuvr]

* (tail) resistor of the PI from 56k to 28k ( paralleled 2 56k's)

That's not a tail resistor. This is a split-load (cathodyne) phase inverter, so the two 56K resistors need to be the same value for balanced output.

Is there a reason why the wiper (output) of the treble pot is connected to the wiper of the MV? That shorts the tone stack and changes the frequency response when the MV is turned down. I would use the top of the MV for this connection and maybe add a resistor between the treble pot and the MV to reduce gain some. Better to control the gain here than by shunting 28K across half the PI.

Jack

[Bieworm]

Remarks noted...
I'll post a schematic with changes this far.
You're right about the tail thing. Just been reading what merlin wrote about cathodyne PI

[TriodeLuvr]

Remarks noted...
I'll post a schematic with changes this far.
You're right about the tail thing. Just been reading what merlin wrote about cathodyne PI

Different authors often describe the various topologies with focus on characteristics most important to their own applications. This is understandable, but it can produce omissions that are critical in understanding and applying the circuit under varying conditions. I have long been a fan of the cathodyne for the reasons pointed out on Merlin's page. Lately, however, I have gained a strong affinity for the long tailed pair. Ironically, it has nothing to do with the 36W I just built, which would have indeed employed a cathodyne if not for the fact I wanted to clone the original Marshall sound as much as possible.

I won't go into the reasons why I believe the LTP is superior, because the advantages it brings to the table are mostly (but not entirely) relevant to achieving low distortion hi-fi reproduction. Nevertheless, I believe it is so much better, I will probably not build another amp with anything other than a long tailed pair for the PI. Low power amplifiers (such as your 1W) are probably the only exception.

I'm only mentioning all this because I think it's important to study a design from different perspectives before reaching hard and fast conclusions. Many articles have been written about the cathodyne going back decades, and many of these are available online. Each one you read will open up your understanding of the circuit and how it operates (and just as important, now it integrates into the surrounding amplifiers).

As a side note, the arc protection Merlin recommends is a technique that's needed in other circuits as well, when solid state rectification is used. Cathode followers with close grid-cathode spacing usually need it (I think he mentions this on a different page), and so does the long tailed pair if its input is DC coupled. I'm generally allergic to the inclusion of solid state devices anywhere in the signal path of vacuum tube audio gear, I use a NE2 bulb, rather than the diode. I seem to remember that Merlin also shows an example of that technique somewhere.

Jack

[Bieworm]

this is the schematic with tweaks this far...

schematic execution with tweaks.png