changing classic 18 watt to 18 watt modern classic

[Bieworm]

Most of the voltages look good, thought the PI is slightly off.

Did you use shielded wire on the inputs?

Get them chopsticks ready.

Yes I used shielded wires.
How can I manipulate the grid voltages of the PI. I suspect the problem is somewhere after the preamp. Both channels hum.
The power section remains untouched from power transformer to cap can. Maybe the PI is somewhere faulty. Because of the thin sound?

But I will start with checking each tube..

[Bieworm]

Nope not the tubes. On both channels when I strum harder there is an ugly fizz overdrive, even on low volume

[JMPGuitars]

Nope not the tubes. On both channels when I strum harder there is an ugly fizz overdrive, even on low volume

Did you try pulling out the preamp tubes 1 at a time to see how it affects the constant buzz?

Do a highlighter test with both the schematic and layout. Make sure your mods make sense too.

[Bieworm]

Nope not the tubes. On both channels when I strum harder there is an ugly fizz overdrive, even on low volume

Did you try pulling out the preamp tubes 1 at a time to see how it affects the constant buzz?

Do a highlighter test with both the schematic and layout. Make sure your mods make sense too.

I will. Hum loud when all tubes are in. Hum loud with only v1 pulled. Same with only V2 pulled.
But the hum is in the background when v3 is pulled. So it's not the preamp. Got to be something with the PI, but I can't see what...

[JMPGuitars]

I will. Hum loud when all tubes are in. Hum loud with only v1 pulled. Same with only V2 pulled.
But the hum is in the background when v3 is pulled. So it's not the preamp. Got to be something with the PI, but I can't see what...

Also look for frayed wires or anything that could be making a connection that shouldn't be there.

Sometimes buzz is related to a ground issue. Which could mean a bad ground connection, or something connecting to ground that shouldn't be.

[Bieworm]

I will. Hum loud when all tubes are in. Hum loud with only v1 pulled. Same with only V2 pulled.
But the hum is in the background when v3 is pulled. So it's not the preamp. Got to be something with the PI, but I can't see what...

Also look for frayed wires or anything that could be making a connection that shouldn't be there.

Sometimes buzz is related to a ground issue. Which could mean a bad ground connection, or something connecting to ground that shouldn't be.

Found the problem...
It were the wires from jack to point x and y for the fx loop. That mod is giving me nothing but trouble ... I removed them...
Amp is shining like a star now. Dead quiet!!! Noisewise that is...

BUT found another issue.. the trem comes on very slow. It takes a few seconds before it starts...

[JMPGuitars]

Found the problem...
It were the wires from jack to point x and y for the fx loop. That mod is giving me nothing but trouble ... I removed them...
Amp is shining like a star now. Dead quiet!!! Noisewise that is...

BUT found another issue.. the trem comes on very slow. It takes a few seconds before it starts...

Did you properly ground the jacks? I generally ground mine on the same ground line as the speaker outs. Probably more out of convenience than anything else.

Here's a recent build of mine with the FX loop:

_JMP7608.jpg

You can hear how clean that amp is here: https://youtu.be/tY9RUIO3KjY and here: https://youtu.be/3oSZVg2STXw

Remember that lead dress is always important. If you have a wire running the wrong way, it will get angry. If something's not grounded correctly, it will be even angrier.

Try swapping to a different tube for the trem. If that doesn't solve it, do the highlighter test for the trem section.

Thanks,
Josh

[Bieworm]

Found the problem...
It were the wires from jack to point x and y for the fx loop. That mod is giving me nothing but trouble ... I removed them...
Amp is shining like a star now. Dead quiet!!! Noisewise that is...

BUT found another issue.. the trem comes on very slow. It takes a few seconds before it starts...

Did you properly ground the jacks? I generally ground mine on the same ground line as the speaker outs. Probably more out of convenience than anything else.

Here's a recent build of mine with the FX loop:
_JMP7608.jpg


You can hear how clean that amp is here: https://youtu.be/tY9RUIO3KjY and here: https://youtu.be/3oSZVg2STXw

Remember that lead dress is always important. If you have a wire running the wrong way, it will get angry. If something's not grounded correctly, it will be even angrier.

Try swapping to a different tube for the trem. If that doesn't solve it, do the highlighter test for the trem section.

Thanks,
Josh

Aha! I didn't ground the jacks. There was no grounding on the layout of the EF86 extra... damn... that's probably it!

Here's the new voltage chart. PI even more off on the plates. What can I do with that?

As for the trem tube.. I tried 5 different tubes from different brands ...even a 12ax7h. The idd thing is that it was the same tube I had in there before the change-over from 18w classis to modern classic.
Anywayz...the trem oscillation is notorious in regard to the ideal voltages. Will look into that thrash online.. there are plenty of threads that treat this issue...
I had to do the snubber cap on the plates of the PI (150pf) to get rid of too much brightness that gave a lot of fizz. Doing the PR mod afterwards when I get the zeners...

Amp sounds awesome by the way!!!!

15887036049724190524317250561343.jpg

Tomorrow I'm gonna raise the B+ on the PT. Wire it for 230V instead of the 240V now.
My tremolo TMB is wired like that too. I like the extra volume ...it's a pretty significant difference. That will probably lift some voltages and maybe kickstart that tremolo sooner. It's worth a try... it's an easy reversible mod...

[JMPGuitars]

lol! Man, how long until you read the schematic, and not just look at a layout?

Check out those cute triangles on the schematic:

FX-Loop.jpg

It's so much easier to understand what's going on with the FX loop looking at this schematic drawing, than guessing looking at the layout.

[Bieworm]

lol! Man, how long until you read the schematic, and not just look at a layout?

Check out those cute triangles on the schematic:
FX-Loop.jpg
I knew you were going to say that..

It's so much easier to understand what's going on with the FX loop looking at this schematic drawing, than guessing looking at the layout.

Any advice on the voltages?

[JMPGuitars]

Any advice on the voltages?

Why? It sounded like you had a decent plan switching the PT wire. See what happens. Test again. Check your PI connections are good, resistors are good, etc...

[Bieworm]

Any advice on the voltages?

Why? It sounded like you had a decent plan switching the PT wire. See what happens. Test again. Check your PI connections are good, resistors are good, etc...

Indeed! But maybe I should first try lovering the 100k resistor from voltage C to pin 6 of V2? That should juice the voltage to the trem too, no?
By the way... I always wondered... why are there 2 voltages on the voltage chart of tremolo tube on pin 1? Are that the oscillation voltage extremes?

[JMPGuitars]

Indeed! But maybe I should first try lovering the 100k resistor from voltage C to pin 6 of V2? That should juice the voltage to the trem too, no?
By the way... I always wondered... why are there 2 voltages on the voltage chart of tremolo tube on pin 1? Are that the oscillation voltage extremes?

Nah, stick with your plan. It already proved to be the better option for your other amp. Adjust V2 after changing the PT wiring if needed, but start with new voltages.

Yup.

[Bieworm]

Indeed! But maybe I should first try lovering the 100k resistor from voltage C to pin 6 of V2? That should juice the voltage to the trem too, no?
By the way... I always wondered... why are there 2 voltages on the voltage chart of tremolo tube on pin 1? Are that the oscillation voltage extremes?

Nah, stick with your plan. It already proved to be the better option for your other amp. Adjust V2 after changing the PT wiring if needed, but start with new voltages.

Yup.

Thx

[Bieworm]

Did all the changes. 230V power trans tap. 100k resistor from point C to 91k (100k + 1M parallel)
Fx loop Works now!!!And I'm really satisfied!!! Muuuuuuch less digital noise ...neglectable in fact.

So this one is ready and shining:)

Here is the New chart. The green is the final state. Powertubes running at 98.3%, 11.8 watts. Should I change anything?

15887928909337086897979929496126.jpg

[JMPGuitars]

Yes, you should drop your B+, and bias the power tubes better.

What's your current cathode resistor value?

[Bieworm]

Yes, you should drop your B+, and bias the power tubes better.

What's your current cathode resistor value?

180 Ohms
For dropping the B+ I might just as well wire the powertrannie back to 240V.. Same result as zeners.. the b+ was 342V with the 240 V wiring

[JMPGuitars]

180 Ohms
For dropping the B+ I might just as well wire the powertrannie back to 240V.. Same result as zeners.. the b+ was 342V with the 240 V wiring

Figure out what's different between your build power sections:

download/file.php?id=12268&mode=view

download/file.php?id=12328&mode=view

They both have similar tube complements. If they're both running the same PT setup, why the voltage disparity?

[Bieworm]

180 Ohms
For dropping the B+ I might just as well wire the powertrannie back to 240V.. Same result as zeners.. the b+ was 342V with the 240 V wiring

Figure out what's different between your build power sections:

download/file.php?id=12268&mode=view

download/file.php?id=12328&mode=view

They both have similar tube complements. If they're both running the same PT setup, why the voltage disparity?

Comparing the voltage charts I don't find them so much different... what's critical in your opinion?

Well let's look at the differences from power transformer to preamp:
1.
TTMB runs through a SS rectifier and a 150R25W resistor to the first 32uf cap, meets a 2.2k 5W resistor to the 2nd cap. It feeds the EL84 with 2 x 1k 3W resistor. Also feeds the PI plates with a 6.6k 3W resistor. The plates of V2 see that voltage through an 82K resistor and the preamp plate of V1 sees that through a 100k resistor.

MCL runs through an EZ81 rectifier to the first 32uf cap and goes to the second through a 1.5k 5W resistor. That feeds the power tubes through a single 100R 5W resistor and feeds PI plates through an 8.2k resistor. The plate of V2 sees the voltage through 91k resistance and the plates of V1 through 100k.

Must say the power tubes of MCL give higher values than those in TTMB. If I swap them between the amps it's vice versa. Then TTMB reads lower values and MCL higher...

[Bieworm]

Also read in some thread that the position of zeners in the B+ line is variable. I think I saw somebody state that in some position it will do a 50% voltage drop of it's value... (if I understand that correctly)
well I have the 24V 5W zeners... where do I put it to decrease voltage by 12V ? That would be an option... (see attachment for the source)

dropping B+ voltage zener position.png